.. include:: replace.txt
.. highlight:: cpp

++++++++++++++++++++
Design Documentation
++++++++++++++++++++


--------
Overview
--------


An overview of the  LTE-EPC simulation model is depicted in
the figure :ref:`fig-epc-topology`. There are two main components:

 * the LTE Model. This model includes the LTE Radio Protocol
   stack (RRC, PDCP, RLC, MAC, PHY). These entities reside entirely within the
   UE and the eNB nodes.

 * the EPC Model. This models includes core network
   interfaces, protocols and entities. These entities and protocols
   reside within the SGW, PGW and MME nodes, and partially within the
   eNB nodes.


.. _fig-epc-topology:
   
.. figure:: figures/epc-topology.*
   :align: center

   Overview of the LTE-EPC simulation model


.. _sec-design-criteria:

---------------
Design Criteria
---------------


LTE Model
+++++++++

The LTE model has been designed to support the evaluation of the following aspects of LTE systems:  

 * Radio Resource Management
 * QoS-aware Packet Scheduling
 * Inter-cell Interference Coordination
 * Dynamic Spectrum Access

In order to model LTE systems to a level of detail that is sufficient to allow a
correct evaluation of the above mentioned aspects, the following requirements
have been considered:

 #. At the radio level, the granularity of the model should be at least that
    of the Resource Block (RB). In fact, this is the fundamental unit being used for
    resource allocation. Without this minimum level of granularity, it is not
    possible to model accurately packet scheduling and
    inter-cell-interference.
    The reason is that, since packet scheduling is done on
    a per-RB basis, an eNB might transmit on a subset only of all the available
    RBs, hence interfering with other eNBs only on those RBs where it is
    transmitting.
    Note that this requirement rules out the adoption of a system level simulation
    approach, which evaluates resource allocation only at the granularity of
    call/bearer establishment.
 #. The simulator should scale up to tens of eNBs and hundreds of User
    Equipments (UEs). This
    rules out the use of a link level simulator, i.e., a simulator whose radio
    interface is modeled with a granularity up to the symbol level. This is because
    to have a symbol level model it is necessary to implement all the PHY
    layer signal processing, whose huge computational complexity severely limits
    simulation. In fact, link-level simulators are normally limited to a single eNB
    and one or a few UEs.
 #. It should be possible within the simulation to configure different cells
    so that they use different carrier frequencies and system bandwidths. The
    bandwidth used by different cells should be allowed to overlap, in order to
    support dynamic spectrum licensing solutions such as those described
    in [Ofcom2600MHz]_ and [RealWireless]_. The calculation of interference should
    handle appropriately this case.
 #. To be more representative of the LTE standard, as well as to be as
    close as possible to real-world implementations, the simulator
    should support the MAC Scheduler API published by the FemtoForum
    [FFAPI]_. This interface is expected to be used by femtocell manufacturers
    for the implementation of scheduling and Radio Resource Management
    (RRM) algorithms. By introducing support for this interface in the
    simulator, we make it possible for LTE equipment vendors and
    operators to test in a simulative environment exactly the same
    algorithms that would be deployed in a real system.  
 #. The LTE simulation model should contain its own implementation of
    the API defined in [FFAPI]_. Neither
    binary nor data structure compatibility with vendor-specific implementations
    of the same interface are expected; hence, a compatibility layer should be
    interposed whenever a vendor-specific MAC scheduler is to be used
    with the simulator. This requirement is necessary to allow the
    simulator to be independent from vendor-specific implementations of this
    interface specification. We note that [FFAPI]_ is a logical
    specification only, and its implementation (e.g., translation to some specific
    programming language) is left to the vendors. 
 #. The model is to be used to simulate the transmission of IP packets
    by the upper layers. With this respect, it shall be considered
    that in LTE the Scheduling and Radio Resource Management do not
    work with IP packets directly, but rather with RLC PDUs, which are
    obtained by segmentation and concatenation of IP packets done by
    the RLC entities. Hence, these functionalities of the RLC layer
    should be modeled accurately.


EPC Model
+++++++++


The main objective of the EPC model is to provides means for the
simulation of end-to-end IP connectivity over the LTE model. 
To this aim, it supports for the
interconnection of multiple UEs to the Internet, via a radio access
network of multiple eNBs connected to a single SGW/PGW node, as shown
in Figure :ref:`fig-epc-topology`.

The following design choices have been made for the EPC model:

 #. The only Packet Data Network (PDN) type supported is IPv4.
 #. The SGW and PGW functional entities are implemented within a single
    node, which is hence referred to as the SGW/PGW node.
 #. The scenarios with inter-SGW mobility are not of interests. Hence, a
    single SGW/PGW node will be present in all simulations scenarios 
 #. A requirement for the EPC model is that it can be used to simulate the
    end-to-end performance of realistic applications. Hence, it should
    be possible to use with the EPC model any regular ns-3 application
    working on top of TCP or UDP.
 #. Another requirement is the possibility of simulating network topologies
    with the presence of multiple eNBs, some of which might be
    equipped with a backhaul connection with limited capabilities. In
    order to simulate such scenarios, the user data plane
    protocols being used between the eNBs and the SGW/PGW should be
    modeled accurately.
 #. It should be possible for a single UE to use different applications
    with different QoS profiles. Hence, multiple EPS bearers should be
    supported for each UE. This includes the necessary classification
    of TCP/UDP traffic over IP done at the UE in the uplink and at the
    PGW in the downlink.
 #. The focus of the EPC model is mainly on the EPC data plane. The
    accurate modeling of the EPC control plane is, 
    for the time being, not a requirement; hence, the necessary control plane
    interactions can be modeled in a simplified way by leveraging on direct
    interaction among the different simulation objects via the
    provided helper objects.
 #. The focus of the EPC model is on simulations of active users in ECM
    connected mode. Hence, all the functionality that is only relevant
    for ECM idle mode (in particular, tracking area update and paging)
    are not modeled at all.
 #. The model should allow the possibility to perform an X2-based
    handover between two eNBs.




.. _sec-overall-architecture:

------------
Architecture
------------





LTE Model
+++++++++



UE architecture
---------------

The architecture of the LTE radio protocol stack model of the UE is
represented in the figures :ref:`fig-lte-arch-ue-data` and
:ref:`fig-lte-arch-ue-ctrl` which highlight respectively the data
plane and the control plane.


.. _fig-lte-arch-ue-data:
 
.. figure:: figures/lte-arch-ue-data.*
   :align: center

   LTE radio protocol stack architecture for the UE on the data plane


.. _fig-lte-arch-ue-ctrl:

.. figure:: figures/lte-arch-ue-ctrl.*
   :align: center

   LTE radio protocol stack architecture for the UE on the control plane


The architecture of the PHY/channel model of the UE is represented in figure :ref:`fig-lte-ue-phy`. 


.. _fig-lte-ue-phy:
   
.. figure:: figures/lte-ue-phy.*
   :align: center

   PHY and channel model architecture for the UE




eNB architecture
----------------

The architecture of the LTE radio protocol stack model of the eNB is
represented in the figures :ref:`fig-lte-arch-enb-data` and
:ref:`fig-lte-arch-enb-ctrl` which highlight respectively the data plane
and the control plane. 


.. _fig-lte-arch-enb-data:
   
.. figure:: figures/lte-arch-enb-data.*
   :align: center

   LTE radio protocol stack architecture for the eNB on the data plane


.. _fig-lte-arch-enb-ctrl:
   
.. figure:: figures/lte-arch-enb-ctrl.*
   :align: center

   LTE radio protocol stack architecture for the eNB on the control plane


The architecture of the PHY/channel model of the eNB is represented in figure :ref:`fig-lte-enb-phy`. 


.. _fig-lte-enb-phy:
   
.. figure:: figures/lte-enb-phy.*
   :align: center

   PHY and channel model architecture for the eNB




EPC Model
+++++++++



EPC data plane
--------------

In Figure :ref:`fig-lte-epc-e2e-data-protocol-stack`, we represent the
end-to-end LTE-EPC data plane protocol stack as it is modeled in the
simulator. From the figure, it is evident that the 
biggest simplification introduced in the data plane model
is the inclusion of the SGW and PGW functionality within a single
SGW/PGW node, which removes the need for the S5 or S8 interfaces 
specified by 3GPP. On the other hand, for both the S1-U protocol stack and
the LTE radio protocol stack all the protocol layers specified by 3GPP
are present. 


.. _fig-lte-epc-e2e-data-protocol-stack:
   
.. figure:: figures/lte-epc-e2e-data-protocol-stack.*
   :align: center

   LTE-EPC data plane protocol stack




EPC control plane
-----------------

The architecture of the implementation of the control plane model is
shown in figure :ref:`fig-epc-ctrl-arch`. The control interfaces that are
modeled explicitly are the S1-AP, the X2-AP and the S11 interfaces. 

We note that the S1-AP and the S11 interfaces are modeled in a simplified
fashion, by using just one pair of interface classes to model the
interaction between entities that reside on different nodes (the eNB
and the MME for the S1-AP interface, and the MME and the SGW for the
S11 interface). In practice, this means that the primitives of these
interfaces are mapped to a direct function call between the two
objects. On the other hand, the X2-AP interface is being modeled using
protocol data units sent over an X2 link (modeled as a point-to-point
link); for this reason, the X2-AP interface model is more realistic.




.. _fig-epc-ctrl-arch:
   
.. figure:: figures/epc-ctrl-arch.*
   :align: center

   EPC control model




.. only:: latex

    .. raw:: latex
      
        \clearpage



-----------------------
Channel and Propagation
-----------------------


For channel modeling purposes, the LTE module uses the ``SpectrumChannel``
interface provided by the spectrum module. At the time of this
writing, two implementations of such interface are available:
``SingleModelSpectrumChannel`` and ``MultiModelSpectrumChannel``, and the
LTE module requires the use of the ``MultiModelSpectrumChannel`` in
order to work properly. This is because of the need to support
different frequency and bandwidth configurations. All the the
propagation models supported by ``MultiModelSpectrumChannel`` can be
used within the LTE module.  



Use of the Buildings model with LTE
+++++++++++++++++++++++++++++++++++

The recommended propagation model to be used with the LTE
module is the one provided by the Buildings module, which was in fact
designed specifically with LTE (though it can be used with other
wireless technologies as well). Please refer to the documentation of
the Buildings module for generic information on the propagation model
it provides. 

In this section we will highlight some considerations that
specifically apply when the Buildings module is used together with the
LTE module.


The naming convention used in the following will be:

 * User equipment:  UE
 * Macro Base Station: MBS
 * Small cell Base Station (e.g., pico/femtocell): SC


The LTE module considers FDD only, and implements downlink and uplink propagation separately. As a consequence, the following pathloss computations are performed

  * MBS <-> UE (indoor and outdoor)
  * SC (indoor and outdoor) <-> UE (indoor and outdoor)
 
The LTE model does not provide the following pathloss computations:

  * UE <-> UE
  * MBS <-> MBS
  * MBS <-> SC
  * SC <-> SC


The Buildings model does not know the actual type of the node; i.e.,
it is not aware of whether a transmitter node is a UE, a MBS, or a
SC. Rather, the Buildings model only cares about the position of the
node: whether it is indoor and outdoor, and what is its z-axis respect
to the rooftop level. As a consequence, for an eNB node that is placed
outdoor and at a z-coordinate above the rooftop level, the propagation
models typical of MBS will be used by the Buildings
module. Conversely, for an eNB that is placed outdoor but below the
rooftop,  or indoor, the propagation models typical of pico and
femtocells will be used.  

For communications involving at least one indoor node, the
corresponding wall penetration losses will be calculated by the
Buildings model. This covers the following use cases: 
 
 * MBS <-> indoor UE
 * outdoor SC <-> indoor UE
 * indoor SC <-> indoor UE
 * indoor SC <-> outdoor UE

Please refer to the documentation of the Buildings module for details
on the actual models used in each case. 


.. _sec-fading-model:

Fading Model
++++++++++++

The LTE module includes a trace-based fading model derived from the one developed during the GSoC 2010 [Piro2011]_. The main characteristic of this model is the fact that the fading evaluation during simulation run-time is based on per-calculated traces. This is done to limit the computational complexity of the simulator. On the other hand, it needs huge structures for storing the traces; therefore, a trade-off between the number of possible parameters and the memory occupancy has to be found. The most important ones are:

 * users' speed: relative speed between users (affects the Doppler frequency, which in turns affects the time-variance property of the fading)
 * number of taps (and relative power): number of multiple paths considered, which affects the frequency property of the fading.
 * time granularity of the trace: sampling time of the trace.
 * frequency granularity of the trace: number of values in frequency to be evaluated.
 * length of trace: ideally large as the simulation time, might be reduced by windowing mechanism.
 * number of users: number of independent traces to be used (ideally one trace per user).

With respect to the mathematical channel propagation model, we suggest the one provided by the ``rayleighchan`` function of Matlab, since it provides a well accepted channel modelization both in time and frequency domain. For more information, the reader is referred to  [mathworks]_.

The simulator provides a matlab script (``src/lte/model/fading-traces/fading-trace-generator.m``) for generating traces based on the format used by the simulator. 
In detail, the channel object created with the rayleighchan function is used for filtering a discrete-time impulse signal in order to obtain the channel impulse response. The filtering is repeated for different TTI, thus yielding subsequent time-correlated channel responses (one per TTI). The channel response is then processed with the ``pwelch`` function for obtaining its power spectral density values, which are then saved in a file with the proper format compatible with the simulator model.

Since the number of variable it is pretty high, generate traces considering all of them might produce a high number of traces of huge size. On this matter, we considered the following assumptions of the parameters based on the 3GPP fading propagation conditions (see Annex B.2 of [TS36104]_):

 * users' speed: typically only a few discrete values are considered, i.e.:

   * 0 and 3 kmph for pedestrian scenarios
   * 30 and 60 kmph for vehicular scenarios
   * 0, 3, 30 and 60 for urban scenarios

 * channel taps: only a limited number of sets of channel taps are normally considered, for example three models are mentioned in Annex B.2 of [TS36104]_.
 * time granularity: we need one fading value per TTI, i.e., every 1 ms (as this is the granularity in time of the ns-3 LTE PHY model).
 * frequency granularity: we need one fading value per RB (which is the frequency granularity of the spectrum model used by the ns-3 LTE model).
 * length of the trace: the simulator includes the windowing mechanism implemented during the GSoC 2011, which consists of picking up a window of the trace each window length in a random fashion.  
 * per-user fading process: users share the same fading trace, but for each user a different starting point in the trace is randomly picked up. This choice was made to avoid the need to provide one fading trace per user.

According to the parameters we considered, the following formula express in detail the total size :math:`S_{traces}` of the fading traces:

.. math::
 S_{traces} = S_{sample} \times N_{RB} \times \frac{T_{trace}}{T_{sample}} \times N_{scenarios} \mbox{ [bytes]}

where :math:`S_{sample}` is the size in bytes of the sample (e.g., 8 in case of double precision, 4 in case of float precision), :math:`N_{RB}` is the number of RB or set of RBs to be considered, :math:`T_{trace}` is the total length of the trace, :math:`T_{sample}` is the time resolution of the trace (1 ms), and :math:`N_{scenarios}` is the number of fading scenarios that are desired (i.e., combinations of different sets of channel taps and user speed values). We provide traces for 3 different scenarios one for each taps configuration defined in Annex B.2 of [TS36104]_:

 * Pedestrian: with nodes' speed of 3 kmph.
 * Vehicular: with nodes' speed of 60 kmph.
 * Urban: with nodes' speed of 3 kmph.

hence :math:`N_{scenarios} = 3`. All traces have :math:`T_{trace} = 10` s and :math:`RB_{NUM} = 100`. This results in a total 24 MB bytes of traces.


Antennas
++++++++

Being based on the ``SpectrumPhy``, the LTE PHY model supports antenna
modeling via the ns-3 ``AntennaModel`` class. Hence, any model based on
this class can be associated with any eNB or UE instance. For
instance, the use of the ``CosineAntennaModel`` associated with an eNB
device allows to model one sector of a macro base station. By default,
the ``IsotropicAntennaModel`` is used for both eNBs and UEs. 



.. only:: latex

    .. raw:: latex

        \clearpage


---
PHY
---


Overview
++++++++

The physical layer model provided in this LTE simulator is based on
the one described in [Piro2011]_, with the following modifications.  The model now includes the 
inter cell intereference calculation and the simulation of uplink traffic, including both packet transmission and CQI generation. 


Subframe Structure
++++++++++++++++++

The subframe is divided into control and data part as described in Figure :ref:`fig-lte-subframe-structure`.

.. _fig-lte-subframe-structure:

.. figure:: figures/lte-subframe-structure.*
   :align: center
   :width: 300px

   LTE subframe division.


Considering the granularity of the simulator based on RB, the control and the reference signaling have to be consequently modeled considering this constraint.  According to the standard [TS36211]_, the downlink control frame starts at the beginning of each subframe and lasts up to three symbols across the whole system bandwidth, where the actual duration is provided by the Physical Control Format Indicator Channel (PCFICH). The information on the allocation are then mapped in the remaining resource up to the duration defined by the PCFICH, in the so called Physical Downlink Control Channel (PDCCH). A PDCCH transports a single message called Downlink Control Information (DCI) coming from the MAC layer, where the scheduler indicates the resource allocation for a specific user.
The PCFICH and PDCCH are modeled with the transmission of the control frame of a fixed duration of 3/14 of milliseconds spanning in the whole available bandwidth, since the scheduler does not estimate the size of the control region. This implies that a single transmission block models the entire control frame with a fixed power (i.e., the one used for the PDSCH) across all the available RBs. According to this feature, this transmission represents also a valuable support for the Reference Signal (RS). This allows of having every TTI an evaluation of the interference scenario since all the eNB are transmitting (simultaneously) the control frame over the respective available bandwidths. We note that, the model does not include the power boosting since it does not reflect any improvement in the implemented model of the channel estimation.


The Sounding Reference Signal (SRS) is modeled similar to the downlink control frame. The SRS is periodically placed in the last symbol of the subframe in the whole system bandwidth. The RRC module already includes an algorithm for dynamically assigning the periodicity as function of the actual number of UEs attached to a eNB according to the UE-specific procedure (see Section 8.2 of [TS36213]_).


MAC to Channel delay
++++++++++++++++++++

To model the latency of real MAC and PHY implementations, the PHY model simulates a MAC-to-channel delay in multiples of TTIs (1ms). The transmission of both data and control packets are delayed by this amount.


CQI feedback
++++++++++++

The generation of CQI feedback is done accordingly to what specified in [FFAPI]_. In detail, we considered the generation
of periodic wideband CQI (i.e., a single value of channel state that is deemed representative of all RBs
in use) and inband CQIs (i.e., a set of value representing the channel state for each RB).

In downlink, the CQI feedbacks are currently evaluated according to the SINR perceived by control channel (i.e., PDCCH + PCFIC) in order to have an estimation of the interference when all the eNB are transmitting simultaneously. In uplink, two types of CQIs are implemented:

 - SRS based, periodically sent by the UEs.
 - PUSCH based, calculated from the actual transmitted data.

The scheduler interface include an attribute system calld ``UlCqiFilter`` for managing the filtering of the CQIs according to their nature, in detail:

  - ``SRS_UL_CQI`` for storing only SRS based CQIs.
  - ``PUSCH_UL_CQI`` for storing only PUSCH based CQIs.
  - ``ALL_UL_CQI`` for storing all the CQIs received.

It has to be noted that, the ``FfMacScheduler`` provides only the interface and it is matter of the actual scheduler implementation to include the code for managing these attibutes (see scheduler related section for more information on this matter).


Interference Model
++++++++++++++++++

The PHY model is based on the well-known Gaussian interference models, according to which the powers of interfering signals (in linear units) are summed up together to determine the overall interference power.

The sequence diagram of Figure :ref:`fig-lte-phy-interference` shows how interfering signals are processed to calculate the SINR, and how SINR is then used for the generation of CQI feedback.


.. _fig-lte-phy-interference:
   
.. figure:: figures/lte-phy-interference.*
   :align: center

   Sequence diagram of the PHY interference calculation procedure



LTE Spectrum Model
++++++++++++++++++

The usage of the radio spectrum by eNBs and UEs in LTE is described in
[TS36101]_. In the simulator, radio spectrum usage is modeled as follows. 
Let :math:`f_c` denote the  LTE Absolute Radio Frequency Channel Number, which
identifies the carrier frequency on a 100 kHz raster; furthermore, let :math:`B` be
the Transmission Bandwidth Configuration in number of Resource Blocks. For every
pair :math:`(f_c,B)` used in the simulation we define a corresponding spectrum
model using the Spectrum framework described
in [Baldo2009]_.  :math:`f_c` and :math:`B` can be configured for every eNB instantiated
in the simulation; hence, each eNB can use a different spectrum model. Every UE
will automatically use the spectrum model of the eNB it is attached to. Using
the MultiModelSpectrumChannel described in [Baldo2009]_, the interference
among eNBs that use different spectrum models is properly accounted for. 
This allows to simulate dynamic spectrum access policies, such as for
example the spectrum licensing policies that are 
discussed in [Ofcom2600MHz]_.



Data PHY Error Model
++++++++++++++++++++

The simulator includes an error model of the data plane (i.e., PDSCH and PUSCH) according to the standard link-to-system mapping (LSM) techniques. The choice is aligned with the standard system simulation methodology of OFDMA  radio transmission technology. Thanks to LSM we are able to maintain a good level of accuracy and at the same time limiting the computational complexity increase. It is based on the mapping of single link layer performance obtained by means of link level simulators to system (in our case network) simulators. In particular link the layer simulator is used for generating the performance of a single link from a PHY layer perspective, usually in terms of code block error rate (BLER), under specific static conditions. LSM allows the usage of these parameters in more complex scenarios, typical of system/network simulators, where we have more links, interference and "colored" channel propagation phenomena (e.g., frequency selective fading).

To do this the Vienna LTE Simulator [ViennaLteSim]_ has been used for what concerns the extraction of link layer performance and the Mutual Information Based Effective SINR (MIESM) as LSM mapping function using part of the work recently published by the Signet Group of University of Padua [PaduaPEM]_.


MIESM
-----

The specific LSM method adopted is the one based on the usage of a mutual information metric, commonly referred to as the mutual information per per coded bit (MIB or MMIB when a mean of multiples MIBs is involved). Another option would be represented by the Exponential ESM (EESM); however, recent studies demonstrate that MIESM outperforms EESM in terms of accuracy [LozanoCost]_.

.. _fig-miesm-architecture:

.. figure:: figures/miesm_scheme.*
   :align: center

   MIESM computational procedure diagram

The mutual information (MI) is dependent on the constellation mapping and can be calculated per transport block (TB) basis, by evaluating the MI over the symbols and the subcarrier. However, this would be too complex for a network simulator. Hence, in our implementation a flat channel response within the RB has been considered; therefore the overall MI of a TB is calculated averaging the MI evaluated per each RB used in the TB. In detail, the implemented scheme is depicted in Figure :ref:`fig-miesm-architecture`, where we see that the model starts by evaluating the MI value for each RB, represented in the figure by the SINR samples. Then the equivalent MI is evaluated per TB basis by averaging the MI values. Finally, a further step has to be done since the link level simulator returns the performance of the link in terms of block error rate (BLER) in a addive white guassian noise  (AWGN) channel, where the blocks are the code blocks (CBs) independently encoded/decoded by the turbo encoder. On this matter the standard 3GPP segmentation scheme has been used for estimating the actual CB size (described in section 5.1.2 of [TS36212]_). This scheme divides the the TB in :math:`N_{K_-}` blocks of size :math:`K_-` and :math:`N_{K+}` blocks of size :math:`K_+`. Therefore the overall TB BLER (TBLER) can be expressed as

.. math::

  TBLER = 1- \prod\limits_{i=1}^{C}(1-CBLER_i)

where the :math:`CBLER_i` is the BLER of the CB :math:`i` obtained according to the link level simulator CB BLER curves.
For estimating the :math:`CBLER_i`, the MI evaluation has been implemented according to its numerical approximation defined in [wimaxEmd]_. Moreover, for reducing the complexity of the computation, the approximation has been converted into lookup tables. In detail, Gaussian cumulative model has been used for approximating the AWGN BLER curves with three parameters which provides a close fit to the standard AWGN performances, in formula:

.. math::

  CBLER_i = \frac{1}{2}\left[1-erf\left(\frac{x-b_{ECR}}{\sqrt{2}c_{ECR}} \right) \right]

where :math:`x` is the MI of the TB, :math:`b_{ECR}` represents the "transition center" and :math:`c_{ECR}` is related to the "transition width" of the Gaussian cumulative distribution for each Effective Code Rate (ECR) which is the actual transmission rate according to the channel coding and MCS. For limiting the computational complexity of the model we considered only a subset of the possible ECRs in fact we would have potentially 5076 possible ECRs (i.e., 27 MCSs and 188 CB sizes). On this respect, we will limit the CB sizes to some representative values (i.e., 40, 140, 160, 256, 512, 1024, 2048, 4032, 6144), while for the others the worst one approximating the real one will be used (i.e., the smaller CB size value available respect to the real one). This choice is aligned to the typical performance of turbo codes, where the CB size is not strongly impacting on the BLER. However, it is to be notes that for CB sizes lower than 1000 bits the effect might be relevant (i.e., till 2 dB); therefore, we adopt this unbalanced sampling interval for having more precision where it is necessary. This behaviour is confirmed by the figures presented in the Annes Section.


BLER Curves
-----------

On this respect, we reused part of the curves obtained within [PaduaPEM]_. In detail, we introduced the CB size dependency to the CB BLER curves with the support of the developers of [PaduaPEM]_ and of the LTE Vienna Simulator. In fact, the module released provides the link layer performance only for what concerns the MCSs (i.e, with a given fixed ECR). In detail the new error rate curves for each has been evaluated with a simulation campaign with the link layer simulator for a single link with AWGN noise and for CB size of 104, 140, 256, 512, 1024, 2048, 4032 and 6144. These curves has been mapped with the Gaussian cumulative model formula presented above for obtaining the correspondents :math:`b_{ECR}` and :math:`c_{ECR}` parameters.

The BLER perfomance of all MCS obtained with the link level simulator are plotted in the following figures (blue lines) together with their correspondent mapping to the Gaussian cumulative distribution (red dashed lines).


.. _fig-mcs-1-4-ber:

.. figure:: figures/MCS_1_4.*
   :width: 900px
   :align: center
   :height: 700px


   BLER for MCS 1, 2, 3 and 4.


.. _fig-mcs-5-8-ber:

.. figure:: figures/MCS_5_8.*
   :width: 900px
   :align: center
   :height: 700px


   BLER for MCS 5, 6, 7 and 8.

.. _fig-mcs-9-12-ber:

.. figure:: figures/MCS_9_12.*
   :width: 900px
   :align: center
   :height: 700px


   BLER for MCS 9, 10, 11 and 12.

.. _fig-mcs-13-16-ber:

.. figure:: figures/MCS_13_16.*
   :width: 900px
   :align: center
   :height: 700px


   BLER for MCS 13, 14, 15 and 16.


.. _fig-mcs-17-20-ber:

.. figure:: figures/MCS_17_20.*
   :width: 900px
   :align: center
   :height: 700px


   BLER for MCS 17, 17, 19 and 20.

.. _fig-mcs-21-24-ber:

.. figure:: figures/MCS_21_24.*
   :width: 900px
   :align: center
   :height: 700px


   BLER for MCS 21, 22, 23 and 24.


.. _fig-mcs-25-28-ber:

.. figure:: figures/MCS_25_28.*
   :width: 900px
   :align: center
   :height: 700px


   BLER for MCS 25, 26, 27 and 28.

.. _fig-mcs-29-29-ber:

.. figure:: figures/MCS_29_29.*
   :width: 900px
   :align: center
   :height: 700px


   BLER for MCS 29.






Integration of the BLER curves in the ns-3 LTE module
-----------------------------------------------------

The model implemented uses the curves for the LSM of the recently LTE PHY Error Model released in the ns3 community by the Signet Group [PaduaPEM]_ and the new ones generated for different CB sizes. The ``LteSpectrumPhy`` class is in charge of evaluating the TB BLER thanks to the methods provided by the ``LteMiErrorModel`` class, which is in charge of evaluating the TB BLER according to the vector of the perceived SINR per RB, the MCS and the size in order to proper model the segmentation of the TB in CBs. In order to obtain the vector of the perceived SINR two instances of ``LtePemSinrChunkProcessor`` (child of ``LteSinrChunkProcessor`` dedicated to evaluate the SINR for obtaining physical error performance) have been attached to UE downlink and eNB uplink ``LteSpectrumPhy`` modules for evaluating the error model distribution respectively of PDSCH (UE side) and ULSCH (eNB side).

The model can be disabled for working with a zero-losses channel by setting the ``PemEnabled`` attribute of the ``LteSpectrumPhy`` class (by default is active). This can be done according to the standard ns3 attribute system procedure, that is::

  Config::SetDefault ("ns3::LteSpectrumPhy::DataErrorModelEnabled", BooleanValue (false));  

Control Channels PHY Error Model
++++++++++++++++++++++++++++++++

The simulator includes the error model for downlink control channels (PCFICH and PDCCH), while in uplink it is assumed and ideal error-free channel. The model is based on the MIESM approach presented before for considering the effects of the frequency selective channel since most of the control channels span the whole available bandwidth.


PCFICH + PDCCH Error Model
--------------------------

The model adopted for the error distribution of these channels is based on an evaluation study carried out in the RAN4 of 3GPP, where different vendors investigated the demodulation performance of the PCFICH jointly with PDCCH. This is due to the fact that the PCFICH is the channel in charge of communicating to the UEs the actual dimension of the PDCCH (which spans between 1 and 3 symbols); therefore the correct decodification of the DCIs  depends on the correct interpretation of both ones. In 3GPP this problem have been evaluated for improving the cell-edge performance [FujitsuWhitePaper]_, where the interference among neighboring cells can be relatively high due to signal degradation. A similar problem has been notices in femto-cell scenario and, more in general, in HetNet scenarios the bottleneck has been detected mainly as the PCFICH channel [Bharucha2011]_, where in case of many eNBs are deployed in the same service area, this channel may collide in frequency, making impossible the correct detection of the PDCCH channel, too. 

In the simulator, the SINR perceived during the reception has been estimated according to the MIESM model presented above in order to evaluate the error distribution of PCFICH and PDCCH. In detail, the SINR samples of all the RBs are included in the evaluation of the MI associated to the control frame and, according to this values, the effective SINR (eSINR) is obtained by inverting the MI evaluation process. It has to be noted that, in case of MIMO transmission, both PCFICH and the PDCCH use always the transmit diversity mode as defined by the standard. According to the eSINR perceived the decodification error probability can be estimated as function of the results presented in [R4-081920]_. In case an error occur, the DCIs discarded and therefore the UE will be not able to receive the correspondent Tbs, therefore resulting lost.


MIMO Model
++++++++++

The use of multiple antennas both at transmitter and receiver side, known as multiple-input and multiple-output (MIMO), is a problem well studied in literature during the past years. Most of the work concentrate on evaluating analytically the gain that the different MIMO schemes might have in term of capacity; however someones provide also information of the gain in terms of received power [CatreuxMIMO]_.

According to the considerations above, a model more flexible can be obtained considering the gain that MIMO schemes bring in the system from a statistical point of view. As highlighted before, [CatreuxMIMO]_ presents the statistical gain of several MIMO solutions respect to the SISO one in case of no correlation between the antennas. In the work the gain is presented as the cumulative distribution function (CDF) of the output SINR for what concern SISO, MIMO-Alamouti, MIMO-MMSE, MIMO-OSIC-MMSE and MIMO-ZF schemes. Elaborating the results, the output SINR distribution can be approximated with a log-normal one with different mean and variance as function of the scheme considered. However, the variances are not so different and they are approximatively equal to the one of the SISO mode already included in the shadowing component of the ``BuildingsPropagationLossModel``, in detail:

 * SISO: :math:`\mu = 13.5` and :math:`\sigma = 20` [dB].
 * MIMO-Alamouti: :math:`\mu = 17.7` and :math:`\sigma = 11.1` [dB].
 * MIMO-MMSE: :math:`\mu = 10.7` and :math:`\sigma = 16.6` [dB].
 * MIMO-OSIC-MMSE: :math:`\mu = 12.6` and :math:`\sigma = 15.5` [dB].
 * MIMO-ZF: :math:`\mu = 10.3` and :math:`\sigma = 12.6` [dB].


Therefore the PHY layer implements the MIMO model as the gain perceived by the receiver when using a MIMO scheme respect to the one obtained using SISO one. We note that, these gains referred to a case where there is no correlation between the antennas in MIMO scheme; therefore do not model degradation due to paths correlation.


.. _sec-phy-ue-measurements:

UE PHY Measurements Model
+++++++++++++++++++++++++

According to [TS36214]_, the UE has to report a set of measurements of the eNBs that the device is able to perceive: the the reference signal received power (RSRP) and the reference signal received quality (RSRQ). The former is a measure of the received power of a specific eNB, while the latter includes also channel interference and thermal noise.
The UE has to report the measurements jointly with the physical cell identity (PCI) of the cell. Both the RSRP and RSRQ measurements are performed during the reception of the RS, while the PCI is obtained with the Primary Synchronization Signal (PSS). The PSS is sent by the eNB each 5 subframes and in detail in the subframes 1 and 6. In real systems, only 504 distinct PCIs are available, and hence it could occur that two nearby eNBs use the same PCI; however, in the simulator we model PCIs using simulation metadata, and we allow up to 65535 distinct PCIs, thereby avoiding PCI collisions provided that less that 65535 eNBs are simulated in the same scenario.

According to [TS36133]_ sections 9.1.4 and 9.1.7, RSRP is reported by PHY layer in dBm while RSRQ in dB. The values of RSRP and RSRQ are provided to higher layers through the C-PHY SAP (by means of ``UeMeasurementsParameters`` struct) every 200 ms as defined in [TS36331]_. Layer 1 filtering is performed by averaging the all the measurements collected during the last window slot. The periodicity of reporting can be adjusted for research purposes by means of the ``LteUePhy::UeMeasurementsFilterPeriod`` attribute.

The formulas of the RSRP and RSRQ can be simplified considering the assumption of the PHY layer that the channel is flat within the RB, the finest level of accuracy. In fact, this implies that all the REs within a RB have the same power, therefore:

.. math::

    RSRP = \frac{\sum_{k=0}^{K-1}\frac{\sum_{m=0}^{M-1}(P(k,m))}{M}}{K}
         = \frac{\sum_{k=0}^{K-1}\frac{(M \times P(k))}{M}}{K}
         = \frac{\sum_{k=0}^{K-1}(P(k))}{K}

where :math:`P(k,m)` represents the signal power of the RE :math:`m` within the RB :math:`k`, which, as observed before, is constant within the same RB and equal to :math:`P(k)`, :math:`M` is the number of REs carrying the RS in a RB and :math:`K` is the number of RBs. It is to be noted that :math:`P(k)`, and in general all the powers defined in this section, is obtained in the simulator from the PSD of the RB (which is provided by  the ``LteInterferencePowerChunkProcessor``), in detail:

.. math::

    P(k) = PSD_{RB}(k)*180000/12

where :math:`PSD_{RB}(k)` is the power spectral density of the RB :math:`k`, :math:`180000` is the bandwidth in Hz of the RB and :math:`12` is the number of REs per RB in an OFDM symbol.
Similarly, for RSSI we have

.. math::
    RSSI = \sum_{k=0}^{K-1} \frac{\sum_{s=0}^{S-1} \sum_{r=0}^{R-1}( P(k,s,r) + I(k,s,r) + N(k,s,r))}{S}

where :math:`S` is the number of OFDM symbols carrying RS in a RB and :math:`R` is the number of REs carrying a RS in a OFDM symbol (which is fixed to :math:`2`) while :math:`P(k,s,r)`, :math:`I(k,s,r)` and :math:`N(k,s,r)` represent respectively the perceived power of the serving cell, the interference power and the noise power of the RE :math:`r` in symbol :math:`s`. As for RSRP, the measurements within a RB are always equals among each others according to the PHY model; therefore :math:`P(k,s,r) = P(k)`, :math:`I(k,s,r) = I(k)` and :math:`N(k,s,r) = N(k)`, which implies that the RSSI can be calculated as:

.. math::
    RSSI = \sum_{k=0}^{K-1} \frac{S \times 2 \times ( P(k) + I(k) + N(k))}{S}
         = \sum_{k=0}^{K-1} 2 \times ( P(k) + I(k) + N (k))

Considering the constraints of the PHY reception chain implementation, and in order to maintain the level of computational complexity low, only RSRP can be directly obtained for all the cells. This is due to the fact that ``LteSpectrumPhy`` is designed for evaluating the interference only respect to the signal of the serving eNB. This implies that the PHY layer is optimized for managing the power signals information with the serving eNB as a reference. However, RSRP and RSRQ of neighbor cell :math:`i` can be extracted by the current information available of the serving cell :math:`j` as detailed in the following:

.. math::

    RSRP_i = \frac{\sum_{k=0}^{K-1}(P_i(k))}{K}

    RSSI_i = RSSI_j = \sum_{k=0}^{K-1} 2 \times ( I_j(k) + P_j(k) + N_j(k) )

    RSRQ_i^j = K \times RSRP_i / RSSI_j

where :math:`RSRP_i` is the RSRP of the neighbor cell :math:`i`, :math:`P_i(k)` is the power perceived at any RE within the RB :math:`k`, :math:`K` is the total number of RBs, :math:`RSSI_i` is the RSSI of the neighbor cell :math:`i` when the UE is attached to cell  :math:`j` (which, since it is the sum of all the received powers, coincides with :math:`RSSI_j`), :math:`I_j(k)` is the total interference perceived by UE in any RE of RB :math:`k` when attached to cell :math:`i` (obtained by the ``LteInterferencePowerChunkProcessor``), :math:`P_j(k)` is the power perceived of cell :math:`j` in any RE of the RB :math:`k` and :math:`N` is the power noise spectral density in any RE. The sample is considered as valid in case of the RSRQ evaluated is above the ``LteUePhy::RsrqUeMeasThreshold`` attribute.




----
HARQ
----

The HARQ scheme implemented is based on a incremental redundancy (IR) solutions combined with multiple stop-and-wait processes for enabling a continuous data flow. In detail, the solution adopted is the *soft combining hybrid IR Full incremental redundancy* (also called IR Type II), which implies that the retransmissions contain only new information respect to the previous ones. The resource allocation algorithm of the HARQ has been implemented within the respective scheduler classes (i.e., ``RrFfMacScheduler`` and ``PfFfMacScheduler``, refer to their correspondent sections for more info), while the decodification part of the HARQ has been implemented in the ``LteSpectrumPhy`` and ``LteHarqPhy`` classes which will be detailed in this section.

According to the standard, the UL retransmissions are synchronous and therefore are allocated 7 ms after the original transmission. On the other hand, for the DL, they are asynchronous and therefore can be allocated in a more flexible way starting from 7 ms and it is a matter of the specific scheduler implementation. The HARQ processes behavior is depicted in Figure:ref:`fig-harq-processes-scheme`.

At the MAC layer, the HARQ entity residing in the scheduler is in charge of controlling the 8 HARQ processes for generating new packets and managing the retransmissions both for the DL and the UL. The scheduler collects the HARQ feedback from eNB and UE PHY layers (respectively for UL and DL connection) by means of the FF API primitives ``SchedUlTriggerReq`` and ``SchedUlTriggerReq``. According to the HARQ feedback and the RLC buffers status, the scheduler generates a set of DCIs including both retransmissions of HARQ blocks received erroneous and new transmissions, in general, giving priority to the former. On this matter, the scheduler has to take into consideration one constraint when allocating the resource for HARQ retransmissions, it must use the same modulation order of the first transmission attempt (i.e., QPSK for MCS :math:`\in [0..9]`, 16QAM for MCS :math:`\in [10..16]` and 64QAM for MCS :math:`\in [17..28]`). This restriction comes from the specification of the rate matcher in the 3GPP standard [TS36212]_, where the algorithm fixes the modulation order for generating the different blocks of the redundancy versions.


The PHY Error Model model (i.e., the ``LteMiErrorModel`` class already presented before) has been extended for considering IR HARQ according to [wimaxEmd]_, where the parameters for the AWGN curves mapping for MIESM mapping in case of retransmissions are given by:

.. math::

    R_{eff} = \frac{X}{\sum\limits_{i=1}^q C_i}

    M_{I eff} = \frac{\sum\limits_{i=1}^q C_i M_i}{\sum\limits_{i=1}^q C_i}

where :math:`X` is the number of original information bits, :math:`C_i` are number of coded bits, :math:`M_i` are the mutual informations per HARQ block received on the total number of :math:`q` retransmissions. Therefore, in order to be able to return the error probability with the error model implemented in the simulator evaluates the :math:`R_{eff}` and the :math:`MI_{I eff}` and return the value of error probability of the ECR of the same modulation with closest lower rate respect to the :math:`R_{eff}`. In order to consider the effect of HARQ retransmissions a new sets of curves have been integrated respect to the standard one used for the original MCS. The new curves are intended for covering the cases when the most conservative MCS of a modulation is used which implies the generation of :math:`R_{eff}` lower respect to the one of standard MCSs. On this matter the curves for 1, 2 and 3 retransmissions have been evaluated for 10 and 17. For MCS 0 we considered only the first retransmission since the produced code rate is already very conservative (i.e., 0.04) and returns an error rate enough robust for the reception (i.e., the downturn of the BLER is centered around -18 dB).
It is to be noted that, the size of first TB transmission has been assumed as containing all the information bits to be coded; therefore :math:`X` is equal to the size of the first TB sent of a an HARQ process. The model assumes that the eventual presence of parity bits in the codewords is already considered in the link level curves. This implies that as soon as the minimum :math:`R_{eff}` is reached the model is not including the gain due to the transmission of further parity bits.


.. _fig-harq-processes-scheme:

.. figure:: figures/lte-harq-processes-scheme.*
   :align: center

   HARQ processes behavior in LTE



The part of HARQ devoted to manage the decodification of the HARQ blocks has been implemented in the ``LteHarqPhy`` and ``LteSpectrumPhy`` classes. The former is in charge of maintaining the HARQ information for each active process . The latter interacts with ``LteMiErrorModel`` class for evaluating the correctness of the blocks received and includes the messaging algorithm in charge of communicating to the HARQ entity in the scheduler the result of the decodifications. These messages are encapsulated in the ``dlInfoListElement`` for DL and ``ulInfoListElement`` for UL and sent through the PUCCH and the PHICH respectively with an ideal error free model according to the assumptions in their implementation. A sketch of the iteration between HARQ and LTE protocol stack in represented in Figure:ref:`fig-harq-architecture`.

Finally, the HARQ engine is always active both at MAC and PHY layer; however, in case of the scheduler does not support HARQ the system will continue to work with the HARQ functions inhibited (i.e., buffers are filled but not used). This implementation characteristic gives backward compatibility with schedulers implemented before HARQ integration.


.. _fig-harq-architecture:

.. figure:: figures/lte-harq-architecture.*
   :align: center

   Interaction between HARQ and LTE protocol stack


---
MAC
---
  

Resource Allocation Model
+++++++++++++++++++++++++


We now briefly describe how resource allocation is handled in LTE,
clarifying how it is modeled in the simulator. The scheduler is in
charge of generating specific structures called *Data Control Indication* (DCI)
which are then transmitted by the PHY of the eNB to the connected UEs, in order
to inform them of the resource allocation on a per subframe basis. In doing this
in the downlink direction, the scheduler has to fill some specific fields of the
DCI structure with all the information, such as: the Modulation and Coding
Scheme (MCS) to be used, the MAC Transport Block (TB) size, and the allocation
bitmap which identifies which RBs will contain the data
transmitted by the eNB to each user. 

For the mapping of resources to
physical RBs, we adopt a *localized mapping* approach
(see [Sesia2009]_, Section 9.2.2.1);
hence in a given subframe each RB is always allocated to the same user in both
slots.
The allocation bitmap can be coded in
different formats; in this implementation, we considered the *Allocation
Type 0* defined in [TS36213]_, according to which the RBs are grouped in
Resource Block Groups (RBG) of different size determined as a function of the
Transmission Bandwidth Configuration in use.

For certain bandwidth
values not all the RBs are usable, since the 
group size is not a common divisor of the group. This is for instance the case
when the bandwidth is equal to 25 RBs, which results in a RBG size of 2 RBs, and
therefore 1 RB will result not addressable. 
In uplink the format of the DCIs is different, since only adjacent RBs
can be used because of the SC-FDMA modulation. As a consequence, all
RBs can be allocated by the eNB regardless of the bandwidth
configuration. 

.. _sec-lte-amc:

Adaptive Modulation and Coding
++++++++++++++++++++++++++++++

The simulator provides two Adaptive Modulation and Coding (AMC) models: one based on the GSoC model [Piro2011]_ and one based on the physical error model (described in the following sections).

The former model is a modified version of the model described in [Piro2011]_,
which in turn is inspired from [Seo2004]_. Our version is described in the
following. Let :math:`i` denote the
generic user, and let :math:`\gamma_i` be its SINR. We get the spectral efficiency
:math:`\eta_i` of user :math:`i` using the following equations:

.. math::

   \mathrm{BER} = 0.00005

   \Gamma = \frac{ -\ln{ (5 * \mathrm{BER}) } }{ 1.5}

   \eta_i = \log_2 { \left( 1 + \frac{ {\gamma}_i }{ \Gamma } \right)}

The procedure described in [R1-081483]_ is used to get
the corresponding MCS scheme. The spectral efficiency is quantized based on the
channel quality indicator (CQI), rounding to the lowest value, and is mapped to the corresponding MCS
scheme. 

Finally, we note that there are some discrepancies between the MCS index
in [R1-081483]_
and that indicated by the standard:  [TS36213]_ Table
7.1.7.1-1 says that the MCS index goes from 0 to 31, and 0 appears to be a valid
MCS scheme (TB size is not 0) but in [R1-081483]_ the first useful MCS
index
is 1. Hence to get the value as intended by the standard we need to subtract 1
from the index reported in [R1-081483]_. 

The alternative model is based on the physical error model developed for this simulator and explained in the following subsections. This scheme is able to adapt the MCS selection to the actual PHY layer performance according to the specific CQI report. According to their definition, a CQI index is assigned when a single PDSCH TB with the modulation coding scheme and code rate correspondent to that CQI index in table 7.2.3-1 of [TS36213]_ can be received with an error probability less than 0.1. In case of wideband CQIs, the reference TB includes all the RBGs available in order to have a reference based on the whole available resources; while, for subband CQIs, the reference TB is sized as the RBGs.


Transport Block model
+++++++++++++++++++++

The model of the MAC Transport Blocks (TBs) provided by the simulator
is simplified with respect to the 3GPP specifications. In particular,
a simulator-specific class (PacketBurst) is used to aggregate 
MAC SDUs in order to achieve the simulator's equivalent of a TB,
without the corresponding implementation complexity. 
The multiplexing of different logical channels to and from the RLC
layer is performed using a dedicated packet tag (LteRadioBearerTag), which
performs a functionality which is partially equivalent to that of the
MAC headers specified by 3GPP. 


.. _sec-ff-mac-scheduler:

The FemtoForum MAC Scheduler Interface
++++++++++++++++++++++++++++++++++++++

This section describes the ns-3 specific version of the LTE MAC
Scheduler Interface Specification published by the FemtoForum [FFAPI]_.

We implemented the ns-3 specific version of the FemtoForum MAC Scheduler
Interface [FFAPI]_ as a set of C++ abstract
classes; in particular, each primitive is translated to a C++ method of a
given class. The term *implemented* here is used with the same
meaning adopted in [FFAPI]_, and hence refers to the process of translating
the logical interface specification to a particular programming language.
The primitives in [FFAPI]_ are grouped in two groups: the CSCHED
primitives, which deal with scheduler configuration, and the SCHED primitives,
which deal with the execution of the scheduler. Furthermore, [FFAPI]_
defines primitives of two different kinds: those of type REQ go from the MAC to
the Scheduler, and those of type IND/CNF go from the scheduler to the MAC. To
translate these characteristics into C++, we define the following abstract
classes that implement Service Access Points (SAPs) to be used to issue the
primitives:

 *  the ``FfMacSchedSapProvider`` class defines all the C++ methods that
    correspond to SCHED primitives of type REQ;
 *  the ``FfMacSchedSapUser`` class defines all the C++ methods that
    correspond to SCHED primitives of type CNF/IND;
 *  the ``FfMacCschedSapProvider`` class defines all the C++ methods that
    correspond to CSCHED primitives of type REQ;
 *  the ``FfMacCschedSapUser`` class defines all the C++ methods that
    correspond to CSCHED primitives of type CNF/IND;


There are 3 blocks involved in the MAC Scheduler interface: Control block,
Subframe block and Scheduler block. Each of these blocks provide one part of the
MAC Scheduler interface. The figure below shows the relationship
between the blocks and the SAPs defined in our implementation of the MAC
Scheduler Interface. 

.. figure:: figures/ff-mac-saps.*
   :align: center

In addition to the above principles, the following design choices have been
taken:

 *  The definition of the MAC Scheduler interface classes follows the naming
    conventions of the |ns3| Coding Style. In particular, we follow the
    CamelCase convention for the primitive names. For example, the primitive
    ``CSCHED_CELL_CONFIG_REQ`` is translated to ``CschedCellConfigReq``
    in the |ns3| code.
 *  The same naming conventions are followed for the primitive parameters. As
    the primitive parameters are member variables of classes, they are also prefixed
    with a ``m_``.
 *  regarding the use of vectors and lists in data structures, we note
    that [FFAPI]_ is a pretty much C-oriented API. However, considered that
    C++ is used in ns-3, and that the use of C arrays is discouraged, we used STL
    vectors (``std::vector``) for the implementation of the MAC Scheduler
    Interface, instead of using C arrays as implicitly suggested by the
    way [FFAPI]_ is written.
 *  In C++, members with constructors and destructors are not allow in
    ``unions``. Hence all those data structures that are said to be
    ``unions`` in [FFAPI]_ have been defined as ``structs`` in our code.

The figure below shows how the MAC Scheduler Interface is
used within the eNB. 

.. figure:: figures/ff-example.*
   :align: center


The User side of both the CSCHED SAP and the SCHED SAP are
implemented within the eNB MAC, i.e., in the file ``lte-enb-mac.cc``.
The eNB MAC can be used with different scheduler implementations without
modifications. The same figure also shows, as an example, how the Round Robin
Scheduler is implemented: to interact with the MAC of the eNB, the Round Robin
scheduler implements the Provider side of the SCHED SAP and CSCHED
SAP interfaces. A similar approach can be used to implement other schedulers as
well. A description of each of the scheduler implementations that we provide as
part of our LTE simulation module is provided in the following subsections.



Round Robin (RR) Scheduler
--------------------------

The Round Robin (RR) scheduler is probably the simplest scheduler found in the literature. It works by dividing the
available resources among the active flows, i.e., those logical channels which have a non-empty RLC queue. If the number of RBGs is greater than the number of active flows, all the flows can be allocated in the same subframe. Otherwise, if the number of active flows is greater than the number of RBGs, not all the flows can be scheduled in a given subframe; then, in the next subframe the allocation will start from the last flow that was not allocated.  The MCS to be adopted for each user is done according to the received wideband CQIs.

For what concern the HARQ, RR implements the non adaptive version, which implies that in allocating the retransmission attempts RR uses the same allocation configuration of the original block, which means maintaining the same RBGs and MCS. UEs that are allocated for HARQ retransmissions are not considered for the transmission of new data in case they have a transmission opportunity available in the same TTI. Finally, HARQ can be disabled with ns3 attribute system for maintaining backward compatibility with old test cases and code, in detail::

   Config::SetDefault ("ns3::RrFfMacScheduler::HarqEnabled", BooleanValue (false));

The scheduler implements the filtering of the uplink CQIs according to their nature with ``UlCqiFilter`` attibute, in detail:

  - ``SRS_UL_CQI``: only SRS based CQI are stored in the internal attributes.
  - ``PUSCH_UL_CQI``: only PUSCH based CQI are stored in the internal attributes.
  - ``ALL_UL_CQI``: all CQIs are stored in the same internal attibute (i.e., the last CQI received is stored independently from its nature).



Proportional Fair (PF) Scheduler
--------------------------------

The Proportional Fair (PF) scheduler [Sesia2009]_ works by scheduling a user
when its
instantaneous channel quality is high relative to its own average channel
condition over time. Let :math:`i,j` denote generic users; let :math:`t` be the
subframe index, and :math:`k` be the resource block index; let :math:`M_{i,k}(t)` be MCS
usable by user :math:`i` on resource block :math:`k` according to what reported by the AMC
model (see `Adaptive Modulation and Coding`_); finally, let :math:`S(M, B)` be the TB
size in bits as defined in [TS36213]_ for the case where a number :math:`B` of
resource blocks is used. The achievable rate :math:`R_{i}(k,t)` in bit/s for user :math:`i`
on resource block group :math:`k` at subframe :math:`t` is defined as 

.. math::

   R_{i}(k,t) =  \frac{S\left( M_{i,k}(t), 1\right)}{\tau} 

where :math:`\tau` is the TTI duration.
At the start of each subframe :math:`t`, each RBG is assigned to a certain user.
In detail, the index :math:`\widehat{i}_{k}(t)` to which RBG :math:`k` is assigned at time
:math:`t` is determined as

.. math::

   \widehat{i}_{k}(t) = \underset{j=1,...,N}{\operatorname{argmax}}
    \left( \frac{ R_{j}(k,t) }{ T_\mathrm{j}(t) } \right) 

where :math:`T_{j}(t)` is the past througput performance perceived by the
user :math:`j`.
According to the above scheduling algorithm, a user can be allocated to
different RBGs, which can be either adjacent or not, depending on the current
condition of the channel and the past throughput performance :math:`T_{j}(t)`. The
latter is determined at the end of the subframe :math:`t` using the following
exponential moving average approach:

.. math::

   T_{j}(t) = 
   (1-\frac{1}{\alpha})T_{j}(t-1)
   +\frac{1}{\alpha} \widehat{T}_{j}(t)

where :math:`\alpha` is the time constant (in number of subframes) of
the exponential moving average, and :math:`\widehat{T}_{j}(t)` is the actual
throughput achieved by the user :math:`i` in the subframe :math:`t`. :math:`\widehat{T}_{j}(t)`
is measured according to the following procedure. First we
determine the MCS :math:`\widehat{M}_j(t)` actually used by user
:math:`j`:

.. math::

   \widehat{M}_j(t) = \min_{k: \widehat{i}_{k}(t) = j}{M_{j,k}(t)}

then we determine the total number :math:`\widehat{B}_j(t)` of RBGs allocated to user
:math:`j`:

.. math::

   \widehat{B}_j(t) = \left| \{ k :  \widehat{i}_{k}(t) = j \} \right|

where :math:`|\cdot|` indicates the cardinality of the set; finally, 

.. math::

   \widehat{T}_{j}(t) = \frac{S\left( \widehat{M}_j(t), \widehat{B}_j(t)
   \right)}{\tau}
   

For what concern the HARQ, PF implements the non adaptive version, which implies that in allocating the retransmission attempts the scheduler uses the same allocation configuration of the original block, which means maintaining the same RBGs and MCS. UEs that are allocated for HARQ retransmissions are not considered for the transmission of new data in case they have a transmission opportunity available in the same TTI. Finally, HARQ can be disabled with ns3 attribute system for maintaining backward compatibility with old test cases and code, in detail::

   Config::SetDefault ("ns3::PfFfMacScheduler::HarqEnabled", BooleanValue (false));



Maximum Throughput (MT) Scheduler
---------------------------------

The Maximum Throughput (MT) scheduler [FCapo2012]_ aims to maximize the overall throughput of eNB.
It allocates each RB to the user that can achieve the maximum achievable rate in the current TTI.
Currently, MT scheduler in NS-3 has two versions: frequency domain (FDMT) and time domain (TDMT).
In FDMT, every TTI, MAC scheduler allocates RBGs to the UE who has highest achievable rate calculated 
by subband CQI. In TDMT, every TTI, MAC scheduler selects one UE which has highest achievable rate 
calculated by wideband CQI. Then MAC scheduler allocates all RBGs to this UE in current TTI.
The calculation of achievable rate in FDMT and TDMT is as same as the one in PF.
Let :math:`i,j` denote generic users; let :math:`t` be the
subframe index, and :math:`k` be the resource block index; let :math:`M_{i,k}(t)` be MCS
usable by user :math:`i` on resource block :math:`k` according to what reported by the AMC
model (see `Adaptive Modulation and Coding`_); finally, let :math:`S(M, B)` be the TB
size in bits as defined in [TS36213]_ for the case where a number :math:`B` of
resource blocks is used. The achievable rate :math:`R_{i}(k,t)` in bit/s for user :math:`i`
on resource block :math:`k` at subframe :math:`t` is defined as 

.. math::

   R_{i}(k,t) =  \frac{S\left( M_{i,k}(t), 1\right)}{\tau} 

where :math:`\tau` is the TTI duration.
At the start of each subframe :math:`t`, each RB is assigned to a certain user.
In detail, the index :math:`\widehat{i}_{k}(t)` to which RB :math:`k` is assigned at time
:math:`t` is determined as

.. math::

   \widehat{i}_{k}(t) = \underset{j=1,...,N}{\operatorname{argmax}}
       \left( { R_{j}(k,t) } \right) 

When there are several UEs having the same achievable rate, current implementation always selects
the first UE created in script. Although MT can maximize cell throughput, it cannot provide
fairness to UEs in poor channel condition.


Throughput to Average (TTA) Scheduler
-------------------------------------

The Throughput to Average (TTA) scheduler [FCapo2012]_ can be considered as an intermediate between MT and PF. 
The metric used in TTA is calculated as follows:

.. math::

   \widehat{i}_{k}(t) = \underset{j=1,...,N}{\operatorname{argmax}}
    \left( \frac{ R_{j}(k,t) }{ R_{j}(t) } \right) 

Here, :math:`R_{i}(k,t)` in bit/s represents the achievable rate for user :math:`i`
on resource block :math:`k` at subframe :math:`t`. The 
calculation method already is shown in MT and PF. Meanwhile, :math:`R_{i}(t)` in bit/s stands 
for the achievable rate for :math:`i` at subframe :math:`t`. The difference between those two 
achievable rates is how to get MCS. For :math:`R_{i}(k,t)`, MCS is calculated by subband CQI while 
:math:`R_{i}(t)` is calculated by wideband CQI. TTA scheduler can only be implemented in frequency domain (FD) because
the achievable rate of particular RBG is only related to FD scheduling.

Blind Average Throughput Scheduler
----------------------------------

The Blind Average Throughput scheduler [FCapo2012]_ aims to provide equal throughput to all UEs under eNB. The metric
used in TTA is calculated as follows:

.. math::

   \widehat{i}_{k}(t) = \underset{j=1,...,N}{\operatorname{argmax}}
    \left( \frac{ 1 }{ T_\mathrm{j}(t) } \right) 

where :math:`T_{j}(t)` is the past throughput performance perceived by the user :math:`j` and can be calculated by the 
same method in PF scheduler. In the time domain blind average throughput (TD-BET), the scheduler selects the UE
with largest priority metric and allocates all RBGs to this UE. On the other hand, in the frequency domain blind 
average throughput (FD-BET), every TTI, the scheduler first selects one UE with lowest pastAverageThroughput (largest 
priority metric). Then scheduler assigns one RBG to this UE, it calculates expected throughput of this UE and uses it
to compare with past average throughput :math:`T_{j}(t)` of other UEs. The scheduler continues 
to allocate RBG to this UE until its expected throughput is not the smallest one among past average throughput :math:`T_{j}(t)` 
of all UE. Then the scheduler will use the same way to allocate RBG for a new UE which has the 
lowest past average throughput :math:`T_{j}(t)` until all RBGs are allocated to UEs. The principle behind this is
that, in every TTI, the scheduler tries the best to achieve the equal throughput among all UEs.

Token Bank Fair Queue Scheduler
-------------------------------

Token Bank Fair Queue (TBFQ) is a QoS aware scheduler which derives from the leaky-bucket mechanism. In TBFQ, 
a traffic flow of user :math:`i` is characterized by following parameters: 

 * :math:`t_{i}`: packet arrival rate (byte/sec )
 * :math:`r_{i}`: token generation rate (byte/sec)
 * :math:`p_{i}`: token pool size (byte)
 * :math:`E_{i}`: counter that records the number of token borrowed from or given to the token bank by flow :math:`i` ;
   :math:`E_{i}` can be smaller than zero 

Each K bytes data consumes k tokens. Also, TBFQ maintains a shared token bank (:math:`B`) so as to balance the traffic
between different flows. If token generation rate :math:`r_{i}` is bigger than packet arrival rate :math:`t_{i}`, then tokens
overflowing from token pool are added to the token bank, and :math:`E_{i}` is increased by the same amount. Otherwise,
flow :math:`i` needs to withdraw tokens from token bank based on a priority metric :math:`frac{E_{i}}{r_{i}}`, and :math:`E_{i}` is decreased.
Obviously, the user contributes more on token bank has higher priority to borrow tokens; on the other hand, the 
user borrows more tokens from bank has lower priority to continue to withdraw tokens. Therefore, in case of several
users having the same token generation rate, traffic rate and token pool size, user suffers from higher interference
has more opportunity to borrow tokens from bank. In addition, TBFQ can police the traffic by setting the token 
generation rate to limit the throughput.  Additionally, TBFQ also maintains following three parameters for each flow: 

 * Debt limit :math:`d_{i}`: if :math:`E_{i}` belows this threshold, user i cannot further borrow tokens from bank. This is for 
   preventing malicious UE to borrow too much tokens.
 * Credit limit :math:`c_{i}`: the maximum number of tokens UE i can borrow from the bank in one time.
 * Credit threshold :math:`C`: once :math:`E_{i}` reaches debt limit, UE i must store :math:`C` tokens to bank in order to further 
   borrow token from bank.

LTE in NS-3 has two versions of TBFQ scheduler: frequency domain TBFQ (FD-TBFQ) and time domain TBFQ (TD-TBFQ). 
In FD-TBFQ, the scheduler always select UE with highest metric and allocates RBG with highest subband CQI until 
there are no packets within UE's RLC buffer or all RBGs are allocated [FABokhari2009]_. In TD-TBFQ, after selecting
UE with maximum metric, it allocates all RBGs to this UE by using wideband CQI [WKWong2004]_. 

Priority Set Scheduler
----------------------

Priority set scheduler (PSS) is a QoS aware scheduler which combines time domain (TD) and frequency domain (FD) 
packet scheduling operations into one scheduler [GMonghal2008]_. It controls the fairness among UEs by a specified 
Target Bit Rate (TBR). 

In TD scheduler part, PSS first selects UEs with non-empty RLC buffer and then divide them into two sets based 
on the TBR: 

* set 1: UE whose past average throughput is smaller than TBR; TD scheduler calculates their priority metric in
  Blind Equal Throughput (BET) style:

.. math::

   \widehat{i}_{k}(t) = \underset{j=1,...,N}{\operatorname{argmax}}
    \left( \frac{ 1 }{ T_\mathrm{j}(t) } \right) 

* set 2: UE whose past average throughput is larger (or equal) than TBR; TD scheduler calculates their priority
  metric in Proportional Fair (PF) style:

.. math::

   \widehat{i}_{k}(t) = \underset{j=1,...,N}{\operatorname{argmax}}
    \left( \frac{ R_{j}(k,t) }{ T_\mathrm{j}(t) } \right) 

UEs belonged to set 1 have higher priority than ones in set 2. Then PSS will select :math:`N_{mux}` UEs with
highest metric in two sets and forward those UE to FD scheduler. In PSS, FD scheduler allocates RBG k to UE n 
that maximums the chosen metric. Two PF schedulers are used in PF scheduler:

* Proportional Fair scheduled (PFsch)

.. math::

   \widehat{Msch}_{k}(t) = \underset{j=1,...,N}{\operatorname{argmax}}
    \left( \frac{ R_{j}(k,t) }{ Tsch_\mathrm{j}(t) } \right) 


* Carrier over Interference to Average (CoIta)

.. math::

   \widehat{Mcoi}_{k}(t) = \underset{j=1,...,N}{\operatorname{argmax}}
    \left( \frac{ CoI[j,k] }{ \sum_{k=0}^{N_{RBG}} CoI[j,k] } \right) 

where :math:`Tsch_{j}(t)` is similar past throughput performance perceived by the user :math:`j`, with the 
difference that it is updated only when the i-th user is actually served. :math:`CoI[j,k]` is an 
estimation of the SINR on the RBG :math:`k` of UE :math:`j`. Both PFsch and CoIta is for decoupling 
FD metric from TD scheduler. In addition, PSS FD scheduler also provide a weight metric W[n] for helping 
controlling fairness in case of low number of UEs.

.. math::

   W[n] =  max (1, \frac{TBR}{ T_{j}(t) })

where :math:`T_{j}(t)` is the past throughput performance perceived by the user :math:`j` . Therefore, on
RBG k, the FD scheduler selects the UE :math:`j` that maximizes the product of the frequency domain 
metric (:math:`Msch`, :math:`MCoI`) by weight :math:`W[n]`. This strategy will guarantee the throughput of lower
quality UE tend towards the TBR. 

::

  Config::SetDefault ("ns3::PfFfMacScheduler::HarqEnabled", BooleanValue (false));

The scheduler implements the filtering of the uplink CQIs according to their nature with ``UlCqiFilter`` attibute, in detail:

  - ``SRS_UL_CQI``: only SRS based CQI are stored in the internal attributes.
  - ``PUSCH_UL_CQI``: only PUSCH based CQI are stored in the internal attributes.
  - ``ALL_UL_CQI``: all CQIs are stored in the same internal attibute (i.e., the last CQI received is stored independently from its nature).


.. _sec-random-access:

Random Access
+++++++++++++

The LTE model includes a model of the Random Access procedure based on
some simplifying assumptions, which are detailed in the following for
each of the messages and signals described in the specs [TS36321]_.

   - **Random Access (RA) preamble**: in real LTE systems this
     corresponds to a Zadoff-Chu (ZC)
     sequence using one of several formats available and sent in the
     PRACH slots which could in principle overlap with PUSCH.
     The RA preamble is modeled using the LteControlMessage class,
     i.e., as an ideal message that does not consume any radio
     resources. The collision of preamble transmission by multiple UEs
     in the same cell are modeled using a protocol interference model,
     i.e., whenever two or more identical preambles are transmitted in
     same cell at the same TTI, no one of these identical preambles
     will be received by the eNB. Other than this collision model, no
     error model is associated with the reception of a RA preamble.

   - **Random Access Response (RAR)**: in real LTE systems, this is a
     special MAC PDU sent on the DL-SCH. Since MAC control elements are not
     accurately modeled in the simulator (only RLC and above PDUs
     are), the RAR is modeled as an LteControlMessage that does not
     consume any radio resources. Still, during the RA procedure, the
     LteEnbMac will request to the scheduler the allocation of
     resources for the RAR using the FF MAC Scheduler primitive
     SCHED_DL_RACH_INFO_REQ. Hence, an enhanced scheduler
     implementation (not available at the moment) could allocate radio
     resources for the RAR, thus modeling the consumption of Radio
     Resources for the transmission of the RAR. 

   - **Message 3**:  in real LTE systems, this is an RLC TM
     SDU sent over resources specified in the UL Grant in the RAR. In
     the simulator, this is modeled as a real RLC TM RLC PDU 
     whose UL resources are allocated by the scheduler upon call to
     SCHED_DL_RACH_INFO_REQ. 

   - **Contention Resolution (CR)**: in real LTE system, the CR phase
     is needed to address the case where two or more UE sent the same
     RA preamble in the same TTI, and the eNB was able to detect this
     preamble in spite of the collision. Since this event does not
     occur due to the protocol interference model used for the
     reception of RA preambles, the CR phase is not modeled in the
     simulator, i.e., the CR MAC CE is never sent by the eNB and the
     UEs consider the RA to be successful upon reception of the
     RAR. As a consequence, the radio resources consumed for the
     transmission of the CR MAC CE are not modeled.



Figure :ref:`fig-mac-random-access-contention` and
:ref:`fig-mac-random-access-noncontention` shows the sequence diagrams
of respectively the contention-based and non-contention-based MAC
random access procedure, highlighting the interactions between the MAC
and the other entities. 


.. _fig-mac-random-access-contention:

.. figure:: figures/mac-random-access-contention.*
   :align: center

   Sequence diagram of the Contention-based MAC Random Access procedure


.. _fig-mac-random-access-noncontention:

.. figure:: figures/mac-random-access-noncontention.*
   :align: center

   Sequence diagram of the Non-contention-based MAC Random Access procedure




.. only:: latex

    .. raw:: latex
      
        \clearpage


---
RLC
---




Overview
++++++++

The RLC entity is specified in the 3GPP technical specification
[TS36322]_, and comprises three different types of RLC: Transparent
Mode (TM), Unacknowledge Mode (UM) and Acknowledged Mode (AM). The
simulator includes one model for each of these entitities

The RLC entities provide the RLC service interface to the upper PDCP layer and the MAC service interface
to the lower MAC layer. The RLC entities use the PDCP service interface from the upper PDCP layer and
the MAC service interface from the lower MAC layer.

Figure :ref:`fig-lte-rlc-implementation-model` shows the
implementation model of the RLC entities and its relationship   
with all the other entities and services in the protocol stack.


.. _fig-lte-rlc-implementation-model:

.. figure:: figures/lte-rlc-implementation-model.*
   :width: 800px

   Implementation Model of PDCP, RLC and MAC entities and SAPs



Service Interfaces
++++++++++++++++++

RLC Service Interface
---------------------

The RLC service interface is divided into two parts:

    * the ``RlcSapProvider`` part is provided by the RLC layer and used by the upper PDCP layer and
    * the ``RlcSapUser`` part is provided by the upper PDCP layer and used by the RLC layer.

Both the UM and the AM RLC entities provide the same RLC service interface to the upper PDCP layer.

RLC Service Primitives
^^^^^^^^^^^^^^^^^^^^^^

The following list specifies which service primitives are provided by the RLC service interfaces:

    * ``RlcSapProvider::TransmitPdcpPdu``

        * The PDCP entity uses this primitive to send a PDCP PDU to the lower RLC entity
          in the transmitter peer

    * ``RlcSapUser::ReceivePdcpPdu``

        * The RLC entity uses this primitive to send a PDCP PDU to the upper PDCP entity
          in the receiver peer

MAC Service Interface
---------------------

The MAC service interface is divided into two parts:

  * the ``MacSapProvider`` part is provided by the MAC layer and used by the upper RLC layer and
  * the ``MacSapUser``  part is provided by the upper RLC layer and used by the MAC layer.

MAC Service Primitives
^^^^^^^^^^^^^^^^^^^^^^

The following list specifies which service primitives are provided by the MAC service interfaces:

    * ``MacSapProvider::TransmitPdu``

        * The RLC entity uses this primitive to send a RLC PDU to the lower MAC entity
          in the transmitter peer

    * ``MacSapProvider::ReportBufferStatus``

        * The RLC entity uses this primitive to report the MAC entity the size of pending buffers
          in the transmitter peer

    * ``MacSapUser::NotifyTxOpportunity``

        * The MAC entity uses this primitive to nofify the RLC entity a transmission opportunity

    * ``MacSapUser::ReceivePdu``

        * The MAC entity uses this primitive to send an RLC PDU to the upper RLC entity
          in the receiver peer


.. _sec-am_data_transfer:

AM RLC
++++++


The processing of the data transfer in the Acknowledge Mode (AM) RLC entity is explained in section 5.1.3 of [TS36322]_.
In this section we describe some details of the implementation of the
RLC entity.


Buffers for the transmit operations
-----------------------------------

Our implementation of the AM RLC entity maintains 3 buffers for the
transmit operations:

    * **Transmission Buffer**: it is the RLC SDU queue. 
      When the AM RLC entity receives a SDU in the TransmitPdcpPdu service primitive from the
      upper PDCP entity, it enqueues it in the Transmission Buffer. We
      put a limit on the RLC buffer size and just silently drop SDUs
      when the buffer is full. 

    * **Transmitted PDUs Buffer**: it is the queue of transmitted RLC PDUs for which an ACK/NACK has not
      been received yet. When the AM RLC entity sends a PDU to the MAC
      entity, it also puts a copy of the transmitted PDU in the Transmitted PDUs Buffer.

    * **Retransmission Buffer**: it is the queue of RLC PDUs which are considered for retransmission
      (i.e., they have been NACKed). The AM RLC entity moves this PDU to the Retransmission Buffer,
      when it retransmits a PDU from the Transmitted Buffer.


.. _sec-rlc-am-tx-operations:

Transmit operations in downlink
-------------------------------

The following sequence diagram shows the interactions between the
different entities (RRC, PDCP, AM RLC, MAC and MAC scheduler) of the
eNB in the downlink to perform data communications.

Figure :ref:`fig-lte-rlc-data-txon-dl` shows how the upper layers send
data PDUs and how the data flow is processed by the different
entities/services of the LTE protocol stack. 

.. _fig-lte-rlc-data-txon-dl:
   
.. figure:: figures/lte-rlc-data-txon-dl.*
   :width: 800px

   Sequence diagram of data PDU transmission in downlink


The PDCP entity calls the ``Transmit_PDCP_PDU service primitive`` in
order to send a data PDU. The AM RLC entity processes this service
primitive according to the AM data transfer procedures defined in 
section 5.1.3 of [TS36322]_.

When the ``Transmit_PDCP_PDU`` service primitive is called, the AM RLC
entity performs the following operations:

    * Put the data SDU in the Transmission Buffer.
    * Compute the size of the buffers (how the size of buffers is
      computed will be explained afterwards).
    * Call the ``Report_Buffer_Status`` service primitive of the eNB
      MAC entity in order to notify to the eNB MAC 
      entity the sizes of the buffers of the AM RLC entity. Then, the
      eNB MAC entity updates the buffer status in the MAC scheduler
      using the SchedDlRlcBufferReq service primitive of the FF MAC
      Scheduler API. 

Afterwards, when the MAC scheduler decides that some data can be sent,
the MAC entity notifies it to the RLC entity, i.e. it calls the
``Notify_Tx_Opportunity`` service primitive, then the AM RLC entity
does the following:

    * Create a single data PDU by segmenting and/or concatenating the
      SDUs in the Transmission Buffer. 
    * Move the data PDU from the Transmission Buffer to the
      Transmitted PDUs Buffer. 
    * Update state variables according section 5.1.3.1.1 of
      [TS36322]_. 
    * Call the ``Transmit_PDU`` primitive in order to send the data
      PDU to the MAC entity. 

Retransmission in downlink
--------------------------

The sequence diagram of Figure :ref:`fig-lte-rlc-data-retx-dl` shows
the interactions between the different entities (AM RLC, MAC and MAC
scheduler) of the eNB in downlink when data PDUs must be retransmitted
by the AM RLC entity.  

.. _fig-lte-rlc-data-retx-dl:
   
.. figure:: figures/lte-rlc-data-retx-dl.*
   :width: 500px

   Sequence diagram of data PDU retransmission in downlink


The transmitting AM RLC entity can receive STATUS PDUs from the peer AM RLC entity. STATUS PDUs are
sent according section 5.3.2 of [TS36322]_ and the processing of reception is made according
section 5.2.1 of [TS36322]_.

When a data PDUs is retransmitted from the Transmitted PDUs Buffer, it is also moved to the
Retransmission Buffer.

Transmit operations in uplink
-----------------------------

The sequence diagram of Figure :ref:`fig-lte-rlc-data-txon-ul` shows
the interactions between the different entities of the UE (RRC, PDCP,
RLC and MAC) and the eNB (MAC and Scheduler) in uplink when data PDUs
are sent by the upper layers.

.. _fig-lte-rlc-data-txon-ul:
   
.. figure:: figures/lte-rlc-data-txon-ul.*
   :width: 800px

   Sequence diagram of data PDU transmission in uplink


It is similar to the sequence diagram in downlink; the main difference
is that in this case the Report_Buffer_Status is sent from the UE MAC
to the MAC Scheduler in the eNB over the air using the control
channel. 

Retransmission in uplink
------------------------

The sequence diagram of Figure :ref:`fig-lte-rlc-data-retx-ul` shows
the interactions between the different entities of the UE (AM RLC and
MAC) and the eNB (MAC) in uplink when data PDUs must be retransmitted
by the AM RLC entity. 

.. _fig-lte-rlc-data-retx-ul:

.. figure:: figures/lte-rlc-data-retx-ul.*
   :width: 500px

   Sequence diagram of data PDU retransmission in uplink


.. _sec-rlc-am-buffer-size:

Calculation of the buffer size
------------------------------

The Transmission Buffer contains RLC SDUs. A RLC PDU is one or more SDU segments plus an RLC header.
The size of the RLC header of one RLC PDU depends on the number of SDU segments the PDU contains.

The 3GPP standard  (section 6.1.3.1 of [TS36321]_) says clearly that,
for the uplink, the RLC and MAC headers are not considered in the
buffer size that is to be report as part of the Buffer Status Report. 
For the downlink, the behavior is not specified. Neither [FFAPI]_ specifies
how to do it. Our RLC model works by assuming that the calculation of
the buffer size in the downlink is done exactly as in the uplink,
i.e., not considering the RLC and MAC header size.

We note that this choice affects the interoperation with the
MAC scheduler, since, in response to the
``Notify_Tx_Opportunity`` service primitive, the RLC is expected to
create a PDU of no more than the size requested by the MAC, including
RLC overhead. Hence, unneeded fragmentation can occur if (for example)
the MAC notifies a transmission exactly equal to the buffer size
previously reported by the RLC. We assume that it is left to the Scheduler
to implement smart strategies for the selection of the size of the
transmission opportunity, in order to eventually avoid the inefficiency
of unneeded fragmentation. 



Concatenation and Segmentation
------------------------------

The AM RLC entity generates and sends exactly one RLC PDU for each transmission opportunity even
if it is smaller than the size reported by the transmission opportunity. So for instance, if a
STATUS PDU is to be sent, then only this PDU will be sent in that transmission opportunity.

The segmentation and concatenation for the SDU queue of the AM RLC entity follows the same philosophy
as the same procedures of the UM RLC entity but there are new state
variables (see [TS36322]_ section 7.1) only present in the AM RLC entity.

It is noted that, according to the 3GPP specs, there is no concatenation for the Retransmission Buffer.

Re-segmentation
---------------

The current model of the AM RLC entity does not support the
re-segmentation of the retransmission buffer. Rather, the AM RLC
entity just waits to receive a big enough transmission
opportunity. 


Unsupported features
--------------------

We do not support the following procedures of [TS36322]_ :

    * “Send an indication of successful delivery of RLC SDU” (See section 5.1.3.1.1)
    * “Indicate to upper layers that max retransmission has been reached” (See section 5.2.1)
    * “SDU discard procedures” (See section 5.3)
    * “Re-establishment procedure” (See section 5.4)

We do not support any of the additional primitives of RLC SAP for AM RLC entity. In particular:

    * no SDU discard notified by PDCP 
    * no notification of successful / failed delivery by AM RLC entity to PDCP entity


UM RLC
++++++

In this section we describe the implemnetation of the Unacknowledge Mode (UM) RLC entity.

Transmit operations in downlink
-------------------------------

The transmit operations of the UM RLC are similar to those of the AM
RLC previously described in Section :ref:`sec-rlc-am-tx-operations`,
with the difference that, following the specifications of [TS36322]_,
retransmission are not performed, and there are no STATUS PDUs.

Transmit operations in uplink
-----------------------------

The transmit operations in the uplink are similar to those of the
downlink, with the main difference that the Report_Buffer_Status is
sent from the UE MAC to the MAC Scheduler in the eNB over the air
using the control channel. 


Calculation of the buffer size
------------------------------

The calculation of the buffer size for the UM RLC is done using the
same approach of the AM RLC, please refer to section
:ref:`sec-rlc-am-buffer-size` for the corresponding description.




TM RLC
++++++

In this section we describe the implementation of the Transparent Mode (TM) RLC entity.


Transmit operations in downlink
-------------------------------

In the simulator, the TM RLC still provides to the upper layers the
same service interface provided by the AM and UM RLC
entities to the PDCP layer; in practice, this interface is used by an RRC
entity (not a PDCP entity) for the transmission of RLC SDUs. This
choice is motivated by the fact that the services provided by the TM
RLC to the upper layers, according to [TS36322]_, is a subset of those
provided by the UM and AM RLC entities to the PDCP layer; hence,
we reused the same interface for simplicity.

The transmit operations in the downlink are performed as follows. When
the ``Transmit_PDCP_PDU service primitive`` is called by the upper
layers, the TM RLC does the following:

 * put the SDU in the Transmission Buffer
 * compute the size of the Transmission Buffer
 * call the ``Report_Buffer_Status`` service primitive of the eNB
   MAC entity
 
Afterwards, when the MAC scheduler decides that some data can be sent
by the logical channel to which the TM RLC entity belongs, the MAC
entity notifies it to the TM RLC entity by calling the
``Notify_Tx_Opportunity`` service primitive. Upon reception of this
primitive, the TM RLC entity does the following:

 * if the TX opportunity has a size that is greater than or equal to
   the size of the head-of-line SDU in the Transmission Buffer
   
   - dequeue the head-of-line SDU from the Transmission Buffer
   
   - create one RLC PDU that contains entirely that SDU, without any
     RLC header 

   -  Call the ``Transmit_PDU`` primitive in order to send the RLC
      PDU to the MAC entity. 

   
Transmit operations in uplink
-----------------------------

The transmit operations in the uplink are similar to those of the
downlink, with the main difference that a transmission opportunity can
also arise from the assignment of the UL GRANT as part of the Random
Access procedure, without an explicit Buffer Status Report issued by
the TM RLC entity.



Calculation of the buffer size
------------------------------

As per the specifications [TS36322]_, the TM RLC does not add any RLC
header to the PDUs being transmitted. Because of this, the buffer size
reported to the MAC layer is calculated simply by summing the size of
all packets in the transmission buffer, thus notifying to the MAC the
exact buffer size.



SM RLC
++++++

In addition to the AM, UM and TM implementations that are modeled
after the 3GPP specifications, a simplified RLC model is provided,
which is called Saturation Mode (SM) RLC. This RLC model does not accept
PDUs from any above layer (such as PDCP); rather, the SM RLC takes care of the
generation of RLC PDUs in response to  
the notification of transmission opportunities notified by the MAC. 
In other words, the SM RLC simulates saturation conditions, i.e., it
assumes that the RLC buffer is always full and can generate a new PDU
whenever notified by the scheduler. 

The SM RLC is used for simplified simulation scenarios in which only the
LTE Radio model is used, without the EPC and hence without any IP
networking support. We note that, although the SM RLC is an
unrealistic traffic model, it still allows for the correct simulation
of scenarios with multiple flows belonging to different (non real-time)
QoS classes, in order to test the QoS performance obtained by different
schedulers. This can be 
done since it is the task of the Scheduler to assign transmission
resources based on the characteristics (e.g., Guaranteed Bit Rate) of
each Radio Bearer, which are specified upon the definition of each
Bearer within the simulation program.

As for schedulers designed to work with real-time QoS
traffic that has delay constraints, the SM RLC is probably not an appropriate choice.
This is because the absence of actual RLC SDUs (replaced by the artificial
generation of Buffer Status Reports) makes it not possible to provide
the Scheduler with meaningful head-of-line-delay information, which is
often the metric of choice for the implementation of scheduling
policies for real-time traffic flows. For the simulation and testing
of such schedulers, it is advisable to use either the UM or the AM RLC
models instead.


----
PDCP
----

PDCP Model Overview
+++++++++++++++++++

The reference document for the specification of the PDCP entity is
[TS36323]_. With respect to this specification, the PDCP model
implemented in the simulator supports only the following features:

 * transfer of data (user plane or control plane);
 * maintenance of PDCP SNs;
 * transfer of SN status (for use upon handover);

The following features are currently not supported:

 * header compression and decompression of IP data flows using the ROHC protocol;
 * in-sequence delivery of upper layer PDUs at re-establishment of lower layers;
 * duplicate elimination of lower layer SDUs at re-establishment of lower layers for radio bearers mapped on RLC AM;
 * ciphering and deciphering of user plane data and control plane data;
 * integrity protection and integrity verification of control plane data;
 * timer based discard;
 * duplicate discarding.


PDCP Service Interface
++++++++++++++++++++++

The PDCP service interface is divided into two parts:

    * the ``PdcpSapProvider`` part is provided by the PDCP layer and used by the upper layer and
    * the ``PdcpSapUser`` part is provided by the upper layer and used by the PDCP layer.

PDCP Service Primitives
-----------------------

The following list specifies which service primitives are provided by the PDCP service interfaces:

    * ``PdcpSapProvider::TransmitPdcpSdu``

        * The RRC entity uses this primitive to send an RRC PDU to the lower PDCP entity
          in the transmitter peer

    * ``PdcpSapUser::ReceivePdcpSdu``

        * The PDCP entity uses this primitive to send an RRC PDU to the upper RRC entity
          in the receiver peer



.. only:: latex

    .. raw:: latex
      
        \clearpage


---
RRC
---

Features
++++++++

The RRC model implemented in the simulator provides the following functionality:

 - generation (at the eNB) and interpretation (at the UE) of System
   Information (in particular the Master Information Block and, at the
   time of this writing, only System Information Block Type 1 and 2)
 - initial cell selection
 - RRC connection establishment procedure
 - RRC reconfiguration procedure, supporting the following use cases:
   + reconfiguration of the SRS configuration index
   + reconfiguration of the PHY TX mode (MIMO)
   + reconfiguration of UE measurements
   + data radio bearer setup
   + handover
 - RRC connection re-establishment, supporting the following use
   cases:
   + handover


Architecture
++++++++++++

The RRC model is divided into the following components:

 - the RRC entities `LteUeRrc` and `LteEnbRrc`, which implement the state
   machines of the RRC entities respectively at the UE and the eNB;
 - the RRC SAPs `LteUeRrcSapProvider`, `LteUeRrcSapUser`,
   `LteEnbRrcSapProvider`, `LteEnbRrcSapUser`, which allow the RRC
   entities to send and receive RRC messages and information
   elmenents;
 - the RRC protocol classes `LteUeRrcProtocolIdeal`,
   `LteEnbRrcProtocolIdeal`, `LteUeRrcProtocolReal`,
   `LteEnbRrcProtocolReal`, which implement two different models for
   the transmission of RRC messages.

Additionally, the RRC components use various other SAPs in order to
interact with the rest of the protocol stack. A representation of all
the SAPs that are used is provided in the figures :ref:`fig-lte-arch-ue-data`,
:ref:`fig-lte-arch-ue-ctrl`, :ref:`fig-lte-arch-enb-data` and
:ref:`fig-lte-arch-enb-ctrl`.


UE RRC State Machine
++++++++++++++++++++

In Figure :ref:`fig-lte-ue-rrc-states` we represent the state machine
as implemented in the RRC UE entity.

.. _fig-lte-ue-rrc-states:

.. figure:: figures/lte-ue-rrc-states.*
   :scale: 70 %
   :align: center

   UE RRC State Machine

It is to be noted that most of the states are transient, i.e., once
the UE goes into one of the CONNECTED states it will never switch back
to any of the IDLE states. This choice is done for the following reasons:

 - as discussed in the section :ref:`sec-design-criteria`, the focus
   of the LTE-EPC simulation model is on CONNECTED mode
 - radio link failure is not currently modeled, as discussed in the
   section :ref:`sec-radio-link-failure`, so an UE cannot go IDLE
   because of radio link failure
 - RRC connection release is currently never triggered neither by the EPC
   nor by the NAS

Still, we chose to model explicitly the IDLE states, because:

 - a realistic UE RRC configuration is needed for handover, which is a
   required feature, and in order to have a cleaner implementation it makes sense to
   use the same UE RRC configuration also for the initial connection
   establishment
 - it makes easier to implement idle mode cell selection in the
   future, which is a highly desirable feature 
 

ENB RRC State Machine
+++++++++++++++++++++

The eNB RRC maintains the state for each UE that is attached to the
cell. From an implementation point of view, the state of each UE is
contained in an instance of the UeManager class. The state machine is
represented in Figure :ref:`fig-lte-enb-rrc-states`.

.. _fig-lte-enb-rrc-states:

.. figure:: figures/lte-enb-rrc-states.*
   :scale: 70 %
   :align: center

   ENB RRC State Machine for each UE


.. _sec-initial-cell-selection:

Initial Cell Selection
++++++++++++++++++++++

Initial cell selection is an IDLE mode procedure, performed by UE when it has
not yet camped or attached to an eNodeB. The objective of the procedure is to
find a suitable cell and attach to it to gain access to the cellular network.

It is typically done at the beginning of simulation, as depicted in Figure
:ref:`fig-lte-cell-selection-timeline` below. The time diagram on the left side
is illustrating the case where initial cell selection succeed on first try,
while the diagram on the right side is for the case where it fails on the first
try and succeed on the second try. The timing assumes the use of real RRC
protocol model (see :ref:`sec-rrc-protocol-models`) and no transmission error.

.. _fig-lte-cell-selection-timeline:

.. figure:: figures/lte-cell-selection-timeline.*
   :scale: 80 %
   :align: center

   Sample runs of initial cell selection in UE and timing of related events

The functionality is based on 3GPP IDLE mode specifications, such as in
[TS36300]_, [TS36304]_, and [TS36331]_. However, a proper implementation of IDLE
mode is still missing in the simulator, so we reserve several simplifying
assumptions:

 - multiple carrier frequency is not supported;
 
 - multiple Public Land Mobile Network (PLMN) identities (i.e. multiple network
   operators) is not supported;
 
 - RSRQ measurements are not utilized;
 
 - stored information cell selection is not supported;

 - "Any Cell Selection" state and camping to an acceptable cell is not
   supported;
 
 - marking a cell as barred or reserved is not supported;

 - cell reselection is not supported, hence it is not possible for UE to camp to
   a different cell after the initial camp has been placed; and
 
 - UE's Closed Subscriber Group (CSG) white list contains only one CSG identity.
   
Also note that initial cell selection is only available for EPC-enabled
simulations. LTE-only simulations must use the manual attachment method. See
section :ref:`sec-network-attachment` of the User Documentation for more
information on their differences in usage.

The next subsections cover different parts of initial cell selection, namely
*cell search*, *broadcast of system information*, and *cell selection evaluation*.

.. _sec-cell-search:

Cell Search
-----------

Cell search aims to detect surrounding cells and measure the strength of
received signal from each of these cells. One of these cells will become the
UE's entry point to join the cellular network.

The measurements are based on the RSRP of the received PSS, averaged by Layer 1
filtering, and performed by the PHY layer, as previously described in more
detail in section :ref:`sec-phy-ue-measurements`. PSS is transmitted by eNodeB
over the central 72 sub-carriers of the DL channel (Section 5.1.7.3 [TS36300]_),
hence we model cell search to operate using a DL bandwidth of 6 RBs. Note that
measurements of RSRQ are not available at this point of time in simulation. As
a consequence, the ``LteUePhy::RsrqUeMeasThreshold`` attribute does not apply
during cell search.

By using the measured RSRP, the PHY entity is able to generate a list of
detected cells, each with its corresponding cell ID and averaged RSRP. This list
is periodically pushed via CPHY SAP to the RRC entity as a measurement report.

The RRC entity inspects the report and simply choose the cell with the strongest
RSRP, as also indicated in Section 5.2.3.1 of [TS36304]_. Then it instructs back
the PHY entity to synchronize to this particular cell. The actual operating
bandwidth of the cell is still unknown at this time, so the PHY entity listens
only to the minimum bandwidth of 6 RBs. Nevertheless, the PHY entity will be
able to receive system broadcast message from this particular eNodeB, which is
the topic of the next subsection.

Broadcast of System Information
-------------------------------

System information blocks are broadcasted by eNodeB to UEs at predefined time
intervals, adapted from Section 5.2.1.2 of [TS36331]_. The supported system
information blocks are:

 - Master Information Block (MIB)
      Contains parameters related to the PHY layer, generated during cell
      configuration and broadcasted every 10 ms at the beginning of radio frame
      as a control message.

 - System Information Block Type 1 (SIB1)
      Contains information regarding network access, broadcasted every 20 ms at
      the middle of radio frame as a control message. Not used in manual
      attachment method. UE must have decoded MIB before it can receive SIB1.
      
 - System Information Block Type 2 (SIB2)
      Contains UL- and RACH-related settings, scheduled to transmit via RRC
      protocol at 16 ms after cell configuration, and then repeats every 80 ms
      (configurable through `LteEnbRrc::SystemInformationPeriodicity` attribute.
      UE must be camped to a cell in order to be able to receive its SIB2.

Reception of system information is fundamental for UE to advance in its
lifecycle. MIB enables the UE to increase the initial DL bandwidth of 6 RBs to
the actual operating bandwidth of the network. SIB1 provides information
necessary for cell selection evaluation (explained in the next section). And
finally SIB2 is required before the UE is allowed to switch to CONNECTED state.

Cell Selection Evaluation
-------------------------

UE RRC reviews the measurement report produced in :ref:`sec-cell-search` and the
cell access information provided by SIB1. Once both information is available for
a specific cell, the UE triggers the evaluation process. The purpose of this
process is to determine whether the cell is a suitable cell to camp to.

The evaluation process is a slightly simplified version of Section 5.2.3.2 of
[TS36304]_. It consists of the following criteria:

 - Rx level criterion; and
 
 - closed subscriber group (CSG) criterion.
 
The first criterion, Rx level, is based on the cell's measured RSRP
:math:`Q_{rxlevmeas}`, which has to be higher than a required minimum
:math:`Q_{rxlevmin}` in order to pass the criterion:

.. math::

   Q_{rxlevmeas} - Q_{rxlevmin} > 0 

where :math:`Q_{rxlevmin}` is determined by each eNodeB and is obtainable by UE
from SIB1.

The last criterion, CSG, is a combination of a true-or-false parameter called
*CSG indication* and a simple number *CSG identity*. The basic rule is that UE
shall not camp to eNodeB with a different CSG identity. But this rule is only
enforced when CSG indication is valued as true. More details are provided in
Section :ref:`sec-network-attachment` of the User Documentation.

When the cell passes all the above criteria, the cell is deemed as *suitable*.
Then UE camps to it (`IDLE_CAMPED_NORMALLY` state).

After this, upper layer may request UE to enter CONNECTED mode. Please refer to
section :ref:`sec-rrc-connection-establishment` for details on this.

On the other hand, when the cell does not pass the CSG criterion, then the cell
is labeled as *acceptable* (Section 10.1.1.1 [TS36300]_). In this case, the RRC
entity will tell the PHY entity to synchronize to the second strongest cell and
repeat the initial cell selection procedure using that cell. As long as no
suitable cell is found, the UE will repeat these steps while avoiding cells that
have been identified as acceptable.

   

Radio Admission Control
+++++++++++++++++++++++

Radio Admission Control is supported by having the eNB RRC
reply to an RRC CONNECTION REQUEST message sent by the UE with either
an RRC CONNECTION SETUP message or an RRC CONNECTION REJECT message,
depending on whether the new UE is to be admitted or not. In the
current implementation, the behavior is determined by the boolean attribute
``ns3::LteEnbRrc::AdmitRrcConnectionRequest``. There is currently no Radio Admission
Control algorithm that dynamically decides whether a new connection
shall be admitted or not. 


Radio Bearer Configuration
++++++++++++++++++++++++++

Some implementation choices have been made in the RRC regarding the setup of radio bearers:

 - three Logical Channel Groups (out of four available) are configured
   for uplink buffer status report purposes, according to the following policy:

   + LCG 0 is for signaling radio bearers
   + LCG 1 is for GBR data radio bearers
   + LCG 2 is for Non-GBR data radio bearers


.. _sec-radio-link-failure:

Radio Link Failure
++++++++++++++++++

Since at this stage the RRC supports the CONNECTED mode only, Radio Link
Failure (RLF) is not handled. The reason is that one of the possible
outcomes of RLF (when RRC re-establishment is unsuccessful) is to
leave RRC CONNECTED notifying the NAS of the RRC connection
failure. In order to model RLF properly, RRC IDLE mode should be
supported, including in particular idle mode cell (re-)selection.

With the current model, an UE that experiences bad link quality will
just stay associated with the same eNB, and the scheduler will stop
allocating resources to it for communications. This is also consistent
with the fact that, at this stage, only handovers explicitly triggered
within the simulation program are supported (network-driven handovers
based on UE measurements are planned only at a later stage).


.. _sec-ue-measurements:

UE RRC Measurements Model
+++++++++++++++++++++++++

UE RRC measurements support
---------------------------

The UE RRC entity provides support for UE measurements; in particular, it
implements the procedures described in Section 5.5 of [TS36331]_, with the
following simplifying assumptions:

 - only E-UTRA intra-frequency measurements are supported, which implies:
   
   - only one measurement object is used during the simulation;
   - measurement gaps are not needed to perform the measurements;
   - Event B1 and B2 are not implemented;
   
 - only `reportStrongestCells` purpose is supported, while `reportCGI` and
   `reportStrongestCellsForSON` purposes are not supported;

 - `s-Measure` is not supported;
 
 - since carrier aggregation is not supported in by the LTE module, the
   following assumptions in UE measurements hold true:
   
   - no notion of secondary cell (`SCell`);
   - primary cell (`PCell`) simply means serving cell;
   - Event A6 is not implemented;
   
 - speed dependant scaling of time-to-trigger (Section 5.5.6.2 of [TS36331]_) is
   not supported.

Overall design
--------------

The model is based on the concept of *UE measurements consumer*, which is an
entity that may request an eNodeB RRC entity to provide UE measurement reports.
Consumers are, for example, :ref:`sec-handover-algorithm`, which compute
handover decision based on UE measurement reports. Test cases and user's
programs may also become consumers. Figure :ref:`fig-ue-meas-consumer` depicts
the relationship between these entities.

.. _fig-ue-meas-consumer:
   
.. figure:: figures/ue-meas-consumer.*
   :scale: 80 %
   :align: center

   Relationship between UE measurements and its consumers

The whole UE measurements function at the RRC level is divided into 4 major
parts:

 #. Measurement configuration (handled by ``LteUeRrc::ApplyMeasConfig``)
 
 #. Performing measurements (handled by ``LteUeRrc::DoReportUeMeasurements``)
 
 #. Measurement report triggering (handled by
    ``LteUeRrc::MeasurementReportTriggering``)
   
 #. Measurement reporting (handled by ``LteUeRrc::SendMeasurementReport``)

The following sections will describe each of the parts above.

Measurement configuration
-------------------------

An eNodeB RRC entity configures UE measurements by sending the configuration
parameters to the UE RRC entity. This set of parameters are defined within the
``MeasConfig`` Information Element (IE) of the RRC Connection Reconfiguration
message (:ref:`sec-rrc-connection-reconfiguration`).

The eNodeB RRC entity implements the configuration parameters and procedures
described in Section 5.5.2 of [TS36331]_, with the following simplifying
assumption:

 - configuration (i.e. addition, modification, and removal) can only be done
   before the simulation begins;
   
 - all UEs attached to the eNodeB will be configured the same way, i.e. there is
   no support for configuring specific measurement for specific UE; and
   
 - it is assumed that there is a one-to-one mapping between the PCI and the
   E-UTRAN Global Cell Identifier (EGCI). This is consistent with the PCI
   modeling assumptions described in :ref:`sec-phy-ue-measurements`.

The eNodeB RRC instance here acts as an intermediary between the consumers and
the attached UEs. At the beginning of simulation, each consumer provides the
eNodeB RRC instance with the UE measurements configuration that it requires.
After that, the eNodeB RRC distributes the configuration to attached UEs.

Users may customize the measurement configuration using several methods. Please
refer to Section :ref:`sec-configure-ue-measurements` of the User Documentation
for the description of these methods.

.. _sec-performing-measurements:

Performing measurements
-----------------------

UE RRC receives both RSRP and RSRQ measurements on periodical basis from UE PHY,
as described in :ref:`sec-phy-ue-measurements`. *Layer 3 filtering* will be
applied to these received measurements. The implementation of the filtering
follows Section 5.5.3.2 of [TS36331]_:

.. math::

   F_n = (1 - a) \times F_{n-1} + a \times M_n

where:

 - :math:`M_n` is the latest received measurement result from the physical
   layer;
 - :math:`F_n` is the updated filtered measurement result;
 - :math:`F_{n-1}` is the old filtered measurement result, where
   :math:`F_0 = M_1` (i.e. the first measurement is not filtered); and
 - :math:`a = (\frac{1}{2})^{\frac{k}{4}}`, where :math:`k` is the configurable
   `filterCoefficent` provided by the ``QuantityConfig``;
   
:math:`k = 4` is the default value, but can be configured by setting the
`RsrpFilterCoefficient` and `RsrqFilterCoefficient` attributes in
``LteEnbRrc``.

Therefore :math:`k = 0` will disable Layer 3 filtering. On the other hand, past
measurements can be granted more influence on the filtering results by using
larger value of :math:`k`.

Measurement reporting triggering
--------------------------------

In this part, UE RRC will go through the list of active measurement
configuration and check whether the triggering condition is fulfilled in
accordance with Section 5.5.4 of [TS36331]_. When at least one triggering
condition from all the active measurement configuration is fulfilled, the
measurement reporting procedure (described in the next subsection) will be
initiated.

3GPP defines two kinds of `triggerType`: *periodical* and *event-based*. At the
moment, only event-based criterion is supported. There are various events that
can be selected, which are briefly described in the table below: 

.. table:: List of supported event-based triggering criteria

   ======== ======================================================
   Name     Description
   ======== ======================================================
   Event A1 Serving cell becomes better than `threshold`
   Event A2 Serving cell becomes worse than `threshold`
   Event A3 Neighbour becomes `offset` dB better than serving cell
   Event A4 Neighbour becomes better than `threshold`
   Event A5 Serving becomes worse than `threshold1`
            *AND* neighbour becomes better than `threshold2`
   ======== ======================================================

Two main conditions to be checked in an event-based trigger are the *entering
condition* and the *leaving condition*. More details on these two can be found
in Section 5.5.4 of [TS36331]_.

An event-based trigger can be further configured by introducing hysteresis and
time-to-trigger. *Hysteresis* (:math:`Hys`) defines the distance between the
entering and leaving conditions in dB. Similarly, *time-to-trigger* introduces
delay to both entering and leaving conditions, but as a unit of time.

The *periodical* type of reporting trigger is not supported, but its behaviour
can be easily obtained by using an event-based trigger. This can be done by
configuring the measurement in such a way that the entering condition is always
fulfilled, for example, by setting the threshold of Event A1 to zero (the
minimum level). As a result, the measurement reports will always be triggered
at every certain interval, as determined by the `reportInterval` field within
``LteRrcSap::ReportConfigEutra``, therefore producing the same behaviour as
periodical reporting.

As a limitation with respect to 3GPP specifications, the current model does not
support any cell-specific configuration. These configuration parameters are
defined in measurement object. As a consequence, incorporating a list of black
cells into the triggering process is not supported. Moreover, cell-specific
offset (i.e., :math:`O_{cn}` and :math:`O_{cp}` in Event A3, A4, and A5) are not
supported as well. The value equal to zero is always assumed in place of them.

Measurement reporting
---------------------

This part handles the submission of measurement report from the UE RRC entity
to the serving eNodeB entity via RRC protocol. Several simplifying assumptions
have been adopted:

 - `reportAmount` is *not* applicable (i.e. always assumed to be infinite);
   
 - in measurement reports, the `reportQuantity` is always assumed to be `BOTH`,
   i.e., both RSRP and RSRQ are always reported, regardless of the
   `triggerQuantity`.


.. _sec-handover:

Handover
++++++++

The RRC model supports UE mobility in CONNECTED mode by invoking the X2-based
handover procedure. The model is intra-EUTRAN and intra-frequency, as based on
Section 10.1.2.1 of [TS36300]_.

This section focuses on the process of triggering a handover. The handover
execution procedure itself is covered in Section :ref:`sec-x2`.

There are two ways to trigger the handover procedure:

 - *explicitly* (or manually) triggered by the simulation program by scheduling
   an execution of the method ``LteEnbRrc::SendHandoverRequest``; or

 - *automatically* triggered by the eNodeB RRC entity based on UE measurements
   and according to the selected handover algorithm.

Section :ref:`sec-x2-based-handover` of the User Documentation provides some
examples on using both explicit and automatic handover triggers in simulation.
The next subsection will take a closer look on the automatic method, by
describing the design aspects of the handover algorithm interface and the
available handover algorithms.

.. _sec-handover-algorithm:

Handover algorithm
------------------

Handover in 3GPP LTE has the following properties:

 - UE-assisted
     The UE provides input to the network in the form of measurement reports.
     This is handled by the :ref:`sec-ue-measurements`.
   
 - Network-controlled
     The network (i.e. the source eNodeB and the target eNodeB) decides when to
     trigger the handover and oversees its execution.

The *handover algorithm* operates at the source eNodeB and is responsible in
making handover decisions in an "automatic" manner. It interacts with an eNodeB
RRC instance via the *Handover Management SAP* interface. These relationships
are illustrated in Figure :ref:`fig-ue-meas-consumer` from the previous section.

The handover algorithm interface consists of the following methods:

 - ``AddUeMeasReportConfigForHandover``
     (Handover Algorithm -> eNodeB RRC) Used by the handover algorithm to
     request measurement reports from the eNodeB RRC entity, by passing the
     desired reporting configuration. The configuration will be applied to
     all future attached UEs.

 - ``ReportUeMeas``
     (eNodeB RRC -> Handover Algorithm) Based on the UE measurements configured
     earlier in ``AddUeMeasReportConfigForHandover``, UE may submit measurement
     reports to the eNodeB. The eNodeB RRC entity uses the ``ReportUeMeas``
     interface to forward these measurement reports to the handover algorithm.
     
 - ``TriggerHandover``
     (Handover Algorithm -> eNodeB RRC) After examining the measurement reports
     (but not necessarily), the handover algorithm may declare a handover. This
     method is used to notify the eNodeB RRC entity about this decision, which
     will then proceed to commence the handover procedure. 

One note for the ``AddUeMeasReportConfigForHandover``. The method will return
the ``measId`` (measurement identity) of the newly created measurement
configuration. Typically a handover algorithm would store this unique number. It
may be useful in the ``ReportUeMeas`` method, for example when more than one
configuration has been requested and the handover algorithm needs to
differentiate incoming reports based on the configuration that triggered them.

A handover algorithm is implemented by writing a subclass of the
``LteHandoverAlgorithm`` abstract superclass and implementing each of the above
mentioned SAP interface methods. Users may develop their own handover algorithm
this way, and then use it in any simulation by following the steps outlined in
Section :ref:`sec-x2-based-handover` of the User Documentation.

Alternatively, users may choose to use one of the 3 built-in handover algorithms
provided by the LTE module: no-op, A2-A4-RSRQ, and strongest cell handover
algorithm. They are ready to be used in simulations or can be taken as an
example of implementing a handover algorithm. Each of these built-in algorithms
is covered in each of the following subsections.

No-op handover algorithm
------------------------

The *no-op handover algorithm* (``NoOpHandoverAlgorithm`` class) is the simplest
possible implementation of handover algorithm. It basically does nothing, i.e.,
does not call any of the Handover Management SAP interface methods. Users may
choose this handover algorithm if they wish to disable automatic handover
trigger in their simulation.

A2-A4-RSRQ handover algorithm
-----------------------------

The *A2-A4-RSRQ handover algorithm* provides the functionality of the default
handover algorithm originally included in LENA M6 (ns-3.18), ported to the
Handover Management SAP interface as the ``A2A4RsrqHandoverAlgorithm`` class.

As the name implies, the algorithm utilizes the Reference Signal Received
Quality (RSRQ) measurements acquired from Event A2 and Event A4. Thus, the
algorithm will add 2 measurement configuration to the corresponding eNodeB RRC
instance. Their intended use are described as follows:

 - *Event A2* (serving cell's RSRQ becomes worse than `threshold`) is leveraged
   to indicate that the UE is experiencing poor signal quality and may benefit
   from a handover.
 
 - *Event A4* (neighbour cell's RSRQ becomes better than `threshold`) is used
   to detect neighbouring cells and acquire their corresponding RSRQ from every
   attached UE, which are then stored internally by the algorithm. By default,
   the algorithm configures Event A4 with a very low threshold, so that the
   trigger criteria are always true.
   
Figure :ref:`fig-lte-legacy-handover-algorithm` below summarizes this procedure.

.. _fig-lte-legacy-handover-algorithm:

.. figure:: figures/lte-legacy-handover-algorithm.*
   :scale: 70 %
   :align: center

   A2-A4-RSRQ handover algorithm
 
Two attributes can be set to tune the algorithm behaviour:

 - ``ServingCellThreshold``
     The `threshold` for Event A2, i.e. a UE must have an RSRQ lower than this
     threshold to be considered for a handover.

 - ``NeighbourCellOffset``
     The `offset` that aims to ensure that the UE would receive better signal
     quality after the handover. A neighbouring cell is considered as a target
     cell for the handover only if its RSRQ is higher than the serving cell's
     RSRQ by the amount of this `offset`.

The value of both attributes are expressed as RSRQ range (Section 9.1.7 of
[TS36133]_), which is an integer between 0 and 34, with 0 as the lowest RSRQ.

Strongest cell handover algorithm
---------------------------------

The *strongest cell handover algorithm*, or also sometimes known as the
*traditional power budget (PBGT) algorithm*, is developed using [Dimou2009]_ as
reference. The idea is to provide each UE with the best possible Reference
Signal Received Power (RSRP). This is done by performing a handover as soon as
a better cell (i.e. with stronger RSRP) is detected.

*Event A3* (neighbour cell's RSRP becomes better than serving cell's RSRP) is
chosen to realize this concept. The ``A3RsrpHandoverAlgorithm`` class is the
result of the implementation. Handover is triggered for the UE to the best cell
in the measurement report.

A simulation which uses this algorithm is usually more vulnerable to ping-pong
handover (consecutive handover to the previous source eNodeB within short period
of time), especially when the :ref:`sec-fading-model` is enabled. This problem
is typically tackled by introducing a certain delay to the handover. The
algorithm does this by including hysteresis and time-to-trigger parameters
(Section 6.3.5 of [TS36331]_) to the UE measurements configuration.

*Hysteresis* (a.k.a. handover margin) delays the handover in regard of RSRP. The
value is expressed in dB, ranges between 0 to 15 dB, and have a 0.5 dB accuracy,
e.g., an input value of 2.7 dB is rounded to 2.5 dB.

On the other hand, *time-to-trigger* delays the handover in regard of time. 3GPP
defines 16 valid values for time-to-trigger (all in milliseconds): 0, 40, 64,    
80, 100, 128, 160, 256, 320, 480, 512, 640, 1024, 1280, 2560, and 5120.

The difference between hysteresis and time-to-trigger is illustrated in Figure
:ref:`fig-lte-strongest-cell-handover-algorithm` below, which is taken from the
`lena-x2-handover-measures` example. It depicts the perceived RSRP of serving
cell and a neighbouring cell by a UE which moves pass the border of the cells.

.. _fig-lte-strongest-cell-handover-algorithm:

.. figure:: figures/lte-strongest-cell-handover-algorithm.*
   :align: center

   Effect of hysteresis and time-to-trigger in strongest cell handover algorithm

By default, the algorithm uses a hysteresis of 3.0 dB and time-to-trigger of
256 ms. These values can be tuned through the ``Hysteresis`` and
``TimeToTrigger`` attributes of the ``A3RsrpHandoverAlgorithm`` class.


Neighbour Relation
++++++++++++++++++

LTE module supports a simplified *Automatic Neighbour Relation* (ANR) function.
This is handled by the ``LteAnr`` class, which interacts with an eNodeB RRC
instance through the ANR SAP interface.

Neighbour Relation Table
------------------------

The ANR holds a *Neighbour Relation Table* (NRT), similar to the description in
Section 22.3.2a of [TS36300]_. Each entry in the table is called a *Neighbour
Relation* (NR) and represents a detected neighbouring cell, which contains the
following boolean fields:

 - `No Remove`
     Indicates that the NR shall *not* be removed from the NRT. This is `true`
     by default for user-provided NR and `false` otherwise.
     
 - `No X2`
     Indicates that the NR shall *not* use an X2 interface in order to initiate
     procedures towards the eNodeB parenting the target cell. This is `false` by
     default for user-provided NR, and `true` otherwise.

 - `No HO`
     Indicates that the NR shall *not* be used by the eNodeB for handover
     reasons. This is `true` in most cases, except when the NR is both
     user-provided and network-detected.

Each NR entry may have at least one of the following properties:

 - User-provided
     This type of NR is created as instructed by the simulation user. For
     example, a NR is created automatically upon a user-initiated establishment
     of X2 connection between 2 eNodeBs, e.g. as described in Section
     :ref:`sec-x2-based-handover`. Another way to create a user-provided NR is
     to call the ``AddNeighbourRelation`` function explicitly.

 - Network-detected
     This type of NR is automatically created during the simulation as a result
     of the discovery of a nearby cell.

In order to automatically create network-detected NR, ANR utilizes UE
measurements. In other words, ANR is a consumer of UE measurements, as depicted
in Figure :ref:`fig-ue-meas-consumer`. RSRQ and Event A4 (neighbour becomes
better than `threshold`) are used for the reporting configuration. The default
Event A4 `threshold` is set to the lowest possible, i.e., maximum detection
capability, but can be changed by setting the ``Threshold`` attribute of
``LteAnr`` class. Note that the A2-A4-RSRQ handover algorithm also utilizes a
similar reporting configuration. Despite the similarity, when both ANR and this
handover algorithm are active in the eNodeB, they use separate reporting
configuration.

Also note that automatic setup of X2 interface is not supported. This is the
reason why the `No X2` and `No HO` fields are true in a network-detected but not
user-detected NR.

Role of ANR in Simulation
-------------------------

The ANR SAP interface provides the means of communication between ANR and eNodeB
RRC. Some interface functions are used by eNodeB RRC to interact with the NRT,
as shown below:

 - ``AddNeighbourRelation``
     (eNodeB RRC -> ANR) Add a new user-provided NR entry into the NRT.

 - ``GetNoRemove``
     (eNodeB RRC -> ANR) Get the value of `No Remove` field of an NR entry of
     the given cell ID.
     
 - ``GetNoHo``
     (eNodeB RRC -> ANR) Get the value of `No HO` field of an NR entry of
     the given cell ID.

 - ``GetNoX2``
     (eNodeB RRC -> ANR) Get the value of `No X2` field of an NR entry of
     the given cell ID.

Other interface functions exist to support the role of ANR as a UE measurements
consumer, as listed below:

 - ``AddUeMeasReportConfigForAnr``
     (ANR -> eNodeB RRC) Used by the ANR to request measurement reports from the
     eNodeB RRC entity, by passing the desired reporting configuration. The
     configuration will be applied to all future attached UEs.
     
 - ``ReportUeMeas``
     (eNodeB RRC -> ANR) Based on the UE measurements configured earlier in
     ``AddUeMeasReportConfigForAnr``, UE may submit measurement reports to the
     eNodeB. The eNodeB RRC entity uses the ``ReportUeMeas`` interface to
     forward these measurement reports to the ANR.

Please refer to the corresponding API documentation for ``LteAnrSap`` class for
more details on the usage and the required parameters.

The ANR is utilized by the eNodeB RRC instance as a data structure to keep track
of the situation of nearby neighbouring cells. The ANR also helps the eNodeB RRC
instance to determine whether it is possible to execute a handover procedure to
a neighbouring cell. This is realized by the fact that eNodeB RRC will only
allow a handover procedure to happen if the NR entry of the target cell has both
`No HO` and `No X2` fields set to `false`. 

ANR is enabled by default in every eNodeB instance in the simulation. It can be
disabled by setting the ``AnrEnabled`` attribute in ``LteHelper`` class to
`false`.


RRC sequence diagrams
+++++++++++++++++++++

In this section we provide some sequence diagrams that explain the
most important RRC procedures being modeled.

.. _sec-rrc-connection-establishment:

RRC connection establishment
----------------------------

Figure :ref:`fig-rrc-connection-establishment` shows how the RRC
Connection Establishment procedure is modeled, highlighting the role
of the RRC layer at both the UE and the eNB, as well as the
interaction with the other layers. 

.. _fig-rrc-connection-establishment:

.. figure:: figures/rrc-connection-establishment.*
   :align: center

   Sequence diagram of the RRC Connection Establishment procedure



.. _sec-rrc-connection-reconfiguration:

RRC connection reconfiguration
------------------------------

Figure :ref:`fig-rrc-connection-reconfiguration` shows how the RRC
Connection Reconfiguration procedure is modeled for the case where
MobilityControlInfo is not provided, i.e., handover is not
performed. 


.. _fig-rrc-connection-reconfiguration:

.. figure:: figures/rrc-connection-reconfiguration.*
   :align: center

   Sequence diagram of the RRC Connection Reconfiguration procedure



Figure :ref:`fig-rrc-connection-reconf-handover` shows how the RRC
Connection Reconfiguration procedure is modeled for the case where
MobilityControlInfo is provided, i.e., handover is to be performed.
As specified in [TS36331]_, *After receiving the handover message,
the UE attempts to access the target cell at the first available RACH
occasion according to Random Access resource selection defined in [TS36321]_,
i.e. the handover is asynchronous. Consequently, when
allocating a dedicated preamble for the random access in the target
cell, E-UTRA shall ensure it is available from the first RACH occasion
the UE may use. Upon successful completion of the handover, the UE
sends a message used to confirm the handover.* Note that the random
access procedure in this case is non-contention based, hence in a real
LTE system it differs slightly from the one used in RRC connection
established. Also note that the RA Preamble ID is signalled via the
Handover Command included in the X2 Handover Request ACK message sent
from the target eNB to the source eNB; in particular, the preamble is
included in the RACH-ConfigDedicated IE which is part of
MobilityControlInfo. 


.. _fig-rrc-connection-reconf-handover:
   
.. figure:: figures/rrc-connection-reconfiguration-handover.*
   :align: center

   Sequence diagram of the RRC Connection Reconfiguration procedure
   for the handover case


.. _sec-rrc-protocol-models:

RRC protocol models
+++++++++++++++++++

As previously anticipated, we provide two different models  for the
transmission and reception of RRC messages: *Ideal*
and *Real*. Each of them is described in one of the following
subsections.

Ideal RRC protocol model
------------------------

According to this model, implemented in the classes and `LteUeRrcProtocolIdeal` and
`LteEnbRrcProtocolIdeal`, all RRC messages and information elements
are transmitted between the eNB and the UE in an ideal fashion,
without consuming radio resources and without errors. From an
implementation point of view, this is achieved by passing the RRC data
structure directly between the UE and eNB RRC entities, without
involving the lower layers (PDCP, RLC, MAC, scheduler).


Real RRC protocol model
-----------------------

This model is implemented in the classes `LteUeRrcProtocolReal` and
`LteEnbRrcProtocolReal` and aims at modeling the transmission of RRC
PDUs as commonly performed in real LTE systems. In particular:

 - for every RRC message being sent, a real RRC PDUs is created
   following the ASN.1 encoding of RRC PDUs and information elements (IEs)
   specified in [TS36331]_. Some simplification are made with respect
   to the IEs included in the PDU, i.e., only those IEs that are
   useful for simulation purposes are included. For a detailed list,
   please see the IEs defined in `lte-rrc-sap.h` and compare with
   [TS36331]_. 
 - the encoded RRC PDUs are sent on Signaling Radio Bearers and are
   subject to the same transmission modeling used for data
   communications, thus including scheduling, radio resource
   consumption, channel errors, delays, retransmissions, etc.


Signaling Radio Bearer model
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We now describe the Signaling Radio Bearer model that is used for the
*Real* RRC protocol model.  

 * **SRB0** messages (over CCCH):

   - **RrcConnectionRequest**: in real LTE systems, this is an RLC TM
     SDU sent over resources specified in the UL Grant in the RAR (not
     in UL DCIs); the reason is that C-RNTI is not known yet at this
     stage. In the simulator, this is modeled as a real RLC TM RLC PDU
     whose UL resources are allocated by the sched upon call to
     SCHED_DL_RACH_INFO_REQ. 

   - **RrcConnectionSetup**: in the simulator this is implemented as in
     real LTE systems, i.e., with an RLC TM SDU sent over resources
     indicated by a regular UL DCI, allocated with
     SCHED_DL_RLC_BUFFER_REQ triggered by the RLC TM instance that is
     mapped to LCID 0 (the CCCH).

 * **SRB1** messages (over DCCH):

   - All the SRB1 messages modeled in the simulator (e.g.,
     **RrcConnectionCompleted**) are implemented as in real LTE systems,
     i.e., with a real RLC SDU sent over RLC AM using DL resources
     allocated via Buffer Status Reports. See the RLC model
     documentation for details.

 * **SRB2** messages (over DCCH):

     - According to [TS36331]_, "*SRB1 is for RRC messages (which may
       include a piggybacked NAS message) as well as for NAS messages
       prior to the establishment of SRB2, all using DCCH logical
       channel*", whereas "*SRB2 is for NAS messages, using DCCH
       logical channel*" and "*SRB2 has a lower-priority than SRB1 and is 
       always configured by E-UTRAN after security
       activation*". Modeling security-related aspects is not a
       requirement of the LTE simulation model, hence we always use
       SRB1 and never activate SRB2.




ASN.1 encoding of RRC IE's
^^^^^^^^^^^^^^^^^^^^^^^^^^


The messages defined in RRC SAP, common to all Ue/Enb SAP Users/Providers, are transported in a transparent container to/from a Ue/Enb. The encoding format for the different Information Elements are specified in [TS36331]_, using ASN.1 rules in the unaligned variant. The implementation in Ns3/Lte has been divided in the following classes:

  * Asn1Header : Contains the encoding / decoding of basic ASN types

  * RrcAsn1Header : Inherits Asn1Header and contains the encoding / decoding of common IE's defined in [TS36331]_
  
  * Rrc specific messages/IEs classes : A class for each of the messages defined in RRC SAP header


Asn1Header class - Implementation of base ASN.1 types
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This class implements the methods to Serialize / Deserialize the ASN.1 types being used in [TS36331]_, according to the packed encoding rules in ITU-T X.691. The types considered are:

  * Boolean : a boolean value uses a single bit (1=true, 0=false).
  
  * Integer : a constrained integer (with min and max values defined) uses the minimum amount of bits to encode its range (max-min+1).
  
  * Bitstring : a bistring will be copied bit by bit to the serialization buffer.
  
  * Octetstring : not being currently used.
  
  * Sequence : the sequence generates a preamble indicating the presence of optional and default fields. It also adds a bit indicating the presence of extension marker.
  
  * Sequence...Of : the sequence...of type encodes the number of elements of the sequence as an integer (the subsequent elements will need to be encoded afterwards).
  
  * Choice : indicates which element among the ones in the choice set is being encoded.
  
  * Enumeration : is serialized as an integer indicating which value is used, among the ones in the enumeration, with the number of elements in the enumeration as upper bound.
  
  * Null : the null value is not encoded, although its serialization function is defined to provide a clearer map between specification and implementation.

The class inherits from ns-3 Header, but Deserialize() function is declared pure virtual, thus inherited classes having to implement it. The reason is that deserialization will retrieve the elements in RRC messages, each of them containing different information elements.

Additionally, it has to be noted that the resulting byte length of a specific type/message can vary, according to the presence of optional fields, and due to the optimized encoding. Hence, the serialized bits will be processed using PreSerialize() function, saving the result in m_serializationResult Buffer. As the methods to read/write in a ns3 buffer are defined in a byte basis, the serialization bits are stored into m_serializationPendingBits attribute, until the 8 bits are set and can be written to buffer iterator. Finally, when invoking Serialize(), the contents of the m_serializationResult attribute will be copied to Buffer::Iterator parameter

RrcAsn1Header : Common IEs
^^^^^^^^^^^^^^^^^^^^^^^^^^

As some Information Elements are being used for several RRC messages, this class implements the following common IE's:

  * SrbToAddModList
  
  * DrbToAddModList
  
  * LogicalChannelConfig
  
  * RadioResourceConfigDedicated
  
  * PhysicalConfigDedicated
  
  * SystemInformationBlockType1
  
  * SystemInformationBlockType2
  
  * RadioResourceConfigCommonSIB


Rrc specific messages/IEs classes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The following RRC SAP have been implemented:

  * RrcConnectionRequest
  
  * RrcConnectionSetup
  
  * RrcConnectionSetupCompleted
  
  * RrcConnectionReconfiguration
  
  * RrcConnectionReconfigurationCompleted
  
  * HandoverPreparationInfo
  
  * RrcConnectionReestablishmentRequest
  
  * RrcConnectionReestablishment
  
  * RrcConnectionReestablishmentComplete
  
  * RrcConnectionReestablishmentReject
  
  * RrcConnectionRelease




.. only:: latex

    .. raw:: latex
      
        \clearpage



---
NAS
---


The focus of the LTE-EPC model is on the NAS Active state, which corresponds to EMM Registered, ECM connected, and RRC connected. Because of this, the following simplifications are made:

 - EMM and ECM are not modeled explicitly; instead, the NAS entity at the UE will interact directy with the MME to perfom actions that are equivalent (with gross simplifications) to taking the UE to the states EMM Connected and ECM Connected; 

 - the NAS also takes care of multiplexing uplink data packets coming from the upper layers into the appropriate EPS bearer by using the Traffic Flow Template classifier (TftClassifier). 

- the NAS does not support PLMN and CSG selection 

- the NAS does not support any location update/paging procedure in idle mode



Figure :ref:`fig-nas-attach` shows how the simplified NAS model
implements the attach procedure. Note that both the default and
eventual dedicated EPS bearers are activated as part of this
procedure. 

.. _fig-nas-attach:
   
.. figure:: figures/nas-attach.*
   :align: center

   Sequence diagram of the attach procedure







.. only:: latex

    .. raw:: latex
      
        \clearpage



-----------------
S1
-----------------

S1-U 
+++++++++

The S1-U interface is modeled in a realistic way by encapsulating
data packets over GTP/UDP/IP, as done in real LTE-EPC systems. The
corresponding protocol stack is shown in Figure
:ref:`fig-lte-epc-e2e-data-protocol-stack`. As shown in the figure,
there are two different layers of 
IP networking. The first one is the end-to-end layer, which provides end-to-end 
connectivity to the users; this layers involves the UEs, the PGW and
the remote host (including eventual internet routers and hosts in
between), but does not involve the eNB. By default, UEs are assigned a public IPv4 address in the 7.0.0.0/8
network, and the PGW gets the address 7.0.0.1, which is used by all
UEs as the gateway to reach the internet. 

The second layer of IP networking is the EPC local area network. This
involves all eNB nodes and the SGW/PGW node. This network is
implemented as a set of point-to-point links which connect each eNB
with the SGW/PGW node; thus, the SGW/PGW has a set of point-to-point
devices, each providing connectivity to a different eNB. By default, a
10.x.y.z/30 subnet is assigned to each point-to-point link (a /30
subnet is the smallest subnet that allows for two distinct host
addresses). 

As specified by 3GPP, the end-to-end IP
communications is tunneled over the local EPC IP network using
GTP/UDP/IP. In the following, we explain how this tunneling is
implemented in the EPC model. The explanation is done by discussing the
end-to-end flow of data packets.  

.. _fig-epc-data-flow-dl:
   
.. figure:: figures/epc-data-flow-dl.*
   :align: center

   Data flow in the downlink between the internet and the UE

To begin with, we consider the case of the downlink, which is depicted
in Figure :ref:`fig-epc-data-flow-dl`.   
Downlink Ipv4 packets are generated from a generic remote host, and
addressed to one of the UE device. Internet routing will take care of
forwarding the packet to the generic NetDevice of the SGW/PGW node
which is connected to the internet (this is the Gi interface according
to 3GPP terminology). The SGW/PGW has a VirtualNetDevice which is
assigned the gateway IP address of the UE subnet; hence, static
routing rules will cause the incoming packet from the internet to be
routed through this VirtualNetDevice. Such device starts the
GTP/UDP/IP tunneling procedure, by forwarding the packet to a
dedicated application in the SGW/PGW  node which is called
EpcSgwPgwApplication. This application does the following operations:

 #. it determines the eNB node to which the UE is attached, by looking
    at the IP destination address (which is the address of the UE);
 #. it classifies the packet using Traffic Flow Templates (TFTs) to
    identify to which EPS Bearer it belongs. EPS bearers have a
    one-to-one mapping to S1-U Bearers, so this operation returns the
    GTP-U Tunnel Endpoint Identifier  (TEID) to which the packet
    belongs;
 #. it adds the corresponding GTP-U protocol header to the packet;
 #. finally, it sends the packet over an UDP socket to the S1-U
    point-to-point NetDevice, addressed to the eNB to which the UE is
    attached.

As a consequence, the end-to-end IP packet with newly added IP, UDP
and GTP headers is sent through one of the S1 links to the eNB, where
it is received and delivered locally (as the destination address of
the outmost IP header matches the eNB IP address). The local delivery
process will forward the packet, via an UDP socket, to a dedicated
application called EpcEnbApplication. This application then performs
the following operations:

 #. it removes the GTP header and retrieves the TEID which is
    contained in it;
 #. leveraging on the one-to-one mapping between S1-U bearers and
    Radio Bearers (which is a 3GPP requirement), it determines the 
    Bearer ID (BID) to which the packet belongs;
 #. it records the BID in a dedicated tag called EpsBearerTag,
    which is added to the packet; 
 #. it forwards the packet to the LteEnbNetDevice of the eNB node via
    a raw packet socket

Note that, at this point, the outmost header of the packet is the
end-to-end IP header, since the IP/UDP/GTP headers of the S1 protocol
stack have already been stripped. Upon reception of
the packet from the EpcEnbApplication, the LteEnbNetDevice will
retrieve the BID from the EpsBearerTag, and based on the BID
will determine the Radio Bearer instance (and the corresponding PDCP
and RLC protocol instances) which are then used to forward the packet
to the UE over the LTE radio interface. Finally, the LteUeNetDevice of
the UE will receive the packet, and delivery it locally to the IP
protocol stack, which will in turn delivery it to the application of
the UE, which is the end point of the downlink communication.



.. _fig-epc-data-flow-ul:
   
.. figure:: figures/epc-data-flow-ul.*
   :align: center

   Data flow in the uplink between the UE and the internet


The case of the uplink is depicted in Figure :ref:`fig-epc-data-flow-ul`.
Uplink IP packets are generated by a generic application inside the UE,
and forwarded by the local TCP/IP stack to the LteUeNetDevice of the
UE. The LteUeNetDevice then performs the following operations:

 #. it classifies the packet using TFTs and determines the
    Radio Bearer to which the packet belongs (and the corresponding
    RBID);
 #. it identifies the corresponding PDCP protocol instance, which is
    the entry point of the LTE Radio Protocol stack for this packet;
 #. it sends the packet to the eNB over the LTE Radio Protocol stack.

The eNB receives the packet via its LteEnbNetDevice. Since there is a
single PDCP and RLC protocol instance for each Radio Bearer, the
LteEnbNetDevice is able to determine the BID of the packet. This BID
is then recorded onto an EpsBearerTag, which is added to the
packet. The LteEnbNetDevice then forwards the packet to the
EpcEnbApplication via a raw packet socket.

Upon receiving the packet, the EpcEnbApplication performs the
following operations:

 #. it retrieves the BID from the EpsBearerTag in the packet;
 #. it determines the corresponding EPS Bearer instance and GTP-U TEID by
    leveraging on the one-to-one mapping between S1-U bearers and Radio
    Bearers;
 #. it adds a GTP-U header on the packet, including the TEID
    determined previously;
 #. it sends the packet to the SGW/PGW node via the UDP socket
    connected to the S1-U point-to-point net device.

At this point, the packet contains the S1-U IP, UDP and GTP headers in
addition to the original end-to-end IP header. When the packet is
received by the corresponding S1-U point-to-point NetDevice of the
SGW/PGW node, it is delivered locally (as the destination address of
the outmost IP header matches the address of the point-to-point net
device). The local delivery process will forward the packet to the
EpcSgwPgwApplication via the correponding UDP socket. The
EpcSgwPgwApplication then removes the GTP header and forwards the
packet to the VirtualNetDevice. At this point, the outmost header
of the packet is the end-to-end IP header. Hence, if the destination
address within this header is a remote host on the internet, the
packet is sent to the internet via the corresponding NetDevice of the
SGW/PGW. In the event that the packet is addressed to another UE, the
IP stack of the SGW/PGW will redirect the packet again to the
VirtualNetDevice, and the packet will go through the dowlink delivery
process in order to reach its destination UE.

Note that the EPS Bearer QoS is not enforced on the S1-U
links, it is assumed that the overprovisioning of the link bandwidth
is sufficient to meet the QoS requirements of all bearers.


S1AP
+++++

The S1-AP interface provides control plane interaction between the eNB
and the MME. In the simulator, this interface is modeled in an ideal
fashion, with direct interaction between the eNB and the MME objects,
without actually implementing the encoding of S1AP messages and
information elements specified in [TS36413]_ and without actually
transmitting any PDU on any link. 

The S1-AP primitives that are modeled are:

 * INITIAL UE MESSAGE
 * INITIAL CONTEXT SETUP REQUEST
 * INITIAL CONTEXT SETUP RESPONSE 
 * PATH SWITCH REQUEST
 * PATH SWITCH REQUEST ACKNOWLEDGE 

.. only:: latex

    .. raw:: latex

        \clearpage



.. _sec-x2:

---
X2 
---

The X2 interface interconnects two eNBs [TS36420]_. From a logical
point of view, the X2 interface is a point-to-point interface between
the two eNBs. In a real E-UTRAN, the logical point-to-point interface
should be feasible even in the absence of a physical direct connection
between the two eNBs. In the X2 model implemented in the simulator,
the X2 interface is a point-to-point link between the two eNBs. A
point-to-point device is created in both eNBs and the two
point-to-point devices are attached to the point-to-point link. 

For a representation of how the X2 interface fits in the overall
architecture of the LENA simulation model, the reader is referred to
the figure :ref:`fig-epc-topology`.

The X2 interface implemented in the simulator provides detailed implementation of the following elementary procedures of the Mobility Management functionality [TS36423]_:

  * Handover Request procedure

  * Handover Request Acknowledgement procedure

  * SN Status Transfer procedure

  * UE Context Release procedure

These procedures are involved in the X2-based handover. You can find
the detailed description of the handover in section 10.1.2.1 of
[TS36300]_. We note that the simulator model currently supports only
the *seamless handover* as defined in Section 2.6.3.1 of [Sesia2009]_;
in particular, *lossless handover* as described in Section 2.6.3.2 of
[Sesia2009]_ is not supported at the time of this writing.

Figure :ref:`fig-x2-based-handover-seq-diagram` below shows the interaction of
the entities of the X2 model in the simulator. The shaded labels indicate the
moments when the UE or eNodeB transition to another RRC state.

.. _fig-x2-based-handover-seq-diagram:

.. figure:: figures/lte-epc-x2-handover-seq-diagram.*
   :scale: 80 %
   :align: center

   Sequence diagram of the X2-based handover

The figure also shows two timers within the handover procedure: the *handover
leaving timer* is maintained by the source eNodeB, while the *handover joining
timer* by the target eNodeB. The duration of the timers can be configured in
the ``HandoverLeavingTimeoutDuration`` and ``HandoverJoiningTimeoutDuration``
attributes of the respective ``LteEnbRrc`` instances. When one of these timers
expire, the handover procedure is considered as failed.

However, there is no proper handling of handover failure in the current version
of LTE module. Users should tune the simulation properly in order to avoid
handover failure, otherwise unexpected behaviour may occur. Please refer to
Section :ref:`sec-tuning-handover-simulation` of the User Documentation for some
tips regarding this matter.

The X2 model is an entity that uses services from:

  * the X2 interfaces,
      
    * They are implemented as Sockets on top of the point-to-point devices.

    * They are used to send/receive X2 messages through the X2-C and X2-U interfaces (i.e. the point-to-point device attached to the point-to-point link) towards the peer eNB.

  * the S1 application.

    * Currently, it is the EpcEnbApplication.

    * It is used to get some information needed for the Elementary Procedures of the X2 messages.

and it provides services to:

  * the RRC entity (X2 SAP)

    * to send/receive RRC messages. The X2 entity sends the RRC message as a transparent container in the X2 message. This RRC message is sent to the UE. 

Figure :ref:`fig-x2-entity-saps` shows the implentation model of the X2 entity and its relationship with all the other entities and services in the protocol stack.

.. _fig-x2-entity-saps:

.. figure:: figures/lte-epc-x2-entity-saps.*
    :width: 700px
    :align: center

    Implementation Model of X2 entity and SAPs

The RRC entity manages the initiation of the handover procedure. This is done in the Handover Management submodule of the eNB RRC entity. The target eNB may perform some Admission Control procedures. This is done in the Admission Control submodule. Initially, this submodule will accept any handover request.

X2 interfaces
+++++++++++++

The X2 model contains two interfaces:

  * the X2-C interface. It is the control interface and it is used to send the X2-AP PDUs
    (i.e. the elementary procedures).

  * the X2-U interface. It is used to send the bearer data when there is `DL forwarding`.

Figure :ref:`fig-lte-epc-x2-interface` shows the protocol stacks of the X2-U interface and X2-C interface modeled in the simulator.

.. _fig-lte-epc-x2-interface:

.. figure:: figures/lte-epc-x2-interface.*          
    :align: center

    X2 interface protocol stacks

X2-C
----

The X2-C interface is the control part of the X2 interface and it is
used to send the X2-AP PDUs (i.e. the elementary procedures). 

In the original X2 interface control plane protocol stack, SCTP is
used as the transport protocol but currently, the SCTP protocol is not
modeled in the ns-3 simulator and its implementation is out-of-scope
of the project. The UDP protocol is used as the datagram oriented
protocol instead of the SCTP protocol.  


X2-U
----

The X2-U interface is used to send the bearer data when there is `DL
forwarding` during the execution of the X2-based handover
procedure. Similarly to what done for the S1-U interface, data packets
are encapsulated over GTP/UDP/IP when being sent over this
interface. Note that the EPS Bearer QoS is not enforced on the X2-U
links, it is assumed that the overprovisioning of the link bandwidth
is sufficient to meet the QoS requirements of all bearers.



X2 Service Interface
++++++++++++++++++++

The X2 service interface is used by the RRC entity to send and receive messages of the X2 procedures. It is divided into two parts:

  * the ``EpcX2SapProvider`` part is provided by the X2 entity and used by the RRC entity and

  * the ``EpcX2SapUser`` part is provided by the RRC entity and used by the RRC enity.

The primitives that are supported in our X2-C model are described in the following subsections.

X2-C primitives for handover execution
--------------------------------------

The following primitives are used for the X2-based
handover:

 - HANDOVER REQUEST
 - HANDOVER REQUEST ACK
 - HANDOVER PREPARATION FAILURE
 - SN STATUS STRANSFER
 - UE CONTEXT RELEASE

all the above primitives are used by the currently implemented RRC
model during the preparation and execution of the handover
procedure. Their usage interacts with the RRC state machine;
therefore, they are not meant to be used for code customization, at
least unless it is desired to modify the RRC state machine.


X2-C SON primitives
-------------------

The following primitives can be used  to implement Self-Organized Network (SON) functionalities:

 - LOAD INFORMATION
 - RESOURCE STATUS UPDATE

note that the current RRC model does not actually use these
primitives, they are included in the model just to make it possible to
develop SON algorithms included in the RRC logic that make use of
them. 

As a first example, we show here how the load information primitive
can be used. We assume that the LteEnbRrc has been modified to include
the following new member variables::

  std::vector<EpcX2Sap::UlInterferenceOverloadIndicationItem> 
    m_currentUlInterferenceOverloadIndicationList;
  std::vector <EpcX2Sap::UlHighInterferenceInformationItem> 
    m_currentUlHighInterferenceInformationList;
  EpcX2Sap::RelativeNarrowbandTxBand m_currentRelativeNarrowbandTxBand;


for a detailed description of the type of these variables, we suggest
to consult the file ``epc-x2-sap.h``, the corresponding doxygen
documentation, and the references therein to the relevant sections of
3GPP TS 36.423. Now, assume that at run time these variables have been
set to meaningful values following the specifications just
mentioned. Then, you can add the following code in the LteEnbRrc class
implementation in order to send a load information primitive::

  EpcX2Sap::CellInformationItem cii;
  cii.sourceCellId = m_cellId;
  cii.ulInterferenceOverloadIndicationList = m_currentUlInterferenceOverloadIndicationList;
  cii.ulHighInterferenceInformationList = m_currentUlHighInterferenceInformationList;
  cii.relativeNarrowbandTxBand = m_currentRelativeNarrowbandTxBand;

  EpcX2Sap::LoadInformationParams params;
  params.targetCellId = cellId;
  params.cellInformationList.push_back (cii);
  m_x2SapProvider->SendLoadInformation (params);


The above code allows the source eNB to send the message. The method
``LteEnbRrc::DoRecvLoadInformation`` will be called when the target
eNB receives the message. The desired processing of the load
information should therefore be implemented within that method.

In the following second example we show how the resource
status update primitive is used. We assume that the LteEnbRrc has been
modified to include the following new member variable::

  EpcX2Sap::CellMeasurementResultItem m_cmri;


similarly to before, we refer to ``epc-x2-sap.h`` and the references
therein for detailed information about this variable type.
Again, we assume that the variable has been already set to a
meaningful value. Then, you can add the following code in order to
send a resource status update:: 

  EpcX2Sap::ResourceStatusUpdateParams params;
  params.targetCellId = cellId;
  params.cellMeasurementResultList.push_back (m_cmri);
  m_x2SapProvider->SendResourceStatusUpdate (params);


The method ``eEnbRrc::DoRecvResourceStatusUpdate`` will be called when
the target eNB receives the resource status update message. The
desired processing of this message should therefore be implemented
within that method. 

Finally, we note that the setting and processing of the appropriate
values for the variable passed to the above described primitives is
deemed to be specific of the SON algorithm being implemented, and
hence is not covered by this documentation. 



Unsupported primitives
----------------------

Mobility Robustness Optimization primitives such as Radio Link Failure
indication and Handover Report are not supported at this stage.




.. only:: latex

    .. raw:: latex

        \clearpage




---
S11
---

The S11 interface provides control plane interaction between the SGW
and the MME using the GTPv2-C protocol specified in [TS29274]_. In the
simulator, this interface is modeled in an ideal 
fashion, with direct interaction between the SGW and the MME objects,
without actually implementing the encoding of the messages and without actually
transmitting any PDU on any link. 

The S11 primitives that are modeled are:

 * CREATE SESSION REQUEST
 * CREATE SESSION RESPONSE
 * MODIFY BEARER REQUEST
 * MODIFY BEARER RESPONSE

Of these primitives, the first two are used upon initial UE attachment
for the establishment of the S1-U bearers; the other two are used
during handover to switch the S1-U bearers from the source eNB to the
target eNB as a consequence of the reception by the MME of a PATH
SWITCH REQUEST S1-AP message.




.. only:: latex

    .. raw:: latex

        \clearpage


-------
Helpers
-------

Two helper objects are use to setup simulations and configure the
various components. These objects are:


 * ``LteHelper``, which takes care of the configuration of the LTE radio
   access network, as well as of coordinating the setup and release of
   EPS bearers. The ``LteHelper`` class provides both the API
   definition and its implementation.  
 * ``EpcHelper``, which takes care of the configuration of the Evolved
   Packet Core. The ``EpcHelper`` class is an abstract base class
   which only provides the API definition; the implementation is delegated
   to child classes in order to allow for different EPC network
   models.

It is possible to create a simple LTE-only simulations by
using ``LteHelper`` alone, or to create complete LTE-EPC simulations by
using both ``LteHelper`` and ``EpcHelper``. When both helpers are used, they
interact in a master-slave fashion, with ``LteHelper`` being the Master
that interacts directly with the user program, and ``EpcHelper`` working
"under the hood" to configure the EPC upon explicit methods called by
``LteHelper``. The exact interactions are displayed in the Figure :ref:`fig-helpers`.

.. _fig-helpers:
   
.. figure:: figures/helpers.*
   :align: center

   Sequence diagram of the interaction between LteHelper and EpcHelper

