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Update fading documentation

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Marco Miozzo
2011-10-27 12:24:54 +02:00
parent bfdd5f7c38
commit d38828affc
2 changed files with 15 additions and 12 deletions
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25
src/lte/doc/source/lte-fading-design.rst
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@@ -6,13 +6,13 @@
++++++++++++++++++++++++++++++++++++++
Jakes Fading Trace Model
++++++++++++++++++++++++
Trace Fading Model
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Fading Traces Model Description
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The fading model is based on 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 for limiting 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:
The fading model is based on 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 for limiting 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.
@@ -26,9 +26,9 @@ Respect to the mathematical channel propagation model, we suggest the one provid
http://www.mathworks.es/help/toolbox/comm/ug/a1069449399.html#bq5zk36
The simulator provides a matlab script (``/lte/model/JakesTraces/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 to filter 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. For sake of clarity, the names of the trace and of the file created can be completed with user defined tag for distinguish them. The file produced is a standard c header file with the definition of the vector containing the trace. This file has to be included in the ``JakesFadingLossModel`` class implementation and the code has to be updated for managing the generated array with the fading trace according to the parameters combination defined by the user at the generation.
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 TS 36.104 Annex B.2):
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 [TS36.104]_):
* users' speed: consider the most common typical value
@@ -48,7 +48,7 @@ According to the parameters we considered, the following formula express in deta
T_{SIZE} = Sample_{size} \times RB_{NUM} \times \frac{T_{total}}{T_{sample}} \times Speed_{NUM} \times Scenarios_{NUM} \mbox{ [bytes]}
where :math:`Sample_{size}` is the size in bytes of the sample (e.g., 8 in case of double precision, 4 in case of float precision), :math:`RB_{NUM}` is the number of RB or set of RBs to be considered, :math:`T_{total}` is the total length of the trace, :math:`T_{sample}` is the sampling period (1 ms for having a new sample each subframe), :math:`Speed_{NUM}` is the number of users relative speeds considered and :math:`Scenarios_{NUM}` is the number of scenarios (i.e., different taps configurations).
According to the formula we have that a typical single channel realization (i.e., independent RB traces at a given speed and given set of number of taps with one sample per ms/TTI)) implies the usage of 8,000,000 bytes (7.6294 MB) considering the precision of double (:math:`1\times10^{-308}` to :math:`1\times10^{308}`). This size can be halved by considering float numbers instead of double, reducing the precision to :math:`1\times10^{-37}` to :math:`1\times10^{37}` (and possibly increasing the computational complexity due to the cast operation between float and double, which is what is used in the simulator; this is compilator dependent and can be evaluated after implementation ``TBD``).
According to the formula we have that a typical single channel realization (i.e., independent RB traces at a given speed and given set of number of taps with one sample per ms/TTI)) implies the usage of 8,000,000 bytes (7.6294 MB) considering the precision of double (:math:`1\times10^{-308}` to :math:`1\times10^{308}`).
A typical set of traces for a simulation will result therefore in:
@@ -63,7 +63,7 @@ which results in 8,000,000 bytes (~8 MB) of traces with double precision.
Fading Traces Generation
~~~~~~~~~~~~~~~~~~~~~~~~
Thanks to the matlab script provided with the code (``/lte/model/JakesTraces/fading-trace-generator.m``) it is possible to generate traces according to specific simulation scenarios. The script already includes the typical taps configurations for three 3GPP scenarios (i.e., pedestrian, vehicular and urban); however users can introduce their specific configurations. The list of the configurable parameters is provided in the following:
Thanks to the matlab script provided with the code (``/lte/model/JakesTraces/fading-trace-generator.m``) it is possible to generate traces according to specific simulation scenarios. The script already includes the typical taps configurations for three 3GPP scenarios (i.e., pedestrian, vehicular and urban as defined in Annex B.2 of [TS36.104]_); however users can introduce their specific configurations. The list of the configurable parameters is provided in the following:
* ``fc`` : the frequency in use (it affects the computation of the dopples speed).
* ``v_km_h`` : the speed of the users
@@ -72,11 +72,12 @@ Thanks to the matlab script provided with the code (``/lte/model/JakesTraces/fad
* ``tag`` : the tag to be applied to the file generated.
The file generated is formatted in a matrix fashion by putting each RBs temporal fading trace samples in different rows.
It has to be noted that, the model is able to manage external fading trace (e.g., generated with dedicated simulators or obtained in experimental way) which respect the format used by the model (i.e., ASCII file with temporal fading trace of each RB distributed in rows).
Fading Traces Usage
~~~~~~~~~~~~~~~~~~~
The proper set of the trace parameters in the simulation is of paramount importance for the correct usage of the trace itself within the simulator.
The proper set of the trace parameters in the simulation is of paramount importance for the correct interpretation of the trace itself within the simulator.
The list of the parameters to be configured are:
* ``TraceFilename`` : the name of the trace to be loaded (absolute path o relative one according to the execution point of the script).
@@ -84,9 +85,10 @@ The list of the parameters to be configured are:
* ``SamplesNum`` : the number of samples.
* ``WindowSize`` : the size of the fading sampling window in seconds.
It is important to highlight that the sampling interval of the fading trace has to me at most of 1 ms or greater and in the latter case it has to be an integer multiple of 1 ms in order to be correctly processed by the fading module.
It is important to highlight that the sampling interval of the fading trace has to me at most of 1 ms or greater and in the latter case it has to be an integer multiple of 1 ms in order to be correctly processed by the fading module.
The default configuration of the matlab script provides a trace 10 seconds long, made of 10,000 samples (i.e., 1 sample per TTI ~ 1ms) and used with a windows size of 0.5 seconds amplitude. These are also the default values of the parameters above used in the simulator; therefore their settage can be avoided in case the fading trace respects them.
In order to activate the fading module (by default disactivated) it has to be explicity specified in the simulation script::
In order to activate the fading module (by default not active) it has to be explicitly specified in the simulation script::
Ptr<LenaHelper> lena = CreateObject<LenaHelper> ();
lena->SetFadingModel("ns3::TraceFadingLossModel");
@@ -99,7 +101,6 @@ While, for setting the parameters::
lena->SetFadingModelAttribute ("WindowSize", TimeValue (Seconds (0.5)));
lena->SetFadingModelAttribute ("RbNum", UintegerValue (100));
Apart of ``TraceFilename``, the parameters have already a default value according to the standard format generated (i.e., the same values used in the example above).
It has to be noted that, ``TraceFilename`` does not have a default value, therefore is has to be always set for using the fading module in the simulator. The simulator provide natively a fading trace generated according to the pedestrian behavior defined in in Annex B.2 of [TS36.104]_ (available at ``src/lte/model/FadingTraces/fading_trace_EPA_3kmph.fad``).

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src/lte/doc/source/lte-references.rst
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@@ -31,6 +31,8 @@
.. [TS25.814] 3GPP TS 25.814 "Physical layer aspect for evolved Universal Terrestrial Radio Access"
.. [TS36.104] 3GPP TS 36.104 "E-UTRA Base Station (BS) radio transmission and reception"
.. [R1-081483] 3GPP R1-081483 "Conveying MCS and TB size via PDCCH" http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1_52b/Docs/R1-081483.zip