21.8 Physical model

The physical layer performs two main operations: (i) It receives a burst from a channel and forwards it to the MAC layer, (ii) it receives a burst from the MAC layer and transmits it on the channel. In order to reduce the simulation complexity of the WiMAX physical layer, we have chosen to model offline part of the physical layer. More specifically we have developed an OFDM simulator to generate trace files used by the reception process to evaluate if a FEC block can be correctly decoded or not.

Transmission Process: A burst is a set of WiMAX MAC PDUs. At the sending process, a burst is converted into bit-streams and then splitted into smaller FEC blocks which are then sent to the channel with a power equal P_tx.

Reception Process: The reception process includes the following operations:

• 1- Receive a FEC block from the channel. 2- Calculate the noise level. 3- Estimate the signal to noise ratio (SNR) with the following formula. 4- Determine if a FEC block can be correctly decoded. 5- Concatenate received FEC blocks to reconstruct the original burst. 6- Forward the burst to the upper layer.

The developed process to evaluate if a FEC block can be correctly received or not uses pre-generated traces. The trace files are generated by an external OFDM simulator (described later). A class named SNRToBlockErrorRateManager handles a repository containing seven trace files (one for each modulation and coding scheme). A repository is specific for a particular channel model.

A trace file is made of 6 columns. The first column provides the SNR value (1), whereas the other columns give respectively the bit error rate BER (2), the block error rate BlcER(3), the standard deviation on BlcER, and the confidence interval (4 and 5). These trace files are loaded into memory by the SNRToBlockErrorRateManager entity at the beginning of the simulation.

Currently, The first process uses the first and third columns to determine if a FEC block is correctly received. When the physical layer receives a packet with an SNR equal to SNR_rx, it asks the SNRToBlockErrorRateManager to return the corresponding block error rate BlcER. A random number RAND between 0 and 1 is then generated. If RAND is greater than BlcER, then the block is correctly received, otherwise the block is considered erroneous and is ignored.

The module provides defaults SNR to block error rate traces in default-traces.h. The traces have been generated by an External WiMAX OFDM simulator. The simulator is based on an external mathematics and signal processing library IT++ and includes : a random block generator, a Reed Solomon (RS) coder, a convolutional coder, an interleaver, a 256 FFT-based OFDM modulator, a multi-path channel simulator and an equalizer. The multipath channel is simulated using the TDL_channel class of the IT++ library.

Users can configure the module to use their own traces generated by another OFDM simulator or ideally by performing experiments in real environment. for this purpose, a path to a repository containing trace files should be provided. If no repository is provided the traces form default-traces.h will be loaded. A valid repository should contain 7 files, one for each modulation and coding scheme.

The names of the files should respect the following format: modulation0.txt for modulation 0, modulation1.txt for modulation 1 and so on... The file format should be as follows

SNR_value1   BER  Blc_ER  STANDARD_DEVIATION  CONFIDENCE_INTERVAL1  CONFIDENCE_INTERVAL2
SNR_value2   BER  Blc_ER  STANDARD_DEVIATION  CONFIDENCE_INTERVAL1  CONFIDENCE_INTERVAL2
 ...          ...  ...     ...                 ...                   ...
 ...          ...  ...     ...                 ...                   ...

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