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HDL OFDM MATLAB References

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This example shows how to model OFDM transmitter, additive white Gaussian noise (AWGN), and OFDM receiver hardware algorithms in MATLAB as a step towards developing a Simulink® HDL implementation. The HDL OFDM MATLAB® References example bridges the gap between a mathematical algorithm and its hardware implementation. This example provides MATLAB references of the HDL OFDM Transmitter, HDL AWGN, and HDL OFDM Receiver algorithms that you can implement on hardware. You can use these MATLAB references to generate test vectors for verifying the HDL implementation of these Simulink models, HDL OFDM Transmitter, HDL Implementation of AWGN Generator and HDL OFDM Receiver.

HDL OFDM Transmitter MATLAB Reference

This section describes the MATLAB reference of HDL OFDM Transmitter.

This MATLAB reference accepts a modulation order, code rate index, number of frames, and data bits to be transmitted as a txParam structure or array of structures. txParam has these fields.

  • modOrder — Specify 2, 4, 16, or 64 for 'BPSK', 'QPSK', '16QAM', and '64QAM', respectively. The default value is 4 ('QPSK').

  • codeRateIndex — Specify 0, 1, 2, or 3 for the rates '1/2', '2/3', '3/4', and '5/6' respectively. The default value is 0 ('1/2').

  • numFrames — Specify a positive integer. The default value is 5.

  • txDataBits — Specify binary values in a row or column vector of length trBlkSize * txParam.numFrames. The default is a column vector containing randomly generated binary values of length trBlkSize * txParam.numFrames.

Calculate the transport block size (trBlkSize) by using these parameters.

  • numSubCar — Number of subcarriers per symbol

  • pilotsPerSym — Number of pilots per symbol

  • numDataOFDMSymbols — Number of data OFDM symbols

  • bitsPerModSym — Number of bits per modulated symbol

  • codeRate — Punctured code rate

  • dataConvK — Constraint length of the convolutional encoder

  • dataCRCLen — CRC length

trBlkSize =
((numSubCar-pilotsPerSym)*numDataOFDMSymbols*bitsPerModSym*codeRate)
-(dataConvK-1)-dataCRCLen

For example, to generate a time-domain OFDM transmitter waveform of 5 frames with a modulation scheme of 16QAM and code rate of 1/2 using random data bits in the transport block, format the inputs as structure.

txParam.modOrder = 16; % Modulation order corresponding to 16-QAM
txParam.codeRateIndex = 0; % Code rate index corresponding to 1/2
txParam.numFrames = 5; % Number of frames to be generated

% Calculate transport block size (trBlkSize) using parameters
numSubCar = 72; % Number of subcarriers per symbol
pilotsPerSym = 12; % Number of pilots per symbol
numDataOFDMSymbols = 32; % Number of data OFDM symbols
bitsPerModSym = log2(txParam.modOrder); % Bits per modulated symbol
codeRate = 1/2; % Punctured code rate
dataConvK = 7; % Constraint length of convolutional code polynomial
dataCRCLen = 32; % Data CRC length
trBlkSize = ((numSubCar-pilotsPerSym)*numDataOFDMSymbols* ...
    bitsPerModSym*codeRate) - (dataConvK-1) - dataCRCLen;
txParam.txDataBits = randi([0 1],txParam.numFrames*trBlkSize,1);

% Generate complex baseband transmitter waveform
fprintf('\n-------------------------\n');
fprintf('\n Transmitting %d frames ...\n',sum(txParam.numFrames));
[txWaveform,txGrid,txDiagnostics] = whdlexamples.OFDMTx(txParam);
fprintf('\n Transmission successful\n');
fprintf('\n-------------------------\n');

-------------------------

 Transmitting 5 frames ...

 Transmission successful

-------------------------

whdlexamples.OFDMTx function returns txWaveform, txGrid, and txDiagnostics are described below.

  • txWaveform is the generated time-domain waveform returned as a column vector of length (((fftLen + cpLen) x txParam.numFrames x numSymPerFrame) + (txFilterLen - 1)), where

  • fftLen is the FFT length.

  • cpLen is the cyclic prefix length.

  • numSymPerFrame is the number of OFDM symbols per frame.

  • txFilterLen is the transmitter filter length.

If txParam is an array of structures, then txParam.numFrames is replaced with the sum of all numFrames attributes present in the array. The frame structure of the generated time-domain waveform txWaveform is similar to the Simulink HDL OFDM Transmitter output waveform. For the detailed explanation of the frame structure, see HDL OFDM Transmitter.

2. txGrid is the transmitter grid output and is returned as a matrix of dimension numSubCar-by-(txParam.numFrames x numSymPerFrame).

3. txDiagnostics is a structure or array of structures and consists of these three fields.

  • headerBits — This field represents the header bits as a column vector of size 22, which includes 3 bits for the FFT length index, 2 bits for the symbol modulation type, 2 bits for the code rate index, and 15 spare bits.

  • dataBits — This field represents actual data bits transmitted in the given number of frames (txParam.numFrames). This field is a binary row or column vector of length (txParam.numFrames x trbBlkSize). The row or column vector depends on the dimension of txparam.dataBits. The default size is a column vector of length txParam.numFrames x trbBlkSize.

  • ofdmModOut — This field epresents the OFDM modulator output as a column vector of length (fftLen + cpLen) x txParam.numFrames x numSymPerFrame.

OFDMTx

whdlexamples.ofdmTx function is used to generate OFDM transmitter waveform with synchronization, reference, header, pilots, and data signals. This function returns txWaveform, txGrid, and diagnostics using transmitter parameters txParam. This function internally calls these individual functions.

  • generateOFDMSyncSignal — This function generates the synchronization signal SyncSignal. This function uses Zadoff-Chu sequence with a root index of 25 and length of 62.

  • generateOFDMRefSignal — This function generates the reference signal refSignal for the given FFT length fftLen. This function uses a BPSK-modulated pseudo random binary sequence.

  • generateOFDMPilotSignal — This function generates the pilot signal pilot. This function uses a BPSK-modulated pseudo random binary sequence.

  • OFDMSymbolModulate — This function modulates input bits to complex modulation symbols based on the specified modulation scheme BPSK, QPSK, 16QAM, and 64QAM.

Plot the resource grid of the transmitter waveform. The plot indicates the magnitude variations of each resource grid elements.

plotResourceGrid(txGrid);

HDL AWGN MATLAB Reference

This section describes the MATLAB reference of HDL AWGN.

This MATLAB reference is used for performance evaluation of the HDL OFDM Transmitter and Receiver algorithms. The HDL AWGN MATLAB reference generates AWGN by accepting the signal-to-noise ratio (SNR) in decibel (dB) and sets of seeds. For more details, see HDL Implementation of AWGN Generator. The generated AWGN is added to the HDL OFDM Transmitter output.

FFTLen = 128;
CPLen = 32;
usedSubCarr = 72; % Out of 128 subcarriers, 72 subcarriers are loaded with data

SNRdB = 30;
SNRdBSimInput = SNRdB*ones(length(txWaveform)+633,1);
seedsURNG1 = [121 719 511]; % Seeds for TausURNG1
seedsURNG2 = [2343 323 833]; % Seeds for TausURNG2
txScaleFactor = FFTLen/sqrt(usedSubCarr);

awgnNoise = whdlexamples.hdlawgn(SNRdBSimInput,seedsURNG1,seedsURNG2);

rxWaveform = txWaveform + (1/txScaleFactor)*awgnNoise(634:end);
fprintf('\n Applying the AWGN channel at %d dB\n', SNRdB);

 Applying the AWGN channel at 30 dB

HDL OFDM Receiver MATLAB Reference

This section describes MATLAB reference of HDL OFDM Receiver.

This MATLAB reference includes time synchronization, CFO estimation and correction, OFDM demodulation, header recovery, CPE estimation and correction, and data recovery.

The whdlexamples.OFDMRx function accepts rxWaveform, a transmitted waveform passed through an AWGN channel.

The whdlexamples.OFDMRx function returns decoded bits rxBits and an array of structures, rxDiagnostics, consisting of these eight fields.

  • estCFO — Estimated carrier frequency offset

  • rxConstellationHeader — Demodulated header constellation symbols

  • rxConstellationData — Demodulated data constellation symbols

  • softLLR — Demodulated soft LLR bits

  • decodedCodeRateIndex — Decoded code rate index from header

  • decodedModOrder — Decoded modulation order from header

  • headerCRCErrorFlag — Status of header CRC

  • dataCRCErrorFlag — Status of data CRC

OFDMRx

The whdlexamples.OFDMRx function is used to demodulate and decode the received rxWaveform. This function internally calls these individual functions.

  • OFDMFrequencyOffset — This function estimates the carrier frequency offset based on cyclic prefix (CP) technique. The cyclic prefix portion of the received time-domain waveform is correlated to estimate frequency offset.

  • OFDMFrequencyCorrect — This function corrects the carrier frequency offset on the received waveform using the estimated frequency offset.

  • OFDMFrameSync — This function synchronizes the received waveform by performing correlation using the reference signal. This step reduces the intersymbol interference while demodulating the received waveform.

  • OFDMDemodulation — This function converts the time-domain waveform to frequency-domain waveform for further decoding. The object dsp.HDLFFT is used for HDL implementation of the receiver.

  • OFDMChannelEstimation — This function performs the estimation of the channel using two reference signals. It uses least squares (LS) estimation technique. LS estimates are averaged to improve channel estimation accuracy.

  • OFDMChannelEqualization — This function performs zero forcing (ZF) equalization using estimated channel. Then the received waveform that is free of the channel is used for header recovery and data recovery.

  • OFDMHeaderRecovery — This function recovers header information by performing symbol demodulation, descrambling, and decoding. The success or failure of header information recovery is indicated by the CRC status. This header recovery CRC status is given as an output to the receiver to indicate frame loss or recovery. When the CRC check fails, the header CRC status is 1. Otherwise, it is 0.

  • OFDMDataRecovery — This function performs symbol demodulation, Viterbi decoding, depuncturing, and descrambling. The data is processed only when the header CRC check passes. After descrambling, CRC check on the recovered data bits to indicate whether the packet is errored. When the CRC check fails, the header CRC status is 1. Otherwise, it is 0.

fprintf('\n Receiving process started...\n');
[rxDataBits,rxDiagnostics] = whdlexamples.OFDMRx(rxWaveform);
fprintf('\n Reception completed\n\n');

% Plot constellation of header and data
scatterplot(rxDiagnostics.rxConstellationHeader(:))
title('Header Constellation')

scatterplot(rxDiagnostics.rxConstellationData(:))
title('Data Constellation');

 Receiving process started...

 Estimating carrier frequency offset ... 

 First four frames are used for carrier frequency offset estimation. 

 Estimated carrier frequency offset is -2.218976e-04 KHz.

 Detected and processing frame 5
------------------------------------------

 Header CRC passed

 Modulation: 16QAM, codeRate=1/2 and FFT Length=128

 Data CRC passed

 Data decoding completed
------------------------------------------

 Reception completed

Verify Simulink Model with MATLAB Reference

In this section, Simulink HDL OFDM Transmitter, AWGN generator, and Simulink HDL OFDM Receiver algorithms implemented in fixed point are compared with the equivalent MATLAB HDL reference models implemented in floating point.

The Simulink model consists of an OFDM Transmitter that generates time-domain waveform for a user-defined modulation order and code rate. The waveform is then passed through the AWGN channel that introduces AWGN noise of the desired SNR in dB. Then, the OFDM Receiver algorithm is used to demodulate and decode information bits. The outputs of the Simulink model are verified with the MATLAB reference at each stage.

open HDLOFDMTxRx;
sim HDLOFDMTxRx;

### Starting serial model reference simulation build
### Successfully updated the model reference simulation target for: whdlOFDMRx
### Successfully updated the model reference simulation target for: whdlOFDMTx

Build Summary

Simulation targets built:

Model       Action                       Rebuild Reason                         
================================================================================
whdlOFDMRx  Code generated and compiled  whdlOFDMRx_msf.mexa64 does not exist.  
whdlOFDMTx  Code generated and compiled  whdlOFDMTx_msf.mexa64 does not exist.  

2 of 2 models built (0 models already up to date)
Build duration: 0h 13m 10.364s

Verify Simulink HDL OFDM Transmitter with MATLAB HDL OFDM Transmitter

In this section, plot the real and imaginary parts of the HDL OFDM Transmitter MATLAB reference function output txWaveform as compared with the output of the HDL OFDM Transmitter block.

matlabTxWaveform = txWaveform;
simulinkTxWaveform = simTxOut;

figure;
plot(real(matlabTxWaveform),'-bo')
hold on
plot(real(simulinkTxWaveform(1:length(matlabTxWaveform))),'-r.')
legend('MATLAB Tx waveform','Simulink Tx waveform');
title('Comparison of MATLAB Tx and Simulink Block Tx (Real Part)');
ylim([-0.2 0.2]);
xlabel('Time-Domain Samples');
ylabel('Amplitude');

figure;
plot(imag(matlabTxWaveform),'-bo')
hold on
plot(imag(simulinkTxWaveform(1:length(matlabTxWaveform))),'-r.')
legend('MATLAB Tx waveform','Simulink Tx waveform');
title('Comparison of MATLAB Tx and Simulink Block Tx (Imaginary Part)');
ylim([-0.2 0.2]);
xlabel('Time-Domain Samples');
ylabel('Amplitude');

Verify Simulink HDL AWGN Generator with MATLAB HDL AWGN

In this section, plot the real and imaginary parts of the MATLAB HDL AWGN is compared with the output of the Simulink AWGN Generator block.

matlabChannelOut= rxWaveform;
simulinkChannelOut = simChannelOut;

figure;
plot(real(matlabChannelOut),'-bo');
hold on;
plot(real(simulinkChannelOut(1:length(matlabChannelOut))),'-r.');
legend('MATLAB channel output','Simulink channel output');
title('Comparison of MATLAB Channel and Simulink Channel (Real Part)');
ylim([-0.2 0.2]);
xlabel('Time-Domain Samples');
ylabel('Amplitude');


figure;
plot(imag(matlabChannelOut),'-bo');
hold on;
plot(imag(simulinkChannelOut(1:length(matlabChannelOut))),'-r.');
legend('MATLAB channel output','Simulink channel output');
title('Comparison of MATLAB Channel and Simulink Channel (Imaginary Part)');
ylim([-0.2 0.2]);
xlabel('Time-Domain Samples');
ylabel('Amplitude');

Verify Simulink HDL OFDM Receiver with MATLAB HDL OFDM Receiver

In this section, plot the decoded bits of the MATLAB receiver as compared with the decoded bits of the Simulink receiver.

matlabRxOut= rxDataBits;
simulinkRxOut = simRxDataBits;

figure;
plot(rxDataBits,'-bo');
hold on;
plot(simulinkRxOut(1:length(rxDataBits)),'-r.');
legend('MATLAB Rx bits','Simulink Rx bits');
title('MATLAB and Simulink decoded bits');
ylim([-0.25 1.25]);
xlabel('Time-domain samples');
ylabel('Amplitude');

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