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Apply OFDM in MIMO Simulation

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Use an OFDM modulator and demodulator in a simple, 2x2 single-user MIMO error rate simulation. The OFDM parameters are based on the 802.11n standard.

Create an OFDM modulator and demodulator pair with user-specified pilot indices, an inserted DC null, two transmit antennas, and two receive antennas. Specify pilot indices that vary across antennas.

ofdmMod = comm.OFDMModulator(FFTLength=128, ...
    PilotInputPort=true, ...
    NumSymbols=3, ...
    PilotCarrierIndices= ...
    cat(3,[12; 40; 54; 76; 90; 118],[13; 39; 55; 75; 91; 117]), ...
    InsertDCNull=true, ...
    NumTransmitAntennas=2);
ofdmDemod = comm.OFDMDemodulator(ofdmMod);
ofdmDemod.NumReceiveAntennas = 2;

Show the resource mapping of pilot subcarriers for each transmit antenna. The gray lines in the figure denote the insertion of null subcarriers to minimize pilot signal interference.

showResourceMapping(ofdmMod)

Figure OFDM Subcarrier Mapping for Tx Antenna 1 contains an axes object. The axes object with title OFDM Subcarrier Mapping for Tx Antenna 1, xlabel OFDM Symbols, ylabel Subcarrier Indices contains an object of type image.

Figure OFDM Subcarrier Mapping for Tx Antenna 2 contains an axes object. The axes object with title OFDM Subcarrier Mapping for Tx Antenna 2, xlabel OFDM Symbols, ylabel Subcarrier Indices contains an object of type image.

Determine the dimensions of the OFDM modulator by using the info method.

ofdmModDim = info(ofdmMod);
numDataSc = ofdmModDim.DataInputSize(1); % Number of data subcarriers
numSym = ofdmModDim.DataInputSize(2);    % Number of OFDM symbols
numTxAnt = ofdmModDim.DataInputSize(3);  % Number of transmit antennas

Set parameters to generate 100 OFDM frames using QPSK modulation.

nframes = 100;
M = 4; % Modulation order to QPSK

Create an error rate counter.

errorRate = comm.ErrorRate;

Simulate the OFDM system over 100 frames assuming a flat, 2x2, Rayleigh fading channel. Remove the effects of multipath fading using a simple, least squares solution, and demodulate the OFDM waveform and QPSK data. Generate error statistics by comparing the original data with the demodulated data.

for k = 1:nframes

    % Generate a frame of user data
    dataIn = randi([0 M-1],numDataSc*numSym*numTxAnt,1);

    % Split the user data into two different streams and map to QPSK
    % symbols
    streamDataOut = reshape(dataIn,numTxAnt,[]).';
    mapperOut = pskmod(streamDataOut,M,pi/4);

    % Generate random OFDM pilot symbols
    pilotData = complex(rand(ofdmModDim.PilotInputSize), ...
        rand(ofdmModDim.PilotInputSize));

    % Reshape the modulated data streams to match the OFDM modulator
    % requirements and modulate the symbols using OFDM
    ofdmModData = reshape(mapperOut,numDataSc,numSym,numTxAnt);
    dataOFDM = ofdmMod(ofdmModData,pilotData);

    % Create flat, i.i.d., Rayleigh fading channel 2-by-2 channel
    chGain = complex(randn(2,2),randn(2,2))/sqrt(2); 

    % Pass OFDM signal through Rayleigh and AWGN channels
    receivedSignal = awgn(dataOFDM*chGain,30);

    % Apply least squares solution to remove effects of fading channel
    rxSigMF = chGain.' \ receivedSignal.';

    % Demodulate OFDM data
    receivedOFDMData = ofdmDemod(rxSigMF.');

    % Form the received streams
    receivedOFDMStreams = reshape(receivedOFDMData,[],numTxAnt);

    % Demodulate QPSK data
    receivedStreams = pskdemod(receivedOFDMStreams,M,pi/4);

    % Demap the data from each rx stream back to the user data
    receivedData = reshape(receivedStreams.',[],1);

    % Compute error statistics
    errors = errorRate(dataIn,receivedData);
end

Display the error statistics.

fprintf('\nSymbol error rate = %d from %d errors in %d symbols\n',errors)
Symbol error rate = 8.996795e-02 from 5614 errors in 62400 symbols

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