OQPSK Modulation-Demodulation Compatibility Considerations

Replace all instances of modem.oqpskmod and modem.oqpskdemod with comm.OQPSKModulator and comm.OQPSKDemodulator.

• This code sample shows you how to simulate multichannel input/output for OQPSK-modulated signals:

  %% Multichannel input/output OQPSK modulation
  m = 4;
  numChan = 3;
  x = randi([0,m-1],10,numChan);
  sps = 2; % Samples per symbol
  
  %% Using modem.oqpskmod modulate the signal
  m_oqpsk = modem.oqpskmod();
  ym1 = m_oqpsk.modulate(x);
  % Process one more frame to verify interframe state handling
  ym2 = m_oqpsk.modulate(x);
  
  %% Using comm.OQPSKModulator modulate the signal
  % Create as many System objects as number of channels
  s1_oqpsk = comm.OQPSKModulator('SymbolMapping','Binary','SamplesPerSymbol',sps);
  s2_oqpsk = comm.OQPSKModulator('SymbolMapping','Binary','SamplesPerSymbol',sps);
  s3_oqpsk = comm.OQPSKModulator('SymbolMapping','Binary','SamplesPerSymbol',sps);
  
  % Preallocate output matrix
  ys1 = complex(zeros(sps*length(x),numChan,'like',x));
  ys2 = ys1;
  
  % Process first frame
  % Process each channel by its specific System object
  ys1(:,1) = s1_oqpsk(x(:,1)); 
  ys1(:,2) = s2_oqpsk(x(:,2)); 
  ys1(:,3) = s3_oqpsk(x(:,3));
  
  % Process second frame
  ys2(:,1) = s1_oqpsk(x(:,1)); 
  ys2(:,2) = s2_oqpsk(x(:,2)); 
  ys2(:,3) = s3_oqpsk(x(:,3)); 
  
  % The modulated signals are not compared because the 
  % comm.OQPSKModulator performs joint filtering and 
  % modulation of the signal, while the modem.oqpskmod 
  % only modulates the signal.
  
  

• The comm.OQPSKDemodulator System object™ does not support soft decision output for OQPSK-modulated signals. However, this code sample shows you how to simulate soft decision output for OQPSK-modulated signals:qamdemod

  %% Soft decision OQPSK demodulation
  % Create an OQPSK signal and add noise to the signal
  sps = 4;
  msg = randi([0 1],1000,1);
  oqpskMod = comm.OQPSKModulator('SamplesPerSymbol',sps,'BitInput',true);
  oqpskSig = oqpskMod(msg);
  
  impairedSig = awgn(oqpskSig,15);
  
  % Soft-demodulate using the qamdemod function
  
  % Align I and Q (create QPSK equivalent signal)
  impairedQPSK = complex(real(impairedSig(1+sps/2:end-sps/2)), imag(impairedSig(sps+1:end)));
  
  % Apply matched filtering to the received OQPSK signal
  halfSinePulse = sin(0:pi/sps:(sps)*pi/sps);
  matchedFilter = dsp.FIRDecimator(sps,halfSinePulse,'DecimationOffset',sps/2);
  filteredQPSK = matchedFilter(impairedQPSK);
  
  % To perform soft demodulation of the filtered OQPSK signal use the 
  % qamdemod function. Align symbol mapping of qamdemod with the symbol
  % mapping used by the comm.OQPSKModulator, then demodulate the signal.
  oqpskModSymbolMapping = [1 3 0 2];
  demodulated = qamdemod(filteredQPSK,4,oqpskModSymbolMapping,'OutputType','llr');