Have You Used Comm.SphereDecoder System Object For Soft Output?

Hi I am Using Comm.SphereDecoder System Object for a simulation. I am using Viterbi Decoder System Object To Decode the Soft Bits.

But BER is Coming Like 0.49.

Anyone is here Experienced the same Problem? Or Any Suggestion? I am Adding my Code Here.

 M     = 16;
 k     = log2(M);
 nBits = 1e3*log2(M);
 noiseVariance = 1e-2;
 EbNo  = 4;
 symMap = [11 10 14 15 9 8 12 13 1 0 4 5 3 2 6 7];
 BitTable = de2bi(symMap, log2(M), 'left-msb');

 codeRate = 1/2;
 constlen = 7;
 codegenpoly  = [171 133];
 tblen    = 32;
 trellis  = poly2trellis(constlen, codegenpoly);
 hConvEnc = comm.ConvolutionalEncoder(trellis);
 trellis = poly2trellis(7, [171 133])

 hMod = comm.RectangularQAMModulator('BitInput', true, ...
        'ModulationOrder', M, 'NormalizationMethod', 'Average power',...
        'SymbolMapping', 'Custom', 'CustomSymbolMapping', symMap);

 hAWGN = comm.AWGNChannel('NoiseMethod', ...
                    'Signal to noise ratio (SNR)',...
                    'SNR',10);
 hMIMO = comm.MIMOChannel('MaximumDopplerShift',       0, ...
                         'NumTransmitAntennas',       4, ...
                         'NumReceiveAntennas',        4, ...
                         'TransmitCorrelationMatrix', eye(4), ...
                         'ReceiveCorrelationMatrix',  eye(4), ...
                         'PathGainsOutputPort',       true);

 hSpDec = comm.SphereDecoder('Constellation', constellation(hMod),...
        'BitTable', BitTable, 'DecisionType', 'Soft');

 hVitDecSD  = comm.ViterbiDecoder(trellis, ...
    'InputFormat', 'Unquantized', 'TracebackDepth', tblen);

 hBER = comm.ErrorRate;

 data = randi([0 1], nBits, 1); 
 encodedData = step(hConvEnc, data);
 yMod = step(hMod, encodedData);
 yTx  = reshape(yMod, [], 4);
 [yFad, yPG] = step(hMIMO, yTx);
 yRec = step(hAWGN, yFad);
 rxBits = step(hSpDec, yRec, squeeze(yPG));
 decDataSD      = step(hVitDecSD, rxBits(:));
 ber        = step(hBER, data, decDataSD);

 disp(ber(1));

Answer 1

[Amit Kansal]

Couple of issues with the code. The Viterbi Decoder is set to the continuous operation which introduces a delay of tblen bits which is not accounted for in the comparison.

Also, the bit polarity assumed by the SphereDecoder is opposite of that used by the Viterbi Decoder.

Update above respective lines to :

    hBER = comm.ErrorRate('ReceiveDelay', tblen);
    decDataSD = step(hVitDecSD, -rxBits(:));

to get better ber values.