Are there some mistakes in the example of function ldpcDecode?

Question

Ping Chun Hu 2022-3-22

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https://ww2.mathworks.cn/matlabcentral/answers/1677854-are-there-some-mistakes-in-the-example-of-function-ldpcdecode

回答： Santosh Fatale 2022-4-7

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Recently, I use the function ldpcDecode to test my LDPC code.

When I run the MATLAB example, the answer is a little bit tricky.

Here is the command.

openExample('comm/DecodeRate34LDPCCodewordsExample')

I append another snr value into the array, like:

snr = [20 6 3 20];

The output would be like:

SNR = 20
Coded: Error rate = 0.00, Number of errors = 0
Noncoded: Error rate = 0.00, Number of errors = 0
SNR =  6
Coded: Error rate = 0.00, Number of errors = 0
Noncoded: Error rate = 0.02, Number of errors = 221
SNR =  3
Coded: Error rate = 0.02, Number of errors = 301
Noncoded: Error rate = 0.07, Number of errors = 956
SNR = 20
Coded: Error rate = 0.02, Number of errors = 301
Noncoded: Error rate = 0.05, Number of errors = 956

The second snr=20 case shows that the error statistics of each case have accumulated.

I thought that the reset of error statistics is required.

Is it appropriate to advise the example like this:

M = 4;
maxnumiter = 10;
snr = [20 6 3 20];
numframes = 10;
qpskmod = comm.PSKModulator(M,'BitInput',true);
qpskmod2 = comm.PSKModulator(M);
ber = comm.ErrorRate;
ber2 = comm.ErrorRate;
for ii = 1:length(snr)
    qpskdemod = comm.PSKDemodulator(M,'BitOutput',true, ...
        'DecisionMethod','Approximate log-likelihood ratio', ...
        'Variance',1/10^(snr(ii)/10));
    qpskdemod2 = comm.PSKDemodulator(M);
    for counter = 1:numframes
        data = randi([0 1],cfgLDPCEnc.NumInformationBits,1,'int8');
        % Transmit and receive with LDPC coding
        encodedData = ldpcEncode(data,cfgLDPCEnc);
        modSignal = qpskmod(encodedData);
        receivedSignal = awgn(modSignal,snr(ii));
        demodSignal = qpskdemod(receivedSignal);
        receivedBits = ldpcDecode(demodSignal,cfgLDPCDec,maxnumiter);
        errStats = ber(data,receivedBits);
        % Transmit and receive with no LDPC coding
        noCoding = qpskmod2(data);
        rxNoCoding = awgn(noCoding,snr(ii));
        rxBitsNoCoding = qpskdemod2(rxNoCoding);
        errStatsNoCoding = ber2(data,int8(rxBitsNoCoding));
    end
    fprintf(['SNR = %2d\n   Coded: Error rate = %1.2f, ' ...
        'Number of errors = %d\n'], ...
        snr(ii),errStats(1),errStats(2))
    fprintf(['Noncoded: Error rate = %1.2f, ' ...
        'Number of errors = %d\n'], ...
        errStatsNoCoding(1),errStatsNoCoding(2))
    reset(ber)
    reset(ber2)
end

We can get the repeatable output.

SNR = 20
Coded: Error rate = 0.00, Number of errors = 0
Noncoded: Error rate = 0.00, Number of errors = 0
SNR =  6
Coded: Error rate = 0.00, Number of errors = 0
Noncoded: Error rate = 0.02, Number of errors = 221
SNR =  3
Coded: Error rate = 0.02, Number of errors = 301
Noncoded: Error rate = 0.07, Number of errors = 956
SNR = 20
Coded: Error rate = 0.00, Number of errors = 0
Noncoded: Error rate = 0.00, Number of errors = 0