Main Content

Decode and Recover Message Using CCSDS LDPC Decoder

This example shows how to decode and recover a message from a codeword using an LDPC decoder designed according to the Consultative Committee for Space Data Systems (CCSDS) standard. This example comprises two Simulink® models, each containing a CCSDS LDPC Decoder block. Use the ccsdsLDPCModelBase model for the (8160,7136) LDPC configuration and the ccsdsLDPCModelAR4JA model for the AR4JA LDPC configuration. To verify the behavior of the block, compare the output of the model with the input provided to the encoder function. The CCSDS LDPC Decoder block used in these models supports HDL code generation.

Set Up Input Variables

Set up workspace variables to generate inputs. These values are tunable and you can modify them according to your requirement. The block length index must be in the range [0, 2] and it is applicable only when configType is set to 'AR4JA LDPC'. The code rate index must be in the range [0, 2] and it is applicable only when configType is set to 'AR4JA LDPC'.

numFrames = 2;
configType = 'AR4JA LDPC'; % Select the configuration type as
                           % '(8160,7136) LDPC' or 'AR4JA LDPC'
blkLenIdx = [0 2];
codeRateIdx = [1 0];
nIter = 8;                 % Number of iterations in the range [1, 63]
EbNo = 4;

Generate Input Data

Generate input data for the Simulink® model and the MATLAB functions that you use in this example.

blkLenSet = [1024 4096 16384];
codeRateSet = {'1/2' '2/3' '4/5'};

msg = {numFrames};

decSampleIn = [];
decValidIn = []; decStartIn = []; decEndIn =[];
blockLenIn = []; codeRateIn = [];

for ii = 1:numFrames

    % Input and output codeword length calculation
    if strcmpi(configType,'AR4JA LDPC')
        blockLen = blkLenSet(blkLenIdx(ii)+1);
        codeRate = codeRateSet{codeRateIdx(ii)+1};
        R = str2num(codeRate); %#ok<*ST2NM>
        blockLen = 7136;
        R = 7/8;

    bps = 2;

    % Input bits generation
    msg{ii} = (randi([0 1],blockLen,1)); % Data before encoding

    % LDPC encoding
    G = generatorMatrix(configType,blockLen,R);
    encData = satcom.internal.ccsds.tmldpcEncode(msg{ii},G); % Encoded data

    % Symbol mapping
    symOut = pskmod(double(encData),bps);

    % Channel addition - AWGN channel
    EsNo = EbNo + 10*log10(bps);
    snrdB = EsNo + 10*log10(R); % Noise in dB
    noiseVar = 1./(10.^(snrdB/10));
    chan = comm.AWGNChannel('NoiseMethod','Variance','Variance',noiseVar);
    rxData = chan(symOut);

    % Symbol demapping
    demodOut = pskdemod(rxData,bps,'OutputType','llr', ...

    ldpcLen = length(demodOut);
    decFrameGap = nIter*5000; % Maximum frame gap considering all block lengths and code rates

    decSampleIn = fi([decSampleIn demodOut.' zeros(1,decFrameGap)],1,4,0); %#ok<*AGROW>
    decStartIn = logical([decStartIn 1 zeros(1,ldpcLen-1) zeros(1,decFrameGap)]);
    decEndIn = logical([decEndIn zeros(1,ldpcLen-1) 1 zeros(1,decFrameGap)]);
    decValidIn = logical([decValidIn ones(1,ldpcLen) zeros(1,decFrameGap)]);
    blockLenIn = fi([blockLenIn repmat(blkLenIdx(ii),1,ldpcLen) zeros(1,decFrameGap)],0,2,0);
    codeRateIn = fi([codeRateIn repmat(codeRateIdx(ii),1,ldpcLen) zeros(1,decFrameGap)],0,2,0);

dataIn = decSampleIn.';
validIn = decValidIn;
startIn = decStartIn;
endIn = decEndIn;
blkLenIdxIn = blockLenIn;
codeRateIdxIn = codeRateIn;

simTime = length(decValidIn) + decFrameGap;

if strcmpi(configType,'AR4JA LDPC')
    modelName = 'ccsdsLDPCModelAR4JA.slx';
    modelName = 'ccsdsLDPCModelBase.slx';

Run Simulink Model

Running the model imports the input signal variables dataIn, startIn, endIn, validIn, blkLenIdxIn, codeRateIdxIn, and simTime from the script. The model exports a stream of decoded output samples dataOut and a control bus containing startOut, endOut, and validOut signals to the MATLAB workspace.

out = sim(modelName);

Compare Simulink Model Output with MATLAB Function Input

Compare the output of the model with the encoder function input data.

startIdx = find(squeeze(out.startOut));
endIdx = find(squeeze(out.endOut));
validOut = (squeeze(out.validOut));
decData = squeeze(out.decOut);
fprintf('Decoded data with the following configuration: \n');
for ii = 1:numFrames
    idx = startIdx(ii):endIdx(ii);
    decHDL = decData(idx);
    validHDL = validOut(idx);

    HDLOutput = logical(decHDL(validHDL));
    error = sum(abs(logical(msg{ii})-HDLOutput(:)));
    fprintf('Frame: %d, The Simulink model output and the MATLAB function input differs by %d bits\n', ii, error);

Decoded data with the following configuration: 
Frame: 1, The Simulink model output and the MATLAB function input differs by 0 bits
Frame: 2, The Simulink model output and the MATLAB function input differs by 0 bits

See Also