Recover Data Bits with GMSK Modulation and Demodulation

Since R2026a
Open Script

This example show how to convert data bits into modulated symbols and then recover the original bits through demodulation by using the GMSK Modulator and GMSK Demodulator blocks, respectively. This example includes two Simulink® models: gmskModDemodScalar for specifying scalar inputs and gmskModDemodVector for specifying vector inputs, enabling you to work with each input mode separately. The GMSKModDemod subsystem in these models supports HDL code generation. To verify the behavior of these blocks, the example compares the input to the GMSK Modulator block with the output from the GMSK Demodulator block.

Set Up Input Parameters

Set up workspace parameters for the Simulink model. The model in this example uses these parameters to configure the GMSK Modulator and GMSK Demodulator blocks. These parameters control the behavior of these blocks. You can modify these variables according to your requirement.

bandwidthTimeProduct = 0.3; % Bandwidth time product
pulseLength = 2;            % Pulse length
samplesPerSymbol = 4;       % Number of samples per symbol
decisionMethod = 'Approximate log-likelihood ratio';
gmskTraceback = 16;
numBitsPerFrame = 1000;
numFrames = 10;
outputType = 'Vector';      % Select 'Scalar' or 'Vector'

gmskMod = comm.GMSKModulator('BitInput', true, ...
    'BandwidthTimeProduct', bandwidthTimeProduct, ...
    'PulseLength',pulseLength, ...
    'SamplesPerSymbol', samplesPerSymbol);
gmskDemod = comm.GMSKDemodulator('BitOutput', true, ...
    'BandwidthTimeProduct', bandwidthTimeProduct, ...
    'PulseLength',pulseLength, ...
    'SamplesPerSymbol', samplesPerSymbol, ...
    'DecisionMethod',decisionMethod,...
    'TracebackDepth', gmskTraceback);

rxFrameSize = samplesPerSymbol*numBitsPerFrame;
rng('default');

numErrs = 0;

Generate Input Data and Perform GMSK Modulation

Generate random data bits and a valid input signal for the GMSK Modulator block.

for frame = 1:numFrames
    % Generate random bits
    data(:,frame) = randi([0 1], numBitsPerFrame, 1); %#ok<*SAGROW>

    % GMSK Modulation
    modSignal(:,frame) = gmskMod(data(:,frame));

end

txBits(:) = data(:)>0;
txWaveformML = modSignal(:);
rxSignal = txWaveformML;

Demodulate GMSK-modulated Data

Demodulate GMSK-modulated data symbols into data bits.

for frame = 1:numFrames
    demodSignal = gmskDemod(rxSignal((frame-1)*rxFrameSize+(1:rxFrameSize)));
     numErrs = numErrs + nnz(xor(data(1:end-gmskTraceback,frame),demodSignal(gmskTraceback+1:end)<0));
end

Run Model

Running the model imports the input signal variables to the model and exports dataOut and a control signal validOut to the MATLAB® workspace.

if strcmp(outputType,'Vector')
    modelName = 'gmskModDemodVector'; %#ok<*UNRCH>
else
    modelName = 'gmskModDemodScalar';
end

load_system(modelName);

if strcmp(decisionMethod, 'Hard decision')
    set_param([modelName '/GMSKModDemod/GMSK Demodulator'], 'DecisionType', 'Hard');
else
    set_param([modelName '/GMSKModDemod/GMSK Demodulator'], 'DecisionType', 'Soft');
end

txBitsSL = txBits(:);
out = sim(modelName);
rxDecBitsSL = out.decBits(out.decBitsValid);

Compare Modulator Input with Demodulator Output

Compare the data bits input to the GMSK Modulator block with the data bits output from the GMSK Demodulator block.

if isequal(txBitsSL(1:end-4*gmskTraceback), rxDecBitsSL)
    fprintf('Comparison of Modulator Input with Demodulator Output\n');
    fprintf('Data bits at the input match with the data bits at the output.\n');
else
    fprintf('Comparison of Modulator Input with Demodulator Output\n');
    fprintf('Data bits at the input do not match with the data bits at the output.\n');
end

Comparison of Modulator Input with Demodulator Output
Data bits at the input match with the data bits at the output.

See Also

Blocks