comm.ViterbiDecoder performing worse than vitdec

Question

Marian Keller 2023-2-13

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https://ww2.mathworks.cn/matlabcentral/answers/1911940-comm-viterbidecoder-performing-worse-than-vitdec

评论： Marian Keller 2023-3-6

I'm trying to get a simple "Hello World" example of convolutional coding and decoding to work. For some reason, the comm.ViterbiDecoder won't ever correctly decode the data, no matter which options I tried, while vitdec works just fine.

trellis = poly2trellis(7,[171 133]);
encoder1 = comm.ConvolutionalEncoder(trellis);
data = randi([0 1],70,1);
codedData = encoder1(data);
tbdepth = 34;
commDecoder = comm.ViterbiDecoder(trellis, 'TracebackDepth', tbdepth, 'InputFormat','Hard','TerminationMethod','Terminated');
decodedData1 = vitdec(codedData,trellis,tbdepth,'trunc','hard');
decodedData2 = commDecoder(codedData);
BER1 = biterr(data,decodedData1)
BER1 = 0
BER2 = biterr(data,decodedData2)
BER2 = 5

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Answer 1

Nadia Shaik 2023-3-6

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https://ww2.mathworks.cn/matlabcentral/answers/1911940-comm-viterbidecoder-performing-worse-than-vitdec#answer_1186115

在 MATLAB Online 中打开

Hi Marian,

I understand that "comm.ViterbiDecoder" is not decoding the data as compared to "vitdec". The different termination methods could explain why the bit error rate differs between the two decoders.

In your case, you are using the "Terminated" method for "comm.ViterbiDecoder", and "trunc" for "vitdec". Consider setting the termination method "Truncated" for "comm.ViterbiDecoder" instead.

Here is the updated code snippet:

trellis = poly2trellis(7,[171 133]);
encoder1 = comm.ConvolutionalEncoder(trellis);
data = randi([0 1],70,1);
codedData = encoder1(data);
tbdepth = 34;
commDecoder = comm.ViterbiDecoder(trellis, 'TracebackDepth', tbdepth, 'InputFormat','Hard','TerminationMethod','Truncated');
decodedData1 = vitdec(codedData,trellis,tbdepth,'trunc','hard');
decodedData2 = commDecoder(codedData);
BER1 = biterr(data,decodedData1)
BER1 = 0
BER2 = biterr(data,decodedData2)
BER2 = 0

I hope this helps!

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Marian Keller 2023-3-6

Thank you very much. Could've sworn I also tried that, apparently not. The alternative solution my professor suggested was to append a few zeros to the data.

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