I am trying to implement the LDPC code for OFDM system, where I input data should be binary of length $1024$ and the output coded data of length $2048$ (means of rate 1/2).

As known in Matlab integrated function, we build the encoder and decoder, respectively, as:

ldpcEncoder = comm.LDPCEncoder;  %Encoder 
ldpcDecoder = comm.LDPCDecoder;  %Decoder

After checking that, I see that parity check matrix is set by default into $32400$ x $64800$ which means the input data length must be a column of length $32400$.

I need the input data to be set manually, for example of length $1024$, how can I modify the parity check of the above encoder?

Thank you

  • 1
    $\begingroup$ Maybe, you should try to build the ldpc manually using your own parameters. $\endgroup$ Jul 17, 2020 at 12:53
  • $\begingroup$ Matlab doesn't have a default 32400 x 64800 size: you've entered that somewhere before. You're not showing us all of your code! $\endgroup$ Jul 17, 2020 at 14:20
  • $\begingroup$ That's all what I entered, I only entered these two commands, and the resulted size is 32400 x 64800. @MarcusMüller $\endgroup$
    – Fatima_Ali
    Jul 17, 2020 at 14:42
  • $\begingroup$ @Fatima_Ali these two lines don't output the size, so that can't be all you've done. Do a clear, then post all lines that you had to type to find out the size. $\endgroup$ Jul 17, 2020 at 14:47
  • 2
    $\begingroup$ @Fatima_Ali the documentation is clear fr.mathworks.com/help/comm/ref/… Specify 'ParityCheckMatrix' in both encoder and decoder. $\endgroup$
    – AlexTP
    Jul 17, 2020 at 15:47

1 Answer 1


As specified in documentation, using 'ParityCheckMatrix' you can configure the Parity Check Matrix (PCM) during the constructions of the encoder/decoder objects.

The syntax is encoder = comm.LDPCEncoder('ParityCheckMatrix',pcm) or simply

encoder = comm.LDPCEncoder(pcm);

where pcm is the desired PCM which must be sparse type.

An example for (probably poor performance) 1024x2048 PCM:

pcm = sparse([eye(1024) eye(1024)]);
encoder = comm.LDPCEncoder(pcm);
decoder = comm.LDPCDecoder(pcm);

m = randi([0 1],1024,1);
%encode then decode. Note the convention of the output of comm.LDPCDecoder
y = 1 - decoder(encoder(m));
%error which equals to 0
  • $\begingroup$ Thank you for you help $\endgroup$
    – Fatima_Ali
    Jul 18, 2020 at 9:46
  • 1
    $\begingroup$ @Fatima_Ali you are welcome $\endgroup$
    – AlexTP
    Jul 18, 2020 at 10:36

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