Setting Parity Check Matrix for LDPC channel coding

Question

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

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);

%data
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
biterr(y,m)