How to use the LDPC encoder/decoder in MATLAB ?

Greetings,

I am simply trying to use the LDPC encoder/decoder in MATLAB. A very small code like the following:

message = randi([0 1],2,1);

ldpcEncoder = comm.LDPCEncoder;
ldpcDecoder = comm.LDPCDecoder;

encoder = ldpcEncoder(message);
decoder = ldpcDecoder(encoder);

is throwing an error :

Error using LDPCEncoder

Input must be a column vector of length K, the message length. K is the number of columns in the parity check matrix minus the number of rows.

I presume when we don't mention a parity check matrix it takes a default value as described in dvbs2ldpc. So here, I really don't know what's happening even with such a simple code. Please guide me through it. Thank you !

Answer 1

[Harikrishnan Balachandran Nair]

Hi,

From my Understanding, you are facing an error related to the size of your message ,when trying to perform LDPC coding. As you have mentioned in the question, when a parity check matrix is not specified, the default sparse parity check matrix is obtained as dvbs2ldpc(1/2), which returns the parity-check matrix H of the LDPC code with code rate (1/2) from the Digital Video Broadcasting standard DVB-S.2. The block length of the code is 64,800. Hence the size of the parity check matrix H is 32400*64800. Thus the size of your message should be (64800-32400), as the dimension of the matrix H is (N-K) by (N) , where K is the size of your message, and N is the block length of the code. You can see that the error no longer exists when the following code is executed. Hope this helps!

message = randi([0 1],32400,1);
ldpcEncoder = comm.LDPCEncoder;
ldpcDecoder = comm.LDPCDecoder;
encoder = ldpcEncoder(message);
decoder = ldpcDecoder(encoder);

[Das Siddharth]

Thank you so much for your response ! If I have to perform LDPC on some of the customized sizes let's say codelength of 7 or 11. Can we set the parity matrix as per our need ? If so how can we do that ? Because I couldn't find proper documentaion for LDPC.

[Harikrishnan Balachandran Nair]

Yes you can set the parity matrix as your need, provided it follows the corresponding properties. You can find more about it here : https://www.mathworks.com/help/comm/ref/comm.ldpcencoder-system-object.html#bsyh0xo-5 .

You can also refer to this file to generate parity check matrix for LDPC code. https://www.mathworks.com/matlabcentral/fileexchange/44454-gallager-s-construction-of-parity-check-matrix-for-ldpc-codes

[Rupali c]

The both arrays message and decoder should be same. but actually why are they different?

[Amit Kansal]

The input to the decoder must be LLRs (log-likelihood ratios), while what is passed in are binary bits.

Using

decoder = ldpcDecoder(1-2*encoder);

to map the logical 0 to a 1 and logical 1 to a -1, gives message as the expected decoded output.