LDPC

Matlab function implementing the LDPC soft decoding algorithm, with the log sum product method

This code was written by studying the paper of Sarah Johnson (http://sigpromu.org/sarah/SJohnsonLDPCintro.pdf or in resources/SJohnsonLDPCintro.pdf), so all the maths explanations of my algorithm can be found there.

USAGE

To use the function, the only paramater is the received sequence (I call it input_frame). You can put bits, or positive/negative floats (it depends of the channel you want to modelize)

Default values

• the decoding matrix is : H = [1 1 0 1 0 0; 0 1 1 0 1 0; 1 0 0 0 1 1; 0 0 1 1 0 1];
• the channel is BSC (Binary Symmetric Channel) with a 0.2 crossover probability. In the code, you can change it to a AWGN channel.

These default values allow to test examples 2.5 and 2.6 of the paper of Sarah Johnson.

Parametrization

• H, the decoding matrix, can be modified ; the code will adapt.
• The impact of the channel on the a priori LLR can be changed. To do that, refer to the differences between fuctions a_priori_log_likehood and a_priori_log_likehood_AWGN

Other variables can be changed too, like the maximum amout of iterations, etc.

Examples

1. BS Channel with the default H matrix command: ldpcdecodersoft([1 0 1 0 1 1]). the 1st bit was flipped, and is wrong

2. AWGN Channel with the default H matrix You have to replace the call of the function a_priori_log_likehood by a_priori_log_likehood-AWGN. command : ldpcdecodersoft([-0.1 0.5 -0.8 1.0 -0.7 0.5]). the recommended SBR is 1.25

These 2 examples are from Sarah's paper (examples 2.5 and 2.6)