I need a decoder written in Matlab for Gallager LDPC codes(Low Density
Parity Check Codes for Mobile communication Systems).
The decoder should decode recieved data (which are in binary format)by
sum-product algorithm.

Hi,

I suppose your decoder receive fixed soft infomation.

There are several links below about LDPC, but the [1] is the best(it
has a PDF doc and deal with SPA). You may read "Efficient
Implementations of the Sum-Product Algorithm for Decoding LDPC" to
understand the [1]'s code. You may find it in scholar.google.com.

Matlab codes on LDPC:
[1]Low Density Parity Check Codes
http://www.lrz-muenchen.de/~prasanna/ldpc.htm
[2]Matlab programms for encoding and decoding of LDPC codes over GF(2)
http://www.kozintsev.net/soft.html
[3]Degree distributions for LDPC forward error correcting codes
http://cm.bell-labs.com/who/poswald/software.html
[4]MATLAB LDPC software
http://arun-10.tripod.com/ldpc/ldpc.htm
[5]Andrew Eckford's Software
http://www.comm.toronto.edu/~eckford/software/

And if you are familiar with C, I recommand you use Neal's C software. 
I used it and found it is the best open source code on LDPC.
I modified it to realize the fixed sum-product and fixed min-sum algorithm.
C codes on LDPC:
[6] Radford Neal's Research: Error Correcting Codes
http://www.cs.toronto.edu/~radford/res-ecc.html

Regards,
Davy