Near Gaussian Multiple Access Channel Capacity Detection and Decoding

We consider the design of low-density paritycheck (LDPC) codes with close-to-capacity performance for interleave division multiple access (IDMA). The degree profile of the LDPC code is optimized using extrinsic information transfer (EXIT) charts to match an IDMA low complexity multiuser detector. Analytical EXIT functions for the IDMA system are derived while verifying that the Gaussian approximation stays valid for a sufficiently large number of users. The resulting LDPC codes are of rather low-rate (e.g., R-c = 0.03 for 32 users) and thus, the practical design of such codes is quite challenging and may, due to the low code-rate, not even yield a performance close to the Gaussian multiple access channel (GMAC) capacity. Adding a serial concatenation with a simple repetition code allows to design matching LDPC codes with a higher rate and, this way, achieving closer to GMAC capacity. This makes IDMA an attractive multiple access scheme for future wireless communication systems.