Iterative Multiuser Detection of Random CDMA Using Partitioned Spreading

Iterative joint detection of random CDMA is considered in conjunction with error control coding, in particular using low-density parity-check (LDPC) codes. The spreading sequences are partitioned into sections, called “partitioned spreading (PS)”, creating an artificial repetition code which interfaces the LDPC code with the multiple access channel. A suboptimal, but efficient cancelation processor is used at the multiple access processing node in the factor graph describing the overall system. This system is then analyzed using Gaussian density evolution and it is shown that partitioned spreading conditions the channels such that the efficiency of the code is increased substantially. A comparison between direct application of LDPC coding (without partition spreading) to a twostage decoding procedure and a full iterative decoding schedule of PS-CDMA reveals that gains of over 100% in spectral efficiencies can be achieved with regular LDPC codes.