On Joint Decoding and Random CDMA Demodulation

We consider a CDMA system on an additive white Gaussian noise (AWGN) channel with K concurrent users. Each user transmits equal-rate data and employs an LDPC error control code (ECC). Each user’s bits are modulated with a random spreading waveform. These spreading waveforms are furthermore partitioned in M sections which are in turn interleaved and spread in time. This creates a rate 1/M repetition code which interfaces the LDPC-ECC with the CDMA channel. This method, called partitioned spreading, significantly improves convergence properties and maximum system load in conjunction with iterative (turbo) detection. Different decoding iteration schedules, viz. a low-complexity two-stage schedule which separates CDMA detection and LDPC decoding into two successive processes, and a full iteration schedule (full decoding) which invokes all message exchanges in a parallel decoding approach are presented and analyzed. We show that, under certain power and rate conditions, two-stage decoding achieves virtually identical performance as full decoding.