Iterative multiuser detection for convolutionally encoded CDMA with differential modulation

This paper explores the use of differential modulation as a rate-1 recursive code to improve performance in an iterative (or turbo) CDMA receiver in a convolutionally encoded system. We would expect an interleaver gain from the recursive nature of the differential encoder, and indeed such a gain over an absolutely encoded CDMA system is easily demonstrated through simulations. The iterative receiver proposed consists of an additional soft-input soft-output (SISO) differential decoder, when compared to turbo multiuser detectors for absolutely modulated systems. Algorithms for iterative decoding with and without phase information at the receiver are developed. The resulting coherent and non-coherent receivers with differential modulation both outperform coherent receivers with absolute modulation at moderate to high signal to noise ratios.