EM Channel Estimation for Iterative DS-CDMA Receiver using LDPC Codes with M-ary Modulation

This paper proposes an iterative pilot-symbol-aided direct-sequence code-division multiple-access receiver. The receiver uses a regular low-density parity-check (LDPC) code, iterative demodulation and decoding, and iterative estimators for the fading amplitude and noise power spectral density (PSD). The expectation-maximization channel-estimation performance is analyzed for an additive white Gaussian noise channel as well as a Rayleigh fading channel. Pilot symbols are periodically distributed throughout each LDPC codeword followed by direct-sequence spreading and an M-ary modulation, e.g., quadriphase-shift keying. After initial estimates of the fading amplitude and noise PSD are obtained by using pilot information, subsequent values of these parameters are iteratively updated by using the soft feedback from the LDPC decoder. The updated fading amplitude and noise PSD estimates are iteratively passed to the LDPC decoder, which results in decoding performance close to that obtained with perfect channel knowledge.