Iterative EM channel estimation for turbo-coded DS-CDMA receiver under time-varying interference

This paper proposes a scheme of iteratively obtaining an estimate of channel coefficients and noise power spectral density (PSD) using expectation maximization (EM) for turbo-coded code-division multiple access receivers. The EM channel estimation is tested under two scenarios - time-varying interference environment with pilot symbol-aided modulation, and a single-user environment without any pilot symbols. For the time-varying interference scenario, the initial estimate of channel coefficient and the noise PSD are obtained from pilots and further estimations are refined using turbo decoder soft outputs. In contrast, pilot symbols are eliminated for the single-user iterative receiver with initial estimation performed from the received code symbols, followed by the iterative updates using the feedback from the turbo decoder as before. The refined channel coefficients and noise PSD are iteratively passed to the turbo decoder resulting in improved decoding results for both time-varying interference and single-user scenarios.