The system under study is a convolutionally coded and orthogonally modulated DS-CDMA system over time-varying frequency-selective Rayleigh fading channels in multiuser environments. Iterative soft demodulation and decoding using the Turbo principle can be applied to such a system to increase the system capacity and performance. To combat multiple access interference (MAI), we incorporate the interference cancellation (IC) and decision-directed channel estimation (CE) in the demodulator. However, both IC and CE are subject to performance degradation due to incorrect decisions. In order to prevent error propagation from the decision feedback, soft interference cancellation and channel estimation assisted demodulation is proposed in this paper. The performance of this strategy is evaluated numerically and proved to be superior to the hard decision-directed approach with a minor increase in complexity.

In time-varying frequency selective channels, to obtain high-rate joint time-frequency diversity, linear dispersion coded orthogonal frequency division multiplexing (LDC-OFDM), has recently been proposed. Compared with OFDM systems, single-carrier systems may retain the advantages of lower PAPR and lower sensitivity to carrier frequency offset (CFO) effects, which motivates this paper to investigate how to achieve joint frequency and time diversity for high-rate single-carrier block transmission systems. Two systems are proposed: linear dispersion coded cyclic-prefix single-carrier modulation (LDC-CP-SCM) and linear dispersion coded zero-padded single-carrier modulation (LDC-ZP-SCM) across either multiple CP-SCM or ZP-SCM blocks, respectively. LDC-SCM may use a layered two-stage LDC decoding with lower complexity. This paper analyzes the diversity properties of LDC-CP-SCM, and provides a sufficient condition for LDC-CP-SCM to maximize all available joint frequency and time diversity gain and coding gain. This paper shows that LDC-ZP-SCM may be effectively equipped with low-complexity minimum mean-squared error (MMSE) equalizers. A lower complexity scheme, linear transformation coded SCM (LTC-SCM), is also proposed with good diversity performance.