Detect-and-Forward Multirelay Systems With Decision-Feedback Differential Coherent Receivers

This paper considers a detect-and-forward multirelay wireless system employing differential MPSK modulation with decision-feedback differential coherent (DFDC) receivers. The DFDC receivers overcome the limitation of conventional differential detectors when used over time-varying fading channels and provide performance gains. Using a selective approach, the relays transmit only when the demodulation is errorless, and hence the destination employs an SNR-dependent threshold-decision rule to determine when the relays are active. The SNR-dependent thresholds ensure good performance over a wide SNR range. We propose the hold-and-estimate and hold-and-combine strategies for proper operation of the DFDC receivers over the relay to destination channels, to address the problem of intermittent transmissions on these links because of the selective protocol. We also demonstrate the necessity of initializing the DFDC receivers with pilot symbols. Finally, a recursive least squares adaptive algorithm is employed with the DFDC receivers to bypass the need of estimating the channel autocorrelation function. Analytical lower bounds to error probability illustrate the potential advantages of using DFDC receivers in such relaying systems. Extensive computer simulation results demonstrate the performance gains achieved by these relaying schemes with respect to comparable systems, especially over fast fading channels.