Cooperative turbo coding with time-varying Rayleigh fading channels

In a cooperative transmission system, a source cooperates with a relay to transmit a message to a local destination. This paper proposes an adaptive cooperative protocol utilizing turbo codes and studies the trade-off between spatial and temporal diversity in increasingly mobile channels. While the advantage of cooperation is most pronounced in slow fading channels on the order of 8-12 dB for the system considered here-the benefit gradually decreases with increasing mobility and vanishes at a normalized Doppler spread of 10(-2). Nevertheless, for typical wireless packet data services, cooperative coding can provide appreciable gain even in moderately mobile environments (e.g., up to 50 km/h). While the protocol implicitly adapts to varying channel conditions, adaptivity based on explicit knowledge of channel state information is also considered, and we find that the adaptive choice of two relays reduces the power requirement in half.