Optimal incremental relaying in cooperative diversity systems

One of the main drawbacks of cooperative communication systems with half-duplex relaying is inefficient use of spectrum. Incremental relaying is an effective technique that overcomes this limitation. In this study, for a single-relay regenerative system, the authors propose a spectrally-efficient incremental relaying scheme that eliminates redundant relay transmissions such that the error propagation from the relay to the destination is effectively mitigated and the full diversity is achieved. The authors examine the impact of the error propagation on the performance of the system and derive closed-form expressions for the end-to-end symbol error probability (SEP) for different modulation schemes. They also derive an approximate closed-form expression for the optimal threshold that minimises the end-to-end SEP. The high-signal-to-noise ratio (SNR) analysis demonstrates that this threshold achieves full diversity whereas neither the bandwidth-consuming cyclic redundancy check code nor the availability of the instantaneous source-relay channel state information at the destination is required in the system. Furthermore, the authors purpose a class of thresholds that are independent of the network geometry and achieve full diversity. The asymptotic behaviour of the system shows that the spectral efficiency for both the optimal and geometry-independent thresholds in the high-SNR regime tends to the maximum achievable value.