Outage-Optimal Transmission Strategies for Rayleigh Fading Relay Channels

We investigate optimal transmission strategies for Rayleigh fading relay channels, which minimize the outage probability of the achievable rate. We consider two cases depending upon whether spatial power allocation at the source and relay is available. We disclose that when no spatial power allocation is available, the optimal transmit signals are not necessarily independent, and their correlation depends on the signal-to-noise ratios of the links and the transmission rate. Moreover, we derive optimal spatial power allocation strategies in a small outage scenario. Our results show that it is not always beneficial to utilize a relay, and the optimal spatial power allocation depends on the total transmit power, the transmission rate, and the variances of the channels. Simulation results validate our theoretical findings.