The Effect of Receiver Diversity Combining on Optimum Energy Allocation and Energy Efficiency of Cooperative Wireless Transmission Systems

This paper considers the problem of how to efficiently allocate transmission energy in a cooperative wireless communication system with one source, one relay, and one destination. The relay assists the source via an orthogonal amplify-and-forward protocol. The channels are assumed to be flat fading and the destination combines the source and relay transmissions using either maximal ratio combining (MRC) or equal gain combining (EGC). We analyze the problem of minimizing average total (source plus relay) transmit energy under a fixed outage probability constraint. We show that the choice of receiver diversity combining technique affects both the optimum energy allocation and the overall energy efficiency of the cooperative communication system. Our results show that the ratio of optimum relay to source transmission energy tends to be greater for EGC than MRC. Our results also show that, while cooperative transmission with MRC is always the most efficient strategy, cooperative transmission with EGC tends to be more energy efficient than direct transmission when the outage probability constraint is small or when the relay experiences an advantaged channel to the destination