A new power allocation scheme for amplify and forward incremental relaying

In this paper, we present a novel power allocation scheme for amplify and forward incremental relaying (IR) systems with the aim to minimize the overall outage probability at the destination. In our problem formulation, we take into account the outage-dependent activation of the relay when writing the power constraint. We provide a simple closed-form solution to the IR power allocation problem that is based on high signal-to-noise (SNR) approximation of the outage probability assuming maximum ratio combining (MRC) at the destination. We investigate the effectiveness of the proposed solution by comparing the achieved outage probabilities with those obtained through global numerical search. We find that the simple closed-form solution is almost optimal in many situations of interest and even at low SNR conditions. We observe that power-optimized IR needs significantly less power than power-optimized fixed relaying to achieve the same outage probability at the same effective rate. Finally, we show that our analytical power optimization for IR-MRC systems is also applicable to IR systems with selection diversity and results in almost optimal outage probability.