Optimal power allocation in a multi-hop decode-and-forward communication system

This paper analyzes optimal power allocation schemes for a multi-hop decode-and-forward relay system in a Rayleigh fading environment at high signal-to-noise ratio. Analysis is done for two kinds of modulation schemes: M-ary phase-shift keying with coherent detection and orthogonal M-ary frequency-shift keying with noncoherent detection. An optimal power allocation scheme based on end-to-end symbol error probability is derived for asymmetric relay placement and it is found that for fixed asymmetric relay locations equal power allocation is not optimal. When we have global channel state information at each node, a scheme for optimal power allocation based on relay link quality is also derived. Simulation results for a multi-hop system with optimal power allocation show significant improvement in performance compared to those of a system with equal power allocation. The gap in performance increases further with increase in number of relay nodes.