Power Allocation for Minimizing Outage Probability of Cognitive Multi-Hop Relay Networks

We consider a cognitive multi-hop relay network consisting of a secondary transmitter (ST), a secondary destination (SD) and multiple secondary relays (SRs), where the ST transmits to the SD with the assistance of SRs. A closed-form expression of outage probability for the secondary transmission relying on realistic spectrum sensing is derived over Rayleigh fading channels. We then formulate an optimization problem to minimize the outage probability of the cognitive multi-hop relay network with a constraint on the total transit power of the ST and SRs. Due to the high complexity of the expression of outage probability, it is very difficult to obtain a closed-form solution of the optimal power allocation. To this end, we propose a power allocation scheme based on particle swarm optimization (PSO) algorithm to obtain the numerical solution of optimal power allocation. Moreover, when the fading coefficients of multi-hop relay channels are independent and identically distributed (i.i.d.), the outage probability expression is greatly simplified, for which a closed-form solution of optimal power allocation is obtained. For comparison purposes, the traditional equal power allocation scheme is considered as a benchmark. Simulation results show that the proposed PSO-based optimal power allocation scheme significantly outperforms the traditional equal power allocation in terms of the outage probability.