Distributed Multi-Relay Selection in Accumulate-then-Forward Energy Harvesting Relay Networks

This paper investigates a wireless-powered cooperative network (WPCN) consisting of one source-destination pair and multiple decode-and-forward relays. Contrary to conventional cooperative networks, we consider the scenario that all relays have no embedded energy supply, but they are equipped with energy harvesting units and rechargeable batteries. As such, they can accumulate sufficient energy harvested from source's signals before helping forward its information to destination. Each relay will adaptively switch between two basic modes, information forwarding mode and energy harvesting mode. A natural yet challenging question for the considered system is "how to determine the operation mode for each relay and select the energy harvesting relays to efficiently assist the source's information transmission?". Motivated by this, we develop an energy threshold based multi-relay selection (ETMRS) scheme for the considered WPCN. The proposed ETMRS scheme can be implemented in a fully distributed manner as the relays only needs local information to switch between energy harvesting and information forwarding modes. By modeling the charging/discharging of the finite-capacity battery at each relay as a finite-state Markov Chain, we derive closed-form expressions for the system outage probability and packet error rate (PER) of the proposed ETMRS scheme over mixed Nakagami-m and Rayleigh fading channels. To gain some useful insights for practical relay design, we also derive the upper bounds for system outage probability and PER corresponding to the case that all relays are equipped with infinite-capacity batteries. Numerical results validate our theoretical analysis and show that the proposed ETMRS scheme outperforms the existing single-relay selection scheme.