On the energy efficiency of a relaying protocol with HARQ-IR and distributed cooperative beamforming

Hybrid automatic repeat request (HARQ) protocols can be employed for reliable transmissions of delay tolerant traffics over fading channels. Relay-aided transmissions can offer various advantages over direct transmissions in wireless communications including transmission power saving. In this paper, we consider the energy efficiency of a relaying protocol that is based on the HARQ protocol with incremental redundancy (HARQ-IR) when distributed cooperative beamforming (DCB) is employed in a wireless relay system. It is assumed that multiple relays between a source node (SN) and a destination node (DN) are available for relaying and some of them are selected for DCB. In the proposed relaying protocol, the signal transmission consists of two phases and each phase can have an independent HARQ-IR protocol. It is shown in this paper that although the performance can be improved when the number of selected relays for DCB increases as the diversity gain increases, it is not true in terms of the energy efficiency. For Rayleigh fading channels, closed-form expressions are derived for the average numbers of transmissions for each phase. From them, under a certain energy efficiency criterion, we can find the optimal number of multiple selected relays for DCB to forward signals from relays to DN.