Energy Efficiency Maximization of AF Relaying SWIPT Systems with Energy Recycling

This paper studies a wireless-powered amplify-and- forward (AF) relaying system, where the relay harvests energy from the source or itself using an antenna switching protocol. At the first time slot, each antenna of the relay either receives information or harvests energy from the source and, then, a subset of antennas is chosen to forward information while the remaining antennas continue to harvest energy at the second time slot, which enables energy recycling. To maximize the energy efficiency of the relaying system, a mixed- integer non- linear fractional-combinatorial problem is formulated, which is however mathematically intractable. Accordingly, a suboptimal low-complexity iterative algorithm is developed, including joint power allocations at the source and relay and antenna switching at the relay. In particular, the joint power allocations is first performed by the BCD and Dinkelbach algorithms for a given antenna combination in the inner iteration while a low-complexity greedy algorithm is adopted to select the optimal antenna subset in the outer iteration. Simulation results demonstrate that energy recycling benefits higher energy efficiency.