Performance of SWIPT in Cooperative Networks With Direct Link and Nonlinear Energy Harvesting at the Battery-Assisted Relay

This paper investigates the performance of time-switching (TS) and power-splitting (PS) energy harvesting (EH) protocols in a two-hop cooperative network with a battery-assisted EH relay. Assuming the presence of the direct link and optimal combining at the destination, and practical nonlinear model for EH, expressions are derived for throughput with selective decode-and-forward (DF) relaying with the direct link (SDFDL) as well incremental relaying (IR). Augmenting the harvested energy with a small amount of battery energy significantly enhances the throughput. Combining direct and relayed signals significantly enhances throughput and decreases the energy consumption at the battery for any desired throughput as compared to the case when no direct link (NDL) from source to destination is used. The IR scheme achieves the highest throughput with very small energy consumption. We demonstrate that, unlike linear EH, the use of nonlinear EH results in loss of diversity, which makes use of the direct link even very important. Also, the use of the linear EH model gives unrealistically low energy consumption figures for any target throughput, making use of nonlinear EH models important. We elaborate on the choice of EH parameters and target rate on throughput and battery energy consumption. Monte Carlo simulations confirm the accuracy of the derived expressions.