Joint Power Splitting and Relay Selection in Energy-Harvesting Communications for IoT Networks

In this article, we consider an energy-harvesting (EH) relay system consisting of a source, a destination, and multiple EH decode-and-forward (DF) relays that can harvest the energy from their received radio signals. A power-splitting ratio is employed at an EH DF relay to characterize a tradeoff between the energy used for decoding the source signal received at the relay and the remaining energy harvested for retransmitting the decode outcome. We propose an optimal power-splitting-based relay selection (OPS-RS) framework and also consider the conventional equal power-splitting-based relay selection (EPS-RS) for comparison purposes. We derive closed-form expressions of outage probability for both the OPS-RS and EPS-RS schemes and characterize their diversity gains through an asymptotic outage analysis in the high signal-to-noise ratio region. We further examine an extension of our OPS-RS framework to an EH battery (EHB)-enabled cooperative relay scenario, where the EH relays are equipped with batteries used to store their harvested energies. We propose an amplify-and-forward (AF)-based EHB-OPS-RS scheme and a DF-based EHB-OPS-RS scheme for AF and DF relay networks, respectively. Numerical results show that the proposed OPS-RS always outperforms EPS-RS in terms of outage probability. Moreover, the outage probabilities of the AF- and DF-based EHB-OPS-RS schemes are much smaller than that of the OPS-RS and EPS-RS methods, demonstrating the benefit of exploiting the batteries in EH relays. Additionally, the DF-based EHB-OPS-RS substantially outperforms the AF-based EHB-OPS-RS and such an outage advantage becomes more significant, as the number of EH relays increases.