Relay Selection for Simultaneous Information Transmission and Wireless Energy Transfer: A Tradeoff Perspective

TL;DR

This paper explores the tradeoff between information transfer efficiency and energy transfer in relay selection, proposing methods and analyzing their performance for simultaneous wireless information and energy transfer scenarios.

Contribution

It introduces new relay selection schemes for balancing information and energy transfer, including an optimal method for two relays and a simpler near-optimal alternative.

Findings

Closed-form expressions for capacity-energy tradeoff

Optimal relay selection for two relays under energy constraints

Simulation results validating the proposed schemes

Abstract

In certain applications, relay terminals can be employed to simultaneously deliver information and energy to a designated receiver and a radio frequency (RF) energy harvester, respectively. In such scenarios, the relay that is preferable for information transmission does not necessarily coincide with the relay with the strongest channel to the energy harvester, since the corresponding channels fade independently. Relay selection thus entails a tradeoff between the efficiency of the information transfer to the receiver and the amount of energy transferred to the energy harvester. The study of this tradeoff is the subject on which this work mainly focuses. Specifically, we investigate the behavior of the ergodic capacity and the outage probability of the information transmission to the receiver, for a given amount of energy transferred to the RF energy harvester. We propose two relay selection methods that apply to any number of available relays. Furthermore, for the case of two relays, we develop the optimal relay selection method in a maximum capacity / minimum outage probability sense, for a given energy transfer constraint. A close-to-optimal selection method that is easier to analyze than the optimal one is also examined. Closed-form expressions for the capacity-energy and the outage-energy tradeoffs of the developed schemes are provided and corroborated by simulations. Interesting insights on the aforementioned tradeoffs are obtained.