Simultaneous information transmission and wireless energy transfer via selecting one out of two relays

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. The relays are efficiently deployed when one out of the set of available relays is activated, as multiple relay activation reduces the overall spectral efficiency. In this regard, since the relays that are preferable for information and energy delivery do not necessarily coincide with one another, relay selection entails a tradeoff between information transmission and energy transfer. Assuming that two relays are available, we investigate the behavior of the ergodic capacity of the information transmission to the receiver, for a given amount of energy transferred to the RF energy harvester. We propose the optimal relay selection method for a given energy transfer constraint (in a maximum achievable capacity sense), as well as three suboptimal schemes which apply to scenarios with limited channel state information.