Optimal energy-efficient antenna selection and power adaptation for underlay spectrum sharing

Abstract

We propose a novel joint antenna selection and power adaptation rule that maximizes the energy-efficiency (EE) of an underlay spectrum sharing system. In an underlay system, a secondary user shares the spectrum with a higher priority primary user, but is subject to tight constraints on the interference it generates. Our approach exploits the spatial diversity benefits of multiple antennas to improve the secondary user’s performance, but with a hardware complexity and cost comparable to a single antenna system. We prove that the proposed rule is EE-optimal for a secondary transmitter that is subject to the interference-outage constraint, which generalizes the widely used peak interference constraint, and the peak transmit power constraint. It has a novel form different from the conventional rules considered in the literature. We present an insightful geometrical characterization that brings out the dependence of the optimal power on the channel gains within the secondary system and between the secondary and primary systems. The proposed rule achieves a markedly higher EE compared to conventional rules.