Optimal Joint Antenna Selection and Power Adaptation for Underlay Spectrum Sharing

Abstract

Underlay spectrum sharing improves spectral utilization by allowing a secondary user to transmit concurrently with a primary user. However, the secondary user's performance is limited by the interference constraint that is imposed on it to protect the primary user. Transmit antenna selection overcomes this limitation with a hardware complexity comparable to a single-antenna system. We present a novel and optimal joint antenna selection and power adaptation rule for a secondary system that is subject to the practically motivated interference-outage constraint, which is more general than the widely studied peak interference constraint. The rule provably minimizes the average symbol error probability (SEP) of the secondary user. We show that it has a fundamentally different and novel structure compared to the ones studied in the literature. We present key geometric insights about its novel structure. We use these to propose a simpler, linearized, and near-optimal variant. Compared to the rules considered in the literature, the proposed rules reduce the average SEP by an order of magnitude.