Joint Antenna Selection and Passive Beamforming for an IRS Assisted Spectrum Sharing System

Combining intelligent reflecting surface (IRS), transmit antenna selection (AS), and spectrum sharing technologies reduces the hardware cost and enables efficient utilization of energy and the scarce spectrum. It improves the performance of an underlay spectrum sharing system by exploiting the diversity benefits of the multiple antennas and passive beamforming of IRS. We propose three different AS rules for a secondary transmitter (STx) that communicates to a secondary receiver with the help of an IRS while sharing spectrum with a primary system. The goal of the maximum signal power rule is to achieve constructive interference of the signals received at the secondary receiver through the direct link and the reflected link through IRS. On the other hand, the destructive interference rule aims at the destructive interference of the interference signals received at the primary receiver through the direct and the IRS reflected links from the STx. The interference satisfying rule aims to satisfy the interference constraint with equality. Our results show that the IRS assisted spectrum sharing system is different from the conventional spectrum sharing system without IRS and interference satisfying rule need not be the best choice always. Furthermore, we show that increase in the IRS elements improves the secondary performance significantly with only a marginal increase in the interference power at the primary.