  In cognitive radio (CR) communication systems, achieving high secondary user
(SU) rate in the presence of strong cross-link interference with the primary
user (PU) is challenging. In this letter, we exploit the emerging intelligent
reflecting surface (IRS) technology to tackle this problem. Specifically, we
investigate an IRS-assisted CR communication system where an IRS is deployed to
assist in the spectrum sharing between a PU link and an SU link. We aim to
maximize the achievable SU rate subject to a given
signal-to-interference-plus-noise ratio target for the PU link, by jointly
optimizing the SU transmit power and IRS reflect beamforming. Since the
formulated problem is difficult to solve due to its non-convexity and coupled
variables, we propose an efficient algorithm based on alternating optimization
and successive convex approximation techniques to solve it sub-optimally, along
with some heuristic designs for lower complexity. Simulation results show that
IRS is able to significantly improve the SU rate, even for the scenarios deemed
most challenging in conventional CR systems without using IRS.
