  Cognitive radio (CR) is an effective solution to improve the spectral
efficiency (SE) of wireless communications by allowing the secondary users
(SUs) to share spectrum with primary users. Meanwhile, intelligent reflecting
surface (IRS), also known as reconfigurable intelligent surface (RIS), has been
recently proposed as a promising approach to enhance energy efficiency (EE) of
wireless communication systems through intelligently reconfiguring the channel
environment. To improve both SE and EE, in this paper, we introduce multiple
IRSs to a downlink multiple-input single-output (MISO) CR system, in which a
single SU coexists with a primary network with multiple primary user receivers
(PU-RXs). Our design objective is to maximize the achievable rate of SU subject
to a total transmit power constraint on the SU transmitter (SU-TX) and
interference temperature constraints on the PU-RXs, by jointly optimizing the
beamforming at SU-TX and the reflecting coefficients at each IRS. Both perfect
and imperfect channel state information (CSI) cases are considered in the
optimization. Numerical results demonstrate that the introduction of IRS can
significantly improve the achievable rate of SU under both perfect and
imperfect CSI cases.
