Beamforming Design for Active IRS-Aided MIMO Integrated Sensing and Communication Systems

This letter investigates active intelligent reconfigurable surface (IRS)-aided MIMO integrated sensing and communication (ISAC) systems. Different from passive IRS, an active IRS with amplifiers is deployed to overcome the product attenuation caused by the reflection of the IRS. We aim to maximize IRS beampattern towards the direction of a point sensing target, while guaranteeing the quality of service (QoS) requirement of the communication user. The target is sensed by the reflected sensing link, and the user can receive communication signals from both the direct link and the reflected link. To solve the formulated problem with coupled variables, an alternating optimization algorithm is proposed. Approximations such as the majorization-minimization (MM) method is used to tackle the BS beamforming subproblem, while some matrix transformations, the semidefinite relaxation and the Gaussian randomization method is applied to solve the reflection coefficient subproblem. Numerical results demonstrate that active IRS can greatly improve the sensing performance compared to passive IRS.