Cramer-Rao Bound Optimization for Active RIS-Empowered ISAC Systems

Abstract

Integrated sensing and communication (ISAC), which simultaneously performs sensing and communication functions using the same frequency band and hardware platform, has emerged as a promising technology for future wireless systems. However, the weak echo signal received by the low-sensitivity ISAC receiver severely limits the sensing performance. Active reconfigurable intelligent surface (RIS) has become a prospective solution by situationally manipulating the wireless propagations and amplifying the signals. In this paper, we investigate the deployment of active RIS-empowered ISAC systems to enhance radar echo signal quality as well as communication performance. In particular, we focus on the joint design of the base station (BS) transmit precoding and the active RIS reflection beamforming to optimize the parameter estimation performance in terms of Cramer-Rao bound (CRB) subject to the service users' signal-to-interference-plus-noise ratio (SINR) requirements. An efficient algorithm based on block coordinate descent (BCD), semidefinite relaxation (SDR), and majorization-minimization (MM) is proposed to solve the formulated challenging non-convex problem. Finally, simulation results validate the effectiveness of the developed algorithm and the potential of employing active RIS in ISAC systems to enhance direct-of-arrival (DoA) estimation performance.Comment: 30 pages, 9 figures, submitted to IEEE journa