Cramér-Rao Bound Minimization for IRS-Enabled Multiuser Integrated Sensing and Communications

• Published in: IEEE transactions on wireless communications Vol. 23; no. 8; pp. 9714 - 9729
• Main Authors: Song, Xianxin, Qin, Xiaoqi, Xu, Jie, Zhang, Rui
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.08.2024; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Algorithms; Antennas; Array signal processing; Beamforming; Communication; Constraints; Copper; Cramer-Rao bounds; Cramér-Rao bound (CRB); Direction of arrival; integrated sensing and communications (ISAC); Intelligent reflecting surface (IRS); Interference; joint transmit and reflective beamforming; Optimization; Receivers; Receivers & amplifiers; Reconfigurable intelligent surfaces; Sensors; Signal to noise ratio; Wireless communication
• ISSN: 1536-1276; 1558-2248
• DOI: 10.1109/TWC.2024.3364913

This paper investigates an intelligent reflecting surface (IRS) enabled multiuser integrated sensing and communications (ISAC) system, which consists of one multi-antenna base station (BS), one IRS, multiple single-antenna communication users (CUs), and one target at the non-line-of-sight (NLoS) region of the BS. The IRS is deployed to not only assist the communication from the BS to the CUs, but also enable the BS's NLoS target sensing based on the echo signals from the BS-IRS-target-IRS-BS link. We consider two types of targets, namely the extended and point targets, for which the BS aims to estimate the complete target response matrix and the target's direction-of-arrival (DoA) with respect to the IRS, respectively. To provide full degrees of freedom for sensing, we consider that the BS sends dedicated sensing signals in addition to the communication signals. Accordingly, we model two types of CU receivers, namely Type-I and Type-II CU receivers, which do not have and have the capability of canceling the interference from the sensing signals, respectively. Under each setup, we jointly optimize the transmit beamforming at the BS and the reflective beamforming at the IRS to minimize the Cramér-Rao bound (CRB) for target estimation, subject to the minimum signal-to-interference-plus-noise ratio (SINR) constraints at the CUs and the maximum transmit power constraint at the BS. We present efficient algorithms to solve the highly non-convex SINR-constrained CRB minimization problems, by using the techniques of alternating optimization, semi-definite relaxation, and successive convex approximation. Numerical results show that the proposed design achieves lower estimation CRB than other benchmark schemes, and the sensing signal interference cancellation at Type-II CU receivers is beneficial when the number of CUs is greater than one.