Enhancing Performance of Integrated Sensing and Communication via Joint Optimization of Hybrid and Passive Reconfigurable Intelligent Surfaces

Recent years have witnessed an increasing interest in leveraging reconfigurable intelligent surfaces (RISs) to enhance the capabilities of integrated sensing and communication (ISAC) systems. RISs are advantageous in improving detection and communication performance, especially in challenging environments characterized by nonLine of Sight (NLOS) conditions and dense urban settings. In this article, a hybrid RIS, comprising passive reflecting elements and active sensors, and multiple fully passive RISs are deployed to enhance an ISAC system, where the direct paths between the base station (BS) and users/targets are blocked. The signal sent from the BS and reflected by RISs is received by the communication user, and simultaneously scattered by the target toward the sensors of the hybrid RIS. A joint optimization of the transmit covariance matrix at the BS and phase-shifting matrices at RISs is formulated, which considers the tradeoff between the communication and sensing performance. The optimization is based on the derived closed-form communication achievable rate by leveraging the free probability theory and positioning error bound (PEB) via the Cramér-Rao lower bound (CRLB) analysis. The block coordinate descent (BCD) algorithm is utilized to tackle the nonconvex problem, where the transmit covariance matrix and phase-shifting matrices are optimized iteratively. Therein, the Riemannian gradient descent algorithm is exploited for optimizing the phase-shifting matrices. Numerical results verify the effectiveness of the proposed algorithm, and both communication and sensing performance gains increase with the number of RIS panels and RIS elements.

I. Introduction

Reconfigurable intelligent surfaces (RISs) and integrated sensing and communication (ISAC) systems represent two pivotal advancements in wireless communication technology. The advent of RISs has marked a significant leap forward in wireless communication technologies, offering novel pathways to manipulate electromagnetic waves for enhanced signal propagation and reception [1], [2]. ISAC systems, on the other hand, are designed to simultaneously perform sensing and communication tasks, optimizing the use of the electromagnetic spectrum and hardware resources [3], [4], [5]. The appeal of ISAC lies in its ability to amplify the utilization of current network infrastructures [6]. The integration of RIS in ISAC systems has garnered significant attention for its potential to enhance the system performance [7]. RISs address critical challenges in ISAC systems, such as signal obstruction and penetrating path loss, energy efficiency [8], interference management, and hardware limitations [9]. This not only promises substantial improvements in spectral efficiency1 but also paves the way for innovative applications across diverse fields. For instance, in vehicular networks [10] and high-speed railway systems [11], RISs can significantly improve navigation and safety by providing precise positioning and reliable communication. In underground coal mines [12], multihop and multipath RISs with switches are applied to maximize the energy efficiency of the joint communication and sensing (JCAS) access point. In dual-function radar communication (DFRC) systems [13], RISs can enhance radar detection capabilities and ensure robust communication, supporting critical safety and operational functions.¹

Spectral efficiency is defined as the rate of data transmission per unit bandwidth, typically measured in bits per second per hertz.