Cramér-Rao Bound and Secure Transmission Trade-Off Design for Semi-IRS-Enabled ISAC

Integrated sensing and communication (ISAC) has evolved into an influential technique to ameliorate energy and spectrum scarcity via co-designing these two functionalities. However, the target can be a potential eavesdropper aiming at wiretapping the information transmitted to the communication user. This paper studies a semi-passive intelligent reflecting surface (IRS) enabled ISAC system, where the IRS is employed to assist the secure communication and simultaneously perform the target sensing based on the echo signals received by the dedicated sensor at the IRS. Specifically, we model two types of targets, namely point targets and extended targets. The direction-of-arrival (DoA) of the former and the complete target response matrix of the latter should be estimated. Under this configuration, we derive the Cramér-Rao bound (CRB) as the performance metric of target estimation. To achieve an optimal performance trade-off, we formulate a weighted optimization problem that balances maximizing the secrecy rate and minimizing the CRB, via jointly optimizing the transmit beamforming and the phase shifts of IRS. Then, we employ the alternating optimization, successive convex approximation and semi-definite relaxation to tackle the proposed non-convex problems for the two target cases. Simulation results show the effectiveness of the proposed schemes compared with benchmarks.

I. Introduction

Integrated sensing and communication (ISAC) has been reckoned as an effective remedy to address the spectrum and resource scarcity for the future sixth-generation (6G) wireless networks, in which a unified spectrum, hardware and signal processing framework can be shared between the radar and communication functions [2]. Therefore, ISAC can provide a new paradigm to support the rising applications that require ubiquitous sensing, connectivity and intelligence, such as robot navigation, virtual reality and vehicle-to-everything [3], [4], [5]. Depending on the deployment of diverse scenarios, ISAC can be categorized as three types of designs: communication-based [6], sensing-based [7], and co-designed [8]. By collaboratively designing the functionalities of radar and communication, ISAC can achieve the communication-aided sensing and sensing-aided communication. It is noteworthy that the wireless propagation environment is a vital factor affecting the performance of communication and sensing. In wireless communications, both the line-of-sight (LoS) and non-LoS (NLoS) links can be utilized to construct strong links to improve the coverage. By contrast, the radar sensing mainly relies on the LoS links to achieve the high-quality target detection and estimation [9]. However, the intricate propagation environment poses challenges to achieving ubiquitous communication and sensing coverage in wireless networks due to the potential attenuation or blocking of transmission links between the base station (BS) and users or targets, leading to severe performance degradation. In addition, including communication information in the sensing waveform may increase the susceptibility of being eavesdropped by the targets as the potential attackers, which raises unique security concerns for ISAC networks [10].