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Integrated sensing and communications (ISAC) technology has attracted increasing attention to enable various emerging applications in the future wireless networks. However, uncontrollable electromagnetic environments still restrict the performance of both radar sensing and wireless communication in ISAC networks. For the above issue, this paper leverages a novel simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) to provide extensive and reliable coverage for ISAC networks. Thereinto, the STAR-RIS can not only elevate the flexibility of the network deployment relying on its full-space coverage, but also reconfigure signal propagation toward both sensing targets and communication users with more degrees-of-freedom over conventional RISs. More specifically, we consider an energy splitting-STAR-RIS-empowered ISAC network in which a multi-antenna base station (BS) broadcasts the combined information-bearing and dedicated sensing waveforms to simultaneously perform single-target detection and multi-user communication. Our objective is to maximize the sensing signal-to-noise ratio (SNR) by jointly optimizing the transmit beamforming at the BS and the transmission&reflection beamforming at the STAR-RIS, subject to the signal-to-interference-plus-noise ratio constraint per user. To solve the formulated non-convex problem, an efficient overall iterative algorithm is developed by invoking the semidefinite relaxation, majorization-minimization, and sequential rank-one constraint relaxation methods. Furthermore, we theoretically prove that whether the STAR-RIS is deployed or not, the dedicated sensing waveforms are always not required from the sensing SNR perspective, which can greatly reduce the implementation complexity of the studied network. Simulations verify our finds, and manifest that the studied STAR-RIS-empowered ISAC scheme significantly outperforms other benchmark schemes in terms of the sensing SNR.
Liu et al. (Tue,) studied this question.