Joint Power Allocation and Discrete Phase-Shift Optimization for SIM-Aided ISAC Systems

Stacked intelligent metasurfaces (SIM) capable of beamforming in the wave domain is a promising technology for realizing low-cost and high-efficiency integrated sensing and communication (ISAC) systems. In this paper, we investigate the trade-off design in SIM-aided ISAC systems with discrete phase shifts. Specifically, we propose an interference management method to improve the multiuser communication quality and a beam gain approximation method to enhance the sensing beam gain. The resulting power allocation and phase-shift optimization problems are intractable discrete non-convex problems. Therefore, we develop an efficient alternating optimization algorithm to decouple the complicated problem and then propose an efficient variable separation approach to obtain closed-form solutions for all subproblems. Simulation results show that the proposed discrete phase-shifted design achieves a 40% performance improvement compared to the quantization of the continuous phase-shifted design in existing studies. Moreover, the proposed method is able to achieve better target sensing and more effective multiuser interference management in real discrete systems compared to continuous phase-shift quantization.