Clutter-Aware Active-RIS-Assisted ISAC: A Joint Optimization Framework for Sensing and Communication

Abstract

This paper characterizes the communication–sensing rate tradeoff of an active reconfigurable intelligent surface (RIS)-assisted integrated sensing and communication system operating under residual environmental clutter. The BS beamformer and active-RIS coefficients are jointly designed while accounting for amplifier noise on the forward and return sensing paths, a global RIS power budget, and per-element amplification limits. Residual clutter introduces a beamformer-dependent term in the sensing denominator, leading to a generally indefinite, nonconvex sensing-QoS constraint in the communication-centric (C-C) design and a fractional quadratic beamformer objective in the sensing-centric (S-C) design. We develop alternating optimization procedures for both formulations based on established MM, Kronecker-vectorization, and Dinkelbach techniques. Their QoS-threshold sweeps are refined and filtered to approximate the nondominated $(R_{C},R_{S})$ boundary. The numerical results show that coordinated BS/RIS design enlarges the achievable region relative to the considered baselines, achieving up to 3 bps/Hz higher communication rate and about 1.6 bps/Hz higher sensing rate than a random baseline in the C-C and S-C designs, respectively, while the gains from transmit power and active amplification diminish when residual clutter, amplifier noise, or the RIS power budget becomes dominant.