  Integrated sensing and communication (ISAC) systems are typically deployed in
multipath environments, which is usually deemed as a challenging issue for
wireless communications. However, the multipath propagation can also provide
extra illumination and observation perspectives for radar sensing, which offers
spatial diversity gain for detecting targets with spatial radar cross-section
(RCS) fluctuations. In this letter, we propose to utilize reconfigurable
intelligent surfaces (RIS) in ISAC systems to provide high-quality and
controllable multipath propagation for improving the performance of fluctuating
target detection and simultaneously enhancing the quality of communication
services. To effectively exploit the spatial diversity offered by RIS-empowered
multipath, the dual-functional transmit beamforming and the RIS reflection
beamforming are jointly designed to maximize the expectation of radar
signal-to-noise ratio (SNR). To solve the resulting complex non-convex
optimization problem, we develop an efficient alternating optimization
algorithm that utilizes majorization-minimization (MM) and alternating
direction method of multipliers (ADMM) algorithms. Simulation results
illustrate the advantages of multipath exploitation and the proposed
beamforming design algorithm for fluctuating target detection in RIS-assisted
ISAC systems.
