Performance and Coverage Enhancement with Frequency Diverse STAR-RIS (FD-STAR-RIS)

Simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) have emerged as a promising technology to achieve full-space coverage. However, existing STAR-RIS schemes primarily rely on angle-domain beamforming. Meanwhile, the frequency diverse RIS (FD-RIS) enables joint distance-angle beamforming, yet its coverage is limited to the half-space. A novel frequency diverse STAR-RIS (FD-STAR-RIS) framework is proposed in this paper by introducing periodic signals for time-modulation on the STAR-RIS. With frequency diversity successfully achieved on the passive STAR-RIS, FD-STAR-RIS enables joint distance-angle beamforming in full-space. In this paper, we first establish the signal processing model for the FD-STAR-RIS. Subsequently, a sum-rate maximization problem is formulated for a dual-user communication system. This is achieved by jointly optimizing the base station (BS) active beamforming, the passive beamforming and time-modulation frequency of the FD-STAR-RIS. To address the formulated problem, an efficient alternating optimization (AO) algorithm is developed. Specifically, the problem is decomposed into subproblems solved via the minimum mean square error (MMSE) criterion, semidefinite relaxation (SDR), and particle swarm optimization (PSO), respectively. Simulation results demonstrate that the proposed FD-STAR-RIS significantly outperforms conventional STAR-RIS by effectively manipulating signals in both distance and angle dimensions.