Robust Beamforming for Active STAR-RIS-Aided Integrated Sensing and Communication

This letter investigates robust beamforming design for active simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-enabled integrated sensing and communication (ISAC) systems under imperfect channel state information (CSI). We formulate a joint optimization problem of receive beamforming, transmit beamforming, and STAR-RIS coefficient matrices to maximize the weighted sum rate. To address the stochastic non-convexity, the sample average approximation (SAA) method is employed to convert the problem into a deterministic form. Building on this reformulation, we develop a block coordinate descent (BCD)–based algorithm that incorporates the weighted minimum mean square error (WMMSE) framework and the quadratic transform technique. Simulation results show that the proposed transmission scheme achieves substantial performance gains over passive STAR-RIS. Moreover, the proposed algorithm closely approaches the performance under perfect CSI, confirming its robustness and effectiveness.