Transceiver beamforming design of RIS-aided active array radar in cluttered environments

This paper equips a reconfigurable intelligent surface (RIS) to assist the active array radar for boosting its interference suppression ability and enhancing the beamforming gain towards target direction simultaneously. The output signal-to-interference-plus-noise ratio (SINR) is chosen as the metric to jointly design the transmit and receive beamformers of radar array and RIS reflection coefficients. In light of SINR performance and implementation complexity, two operation modes using identical or distinct RIS reflection coefficients in transmit and receive stages (ITR or DTR) are investigated. In each mode, the proposed joint design is formulated into a nonconvex constrained fractional programming problem and the solving algorithm is customized under the block coordinate descent framework. Specifically, the RIS reflection coefficients are respectively optimized by the quartic Riemannian Newton method (RNM) in ITR mode and by the quadratic RNM in DTR mode after Dinkelbach transform. Moreover, a simplified scheme under DTR mode is also given to speed up processing, where the beamforming of radar array and RIS separately concentrates on the beamforming gain enhancement and interference suppression in transmit and receive stages. Numerical results display that both ITR and DTR modes significantly outperform the array radars using an RIS only in receive or transmit stage.