Robust Beamformer Design for Backscatter-Enabled RIS-Assisted NOMA ISAC

In this paper, we propose a robust beamformer design for a backscatter-enabled hybrid reconfigurable intelligent surface (RIS)-assisted non-orthogonal multiple access (NOMA) network for integrated sensing and communication (ISAC). We formulate a transmit power minimization problem that jointly optimizes the transmit beamformer at the base station (BS) and the phase shifts at the RIS under a strict power budget, quality of service constraints at each receiving node, and unit modulus phase shift at each reflecting element of the RIS. Due to the non-convex nature of the constraints, we propose an alternate optimization (AO) based iterative algorithm that solves the problem using semi-definite programming (SDP) and successive convex approximation (SCA). The simulation results validate the convergence and efficacy of the proposed algorithm. Furthermore, we illustrate the influence of critical system parameters, including the number of reflecting elements at the RIS, the number of antennas at the BS, and channel state information (CSI) error.