QoS Aware Transmit Beamforming for Secure Backscattering in Symbiotic Radio Systems

This article presents secure backscatter transmission against a multiantenna eavesdropper in a symbiotic radio (SR) system, where a single-antenna backscatter device (BD) can transmit confidential information to a primary receiver via a multiantenna primary transmitter's signal. The primary receiver jointly decodes the received signals. The aim is to ensure secure communication for BD while meeting the quality of service (QoS) requirements of the primary system. This article investigates two practical SR systems based on the eavesdropper's available channel state information (CSI). First, when the full CSI is known, an alternating optimization algorithm is proposed that maximizes the achievable secrecy rate of BD by jointly optimizing primary transmit beamforming and power sharing between information and artificial noise (AN) signals. Second, when no eavesdropper CSI is available, it is impossible to determine a secrecy rate expression. Consequently, secrecy communication is achieved by an AN-assisted optimal beamforming scheme where a minimum transmit power is used to meet the QoS constraints of the primary and backscatter systems before utilizing the remaining power for random AN to jam the eavesdropper. Successive convex approximation-based solutions are also obtained for comparison. Simulations verify the key analytical claims, provide nontrivial design insights, and quantify the achievable gains over the relevant benchmarks.