Beamforming and Power Allocation Design for Secure Backscatter Communication

In this paper, we investigate the physical layer secrecy (PLS) performance of a bi-static backscatter communication (BackCom) system, where a tag reliably communicates with a reader node by reflecting the radio frequency (RF) signal emitted by a source, while a malicious eavesdropper (Eve) attempts to intercept the transmission. To enhance the system's security, we propose an artificial noise (AN) injection scheme, in which the RF source transmits AN to confuse Eve while concurrently serving a legitimate communication user (CU). The objective is to maximize the tag's secrecy spectral efficiency (SSE) by jointly optimizing power allocation and beamforming for both the information signal and AN, subject to practical constraints including a minimum spectral efficiency (SE) requirement for the CU and a total power budget at the RF source. To tackle the resulting non-convex optimization problem, we develop an efficient algorithm based on alternating optimization (AO) to obtain a high-quality suboptimal solution. Simulation results confirm that the proposed design significantly improves secrecy performance compared to conventional schemes based on the null-space projection (NSP) technique.