Ergodic Secrecy Rate Analysis and Optimal Power Allocation for Symbiotic Radio Networks

In this paper, we address the challenge of establishing secure communication within a symbiotic radio (SR) network. This network comprises a primary transmitter (PT), a primary receiver (PR), a passive backscatter device (BD), and an eavesdropper (ED) attempting to intercept the BD’s transmitted information signal. The PT simultaneously transmits an information-bearing signal to the PR and artificial noise (AN) to confound the ED. The objective is that the BD conveys confidential information to the PR by leveraging the PT’s signal. The PR performs joint decoding of both the symbols transmitted by the PT and the BD. In this system configuration, we derive a closed-form expression for the ergodic secrecy rate of the BD, providing an analytical framework for evaluating its security performance. Furthermore, we derive an expression for the secrecy rate in the asymptotic regime characterized by a large number of transmit antennas. These derived expressions allow us to optimize both the reflection coefficient and power allocation factor, enabling the maximization of the BD’s ergodic secrecy rate while considering the quality of service (QoS) requirements of the primary system. The derived analytical results provide valuable insights into the influence of key system parameters on the secrecy performance. Notably, the derived analytical results quantify the effect of key system parameters on the secrecy performance. In particular, we show that the using AN can always improve the secrecy rate given the QoS constraints of the primary user. Moreover, we show that the secrecy rate can be improved by increasing the reflection coefficient at the BD even with better channel condition for the ED than the legitimate link.