On the Strict Secrecy Outage Probability of Wirelessly Powered Backscatter Communications

Physical layer security (PLS) has been studied in wirelessly powered backscatter communications (WPBCs) based on secrecy outage probability (SOP), which however does not consider whether or not a backscatter device's (BD's) information can be decoded at the legitimate receiver (LR). In this letter, we adopt the strict SOP (S2OP), i.e., the SOP conditioned on the LR successfully decoding the BD's information, and design an on-off reflection scheme for a WPBC network in the presence of an eavesdropper. To this end, we formulate a problem to maximize the average secrecy rate (ASR) of the BD by jointly optimizing its power reflection coefficient (PRC) and two key parameters of Wyner's encoder, i.e., the rate of the transmitted codewords (TC) and the rate of the confidential information (CI), subject to constraints on the S2OP and the successful decoding probability (SDP) of the BD's information. We first determine the optimal rate of TC and then derive closed-form expressions of the S2OP and SDP, based on which the optimal PRC is obtained and a locally optimal rate of CI is attained by a projected gradient descent method. Simulations verify our analytical results and reveal the impact of S2OP on the ASR of WPBC.