A joint weighted power detector for Willie in two‐hop covert communication system

Abstract

This letter proposes a joint weighted power detector based on maximum a posteriori probability criterion for Willie aiming at two-hop covert communication scenario, which is a near optimal detector. Instead of only supervising one single phase, Willie combines the observations of two phases to make joint decision in the proposed scheme. The proposed scheme achieves lower probability of detection error than addingpower-directly-detector scheme and the existing single-phase-detector scheme due to sufficient utilization of the two-phase observations. Numerical results demonstrate the benefit of authors' proposed scheme.Introduction: Covert communication, also known as low probability of detection (LPD) communication, is regarded as the first barrier to defend privacy security in wireless networks. The information-theoretic fundamental limits in additive white Gaussian noise (AWGN) channels are derived in [1], which also illustrates the benefits can be achieved by introducing uncertainty at Willie [2]. As reported in [3], the transmitted signal is transformed into Gaussian noise to confuse Willie's detector. In [4], an optimal detector for Willie under AWGN and block fading channels is proposed in single-hop system, [4] also reveals that covert communication can be achieved by varying the transmit power at Alice. Moreover, the randomness of fading channel and nodes like Jammer and Relay can be also treated as interference for Willie's detection [5][6][7]. However, the scenario considered in [6][7][8] is that Willie can only detect the forward link (Phase 2), whereas ignoring the link between Alice and Relay (Phase 1). This is not practical for implementation. In addition, the authors in [9] employ the single-phase-detection (SPD) scheme to analyse the covertness of two-hop scenario that Willie detects the two phases independently. Nevertheless, the joint power detector for two-hop system has rarely been studied.This letter proposes, to make Willie's detection more efficient and practical, a joint weighted power detector (JWPD) based on two-phase observation. Different from the adding-power-directly-detector (APDD) scheme that adding Willie's total received power in two phases straightforwardly, the proposed JWPD scheme combines two-phase observations efficiently based on maximum a posteriori probability (MAP) criterion to achieve a near optimal performance. Numerical results coincide with the analysis and show that JWPD scheme achieves the lowest PDE compared to SPD and APDD schemes. Therefore, the covertness of two-hop system can be further analysed with tighter covert constraint with the proposed JWPD scheme.System model: As depicted in Figure 1, considering a typical twohop covert communication system consisting of a legitimate transmitter Alice (A), a legitimate receiver Bob (B), a friendly Jammer (J ) and a cooperative decode-and-forward (DF) Relay (R) assisting Alice for covert communication (Figure 1). A malicious Willie (W ) is monitoring whether covert communication between A and B exists. Let us assume that...