Does D2D Communication Always Benefit Physical Layer Security?

Device-to-device (D2D) communication is able to enhance the spectrum efficiency and coverage of wireless networks but its transmission secrecy is vulnerable since D2D terminals are usually resource/power constrained. Various existing endeavors propose physical-layer security (PLS) means to protect D2D transmissions, under the default prerequisite that D2D transmissions always improve the secrecy performance. In this article, we show that this assumption/conclusion is invalid. We investigate a so-called cellular transmission mode assisted by the base station (cellular mode) and compare its secrecy performance with that of the D2D direct transmission (D2D mode). In both modes, subject to the secrecy outage probability constraints, we optimize the transceiver parameters, such as the signal power, secrecy rates, and transmission thresholds, to maximize the secrecy throughputs. All of the optimal solutions are obtained by closed-form expressions in both modes. Numerical results show that neither of the two transmission modes is always the better but highly depends on the system parameters, such as the locations of nodes, transmit power, and secrecy requirements, and consequently provides the answer that D2D communication is not always beneficial for PLS. This immediately suggests that an adaptive switching transmission mode could achieve an enhanced secrecy performance, which is also validated by simulations.