Security Enhancement and QoS Provisioning for NOMA-Based Cooperative D2D Networks

The characteristic of heterogeneous power utilization in device-to-device (D2D) enabled cellular networks provides a natural fit for the adoption of non-orthogonal multiple access (NOMA). It contributes to a novel D2D framework, NOMA based cooperative D2D communications, with which the heterogeneous interference among different types of nodes can be tackled well. In the framework, a D2D transmitter not only acts as a relay of a pair of cellular transceiver but also transmits its own information to the paired receiver occupying the spectrum of cellular networks with NOMA technique. Apparently, this setup makes the reliability and security of the networks more challenging, in the presence of eavesdropper. To tackle this issue, a collaborative protocol with joint power optimization is built up. Specifically, to prevent information leakage, full-duplex (FD) cellular receiver injects the artificial noise (AN) signals to deteriorate the eavesdropper's channel while performing beamforming to suppress AN in the legitimate users' directions. Meanwhile, D2D transmitter brings desirable QoS provisions for the networks by power scheduling. The joint power optimization problem is formulated by maximizing the secrecy sum rate (SSR) of the overall system, subject to the quality of service (QoS) requirements of cellular and D2D receivers. Due to the non-convexity of the problem, a double-layer optimization method is proposed and then two optimal power allocation ratios are generated. Furthermore, connection outage probability (COP) and secrecy outage probability of the system (SOPS) are respectively characterized and their closed-form expressions are derived. Finally, numerical results are employed to validate the accuracy of theoretical results and the superiority of the proposed protocol in terms of joint security enhancement and QoS provisioning.