Securing ultra reliability low latency communication in cooperative NOMA network with untrusted UAV relay

With the advent of 5G and beyond, the requirements for latency and reliability in ultra reliability low latency communication (URLLC) have become more stringent than ever. Unlike the assumption of infinite packet length based on information theory, URLLC needs short packet transmission to achieve the stringent requirements. In this background, conventional expression of secrecy rate based on Shannon capacity achieved with infinite packet length is not applicable. Moreover, due to the openness characteristics of wireless channel, security problem is a pronounced challenge for secure URLLC. In this paper, we investigate the physical layer security (PLS) of URLLC in a cooperative non-orthogonal multiple access (NOMA) network with an untrusted unmanned aerial vehicle (UAV) relay. To enhance the PLS in the cooperative NOMA network, we propose an Effective Secrecy Throughput (EST) maximization problem which includes the joint optimization of packet length, power allocation and UAV relay trajectory. Specially, the expression of secrecy rate based on decoding error probability is utilized to construct the expression of EST. Since the EST maximum problem is nonconvex and difficult to find an optimal solution, we propose an EST optimization algorithm (ESTO) based on the block successive convex approximation (BSCA) by dividing the EST maximum problem into three subproblems: packet length optimization, power allocation optimization and UAV relay trajectory optimization. We solve the three subproblems alternately until the objective function convergences. Simulation results confirm the convergence of our proposed algorithm and demonstrate the secrecy performance of our cooperative NOMA network.