Toward a Green Secure Relay System for Mission-Critical IoT: Hybrid Duplex Relay Selection and Resource Allocation in the Finite Block Length Regime

In this paper, we investigate the physical layer security of a relay system toward secure and green mission-critical Internet of Things, in the finite block length regime, which was hardly considered in the literature. Closed-form expressions for statistical secrecy throughput of full-duplex (FD) and half-duplex (HD) relay systems are respectively derived and verified by simulation results. To maximize the secrecy throughput derived, joint optimization of block length and transmit powers at source and relay is conducted, and a hybrid duplex relay selection (HyDRS) scheme is also proposed, where a relay out of multiple candidates and its duplex mode is selected. Simulation results demonstrate the following findings. a) Together with the HyD relaying scheme, the proposed joint power allocation and block length adaptation scheme achieves much higher secrecy throughput than the conventional sole FD/HD mode relay systems. b) With the proposed HyDRS scheme, a higher degree of freedom in transmission is achieved than the single relay candidate scenario, leading to higher secrecy throughput and energy efficiency. c) It is revealed that increasing block length or transmit power may not always make a positive contribution to secrecy throughput and energy efficiency.