Energy-Efficient Secure Transmission Design for the Internet of Things With an Untrusted Relay

In this paper, we investigate secure uplink transmission in a typical Internet of Things (IoT) deployment, where multiple sensors communicate with a controller via the assistance of an untrusted relay. By taking both the relay and direct links into account, three different scheduling schemes, e.g., optimal scheduling (OS) scheme, threshold-based scheduling (TS) scheme, and random scheduling (RS) scheme, are proposed to cope with the implementation complexity of user scheduling in IoT communications. For each scheduling scheme, we first derive the closed-form expressions for the secrecy outage probability and the secrecy throughput, as well as characterizing the secure energy efficiency to help facilitate an energyefficient secure transmission design. Finally, numerical simulations demonstrate that increasing the number of sensors is an efficient method to boost the security and energy efficiency under the OS scheme. Moreover, the TS scheme provides a good tradeoff between implementation complexity and secrecy performance.