Decoding Order and Power Control for Securing Priority Users in Cooperative NOMA-Enabled Industrial IoT Networks

The advancement of industrial Internet of Things (IIoT) networks has brought challenges in terms of connectivity, efficient spectrum usage, and low latency. To tackle these challenges, advanced multiple access techniques have been developed. nonorthogonal multiple access (NOMA) is a promising multiple access technique due to its high-energy efficiency and fairness for devices. However, NOMA has inherent security issues due to wireless transmission and complex successive interference cancelation (SIC)-based decoding, which can negatively impact system performance. Furthermore, achieving perfect SIC is also a challenging task due to implementation complexity. This study investigates the effects of imperfect SIC in a dual-device cooperative NOMA system. Our system includes direct links between the source and devices and uses both decode-and-forward and amplify-and-forward relays. The overall objective is to optimize decoding order and power allocation coefficients in order to maximize the near or priority device’s secrecy rate while meeting the quality-of-service (QoS) requirements of the far or normal device. Observing the underlying optimization problems to be nonconvex, a low-complexity algorithm yielding optimal solutions is developed. Our findings reveal how imperfect SIC can impact massive access systems and secrecy performance. Extensive simulations provide novel design insights into the achievable secrecy rate and optimal power allocation coefficients. We also explore the tradeoff between the priority device’s secrecy rate and the normal device’s rate, along with the impact of residual interference. Finally, our proposed solution has been shown to significantly improve the QoS-constrained secrecy rate of priority devices when compared to relevant benchmarks.