Security improvement with QoS provisioning using service priority and power allocation for NOMA-IoT networks

Non-orthogonal multiple access (NOMA) has gained attention as a promising multiple access scheme for the Internet of Things (IoT). A typical setting of user ordering in NOMA networks with user priority difference allows a service priority for solely low-rate high-priority users. In contrast, the diverse quality of service (QoS) requirements and service priorities are prerequisite features of users in the IoT. In this paper, we consider a downlink transmission scenario for NOMA-IoT networks in which the base station (BS) simultaneously serves the two users with a priority difference. To tackle the requirements of the IoT, we consider two schemes: a service priority scheme for high-priority user (SP-HP), and a service priority scheme for low-priority user (SP-LP). Meanwhile, the BS adopts a power allocation strategy to realize the desirable QoS provision for high-priority user and optimize the outage experience of low-priority user in an opportunistic manner. It is novel and interesting to extend the NOMA-IoT framework for a malicious attempt of a passive eavesdropper. To investigate the efficiency and security performances of both schemes, the connection and secrecy outage probabilities of both users are characterized, and their closed-form expressions are derived over Rayleigh fading channels. An effective secrecy throughput (EST) is presented to holistically characterize the performance of the system. Numerical results validate the accuracy of the theoretical results. The results suggest that the transmit power of both users in each scheme can be optimized for the maximum EST, and a selection of an optimal scheme for the reliable and secure transmissions of both users is possible under certain channel conditions.