Tailoring Pairwise Non-Orthogonal Multiple Access to the Requirements of Critical Cyber-Physical Systems

Sammanfattning: Within the context of Industry 4.0, many devices have become more intelligent and connected, leading to challenges on how to meet the stringent requirements on latency and reliability in networks of critical cyber-physical systems. In particular, timely channel access and high reliability are of essence to guarantee real-time deadlines. To this end, time-division multiple-access (TDMA) schemes are often used in industrial applications to get contention-free access to the channel. Using pairwise non-orthogonal multiple access (NOMA) on top of such an existing TDMA scheme has recently emerged as a promising solution. With pairwise NOMA, two nodes are served simultaneously using the same time-frequency resources but with different power levels. To separate the signals, successive interference cancellation is used at the receiver. In addition, by adjusting the power allocation, pairwise NOMA can easily switch to TDMA by assigning zero power to one user, if and when needed. Due to this flexibility, pairwise NOMA can be integrated into existing wireless networks and schedulers with improved performance as a result. In particular, if pairwise NOMA could be tailored to the requirements of systems of collaborating cyber-physical systems, it would be possible to enhance performance in terms of latency and reliability, while still providing timely channel access to critical users using TDMA. This is the scope of the thesis work.In order to evaluate the communication reliability for each user in the system as well as for the overall system, the individual outage probability (IOP) and the overall outage probability (OOP) are of essence, but have so far not been available for pairwise NOMA used on top of TDMA. In this thesis work, closed-form expressions for the IOP and the OOP of both uplink and downlink NOMA are derived – also in the presence of a mobile smart jammer. Using these performance metrics, insightful guidelines on the impact of some key parameters on the communication reliability such as power allocation, decoding order, node placements and so on are provided. It should be noted that the conclusions on node placement can be used for smart user pairing, but also for placement of access points (AP) or even mobile APs, using a UAV. Moreover, by formulating a non-cooperative game between a malicious smart mobile jammer and a friendly mobile AP serving two friendly sensor nodes simultaneously, Nash equilibrium points are obtained to reduce power consumption for the AP, while satisfying the communication reliability requirements. Using the derived expressions for OOP and IOP to select proper settings for pairwise NOMA, it is shown that NOMA can be tailored to ensure user fairness, provide timely channel access and high reliability, which is useful for enhancing performance of critical cyber-physical systems even in the presence of jamming.