Performance Analysis of Hybrid Scheme of Slotted ALOHA and PD-NOMA for Massive Machine-Type Communications

This paper investigates the characteristics of slotted ALOHA and power-domain nonorthogonal multiple access (PD-NOMA) hybrid schemes, focusing on two key performance metrics, namely device transmission success probability and base station throughput, while also considering distance-based unfairness among devices. We analyse three distinct hybrid schemes: a scheme where all devices transmit signals with equal power without utilising received signal strength (RSS) targets (Scheme 1), a scheme where devices randomly select one of multiple RSS targets at the nearest base station (Scheme 2), and a scheme where devices select one of multiple RSS targets based on their proximity to the nearest base station (Scheme 3). Our findings reveal that while Scheme 1 has the highest performance, it also has the most pronounced distance-based unfairness. Conversely, Scheme 2 has the lowest performance but effectively mitigates distance-based unfairness. The performance of Scheme 3 improves as the number of RSS targets increases; however, this coincides with an increase in distance-based unfairness. These results suggest that achieving a balance between fairness and performance improvement may be inherently challenging in grant-free access schemes.