GNC-MAC: An Efficient Geo-Routing-Aware and Cross-Layer MAC Protocol Based on NOMA for Underwater Acoustic Networks

Underwater Acoustic Networks (UANs) play a critical role in marine monitoring and exploration. The integration of mobile nodes can extend the monitoring range and enhance network flexibility and real-time performance. UANs that combine both fixed and mobile nodes typically employ geographic routing protocols and dynamic communication techniques. However, in geographic routing protocols, the uncertainty of the next-hop node leads to several challenges, including access conflicts in the underwater acoustic medium, data collisions, and resource allocation difficulties. Non-Orthogonal Multiple Access (NOMA) technology allows for more users to access the network under limited frequency spectrum, thereby improving network performance. However, due to the complexity of underwater acoustic channels, existing NOMA solutions are difficult to be directly applied in marine environments. Moreover, many existing NOMA techniques rely on centralized control and require real-time global network information, which is challenging to be implemented in UANs. Currently, research on physical-layer and network-layer integrated Medium Access Control (MAC) protocols to address resource allocation and data collision issues remains limited. To address these challenges, this paper proposes the GNC-MAC protocol, compatible with geographic routing protocols and NOMA access techniques. First, we propose a dynamic programming-based grouping algorithm to address the user grouping problem in the uplink of underwater NOMA. Next, to tackle the resource allocation issue, we transform it into a problem of maximizing the sum of the system data transmission rates, and design a distributed dynamic power selection algorithm considering node constraints to improve system throughput. Finally, considering six dynamic factors, including channel quality, energy status, link stability, queue load, node mobility, and interruption probability, we design an adaptive backoff algorithm in the GNC-MAC protocol to optimize next-hop selection in geographic routing protocols and reduce packet collisions. The simulation results indicate that the overall performance of GNC-MAC outperforms existing advanced protocols and offers higher reliability.