Priority Access in NOMA-Based Slotted ALOHA for Overload 6G Massive IoT

This letter proposes a priority access (PA) in non-orthogonal multiple access (NOMA)-based slotted ALOHA (SA) (PA-NOMA-SA) for overload 6G massive Internet of Things (IoT). First, a NOMA-SA is considered, where devices in massive machine-type communication exploit uncoordinated uplink NOMA for packet transmissions in physical layer and SA protocol in medium access control layer. Second, PA is proposed to prioritize emergency devices over regular devices to keep stable throughput at an overload network condition. Each device estimates the traffic of the IoT network via traffic load estimation and adopts an adaptive traffic load scheme to send packets. The PA controls the load of the IoT network above critical traffic load, and it is demonstrated that by appropriately employing priority access, the normalized throughput is enhanced. Last, the lower-bound of normalized throughput is derived to demonstrate the superiority of PA-NOMA-SA over the existing random access protocol.