Graph-based Spatially Coupled IRSA Random Access for Age-Critical Grant-Free Massive Access

In this paper, we focus on an age-critical grant-free massive access setup and analyze the freshness of information via a new metric named age of information (AoI), where a large number of user equipments (UEs) are randomly activated and attempt transmitting packets of status update to a base station (BS) over a common shared channel. We propose a graph-based spatially coupled irregular repetition slotted ALOHA (G-SC-IRSA) random access protocol combined with the pseudo-random access method and coupled frames to support massive access. In order to analyze the packet loss rate (PLR) performance of the G-SC-IRSA protocol, we first utilize the density evolution (DE) with a bipartite graph to evaluate the system load threshold of G-SC-IRSA. Then, we derive an analytical expression of average AoI as a function of the active probability of UEs, frame length and PLR. Simulation results validate the accuracy of our theoretical analysis and show the great advantages of G-SC-IRSA in better PLR and AAoI performance compared to the existing benchmark schemes.