G-SC-IRSA: Graph-Based Spatially Coupled IRSA for Age-Critical Grant-Free Massive Access

In this article, we focus on a grant-free massive access setup and analyze its Age of Information (AoI), where a large number of user equipments (UEs) are randomly activated and attempt to transmit status update packets to a base station (BS) over a common shared channel. To support this age-critical grant-free massive access, we propose a graph-based spatially coupled irregular repetition slotted ALOHA (G-SC-IRSA) random access protocol, which utilizes the pseudo-random access pattern (PRAP), coupled frames, and sliding window decoder (SWD) to improve the packet loss rate (PLR) and AoI performance. Specifically, we derive the approximate expressions to the normalized Average AoI (AAoI) as a function of the PRAP and system load. Then, we establish the problem of minimizing the AAoI under the G-SC-IRSA protocol. Furthermore, we utilize the density evolution (DE) with a bipartite graph to evaluate the system load threshold of G-SC-IRSA in asymptotic regime, achieve an optimal degree distribution via the differential evolution algorithm, and finally obtain the optimal PRAP with progressive edge-growth algorithm. Simulation results validate the accuracy of our theoretical derivations and show that the G-SC-IRSA can achieve the minimum AAoI with the optimal PRAP and outperforms the existing benchmark schemes in terms of PLR and AAoI.