Joint Delay And User Activity Detection in Asynchronous Massive Access

Massive access plays an important role in establishing future 6G networks. In an asynchronous massive multiple-access channel (MAC) where user activities and channel delays are unknown, it is critical to detect and estimate them in order to ensure reliable data transmission. Even in asynchronous unsourced MAC, a promising data transmission scheme for massive MAC, this information is also crucial. In this paper, we propose a tropical-arithmetic method for a joint delay and user activity detection, which can support a massive number of users with only a few orthogonal wireless resources. A necessary condition for the maximum path delay to ensure correct detection is also identified. Simulation results verify that our designs have zero detection errors even when the number of users scales quadratically with the number of orthogonal resources.