An Approach to Asynchronous Unsourced Random Access (URA)

In this paper, we propose an approach to construct a fully asynchronous unsourced random access (URA) communication system. In addition to the typical characteristics of a URA system, such as absence of the user identification in the packet and focus on decoding of the message content, there is no limitation on the user transmission timing. The active users can transmit their packets at any time, on demand. The proposed system belongs to the class of preamble-payload URA formats. Contrary to the typical role of the preamble to serve as temporary user identifier in a URA system, in our proposed system the preamble serves the purpose of timing acquisition. The natural transmission asynchronicity reduces packet collisions. The remaining collisions are tackled by use of a small pool of distinct preambles. Our results demonstrate that, over a wide range of signal-to-noise ratios (SNRs), the number of active users supported by the proposed system is close to the number of users supported by an asynchronous URA system with genie-aided nacket timing.