Asynchronous Repetition Slotted ALOHA for Massive Random Access

This paper studies asynchronous repetition slotted ALOHA (a-ReSA) for massive random access. Each user’s packet is repeated and allocated to randomly selected slots. Then, the users transmit simultaneously and arrive at the base station (BS) receiver with different delays, i.e., without user-synchronization. The BS receiver carries out an over-sampling-based operation, yielding an over-sampled signal space. We show that owing to user-asynchrony, the channel state information (CSI) can be acquired even when some users utilize an identical pilot sequence, i.e., pilot collision happens. Further, we develop an iterative soft cancellation detection and decoding that exploits the interference structure of the over-sampled signal for powerful multi-user decoding. Afterwards, packet cancellation for a-ReSA is utilized to solve packet collision. To characterize the performance of a-ReSA, we analyze the achievable channel parameter region (ACPR) and outage probability. Our analysis shows that the ACPR of the user-asynchronous scenario is considerably larger than that of user-synchronous scenarios. Further, we present an asymptotic analysis of the throughput of a-ReSA system. It is demonstrated that the normalized throughput of a-ReSA exceeds that of traditional ReSA by about 20% ~ 80%. We also show that the a-ReSA scheme is more robust than traditional ReSA in imperfect CSI scenarios. Numerical results are verified to agree with the analyzed results.