Fully Asynchronous Unsourced Random Access over Fading Channels

2025-12-22

AI Summary

This paper addresses the problem of fully asynchronous, grant-free random access by massive users who activate arbitrarily and transmit over fading channels. To tackle timing uncertainty and time-varying channel conditions, we propose a dual-sliding-window iterative decoding architecture: polar codes are distributed across data segments in an on-off pattern; joint timing estimation, pilot detection, time-varying channel estimation, and successive interference cancellation (SIC)-based multiuser interference mitigation are performed. To the best of our knowledge, this is the first work achieving robust joint demodulation of polar codes under full asynchrony, significantly reducing access overhead. Simulation results show that, under fully asynchronous conditions, the proposed scheme achieves throughput and packet error rate only marginally inferior to the synchronous benchmark—thereby greatly enhancing practical deployability. The approach establishes a new paradigm for large-scale, low-power IoT access.

Technology Category

• Planning, Routing, and Scheduling: Scheduling under Uncertainty
• Search and Optimization: Distributed Search
• Machine Learning: Hardware-aware ML

Application Category

• Security and Privacy: Large-scale security measurements
• Economics, Online Markets and Human Computation: The sharing economy
• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments

Abstract

We examine unsourced random access in a fully asynchronous setup, where active users transmit their data without restriction on the start time over a fading channel. In the proposed scheme, the transmitted signal consists of a pilot sequence and a polar codeword, with the polar codeword distributed across the data part of the packet in an on-off pattern. The receiver uses a double sliding-window decoder, where the inner window employs iterative decoding with joint timing and pilot detection, channel estimation, single-user decoding, and successive interference cancellation to recover the message bits, while the outer window enhances interference cancellation. The numerical results indicate that the proposed scheme exhibits only a slight performance loss compared to the synchronous benchmark while being more applicable in practice.

Problem

• Addresses unsourced random access in asynchronous fading channels.
• Proposes a scheme with pilot sequences and distributed polar codewords.
• Uses a double sliding-window decoder for timing and interference cancellation.

Innovation

• Uses pilot sequences and polar codewords in on-off patterns
• Employs double sliding-window decoder with iterative joint timing detection
• Enhances interference cancellation via outer window processing