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.

翻译：本文研究了一种完全异步的无源随机接入方案，其中活跃用户可在任意起始时间通过衰落信道传输数据，不受时间同步限制。所提方案的发射信号由导频序列和极化码字组成，其中极化码字以开关模式分布在数据包的数据部分。接收机采用双滑动窗解码器：内层窗口通过迭代解码联合执行定时与导频检测、信道估计、单用户解码以及连续干扰消除以恢复信息比特；外层窗口则用于增强干扰消除效果。数值结果表明，与同步基准方案相比，所提方案仅存在轻微性能损失，同时具有更强的实际适用性。