A Joint User Activity Detection and Channel Estimation Scheme for Packet-Asynchronous Grant-Free Access

Grant-free (GF) access has been viewed as a promising solution to meet the low-latency requirement in massive access of short packets due to the cancellation of the procedure of requesting for the grant. While most GF schemes are realized in a synchronous way, a recently proposed packet-asynchronous GF (PA-GF) scheme can further reduce the overall delay. However, its performance was only validated under the ideal conditions, i.e., perfect active user detection (AUD) and channel estimation (CE). In this letter, in order to realize timely AUD and CE for PA-GF practically, a novel frame structure for multiplexing pilot sequences and data is proposed together with a sliding window detection algorithm. A modification of the orthogonal matching pursuit (OMP)-the prior-aided OMP (PA-OMP)-is also proposed. Numerical results show that the proposed algorithm achieves the low-latency advantage of PA-GF under imperfect AUD and CE conditions.