Activity Detection for Uplink Grant-Free NOMA in the Presence of Carrier Frequency Offsets

In this paper, we propose an activity detection method for uplink grant-free non-orthogonal multiple access (GF-NOMA) systems under imperfect frequency synchronization between uplink users and a common base station. Due to the carrier frequency offsets caused by the asynchronous system clock, the performance of the activity detection would be degraded significantly. Although some countermeasures have been studied in the literature, those approaches assume the perfect knowledge of the maximum number of active users at the base station, which is not reasonable in grant-free systems. In our approach, frequency offsets are efficiently estimated using an idea of array-signal processing, and then activity detection is efficiently performed via box-constrained coordinate descent. Computer simulations confirm the superior performance of our proposed activity detection.