  In this paper we focus on a feedback mechanism for unsourced random access
(URA) communications. We propose an algorithm to construct feedback packets
broadcasted to the users by the base station (BS) as well as the feedback
packet format that allows the users to estimate their channels and infer
positive or negative feedback based on the presented thresholding algorithm. We
demonstrate that the proposed feedback imposes a much smaller complexity burden
on the users compared to the feedback that positively acknowledges all
successful or negatively acknowledges all undecoded users. We also show that
the proposed feedback technique can lead to a substantial reduction in the
packet error rates and signal-to-noise ratios (SNR)s required to support
various numbers of active users in the system.
