  Ultra-reliable and low-latency communication (URLLC) is one of three major
application scenarios of the 5G new radio, which has strict latency and
reliability requirements. Contention-based grant-free (GF) access protocols,
such as Reactive, K-Repetition, and Proactive, have been proposed for uplink
URLLC service. In the GF access, user equipment (UE) resends packet immediately
after an unsuccessful transmission such that the latency requirement can be
satisfied. Taking Reactive as an example, this paper studies the impact of 1-
persistent retransmission (1-pR) on the distribution of user-plane delay. We
define the number of UEs that try to send packets in each mini-slot as attempt
rate. We show that the 1-pR makes the attempt rate seen by the packet in
retransmission larger than that seen by the packet in the first transmission.
As a result, the successful probability of retransmission is lower than that of
the first transmission. Based on this observation, we derive the distribution
of user-plane delay, which also takes into account the delay incurred by
queueing process. We demonstrate that whether to include the effect of 1-pR and
queueing process in the analysis would have a significant impact on the
prediction accuracy of delay distribution.
