  This paper presents a novel opportunistic spectrum sharing scheme that
applies ARQ protocol to achieve ultra reliability in the finite blocklength
regime. A primary user shares its licensed spectrum to a secondary user, where
both communicate to the same base station. The base station applies ARQ with
the secondary user, which possess a limited number of trials to transmit each
packet. We resort to the interweave model in which the secondary user senses
the primary user activity and accesses the channel with access probabilities
which depend on the primary user arrival rate and the number of available
trials. We characterize the secondary user access probabilities and transmit
power in order to achieve target error constraints for both users. Furthermore,
we analyze the primary user performance in terms of outage probability and
delay. The results show that our proposed scheme outperforms the open loop and
non-opportunistic scenarios in terms of secondary user transmit power saving
and primary user reliability.
