  5G cellular networks are designed to support a new range of applications not
supported by previous standards. Among these, ultra-reliable low-latency
communication (URLLC) applications are arguably the most challenging. URLLC
service requires the user equipment (UE) to be able to transmit its data under
strict latency constraints with high reliability. To address these
requirements, new technologies, such as mini-slots, semi-persistent scheduling
and grant-free access were introduced in 5G standards. In this work, we
formulate a spatiotemporal mathematical model to evaluate the user-plane
latency and reliability performance of millimetre wave (mmWave) massive
multiple-input multiple-output (MIMO) URLLC with reactive and K-repetition
hybrid automatic repeat request (HARQ) protocols. We derive closed-form
approximate expressions for the latent access failure probability and validate
them using numerical simulations. The results show that, under certain
conditions, mmWave massive MIMO can reduce the failure probability by a factor
of 32. Moreover, we identify that beyond a certain number of antennas there is
no significant improvement in reliability. Finally, we conclude that mmWave
massive MIMO alone is not enough to provide the performance guarantees required
by the most stringent URLLC applications.
