  One of the key challenges in realizing ultra-reliable low-latency
communications (uRLLC) for factories-of-the-future (FoF) applications is to
enhance the cellular random access channel (RACH) procedure. The
state-of-the-art LTE RACH procedure does not fulfil the latency requirements
for envisioned FoF applications. Moreover, it becomes challenging due to
congestion and overloading from massive machine type communication (mMTC)
devices leading to collisions especially in a densely populated factory
scenarios. The main objective of this paper is to conduct a comprehensive
performance evaluation of different random access (RA) enhancements for uRLLC
over 5G wireless networks. Our performance evaluation is based on a realistic
system-level simulator. The core enhancements considered in this work include
early data transmission (EDT), reserved preambles and the use of flexible
physical (PHY) layer numerology. We also propose three new RA enhancements for
uRLLC. Performance evaluation demonstrates that the proposed RA enhancements
can fulfil the 3GPP control plane target of less than 10 ms latency with 99.99%
reliability in factory environments.
