Tuning Access Class Barring Factor for Machine-Type Devices in Random Access: A Tradeoff Between Preamble Detection and Collision Resolution

Access control mechanisms for massive Machine-Type Communication (MTC) devices are essential for reducing the collision probability caused by the large number of access attempts during the contention-based Random Access (RA) procedure, which begins with preamble transmission. These mechanisms play a critical role in ensuring reliable communication services in fifth-generation (5G) cellular networks. However, the optimum number of admissible devices under given channel conditions that maximizes RA throughput, referred to in this paper as the effective access capacity, and its role in computing the Access Class Barring (ACB) factor have largely been overlooked. This study presents a detailed evaluation of RA throughput under varying channel conditions, focusing on two primary sources of failure during RA, namely undetected preambles due to weak signal strength and unresolved collisions resulting from mismatched Timing Advance (TA) values. These impairments degrade RA performance and constrain the achievable throughput. The trade-off between these two types of errors is analyzed to determine the effective access capacity that maximizes throughput. Simulation results show that, given specific transmission and reception capabilities of MTC devices and the Base Station (BS), wireless channel characteristics significantly influence the optimal admission level and overall system performance.