On fast retrial for two-step random access in MTC

In machine-type communication (MTC), a group of devices or sensors may need to send their data packets with certain access delay limits for delay-sensitive applications or real-time Internet-of-Things (IoT) applications. In this case, 2-step random access approaches would be preferable to 4-step random access approaches that are employed for most MTC standards in cellular systems. While 2-step approaches are efficient in terms of access delay, their access delay is still dependent on re-transmission strategies. Thus, for a low access delay, fast retrial that allows immediate re-transmissions can be employed as a re-transmission strategy. In this paper, we study 2-step random access approaches with fast retrial as a buffered multichannel ALOHA with fast retrial, and derive an analytical way to obtain the quality-of-service (QoS) exponent for the distribution of queue length so that key parameters can be decided to meet QoS requirements in terms of access delay. Simulation results confirm that the derived analytical approach can provide a good approximation of QoS exponent.