Exploiting Propagation Delay Difference in Collided Preambles for Efficient Random Access in NB-IoT

To meet the tremendous demand of Internet of Things (IoT) applications, the 3rd Generation Partnership Project (3GPP) has specified the Narrowband IoT (NB-IoT) standard. However, collisions in the random access (RA) channel of NB-IoT can be severe due to mismatch between frequent random access attempts by a huge number of devices and the extremely limited radio resources. In this paper, a new random access mechanism called time-alignment-value based random access (TARA) is designed to increase the efficiency of the random access channel. The key idea of TARA is to conduct quick retries by an RA-failure device based on the time-alignment (TA) values of colliding UEs in a random access response (RAR) message. Simulation results show that TARA performs much better than the original slotted Aloha mechanism, i.e., higher success probability, higher throughput, and lower access delay. Comparison with other existing schemes further demonstrates high performance and advantages of TARA.