Repetitions Versus Retransmissions: Tradeoff in Configuring NB-IoT Random Access Channels

• Published in: IEEE internet of things journal Vol. 6; no. 2; pp. 3796 - 3805
• Main Authors: Harwahyu, Ruki, Cheng, Ray-Guang, Tsai, Wan-Jung, Hwang, Jeng-Kuang, Bianchi, Giuseppe
• Format: Journal Article
• Language: English
• Published: Piscataway IEEE 01.04.2019; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: 3GPP; Delays; Internet of Things; Machine-to-machine communications; medium access control (MAC) protocols; Narrowband; Optimization; Performance evaluation; Random access; Repetition; Signal to noise ratio
• ISSN: 2327-4662; 2327-4662
• DOI: 10.1109/JIOT.2019.2891366

Narrowband Internet of Things (NB-IoT) is a cellular standard supporting IoT applications in wide area. NB-IoT utilizes "repetition" to extend the coverage of the base station. The repetition value in NB-IoT physical random access channel (NPRACH) is set to ensure 99% detection probability without considering the inherent "retransmission" nature of NPRACH. In this paper, we try to optimally trade extra time-diversity gain in preamble transmission obtained from retransmission to reduce the repetition requirement in NB-IoT. The preferred regions for pure repetition and retransmission schemes sharing the same amount of NPRACH resource under various offered loads and channel conditions are first investigated. The optimal amount of NPRACH resource, repetition value, and retransmission value are then obtained subject to a target successful probability. The results show that the embedded retransmission nature of NPRACH can effectively reduce the number of repetitions. High number of repetitions is required only under worse channel condition.