  Narrowband Internet of Things (NB-IoT) is a new cellular technology
introduced in 3GPP Release 13 for providing wide-area coverage for the Internet
of Things (IoT). This article provides an overview of the air interface of
NB-IoT. We describe how NB-IoT addresses key IoT requirements such as
deployment flexibility, low device complexity, long battery life time, support
of massive number of devices in a cell, and significant coverage extension
beyond existing cellular technologies. We also share the various design
rationales during the standardization of NB-IoT in Release 13 and point out
several open areas for future evolution of NB-IoT.
