# Narrowband Internet of Things (NB-IoT):
## 1 Introduction  
According to Information Handling Services (IHS) technology forecast, the Internet of Things (IoT) market is expected to grow to billions of devices by 2020 [1]. Massive connections are expected to respond to different IoT use cases such as smart city, smart wearables, smart home, etc. [2]. For these applications, latency-insensitive devices can be positioned in hard-to-reach areas and do not require high throughput or frequent reporting. Therefore, to cope with such tremendous IoT trends, the Third-Generation Partnership Project (3 GPP) introduced the Narrowband Internet of Things (NB-IoT) standard as a communication technology enabler. NB-IoT is categorized as one of the licensed Low-Power Wide-Area Networks (LPWAN) cellular technologies based on Long-Term Evolution (LTE) with long range and low cost. In the LPWAN category, there exist other licensed technologies, i.e., Long-Term Evolution Category M1 (LTE-M), and unlicensed technologies, i.e., Long Range (LoRa), SigFox, Ingenu, etc. [3; 4; 5; 6; 7], but they are not the focus of the current work since they are not based on cellular technology. The term Narrowband refers to NB-IoT's bandwidth of maximum 200 kHz thanks to which it can coexist either in the Global System for Mobile Communications (GSM) spectrum or by occupying one of the legacy LTE Physical Resource Blocks (PRBs) as in-band or as guard-band. Since it coexists in the LTE spectrum, NB-IoT follows the legacy LTE numerologies as it uses Orthogonal Frequency Division Multiplexing (OFDM) and Single-Carrier Frequency Division Multiple Access (SC-FDMA) in the downlink and uplink transmission schemes, respectively. Some modifications in the physical (PHY) and medium access control (MAC) layers are implemented to support the long-range massive machine-type (mMTC) connections with low power, low data rates, low complexity, and hence low cost. However, despite its low complexity, this new radio access technology (RAT) delivers better performance in terms of the supported number of devices, and coverage enhancements for latency-insensitive applications with maximum coupling loss (MCL) of about 20 dB higher than LTE (i.e., 164 dB) [5; 6; 7; 8; 9; 10; 11]. With flexible deployment as well as the possibility to implement over-the-air (OTA) firmware upgrades, many telecommunication operators across the globe (as shown in Figure 1) deployed NB-IoT to test its practical feasibility on diverse use cases with real-life trials such as connected sheep in Norway [12], smart metering and tracking in Brazil [13], NB-IoT at sea in Norway [14], smart city in Las Vegas, USA [15], etc. The trials are enabled by different NB-IoT software and hardware solutions from different chip or module vendors such as Skyworks [16], Media tek [17], Neul (Huawei) [18], Quectel [19], Nordic Semiconductors [20], Intel [21], Sequans [22], Qualcomm [23], Siera wireless [24], Samsung [25], Altair [26], U-Blox [27], and so on. Figure 1: The geographical representation of countries with the ongoing NB-IoT real-life deployments for diverse use cases (May 2019). The availability of such commercial off-the-shelf solutions speeds up the adoption of NB-IoT. For this reason, numerous studies addressing segmented enhancement criteria including survey articles emerged to analyze NB-IoT performance and implementation. Table 1 presents, in a nutshell, the main differences and similarities between this survey and the other existing ones by displaying the key focus features. For example, in [32], the authors surveyed the development path of MTC and elaborated the NB-IoT evolution in Release 13. Similarly, in [28], the authors discussed the Release 13 features and compared its performance with respect to other communication technologies such as LTE-M, SigFox, Lora and Wireless-Fidelity (WiFi), etc. In [29; 30], the authors gave an overview of NB-IoT Release 14; however, in [30], the authors elaborated more on the expectations for NB-IoT Release 15 agenda.