# On Channel Estimation in LTE-Based Downlink Narrowband Internet of Things Systems
## 1 Introduction  
With the in-depth research of the emerging 5 th generation cellular network (5G), the Internet of Things (IoT) technologies are gaining ground as a promising paradigm. Conceptually and practically, almost every class of smart devices such as consumer electronics, actuators, sensors, and mobile phones can be connected and incorporated among them through unique addressing modes with IoT technologies [1, 2]. The significant advantages in performance [3], heterogeneity [4], and big data processing [5] of IoT technology has made it a key technology of the revolutionary 5G network. IoT communication is classified into high data rate (e.g., video signal streaming) and low data rate (e.g., meter reading) services in consideration of transmission rate [6]. In view of the range of transmission, IoT technology is divided into short-range and long-range communication [7, 8]. Wi-Fi, Zigbee, and Bluetooth represent short-range IoT communications and are usually appliedin smart home applications. On the other hand, the long-range IoT is preferred for low data rate applications like deep indoor coverage and smart parking, which is recognized as a low power wide area network (LPWAN) by the industries. LPWAN technologies are divided into authorized and unauthorized spectrum technology regarding the view of spectrum licensing [9; 10]. Sigfox and Lora are non-standard and custom-implemented unauthorized IoT technology. On the other hand, the authorized IoT technology is represented by some well-known technology such as GSM, CDMA, WCDMA, which are extended up to 4 th generation (4G) long-term evolution (LTE) and evolved LTE (eLTE) technology [11]. The authorized spectrum technologies are standardized, developed, and regulated by international standardization organizations like the 3 rd generation partnership project (3 GPP). For example, GSM, CDMA, LTE, and eLTE are standardized by 3 GPP, and CDMA is standardized by 3 GPP2 [12]. GSM, LTE, and the upcoming 5 th generation cellular communication systems are designed to offer voice and high throughput with multimedia services. Furthermore, it is possible to use these technologies to provide a wide area coverage for various IoT uses by modifying the baseband numerology and upgrading the software for the purpose of LPWAN [13; 14]: low cost and complexities, low throughput, deep indoor coverage, small delay sensitivity, massive connectivity, and long battery life. A brand new promising and emerging IoT technology named Narrowband Internet of Things (NB-IoT) was standardized in release 13 by 3 GPP for enabling ultra-low end communication among IoT devices [15]. As NB-IoT is designed based on the existing LTE network, its signals and channels are also inherited from LTE with the specified simplification and required modification. 3 GPP specified a narrow bandwidth of 180 kHz for both the uplink and downlink of NB-IoT systems. NB-IoT finds its diverse applications in the case of smart grids, smart cities, smart homes, logistics, health care, supply chain management, and manufacturing [16]. The principal design objectives of NB-IoT were to achieve deep indoor coverage, low throughput, low latency, low delay sensitivity, longer battery life (>10 years), and low-cost user equipment (UE) devices [17], according to release-13. The technique of repetition of the associated control signal and user data for an extended time period has been taken to get wide-area coverage for NB-IoT devices that are in the extreme coverage area. This technique indicates that the NB-IoT receiver will be capable of decoding the received signal even when the noise power will be higher than the signal power. Therefore, the expected signal-to-noise ratio (SNR) is always less than zero in NB-IoT systems. Hence, in such a repetition-based and extreme area coverage scenario, an efficient channel estimation is the vital prerequisite for the signal detection reliability at the receiver. As the NB-IoT system is still in its growing stage, the current literature of channel estimation is still minimal and inadequate.