###### Abstract  
As a dedicated communication protocol for Internet-of-Things, narrowband internet of things (NB-IoT) needs to establish the communication link rapidly and reduce retransmissions as much as possible to achieve low power consumption and stable performance. To achieve these targets, the low-power scheme of the initial cell search and frequency tracking is investigated in this paper. The cell search process can be subdivided into narrowband primary synchronization signal (NPSS) detection and narrowband secondary synchronization signal (NSSS) detection. We present an NPSS detection method whose timing metric is composed of symbol-wise autocorrelation and a dedicated normalization factor. After the detection of NPSS, the symbol timing and fractional frequency offset estimation is implemented in a resource-efficient way. NSSS detection is conducted in the frequency domain with a calculation-reduced algorithm based on the features of NSSS sequences. To compensate the accumulated frequency offset during uplink transmission, a pilot-aided rapid frequency tracking algorithm is proposed. The simulation results of the proposed cell search scheme are outstanding in both normal coverage and extended coverage NB-IoT scenarios, and the accumulated frequency offset can be estimated with high efficiency. Internet-of-Things; NB-IoT; cell search; NPSS; NSSS; frequency tracking 10.18.3274; doi:10.3390/s18103274103  
namely narrowband internet-of-things (NB-IoT), is supplemented to the Long-Term Evolution (LTE) which is complemented in 3 GPP Release-14 [2]. As a specified machine type communication technology, NB-IoT not only boasts low energy consumption [3], but can also access the cellular network. This means that NB-IoT can be directly deployed in GSM, Universal Mobile Telecommunications System (UMTS) and LTE. NB-IoT has four primary characteristics: (1) In the same frequency band, NB-IoT gains 20 dB minimum coupling loss (MCL) compared with legacy LTE, so the coverage is expanded by 100 times theoretically. (2) The mass connection capacity allows one NB-IoT sector to support 100 thousand connections at most. (3) Low power consumption design guarantees that the standby time of NB-IoT devices can be up to 10 years. (4) By reducing cost, the price of a single NB-IoT module is expected to be no more than 5 US dollars. The IoT scenarios deploying NB-IoT devices can gain special advantages. Hospitals, banks, and other sites that need high trustworthiness should consider privacy and security at first before deploying IoT services. NB-IoT has an innate feature to counteract this challenge: its spectrum is licensed. Moreover, the authors of [4] also consider a specific scenario and model the reliability of NB-IoT communication, and they give a trust-based solution with NB-IoT. Another area in need of IoT is the automobile field, including connection among cars and self-driving. [PERSON] et al. [5] present an opportunistic crowdsensing scenario where traffic from a large number of connected sensors is transmitted over NB-IoT, and the results show that NB-IoT technology has the ability of sharing dynamic radio resources between vehicular base stations. In addition to the above, the positioning function supplemented into NB-IoT in Release-14 is also proved capable of offering reliable positioning function by [PERSON] et al. in [6]; they also improve the positioning performance in NB-IoT with the observed-time-difference-of-arrival (OTDOA) algorithm. Although NB-IoT has many advantages, considering that billions of NB-IoT devices will be deployed to every corner of the world, how to construct green IoT wireless communication networks with NB-IoT is a challenging issue. Since green IoT wireless communication networks are required to be built as economically and energy-efficient as possible, the massive NB-IoT terminals which make up the core parts of green IoT networks still need to be further optimized with low power consumption techniques.