###### Abstract
One significant majorization point is to optimize the process of the initial cell search and frequency tracking. More specifically, novel detection and synchronization methods should be presented to reduce the initial cell search and frequency tracking time in NB-IoT systems, and this operation shortens the working time of each transmission, which will extend the battery life of NB-IoT terminals. In this paper, we investigate the efficient cell search and frequency tracking schemes to reduce the communication link setup time and retransmissions. Everything is double-edged: if we want to shorten the operation time, a certain amount of hardware resources and computation complexity will be sacrificed. Nevertheless, the increased cost introduced by the digital circuit resources is almost negligible, and the added power consumption caused by computation complexity is incomparable when compared with the RF transceiver. That is because the RF transceiver consumes most of the power of the whole transceiver. So, it is reasonable to exchange operation efficiency with computation complexity. In this way, low-power IoT networks are obtained. For NB-IoT cell search, robust performance and rapid estimation are two major objectives. The NB-IoT cell search process can be divided into narrowband primary synchronization signal (NPSS) detection and narrowband secondary synchronization signal (NSSS) detection. During the above procedures, coarse time of arrival (ToA) detection, timing synchronization and carrier frequency offset (CFO) recovery are also essential to ensure reliable cell ID detection. The MCL of NB-IoT can be 164 dB, which demands that NB-IoT terminals can build communication links with the base station at a very low signal-to-noise ratio (SNR). On the other hand, under economical considerations, a low precision oscillator is adopted by NB-IoT devices. For this reason, frequency tracking must be conducted during the transmission. To describe the whole process and the problems encountered clearly, an overview of each procedure is presented. Firstly, when the downlink NB-IoT frames arrive, the NPSS detector will perform coarse ToA detection with the assistance of NPSS [7], and the detector is conventionally constructed with a timing metric function which is the ratio of sliding autocorrelation and a normalization factor. However, the NB-IoT needs to adapt a low SNR environment and large frequency offset, but the performance of the traditional timing metric function is seriously affected by noise and frequency offset. Secondly, after the coarse timing synchronization is completed, fine timing synchronization and frequency offset estimation will also be done in the time domain. Then, the NB-IoT cell ID is detected in the frequency domain. In legacy LTE, the cell ID is jointly determined by primary synchronization signal (PSS) and secondary synchronization signal (SSS) [2]. However, the cell ID of NB-IoT is decided by NSSS entirely, which has more diverse sequences compared with LTE [8]. Finally, when a stable communication link has been built up, frequency tracking is essential for NB-IoT systems because of the residual frequency offset after CFO recovery and the continuous frequency drift caused by low-cost oscillators. In the proposed cell search scheme, the fractional frequency offset (FFO) pre-estimator and noise elimination circuit are applied to promote the performance of NPSS detection. The IFO is compensated with a maximum likelihood (ML) cross correlation method in the time domain. Additionally, considering the timing uncertainty introduced by the multi-path fading channel, fine timing offset is estimated jointly by the aforementioned ML solution. Then, the fast Fourier transform (FFT) transforms the time domain signals into frequency domain signals. NSSS detection is conducted under the premise that symbol timing synchronization and CFO recovery have been completed.