Cell-search and tracking of residual time and frequency offsets in low power NB-IoT devices

摘要

Initial synchronization and cell-search in cellular based systems is the first step of establishing communication link between user equipment (UE) and base station (BS). Under this process UE aligns to the timing and frequency of the BS and acquires critical information like cell-ID, frame number and system bandwidth. Narrow band IoT (NB-IoT) is 3GPP defined cellular based IoT technology in which cell-search is facilitated through a set of two signals namely NB primary synchronization signal (NPSS) and NB secondary synchronization signal (NSSS). Across all BS, the same NPSS sequence is used. However, the NSSS sequences is used to distinguish them using a cell identity. We assume NPSS based initial synchronization is carried out to estimate timing and frequency offsets using existing traditional algorithms. In this paper, we first propose NSSS detection algorithm which is used to detect cell-ID and partial information of frame numbering. Post initial cell-search, UE has to be constantly synchronized to the BS in order to transmit or receive the data. For this purpose of periodic tracking of the timing and frequency offsets we make use of NB reference signals (NRSs). In the second part of the paper, we present an algorithm to track the residual time and frequency offsets using NRS which are distributed across frequency and time grid. We show novel algorithms that achieve significant detection performance even at low SNR values like - 12 dB which is the typical operating range for low power NB-IoT devices. The simulation results prove these claims.