# Narrowband Internet of Things (NB-IoT):
## 3 Narrowband-IoT Protocol Stack
### Physical Layer
#### 3.1.1 Cell Acquisition and Synchronization  
NB-IoT UE goes through the same process as LTE UE where to camp on a cell, it goes through frequency and timing synchronization to obtain the center carrier frequency as well as the allocated slot and frame timing used for the cell acquisition. In general, if MIB and SIB are properly decoded, cell ID, a subframe number, scheduling information, and system bandwidth can be detected successfully. In NB-IoT, the low complexity of devices may lead to poor synchronization and cell acquisition performance, especially due to carrier frequency offsets and poor channel estimation capacity. The following are the papers that have proposed different solutions to optimize the initial cell acquisition and initial synchronization procedure. In [49], the authors presented a Maximum-Likelihood (ML) NPSS detector which is based on frequency domain cross-correlation metrics by using an overlap-save method. Their method achieves an average timing synchronization latency of 140 ms for the in-band deployed mode with SNR of \(-\)12.6 dB. Their proposed method showed a 34% reduction of the energy that is required for NPSS detection. However, their work showed only how much energy could be reduced with respect to the autocorrelation NPSS detection methods. It could be better to show how much of the total device's energy is consumed by their proposed computationally complex detector, i.e., it could be more realistic to include analysis in terms of reduction with respect to the energy consumed during time synchronization but also in terms of energy optimization over the total device consumption. In [50], the authors presented an algorithm for initial synchronization and cell search. The proposed algorithm uses NPSS for timing acquisition and initial Carrier Frequency Offset (CFO) estimation called the two-stage time domain NPSS correlation. They also used NSSS sequences for the cell ID and frame timing. Their proposed algorithm showed that under extremely low SNR and different fading conditions, NB-IoT could provide the required performance and could also quickly camp on the cell, if any. However, practical experiments are still needed to prove the feasibility of these simulations especially on how the newly introduced NB-IoT power class and actual channel variations could have an impact on the detected SNR at the base station. In [51], the authors presented an NB-IoT framework by using an advanced signal waveform called non-orthogonal spectral efficient frequency division multiplexing (SEFDM). This waveform uses less bandwidth as compared to OFDM waveform. The designed signal could improve the data rate without the need for more bandwidth. At the base station, the minimum Euclidian norm search detector is used for better error correction. The simulation results reveal that the proposed advanced signal waveform could achieve 25% improvement on data rate as compared to the OFDM signal waveform. The work also proposed an overlapped sphere decoding (OSD) detector which reduces the computation complexity as compared to the single sphere decoding detector while guaranteeing the needed performance. However, the model does not explain the impact of CFO due to the non-orthogonality of the subcarriers on the received signal.