# Narrowband Internet of Things (NB-IoT):
## 3 Narrowband-IoT Protocol Stack
### Physical Layer
#### 3.1.1 Cell Acquisition and Synchronization
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Feature** & **Article** & **Technique Used** & **Enhancement Criteria** & **Limitation** \\ \hline \multirow{6}{*}{Interference mitigation} & \multirow{3}{*}{[65]} & \multirow{3}{*}{Channel Equalization algorithm} & Intersymbol Interference mitigation by the phase-shifted channel frequency responses (CFR) to conquer the sampling mismatch between NB-IoT and base station & The proposed model did not consider the NPSS and NSSS impact ON time and frequency synchronization and base station & The proposed model did not consider the NPSS and NSSS impact ON time and frequency synchronization \\ \cline{1-1} \cline{2-4}  & & & & \\ \cline{1-1} \cline{2-4}  & \multirow{3}{*}{[66]} & \multirow{3}{*}{Mathematical model for sample duration in LTE and NB-IoT system} & Interference and close-form interference analysis due to sampling mismatch between NB-IoT and base station & The model is computational complex when implemented in NB-IoT systems \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._In [52], the work investigated the downlink synchronization signal design and proposed the novel general synchronization signal structure with a couple of Zadoff-Chu (ZC) conjugated sequences in order to remove the potential timing errors caused by large frequency offsets. Their new synchronization signal structure demonstrated better functionality with the frequency offset tolerance of up to 40 kHz. However, the model does not explain the number of samples per symbol involved in synchronization operation and decision. In [53], the random access preamble is discussed based on the design of NPRACH for single-tone frequency hopping only. It introduces the new single-tone frequency hopping random access signal used by NPRACH in NB-IoT systems. It further explains the design rationale and proposes some possible receiver algorithms for NPRACH detection and ToA estimation. The simulation results show that NPRACH performance is improved. However, the paper did not discuss the impact of massive interference which may result in lower received Signal Interference plus Noise Ratio (SINR) at the base station such that the lower the SINR the lower the detection probability of NPRACH. In addition to that the higher the number of devices the higher probability of NPRACH preamble collision. So lower SINR detected at base station and higher collision probability both affect NPRACH detection negatively. In [54], the authors described a NPRACH design as specified in 3 GPP in standard in Release 13. They proposed a receiver algorithm for the NPRACH timing advance estimation as well as detection. The simulation results for the NPRACH detection shows that if one preamble sequence is transmitted, the detection threshold should be set between 55% to 70% of the average value to satisfy the desired NPRACH performance at the lowest SNR. The results also showed that at 5 and 11 preamble sequence transmission, the detection threshold should be 50% and 35% of the average value, respectively. It is noted that increasing the detection threshold lowers the false alarm probability, which leads to an increased likelihood of misdetection. In [55], the authors provided a mathematical model of an NB-IoT network in order to predict the optimum performance with a specific configuration of some design parameters (i.e., repetition, number of the preamble in NPRACH per second, coverage classes and intersite distance). The paper analyzes the effects of parameter choice in outdoor, indoor, and deep indoor. The work finally proposes how to choose the optimal configuration i.e., by providing the highest throughput, as well as success probability higher than minimal success probability with minimal one being of 90%. The work showed that even though the success probability has a maximum limit, it can still be altered by modifying the number of repetitions to enhance the coverage or the system capacity in terms of throughput.