# Narrowband Internet of Things (NB-IoT):
## 3 Narrowband-IoT Protocol Stack
### Physical Layer
\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** & **Enhance Criteria** & **Limitation** \\ \hline \multirow{6}{*}{[50]} & [49] & Maximum-Likelihood (ML) & Average latency reduction for timing synchronization & It is a computationally complex detection method \\ \cline{2-5}  & & & Time and frequency synchronization by using NPSS and NSSS with two-stage time domain & mobility and new NB-IoT transmit power are not considered which have a direct impact on inter-RAT mapping and the detected SNR, respectively \\ \cline{2-5}  & & Non-orthogonal spectral efficient frequency division multiplexing (SEPDM) waveform and an overlapped sphere decoding (OSD) detector & Resource optimization by the use of less bandwidth with better data rates compared to OFDM signal waveform & The proposed method would lead to mismatch, carrier frequency offset and also will need to raise the computation complexity to NB-IoT UE \\ \cline{2-5}  & & New synchronization signal structure with Zadoff-Chu conjugates & Minimization of timing errors due to low-complexity NB-IoT & If the same model is used for uplink synchronization it might lead to estimation errors if mobility is involved in NB-IoT \\ \cline{2-5}  & & & The algorithm might not work for multi-tone allocation. Also, frequency hopping may raise power consumption as well as device complexity \\ \cline{2-5}  & & & Modeling the detection threshold to satisfy the NPRACH performance by lowering the probabilities of false alarm & The paper did not explain how receiver sensitivity can affect the NPRACH detection \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Articles on the proposed PHY layer enhancement techniques. Figure 4: NB-IoT Uplink Frame Structure, **(A)** when 15 kHz spacing is used with different tone-allocation possibilities with slot duration of 0.5 ms and **(B)** when 3.75 kHz is used only single-tone allocation is supported with 4 times longer slot duration (2 ms). \begin{table}
\begin{tabular}{p{108.4 pt} p{113.8 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}{*}{Cell Acquisition} & \multirow{3}{*}{[55]} & \multirow{3}{*}{Mathematical modeling of NB-IoT performance} & \multirow{3}{*}{Throughput enhancement and NPRACH optimization by the use of repetition number, NPRACH} & The work did not include some parameters such as the impact of mobility and how the achieved MCL for different coverage classes can impact the repetition assignment \\  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \hline \multirow{6}{*}{[56]} & \multirow{3}{*}{[57]} & \multirow{3}{*}{Configurable signal propagation model} & \multirow{3}{*}{System performance analysis in terms of number of supported devices, BER performance, preamble retransmissions, etc.} & The impact of preamble retransmissions on the overall transmission latency is not considered \\ \cline{3-5}  & &