# Narrowband Internet of Things (NB-IoT):
## 3 Narrowband-IoT Protocol Stack
### Media Access Control Layer  
Handling retransmissions (HARQ), multiplexing, random access, timing advance, choice of transport block formats, priority management, and scheduling are the tasks executed by the MAC layer. The discussion on this part focuses on features such as radio resource management, link adaptation, coverage, and capacity improvement, power, and energy consumption reduction, as summarized in Table 3. \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** & **Limitation** \\ \hline \hline \multirow{9}{*}{[68]} & \multirow{3}{*}{Resource blanking} & \multirow{3}{*}{Resource blanking} & \multirow{3}{*}{Interference cancellation by resource blanking} & The proposed technique may lead to performance degradation in terms of spectral efficiency, especially for NB-IoT massive deployment. \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5} Resource allocation & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \hline \multirow{9}{*}{[71]} & Resource allocation & \multirow{3}{*}{[70]} & Resource allocation & power consumption reduction for NB-IoT UE during paging loading and offloading & The use of specific PRB for paging offloading is not an efficient use of the existing resource blocks. Also, the model is not applicable in standalone mode. \\ \cline{3-5}  & & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \hline \multirow{9}{*}{[72]} & NB-IoT scheduling algorithm & \multirow{3}{*}{\begin{tabular}{} \end{tabular} } & \multirow{3}{*}{\begin{tabular}{} \end{tabular} } & \begin{tabular}{} \end{tabular} \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \hline \multirow{9}{*}{[73]} & \multirow{3}{*}{\
\begin{tabular}{} \end{tabular} } & Offset index selection and UE specific and common search spaces for NB-IoT dense networks & Cell capacity enhancement by means of optimal scheduling & Did not consider the number of sessions that each device has to transmit with respect to different requirements and use cases \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \hline \multirow{9}{*}{
\begin{tabular}{} \end{tabular} } & Two-dimensional NB-IoT dynamic link adaptation algorithm by using the preferential application number and NB-IoT supported & Optimization of repetition number by dynamically adjusting MCS to ensure better BLER and BER performance & the model does not encompass the effect of speed and the deployment of the optional HARQ process to ensure better channel modeling \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \hline \end{tabular}
\end{table} 
Table 3: Articles on proposed MAC layer enhancement techniques