# MAC Protocols for mmWave Communication: A Comparative Survey +
## 5 Distributed Protocols
### HetSNet in mmWave  
Heterogeneous and small cell networks (HetSNets) use hierarchical deployments to improve the spectrum efficiency and throughput in mmWave networks. Considering this, the authors in [52] proposed a new frame scheme on the basis of time division duplex, and \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Ref.** & **Protocol Name** & **Spatial Reuse** & **Anti-Blockage** & **Targeted Wavelength** & **Key Idea** & **Limitations** \\ \hline \hline
[37, 38] & Directive CSMA/CA & \
\begin{tabular}{c} Directive CSPMA/CA \\ \end{tabular} & Not specified & 60 GHz &
\begin{tabular}{c} Executing low overhead \\ action in congested networks, utilizing Markov decision process (MDP) to achieve high throughput \\ \end{tabular} & No priority assignments in the nodes \\ \hline
[39] & MRD-MAC & Not specified & \
\begin{tabular}{c} Support- \\ tied \\ \end{tabular} & 60 GHz &
\begin{tabular}{c} Multi-hoping feature enables robust linking \\ \end{tabular} & No consideration for concurrent connectivity \\ \hline
[40] & FD-MAC & \
\begin{tabular}{c} Support- \\ tied \\ \end{tabular} & Not supported & 60 GHz &
\begin{tabular}{c} Low complexity, great \\ fairness performance, considers psuedowired \\ interference models \\ \end{tabular} & Antiblockage related solutions are unavailable \\ \hline
[42] & CAD-MAC & Not specified & Not specified & 60 GHz & \
\begin{tabular}{c} Exhibits high network \\ throughput and energy efficiency \\ \end{tabular} &
\begin{tabular}{c} Performance at 60\({}^{\circ}\) and \\ 90\({}^{\circ}\) beamwidth is poor, consumes high power, exhibits long delay \\ \end{tabular} \\ \hline
[43] & RD-MAC & \begin{tabular}{c} Support- \\ tied \\ \end{tabular} & \
\begin{tabular}{c} Support- \\ tied \\ \end{tabular} & 60 GHz &
\begin{tabular}{c} Concurrent transmissions are exploited \\ \end{tabular} & No consideration of allocation capacity \\ \hline
[44] & BRD-MAC & \begin{tabular}{c} Support- \\ tied \\ \end{tabular} & \begin{tabular}{c} Support- \\ tied \\ \end{tabular} & 60 GHz & \
\begin{tabular}{c} Utilizing relay selection \\ to overcome blockage \\ \end{tabular} &
\begin{tabular}{c} Maximum two-hops are \\ allowed, and complex \\ blockage might need \\ more than two hop to \\ ensure robust link \\ \end{tabular} \\ \hline
[45] & D2D-MAC & \begin{tabular}{c} Support- \\ tied \\ \end{tabular} & \begin{tabular}{c} Support- \\ tied \\ \end{tabular} & 60 GHz & \
\begin{tabular}{c} Achieving near optimal \\ delay and throughput \\ \end{tabular} &
\begin{tabular}{c} NLoS transmission is not \\ considered \\ \end{tabular} \\ \hline
[47] & D-Coop MAC & Not specified & Not specified & 60 GHz &
\begin{tabular}{c} Creating two-hop links \\ via relay station \\ \end{tabular} & No backhaul networking scheme was integrated \\ \hline
[6] & VTSA & \
\begin{tabular}{c} Support- \\ tied \\ \end{tabular} & Not specified & 60 GHz &
\begin{tabular}{c} Maintaining low overhead and computational \\ complexity \\ \end{tabular} & Optimization is not incorporated \\ \hline
[48] & CTA-PSO & Not specified & \
\begin{tabular}{c} Support- \\ tied \\ \end{tabular} & 60 GHz &
\begin{tabular}{c} Efficiently distributing \\ resources even when \\ there is a blockage, reducing delay \\ \end{tabular} & High computational complexity \\ \hline
[49] & MHCT & \begin{tabular}{c} Support- \\ tied \\ \end{tabular} & \
\begin{tabular}{c} Support- \\ tied \\ \end{tabular} & 60 GHz &
\begin{tabular}{c} No novel relay selection \\ matrix is presented \\ \end{tabular} & Complex scheduling algorithm \\ \hline
[50] & REX & \begin{tabular}{c} Support- \\ tied \\ \end{tabular} & Not specified & 60 GHz & \
\begin{tabular}{c} Obtaining a significant \\ spatial multiplexing gain \\ \end{tabular} &
\begin{tabular}{c} Owing to scheduling \\ repatations computational complexity \\ increases with time \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparative analysis of centralized MAC protocols. the scheme is 3 GPP backward compatible.