# MAC Protocols for mmWave Communication: A Comparative Survey +
## 5 Distributed Protocols
### DtD-Mac  
Traditionally, it is considered that the wireless nodes in a network can perform both omnidirectional and directional transmission. Nonetheless, a node is unable to sense multiple directions simultaneously; thus, this leads to the deafness problem and increases the chance of collision. Consequently, in 60 GHz ad hoc networks, establishing communication links with an accurate directional network allocation vector (DNAV) is not feasible.  
Addressing these issues, [PERSON] et al. in [71] introduced a DtD-MAC for mmWave ad hoc networks. DtD-MAC stands for directional-to-directional (DtD) MAC protocol. In this architecture, the sending node collects the angle of arrival (AoA) of any existing incoming message. Concurrently, all nodes in the architecture also estimate the AoA of any incoming transmission. Afterwards, to predict the next-hop node's location, the node analyzes the cache before any sensing or transmission occurs. Moreover, the idle nodes of the networks reduce the crucial directional deafness problem by continuously swapping their sensing directions, be it clockwise or anticlockwise. In addition, the protocol also ensures that the receiver receives the information. A node sends multiple directional RTS before receiving a CTS from the receiver.  
* Advantages: The directional properties of the DtD-MAC enables both the senders and receivers to solve the asymmetry-in-gain problem. To capture the continually scanning idle receiver, the sender sends several DRTS packets to the receiver. These behaviors lead the protocol to mitigate the impact of the problems of deafness and collision of mmWave bands.
* Disadvantages: The primary aim of the DtD-MAC or neighbor discovery is associated with a disadvantage. Idle nodes must switch their sensing orientation either in clockwise or anticlockwise, and this makes it a power intensive protocol. During this process, extensive DRTS and CTS packets are exchanged, which may produce numerous handshake messages at the sender's end, and limit the performance of the protocol as the beam count increases.  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Ref.** & \
\begin{tabular}{l} **Protocol** \\ **Name** \\ \end{tabular} & **Scenario** & **Application** &
\begin{tabular}{l} **Simulation** \\ **Tool** \\ \end{tabular} \\ \hline
[69] & MD-MAC & \begin{tabular}{l} Outdoor mesh networks \\ \end{tabular} & \
\begin{tabular}{l} Deafness and interference reduction \\ \end{tabular} &
\begin{tabular}{l} QualNet \\ \end{tabular} \\ \hline
[70] & ALD-MAC & Not specified & Neighbor discovery &
\begin{tabular}{l} Not specified \\ \end{tabular} \\ \hline
[71] & DtD-MAC & Ad hoc networks & \
\begin{tabular}{l} Deafness and collision avoidance \\ \end{tabular} &
\begin{tabular}{l} QualNet \\ \end{tabular} \\ \hline
[30, 56, 57] & ALOHA & \begin{tabular}{l} Ad hoc networks, sensor networks, homogeneous mobile networks \\ \end{tabular} & \
\begin{tabular}{l} Multi-hop selection \\ \end{tabular} &
\begin{tabular}{l} Not specified \\ \end{tabular} \\ \hline
[61, 62, 64] & TDD, TDMA & Ad hoc networks & Access, slot allocation & Ns-3 \\ \hline
[52] & HetSNet & \
\begin{tabular}{l} Heterogeneous small cells \\ \end{tabular} & Access, backhaul &
\begin{tabular}{l} Not specified \\ \end{tabular} \\ \hline
[67] & PCDS & Small cells & D2D, content delivery & QualNet \\ \hline \hline \end{tabular}
\end{table}
Table 4: Applications of distributed MAC protocols.