# MAC Protocols for mmWave Communication: A Comparative Survey +
## 5 Distributed Protocols
### HetSNet in mmWave
The proposed method ensures backhaul links and high-capacity access. Therefore, in future 5G cellular networks, a combination HetSNets with mmWave bands will play a critical role. However, there are numerous issues with deploying HetSNets with the mmWave band 5G networks. Even though there is abundant literature on the PHY layer [53], only a few published studies in the literature have addressed the communication difficulties from the perspective of access and networking [32]. With the integration of mmWave communications with the HetSNet, a variety of deployment opportunities involving mmWave communication of backhaul user access lines have emerged [54]. Macrocell eNB (MeNB) is a standard HetSNet component. There are also other components such as multiple small cell eNBs (SeNBs), which can be further divided into relay eNBs, picocells, and femtocells. Before forwarding data to the MeNB in such a network, a combination process takes place. The backhaul data of every SeNB combines with the received data from the other nodes which exist in the network. Short distances (approximately 100-200 m) are supposed to separate the SeNBs, which helps mitigate high propagation losses. A mmWave radio may also provide user access coverage within small cells by lowering the interference level, which can be observed on the typical sub-3 GHz frequency ranges. Figure 7 presents a traditional HetSNet architecture where several scenarios are illustrated. In scenario 1, an SeNB is connected to its donor MeNB through a wired backhaul. Moreover, UEs are served by both MeNB and SeNB. In scenario 2, UE's communication with the MeNB takes place on the traditional microwave bands and with SeNBs, mmWave radio. In scenario 3, there is no wired backhaul where a single-hop mechanism is permitted for executing backhaul with mmWave band. For scenario 4, through single-hop mmWave wireless backhaul executes the backhaul for the SeNBs, and the SeNBs serve the UEs with mmWaves. Finally, in the 5 th scenario, dense small cells are deployed to connect multi-hop wireless backhaul between SeNBs and MeNB. Here, scenario 1 is the baseline, and scenarios 2-4 form a subset of scenario 5. Figure 7: Architecture of HetSNet. Figure 8 represents a standard routing protocol scheme. It demonstrates an example of a possible path from a SeNB to the MeNB, as well as the tandem queue associated with it. Each queue may receive data traffic from a variety of sources. Traffic from connections other than the one being considered can enter and exit the tandem system at any time. By thoroughly searching all possible routes, the best practicable path can be discovered. This technique, on the other hand, has a substantial signaling overhead and prohibitively high computing complexity. * Advantages: The HetSNet protocol implementation in the 3 GPP standards enables us to overcome numerous key mmWave communication difficulties while also achieving an aggregated cell throughput of almost 13 Gb/s, which is an order of magnitude higher than that of the current best 5G system design [55]. * Disadvantages: Routing in the HetSNet protocol can result in a substantial signaling overhead and a prohibitively high computing complexity. Even though the control overhead and delay may be reduced by applying a hierarchical routing scheme, however, the complexity still remains an open issue.