# An Overview of Emergency Communication Networks
## 5 Sea Ad Hoc Network (SANET)
#### 3.1.3 Cellular Networks  
Cellular mobile communication adopts cellular wireless networking mode to connect terminals and network devices through wireless channels, so as to realize mutual communication between users. After developing the 1G, 2G, 3G, and 4G technologies, cellular mobile communication technology has entered the 5G era, and the development vision of 6G was proposed at the same time [87; 88; 89; 90]. The development trend from 1G to 6G is shown in Figure 7. Modern mobile communication technology can be mainly divided into low frequency, medium frequency, high frequency, VHF, and UHF frequency bands. In these bands, technicians can connect terminal devices in the mobile communication network, using mobile station technology, base station technology, and mobile switching technology to meet people's mobile communication needs. In emergency communication networks, the most significant advantage of mobile communication technology is the large communication distance, wide coverage area, and low cost. However, in the case of major natural disasters and public safety accidents, the public communication network may be interrupted and unavailable, and seriously affect the rescue effect.  
With the advent of MIMO technology and D2D communication, 5G is widely used in disaster management. 5G networks will integrate a variety of promising technologies, such as software-defined networks (SDN), software-defined wireless sensor networks (SD-WSN), coordinated multi-point processing (CoMP), MIMO, network function virtualization (NFV), and centralized radio access networks (CRANs) [91]. D2D communication in cellular networks supports direct communication between near-end mobile devices without the need for base stations (BSs) and core networks, and can be used to extend network coverage and enable infrastructure free connectivity. Therefore, D2D communication has become an indispensable candidate technology for next-generation wireless networks and an effective alternative to cellular communication during a disaster.  
Because of their effectiveness, hybrid networks consisting of existing cellular networks and D2D networks are popular in disaster recovery. [PERSON] et al. [92] proposed a hybrid network based on D2D and a ground infrastructure system. [PERSON] et al. [93] proposed a cluster-centered D2D network structure, which is based on cellular and ad hoc networks, and uses D2D communication as the LTE underlying network to obtain hop gain. [PERSON] et al. [11] integrated clustering technology with D2D communication into a cellular network so that communication services could be maintained if parts of the cellular infrastructure become dysfunctional. As part of the auxiliary network of emergency communication, UAVs meet the urgent needs of public security services. [PERSON] et al. [94] discussed the use of UAVs to broadcast an emergency alarm through a D2D-assisted multi-hop network, and established a multi-hop multi-cast D2D auxiliary network to  
Figure 7: Development trend from 1G to 6G.  
broadcast emergency information to victims. [PERSON] et al. [95] designed a framework called Finding Isolated Nodes Using D2D for Emergency Response (FINDER) to locate and reconnect isolated mobile nodes (MNs) in disaster areas, so as to avoid damage to assets and loss of life.  
Currently, 5G wireless communication networks are being deployed worldwide. However, in 2030 and beyond, 5G will not be able to meet all future requirements, and 6G wireless communication networks are expected to provide global coverage. Compared with 5G networks, 6G wireless communication networks are expected to provide higher spectrum/energy/cost efficiency, higher data rate, lower delay, and sub-centimeter geographic positioning accuracy.