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.

2.4. Wireless Private Networks

In the case of natural disasters and public security and other emergencies, due to the interruption of public communication networks or the inability to use public communication networks because of security reasons, it is often necessary to restore or supplement on-site communication in a short time using private networks. According to the technology used, emergency private wireless networks can be divided into five stages, namely, analogue cluster/conventional, narrowband digital cluster/conventional, 3G cluster, 4G broadband cluster, and 5G evolution. With digital wireless communication technology, digital cluster and conventional digital technology have been developed rapidly. PDT in China, Tetra and Digital Mobile Radio (DMR) in Europe, and P25 technology in the United States represent the narrowband digital cluster. As public network communication is evolving from 4G to 5G, digital cluster private networks are also gradually evolving in the direction of broadband multimedia [96]. 5G applications in the emergency industry will use 5G and artificial intelligence (AI) as the main approach and vigorously develop related applications. Wireless private networks, represented by digital trunking wireless communication systems, have the characteristics of high confidentiality, high reliability, and low service cost. They are mainly used in specific fields such as public security, fire protection, rescue services, and the petrochemical industry. Private network mobile communication systems include interphones, non-central digital systems, and trunking communication systems. An interphone is a two-way communication tool that is suitable for real-time communication, emergency dispatching, and collective cooperation. If natural disasters destroy the public network infrastructure, thereby preventing communication, the role of interphones will be more obvious. A trunked mobile system is a special dispatching communication system that is part of the advanced development stage of the private radio dispatching network. The system can dynamically, automatically, quickly, and optimally distribute the limited channels to all users of the system, so as to make maximum use of the channel frequency resources of the whole system. To date, an 800 MHz wireless trunking mobile communication system has been gradually established in various cities in China. Trunked mobile communication networks can be divided into two types: Public Access Mobile Radio (PAMR) networks and Private Mobile Radio (PMR) networks. To date, the commonly used standards in digital private communication include DMR, Digital Private Mobile Radio (DPMR), and PDT. The DMR standard adopts Time Division Multiple Access (TDMA) technology, supports single call, full call, group call, and selective call modes, and also supports direct mode communication without a base station. DPMA is a narrowband (6.25 KHz) Frequency Division Multiple Access (FDMA) technology that can provide various forms of voice/data applications. The PDT standard adopts TDMA multiple access mode and is based on the Chinese public security market. It not only supports low-cost single base station system communication, but can also achieve efficient regional coverage. While satisfying basic business requirements, it also adds innovative functions such as simulcast and dynamic frequency resource management. In earthquakes, wind disasters, fires, and other emergency events, the private communication network can quickly access the rescue dispatching platform, to realize flexible networking, efficient command and dispatching, high-quality voice and data transmission, and other functions. LTE is a long-term evolution technology based on 3GPP. LTE networking has the following advantages: it has a fast wireless network communication rate and high throughput, which can provide strong support for extensive services; with a slight delay, it can meet most application scenarios having high real-time requirements; and the LTE base station can be conveniently deployed and used to supplement the existing wireless communication infrastructure [97]. LTE is mainly applied in private networks, such as for emergency communication, smart construction sites, and forest fire prevention. In critical areas, the 4G-LTE broadband cluster is used for signal coverage, and the high bandwidth of the LTE system is used to realize video and high-speed data interaction and other services.

3. Summary of Network Development Status

3.1. 370M Narrowband Private Networks

The 370M narrowband private network was the first narrowband wireless communication network used for emergency command in China. When major disasters occur, there are often “three noes”, of no power, no network, and no road. In this case, the narrowband private wireless network significantly improves the capability of ensuring communication. China adopted the domestic PDT standard with independent intellectual property rights to build a 370M emergency command wireless communication network, as well as a public network cluster and emergency communication equipment at the rescue site, to meet the demand of daily wireless command scheduling and sudden emergency communication, and finally connect the 370M PDT communication system of the national emergency management department to the Internet. The PDT communication system is deployed as a combination of fixed and mobile networks, with mobile base stations as the main part and fixed base stations as the auxiliary for digital cluster networking. It can be applied in fire protection, earthquake, and other fields to provide communication support for emergency rescue.

3.2. Mobile Satellite Networks

The International Telecommunications Union (ITU) divides satellite communication services into three categories: fixed-satellite service (FSS), mobile satellite service (MSS), and broadcasting satellite service (BSS). The traditional emergency satellite communication is mainly an FSS system, which uses a C/Ku high orbit satellite to construct the VSAT satellite communication system. However, in recent years, the MSS system has played an increasingly important role in major natural disasters, especially within 72 h of the disaster. When the ground network cannot be used, the rescue organization needs to use the MSS system to understand the real-time messages in the disaster area, so as to organize effective rescue activities. 目前，世界上大多数通信卫星主要是At present, most communication satellites in the world are mainly GEO卫星，覆盖范围广，位置相对稳定。然而，随着轨道空间日益拥挤以及蜂窝通信、多址、点波束等小卫星技术的发展，低地球轨道卫星已逐渐应用于天基通信系统;这些卫星具有传输延迟短、路径损耗小、频率重用更有效等特点。一些国家相继发射了大规模的低地球轨道卫星系统。 satellites, which have wide coverage and a relatively stable location. However, with the increasingly crowded orbital space and the development of cellular communication, multiple access, spot beam, and other small satellite technologies, LEO satellites have gradually been application in space-based communication systems; these satellites have short transmission delay, small path loss, and more effective frequency reuse. A number of countries have successively launched large-scale LEO satellite systems. 国际海事卫星组织是世界上第一个全球移动服务卫星通信系统。用户链路使用Inmarsat was the first global mobile service satellite communication system in the world. The user link uses the L波段提供低速数字语音、数据通信、传真和电报服务。-band to provide low speed digital voice, data communication, facsimile, and telegraphy services. Inmarsat的第一代至第四代卫星用户链路均采用L波段，而’s first to fourth generation satellite user links all adopted the L-band, while the Inmarsat第五代卫星使用基于 fifth-generation satellites use the Ka-band, based on DVB-S2技术的Ka波段，并支持上行链路为 technology, and support broadband services having uplink of 5 Mbit/s和下行链路为 and downlink of 50 Mbit/s的宽带业务[100]。.

3.3. 混合网络

应急通信网络是典型的复杂网络，涉及多种通信技术。它们的网络组成是多种多样的，并且是动态变化的。WIridium utilizes on-board processing, on-board switching, and interplanetary link technologies, and uses TDMA multiple access for direct communication between handheld terminals and satellites, providing users with voice, data, paging, fax, and other services. “Iridium N作为一种新型的自组织无线网络，不仅可以与卫星、短波等应急通信方式有效结合，还可以作为移动通信网络和城域网的延伸，提供更广泛的无线接入覆盖，全面提升应急通信质量。EXT” mainly aims at IP broadband networking and the scalability and upgrading of load capacity, which will enable it to meet the complex needs of future spatial information applications. 目前，Starlink adopts a new connection protocol that is more lightweight than IPv6 and supports P2P. It is a decentralized network similar to blockchain. Satellites can accurately understand the longitude and latitude of each user’s physical location and reasonably distribute their data links. Starlink’s second generation system will employ inter-satellite laser links to provide seamless network management and service continuity. Starlink users access the Internet by converting the satellite signal into a WMN已应用于消防、水利防洪、电力救援、铁路救援、海上执法、海洋监视评估、森林防火等领域。中国地震局[102]提出了一种基于卫星和i-Fi signal through a WMN技术的地震应急场通信系统。通过灾区多跳网状网络、卫星通信车、后地面固定通信基站，实现了i-Fi router. The Oneweb system has set up more than 50 gateway stations around the world, and its user link adopts the Ku-band. The user terminal can be used as a local Internet access point, and users can access the Internet through 3G, LTE, WMi-Fi, or satellite links. The BeiDou satellite system is an active satellite positioning and communication system independently developed by China. It is the third satellite navigation system covering the world after the Global Positioning System (GPS) of the United States and the Global N与后方指挥中心的互联互通。对于煤矿应急通信系统，关超[103]提出了一种基于WMavigation Satellite System (GLON的地下救灾通信系统。该系统以由多个网状接入点、路由器和网关构建的无线网状骨干网为网络架构，利用地下通信功能系统和地面指挥中心实现煤矿应急救灾，满足现代矿山需求。网状和LASS) of Russia. BeiDou satellites can not only provide rapid positioning and timing services for ground users worldwide, but also enable short digital message communication, which is one of the means of ensuring communication in an emergency. The Hongyan global satellite constellation communication system is planned to be composed of 300 low orbit small satellites and a global data service processing center, with all-time two-way communication capability under complex terrain conditions. It will provide users with global real-time data communication and comprehensive information services. The Hongyan constellation will integrate a number of satellite application functions. Its satellite data acquisition function can realize large-scale regional information collection and meet the needs of monitoring data information transmission in the fields of oceanography, meteorology, transportation, environmental protection, geology, and disaster prevention and reduction. According to the existing information, it is expected that the system will include a communication network of 60 artificial satellites by 2023. By 2025, the second phase will be completed, and a mobile satellite communication and network access system having space-air-ground-sea integration will be built. The Hongyun project is a “star chain plan” independently developed by China. It is expected that 156 satellites will be launched into orbit 1000 km above the ground. The project is committed to building a satellite-borne broadband global mobile Internet that can provide integrated services for communication, navigation, and remote sensing for all types of users around the world. Because of its extremely low communication delay, extremely high frequency reuse rate, and real global coverage, the Hongyun project can meet the needs of China, underdeveloped Internet areas, and large-scale user units to share broadband Internet access. At the same time, it can also meet the application requirements of high real-time information interaction, such as in emergency communication, sensor data acquisition, industrial Internet of TE混合组网因其组网灵活、网络鲁棒性好、无线网络带宽高、支持非视距通信、网络规模大等优点，被广泛应用于应急通信。hings, and remote control of unmanned equipment. 目前，应急通信行业的专网项目正逐步从窄带数字集群向LThe Tiantong-1 satellite mobile communication system is a satellite mobile communication system independently developed and constructed by China. It is also an important part of China’s space information infrastructure, and solves the problem of seamless signal coverage in a particularly complex environment. The Tiantong-1 01, Tiantong-1 02, and Tiantong-1 03 satellites were successfully launched in 2016, 2020, and 2021 respectively. After the Tiantong-1 03 satellite was launched into orbit, it was networked in orbit to the Tiantong-1 01 and Tiantong-1 02 satellites. The TE宽带集群转变，宽带-窄带融合也成为重要的发展方向。采用窄带集群系统实现全覆盖，实现广域语音通信，4G-LTiantong-1 satellite mobile communication system will achieve regional coverage in the Asia-Pacific region. It has the advantages of strong reliability, strong real-time performance, strong confidentiality, wide coverage, and long communication distance. It can provide stable and reliable voice, short message, video, and other mobile communication services, and improve the service level of satellite communication and emergency communication support capacity. The Tiantong-1 satellite system is a narrowband mobile satellite communication system. In case of natural disasters, the emergency communication capability of Tiantong-1 can play a significant role.

3.3. Hybrid Networks

E宽带集群增加关键区域范围。利用Lmergency communication networks are typical complex networks, involving a variety of communication technologies. Their network composition is diverse and changes dynamically. As a new type of self-organizing wireless network, WMN can not only effectively combine with emergency communication methods such as satellites and short-wave, but can also be used as an extension of mobile communication networks and metropolitan area networks to provide more extensive wireless access coverage and comprehensively improve the quality of emergency communication. Under 5G with high bandwidth and low delay, WMN has more technical advantages, improving the synthesis, application, and systematization. TE系统的高带宽实现视频和高速数据业务。4he unique advantages of the ad hoc network enable it to occupy a position in the field of emergency communication. In the future, ad hoc networks will have an ideal space for improvement in miniaturization and portability. In the application of emergency communication, the integration of mesh networks with other private network communication and satellite communication systems will also be the focus of future development. Future development will also include the integration of 5G专网通信可以提供矿井全无线覆盖，实现语音、数据、视频网络的高带宽无线集成，为煤矿数字化提供无线平台。针对地震现场工作，中国地震局提出，通过在重灾区建设4G应急指挥调度系统，通过构建宽带应急集群专网系统，实现现场人员调度、实时视频传输、现场数据传输等功能，缓解临时通信终端问题[116]。 with satellite networks, 5G with UAV integration, and the space-ground convergence network.

3.4. 5G 网络Networks

5G网络技术[117]包括网络切片技术、超密集异构网络技术和多接入边缘计算技术。在应急救援工作中，可以利用网络切片技术实现各种设备的互联互通，从而全面提高应急救援指挥效率，促进救援工作的发展[118]。在 network technology includes network slicing technology, ultra-dense heterogeneous network technology, and multi-access edge computing technology. In emergency rescue work, the network slicing technology can be used to realize the interconnection of various devices, thus comprehensively improving the efficiency of emergency rescue command and promoting the development of rescue work. With the support of 5G网络技术支撑下，各地需要加快应急救援通信设备建设，特别是用于网络控制中心、卫星地面站、融合通信平台的设备。同时，还要利用应急通信设备扩大网络覆盖，推动应急救援工作的发展。 network technology, all regions need to speed up the construction of emergency rescue communication equipment, especially that used for the network control center, satellite ground station, and converged communication platform. At the same time, it is also necessary to use emergency communication equipment to expand network coverage and promote the development of emergency rescue work.

4. Future Development Directions

With the rapid development of network technology and information technology, future emergency networks will develop in the direction of intelligence, integration, popularization, and low cost. This will mainly introduce network intelligence and integration, and lead to the development trend of space-air-ground-sea integration networks.

Due to limitations in the radio spectrum, service geographic coverage, and operating costs, 5G ground communication networks alone cannot provide high-quality and reliable services at any time and in any place. This is especially the case in remote areas such as oceans, forests, and deserts. In order to provide truly ubiquitous wireless communication services on a global scale, it is necessary to develop an integrated space-air-ground-sea network to achieve global connectivity. The rapid development of wireless, broadband, fusion, and three-dimensional network technology provides the construction foundation for the integration of the space-air-ground-sea network with information sharing, interconnection, and unified command. Integrated use of big data, cloud computing, AI, satellite communications, multimedia communications, mobile communications, and other advanced technologies, and integration of space-based, air-based, ground-based, and sea-based platforms, can be used to construct the space-air-ground-sea integrated emergency communication network architecture. This architecture can realize the overall emergency resource planning and collaborative services of multiple platforms. The network architecture is shown in Figure 7. The integrated space-air-ground-sea network is based on a ground network, extended by air-based and sea-based networks, and connected and supplemented by a space-based network; thus, it covers the natural space including space, the sky, the ground, and the ocean. The air-based network consists of satellites, which can be divided into GEO, MEO, and LEO satellites. The space-based network is composed of aerial platforms such as near-space airships, airplanes, UAVs, and hot-air balloons. The ground-based network is mainly composed of LANs and cellular mobile networks, including all wired and wireless network facilities on the ground. Currently, this mainly relies on 5G network technology. The sea-based network consists of coastal base stations, surface ships, buoys, etc.

Figure 7. System topology of the space-air-ground-sea integrated emergency communication network.