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应急通信[8，9，10]是指广泛使用各种通信资源支持救援（包括紧急救援）以及在自然或人为紧急情况下的必要通信所需的通信手段和方法。由于灾害的特点，有效利用有限的资源提供最佳的通信服务是一个关键问题。与传统通信网络不同，应急通信网络是一个复杂的网络，能够实施应急救援活动，为实现应急救援功能提供首要条件。其网络结构对救援效率有重大影响。在不同的紧急情况下，对应急通信的需求不同，使用的技术手段也不同。应急通信主要具有以下特点：时间和地点的不确定性;容量需求的不确定性;应急通信的及时性高;环境的复杂性;通信网络本身损坏程度的不确定性;以及信息的多样化。
- broadband-narrowband integration
- emergency communication network
- narrowband private network
- satellite network
- space-air-ground-sea integrated networks
1. 简介
灾害发生前，应急通信网络可以结合不同系统获得的数据，为预警、减灾和预测提供技术平台，从而提高事故灾害的监测、管理和控制能力。灾后，应急通信网络可以快速可靠地恢复灾区通信，协助用户，用于救援人员，支持救灾指挥部，并实现现场人员调度、现场数据传输等功能，促进应急救援工作。这可以为救援提供宝贵的时间和第一手信息，从而挽救更多生命。随着自然灾害频发和国际安全形势的收紧，世界各国都在加强和完善应急通信系统建设和相关技术研究，努力部署更加可靠稳定的应急通信网络。
在公共通信网络设施损坏，性能突然下降和流量增加的情况下，有必要使用非常规和多通信手段的组合来恢复通信能力。应急通信网络[11]的重要特征如下所示：
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能够在灾难情况下和正常情况下工作;
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动态扩展网络以提供更多覆盖范围;
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准确传输灾害信息，提供综合服务;
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具有高可靠性、高冗余、高安全性、高抗干扰能力。
应急通信系统包括在紧急情况下各种通信技术和方法的集成和应用。因此，应急通信系统应充分利用成熟的通信技术和网络设备，并将它们集成到可扩展可靠的安全通信网络中。通常，应急通信网络使用现有的固定有线网络，移动蜂窝网络[12]，Internet和其他公共通信网络。还将使用专用通信网络，例如集群通信网络[13]，卫星通信网络[14，15]和短波通信网络[16，17，18，19]。此外，还使用公共媒体网络，如广播、电视和报纸，以及现场监控和救援网络，如传感器网络[20]和自组网络[21，22，23，24]。
应急通信网络是涉及多种通信技术的异构网络，其组成是多样的、动态的。近年来，通过对公共事件和自然灾害的研究，发现应急通信平台需要使用现有的公共网络或专用网络进行通信[25，26，27]，各级救援单位之间的通信需要互联互通。为了解决各种应急通信平台的互通问题，我国提出了一种基于互联网协议的综合通信系统，用于建设灾后应急救援通信平台[28，29，30]。一些研究人员提出了一种集成了卫星通信、4G和网状网络的互联通信解决方案[31，32]。目前，应急无线专用网也在向专业数字集群（PDT）窄带数字集群和B-TrunC宽带集群专网演进。此外，还提出了固定-移动一体化、窄带融合、公私网互补等多系统融合通信技术。鉴于5G通信网络具有高速率、高可靠、低时延、低功耗等特点，以及组网技术的快速发展[33]，无线自组网技术与5G通信技术的融合已逐步进入应急通信领域[34，35].未来应急网络将向融合化、普及化、低成本化、移动互联网融合、天-空地-海一体化网络方向发展[36，37]。新一代应急通信系统应集成多种通信技术，确保抢险救灾、重大活动等工作提供及时、高效、安全的应急通信服务。
2. 网络分类
应急通信网络为各类突发事件提供及时有效的通信保障，是综合应急保障体系的重要组成部分。对于任何国家来说，建立基于灾害通信骨干的有效应急通信是救灾的重要组成部分。应急通信网络根据不同的标准分为不同的类别。常用的分类标准如下：
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根据传输介质，网络可分为有线通信网络和无线通信网络。
有线通信网络以通信的形式使用有线传输介质。常见的通信形式是固定电话通信、电话线/网线接入互联网和光纤通信。无线通信网络使用无线传输介质。常见的通信形式包括微波通信、移动通信和卫星通信。
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根据属性，网络可分为公网和专用网。
公网是由运营商运营和管理的网络，通过公共用户网络接口连接各专网和用户终端。专用网络是指单位或部门内的网络。目前，专用网络主要使用以太网，异步传输模式（ATM）和各种无线技术。
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根据地理分类，网络可分为局域网 （LAN）、城域网 （MAN） 和广域网 （WAN）。
LAN 是一种分布范围从数百米到几公里的通信网络。城域网的传输覆盖区域主要在城市内，分布区域从数百公里到几十公里不等。广域网的传输覆盖区域包括各省、国家，甚至世界，范围从数百公里到几万公里不等。
2.1. 卫星网络
卫星通信技术是两个或多个地球站之间的通信方法，使用人造地球卫星作为中继站来转发无线电波。卫星通信系统通常按卫星轨道高度分类，可分为地球同步轨道（GEO）卫星、中地球轨道（MEO）卫星和低地球轨道（LEO）卫星。
卫星通信系统主要由空间、地面和用户段组成，如图1所示。航天段：以通信卫星为主体，用于接收和转发来自卫星通信地面站的信号，实现地球站之间或地球站与航天器之间的通信。地面部分：包括卫星转发器，网关站，卫星控制中心（SCC）和跟踪，遥测和指挥站（TT&C），实现用户之间的通信。用户细分：主要由各种终端用户设备组成，包括安装在汽车、船舶、飞机上的甚小型天线终端（VSAT）站、手持终端和移动终端，以及基于卫星通信的各种应用和服务。
图1.卫星网络通信系统（PLMN：公共陆地移动网络，PSTN：公共交换电话网络）。
卫星通信具有不受地面条件限制、通信覆盖范围广、容量大、可靠性高、传输距离远、独立通信能力强、抗损坏能力强等优点，非常适合应急通信。Ran等人[59]彻底分析了灾难后卫星通信的有用性。Iapichino等人[60]提出了一种基于卫星的混合网络，其中地面网络和卫星网络混合在一个网状无线通信协议平台上。
2.2. 自组网络
1991年，IEEE正式提出“自组网”一词[64]。自组网技术采用"点对点结构"的移动通信模式，无需基站。与其他网络形式相比，自组网是一种特殊的移动通信网络，无需相关基础设施预设即可快速联网。自组网具有高破坏性、自愈性和自组织性的特点，特别适合在临时或突发突发事件中进行通信。在临时基础上，已经扩展了各种网络：无线网状网络（WMN）[65，66]，无线自组织网络（MANET）[54]，车辆自组网（VANET）[67]，飞行自组网（FANET）[68，69]和海上自组网（SANET）。
1. 无线网状网络
WMN [70]是基于无线网络发展和自组网扩展的新型网络产品。其网络架构如图2所示。WMN的主要优点是：
图2.WMN的结构图。
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WMN是一种多跳网络，自动选择最佳路径，避免因障碍物导致通信故障，从而为不在视线范围内的用户提供服务传输能力。
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WMN具有灵活多样的组网模式，支持动态拓扑组网，为高移动性需求的应用场景提供有效的组网能力，保证用户间连接质量，扩大网络覆盖范围。
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WMN具有自组织、自愈、自平衡能力，网络可靠性强。
WMN作为一种新型的无线自组织网络，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4G等移动通信网络和无线个人局域网（WPAN）等城域网的延伸，提供更广泛的无线接入覆盖，全面提升应急通信质量。WMN主要用于矿山应急救援。近年来，它在消防[74，75]和地震和其他灾害方面的应用也得到了更广泛的应用。
2. 移动自组织网络
MANET是一种基于自组网模式的新型宽带WMN。其结构如图3所示。MANET利用移动终端的路由和转发功能，在没有基础设施的情况下进行通信，从而弥补了可用的网络通信基础设施的不足。马奈的特点是：
Figure 3. Structure diagram of MANET.
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MANET is a distributed, multi-hop relay wireless broadband system without a center, and provides dynamic routing, strong damage resistance, and good scalability. Its network topology changes dynamically, and it internally uses its own routing protocol to complete wireless communication among nodes through wireless multi-hop forwarding.
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MANET has the advantages of having a low deployment and maintenance cost, large coverage, high rate, strong network robustness and adaptability, and link self-sensing and self-healing. It can not only exist as an independent wireless self-organizing network, but also serve as an effective supplement and expansion of an existing heterogeneous network system.
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MANET can be widely used in military communication private networks, public security private networks, emergency communication private networks, regional broadband private networks, and wireless monitoring private networks.
Due to its significant advantages, MANET is widely used in emergency communications in order to provide effective communications during natural disasters. For example, Chen et al. [76] developed a hybrid MANET topology with the advantages of fast deployment, simple configuration, and convenient interoperability. The proposed solution can provide network access for post-earthquake emergency rescue services to meet the communication requirements of rescue work.
3. Vehicular Ad Hoc Network (VANET)
VANET is a new mobile ad hoc network created by applying the principles of MANET to vehicles. VANET is composed of self-organizing vehicles as mobile nodes, and the communication between nodes can be divided into vehicle-to-vehicle (V2V), vehicle-to-roadside (V2R), and vehicle-to-infrastructure (V2I). Its network structure is shown in Figure 4. VANET is a wireless communication technology that can be deployed quickly, which simplifies data exchange between vehicles and provides a variety of data services [79]. VANET is expected to enter the application field of disaster communication due to its features of connecting emergency vehicles and road navigation.
Figure 4. Structure diagram of VANET.
In recent years, as automotive technology has advanced, VANET has played an important role in saving drivers’ lives and property by disseminating critical incident information. The important purpose of VANET architecture is to provide efficient and effective emergency and warning alerts for vehicles, so that vehicles can make appropriate decisions without delay to safeguard passenger safety [80].
4. Fly Ad Hoc Network (FANET)
FANET is a self-organizing wireless network that enables communication between flying nodes, such as UAVs, without a fixed network infrastructure, making it low cost, flexible, and easy to deploy. FANET can be regarded as an extended paradigm of MANET, realizing communication between UAVs and between UAVs and control stations. FANET not only has the inherent characteristics of a wireless ad hoc network, such as multiple hops, self-organization, and centrality, but also has some functions related to UAVs, such as the rapid change in network topology caused by high-speed moving nodes, sparsity of network nodes and network heterogeneity, network node peers, and data relay capability. UAVs have potential advantages in wireless communications because they can transfer data from the air to one or more ground stations [82], so they can be widely used in disaster areas where communications are completely shut down due to an unexpected disaster.
5. Sea Ad Hoc Network (SANET)
SANET is a wireless multi-hop network composed of nodes (e.g., ships, offshore buoys, and beacons). As shown in Figure 6, connections can be established between these nodes, or with coastal base stations and satellites [85,86]. SANET has characteristics that are similar to those of other wireless networks: composition made up of mobile nodes; high processing capacity; no energy restrictions; and high data storage capacity. However, there are some differences in many areas, such as power consumption, topology, speed, and deployment changes.
Figure 6. Structure diagram of SANET.
SANET can be used for maritime communications, navigation, and emergency response.
6. Comparison with other ad hoc networks
Ad hoc networks are also known as infrastructure-free networks, non-central networks, multi-hop networks, and self-organizing wireless networks. Ad hoc networks have no fixed infrastructure, and their nodes can be vehicles, mobile devices, aircraft, and ships, which can dynamically maintain contact with other nodes in any way. Ad hoc networks include MANET, VANET, FANET, and SANET. VANET is a subset of MANET, FANET is a subset of VANET, and SANET is a popular type of self-organizing network. Table 2 summarizes the differences between different ad hoc wireless networks. An ad hoc network is an ad hoc wireless network composed of communication nodes that do not depend on the infrastructure, in which each node can act as a host and router. Ad hoc networks have been gradually applied to many key scenarios, such as emergency communication, rescue and disaster relief, and expanding the coverage of existing cellular networks, due to their fast network speed and robustness. Therefore, ad hoc networks are an important part of next-generation wireless communication network systems.
Table 2. Comparison of ad hoc networks.
| Parameter | MANET | VANET | FANET | SANET |
|---|---|---|---|---|
| Number of nodes | high | high | low | low |
| Propagation model | on the ground | on the ground | in the air | on the sea |
| Topology | ad hoc and random | star with roadside infrastructure and ad hoc among vehicles | star and mesh (with base station, ad hoc among UAVs) | star and mesh (with base station, ad hoc among ships) |
| Topology change | dynamic, unpredictable, and low | average speed | fast | medium |
| Line of sight | not available for all cases | available in some cases | available in most cases | available in some cases |
| Power consumes | low | high | high for mini-UAV, small UAV not needed | high |
| Mobility | low | high | very high | medium |
| Mobility model | random | regular | regular under the condition | random |
| Main functionality | real-time communication | real-time communication | real-time communication | real-time communication |
| Computational power | limited | average | high | high |
| Density | low | high | very low | high |
| Localization | GPS | GPS, DGPS, AGPS | IMU, GPS, DGPS, AGPS | GPS, DGPS |
2.3. 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.
Figure 7. Development trend from 1G to 6G.
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。
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L波段提供低速数字语音、数据通信、传真和电报服务。Inmarsat的第一代至第四代卫星用户链路均采用L波段，而Inmarsat第五代卫星使用基于DVB-S2技术的Ka波段，并支持上行链路为5 Mbit/s和下行链路为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。WMN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。
目前，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WMN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WMN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LTE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。
目前，应急通信行业的专网项目正逐步从窄带数字集群向LTE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-LTE宽带集群增加关键区域范围。利用LTE系统的高带宽实现视频和高速数据业务。4G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]。
This entry is adapted from the peer-reviewed paper 10.3390/rs15061595
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