# Communication Technologies in Emergency Situations
## 4 Emergency Communications Technologies--A Systematic Literature Review
### Results
#### 4.2.1 Short Range Communications  
Most of the research works identified in this systematic review are addressing this category. As stated previously, not only it is crucial for saving lives and alleviating post-disaster consequences, but these are the most affected communications within an emergency. Within this category, Aerial Wireless Networks is the most researched topic, but, as it will be described herein, it is addressed from many different perspectives by the research community, as, indeed, the most suitable technology and the main issue to be solved will depend on the size of the stricken-area to be covered, the density and mobility of people/objects to be connected, the type and amount of data to be transmitted, and the available information regarding the location of all the network nodes, etc. For example, in [24], they focus on a large emergency area (4 km \(\times\) 4 km) and, thus, use multicopters to construct a wireless relay network in the sky, which serves as a backbone network. Each multicopter operates as an access point and accommodates user nodes on the ground. Then, a packet that is generated by a source node is transmitted to a destination node through the AWRN. If a multicopter exists in the communication range of another multicopter, they can forward packets to each other. Otherwise, the packets are conveyed by the movement of the multicopters. In this scenario, it is not easy to supply many multicopters to cover the emergency area, and thus, the main challenge is how to reduce the significant delay time of packet transmission caused by multicopters' movement. Minimizing the delay time depends on how the multicopters move, which is referred to as flight schemes in the literature, and ref. [24] (considering 16 multicopters) is a good reference for any researcher interested in this topic. Finally, it also includes a good comparison between balloon-based AWRNs and multicopters' UAVs and AWRNs. On the other hand, in [44], they consider an emergency communication network where a single UAV aims to achieve complete coverage of potential underlaying device-to-device (D2D) users. In their work, trajectory planning issues are grouped into clustering and supplementary phases for optimization, and thus, UAVs' energy consumption is reduced and the quality of D2D users' communication is improved. However, this work is not applicable if the ground terminals dynamically change their position. Furthermore, in [41], they explore the use of a single UAV, but to provide backhaul connectivity to truck-mounted Base Stations (BSs) that have been deployed within a disaster zone to provide network coverage to users based on the principle of delay-tolerant communications. They propose a trajectory design that uses genetic algorithm to find the trajectory with the least energy requirement for the UAV to visit all the BSs and return to a central node that acts as a gateway to the core network. Delay tolerant networks (DTNs) use the method of a \"store-carry-forward\" to transfer messages, which is suitable for the large transmission delay and intermittent link communication. In [43], the DTNs based on a regional centre node and a UAV as ferry node are proposed to build the post-disaster emergency communication network. Further, a region-based message selective delivery routing policy is proposed for the emergency communication network. The received messages are classified according to their destination address at the regional centre node and the UAV ferry node. In this way, the data packets can be accurately delivered to the corresponding area, and redundant data packets will be reduced in the network, reducing delay. Another important research goal of UAVs equipped with BS network coverage control is to maximize the network coverage under the condition of maintaining the service quality. In view of the low dynamic coverage ratio of UAVs equipped with base station network, ref. [56] proposes a relative distance-based UAV equipped with base station deployment method.