# Adaptive Emergency Call Service for Disaster Management
## 1 Introduction  
A self-adaptive, efficient, and easy-to-access emergency call service is paramount during a large disaster. During a disaster, impacted people face the risk of not being rescued on time when the number of victims increase and the network infrastructure itself is impacted. Although it is difficult to prevent natural disasters and pandemics such as Tsunami in 2004 and COVID-19, a proper disaster management scheme allows us to mitigate their effects and improve the impact assessment and response phases in the disaster management cycle [1; 2; 3], as depicted in Figure 1. In this context, cellular networks play a key role in providing the easiest and fastest ways to receive emergency data and provide safety services. Many applications and protocols [4; 5] are proposed for public safety services based on cellular network technologies [6]. Since Release 11, the third-generation partnership project (3 GPP) developed the specifications of new protocols and services for supporting public safety services. The evolution of such specifications with their respective 3 GPP release are illustrated in Figure 2. In general, public safety services use a narrow band terrestrial trunked radio (TETRA)-based system that can only support voice services [7] and concern-specific devices. In addition, the emergency data about the victims provides crucial information for rescuing actions in the impact assessment phase. Hence, it is important to maintain connectivity among user devices, i.e., potential victims, in such a manner that all the emergency messages get collected reliably. Figure 1: The phases of disaster management cycle during pre-disaster and post-disaster stages. Figure 2: Evolution of the 3 GPP specifications for missions critical/emergency scenarios using proximity-based services under each release from 11–17. Furthermore, 4G/5G device-to-device (D2D) technology allows the connectivity among user devices enabling the extension of the radio coverage of gNodeBs (gNBs) during disaster scenarios. Despite the works on D2D during disaster scenarios, the real implementation of such solutions faces two major challenges. The first challenge is the cohabitation of D2D communications and the traditional uplink/downlink communications [8]. Emergency call service should be light enough and should not interrupt the normal functioning of the user phones. Secondly, the proposed emergency call protocol should manage traffic fluctuation and radio resource access in a fully distributed way, when the network infrastructure may be damaged. Thus, a fully distributed, feasible, and adaptive emergency service is required when network infrastructure failures occur during large disaster scenarios. Hence, in this paper, an adaptive call protocol is proposed for emergency scenarios. On the whole, the main contributions of this work are summarized as follows:  
* A novel 3 GPP and 5G New Radio (5G-NR) compatible emergency call protocol named 5G StandalOne Service (5G-SOS) is proposed. This protocol is designed to forward emergency calls autonomously and reliably to the network infrastructure during massive network failures using multi-hop D2D relaying across the sidelink channel. This protocol generates zero additional control traffic, is fully distributed, and is adaptive to local emergency call congestion. * The proposed 5G-SOS protocol is designed to utilize distributed learning and adapt its parameters according to the observed neighborhood emergency call congestion data. Further, it is modeled to use the prior knowledge of local network congestion limits to assign values to the protocol parameters such that the transmission reliability and efficiency are enhanced. * Feasible algorithms are proposed for the user devices to follow the 5G-SOS protocol and function as either an emergency or a relay device at a given instance.