# UAV Support for Mission Critical Services
## 1 Introduction  
A Mission-Critical System (MCS) provides communication in an area where the classical, infrastructure-based solutions do not provide connectivity due to a lack of coverage (forests or jungles), power grid failure and as a result of disasters (earthquakes, floods and fires), etc. The most popular at present MCSs were developed nearly 30 years ago, mostly to handle voice services. These solutions include Terrestrial Trunked Radio (TETRA) [1], Terrestrial Trunked Radio Police (TETRAPOL) [2] and Project 25 (P25) [3]. These systems are deployed in several countries on a country-wide level, regional level or locally (airports, etc.). Their key features are the capability to work without infrastructure support, direct communications between terminals and so-called \"Push-to-Talk\" functionality. Such systems can be installed on demand or as permanent, private networks. A problem with the mentioned solutions is a limited set of narrowband services and a lack of data transmission capability. Moreover, these solutions use outdated, inflexible and ineffective technological solutions. It is surprising, but almost two decades after the development of the solutions mentioned above, there has been no research or standardization activity related to the development of new, more functionally rich MCS. Recently, as a result of technological progress, widespread deployment of broadband telecommunications, including the global deployment of LTEs [4], interest in the deployment of broadband MCSs has grown. The main driver is the limited data transmission capability of narrowband MCS. As developing a completely new solution is costly, some research groups have looked into adapting existing solutions for broadband MCS. The selected solution is LTE-MC [5], a modification of LTE that is standardized by 3 GPP. LTE has an efficient radio interface, relatively simple core network and the capability to deploy services in the \"over IP\" mode. However, some modifications are needed to support direct communication between User Equipments (UEs) and services in a \"Push-to-X\" style. The concept at the time of writing the paper has not been deployed yet. LTE-MC is a broadband solution designed to provide several services only. However, the modern Information and Communications Technology (ICT) solutions require more support. In that context, IoT solutions (including the Smart City concept), Vehicle to Everything (V2X) services, e-Health, Industry 4.0 and IP-based client-server applications are worth mentioning. The solutions mentioned above require reliable communication and a reliable service platform. Unfortunately, most commercial service platforms, including clouds, are prone to disasters, terrorist attacks or power grid failure. The battery-powered backup power supplies can handle only a temporary lack of power (typically a few hours). The network virtualization and UAV systems can be seen as technological enablers for a new generation of MCS. This paper presents a concept called Virtualized Mission Critical System (VMCS) that combines both technologies. It has the following key features:  
* Exploits UAV mobility for on-demand deployments of MCS. This covers both the delivery capability of UAVs and using UAVs as a mobile communications node. * Exploits the virtualization of commercial mobile network terminals to use them for MCS communications and ad hoc implementation of service platforms. These service platforms can replace platforms that stopped functioning due to disaster or power grid failure but also platforms that are dedicated solely to MCS services. The VMCS concept goal is to exploit all the existing resources efficiently and, by using the resources virtualization technology, also flexibly implement services. This goes far beyond LTE-MC capabilities; however, it comes with the price of complexity and many new research challenges. The paper's primary goal is to describe the overall concept and indicate solutions to some VMCS-related problems.