# UAV Support for Mission Critical Services
## 2 Status of Mission-Critical Solutions  
TETRA [1], TETRAPOL [2] and Project 25 (P25 or APCO-25) [3] are the narrowband networking solutions that belong to the MCS category. The most popular out of the three is TETRA, which is a standard for digital radio-telephony dispatcher (trunked) communication developed by European Telecommunications Standards Institute (ETSI) in 1995 and designed particularly for public safety and rescue services. TETRA uses Time-Division Multiple Access (TDMA) for the simultaneous handling of four separate conversations or data transmission sessions on a single 25 kHz radio channel. The system can operate in several radio bands, including 380/410/450/870 MHz. TETRA is mainly used for voice communication (including group and peer-to-peer communication). The voice channel has a bitrate of 7.2 kbps, and data transmission (depending on the signal level, i.e., the distance from the antenna) ranges from 2.8 to 28 kbps in the 25 kHz channel and 80-157 kbps in the latest versions of the standard named TETRA Enhanced Data Service (TEDS). All calls are half-duplex, and only up to two calls per frequency carrier are allowed, which impacts the system capacity. Figure 1[6] shows a simplified TETRA architecture. TETRA works in two modes:  
* Trunked Mode Operation (TMO). In this mode, a fixed private TETRA infrastructure with TETRA base stations is used. This mode, in general, cannot be used in disasters. * Direct Mode Operation (DMO). This mode allows direct communication between the TETRA mobile nodes (TMNs) without support from the TETRA infrastructure. The main advantages of the TETRA are the direct communication between terminals, group communication, the ability to operate the terminal as a connection relay to an infrastructure network and Push-to-Talk calls. TETRA also allows the exchange of short messages (SDS) that can be seen as the TETRA version of Short Message Service (SMS). The system provides authentication of connections and a high level of security. TETRA is undoubtedly successful--it has been installed on the island basis in 114 countries and is used by the police, emergency services, airports, railways, etc. It covers the entire territory of Malaysia, Sudan, Egypt, Namibia, Belgium, the Netherlands, Croatia, Serbia, Denmark, Germany, Greece, Finland, Sweden, Ireland, Romania, Great Britain, Israel and Brazil. The narrowband MCS solutions were designed long ago, mostly for classical Plain Old Telephone Service (POTS) services, and the progress of communication technologies has made them outdated. The deployment of broadband access (wired and mobile network-based), Internet of Things (IoT) systems, Smart City and Industry 4.0 concepts, and the automation of power grid and gas systems have created new MCS requirements that narrowband MCS solutions cannot meet. The most important feature, apart from voice and video services, has become data transmission, which is the Achilles' heel of narrowband MCS. To solve the lack of a broadband MCS, the EU has started projects on broadband Public Protection & Disaster Relief (PPDR). In February 2019, as part of the BroadWay project, a call for tenders was published to find solutions for introducing a pan-European mobile broadband public safety system. A natural candidate for such a network is LTE, a relatively simple and efficient wireless network. LTE, however, requires modifications to cope with MCS requirements. Such changes should enable the proper working of the new solution even if part of the network will fail and should support push-to-talk and other services that TETRA already supports. In such applications, the advantages of LTE include video communication, low-latency IoT traffic handling and broadband data transmission. The standardization of the new mission-critical communication solution, called LTE-MC, is performed using 3 GPP and is still in progress [5]; however, LTE-MC is the only broadband MCS with significant  
Figure 1: TETRA network architecture. international and standardization support.