## 3 Toward Next-Generation Public Safety Networks
### Public-Safety-Targeted Technologies  
In Table 1, we summarize the main cellular technologies that have been ratified by 3 GPP as a result of the public-safety-targeted standardization efforts. Group call system enablers (GCSE) provide a collection of mechanisms that enable both unicast and multicast transmissions for group communications. Proximity services (ProSe) were also among the main standardization items ratified in 3 GPP Rel-12. They present the architecture and the radio interface for device-to-device (D2D) communications in cellular networks [5]. The new D2D interface is known as sidelink and was introduced as part of the support of public safety ProSe in LTE networks. As proved by several research works dealing with this technology, D2D communications allow cellular networks to take advantage of three gains (i.e., proximity, hop, and reuse) and several network performance improvements (i.e., high data rates, low delays, high reliability, and low power consumption) [21]. Additionally, establishing direct links between devices in out-of-coverage scenarios helps to provide first responders with the needed communications, especially in dangerous situations [22]. To support such scenarios, two LTE sidelink transmission modes were ratified by 3 GPP for public safety. The sidelink transmission mode defines the entity responsible for the sidelink radio resource allocation: (i) mode-1, also called scheduled mode, in which the sidelink resource scheduling is monitored by the base station; (ii) mode-2, known as autonomous mode, where user equipments (UEs) rely on sidelink preconfigurations stored in the devices [23]. While in-coverage UEs can operate in mode-1 or mode-2 as decided by the network, out-of-coverage UEs are restricted to using mode-2. As depicted in Table 1, the isolated E-UTRAN operation for public safety (IOPS) was ratified in 3 GPP Rel-13. The aim behind its introduction is to provide public safety users with network access in challenging situations. The latter include cases where the network is overloaded due to increased user demands and where the access network part loses its backhaul connection with the core network in disaster areas [24]. While only voice-based communications are exchanged in LMR networks, public safety LTE (PS-LTE) networks offer a set of mission-critical services. These were introduced by 3 GPP among its public-safety-related standardization items. Based on the market demands, MCPTT was the first major step in the series of mission-critical services introduced in Rel-13. 3 GPP then added certain enhancements to the MCPTT specification in Rel-14. It also enriched its repertoire of standardized public safety applications by introducing mission-critical data (MCData) and mission-critical video (MCVideo) [25]. These three applications are also called MCX services. A general framework for MCX was provided in 3 GPP Rel-14 to facilitate the standardization of additional services in the upcoming releases. This framework includes a common architecture for the support of MCX services. Two functional or architectural modes are defined based on the existence of a mission-critical server in the network: on-network and off-network modes [26]. Figure 1 shows the differences between the two modes. In on-network mode, the communications are based on a client/server setup. Client/server communications are established via the LTE core network. However, in off-network mode, the communications are only supported by UEs in a peer-to-peer setup [26]. The network coverage requirement in the two MCX architectures is also illustrated in Figure 1. To deploy the on-network mode, MCX users have to be within the coverage of the cellular network. On the contrary, off-network MCX users can establish D2D links in different network coverage situations (i.e., in-coverage, out-of-coverage, and partial coverage).