Business and mission-critical communications are communications between professional users either from the public safety and security sector (police, army, fire fighters), operating critical infrastructures (like metro or railway companies, airports…), or public utilities (electricity, gas, water) .

Effective communications are the key to a successful response to emergency and disaster situations. Indeed, the ability of the first responder emergency services to communicate among themselves affects the ability to save lives. This is reflected in increasing public investment in mission-critical communication systems. Therefore, these systems have some specific and severe requirements, such that:

• High reliability and availability: as specified in 3rd Generation Partnership Project (3GPP) Release 16, the system shall be available for 99.9999% of time [1].
• Call priority and preemption: the system shall assign different levels of priority to calls and interrupt low priority calls on arrival of high priority calls that do not find available resources.
• Coverage: coverage plays such a large role in mission-critical communication networks, since losing the signal can mean a life or death situation. In order to extend coverage range in uplink channel, high transmit power has been enabled by the system in specific bands [2].
• Resilient/Isolated network: any Base Station (BS) should be able to act alone in routing calls between network entities that stay operational, e.g., after a disaster had partially caused some network equipment to fail [3].
• Fast call setup time lower than 300 ms [4].
• Network inter-operability: communications with users located on external networks.

Moreover, unlike public cellular networks like Global System for Mobile communications (GSM), Third Generation (3G) or Fourth Generation (4G) mobile networks, mission-critical communication networks are characterized by additional services, e.g. Mission-Critical Push-To-Talk (MCPTT) [5], Mission-Critical Video (MCVideo) [6] and Mission-Critical Data (MCData) [7] including group call communication with low call setup time, mobile communication systems for railways [8], maritime communication services [9], etc.

Besides these services, group communications are one of the most important and indispensable services of mission-critical communication networks. Group communications provide an efficient management of the rescue teams, and allow sending commands and sharing information with all contributors in a disaster area. Indeed, a public safety communications system provides the means for first responders to accomplish their mission by communicating simultaneously with their collaborators in a variety of media. In such systems, individuals in a fire brigade or a police department are typically organized into groups, with different responsibilities. These groups can be predefined or formed on-demand, may have geographic areas to cover, and may be organized based on types of skills or activities to be performed. Within and across those groups, some individuals with supervisory or dispatch authority and responsibility should be able to manage and coordinate the efforts of the first responders. Further, some individuals may be able to receive multiple group communications simultaneously, using their device to listen to the one with the highest priority as signalled by the system. The network shall then allow the coexistence of many active groups at the same time, and each individual can be registered to many groups at same time. It’s worthwhile to note that communication to group members is not confined to speech, as data messaging (e.g. text, image, video…) can also be sent in parallel to speech, and may be sent from a group member who is not currently speaking.

To meet these critical additional requirements and services, mission-critical communications rely on reliable and secure Professional Mobile Radio (PMR) networks.

Most of the deployed wireless systems for mission-critical communications are today based on PMR technologies, such as TErrestrial Trunked RAdio (TETRA), TETRA for POLice (TETRAPOL), or Association of Public-safety Communications Officials Project 25 (APCO P25), meet most of the aforementioned requirements. However, these networks are mainly devoted to provide a wide range of voice services, but have a limited possibility to provide high data rates mobile services like video streaming, files transmission (maps, databases, pictures…), ubiquitous Internet and Intranet access or Device-To-Device (D2D) communication, which have a strong impact on the efficiency and the responsiveness of the emergency services. Therefore, worldwide there is a great interest of governments and organizations involved in public safety and security towards the provision of such wide range added-value services, in PMR networks, in order to improve the situational awareness and enhance the life-saving operations.

Even if some efforts have been done to enhance PMR systems and to offer higher communication capacity, achievements are still behind those made in the commercial world that recently has developed the 3GPP Long Term Evolution (LTE) technology. Hence, there is a great consensus in adapting the LTE technology to provide IP-based broadband services with the security and reliability typical of PMR networks, which answer to the professional and critical communication needs.

Indeed, many railway researches, including the Future Railway Mobile Communication System (FRMCS) project triggered by the International Union of Railway (UIC), estimate that LTE can meet the needs for transferring railway data in the long term [10]. Moreover, governments in many countries, including the United States, Belgium and the Republic of Korea have also been surveying how to utilize the LTE system for public safety communications, either to augment their existing systems, or to provide a future migration path .

The adoption of mainstream commercial technologies, such as LTE and Long Term Evolution Advanced (LTE-A), for business- and mission- critical communications has been discussed in literature [13–17]. Initially designed for public networks, the LTE standard needs some specific enhancements to meet the requirements of PMR. In this context, adopting LTE as PMR broadband technology needs that some specific applications and functionalities requested by railway as well as public safety and security operators, such as MCPTT, dispatch services, priority management, group communications and direct communications, be included in the releases of the 3GPP standard, also guaranteeing interoperability with actual PMR systems. Thus, the 3GPP has been handling various specification items for supporting the public safety features, and technical specification groups in the 3GPP have established such improvements in Release 12 and 13 specifications, which include:

• Group communications: Group Communication System Enabler (GCSE) for LTE [18] and Mission-Critical Push-To-Talk (MCPTT) over LTE [5] have been introduced into 3GPP specifications in Release 12 and Release 13 respectively, in order to support group communications together with Push-To-Talk (PTT) voice application and its evolution toward multimedia services. Further, the MBMS, introduced in Release 6, to support such communications (see Section 1.2).
• Proximity Services (ProSe): in order to enable D2D communications, without the need to have coverage from a network infrastructure, Proximity Services (ProSe) has been introduced by 3GPP under Release 12 [19, 20]. Together with group communications, D2D communications are among the key requirements for missioncritical voice services.
• Coverage: 3GPP has specified a higher power transmit class in Release 11 to improve system coverage [2].
• Enhanced Evolved Universal Terrestrial Radio Access Network (EUTRAN) sharing: intended to add flexibility in sharing network resources, 3GPP has introduced in Release 12 an enhanced resources sharing mechanism between critical and noncritical users [21].
• Isolated EUTRAN operation: in order to improve network resiliency, 3GPP has introduced in Release 13 the isolated EUTRAN operation for public safety to enable any BS to act alone in case of disasters [22].