6.1 Role of LTE

Nowadays Long-Term Evolution (LTE) is the worldwide de facto standard for mobile broadband communication and it is expected to be in this position for the next decade, although research work is ongoing in the industry for a new radio technology (working title 5G). For commercial network operators, spectrum flexibility and efficiency, Operational Expenditure (OPEX) and Capital Expenditure (CAPEX) reductions due to a simplified architecture, and consequent usage of the Internet Protocol suite throughout all network interfaces are key points (i.e., there is no need to have a team of operational experts for Internet Protocol (IP) and non-IP protocols any more, for example, ITU-T signaling system No. 7 based protocols). Public Safety networks adopting LTE will benefit from these advantages. They can deploy a mobile system that has proven its reliability, flexibility, security, and feature-richness in mass deployments (forecasts predict 2.5 Billion LTE subscriptions by end of 2020). As the de facto mobile broadband standard in all parts of the world, it provides the necessary economy of scale in terms of shipped chipsets, devices, and network equipments. Backed by a strong standardization organization like 3rd Generation Partnership Project (3GPP), necessary changes to the LTE system are provided rapidly and based on consensus among all major device and infrastructure vendors. The development of LTE is far from an end. 3GPP is constantly improving and optimizing the whole system. New features are added with each release, new frequencies are added to the LTE bands, and bandwidth is even increasing with LTE Advanced (LTE-A), Carrier Aggregation (CA), and Multiple-Input-Multiple-Output (MIMO). As a matter of fact, 3GPP's efforts are very much focused on improving LTE, not so much anymore on improving older technologies such as Universal Mobile Telecommunications System (UMTS) or General Packet Radio Service (GPRS). This provides confidence to network operators that necessary resources in 3GPP are available to correct errors, optimize, and evolve LTE. Optimizing LTE for Public Safety in Release 12 has proven 3GPP's capability and willingness to take new requirements on board and optimize or adapt LTE also for new use cases. Other examples are optimizations to fit LTE for new market opportunities around Machine-to-Machine (M2M) communication and the Internet-of-Things (IoT). Future enhancements of LTE (more specifically to the Evolved Packet Core) due to the introduction of Network Function Virtualization (NFV) and Software Defined Networking (SDN) techniques will allow Public Safety network operators to benefit from increased deployment flexibility, reduced CAPEX, and OPEX due to ease of scaling up and down network resources and capabilities. Other features 3GPP is currently working on in Release 13 like paging optimizations will help Public Safety networks to prioritize voice calls, thereby reduce call setup times and help to meet their strict latency requirements. Such feature enhancements will come “for free” for Public Safety networks due to the fact that LTE is a system that is specified by a global standardization organization.

Network sharing and (national/international) roaming will allow Public Safety operators to deploy dedicated networks only in some parts of a country but have sharing or roaming agreements with commercial LTE operators to use their network (especially Radio Access) infrastructure. This can lead to significant cost reductions while building up a nationwide Public Safety network. On the other hand, commercial operators will benefit from providing their network capacities, infrastructure, and know-how to the Public Safety community.

The end user is mainly interested in getting fast and reliable access to IP-based services such as web browsing, voice and video calls, video streaming, online gaming, and much more. LTE provides access to this kind of services with high bandwidth and low latency, combined with the benefits of a standardized cellular system, including nomadicity, mobility, and worldwide roaming agreements. For many people, LTE in combination with innovative new tariff structures has brought the vision of being “always-on” (e.g., connected to the Internet at any time) to reality. Once LTE is rolled out in more countries this vision will become reality for much more people in the future.

6.2 Public Safety Features

As explained throughout this book, LTE provides a big variety of capabilities enhancing people's safety in their daily life. Although the focus of this book was on LTE for “Public Safety networks” we provided also insights about other public safety-related features that are available in LTE networks. One of the most important ones for the regular public is certainly the emergency call. Especially when provided in future via 3GPP's IP Multimedia Subsystem (IMS), it allows people not only to setup an emergency voice call but also using other media such as video call, messages or voice, and video clips to communicate with emergency call centers. In combination with enhanced location services (e.g., Location-Based Services, Assisted Global Positioning System (GPS)), this new kind of multimedia emergency services will allow for more accurate and faster assistance for people in emergency situations. Even before rescue personnel has reached the location where an accident or a disaster has occurred, pictures and videos were sent via a multimedia emergency service to the emergency service center and eventually to the rescue personnel on their way to the accident and will help to prepare them with more precise information, for example, what situation they can expect when arriving at the accident or disaster location. This can save the lives of people involved in an accident but also the lives of the rescue personnel as they have a clearer and more accurate view of the situation in advance.

The eCall system (see Ref.[1] for more information) that will be introduced in the European Union provides information to the emergency center automatically from cars that are involved in an accident. Besides providing data such as car's position, number of passengers, and speed before the accident, eCall can call rescue personnel directly even when the passengers of the car are not able to communicate anymore. Once eCall is available via LTE and IMS much more data at higher speeds can be sent to the emergency center or to a data center. Cars that may be equipped in the future with lot of sensors and cameras can send precise pictures or real-time videos to the emergency service center or to rescue personnel directly. Similar features like eCall for cars can be also foreseen for buildings or certain confined places. Once a fire is detected in a house, for example, in a public building, not only a simple fire alarm but also pictures or a real-time video stream showing the fire and the rooms where it is ongoing can be sent. This will help firemen to coordinate their operation before and after arrival at the accident site.

Public Warning System (PWS) allows an official authority to send warning messages over LTE to people in a certain area where a disaster (tsunami, earthquake, or volcanic eruption) may occur in the near future. Again, public warning messages in combination with a mobile broadband system such as LTE are a much more powerful method to make people aware of the dangers that are ahead of them, and more precise data regarding the area and degree of the disaster can be delivered to the endangered people.

Looking ahead we can even assume new Smartphone Apps are available, providing safety-related information to users: information coming directly from authorities where the App is registered but also information from other users who may be near a disaster or accident and inform either everyone or only friends in real time about the situation. This could lead to a setup of new kinds of social networks: social networks providing participants with safety relevant information wherever they are.

6.3 LTE for Public Safety

Proximity Services (ProSe) and Group Communication System Enablers (GCSE) are the two remarkable features standardized by 3GPP to allow Public Safety communication over LTE. With proximity services, fire and policemen and other Public Safety personnel are able to detect and communicate with members of a group they are belonging to without network coverage. The principle of device-to-device communication using LTE frequencies is a new kind of feature to 3GPP. Commercial network operators offer their services usually or only via their network infrastructure using licensed spectrum allocated to them by national authorities. This type of walkie-talkie function (i.e., 1-1 or 1-many communication) is essential for public safety personnel in a disaster region where none of the devices or only some of them are in network coverage. Devices in coverage can act as communication relays for devices out of coverage. The group communication enabler based on Multimedia Broadcast/Multicast Service (MBMS) as described in Chapters 1 and 5 is important for providing a resource efficient way for “Push-to-Talk” (PTT) communication where only one person can speak at a given time to all other members of his or her group. Some improvements of the MBMS feature were necessary to allow small talk bursts to be delivered to a certain area (a probably small area like a single cell) in a fast and efficient way. Fast and efficient communication refers to communication with low (“mouth to ear”) latency and without wasting network resources so that many different groups, for example, groups of fire and policemen, can use the network in a distinct area while communication of other people is still possible.

An important aspect for Public Safety is the reliability of the underlying network and applications on top. LTE provides in-built reliability and robustness, for example, through restoration features on a per node (Mobility Management Entity (MME), Serving Gateway (S-GW), Packet Data Network Gateway (P-GW), Policy and Charging Rules Function (PCRF), Multimedia Broadcast/Multicast Service Gateway (MBMS GW)) level. Restoration features provide capabilities on protocol level to restore certain context data and sessions in a node such as the MME, S-GW, or MBMS GW after restart of this node.

The PTT application as such is not part of 3GPP's Release 12 activities. However, PTT is an integral part of existing Public Safety systems such as P25 (Project 25, a set of digital radio standards for use by Public Safety agencies in North America) and TETRA (Terrestrial Trunked Radio, a time division multiplex system standardized by ETSI), which are narrowband mobile radio systems. As a consequence, there is a strong desire to develop the PTT application on top of the group communication system enabler specified in Release 12.

The application known as Mission Critical Push-To-Talk (MCPTT) is currently being worked on in 3GPP Release 13. Completion of this work can be expected in late 2015. Service requirements for MCPTT are listed in 3GPP TS 22.179[2]. These requirements consist of a general description of the PTT feature including floor control, group call, announcement of group call, group management, and priority of group calls but also requirements related to the allowed latency of group calls and the possibility to interwork with existing systems such as P25 and TETRA. Strict latency requirements for the MCPTT service will require using special bearer Quality of Service (QoS) for MCPTT signaling and user plane fulfilling strict QoS (low packet delays) and priority requirements. At the time of writing this book, it seems quite likely that MCPTT will be based on the Push-To-Talk-Over-Cellular (PoC) application specified by the Open Mobile Alliance (OMA) with several enhancements and optimizations to allow for mission critical communication in the field of Public Safety. Obviously, the MCPTT application has to run on top of the Group Communication System Enabler as specified in 3GPP Release 12 and has to implement the respective interfaces, that is, the MCPTT application server has to implement the 3GPP-specified interface toward the BM-SC (MB2 interface for control and user plane). OMA PoC control plane is based on SIP and it can particularly rely on IMS capabilities (e.g., with regard to reuse IMS security, registration, session establishment), which would make it easier to integrate an IMS-based MCPTT service with VoLTE on the same device and chipset. OMA PoC specifies how a PoC client registers for the service with a PoC server, initiates a PoC (on-demand or pre-established) session, joins or leaves a session, and terminates a session. The OMA PoC user plane specifications define, for example, the media/talk burst protocols allowing a PoC client to request the right to send talk and multimedia bursts to other clients. The latest versions of OMA PoC enabler specifications version 2.1 can be found in Ref.[3].

In addition, MCPTT need to make use of ProSe capabilities when device-to-device or device-to-network relay communication is required. Several possible solutions for the relay of MCPTT communication from device to device are discussed: either a Layer 2 (i.e., relay on the link layer) or Layer 3 relay (i.e., relay on the IP layer, this can, for example, make use of the well-known IPv6 prefix delegation feature), an application layer gateway (i.e., a MCPTT gateway/proxy function) running in the relay device, or a relay of the multicast transport (i.e., the relay device acts like a base station forwarding data received on a multicast channel to surrounding devices on another multicast channel). Discussions on possible solutions have just started at the time of writing this book.

3GPP has also started work on another feature called “Isolated E-UTRAN Operation for Public Safety (IOPS).” IOPS use cases are listed in 3GPP TR 22.897[4]. The requirements for IOPS can be found in 3GPP TS 22.346[5]. At the time of writing this book, work on stage 2 (architecture work) and stage 3 (protocol work) has not commenced yet so the description about IOPS is subject to future changes.

IOPS aims at providing services to Public Safety users even when the network is not fully functional, for example, when eNodeBs have become disconnected from other parts of the network and form an “Isolated E-UTRAN.” Situations like that predominantly occur when there are natural disasters such as earthquakes or tsunamis. Under these conditions, ensuring the continued ability of Public Safety users for mission critical communication is of utmost importance.

IOPS also aims to provide the ability to deliberately create a local serving radio access network without backhaul connections, by deploying one or more additional so-called Nomadic eNBs (NeNBs) working in a stand-alone mode. This is intended for use in areas where there is no or very limited coverage, for example, in case of a bush fire in a remote region. In this case, an Isolated E-UTRAN may comprise of one or more NeNBs. A NeNB should be able to initiate Isolated E-UTRAN operation under the control of the operator (details of how this can be realized are left to further work in 3GPP). An Isolated E-UTRAN derived from NeNBs is expected to exhibit similar behavior like an Isolated E-UTRAN derived from eNBs including: support for Public Safety UEs in the coverage area, communication between NeNBs, and support for limited backhaul connectivity.

In addition, the IOPS feature also aims to address a scenario where a fixed or nomadic set of eNBs is without normal backhaul communications but has been provided with an alternative (non-ideal) limited bandwidth backhaul, for example, only able to support signaling traffic but very little or no user plane traffic. Again, its purpose is mainly for use in areas where there is very limited or no coverage and there is, for example, only a satellite backhaul connection available for backhauling.

The Isolated E-UTRAN may comprise a single or multiple eNBs, a single or multiple NeNBs, or a mixed group of eNBs and NeNBs. This mixture of eNB and NeNB can occur when there is an outage due to a disaster and additional NeNBs are deployed for improving coverage and capacity in the Isolated E-UTRAN area. An Isolated E-UTRAN comprising multiple (N)eNBs, with connections between the (N)eNBs, can provide communication between UEs across a wider area of coverage that can be provided by a single isolated (N)eNB. The UEs in the coverage of the Isolated E-UTRAN are expected to continue communication and provide a restricted set of services supporting voice, data, and group communications to their Public Safety users. IOPS should be able to support Isolated E-UTRAN networks joining with each other to form larger, but still Isolated E-UTRANs, for example, when recovery is ongoing after a natural disaster and connections between eNodeBs are reestablished again step by step.

While under coverage of an Isolated E-UTRAN, MCPTT users should be offered some limited functionality to make use of the Isolated E-UTRAN for Public Safety communication. As there is no EPC available, group memberships and affiliation to these groups has to be pre-provisioned to take effect when operating in an Isolated E-UTRAN scenario. As no dynamic setup of groups is possible under this scenario, these groups will have to be fairly generic (e.g., some of them even including users from other organizations). For example, there may be a group defined including police, firefighters, and paramedic personnel, something that would usually be created on the spot and only if needed at an accident scene.

A Public Safety user should also be offered the means to initiate a MCPTT Emergency Group call that may reach a dispatcher (the dispatcher can be connected directly to the (N)eNB or via a UE), members of the group under the coverage of the Isolated E-UTRAN, and/or all UEs under the coverage of the Isolated E-UTRAN.

During Isolated E-UTRAN operation, each group can initiate at least one MCPTT voice group communication that all authorized members are allowed to join, up to the limit of available radio resources.

6.4 Outlook

We have seen that 3GPP has made remarkable efforts within only one release cycle to provide necessary features and enablers for the Public Safety community. These features are called Proximity Services, that is, device-to-device communication over LTE and Group Communication System Enablers. The latter one is based on MBMS for LTE. Owing to time constraints, not all aspects of these features could be developed within the Release 12 time frame. In the next 3GPP Release(s), an MCPTT application, enhancements, and optimizations for Proximity Services and Group Communication will follow. We expect first deployments of Public Safety networks in 2015, although trial networks in specific regions (e.g., in some US states or counties) may already be operated in 2014. The status of ongoing and planned 3GPP work items regarding LTE for Public Safety (authority-to-authority) communications can be found under the link in Ref.[6]. Once Public Safety networks based on LTE technology are deployed, new requirements may arise leading to new features standardized by 3GPP or other standardization organizations. It will be interesting to see how a standardized broadband wireless system such as LTE will help to build reliable Public Safety networks in a cost-efficient manner. These networks and their users will benefit from a radio/network technology and from devices such as Smartphones that have already changed the way people communicate with each other significantly over the last few years.

References

1. [1] European Union regulation: http://ec.europa.eu/enterprise/sectors/automotive/safety/ecall/index_en.htm.
2. [2] 3GPP TS 22.179: “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Mission Critical Push to Talk MCPTT (Release 13)”.
3. [3] OMA Push to Talk Over Cellular V2.1 Enabler: http://technical.openmobilealliance.org/Technical/technical-information/release-program/current-releases/poc-v2-1.
4. [4] 3GPP TR 22.897: “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Study on Isolated Evolved Universal Terrestrial Radio Access Network (E-UTRAN) Operation for Public Safety (Release 13)”.
5. [5] 3GPP TS 22.346: “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Isolated E-UTRAN Operation for Public Safety (Release 13)”.
6. [6] 3GPP Work Items on LTE for Public Safety (Authority-to-Authority) Communications: http://www.3gpp.org/ftp/information/work_plan/description_releases/Previous_versions/.