It has been a long haul for mobile services to gain sufficient capability and trust for widespread use in critical and emergency communications. For many years, consumers were advised to rely on fixed rather than mobile phones to call emergency services, while various dedicated critical wireless systems were used by those service themselves.
The latter included Project 25 (P25 or APCO-25) adopted by public safety organizations in north America, and Tetra in Europe. These are both examples of land mobile radio systems (LMRS) for person-to-person communication, using frequencies reserved for these services, dating back long before the advent of cellular services in the 1980s. However, such services have failed to innovate and add the sort of features, such as support for video and even broadband data, that have come along with 3G and then 4G cellular services. This deficit has added impetus to efforts to enhance cellular services for critical communications, especially as this would avoid the need for separate dedicated devices and networks.
This process of adding support for critical communications to cellular had earlier roots but really started to take off with the launch of LTE Sidelink under Release 12 in 2015. Sidelink simply refers to direct communication between devices without going through the base station. But that raises questions of which form to adopt, as well as challenges such as synchronization and discovery among peers, resource allocation, and mitigation of interference.
Other questions to resolve include whether such sidelinks should be inside or outside the allocated bands, with variants within each. There is also a distinction between communication where all peers are within a cell or service area, where some are and where none are.
The ultimate goal is naturally to facilitate critical communications under as many circumstances as possible, including where none of the peers are actually within range of a cellular signal. Indeed, a key objective has been to overcome the longstanding handicap of mobile services from the critical perspective, which is intermittent and inconsistent coverage. By enabling direct communication between peers, not only can coverage areas by extended, but availability within cells can be increased to the level previously enabled by fixed line phones. That, after all, is essential to bring mobile services up the mark for emergency calling.
Another challenge is battery life and while that will always be a point of weakness, improvements have been made to support emergency modes where power drain is reduced to much lower levels than before. Otherwise, the only recourse for devices in the field is solar charging, which at least ensures that intermittent emergency communications can be maintained.
On the power saving front, a key feature coming on stream is Survival on Sharing (SoS), which has been designed to work with D2D (device-to-device) communications in disaster and emergency situations, when normal cellular connectivity is temporarily disabled. The aim is to optimize battery life at a network level across multiple devices within a disaster zone, so that no single user experiences disproportionate power drain. Simulations so far have shown that this can increase battery life by around 12%, compared with just disabling cellular communications.
Yet the advance of mobile services into critical communications is not just about catching up with the proven legacy alternatives, nor just mitigating handicaps such as reliance on rapidly draining batteries. There is also the positive aspect of adding new value, which is primarily in the two areas of video and position awareness. Some operators have already taken advantage of video capabilities to offer critical services, with the aim of being better prepared to take the best emergency action when they arrive on the scene.
In South Korea in December 2019, KT launched emergency video call centers over its 5G network, after two months of trials that found significant improvements in responses to calls via the emergency number, 119, were achieved. This will also be extended to medical services themselves after trials with one hospital over use of 360-degree cameras inside ambulances to allow staff there to give advance information on victims of accidents or health emergencies to A&E doctors.
Determination of location can also be critical for responding to emergencies and already the major OS providers have come up with solutions. In some densely populated MDUs (multi-dwelling units) in urban areas, even being a few feet off can mean vital minutes being lost if emergency services first enter the wrong dwelling.
Google’s Android Emergency Location Service (ELS) provides enhanced location detection directly from handsets only in the event of emergency calls. It works on around 99% of active Android devices that run Android OS version 4.1 /Jelly Bean and above, and uses what it calls its fused location provider (FLP) API in Google Play services to combine signals from multiple radio or other sources, including GPS, cellular, WiFi and Bluetooth, to zoom in on position. The point is that while say GPS can be accurate enough on its own outdoors, it cannot be relied on indoors where many emergencies happen.
Position fixing indoors can also be compromised by multi-path reflections and multiple sources of interference. Other remedies include analysis of enhanced cell ID (ECID), observed time difference of arrival (OTDOA), and use of WiFi beacons to augment satellite for device-based position calculation.
ECID involves trigonometric calculations based on analysis of timing information from several base stations, using a learning algorithm to minimize the errors. OTDOA is similar in principle, measuring differences in arrival times of downlink positioning reference signal (PRS) from several neighboring cells in comparison to a reference cell, in order to perform the trigonometry.
Support for such hybrid techniques was built into LTE. Under 5G, one step has been to extend the ambit of critical services to general enterprises as demand for those levels of service become widespread, for example in autonomous driving and the Industrial IoT. The market demand for such mission-critical national security and public safety services is being assessed, as well as provision of more general NR proximity services (ProSe) in Release 17 of 5G, due for commercial availability from late 2021.
One idea here is to exploit the high reliability and low latency features of the NR cellular (Uu) interface and also of the PC5 interface developed for automotive V2X applications. For ProSe, there is a need to provide support for out-of-coverage services and be able to connect them as quickly as possible to infrastructure nodes. To meet such requirements, 3GPP will extend the capabilities of the Release 16 NR PC5 interface by enabling user equipment (UE)-based relaying to establish multihop communication paths, where this is possible.
However, the latter is work in progress and will not all be delivered even under Release 17. The relaying may need some additional components for network control, resource allocation, end-to-end QoS management and UE/relay discovery. At the physical and MAC level, all the components will be there with the ECID and OTDOA already in place to augment A-GNSS for device-based positioning. Additionally, new indoor positioning methods such as the WiFi beacons are now being deployed.
It is worth emphasizing that D2D communications are not confined to critical services and will play a major part across the whole 5G spectrum, because of the scope for increasing energy efficiency, reducing latency and making more effective use of spectrum at a local level. It dovetails with the growing focus on edge computing and will feature prominently in those use cases that call for ultra-low latency. But it will also be occurring transparently for other services as part of the overall optimization of communications at the RAN level.
Such optimization will indeed occur at the cell level, especially for outband D2D communication, where links suffer interference from each other, as well as from other devices in the vicinity operating in the same band. Interference can be reduced if the devices transmit at lower power levels, but to ensure that QoS at the receiver is high enough in all cases, this has to be optimized at the network level. Such improvements will take several more 5G releases to come through.