One of the key attributes of land mobile radio systems is their ability to provide off-network, device-to-device services, generally known as direct mode operation (DMO) communications.
DMO enables radios within range of each other to communicate directly, either inside or outside of main network coverage to support both individual and group calling. DMO can also be used to extend communications to radios outside network coverage area via a relay device, such as a vehicle-mounted radio with gateway functionality connected to the main network.
Relays allow communications to be maintained when some users are within network coverage and others are not, and when all users are out of network range. This is similar to LTE, UMTS and GSM repeaters.
Most importantly, DMO enables mission-critical communications users to maintain contact when the network is overloaded or fails completely.
“Most emergency services use DMO today,” says Jeppe Jepsen, director of international business relations at Motorola Solutions. “It is the ultimate assurance that you will be able to connect to people around you.”
In the UK, the operational fire teams switch to DMO on the fireground using analogue or DMR radios on six Fire Service UHF channels, rather than the national Airwave TETRA network. UK ambulance services also use DMO on a regular basis.
“Crews routinely switch their hand-portable devices to DMO on arrival at an incident scene,” says Tony Gray, chief executive of TCCA. “In addition, some specialist units use DMO as a basis for establishing off-network communications in a confined area.”
Proximity Services
When 3GPP, the cellular standards body, was asked to introduce mission-critical (MC) functionality to the 4G LTE standard, it addressed DMO by creating what it refers to as Proximity Services (ProSe). The ProSe specifications, which largely replicate the narrowband voice and messaging capabilities of two-way radio, were defined in LTE Releases 12 and 13, grouped under ‘direct discovery’ and ‘direct communications’.
Francois Piroard, senior expert for standardisation, Secure Land Communications at Airbus, observes: “There are two flavours of ProSe: one for commercial use where D2D services are fully under the control of the network operator; and another for public safety where D2D can be used automatically without network coverage or control.”
Direct discovery is a standalone feature and is not necessarily tied to direct communication. In ‘network assisted discovery’, a ProSe-enabled device uses the LTE network to ‘discover’ other nearby enabled devices.
The device has to contact the network ProSe function to get service authorisation, provisioning and the required radio and security parameters. Once devices have discovered each other, they can then establish a direct radio link, referred to by 3GPP as the ‘sidelink’, and communicate.
The ‘direct communication with ProSe’ mode is aimed at public safety users as it bypasses both the local LTE cell site and the network to enable devices to automatically establish D2D communications. Alternatively, the user can manually set the device to use direct discovery and direct communication even when LTE coverage is available.
Another variation is ‘locally routed communication with ProSe’, also known as indirect network connection mode. This uses the local LTE cell site to establish the ProSe connection between devices without being routed via the main network. In LTE Release 13, 3GPP added unicast and groupcast direct communications and ‘device to network relay ProSe’. This enables an out-of-coverage device to connect to the network via an in-coverage device.
So far, so good. However, ProSe as it stands has not taken off for either public safety or consumer applications. The first stumbling block as far as public safety organisations are concerned is the coverage range.
“As was highlighted in a TCCA report [2016’s TETRA Direct Mode and LTE Proximity (ProSe) services compared] commissioned by P3 Group, there are fundamentally two key areas of difference between DMO and ProSe, ie, the frequencies used by, and the transmitted power of, the devices,” says Gray.
“For example, TETRA DMO radios typically operate in frequency bands from 350-470MHz, while the bands assigned for LTE ProSe devices are usually at least twice those, and often several orders higher. TETRA hand portables are specified for 1Watt transmitted power [or more], while LTE devices are constrained to a maximum of 200mWatt.”
The TCCA report cited the LTR direct discovery trial from September 2014 involving Deutsche Telekom, Huawei and Qualcomm, which found that: “The discovery ranges observed are 550m in a rural area, 350m in an urban area with line of sight, and 170m in an urban area with non-line of sight.”
Airbus’s Piroard adds: “We now have a transport layer that can support D2D, but the range is still very limited even with higher-power devices. They are far from fulfilling the expectations of public safety organisations.”
“ProSe is probably the most challenging output from 3GPP in terms of meeting the needs of first-responders,” agrees Jepsen. “If the standards were implemented in their current form, they would not satisfy the longer-range use-cases.”
Chipsets
The other major problem with ProSe is the lack of chipsets. 3GPP envisioned that ProSe could be used for a range of commercial services that rely on proximity, such as advertising, social networking, gaming, relaying traffic for wearable devices and for Vehicle-to-Everything (V2X) connectivity. The idea was that these services would create mass-market demand to persuade chipset vendors to invest in ProSe.
However, the anecdotal evidence is that among the major chipset vendors, only Samsung has adopted ProSe. Ken Rehbehn, founder and principal analyst at CritComm Insights, says: “Silicon vendors have not incorporated ProSe into modems, so modules and handsets have no access to the capability. I’m sure Qualcomm and the other vendors evaluate ProSe in terms of the wider market. It adds capabilities to the modem, but also more complexity, and that means cost.
“So, what is the motivation for the silicon supplier to make that investment? The possibility is that the commercial sector will never be interested in ProSe capabilities.”
Jepsen thinks that any further development of the ProSe standard will need to be driven by other use-cases to bring the demand up to a level where chip vendors see a business case. “It’s possible that some of the 5G work for low-power, long-range communications might become a basis for development of ProSe in the future,” he says.
It may be that the automotive industry will provide the business case. 3GPP is addressing D2D applications in relation to V2X in Release 15, where it identified 25 use-cases for advanced V2X services grouped under four categories based on 4G LTE D2D specifications: Vehicle Platooning; Extended Sensors; Advanced Driving; and Remote Driving.
These V2X enhancements may not do much to address the range issue. After all, the thrust of 3GPP’s work is to support intelligent transport systems for safety and efficiency on roads. The V2X specifications are aimed at supporting applications between vehicles and between vehicles and road-side units in relatively close proximity to each other (mostly under 500m).
However, at the 3GPP meeting in California in June, a decision was made to add a study item to Release 16 to identify technical solutions for a 5G New Radio (NR) D2D sidelink, designed to meet the requirements of advanced V2X services. This includes support for sidelink unicast, groupcast and broadcast of D2D broadband video and media data streams where mission-critical levels of ultra-low latency and high reliability are required.
Erik Guttman, chairman of 3GPP TSG-SA (Technical Specification Group – Service & System Aspects), points out that although 3GPP’s focus is on V2X at this moment, there could be benefits for public safety to follow with NR D2D. “First, the market is potentially much larger, which could lead to greater economy of scale for equipment,” he says.
“Second, the range and mobility requirements are much stricter and clearer than LTE D2D. Third, NR D2D will continue to evolve with the needs of a demanding market. 5G NR D2D may prove important for public safety and work has already begun within the Release 16 timeframe [due for completion at the end of 2019] to investigate how mission-critical applications (MCPTT, MC video, MC data, etc) will operate over 5G accesses and core network.”
One possible future route to greater D2D range is LTE Maritime, which proposes D2D services of up to 100km for ship-to-shore communications and 15km, or 7.5km for speedboats, for ship-to-shop communications. Guttman reports that there is active development of the Maritime stage 1 (requirements) specification under way, but warns that international maritime standards involving regulatory negotiation at the UN IMO level come to fruition at a slower rate than telecommunication standards.
Piroard says: “We are looking at all the evolutions 3GPP is working on and trying to detect what solutions are being developed that might meet public safety organisations’ D2D range requirements. Ultra-reliable and low-latency communications for massive IoT might provide an answer.”
Interim solutions
In the meantime, those countries looking to migrate to public safety MC LTE will have to find proprietary interim solutions for D2D operations. Gray points to solutions such as dual-mode LMR/LTE devices, single-cell tactical LTE networks, and possible use of the 3GPP IOPS (Isolated E-UTRAN Operation for Public Safety) features standardised in Release 14.
“The IOPS mode provides the ability to create an LTE radio access network without backhaul communications to the core network from a deployment of one or more standalone nomadic eNodeBs (LTE base stations),” explains Gray. “IOPS also addresses the situation where a fixed or nomadic set of eNodeBs is without normal backhaul communications, but has access to alternative non-ideal limited bandwidth backhaul.”
There are dual-mode LMR/LTE handsets available from LMR vendors in the familiar two-way radio form factor with large buttons, external antennas and display screen. Sepura offers its SC20 and SC21 TETRA handsets, which can also support a high-speed bearer; Hytera has its PDC760 Multi-mode Advanced Radio supporting DMR/TETRA and LTE; Airbus has the Tactilon Dabat TETRA/LTE device; and Leonardo its Puma T4 TETRA/LTE hand portable. Motorola Solutions has not opted to release a dual-mode device, concentrating on rugged LTE smartphone devices, such as the LEX F10, L10 and L11.
Harris has released the XL-185P and XL-200P hybrid devices, which operate in P25 and LTE frequencies, as well as Wi-Fi and Bluetooth. “A police officer can pair the radio with other devices such as a tablet over Bluetooth to provide a nice combination of trusted rugged voice handset and LTE tablet. What’s missing in Harris’s approach is automatically moving between bearers from trunked legacy PTT to LTE PTT. That has to be a decision taken by the user,” says Rehbehn.
Harris announced an ambitious proposal in late 2017 “to develop solutions that both extend range and the reliability of ProSe” using the new capabilities developed by 3GPP standards for V2X and Internet of Things. It claimed it “is confident that [we] can develop a DMO solution that meets user expectations”. We shall see.
Rugged smartphone supplier Sonim offers the XP8 and XP5 and has developed the XPand Interface, which extends the capabilities of the handsets through rugged attached peripherals, including direct mode modules.
“It doesn’t make a bet in one domain other than LTE, but it recognises there remains a demand for PMR-type applications,” says Rehbehn. “You could use the Sonim LTE phone as your primary device and then have a DMO capability attached via a module, or even a full implementation of P25, or perhaps DMR. It’s very flexible.”
In the UK, the Home Office indicated in 2017 that it was looking at a similar proposition combining a 4G device with a remote speaker microphone housing a TETRA radio for users of the new Emergency Services Network (ESN). This would use TETRA spectrum, but not the Airwave TETRA network.
Rehbehn cites another alternative being considered by the SafeNet project in South Korea. “At Mobile World Congress 2018, [Korean mobile operator] SK Telecom showcased a hybrid LTE with DMR direct mode device to address the concerns of the Korean emergency service agencies. They would use the ubiquitous LTE network for straightforward MC voice communications, but use DMR to ensure that when they are off-network, they get a trusted and effective technology solution.”
John Swarbrick, founder and CEO of Andromeda Digital, a UK Push-to-talk over Cellular company, backs that idea. “We view DMO as an emergency position rather than a day-to-day requirement. I think the number-one challenge is the effective operational range. ProSe will get there, but I think it is several years away in terms of the necessary maturity of the silicon and vendor ecosystem,” he says.
In his view, DMR is the obvious interim choice, at least for the UK. “DMR is available with protected channels for fire service already allocated by the Home Office, for example. It is a developed and trusted open standard with a multi-vendor ecosystem. If you used DMR operating in public safety spectrum for D2D, you could also provide interoperability between multiple vendor devices.”
Andromeda is starting field trials for its SmartRadio, which supports LTE, Wi-Fi and a UHF PMR-based direct mode application. “The goal is for SmartRadio to do all mode switching automatically, freeing the user to focus on the task in hand,” says Swarbrick. Andromeda aims to have the device commercially available in September.
As it stands today, to support longer-range D2D use-cases, public safety users will have to extend LTE network coverage via mobile eNodeBs in vehicles or perhaps drones, or use some form of ad hoc autonomous network.
This still requires chipset vendors to adopt ProSe on their silicon to ensure ProSe-enabled LTE handsets are available, but most seem unwilling to do so. The specifications in Release 15 and proposed for Release 16 bring the promise of major ProSe enhancements for broadband D2D data and video streaming, but these are some years away from commercial availability.
The other main alternative is for public safety personnel to continue to carry a separate LMR and LTE device or use a hybrid LMR/LTE device. But that means either maintaining legacy LMR emergency services networks with the additional cost that implies, or at least making available VHS or UHF frequencies to enable off-network D2D communications using an LMR-style radio of some description.
For countries looking to migrate their emergency services communications over to LTE, it seems that some kind of interim D2D measure will be necessary, and for quite some time to come.
