Summer 2017 is emerging as a defining moment for LTE-based mission critical push-to-talk (MCPTT) technology. Kicking off summer, ETSI conducted an ambitious Mission Critical Push to Talk (MCPTT) Plugtest event that brought together 19 vendors exercising 47 test cases to expose interoperability issues with nascent 3GPP Release 13 specification implementations. Perhaps more significantly, the UK Emergency Services Mobile Communications Programme (ESMCP) publicly tipped its hand on its plans to address the important gap in MCPTT off-network capabilities with a June announcement of a Request for Information (RFI) seeking answers to questions about alternative approaches to off-network MCPTT. At the June PCSR Conference in San Antonio, ESMCP program director Gordon Shipley described a bolt-on remote speaker microphone (RSM) with a second TETRA direct mode radio as one possible approach. The ESMCP move is profoundly significant and will have an impact that reverberates around the globe.

LTE is the Future but Device-to-Device Stands in the Way

Narrowband radio systems used by emergency services today stood the test of time by providing reliable voice transmissions in a wide range of challenging incident operations. Voice-only network support, however, is no longer adequate for incident command, control and communications in an age of sophisticated cloud-enabled applications offering rich information sets and video to incident commanders and field personnel. By embracing LTE as the foundation of critical communications, governments provide an important foundation for new capabilities that speed response, improve situational awareness and expedite incident closure. Naturally, moving to a single holistic network architecture that extends the benefits of LTE transmission to MCPTT is an important goal. In the UK, the government expects to save £3.6 billion over a 17 year period thanks to the shuttering of an expensive legacy Airwave TETRA network. 

There is no question that LTE is the radio access network of the future for public safety. But the challenges of LTE-based off-network operation remains a stubborn obstacle. Legacy trunked voice systems based on TETRA or APCO Project 25 provide an integrated direct mode capability that allows users to communicate on a simplex channel without network support. The process is not automatic and requires users to switch to a radio setting to initiate the communication. But the systems do provide scan functionality that allows other users to automatically hear direct mode transmissions when they monitor a trunked talk group. The feature is simple but essential. Emergency workers -- most notably fire fighters -- often work as teams in network-deprived environments such as sub-basements or inside shielded building structures. 

Unfortunately, progress bringing public safety grade LTE-based direct mode capabilities to market remains stalled. Two issues stand in the way. First, 3GPP Proximity Services (ProSe) standards needed to coordinate interactions between two LTE devices that are not connected to a network remain unproven and, more importantly, absent from mainstream LTE chipset implementations. Secondly, the transmission characteristics of LTE terminals suffer from much lower uplink propagation range due to higher frequency bands and low transmit power. Emergency services personal cannot punch outside of building structures with the same success allowed by legacy TETRA or Project 25 devices. For a fire-fighting team attempting to reach an incident commander at street level, failure to communicate becomes a serious safety issue.

Driven by ambitious cost-cutting goals, the UK Emergency Services Mobile Communications Programme is the first to grapple with the limits of LTE-based off-network MCPTT. Facing mounting costs of the legacy Airwave network and hemmed in by an Airwave contract sunset date, the UK embraced LTE as a replacement technology. But the move was made before standards efforts were completed and products commercialized. Now, at an advanced stage in the program's roadmap, the ESMCP must find an alternative approach that retains its LTE network architecture as a foundation while delivering a trusted off-network solution. Complicating matters, the ESMCP has already issued a tender for devices based on 3GPP Release 12. This means that the alternative approach may need to be a "bolt-on" addition to coming devices or an entirely separate radio unit. 

Comments by the head of the UK ESMCP at the June Public Safety Communications Research (PSCR) Conference hosted by the US National Institute of Standards and Technology suggested the possibility of a remote speaker microphone (RSM) radio device attached to an LTE device. This is an example of a bolt-on approach as the RSM would incorporate a TETRA radio operating in the 380 to 400 MHz band and restricted to direct mode functionality.

Moving Away from LTE for Direct Mode

The approach of adding support for TETRA (or Project 25) direct mode functionality to an LTE device makes sense. It combines the strengths and economic value of LTE networking with the simple, but proven direct mode operation of a trusted device-to-device interface operating at higher power in lower band spectrum. But challenges remain. For a bolt-on RSM radio, the power draw required by a radio will likely exceed the power supplied by a typical RSM cable. A further complication, however, is that with no industry-wide consensus on how to blend the two technologies, the UK may end up with a market outlier that enjoys no economy-of-scale and -- worse -- gives end users a cumbersome or unreliable experience. With only 282,000 devices for police, fire and EMS across the UK, ESMCP may pay a premium for the late addition of vital functionality.

Challenges aside, the move serves as a clarion call to the industry and points the way to a rational approach for direct mode support that bypasses serious limitations accompanying LTE-based mechanisms. What we look for, however, is tight integration of simple device-to-device function with two internal radio modules supervised by the device operating system. One radio module will provide all the LTE functionality and handle MCPTT over virtual talk groups. The other can be a simpler analog radio operating on a simplex channel or the more complex mechanism provided by TETRA's direct mode operation capabilities. But the ecosystem needs to formulate a harmonized approach that ensures interoperability between devices of different vendors.

Clearly, when it comes to LTE-based MCPTT, the UK is at the tip of the sword. The decisions now being made will shape the broader global ecosystem approach to MCPTT. Governments, ultimately, will be purchasing these systems to boost the safety of citizens. Now is the time for governments around the globe to take a unified stand and demand a harmonized approach for MCPTT direct mode operation. The UK may have limited options due to the time pressures of the impending Airwave retirement. But the European Commission and the US government must take a procurement stand now to ensure device availability that enables retirement of voice-centric radio systems. This means the establishment of non-LTE supplemental direct mode operation standards that can be incorporated into requirements for future LTE device procurement. With anticipated 3GPP mechanisms for proximity services (ProSe) in the wings, the UK failed to provide this key element. Nations that follow the UK, however, have no excuse. The need is clear and the time to act is now.

Operational Impact

The end users of MCPTT are remarkably varied. Some are administrative users. Others are security guards or bus drivers. But for emergency services workers that must enter structures or remote areas outside the coverage of the LTE network, an ability to communicate in the absence of a supervising network becomes a life-safety issue. 

Deployable systems are a valuable adjunct that can address network coverage gaps. But these systems are not ideal in the early stages of a fluid, fast-moving incident. Structure fires are prime example of an environment where the first arriving personnel will not have the resources or time to establish a temporary network. 

In these cases, having a reliable device-to-device communication channel that bypasses the network is essential. Devices must be able to operate at higher power levels than an LTE device and, if possible, at lower frequencies. The downside operationally, however, is that the device will be nominally heavier than a consumer-grade LTE device or a typical TETRA device. An external antenna attached to a RSM will also have an impact on the device form factor. While US emergency services workers are used to heavy radio units and RSMs, users in other parts of the world tend to use smaller, lighter devices.