Is Direct Mode the Holy Grail of Critical Broadband Communications?
By Tero Pesonen
Monday, March 30, 2020 | Comments
Direct mode, also called device-to-device (D2D) communications — or more precisely, the lack of it — is seen to be one of the main risks in moving from narrowband professional mobile radio (PMR) technologies exclusively to broadband Third Generation Partnership Project (3GPP)-based solutions. However, at times, the root cause is lost in the discussion.

Why is the public-safety and wider critical communications sector so keen on direct mode in the first place? Their fundamental requirement is to have a connection at all times in all locations. Direct mode as we know it from the narrowband world has on one side enabled communications outside the network coverage area, be it rural or inside buildings, and on the other provided redundancy in case the network wasn’t available. Direct mode has offered the core group voice, some messaging, and via a gateway, network access. Despite offering inferior service compared to the network mode, at least in digital technologies such as TETRA and Project 25 (P25), the service has been acceptable, and the operational mode has been adapted to use it when needed.

The same fundamental requirement remains applicable in the broadband era: connection at all times in all locations. However, now plain voice is not quite sufficient, as the entire idea of critical broadband communications is to move from voice-centric to information-centric operations. On a philosophical level, how the connection is established is irrelevant as long as the users on the field can rely on it, it does its job, and it is affordable. So, what are the options for providing connections to places without critical 4G/5G network coverage?

One category is to bring the network to the users. The normal solution is to extend the network coverage permanently by building new sites and introducing leaky feeders and other solutions for indoor coverage. More temporary solutions include the traditional cell on wheels or more advanced cell on wings (both known as COW). 5G introduces promising integrated access backhaul (IAB) for extending backhaul. Private Long Term Evolution (LTE)/5G networks and isolated E-Utran operation for public safety (IOPS) essentially provide a private local network at the scene.

Another category is to look for alternative access methods. For rural areas beyond financially viable network coverage, an interesting option is 5G satellite connectivity, albeit with limited bandwidth but potentially large coverage areas. However, there are challenges to provide coverage to locations where satellite is not viable, such as in-building and underground.

For local area coverage, especially indoors, direct mode remains the last lifeline. The work is ongoing in 3GPP using vehicle to everything (V2X) new radio (NR) sidelink capability for D2D communications. This includes potential for network gateways and D2D repeaters (sidelink relay). By reusing V2X capability, the goal is to benefit from the economies of scale provided by the vehicle industry and thus catalyze the interest of industry to produce the required components and products.

However, a great deal of work needs to be done to reach the point at which the fire chief, police officer and medic can trust the lives of personnel with this solution. Connection availability is what matters. For many operational scenarios, there are multiple alternatives to provide connectivity. The single most important aspect to consider is always resilience, to ensure first responders will be able to carry out their critical work safely.

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Tero Pesonen is chair of TCCA’s Critical Communications Broadband Group (CCBG) and vice chair of TCCA’s board, representing Finland’s Erillisverkot. He has been developing critical communications plans for more than 20 years. He serves as senior advisor to Suomen Virveverkko, the national operator for the VIRVE shared-authority network in Finland.

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On 8/26/20, Sanaa said:
I am surprised to read comments stating that communication will remain voice centric for first responders. What about the increasing use of Unmanned vehicles either ground or aerial That may bring new communication needs as crucial as human speech. With certainly considerable amounts of data transfer and video streaming Data communication is not generated only by screens applications or text messages.

On 4/8/20, A.V. Rutter said:
I see one issue missing in the discussion: the reason direct mode or simplex communications is still needed. Voice and voice only is the sole means of communications during the most life-threatening moments. In the midst of a chase, firearm exchange, fire suppression or life/limb-saving medical operation, the front-line person cannot spare the time and attention needed to interact with visual or data-intense input, even if it is available. During those times voice communications is vital. If beyond the range of normal network coverage, for any reason including old-fashioned LMR repeaters, simplex voice communication is vital. At that point in the emergency response, all else is an extra. There often is no time to set up any backup system. This is the crux of why direct mode/simplex is still, and in my opinion, always will be an absolute requirement.

On 4/8/20, Tero Pesonen said:
I am happy to see an active discussion. To keep it flowing, here are a couple of complementary remarks.
• It has been said that cell on wheels (COWs) take too long. How about if every vehicle had one? What would that change?
• Mesh networking technologies are certainly an option that needs to be taken seriously. Who would drive global standards for technology and spectrum harmonization to have a proper international interoperable multivendor market? Should it also have a standardized gateway to 3GPP networks for interconnection? Or is it acceptable to use a proprietary solution?
• If operational capabilities change and with that the way of working changes when moving from network service to off-network, is that an operational safety risk to the user? Is it acceptable or does the operational model need to be designed according to the lowest service level, potentially leading that anything else is just nice to have, i.e. non-critical?
• What are natural means for delivering information and building situational awareness? With that what is the role of machine-to-machine (M2M) communications? Is it for instance there to deliver sensor information from a firefighter in a building?
• What is the smart society we want to live in during the next decade? Is the society so dependent on connection that it makes sense to ensure coverage even in the darkest basement? Or is it just an unthankful task of first responders to go there?

On 4/7/20, Leon van der Linde said:
Have they ever thought of looking at mesh networking? Something similar to Rajant's idea where the POTC units can build their own mesh network on a limited basis. This could be for a few kilometers between them. This will work brilliantly. Think out of the box. The technology is there, and it works. There are several manufacturers that build these type of products for mining and the military and such industries. I suggest they look into it. Don't look at only one option. There are so many different options.

On 4/7/20, Andrew Seybold SR said:
Interesting article which misses most of the points of off-network. There is no way to provide coverage wherever and WHENever it is needed — to sub-basements, sub-ground parking garages, and into caves or the depth of a well. Waiting for a cell on wheels (COW) or a flying COW just does not cut it during an emergency. The one thing first responders do not have is time to wait.

Why this big push to get rid of LMR radio systems so that all that is left are Third Generation Partnership Project (3GPP) systems? Off-network will continue to be a vital requirement for public safety for a long time to come. If the goal is to migrate everyone to Long Term Evolution (LTE)/5G, how about if we let those in the field and those whose lives are in the balance make the decision, if and when there comes a time for that decision to be made? I do not believe for a minute that networks will ever replace off-network capabilities no matter where the incident is. Now is not the time to talk about doing away with off-network. Today two people can yell farther than than proximity services (ProSe) works. Don't be in such a hurry to obsolete something that is today the last line of communications defense for our first responders.

On 4/1/20, John Facella P.E. said:
The use of direct mode in narrowband voice is pretty clear to the fire service. It is the communications choice of last resort. However, in some U.S. cities, trunking is the standard means of communications once units are on scene. In the TETRA world of trunking, this may be less the case. FirstNet and its international equivalents have added mission-critical data to voice. However the last-resort communications will always be voice, so having direct mode voice available, especially to the fire service, remains a requirement.

One must then ask what does direct mode mean in a data world? My answer is probably very little. Firefighters with gloves on in air masks carrying tools in a smoke-filled environment cannot look at screens and type on keyboards. Further, other than for texting, data means very little in a confined network with no access to data servers and the databases for building preplans, hazardous materials information and the like.

My point is that broadband must provide a simple solution to direct mode voice, which was never envisioned in the use of complex Long Term Evoution (LTE) technology dependent on multicarrier orthogonal frequency division multiple (OFDM) technology, multiple input multiple output (MIMO) and the like. It probably can only be provided by a separate simple FM narrowband transceiver embedded in the broadband user device, and it doesn't need, at this juncture in the evolution of public-safety communications, to have data beyond an emergency alarm signal man down.

On 3/31/20, Christopher Suter said:
In general I understand the concept of data communications instead of voice but until the ability to send a verbal mayday from a basement level or anywhere else is achieved, for example, I don't see how it could work.

The concept has always been and probably always will be to get help to that person immediately from the people immediately adjacent to them. The public-safety responder will not be texting, I can pretty much assure you.

I think the whole concept of no direct mode is a non starter for the fire service. There are enormous areas of the U.S. with poor street-level broadband data, let alone subgrade coverage. Add that to the thousands of square miles with no coverage at all and you have a recipe for disaster.

A cell on a wheels (COW) is a great idea 12 hours from now but what about now? Any solution has to have a last line of defense of simple device to device, no bells and whistles, just basic extremely reliable communications.


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