MCPTT ‘no longer just a theory,’ but off-network questions remain, according to webinar speakers

Mission-critical-push-to-talk (MCPTT) services already have been deployed in real-world situations, and users should be confident in MCPTT voice when on a purpose-built LTE network, although the path to robust MCPTT when users are off network remains unclear, industry experts said during a recent webinar.

AT&T has announced that FirstNet users should be able to subscribe to MCPTT services that are “fully compliant” with the 3GPP standard during the second half of next year. However, MCPTT services already are being provided in South Korea, according to Sagi Subocki, vice president of products and marketing for Softil, which has developed the core of an MCPTT software client.

In addition to participating in MCPTT plugtests that are designed to validate that vendors’ solutions work and interoperate, Softil’s solution is part of two deployments of the technology—not just MCPTT, but also mission-critical data and video (MCX)—associated with South Korea railway communications, Subocki said.

“This is not proof-of-concept or field testing,” Subocki said during last week’s webinar (click here for free registration to an on-demand archive of the event). “This is actually commercially deployed, running MCPTT communications.

“What we’re trying to say here is that the 3GPP MCPTT/MCX standard is no longer just a theory. This is a reality in the field running. It’s basically taking mission-critical communications to the next level.”

In one implementation, the Softil client offering is running on the Korea Telecom network that utilizes a Samsung server, Subocki said. If all components meet the 3GPP standard, there are many benefits, he said.

“Once you implement your solution standard, then you are able to provide the end user—the agencies, the customer—[the ability to] pick and choose,” Subocki said. “They can take our client and have it running on a Sonim device, a Nemergent server.

“Everything will run seamlessly, and it’s all standard-compliant and tested to be interoperable; there’s no vendor lockdown. And, in the future, if you want to change any component with internetwork, it’s not a problem, because you are changing one standards component from one vendor to another that is also standards-compliant.”

Fidel Liberal, coordinator of the Mission Critical Open Platform (MCOP) initiative, also emphasized the importance of open standards for MCPTT, a standard that includes key performance indicators (KPIs) that are almost identical to those of P25, the primary public-safety LMR protocol in the U.S.

“The performance targets defined for MCPTT are comparable with PMR,” Liberal said. “It’s doable, it’s achievable, and we will be seeing this in the near future.”

Given this baseline target for MCPTT functionality, all four webinar panelists said they are confident that standards-compliant MCPTT will meet the voice-communications needs of public safety when the user is on a broadband network that provides the appropriate priority and quality of service to push-to-talk voice.

“I’d feel pretty comfortable with that,” according to Chris Kindelspire, who chairs the National Public Safety Telecommunications Council (NPSTC) LMR LTE Integration and Interoperability Working Group. “You said the key piece: I’m in [network] coverage, I have all of the different pieces I need and mechanisms to make it all work … If it’s a perfect environment, I’d feel pretty confident.”

Bob Escalle, Sonim Technologies’ vice president for public-safety market segments, echoed this sentiment.

“On network, absolutely [MCPTT will work],” Escalle said. “I think, with priority, preemption, QoS and a dedicated EPC core, I think the confidence level is growing and building and is there today.”

But the speakers’ confidence level with MCPTT is very different when a robust broadband network is not available, as there is no off-network solution within the 3GPP standard that can provide the kind of long-range communications that LMR can provide on a radio-to-radio basis when the network is unavailable.

“Public safety is a heavy user of direct-mode, off-network [communications],” Kindelspire said. “That’s one key piece that I feel like we’re trying to hold back and wait and see what that piece is going to be. How is public safety going to adopt those pieces?

“Today, we work in two different environments. We work on a wide-area trunked network for dispatch, but we still go off network for major incidents.”

The MCPTT standard includes a Proximity Services (ProSe) standard to support device-to-device communications—not just for voice (MCPTT), but also for data (MCDATA) and video (MCVIDEO). However, the range of ProSe is limited, as LTE devices typically use only 0.25 watts to transmit signals with devices that usually have an internal antenna.

By comparison, LMR portables feature an external antenna that supports signals being transmitted at much higher power levels of between 1 watt and 5 watts—in other words, four to 20 times the power levels of a typical LTE device—often on spectrum that provides better propagation characteristics than the airwaves used by commercial carriers.

In addition, Escalle and Liberal said that chipmakers are focused on 5G and vehicle-based communications (V2X) needed to support autonomous cars, so they have not demonstrated much interest or support for ProSe, Samsung has said that it has developed a chip that supports ProSe and has embedded it in devices, but the company has not disclosed information about the performance of those chips in trials to date.

These challenges mean that public safety and other mission-critical users “may be waiting for a very, very long time” for a ProSe solution to be developed that is comparable to direct-mode LMR communications, Escalle said.

With this in mind, Sonim Technologies has been pursuing alternative solutions to the direct-mode dilemma, according to Escalle. One is a 900 MHz accessory to Sonim LTE devices that leverages LoRa technology for device-to-device communications. This solution supports a range of 1-2 miles, but it does not interoperate with existing LMR protocols, Escalle said.

Sonim also is developing the Sonim LMR Enabled Detachable (SLED), which the company expects to showcase during the IWCE 2019 event in March. The SLED is designed to attach to a Sonim XP8 rugged LTE smartphone, resulting in an LMR-like form factor and allowing the user to take advantage of high-powered LMR direct-mode communications when a broadband network is not available.

While the SLED could be designed to utilize any LMR protocol—P25, DMR or TETRA—one unanswered question is what spectrum should be used for such alternative direct-mode operations, Escalle said. Leveraging existing LTE airwave for this purpose would limit the power output and could create significant interference issues, but utilizing existing LMR spectrum—notably, interoperability and mutual-aid channels—could work effectively, he said.

“Can we dedicate a few of those channels to this work effort on device-to-device, so we can have an easier way to have interoperability between our existing narrowband assets [and MCPTT devices],” Escalle said. “I think, if we don’t solve and implement some standards-compliant or standard-based device-to-device [solution] outside of ProSe being available to us, I think we’re not going to get to the adoption of mission-critical PTT as quickly as we’d all like.”

In addition to device-based solutions, panelists noted that industry is developing other approaches to tackle the issue of ensuring that users can have reliable push-to-talk communications in “off-network” scenarios when a broadband network is not available. Some notable possibilities include “bring the network with you” solutions that have been proposed in form factors as small as backpacks or Pelican cases to large vehicle-driven deployable solutions.

“I don’t really see that as a technology hurdle,” Kindelspire said. “I think right now it’s really trying to figure out what the ecosystem’s going to be.”