Sonim Technologies is developing an accessory for its rugged LTE devices that would let users engage in direct-mode P25 push-to-talk communications while operating on licensed LMR spectrum in the VHF band, according to company officials.

Sonim Technologies officials privately demonstrated a working prototype of a P25 direct-mode accessory—often referenced as a “sled” by company representatives—last month to some attendees during IWCE 2018 in Orlando. Sonim Technologies CEO Bob Plaschke said the company plans to complete development of a P25 direct-mode accessory “in the next few months,” although the final design could differ from the “sled” prototype demonstrated during IWCE 2018.

“There’s a lot of different ways of doing this,” Plaschke said during an interview with IWCE’s Urgent Communications.

Most industry analysts believe that direct-mode communications—the ability for users to send and receive signals directly between users’ devices when the LTE system is unavailable—represents the toughest challenge for the mission-critical-push-to-talk (MCPTT) standard will face when trying to replicate LMR push-to-talk functionality.

MCPTT is expected to perform well when the user device is operating on an LTE network—particularly one that enables quality of service, priority and preemption (QPP), such as FirstNet. However, MCPTT’s proximity-services (ProSe) functionality—the direct-mode standard within MCPTT—is not expected to provide the same kind of communications range as first responders experience with higher-powered LMR radios that often operate on spectrum that propagates better than the 700 MHz airwaves licensed to FirstNet.

If the proposed P25 accessory works as designed, the direct-mode performance should be comparable to public-safety LMR direct-mode communications, because it would leverage the same over-the-air protocol and the same licensed spectrum that is used by many public-safety P25 users today. The solution also would provide another method for LTE users to interoperate with LMR users.

Prior to IWCE 2018, Sonim Technologies announced its new XP8 and XP5s rugged LTE handsets. Both devices support a module accessory—located on top of the handsets—that enable direct-mode communications that leverage LoRa technology and 900 MHz unlicensed spectrum.

Many IWCE attendees expressed interested in the 900 MHz direct-mode module from a conceptual perspective—as an alternative to ProSe—but several expressed doubts about its practical usefulness. Not only does the 900 MHz module communications operate on unlicensed spectrum—something most public-safety officials would prefer to avoid—the LoRa technology is not used for voice by public safety, so it could create an interoperability problem.

Bob Escalle, vice president of Sonim Technologies’ public-safety market segment, acknowledged these issues when making a public reference to the P25 accessory effort during an IWCE conference session.

“We’ve actually taken a P25 module and put it onto a smartphone,” Escalle said during the session. “We’ll be supporting multiple bands on a smartphone device as an option. So, we have 900 MHz ISM unlicensed band direct-mode capability, and we will also have a P25 option, as well.”