Officials provide more specifics about mission-critical push-to-talk (MCPTT) standard for LTE, FirstNet deployment

Under the new MCPTT capability, direct-mode communications—known as proximity services, or “ProSe,” in the 3GPP standard—will be available to users when the macro LTE network is unavailable. This direct-mode communications also will be available by user choice, with Thiessen noting that many fire departments prefer to use direct-mode communications while at an incident scene, even if they are within the coverage provided by the LTE macro network.

Rectifying potential interference that could be caused by direct-mode subscriber devices—known as user equipment (UE) in 3GPP—in the latter scenario was a significant challenge within 3GPP, according to Thiessen.

“This [interference from direct-mode users on the macro LTE network] was a chief concern of us driving this technology into 3GPP from the commercial mobile-network operators,” he said. “They were worried that, ‘Oh my gosh, we’re going to have a bunch of devices communicating in the same spectrum that we have the macro network, and they’re going to interfere.’

“What we did was we drove in a lot of control. When that UE is in the macro-network footprint, that macro network is going to control that UE’s use of direct mode, so that they don’t mutually interfere with each other.”

While the MCPTT-over-LTE standard is designed to support direct-mode voice communications, some have questioned whether it can perform as well as LMR, which features subscriber units that typically operate at higher power (at least 1 watt compared to 0.25 watts for LTE) and often utilize lower-band VHF/UHF frequencies that propagate signals farther than FirstNet’s 700 MHz spectrum.

Thiessen acknowledged the physics issues, and said that PSCR officials are anxious to begin testing of equipment in real-world settings to determine the performance characteristics of LTE in comparison with LMR systems. But at least one study indicates that the performance differences between the two direct-mode approaches may not be as great as originally thought, he said.

“We’re going to benchmark what LMR can do and what LTE can do,” Thiessen said during an interview with IWCE’s Urgent Communications after the IWCE session. “Harris Corporation—I think in Release 12—actually submitted into the RAN a link-budget analysis comparing LMR and LTE. It found that, because some of the advanced stuff that LTE can do, there wasn’t a whole lot of problems link-budget wise.

“It was a paper calculations, which is why we’re going to try to get real radios and test it. But it was heartening to see that the gap—even on paper—wasn’t huge.”