LTE ProSe direct-mode communications works with limited range, Samsung official says

Proximity Services (ProSe)—an LTE standard for direct-mode communications when network connectivity is not available—works today, but its range is limited to “maybe half a mile” because of the low power levels of LTE devices, according to a Samsung official.

“ProSe is already available today,” Timothy Paul, senior sales manager for Samsung Electronics America, said during an IWCE 2020 Virtual session webcast last week. “[But] It’s not launched, or it’s not fully tested.

“I will say that ProSe is at the early stages. There have to be a lot of developments as we go along to handle off-net kind of solutions.”

On March 31, AT&T announced FirstNet PTT, the first push-to-talk-over-cellular (PoC) offering based on the mission-critical-push-to-talk (MCPTT) standard developed by 3GPP. Those who have used FirstNet PTT when connected to the FirstNet system have applauded its audio quality and low-latency performance, which many have judged as being superior to push-to-talk communications on LMR networks used by public-safety agencies.

But most LTE devices do not support communications when they are not connected to a broadband network, typically an LTE or Wi-Fi system. ProSe was created to address this communications gap, so LTE users could communicate directly to each other, without the need for a network connection—the kind of direct-mode communications supported by LMR devices.

Although the ProSe standard was established more than four years ago, most vendors did not make chipsets supporting the functionality. A notable exception is Samsung, which developed a working ProSe chipset years ago for use in South Korea but historically declined to discuss its performance—until Paul spoke at the IWCE session last week.

“It [ProSe] works today, but the range depends on the surroundings and what kind of building situations you are in,” Paul said.

“The range of ProSe … I would not say that it’s ready. I mean, it’s maybe half a mile. But if you’re in a building or you’re in a dense situation, you know that gets significantly reduced, right? It’s not a high-power device, so those aspects need to be considered.”

Indeed, the physics associated with ProSe on a typical LTE device has long been a concern within public safety, which often needs to communicate when users are outside of terrestrial network coverage.

LTE devices typically use only 0.200 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.

Paul said that ProSe is being used operationally in South Korea. ProSe is not yet offered in the United States, although Samsung Galaxy XCover FieldPro—the rugged device that supports FirstNet PTT—includes a ProSe chipset that makes it technically capable of direct-mode communications, if allowed by a network operator.

From a functionality standpoint, LMR direct-mode communications supports only push-to-talk voice, while ProSe device-to-device communications enables not just voice (MCPTT) but also data (MCDATA) and video (MCVIDEO).

Paul also noted that ProSe currently supports only direct-mode communications where users are off network—have no network connection. Although the 3GPP standard envisions the possibility of “daisy-chain” users via ProSe to link to a network connection, that functionality is not yet available, he said.

“If you go into an area where there is no coverage, ProSe-enabled devices can detect who are the other devices nearby, and—if they are already provisioned—you can set up a group call amongst them,” Paul said. “That’s the capability that you will have today when it comes to ProSe.”

“It [ProSe] is strictly off-net—it is not network-assisted or transitory between network coverage and off-net coverage. It’s strictly off-net today.”

Public-safety representatives have focused on the lack of direct-mode communications as a primary reason why MCPTT should not be considered an alternative to LMR for push-to-talk voice communications. However, many MCPTT proponents have noted that the availability of LTE network coverage continues to increase, and myriad deployable solutions—from vehicle-deployed LTE systems to those that can carried in a Pelican case or backpack—are available, meaning users may not be off network very often.

This also could be true inside buildings and other facilities if public safety were able to leverage Wi-Fi systems automatically, because Samsung’s MCPTT works on Wi-Fi, Paul said.

“We have Wi-Fi support on MCPTT,” Paul said. “That helps a lot, because Wi-Fi is more universal [in buildings than LTE] and MCPTT can run over Wi-Fi. ProSe is an addition to it.”