AT&T expects its mobile 5G deployments operating on millimeter-wave spectrum to have transmitter sites that will be within 150-250 meters of each other, based on the performance results from the carrier’s fixed millimeter-wave trials to date, according to company officials.

AT&T’s current mobile broadband offerings typically are delivered via network infrastructure utilizing spectrum from the lower 700 MHz band to the lower 2 GHz band. In contrast, AT&T plans to provide 5G services—featuring high speeds and latencies below 10 milliseconds—that utilize millimeter-wave spectrum at 39 GHz, which does not propagate signals nearly as well as the lower-band airwaves.

AT&T CFO John Stephens said company officials are encouraged by the results of fixed 5G trials that have been conducted in four cities during the past two years.

“We're seeing gigabit-plus speeds under line-of-sight conditions to distances up to 900 feet and with extremely low latency rates, some as low as 9 milliseconds,” Stephens said during AT&T’s recent quarterly conference call. “These trials have shown that millimeter-wave [signals are] able to penetrate foliage, glass—and even walls—better than anticipated with no discernible signal performance impacts due to rain, snow or other weather issues.”

Dave Wolter, assistant vice president of radio technology and strategy for AT&T Labs, said company engineers are confident that fixed-broadband connections that leverage millimeter-wave spectrum—swaths of several hundred MHz—will deliver desired performance at ranges exceeding 300 meters under line-of-sight conditions.

With fixed wireless at millimeter wave, we have a lot of antenna gain to work with,” Wolter said during an interview with IWCE’s Urgent Communications. “At the base station, we have an antenna array of maybe 64 elements that are beam-forming, so they concentrate the energy in the direction of interest toward your user. The user also has an antenna array with higher gain than what you would have in a mobile device, and it uses higher power than typical mobile devices are using.”

AT&T did not use hardened antennas for its fixed 5G trials, so the receiving device was located inside of a standard construction window, Wolter said. When deployed commercially, AT&T expects the antennas to be located outside, he said.

“With millimeter waves, you lose quite a bit of power just going through a window, so we’re having to deal with that, as well,” Wolter said. “If you can put [the antennas] outside, you’ll get substantially more range, just because you don’t have that loss through the window.”

In fact, if a building has certain energy-efficient windows, the weak propagation characteristics of millimeter-wave signals will not be enough to penetrate the building, Wolter said.

“If you have something like Low-E glass, it’s going to kill it [the millimeter-wave] signal altogether,” he said. “I would anticipate that, for most fixed-wireless uses, you’re going to have the antennas on the outside and bring the signal inside in whatever way makes the most sense.”