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AT&T, however, says its early trials show that these problems can be overcome, at least in certain conditions. In South Bend, Indiana, AT&T set up a 5G trial for small business and residential customers. The trial "Successfully provided gigabit wireless speeds on mmWave spectrum in both line-of-sight and some non-line-of-sight conditions," AT&T said in an announcement yesterday.

That qualifier—"some non-line-of-sight conditions"—suggests that AT&T has been able to overcome the physical limitations of millimeter wave spectrum in only limited circumstances. AT&T didn't say what circumstances produced the positive result—we've asked the company for more details and will update this post if we get answers.

AT&T had other good news to report. In a trial involving small businesses in Kalamazoo, Michigan, AT&T "Observed no impacts on 5G mmWave signal performance due to rain, snow, or other weather events." AT&T said it also "learned [that] mmWave signals can penetrate materials such as significant foliage, glass, and even walls better than initially anticipated."

There was no suggestion that the Kalamazoo trial succeeded in any non-line-of-sight conditions. But AT&T said the trial produced gigabit speeds in line-of-sight conditions to users up to 900 feet away from the cell sites.

The other trial in Waco, Texas, "Provided 5G mmWave service to a retail location more than 150 meters away from the cell site and observed wireless speeds of approximately 1.2Gbps in a 400MHz channel," AT&T said.

The Waco trial also produced "latency rates at 9-12 milliseconds" and "supported hundreds of simultaneous connected users using the 5G network," AT&T said. It's not clear how many users were able to receive gigabit speeds simultaneously.

AT&T's average download speeds over 4G are below 15Mbps, according to a January 2018 reportby testing firm OpenSignal.

Latency of 9 or 12 milliseconds could be a big improvement over 4G. AT&T led the industry "with an LTE network ping time of 58.3 milliseconds," the OpenSignal report said.

AT&T presumably only reported the best results from its trials, so the average 5G latency from the trials might have been higher than 9 or 12ms. Still, the shift from 4G to 5G is expected to reduce latency and boost speeds industry-wide. AT&T says that 5G's low latency will help support communication between self-driving cars, for example. And the best 5G latencies could end up being lower than 9ms, as AT&T has said that "the industry expectation for 5G is latency less than 5 milliseconds."

Even after mobile services over millimeter-wave spectrum are widely available, the high frequencies will be a complement to existing networks rather than a full replacement. Carriers will use millimeter waves to improve their networks where they're able to, but they won't abandon the sub-1GHz spectrum that's so effective at covering virtually the entire United States.

UPDATE: An AT&T spokesperson some of our questions after this article published. The 5G trials established connectivity at each site with fixed wireless networks, and individual users accessed the networks via Wi-Fi from "a variety of mobile devices" including smartphones and tablets, AT&T said. While fixed wireless is obviously different from mobile, AT&T said that "the learnings about mmWave that we gained during the trials will guide our mobile deployments, since those will also use mmWave spectrum."

AT&T also gave us more detail on how the test networks performed in line-of-sight and non-line-of-sight conditions:

Both propagation measurements and our trials are showing mmWave performing better than expected in the 28 and 39Ghz band in both line of sight and non-line of sight conditions. Specifically in NLOS conditions, we’ve seen the signal perform well despite foliage and building structures. We’ve also been testing adaptive beamforming and beam tracking to enable non-line-of-sight 5G services in our trials. In terms of bad weather, especially at our Austin trial, we have not seen any observable signal degradation due to weather and the trials have lasted multiple months through all kinds of rainfall.