Low-emissivity glass used in modern homes and apartments could become an impediment to delivering 5G connectivity to users indoors, Light Reading hears.

US carriers are currently testing "millimeter wave" radios for use in fixed and mobile 5G services. Light Reading has now been told that the 28GHz radios have trouble penetrating low-emissivity (low-e) glass, which is designed to insulate the home while cutting UV rays from the sun.

Low-e glass works by using an extremely thin metal-oxide coating on the window. Other window designs also use gas fillings and reflective coatings for better energy efficiency.

Multiple people have confirmed to Light Reading that low-e glass nearly completely blocks mmWave 5G signals. The lower-band cellular signals used today -- from 800MHz to 2.5GHz in the US -- are much better at penetrating common building materials, albeit by no means perfect.

In the US, AT&T Inc. (NYSE: T) and Verizon Communications Inc. (NYSE: VZ) are already conducting field tests with 28GHz radios. Both carriers are aiming to deploy commercial fixed wireless, likely in 2018, with initial mobile 5G services to follow in 2019-2020.

Fixed wireless services had been expected to use a 5G radio -- delivering around 1-Gbit/s downloads -- self-installed by the user, via a unit that sits in the window of the domicile. This approach would reduce costs for the carrier versus cable -- as it means they don't have to run fiber to the home -- as long as the signal can get into the home.

Some trials have apparently boosted the mmWave signal to try and improve test performance, but the FCC has strict limits written into radio test licenses. NYU Wireless, meanwhile, is working on more tests and research into beam steering and being able to look for other reflective paths if the line-of-sight path is blocked.

The simplest solution, however, would be to mount the 5G antenna outside the house, run power to that box and then use a WiFi router to distribute the connectivity indoors.

This would get ambitious as a self-install project for users, particularly as they would need to get the best line-of-sight signal while clambering around the roof of their home, assuming they even have access to the roof. This then starts to cut into 5G's cost advantage over a fiber deployment, one of its selling points for both carrier -- and presumably -- the end user.

We've asked AT&T, Verizon and a 5G startup, Phazr, for comment on the e-glass issue. We'll update the story if they come back with new details.

— Dan Jones, Mobile Editor, Light Reading