5G should be 10 to 20 times faster than today's cell-phone networks. 5G will operate in a high-frequency portion of the radio spectrum, known as millimeter wave. It has a lot of available bandwidth and should make it possible for wireless devices to process data with minimal delays. But since its wavelengths are much shorter, it is more easily obstructed. And because it has never been used for consumer mobile services, carriers are still learning how 5G signals will behave in different types of terrain and weather. "We need to look at how the signals are affected by things like snow, rain, sleet, hail, maple trees, oak trees, and spruce trees, because each of those will be different," says AT&T research engineer Bob Bennett.

The problem: most 5G measurement equipment is so expensive, fragile, and bulky that it can be deployed outdoors for only a few hours at a time. ­Bennett and colleagues say that far more real-world data is needed to properly develop the technology, so they have created weatherproof radios the size of toaster ovens and installed them across AT and T's 260-acre campus in Middletown, New Jersey, which was once part of Bell Labs.

Since deploying the radios last September, the engineers have seen how tree leaves, heavy rain, and truck traffic all obstruct millimeter-­wave signals to some extent. AT and T plans to share the information with the rest of the telecom industry to aid in the design of 5G technical specifications, base stations, modems, smartphone chips, and more. The new technology won't be commercially widespread until after 2020, but these small, homemade radios are a crucial step toward making it real.