The next generation of wireless tech, 5G, promises a frictionless future: We'll be able to do whatever we do on our phones much, much faster, and more devices can come online without slowing down the works. Self-driving cars, smart meters that track electricity usage, and health-monitoring devices may all take a big leap from childhood to adolescence. 5G will happen in the airy realm of radio waves. To get there, big telecoms have to harness underused parts of the spectrum.