Not only are these frequencies limited to line-of-sight paths, low-power signals can traverse only a few hundred meters under ideal conditions (which are rare), and are attenuated by almost anything, from precipitation to leaves. They won’t penetrate common building materials either, including the low-emissivity glass used in new construction and replacement windows. “Low-e” glass works fantastically for reducing UV rays from the Sun, but it’s metal-oxide coating is equally effective in attenuating sometimes entirely blocking millimeter-wave signals.
In addition to its propagation challenges, millimeter-wave operation requires further development of semiconductor technologies such as silicon-on-insulator and silicon germanium, greater integration with baseband components, massive MIMO on a scale that befits its name, and Active Electronically-steered Array (AESA) antennas currently the exclusive domain of next-generation military radar systems.Considering the challenges presented by millimeter-wave operation, it may seem odd that one of the first applications of 5G will be at 24 and 28 GHz in the form of fixed wireless access (FWA). On one level, delivering residential wireless broadband at these frequencies makes sense, because it’s a good “beta application” for millimeter-wave technology. It will allow carriers to further develop millimeter-wave wireless systems based on insights from actual operation rather than trials or simulations. It’s also a point-to-multipoint application, so it won’t have to serve mobile devices or deal with the integrating these bands into smartphones that are already cramped for space.
The small cell scenario
One of the issues whose challenges are often understated is just how much infrastructure or “densification” will be required to support 5G in the next few years and later as millimeter-wave frequencies come online. Industry organizations typically state the increase in base stations at ten times those in use today for 4G (for “hyper densification” 150 small cells per km2). More than 10 million small cells have already been deployed throughout the world to serve 4G, but this is just the beginning.