The use of very high frequencies for communication has always been “just around the corner,” where, because of significant technical challenges, it’s remained for 50 years. Now, faced with scarce spectrum available at lower frequencies, the wireless industry is determined to overcome those challenges.
The fifth generation of wireless communications, commonly called “5G”, will be a technology tour de force once it’s rolled out after 2020. One of its most impressive accomplishments will be the use of radio frequencies far, far higher than have ever been used for cellular networks or almost anything else, a region called millimeter wavelengths. This is a very big deal for many reasons. But first, it’s important to understand how these frequencies fit into the spectral landscape, how they differ from their lower-frequency counterparts, and why except for satellite communications, vehicle radar, and defense systems, they’ve not been used.
The electromagnetic spectrum is massive in its breadth, covering frequencies from 1Hz (the domain of lightning) to above 10 exahertz (10EHz, or 10 quintillion hertz, where gamma rays live). The region between about 1MHz and 300GHz is where communications can take place, of which only a tiny portion is actually used (1MHz to 30GHz). And of that, less than 20 percent (about 6GHz) is densely populated by wireless communications systems including shortwave, AM, FM, TV broadcasting, cellular, amateur radio, and land mobile radio, as well as assorted unlicensed applications like Wi-Fi, Bluetooth, ZigBee, microwave ovens, and industrial and medical systems. This is the “sweet spot” where communications over significant distances can be accomplished, ranging from spanning the world at the low end to a few miles at the high end (Figure 1).
Figure 1: The millimeter-wave portion of the spectrum, also called Extremely High Frequency (EHF) is generally defined as beginning at 30GHz and extending to 300GHz. (Source: NASA)
However, the swath of spectrum between about 150MHz and 1GHz is prime spectral real estate because it poses fewer challenges such as signal loss, the ability to penetrate buildings, glass, and other impediments, while offering wide-area coverage. So it’s not surprising that this region accounts for almost all communications throughout the world, an increasing problem as there is precious little unused spectrum left. Thus, the FCC can auction remarkably narrow slices of this spectrum for billions of dollars.
So now the powers that determine the future of wireless communications have determined that it will collectively venture into the relatively unexplored spectral frontier as high as 60 GHz and eventually even 100 GHz, which is 10 to 100 times higher than ever before. On the face of it this is certainly logical, but as always the devil is in the details, and they account for why millimeter-wave frequencies have never been widely used.
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Barry Manz is president of Manz Communications, Inc., a technical media relations agency he founded in 1987. He has since worked with more than 100 companies in the RF and microwave, defense, test and measurement, semiconductor, embedded systems, lightwave, and other markets. Barry writes articles for print and online trade publications, as well as white papers, application notes, symposium papers, technical references guides, and Web content. He is also a contributing editor for the Journal of Electronic Defense, editor of Military Microwave Digest, co-founder of MilCOTS Digest magazine, and was editor in chief of Microwaves & RF magazine.