28 GHz is the most popular band of millimetre wave high-frequency spectrum. It was generally little used in the U.S. but now Verizon plans to pass 30 million homes in the first phase of the buildout. Europe chose 26 GHz because satellites were using 28 GHz. There is little gear available other than for 26-30 GHz. Verizon controls 800 MHz of the band in the U.S.
39 GHz, some AT&T controlled, requires 50% more antennas than 28 GHz and is not as popular. AT&T cutbacks in mmWave mean 39 GHz mostly will be on the shelf for years.
60 GHz, shared in the U.S., was developed by Intel as WiGig but didn't find a market. CCS, Facebook, Qualcomm, and others are reviving it, especially for backhaul. The FCC has allocated 14 GHz. With enough very small antennas, performance looks good.
Other bands above 15 GHz lack equipment or supporters, so are years away. The U.S. FCC is about to auction 24 GHz.
Low and mid-band
3.3 GHz to 4.2 GHz is the emerging new band, with enough available spectrum for several companies to expect the ideal 100 MHz.
Over $10 billion of auctions are completed or coming soon, including Korea, Spain, and the UK. Until Massive MIMO reached production in 2016, these bands didn't have enough reach for most practical purposes. With MM, they have become prime, although phones are few.
2.5 GHz to 3.0 GHz has seen limited use until recently but now is the core of Sprint's network buildout. Performance is much improved with Massive MIMO.
2.3 GHz is also now coming into wider use, including at AT&T.
1.8 GHz to 2.1 GHz have been primary bands in 3G & 4G. They will be re-farmed for 5G as available. 4x4 MIMO is very effective, but Massive MIMO less so. As frequencies get lower, antennas get larger and sometimes impractical. These bands are designated FDD, which severely limits performance.
600 MHz to 1 GHz are ideal 4G bands with excellent reach and indoor coverage. Add NR software, and these frequencies become "5G," The antennas are larger and FDD does not work well for Massive MIMO.
Why many engineers consider only high frequencies 5G
Until late 2017 only mmWave was considered 5G, including in the ITU IMT-2020 definition. Theoretical peak speeds are up to 20 gigabits, although networks like Verizon top out at 5-7 gigabits. The speed is due to the large amount of spectrum available. 5G is designed to use 400 MHz in high bands; Verizon has 800 MHz.
About a year ago, the marketing people took over. They wanted to call everything 5G and pushed through the 3GPP standards group a definition of 5G that included all bands as long as a modest software tweak (NR) was included. Suddenly, even the lowest frequencies in active use - T-Mobile's 600 MHz in the U.S. - could be called "5G." The "NR" software adds only a little performance compared to 4G using the same frequency and antennas. These bands are best considered 4G-like. Whether called 4G or 5G, these bands use a maximum of 100 MHz and peak at ~ 2 gigabits.
Fortunately, 4G reaches well into the hundreds of megabits to typical users and has enough capacity to meet projected needs well into next decade.