The FCC has started its auction for 3.5 GHz mid-band spectrum for 5G.

Many experts consider mid-band spectrum to be the sweet spot for 5G, offering the best compromise between speed and building penetration. Many international carriers have focused primarily on mid-band spectrum as they have rolled out their own 5G networks.

In the US, however, carriers’ efforts have largely focused on the low and high-band (mmWave) varieties. All three carriers have deployed some mmWave offerings in cities, but mmWave requires repeaters and base stations every couple hundred meters and has very poor penetration. These factors mean it will likely never be used as a nationwide option.

As a result, T-Mobile and AT&T have both rolled out low-band networks to provide nationwide 5G coverage. Low-band, while offering excellent range and penetration, is only marginally faster than 4G. Verizon, on the other hand, does not have enough low-band spectrum to roll out a dedicated, nationwide 5G network and will have to rely on switching technology to allow 4G and 5G to be served from the same towers, using the same spectrum.

T-Mobile currently is the only one of the three carriers that has a wealth of mid-band spectrum, thanks to its Sprint acquisition. T-Mobile’s spectrum is in the 2.5 GHz range, and should offer speeds in the 600 – 700Mbps range.

The FCC is auctioning spectrum in the 3.55 to 3.65 GHz range, which should offer slightly higher speeds than T-Mobile’s mid-band, although slightly less range and penetration as well.

“5G is critical to America’s global economic and technological leadership, and the start of the 3.5 GHz auction today is a key milestone in our work advancing this national priority,” said FCC Chairman Ajit Pai.

It’s a safe bet this will be one of the most important spectrum auctions in recent years, with Verizon and AT&T eager to flesh out their 5G networks. It remains to be seen if T-Mobile will purchase some of the spectrum, or if they are content with the mid-band they already have.