The long-awaited 600MHz incentive auction has finally started but it could still be months before we know which operators have won the low-band spectrum.

The auction started on March 29 but -- unlike the AWS and 700MHz auctions -- this one doesn't simply involve selling off a bunch of empty airwaves for mobile use. This is because broadcast TV runs over the 600MHz spectrum now. (See FCC's Monster Auction Ends at $45B in Bids.)

So, the initial reverse auction action is intended to set the prices for broadcasters relinquishing 600MHz spectrum. The UHF spectrum will then be repackaged so broadcasters transmit in smaller portions of the bandwidth, while mobile operators will be offered neighboring (contiguous) spectrum for mobile services.

This re-banding exercise could take the Federal Communications Commission (FCC) several weeks at least. The spectrum will then be auctioned off to mobile operators and other interested parties. There are 104 companies and organizations that are eligible to bid in the auction. (See FCC's Wheeler: Go High for 5G, 600MHz Switch Will Be Slow.)

AT&T Inc. (NYSE: T), T-Mobile US Inc. and Verizon Wireless are all expected to bid on the auction. Sprint Corp. (NYSE: S) says it doesn't need low-band spectrum and is sitting the auction out. (See T-Mobile CEO: We'll be a 600MHz Auction Winner and Sprint Might Sit Out 600MHz Spectrum Auction.)

The low-band 600MHz spectrum is good for combining with other high-frequency bands to improve overall network coverage and in-building penetration. The FCC has already suggested that the 600MHz spectrum will be a bedrock frequency for forthcoming 5G services.

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"The timing of the incentive auction makes the 600MHz band a prime candidate for deployment of a wide-area 5G coverage layer," wrote FCC chairman Tom Wheeler in a blog. "In much the same way that 700MHz paved the way for America's world-leading deployment of 4G, so could 600MHz accelerate US deployment of 5G."

This is because it usually takes at least a couple of years to open up spectrum for commercial use after auction. Initial commercial 5G services are expected to start around 2020.

— Dan Jones, Mobile Editor, Light Reading