This spectrum being called white space has historically been used for the transmission of television through the air. In the recent FCC incentive auction the FCC got a lot of TV stations to migrate their signals elsewhere to free up this spectrum for broadband uses. And in very rural America much of this spectrum has been unused for decades.
Before Microsoft or anybody can use this spectrum on a widespread basis the FCC needs to determine how much of the spectrum will be available for unlicensed use. The FCC has said for several years that they want to allocate at least one channel of the spectrum for unlicensed usage in every market. But Microsoft and others have been pushing the FCC to allocate at least three channels per market and argue that the white space spectrum, if used correctly, could become as valuable as WiFi. It’s certainly possible that the Microsoft announcement was aimed at putting pressure on the FCC to provide more than one channel of spectrum per market.
The biggest issue that the FCC is wrestling with is interference. One of the best characteristics of white space spectrum is that it can travel great distances. The spectrum passes easily through things that kill higher frequencies. I remember as a kid being able to watch UHF TV stations in our basement that were broadcast from 90 miles away from a tall tower in Baltimore. It is the ability to travel significant distances that makes the spectrum promising for rural broadband. Yet these great distances also exacerbate the interference issues.
Today the spectrum has numerous users. There are still some TV stations that did not abandon the spectrum. There are two bands used for wireless microphones. There was a huge swath of this spectrum just sold to various carriers in the incentive auction that will probably be used to provide cellular data. And the FCC wants to create the unlicensed bands. To confound things, the mix between the various users varies widely by market.
Perhaps the best way to understand white space interference issues is to compare it to WiFi. One of the best characteristics (and many would also say the worse characteristics) of WiFi is that it allows multiple users to share the bandwidth at the same time. These multiple uses cause interference and so no user gets full use of the spectrum, but this sharing philosophy is what made WiFi so popular – except for the most crowded environments anybody can create an application using WiFi and knows that in most cases the bandwidth will be adequate.
But licensed spectrum doesn’t work that way and the FCC is obligated to protect all spectrum license holders. The FCC has proposed to solve the interference issues by requiring that radios be equipped so that unlicensed users will first dynamically check to make sure there are no licensed uses of the spectrum in the area. If they sense interference they cannot broadcast, or, once broadcasting, if they sense a licensed use they must abandon the signal.
This would all be done by using a database that identifies the licensed users in any given area along with radios that can search for licensed usage before making a connection. This sort of frequency scheme has never been tried before. Rather than sharing spectrum, like WiFi, the unlicensed user will be only allowed to use the spectrum when there is no interference. As you can imagine the licensed cellular companies, which just spent billions for this spectrum are worried about interference. But there are also concerns by churches, city halls and musicians who use wireless microphones.
It seems unlikely to me that in an urban area with a lot of usage on the spectrum that unlicensed white space spectrum is going to be very attractive. If it’s hard to make or maintain an unlicensed connection then nobody is going to try to use the spectrum in a crowded-spectrum environment.
The question that has yet to be answered is if this kind of frequency plan will work in rural environments. There have been a few trials of this spectrum over the past five years, but those tests really proved the viability of the spectrum for providing broadband and did not test the databases or the interference issue in a busy spectrum environnment. We’ll have to see what happens in rural America once the cellular companies start using the spectrum they just purchased. Because of the great distances in which the spectrum is viable, I can imagine a scenario where the use of licensed white space in a county seat might make it hard to use the spectrum in adjoining rural areas.
And like any new spectrum, there is a chicken and egg situation with the wireless equipment manufacturers. They are not likely to commit to making huge amounts of equipment, which would make this affordable, until they know that this is really going to work in rural areas. And we might not know if this is going to work in rural areas until there have been mass deployments. This same dilemma largely sunk the use fifteen years ago of the LMDS and the MMDS spectrums.
The white space spectrum has huge potential. One channel can deliver 30 Mbps to the horizon on a point-to-point basis. But there is no guarantee that the unlicensed use of the spectrum is going to work well under the frequency plan the FCC is proposing.