Thursday, March 27, 2008

Looking For Spectrum In All The White Spaces

Now that the winners of the recent 700 MHz spectrum auction are being announced, a cold chill is descending over the wireless industry. It's gone. It's all gone. Once the analog TV transmitters are shut down and the auction winners claim their new slices of the electromagnetic spectrum, there will be no decent place to go for over-the-air real estate. Like beach-front property, it's all bought up and they ain't makin' any more of it. Or is it?

Instead of a land-rush that's over with all property staked out and registered, think of the spectrum as more like choice territories in the Cretaceous period, some 65 million years ago. Those T-Rex's, the guys with the big teeth, each ruled their piece of territory to the exclusion of all others. Think of each powerful TV station as one of these T-Rex's. Once they've multiplied to the capacity of the land, there's really no room for any new predators. They'll be stomped-and-chomped as soon as they stick their heads up. Or will they?

While it's true that the spectrum seems full of thunder lizards, there is really lots of territory available if you are small and nimble like a furry little mammal. You have the world at your disposal as long as you are smart enough to stay out of the way of those ferocious big guys. This is what advocates of using the "white spaces" in the TV band are thinking. It should be OK to squat on any frequencies that aren't being blasted with RF from those thundering over-the-air digital TV stations.

Google is the latest to point out that the broadcast television bands are not fully occupied. Part of the reason is the legacy of analog television standards that divide the spectrum into 6 MHz channels. In any given area, you won't find TV stations on adjacent channels. There's always at least one blank channel separating TV transmissions. That's because early tuner designs based on coils and capacitors weren't so precisely selective. For a receiver to pull in a powerful station on one channel and completely reject an equally powerful station on the next channel over was too much to ask in the age of vacuum tubes and rabbit ears. Even today, you can sometimes see a TV station bleeding into an unused adjacent channel if you are close enough to the transmitter.

When channels 2 through 13 in the VHF band filled up, the FCC simply opened up a new UHF band with 56 more channels. It was no problem, because those ultra high frequencies weren't much in demand right after WWII. Over the decades, the UHF band also began to fill up. But it never filled up entirely. The construction, operating and programming costs for high power television stations became less attractive after cable and satellite delivery gained popularity. So much so, that the FCC felt justified in carving off the chunk of UHF spectrum from channels 52 to 69 and putting them up for auction to the wireless companies.

Even what's left of the broadcast spectrum won't be fully occupied with high power TV transmissions or even low power repeaters and translators or limited coverage stations. Right now it's as full as it's going to get, with each broadcaster spreading out with an analog transmitter on one channel and a digital transmitter on another. But half of those are going bye-bye next February 17 when the analog transmitters get shut down for good. The auctioned spectrum is given to the winners for other uses and the broadcasters settle into their assigned DTV channels. The channels that aren't assigned or are used as "guard" channels between local broadcasters will sit there empty. These are the white spaces that might be pressed into service.

What could this white space spectrum be good for? How about meshed networks of WiFi hotspots that could spread to provide broadband service in rural areas? WiFi currently has just a handful of channels available and they are located in the 2.4 GHz and 5 GHz bands where they don't carry or propagate very far. The VHF band from 54 to 216 MHz and reduced UHF band between 470 and 698 MHz can penetrate buildings much easier. Why not press those white space frequencies into service and essentially get a new "free" WiFi band?

Broadcasters are suspicious. They fear that thousands of unlicensed transmitters operating right next to their signals or, worse, right on top of their signals will ruin their over the air business completely. There are a couple of work-arounds to prevent this. First, load a database of all licensed and occupied channels into each white space transmitter along with a GPS receiver. The transmitter will then avoid channels known to be in use in a given area. Another approach is to use "spectrum sensing" to listen for transmissions on a channel before using it yourself. The sensing would have to be done a level that ensures any receivable digital TV signal would be left alone.

Early implementations of proposed white space transmitters failed miserably and were withdrawn from consideration. But that's more of an engineering development problem than a fatal flaw in the theory. With demand for new wireless applications increasing and available spectrum being locked up by carriers with deep pockets, it seems reasonable to at least seriously consider getting the most value from every MHz out there. With Google now advocating for white space usage, the idea has a good chance of picking up steam and getting the nod for deployment in the next few years.

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