Thursday, March 03, 2005

Recycling TV Bands Into Broadband

Wireless regional broadband service may be coming to a TV channel near you. No, this isn't the Internet on TV, it's using channels originally assigned to TV stations to transmit a new type of wireless broadband service with a range as far as 30 miles and the ability to penetrate walls.

The new standard is just starting development now. It's designated 802.22 and called WRAN for Wireless Regional Area Networks by the IEEE. It's also picked up the nickname WiTV for WIreless over the TV band.

If you think that WiTV sounds like something similar to WiMax, you are right. They are both new wireless networking standards and both with the ability to operate over a range of miles rather than the 300 ft. radius typical of WiFi networking. So why yet another standard? To be sure, WiTV and WiMAX have some overlap in their results, but they also have some key differences. WiMAX will operate on the microwave bands of 2-11 GHz and 10 to 66 GHz. WiTV is targeting the UHF and VHF television bands that are located between 54 and 862 MHz.

The significance of these bands is that radio waves propagate differently at different frequencies. Lower frequencies tend to travel farther and more easily penetrate structures. For instance, a hard rain will break up your satellite TV reception, but doesn't seem to affect over the air broadcasts. When there is a storm pattern with low clouds, you might even clearly pick up a station hundreds of miles away for a short period of time.

WiTV offers the possibility of true regional wireless Internet service that will be just as easily accessed in rural areas as it is in town. With WiTV in your laptop computer, you could log onto your VPN, send and receive email, surf the Web, or enter data into a website sales form in the customer's office, in your car (stopped, I hope), on a table at home, or in a motel room that offers no Internet service.

The other big appeal of WiTV is the allure of using spectrum that isn't already in service for wireless data networks. Truth is that the UHF television band never really filled in with broadcast stations. After the VHF band became fully occupied and broadcasters were pushed up the dial into the much wider UHF band, cable and satellite broadcasting started to pick up steam. So how about giving someone else a shot at putting that spectrum to work?

Sounds good, but at the moment a lot of stations are squatting on two channels as they begin their digital HDTV broadcasts. The plan was that broadcasters would establish themselves on their new digital channels and then abandon their traditional analog channels. The cutoff was supposed to be 2006. There's not much chance of that actually happening on schedule, but the FCC is not waiting around to get better use utilization of the UHF TV spectrum. It's given the nod to wireless networking on those channels, with the provision that they can't interfere with stations already there.

This is where the 802.22 standard is expected to get very clever. There are a couple of innovative ways that new signals can be introduced into already assigned spectrum and not cause a problem. The first is to include a GPS locating system in each access point and compare the physical location of the transmitter with a database of licensed services assigned in that area. The frequencies that a particular wireless access point could use would be limited to protect existing services. Move the access point and you change the allowed channels it can transmit on.

An even more radical plan is to develop what is called a Cognitive Radio (CR). A radio that can think? No, actually the cognitive part means a radio that is aware of what is going on in the radio spectrum around it. It searches for unused channels and uses those while avoiding channels with existing signals. This sounds a little like the original Ethernet CSMA/CD or Carrier Sense Multiple Access/Collision Detect system, doesn't it? I guess if you consider radio signals to be something akin to digital packets, it makes sense to actively avoid the interference that is caused by colliding signals.

The cognitive radio approach offers a new dimension in spectrum utilization and one that might spread beyond sharing channels between television and broadband. This concept could be extended to other parts of the spectrum so that new services have some place to go well before the legacy services reach the end of their useful life.

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