Wednesday, July 30, 2008

I'm Sorry, Dave. I Can't Let You Use That Channel

After the high dollar feeding frenzy of the last spectrum auction, you might have the impression that all the good frequencies are spoken for. From DC to light, there isn't a spare kHz to squeeze in one more carrier. Or is there?

If you call the FCC office and tell them you want to start a new radio-based service and then ask what band of frequencies they can give you exclusive use of, they'll probably be polite enough not to laugh in your face. Unassigned swaths of spectrum are as rare as unclaimed swaths of land. But that doesn't mean that occupied communications channels are packed cheek to jowl up and down the electromagnetic spectrum. In fact, at any given time in any given place there is plenty of room to communicate.

The problem with making more efficient use of the various RF (Radio Frequency) bands lies in the way that transmitters and receivers operate. A transmitter sends out a signal on one or more channels, which is then picked up by a receiver listening on the same channel or channels. If two transmitters try to occupy exactly the same space at the same time, interference occurs and any listening receivers hear gibberish.

You may have experienced this using walkie-talkies, or hearing two broadcast radio stations cutting in and out on the same channel. WiFi networks and wireless phones using the same 2.4 GHz spectrum can also interfere to the point where one or both won't function properly.

Some services, such as radio and TV stations, transmit continuously 24/7. Those channels truly are spoken for. But other communications and data services only transmit some of the time. When nobody is transmitting, those channels might be used by another service. Even TV and Radio station channels aren't used to the fullest. Not every channel has a listenable signal in each and every location.

The major obstacle in the way of letting more services use the currently assigned spectrum is who's going to manage the traffic? If you just let everybody do what they want on any channel they want, you'll soon have little more than a bar room brawl where the commotion is so loud that nobody can hear anything. It's channels assigned and policed by a central authority that keeps things civil.

The way cellular telephony works gives a hint at one way to let many, many users use only a few channels without interference. A central switching system tells each phone what channel to use. When that call is done or the user moves out of range of a cellular base station, the channel is reassigned to another caller.

So is the answer to establish a central switching system for all communications and have it assign channels to whomever wishes to transmit at a given time? Very Orwellian or perhaps something reminiscent of HAL in 2001: A Space Odyssey, but not very practical and unbelievably expensive.

But what about building intelligence into the equipment that will use the spectrum? The rule could be that you can use any channel allocated for a particular purpose, but you have to listen before you transmit and only transmit if the channel is clear. In deference to established licensees, if the assigned user wants the channel, you have to get off immediately.

This is the idea behind proposed "white space" systems that would squat on unused TV channels to deliver services such as rural broadband. But it can be taken further and generalized into a concept known as cognitive radio. These would be smart radios based primarily on software that would be programmed to take a number of factors into account to avoid interfering with licensed or even unlicensed users of a communications channel. As long as there is some open spectrum, cognitive radios could find it and put it to good use.

Cognitive radio is a far cry from the crystal sets that pioneered radio broadcasting. Even the latest in spread spectrum systems that use sophisticated technology fall far short of radios that could be considered deep thinkers. White space transmitters still need to prove they can work reliably before anybody is going to unleash them on even TV channels where interference is unlikely. Expanding cognitive radio technology beyond that well-defined and limited application will be done slowly and carefully.

The sophistication of low cost processing chips may well be at the point where every radio can have a brain that is so agile and polite that shared usage of scarce spectrum is now practical. What's hard is accepting the idea that nothing is going to go wrong - go wrong - go wrong.

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