When the Federal Communications Commission (FCC) made the momentous decision late in 2008 to allow unlicensed broadcasting devices access to "white spaces" in the television spectrum, backers hailed the move as a major step forward for US wireless networking. "WiFi on steroids," was how one engineer put it during the debate.

But for white space devices to move from laboratory concept to store shelves, they would need more than steroids; they would need some sophisticated engineering. That's because the FCC imposed two critical conditions: whitespace devices must sense local transmissions from televisions and wireless microphones in order to avoid transmitting on those frequencies, and the devices must also access a geolocation database of known transmitters as a backup solution in case spectrum sensing failed.

(To see the available white space TV channels in your area, check showmywhitespace.com.)

Google, Microsoft, and others promptly got to work on the database, and spectrum sensing technology has already existed commercially for years. But how would a white space device—say, one located in the kitchen next to the family computer—actually communicate with the access point providing a connection to the Internet?

Such problems have already been solved many times over by wireless networking solutions as diverse as Bluetooth, 3G data transmission, and WiFi, so adapting one of the existing networking techniques to the new spectrum might seem like a no-brainer. But white space transmission features a host of challenges not found in conventional WiFi installations, challenges that meant existing WiFi protocols would not be optimal without some tweaking.

Microsoft Research's KNOWS team

Enter "WhiteFi," a proposal from Microsoft Research to create a WiFi-style system in which multiple clients can connect to a local access point operating in the UHF TV band. WhiteFi is the latest project to emerge from Microsoft Research's "Networking Over White Spaces" (KNOWS) project.

For its first trick, the research team handled mesh networking, in which each whitespace device connects to its neighbors, which connected their neighbors, to create a spontaneous local network. Recently, the team has turned its attention to replicating the traditional WiFi model (PDF) of a central access point with multiple client devices connected to it, but getting there required quite a bit of work.

The problems

The differences between WiFi and the white spaces go far beyond the difference in spectrum (WiFi operates most commonly at 2.4GHz and 5.0GHz, while white space devices can operate over 30 separate 6MHz TV channels in the UHF band).

For one thing, white space devices have to contend with far more "spatial variation" than does WiFi. As transmitters are moved around the neighborhood, city, or the country, the channels on which they can broadcast will change to avoid existing TV transmitters.

This creates more than regional variation. Imagine an access point that finds a clear space on channel 27 and begins accepting connections from client devices. But one of the client devices is one kilometer away in a location where a TV broadcast channel 27 can be faintly detected. The channel can't be used. The access point therefore needs to query all of the client devices when negotiating its data channels, making the system more complex than WiFi, which may need to worry about interference, but does not have to hop off any particular channel because of it.

A second (and related problem) is the variation of the data channels over time. Imagine an access point merrily transmitting data to five white space client devices in the neighborhood. Everything progresses swimmingly until someone at the church next door switches on a wireless microphone. The access point must immediately drop its signal on that channel and find a way to notify all of its clients about a move to some new (and clear) channel.

The prototype device

To find out just how many packets of data such wireless mics could handle before interference became audible, Microsoft researchers hauled one of their trial white space devices and a wireless mic down to an anechoic chamber. The results were not good; as the research paper puts it, "even a single packet transmission causes audible interference during wireless microphone transmissions."

In the real world, the problem might not be quite as bad, since transmitters and microphones will rarely sit within a few feet of one another, but it's still a serious issue.

Finally, white space devices face issues with spectrum fragmentation. While WiFi broadcasts on a single channel, there's no particular reason to do this when designing a white space device. In many locations, the available spectrum will feature several contiguous free channels, and if channels 25, 26, and 27 are all open, the device can get the most throughput by broadcasting on a wider spectrum or by channel bonding.

But this also poses problems; signals of varying widths take more time to detect, and the channel assignment problems noted above all get more difficult to deal with.

The solutions

The Microsoft Research team has addressed many of these issues with WhiteFi. First, the team crafted an efficient way to detect "variable-bandwidth signals," called SIFT. Then, it tackled spectrum assignment algorithms (finding and using the largest block of contiguous free channels isn't always the best technique, as some other white space devices might be transmitting on one or more of those channels, creating interference that could reduce speeds).

Handling disconnections, such as when a wireless mic suddenly pops on and forces a channel change, takes place using a special 5MHz backup channel. Whenever an access point or a client device detects an TV or wireless mic signal on a channel it is using, it stops transmission immediately and sends out a series of "chirps" on the backup channel. All access points and client devices check this channel every few seconds, and the chirps will help all the devices work out where to transmit next.

(Ah, but what happens when a wireless mic pops up on the "backup channel" itself? The researchers propose a secondary backup channel to deal with this situation.)

How does it all work? Pretty well, it seems. In the team's testing, the SIFT approach to channel scanning was 34 percent faster than the baseline approach. Channel disconnections were dealt with in under four seconds.

Such work is going on among various white space developers (Motorola and Philips are working on devices, for example), but it's also taking place at the IEEE, where engineers on the 802.22 working group are developing techniques for Wireless Regional Area Networks over the TV white spaces.

We're still some ways from seeing devices on store shelves, but with a projected 1km transmission distance and the promise of broader transmission channels, white space devices are shaping up into some promising technology. Once seen largely as a way to offer fixed wireless connections to rural homes, white space devices will soon be able to create mesh networks and WiFi-like connections—that early promise of "WiFi on steroids" might turn out to be surprisingly accurate, after all.