Wednesday, August 19, 2009

WhiteFi: Wi-fi like networking in the UHF White Spaces

This week our paper, joint with Microsoft Research, on White Space Networking with Wi-Fi Like Connectivity was presented at SIGCOMM 2009, where it actually won the best paper award. This paper lays the foundations for the first Wi-Fi like network operating in the UHF white spaces (that is, the portions of the TV spectrum unoccupied by TV channels, wireless mics, and other devices). There's been some press on this work from Technology Review, Engadget, and other sites. My student, Rohan Murty, gave the talk. He is pictured to the right, apparently wearing the UHF antenna on his head -- I am not sure whether this improves his mental capacity or not. (Update 8/24/09: The slides are now available.)

By way of background, in 2008 the FCC issued a ruling allowing unlicensed devices to operate in the UHF white spaces, under certain restrictions. Opening up this spectrum for unlicensed wireless networks is a huge opportunity -- for example, UHF devices would achieve much longer range than networks operating in the 2.4 GHz and 5 GHz ISM bands. There's been a lot of recent research on establishing individual links in the UHF white spaces, but to our knowledge nobody has proposed a network design allowing multiple clients to communicate via an access point. That's where WhiteFi comes in.

Networking in the UHF white spaces raises a number of interesting challenges. The first is that the spectrum is fairly fragmented, and we can make use of variable-width channels (unlike the standard 5 MHz channels used by existing 802.11 networks). This makes AP discovery more difficult since there are many combinations of center frequencies and channel widths that would require scanning.

The second is that, by FCC mandate, a white space device must avoid interfering with any "primary users" of the spectrum. TV channels are relatively easy to avoid, given that they don't tend to come and go (although a mobile device would need to determine when it is coming in range of a new station). It turns out that wireless microphones also operate in this band, and of course you can't predict when one might be turned on. This requires the use of channel sensing to rapidly determine the presence of a wireless mic and mechanisms for switching an access point and any associated clients over to a new channel when interference is detected.

In WhiteFi, the key idea is to use a software-defined radio to scan the physical RF channel and use an efficient algorithm for performing AP discovery without performing a full decode on the signal. The SIFT technique (described in the paper) is a simple time-series analysis of the raw samples from the SDR that quickly determines if there is an AP operating at the chosen center frequency, as well as its probable channel width. The SDR is also used to detect incumbents. WhiteFi also includes algorithms for assigning channels to APs based on spectrum availability, as well as for handling disconnections due to interference or station mobility.

Going forward, we are continuing to collaborate with Microsoft Research and are developing a white space testbed here at Harvard that will allow us to experiment with these ideas at larger scales. Ranveer Chandra, Thomas Moscibroda, and Victor Bahl from the Microsoft Networking Research group are all involved in this effort.

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