White Space Networking with Wi-Fi like Connectivity

[paper]

The problem: How can we use UHF white spaces (unregistered portions of the wireless spectrum) for network connectivity? This is more challenging than standard WiFi because:

• Spatial variation in spectrum matters more in white spaces because the FCC requires non-interference with wireless transmissions of incumbent users (e.g., TV).
• White spaces are more likely to have spectrum fragmentation, which each fragment potentially being a different width.
• Temporal variations because of wireless microphones. Need to immediately switch to another channel when wireless mic is transmitting.

Their proposal: WhiteFi adaptively configures itself to handle the above challenges.

• They propose an adaptive spectrum assignment algorithm that periodically checks space availability of both the access point AND the clients. Every AP & client maintains a spectrum map that knows what channels are in use and an estimate of how much each channel has been used. Clients periodically send the AP updates on their spectrum map. An AP looks for a new channel if an incumbent starts to use part of the channel: this can either be voluntary (incumbent is causing minor performance degradation) or involuntary (a wireless mic has completely killed the channel). When a switch happens, the AP looks for the best available space as determined by both the AP's and its clients' spectrum & availability maps. The AP then broadcasts the choice to all users. (Question: how does this broadcast work if the wireless mic is totally killing the channel?)
• Variable width channels makes AP discovery hard. They propose SIFT (Signal Interpretation before Fourier Transform) to determine packet widths based on signal amplitudes. They can then search the space using SIFT, which will figure out the width of the channels.
• This one answers my earlier question about how to handle an involuntary disconnection. Each AP maintains a backup channel. If there is a disconnection, the AP will broadcast the new channel information on the backup channel and/or the client will chirp that it's been disconnected (depending on who first discovers the disconnection).

Evaluation: In a small experiment, they showed that switching on a mic causes a disconnection between a client and an AP of at most 4 seconds. They also used a simulator to examine their spectrum assignment algorithm and find that, in practice, it performs nearly as well as the theoretical optimal. (Figures 11 and 12 look pretty impressive.)

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Class notes:

White space: Lower frequency. Many frequencies are assigned but not used widely. Digital TV opened up a lot of space between channels because it is compressed with coding. Great frequencies for wide area transmissions.

Wifi = ISM