White Space Networking with Wi-Fi like Connectivity

P. Bahl, R. Chandra, T. Moscibroda, R. Murty, M. Welsh, "White Space Networking with Wi-Fi like Connectivity", ACM SIGCOMM Conference, (August 2009).

This paper presented WhiteFi, the first WiFi like system constructed on top of UHF white spaces. WiFi builds on a technique called SIFT (Signal Interpretation before Fourier Transform) that helps in reducing the time to detect transmissions in variable width systems by analyzing raw signals in time domain. To elaborate the problem in a bit more detail, FCC recently has permitted the use of unlicensed devices in the frequency band of 180 MHz to 512 MHz comprising of 30 channels (21 to 51 with the exception of 37) of 6 MHz each. This opened up a great opportunity of have a WiFi like system on this White Space instead of conventional ISM bands. However, FCC directive requires that communication must not interfere with already existing communication on the channel due to TV channels or microphones. This makes the problem very interesting due to 3 characteristics:

• Spatial Variation: 'Channel use'  is dependent on the location of sender and receiver. Hence there should be a way of communicating as to which channels are busy and which of them are available.
• Spectrum Fragmentation: The UHF white space are often fragmented due to presence of incumbents. The size of each fragment can vary from 1 channel to several. A bigger contiguous channel = More Throughput! So it is the responsibility of the protocol to look for contiguous channels.
• Temporal Variation: UHF band also suffers from temporal variation due to widespread use of microphones. Hence the protocol was designed to be robust to these changes in band availability.

The WhiteFi protocol takes all the above characteristics into account and was developed on the KNOWS hardware prototype. The spectrum assignment policy involves the exchange of a spectrum map and airtime utilization vector between the AP and the client to find out which of the channels are free for use at both the ends. Such a design tackles the problem of spatial variation. Further, once the AP gets the utilization vector, given a range of UHF channels, it calculates the MCham (multichannel airtime metric) and chooses the set of channels with maximum MCham. Further, SIFT is used for efficient variable bandwidth signal detection. The authors propose 2 algorithm choices for the same -- the Linear SIFT-Discovery Algorithm (L-SIFT) or the Jump SIFT-Discovery Algorithm (J-SIFT). J-SIFT outperforms L-SIFT when number of channels is greater than 10.

The evaluation was done using a combination of simulations and experiments and the results pretty much matched the expectations. J-SIFT pretty outperformed both Baseline non-SIFT and Linear SIFT in time taken to discover the AP. Overall WhiteFi due to its adaptive nature was successful in asserting its supremacy over any other static optimal implementation of channel communication policy. Particularly, figure 14 was a treat to watch that highlighted how well the protocol behaved to changes temporal changes in spectrum.

Critiques

This is definitely one of the very good papers we have read as part of this course. Not only does it tackle a very real problem, it presents a efficient and optimal solution which works despite the stringent constraints laid down by FCC. However, I have few comments on this paper:

1. I thought the paper lacked some analysis on how does the protocol behaves when multiple microphone users interfere on a regular basis (that is how efficient is the J-SIFT based channel hopping). Though Fig. 14 provided some insight, it would have been great to have a deeper analysis involving throughput/overhead over a relatively long period of time.
2. Theoretically, all the channels could be busy when someone wants to communicate. Isn't that a big usability issue -- 'My Internet may or may not work'! Is it really the case? Can all frequencies between 180-512MHz be assigned for TV/Microphone transmissions?