White Space Networking with Wi-Fi like Connectivity; Bahl, Chandra, Moscibroda, Murty, & Welsh

This paper discusses WhiteFi, a Wi-Fi like protocol meant for use in UHF white spaces. The difficulty in configuring networks in the UHF realm is efficient channel assignment and Access Point (AP) discovery without causing interference from packet transmissions. WhiteFi must deal with spatial/temporal variation (different channels available in different locations/times) and spectrum fragmentation (non-contiguous channels).

WhiteFi uses special hardware called KNOWS which consists of a UHF translator that translates Wi-Fi signal to the proper UHF band, scanner that searches for signals, and processor that performs a FFT on the incoming signals. APs select the channel to operate on by finding the channel that maximizes the metric MCham, their expected share of the channel as calculated from all associated clients; channel selection happens periodically.

Variable channel widths complicate the process of clients finding an AP. Instead of scanning every possible channel at every channel width, clients scan a subset of channels and use the SIFT technique to detect an AP in hardware. Moving this process from software to hardware and reducing the number of scans improved performance significantly in simulation. One important aspect of channel assignment the paper neglects is incumbent detection; the authors bank on future research solving this problem and assume that part is dealt with.

When an AP or client senses an incumbent on the channel, it disconnects. WhiteFi handles these disconnections via a chirping protocol. In the APs normal beacons, it advertises a backup channel. In the case of disconnection, AP and client switch to the backup channel and transmit chirps until a new channel can be decided selected.

This paper had a number of aspects that were mysterious to me. First off, I didn't have much previous knowledge of the UHF spectrum, and the paper's introduction didn't really help that much. (Thanks to Wikipedia for a very helpful summary and description of UHF.) Secondly, why all this focus on wireless microphones? Out of all the electronic equipment out there, are these really the main concern in terms of wireless transmitters that could be impacted by interference from packets? Maybe it's just that in particular they are prevalently used with difficult-to-predict usage. An unfortunate fact of research on the UHF spectrum is that full-scale evaluations cannot be done without FCC approval, so simulations have to suffice.