The 100 MHz of spectrum around 2.45 GHz is shared by all sorts of technologies, from wireless LAN and Bluetooth, through video streaming to the yesterday's meatloaf you are heating up in the microwave oven. It's not hard to see the potential for it being overused. Couple this with ubiquitous complaints about non-working Wi-Fi at conferences and overuse is generally taken as a fact.
The assumption that existing unlicensed spectrum, including the 2.4 GHz band, is not enough to support all the igadgets of tomorrow is pretty much central in all sorts of efforts that push for new radio technologies. These try to introduce regulatory changes or develop smarter radios. While I don't have anything against these projects (in fact, some of them pay for my lunch), it seems there's a lack of up-to-date surveys of how much the band is actually used in the real world. It's always nice to double-check the assumptions before building upon them.
Back in April I've already written about using VESNA sensor nodes to monitor the usage of radio spectrum. Since then I have placed my stand-alone sensor at several more locations in or around Ljubljana and recorded spectrogram data for intervals ranging between a few hours to a few months. You might remember the sensor box and my lightning talk about it from WebCamp Ljubljana. All together it resulted in a pretty comprehensive dataset that covers some typical in-door environments where you usually hear most complaints about bad quality of service.
(At this point, I would like to thank everyone that ranted about their Wi-Fi and allowed me to put a ugly plastic spy box in their living room for a week. You know who you are).
A few weeks ago I have finally managed to put together a relatively comprehensive report on these measurements. Typically, such surveys are done with professional equipment in the five-digit price range instead of cheap sensor nodes. Because of that a lot of the paper is dedicated to ensuring that the results are trustworthy. While there are still some unknowns regarding how the spectrum measurement with CC2500 behaves, I'm pretty confident at this point that what's presented is not completely wrong.
To spare you the reading if you are in a hurry, here's the relevant paragraph from the conclusion. Please bear in mind that I'm talking about the physical layer here. Whether or not various upper-layer protocols were able to efficiently use this spectrum is another matter.
According to our study, more than 90% of spectrum is available more than 95% of the time in residential areas in Ljubljana, Slovenia. Daily variations in occupancy exist, but are limited to approximately 2%. In a conference environment, overall occupancy reached at most 40%.
For another view of this data set, check also animated histograms on YouTube.