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Note that sensing-based whitespace radio has a number of issues. The biggest one is the hidden node problem - you can interfere with receivers without being able to hear the transmitter. There is also the issue that the licensed users of the band need to be very interference-resilient for the first few ms of their transmissions, until all of the sensing-radio users realise that they aren't allowed to be there anymore (and then the sensing radios need to have a consistent way to keep the link up). The first problem can be mitigated by making the sensing receiver significantly more powerful than the transmitter, although you still have issues with specific terrain layouts (e.g. something big and RF-attenuating between you and the transmitter). The second problem makes it very difficult to interoperate with existing licensees (who assume exclusive use of the spectrum); it may be possible to make it work better with new licensees. There is another interesting approach to cognitive radio, which is to use a centralised database of spectrum allocation, and then use RF propagation models & surveys to find gaps in space & frequency where you can let people transmit - for example, there are big gaps between terrestrial TV transmitter zones, to avoid the multi-kW transmitters interfering with each other, where you can easily fit "big WiFi"-style transmitters transmitting at a few watts. Obviously, there are a number of issues here (need GPS or an accurate location on installation, need internet access for the DB, need to restrict movement, inefficient without really good RF models, weather affecting RF propagation) which make this less of a panacea than sensing-based cognitive radio, but it is relatively simple and robust to implement. In particular, a lot of the issues disappear if you use it for "big WiFi" applications - access points already have a fixed location, internet access, and mains power. It also opens the possibility of a much more dynamic marketplace for spectrum - if all spectrum users are checking with a centralised DB (at least in a particular band), then it becomes much easier to handle short-term/local licenses - for example, providing massive short-term additional cell/wifi service to big events and festivals. tl;dr Cognitive radio is a very interesting and promising development, but it's much harder than initial intuition suggests. |