Spectral Sensing: A cognitive radio technique that monitors transmissions on particular channels and reports if they are busy. A spectral sensing device monitors while using the spectrum itself. This results in an autonomous, self-contained device that is simpler and less costly to build and may be used anywhere. The device must be able to sense signals that are hundreds of times weaker than a TV station, so this makes it more costly in the short term, in terms of hardware.
Database Look-up: A device determines its location through GPS or other means and consults a dynamically updated database with information on which channels are free in the region. The device then knows which channel to use. Although this is more expensive and complex to implement, the FCC has established this method for all white space devices, as there is considerable disagreement about whether spectral sensing devices can be made to work reliably. But this method has inherent limitations that reduce the scope of the service, at least in the short term.
