The main reason is that the access to the radio spectrum is ruled by a restrictive regulatory regime that emerged when the Radio Act of 1927 declared the "ether" to be a publicly owned resource. Hence the spectrum is mainly allocated to licensed radio services and cannot be freely used by other users. This happens even when the legal "owner" is not using its assigned portion of the spectrum.
Cognitive radio is a new technology that address the drawbacks of the license based spectrum regulation. Cognitive radios are intended to openly access a wide range of unused spectrum, both licensed and unlicensed, while they respect the licenses of primary users. To this end
- Cognitive Radios are aware of their environment: geographical position, spatial/temporal presence of licensed users, modulations employed,...
- They adapt their transmission schemes, bandwidth and modulation to avoid primary users and maximize the spectral efficiency.
- They collaborate with other cognitive users to increase their funcionality. They may establish complex iterations with other elements.
- Cognitive users may perform complex processing of the available information to infere the best response to the environment.