Spectrum sharing refers to a wide variety of techniques aiming at using spectral resources in geographical locations where any particular part of the spectrum can be used simultaneously by at least two systems. Signals from other systems can be separated by detecting unused time or frequency slots, for instance. This closely resembles cognitive radio, where white-space detection is an essential feature.
A very well-known example is the use of the frequency band reserved for fixed satellite communications. Satellite stations transmit beacons providing information about spectrum usage. If the cognitive system does not receive this beacon then it is free to use the spectrum. A second example is femtocells. A common requirement with femtocells is that they do not cause interference to the primary system, which has rights to use the band. Especially for intersystem sharing, where the primary system (cellular network) and the femtocell belong to different operators, it is important to somehow guarantee that the femto-to-macro interference is minimal. In intrasystem sharing we have more options: by allowing controlled amount of interference to the macro system, we can maximize the sum rate of the macro and femto layers.
Spectrum sharing has become a high-priority research topic over the past few years. The motivation behind this lies in the fact that the limited spectrum is currently inefficiently utilized. ...