The success of license exempt spectrum in Europe is obvious. A multitude of very diverse wireless applications is available for e.g. the 2.4 - 2.4835 GHz license exempt band, using just as diverse techniques and implementations. The contribution of license exempt spectrum to the European (and Dutch) economy is substantial. This success has his adverse sides too. The fast growing number of consumer wireless devices in license exempt bands threaten to clog up the available spectrum.
The usage of common spectrum by highly heterogeneous radio applications with ad-hoc spectrum access is by itself not very efficient. Higher spectrum efficiency and lower interference levels can be achieved by the implementation of a spectrum access mechanism. This mechanism should sensing of the presence of others, and use social rules to achieve an optimal distribution of the available spectrum in terms of frequency space, time space and coverage area. A reaction to the detection of another device could be:
- Reduction of coverage overlap by power reduction, polarisation change or directivity change;
- Reduction of spectral overlap by frequency change or bandwidth change;
- Reduction of overlap in time by adapting the timing of transmissions;
- Reduction of interference intensity by changing modulation schemes and time sequences.
If no other spectrum users are present, the radio system may use the entire allocated frequency band. When other radio systems are present, the radio system should avoid causing interference to these other systems. When congestion occurs, the radio system should restrict itself in such a way that all devices have an equal or fair chance to access the spectrum. The research questions to be answered are:
What Spectrum Access Mechanism for license exempt radio systems performs best?
Is it necessary that all devices have an identical Spectrum Access Mechanism?
Will the implementation of a Spectrum Access Mechanism result in a better group spectrum efficiency than without?
The spectrum access mechanism should be optimized for best group spectrum efficiency. The requirements imposed by the spectrum access mechanism must be specified in a generic way, and should not have preference to a technology specific. Spectrum access mechanisms for license exempt device can only regulate behavior in the physical layer, as nearly all other implementational decisions are free - and must remain free - to the designer.