Spectrum sharing takes place when multiple systems or services operate in the same frequency band. The challenge is to enable each system to operate successfully without interfering with, or receiving interference from, other systems using the same band. Sharing agreements can be static (e.g., fixed geographic separation between systems) or dynamic (e.g., priority access to shared spectrum). The sharing agreement should minimize the areas/times for which spectrum is unused. Effective, efficient spectrum sharing is enabled by four technical capabilities:
- Spectrum surveying/monitoring. Spectrum surveys and spectrum monitoring are necessary to determine which bands are good candidates for sharing in space or time. Spectrum surveys measure and analyze the geographic and temporal distribution of RF signals within a spectrum band. If a band is unused within an area, then geographic sharing of that band is possible. Within a given geographic area, the percentage of time that the band is being used is the occupancy. Spectrum monitoring produces continuous observations of band occupancy. If band occupancy is low, then that band is a candidate for sharing; a newcomer system could use the band when it is unoccupied by incumbent systems.
- Electromagnetic compatibility analysis. Two RF spectrum-dependent systems with overlapping coverage in the same RF band may or may not interfere with each other. Electromagnetic compatibility analysis (EMC) determines the interference protection criteria (IPC), the engineering parameters that specify combinations of frequency and geographic separation that, when met, enable two RF systems to co-exist.
- Spectrum efficiency. Because RF spectrum is a both a limited and shared resource, it should be used efficiently. In practical terms, this means that RF systems should reduce their demand on spectrum (in terms of area and time), to the lowest level consistent with acceptable system performance. Spectrum efficiency has been studied for at least 50 years, yielding significant insight into the complex factors that determine spectrum efficiency.