Defining white space

Most of the spectrum below 10GHz across most of the world is allocated for particular applications and assigned to certain users. These include broadcasters, mobile phone network operators, defence departments and many, many more. The net result is that there is little obviously spare spectrum, particularly in the preferred frequency bands between around 300MHz and 3GHz where propagation is favourable but antennas are conveniently small. However, measurements of the actual utilisation of this assigned spectrum suggest that it is typically only used in around 20% of the locations. Such measurements undoubtedly underestimate usage but nevertheless it is clear that there is some potential for more efficient use of the spectrum.

One way to visualise where there might be underused spectrum is to plot on a map the strength of signal from the licensed user of that band. If colours are used to represent signal strength then those parts with no coverage at a given frequency will be uncoloured and appear white on a black and white map. Hence the term ‘white space’ for areas where there is potential for others to use the spectrum.