In radio communications, spectrum is a scarce resource. Often, there can be more than one non-interworking users for a particular channel or frequency. In general, in such a case one of those users is designated as the primary user and the other as the secondary user. The secondary user of a frequency would have additional constraints defined for its operation including regulations for power and constraints put on the spectral emission mask. The primary goal of all these restrictions is to allow the secondary users access to the spectrum as long as they do not interfere with the operation of the primary user.
The 5 GHz spectrum is typically put to three distinct primary uses: maritime communications radars; weather radars; and military communications. The interesting thing about these users is that even though they are the primary users of a large chunk of spectrum, they use it very sporadically and typically require very low bandwidths. So, in order to make best use of this underutilized spectrum, regulatory authorities in the European Union have developed a concept called dynamic frequency selection (DFS). The DFS regulations impose very strict timing and non-interference requirements on the secondary users of the spectrum, which also make the spectrum useful only in an unlicensed/lightly-licensed scenario. Over the years, the use of DFS has spread throughout the world. It has been one of the major drivers for adoption of Wi-Fi as DFS provides it with a huge spectrum which is lot less noisy than the 2.4 GHz ISM band.
Each regulatory regime has defined pulse patterns for radars, which a secondary user is expected to detect in real time and take corrective action so as to not interfere with them.