A spectrum overlay technique is a spectrum management principle whereby a secondary user uses a channel from a primary user only when it is not occupied. Spectrum overlay techniques are based on a detect and avoid mechanism such as DFS. The secondary user senses the frequency spectrum, if a primary user is active the channel will not be used.
This mechanism is e.g. used by RLANs in the 5 GHz range. The RLANs use a frequency band that is also used by radar systems.