Underlay and overlay is the coexistence of two signal sources together which are active in the same time ans at the same frequency but have different transmitting powers. In case of the underlay one is very weal and the other is very strong. So the weak will very slightly disturb the strong one. In case of overlay the the overlay is the stronger source and the other is the weaker source. In fact Overlay is the stronger and the underlay is the weaker. The stronger can detect his signal with acceptable bit error rate. The weaker in order to detect his signal he must detect the strong signal at first and then cancel it and detect his signal. It may be similar to NOMA nonorthogonal multiple accessing. Also one can make the underlay signal very wide band by spreading it on this wide band while strong signal is narrow band. In this way one can also detect the wideband signal in existence of the strong signal.

Best wishes

Thanks, Abdelhalim abdelnaby Zekry for your response.

In underlay, the secondary user controls his transmitting power to a certain limit.

In overlay, the secondary user utilizes channels, which are not used by the primary users with out having any restriction over power. They will use until the primary users demand them or trying to use.

thanks @Anveshkumar It is interview paradigm and not overlay in which the SU utilizes the empty channels of PU.