secondary users from accessing that band. In PUE attacks, the attacker only transmits on the channels that are not used by
primary users. Therefore, the secondary users regard the attackers as primary users and do not try to access the channels
that are not used by primary users. As pointed out in there are several types of PUE attacks. In a selfish PUE
attack, an attacker tries to make use of the unused spectrum.When a selfish PUE attacker detects an unused spectrum
band, it transmits signals that emulate the signal characteristics of a primary user and prevent the secondary users from
using it. Thus, the attacker can make use of the vacant channels that are not used by primary users. However, for a malicious
PUE attack, the malicious attacker just tries to prevent the transmission of the secondary users without using it.
There exist some more complicated PUE attacks. Some attackers can even attack only when the primary user is off,
which means that attackers can save energy.