3.2.1.1. Primary User Emulation Attack
Primary User Emulation (PUE) attack is one of security problems that physical layer needs facing, which has great threat to Spectrum Sensing. The attacker sends CR signal by imitating the primary user’s signal characteristic. This kind of attack method can realize a highly flexible and software based air interface under the circumstance of CR. In the environment of Dynamic Spectrum Access (DSA), the primary user can utilize the authorized frequency band free of all times. The authorized frequency band turns into idle state when the primary user releases the resources, so the subprime users can attempt to access [8]. One necessary condition is the sub-prime users must be able to perceive the existence of free frequency band. Hence, it needs Spectrum Sensing algorithm to carry out real-time perception for spectrum state by detection devices. At this time the attacker creates fully similar signal as the primary user does to cause an error frequency spectrum, which lead subprime users to make mistakes for the spectrum state. This will let the channel free in the system, and give attackers have the opportunity to access such channels. This kind of attack is referred to as Primary User Emulation attack [9].