To defend against this threat, a transmitter verificationscheme called localization-based defense (LocDef) was proposed
in , which verifies whether a given signal is that of an incumbent transmitter by estimating its location and
observing its signal characteristics. In a practical case of cognitive radio networks, the primary users can mainly be
composed of TV signal transmitters (i.e. TV broadcast towers)and receivers. Their locations are typically determined.
If a malicious user wants to emulate the primary user and its location is almost the same as the primary user, secondary
users would not receive the signal of the malicious user since the transmit power of the malicious node is much
smaller than a TV tower. If the secondary users receive a high power signal from the malicious user, it means that the
malicious user must be very close to the secondary user.Thus, the secondary user can determine whether a transmitter
is a primary user or malicious user just by estimating thelocation of the transmitter. The transmitter verification scheme includes three steps: verification of signal characteristics,measurement of received signal energy level, and
localization of the signal source. The first two steps have been investigated thoroughly. For the third step, there are
many techniques that can be used to estimate the location of the transmitter, such as Time of Arrival (TOA), Time
Difference Of Arrival (TDOA), Angle of Arrival (AOA),and Received Signal Strength (RSS). Take RSS as an example:
there is a strong correlation between the distance of a wireless link and RSS. Therefore, if multiple secondary
users take RSS measurements from a transmitter, the transmitter location can be estimated using the relationship between distance and RSS. Thus, the key to counter against PUE attack is to determine whether the transmitter is a primary
user or a malicious user.