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Interference is typically regulated in a transmitter- centric way, which means interference can be controlled at the transmitter through the radiated power, the out-of-band emissions and location of individual transmitters. However, interference actually takes place at the receivers, as shown in Fig. 10(a) and (b).

Therefore recently, a new model for measuring interference, referred to as interference temperature shown in Fig. 11 has been introduced by the FCC . The model shows the signal of a radio station designed to operate in a range at which the received power approaches the level of the noise floor. As additional interfering signals appear, the noise floor increases at various points within the service area, as indicated by the peaks above the original noise floor. Unlike the traditional transmitter-centric approach, the interference temperature model manages interference at the receiver through the interference temperature limit, which is represented by the amount of new interference that the receiver could tolerate. In other words, the interference temperature model accounts for the cumulative RF energy from multiple transmissions and sets a maximum cap on their aggregate level. As long as xG users do not exceed this limit by their transmissions, they can use this spectrum band. However, there exist some limitations in measuring the interference temperature. In some articles, the interference is defined as the expected fraction of primary users with service disrupted by the xG operations. This method considers factors such as the type of unlicensed signal modulation, antennas, ability to detect active licensed channels, power control, and activity levels of the licensed and unlicensed users. However, this model describes the interference disrupted by a single xG user and does not consider the effect of multiple xG users. In addition, if xG users are unaware of the location of the nearby primary users, the actual interference cannot be measured using this method. In [33], a direct receiver detection method is presented, where the local oscillator (LO) leakage power emitted by the RF front-end of the primary receiver is exploited for the detection of primary receivers. In order to detect the LO leakage power, low-cost sensor nodes can be mounted close to the primary receivers. The sensor nodes detect the leakage LO power to determine the channel used by the primary receiver and this information is used by the unlicensed users to determine the operation spectrum.