A method of modeling a characteristic of a non-linear system, comprises feeding testing input signals into the non-linear system to obtain testing output signals corresponding to the testing input signals, wherein the testing input signals include a first testing input signal and the testing output signals include a first testing output signal, identifying occurrences when an output level state in at least one specific frequency band of the first testing output signal significantly changes under the first testing input signal so as to obtain a first profile, and modeling the characteristic based on the first profile.