A communication system comprises a predistorter for distorting an input signal according to at least one parameter to generate a distorted signal, an amplifier for amplifying the distorted signal according to an input-output characteristic to generate an output signal, and a frequency-domain adaptive calibration module for adaptively adjusting the parameter of the predistorter according to a frequency characteristic of the output signal. Because the calibration is performed in the frequency domain, there is no need to precisely estimate the group delay formed by the feedback path. The system complexity is therefore reduced without loss of performance.