A communication system is provided. The communication system includes an antenna, a power amplifier, a detector and a signal-processing module. The power amplifier provides an output signal to the antenna according to an input signal. The detector provides power information and voltage information according to the output signal. The signal-processing module obtains a loading variation according to the power information and the voltage information, and provides the input signal according to the loading variation. When the loading variation exceeds a threshold value, the signal-processing module performs a digital pre-distortion calibration for the input signal.