In a receiver used in wireless communications, any of the following signal processing methods are performed in order to keep the reception signal to a constant level.    (1) Performing automatic gain control (AGC) on an amplifier in response to the reception level and keeping the input level of the reception signal to a demodulator constant (Patent Literature 1)    (2) Adding a pilot signal different from a main signal in frequency and monitoring only the pilot signal thereby to perform the above-mentioned AGC (Non-Patent Literature 1).    (3) Generating and transmitting a training signal with a unique word, receiving and demodulating the signal thereby to determine the bit error rate so as to control a threshold or AGC (Patent Literature 2).
As to the above-mentioned (1), the system of controlling a variable attenuator in a reception signal input unit by monitoring an error rate correction degree of a reception demodulated signal or monitoring the reception level of a reception signal (baseband signal) so as to demodulate the reception signal in the radio receiver in s stable manner is disclosed in the patent literature 1. Besides, as to the above-mentioned (2), the ADSL of determining a specific fixed frequency, for example, 276 kHz as a pilot frequency, monitoring the reception level of the specific frequency constantly thereby to estimate the transmission state of a main signal and controlling the reception level to be optimal according to need is disclosed in the non-patent literature 1. Further, as for the above-mentioned (3), the system of inserting a pilot signal with a known unique word into a transmission signal periodically over plural periods by a transmitter, monitoring the pilot signal contained in a reception signal by a receiver and determining the reception state of the signal thereby to correct and demodulate the data is disclosed in the patent literature 2.