Communication system, transmitter, communication method, and transmitter detection method

A communication method comprising:

• transmitting, by a transmitter, a first signal generated by converting a frequency of a provided signal to a first frequency band, and a second signal generated by converting the frequency of the provided signal to a second frequency band having a given frequency difference against the first frequency band collectively as a transmission signal;
• converting, by a frequency converter of a receiver, the transmission signal to a frequency-shifting signal shifted by the given frequency difference;
• multiplying, by a multiplier of the receiver, the transmission signal and the frequency-shifting signal together to output a multiplied signal;
• extracting, by a filter of the receiver, a baseband signal from the multiplied signal;
• and
• detecting, by a detector of the receiver, significant information from the baseband signal.

A transmitter detecting method, comprising:

• converting, by a frequency converter, a transmission signal having a first signal generated by converting a frequency of a provided signal to a first frequency band and a second signal generated by converting the frequency of the provided signal to a second frequency band having a given frequency difference against the first frequency band, to a frequency-shifting signal shifted by the given frequency difference;
• multiplying, by a multiplier, the transmission signal and the frequency-shifting signal together to output a multiplied signal;
• extracting, by a filter, a baseband signal from the multiplied signal;
• and
• detecting, by a detector, significant information from the baseband signal.