US Patent 8665985 Secondary communication signal method and apparatus

A low level secondary communication signal is summed with an existing primary communication signal in a manner that prevents interference to the existing primary communication signal while providing secondary communication signal benefits such as increased data rate, range, or interference immunity. Examples are presented in which a M-QAM secondary signal is summed with either an ATC Mode S PPM reply or DPSK interrogation primary signal. Legacy Mode S transponders, TCAS, and ADS-B equipment continue to demodulate and decode the primary signal information in accordance with preexisting formats while new enhanced equipment obtains the benefits of the secondary signal.