Covert/frequency re-use communications using adaptive locally optimum processing (ALOP)

Abstract

Adaptive locally optimum processing techniques have been used to detect a weak signal in the presence of interference. These algorithms have been shown to work well against broadband as well as narrowband interference. This fact can be exploited to provide covert communications by noncoherently adding a weak communications signal to a much stronger primary communications signal which can also carry information. The primary communications signal is demodulated normally, and the weak signal is demodulated after adaptive locally optimum processing. The weaker signal can be sufficiently low in power relative to that of the primary signal so as to be undetectable by receivers unaware of its presence. The phase of the combined signal carries primary signal information for the covert signal and the amplitude of the combined signal carries the secondary signal information. The primary communications signal is recovered from the combined signal by use of an ordinary radio while the covert communications signal is recovered by processing the combined baseband signal to reveal the projection of the covert signal onto the primary signal and then demodulating this signal as done in a standard radio. This paper describes a simple way to transmit a covert communications signal over the same communication channel as a primary communications signal. The device described can also be used to increase the throughput of the channel without necessarily providing a covert communications channel.