Orthogonal detection to reduce common channel interference

An orthogonal detection system has been developed which can extract any one of two double-sideband signals sharing very nearly the same radio-frequency spectrum. No specific relationships in frequency, phase, or amplitude between the two double-sideband carrier waveforms are required. The presence of thermal noise or other uncorrelated waveforms degrades the signal-to-noise ratio of the desired separated signals from about 3 db to 10 db for 90 per cent of the possible co-channel carrier frequency differences. Departures from ideal component characteristics and signal processing may introduce further degradation. An experimental breadboard was developed and tested. An improvement over a conventional diode envelope detector of +25 db was measured for -12 db signal-to-interference ratio. If a sampling technique were employed, it is estimated that an additional 15 db improvement could be realized. The implication is that signals associated with other types of conventionally modulated waveforms sharing very nearly the same spectrum are reasonably separable provided that the channel capacity is not fully utilized.