We have already seen in the case of M-ary FSK that an orthogonal symbol set makes it possible to send two or more symbols simultaneously over a channel, without their affecting the coherent detection performance of any individual symbol. It turns out that as well as the M-ary FSK orthogonal symbol set, there is orthogonality between a cosine and sine carrier term when averaged over a full number of carrier cycles. This would imply that if we were to signal using binary PSK on the cosine of a carrier, and simultaneously send a second PSK signal using the sine of a carrier, then it would be possible to detect each one independently of the other, (as if the other were not there), providing that each detector averaged over a symbol period that contained a whole number of carrier cycles. |
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