Can't each transmitter/radio add some type of a 2-nd order frequency onto the transmitting signal (imagine a sin-wave, except it's line is not smooth, the line has its own "frequency") and detect that to filter the incoming signal?
what you proposing is equivalent to increasing the frequency of the channel. If both do the same "2-nd" order frequency - we're back to the same "1-frequency" channel problem. If different - that's outside of the "1-frequency" channel problem.
Each radio would have a unique 2-nd order frequency for it's own filtering use, as a "tag" of it's signal. The primary frequency is known and used for the transmition, and the 2-nd order one is piggybacked on it. I'm not saying there are 2 transmiting frequencies.
> I'm not saying there are 2 transmiting frequencies.
200 years ago, people might have believed you. Then Fourier showed that your statements contradict each other. He said a lot of other interesting things - Bracewell's book is a good introduction.
