SatSig topic:

Bandwidth saving in Carrier-in-Carrier system

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Jul 2^nd, 2019 at 8:16am

Dear Members
My query is about stated saving of bandwidth in Carrier-in-Carrier system. Suppose two signals S1 and S2 need to be transmitted. A typical Carrier-in-Carrier system would do this by transmitting combined signal S1+S2. At the receiving end, signal S1 would be extracted by subtracting signal S2 from the received combined signal S1+S2. That is to say for Carrier-in-Carrier cancellation, it is necessary to provide each demodulator with a copy of its local modulator's output.
Meaning thereby, it is necessary to provide a copy of signal S2 to the receiving end (probably by transmission of S2 over alternate path and media). So actually, one has to transmit S1+S2 as well as S2 to enable receiver to get S1. Please explain how does it saves bandwidth in such a scenario (of course if I have understood it correctly). I mean the net effective transmission is only S1 whereas transmit bandwidth consumed is for S1+S2 and also for S2.

Thanks and regards

Jul 2^nd, 2019 at 7:04pm

Each site transmits only one carrier.

The carriers superimpose when they arrive at the satellite.

On the downlink, each site receives its wanted (from far end) signal, plus a delayed version of its own transmitted signal. Since each receive site knows what it transmitted earlier it can create an equal and opposite version to cancel its own transmitted signal visible in the downlink. Thus the receive site get the full wanted signal plus a greatly suppressed version of its own transmit signal.

Note that the system requires that a C-in-C site can see its own signal via the satellite (i.e. in a looped back up/down coverage beam.)

The C-in-C system can almost double the throughput per MHz and gives big space segment cost savings when the link budgets are good enough to make C-in-C possible.

See: https://www.comtechefdata.com/technologies/doubletalk

There are a couple of links to two pdf white papers at the bottom of that page.

Best regards, Eric.

Jul 3^rd, 2019 at 5:21pm

Thanks Eric!

You have explained very clearly and also cleared my doubt about CnC. I had missed the point that CnC links are duplex links.

Regards