SatSig topic:

Carrier in Carrier Tech. Information wanted.

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Mar 30^th, 2007 at 1:52pm

I have learnt of a new technology on satellite modems called the Carrier-In-Carrier. I understand a modem like ComTech QX can use a single frequency for its Rx and Tx, therefore cutting bandwidth cost to half! Can anyone better explain the technology behind this innovation and any other implication this development has or will have in present or future.

Ime

Mar 30^th, 2007 at 6:24pm

Meghan posts interesting documents. I must however point you to page 8 on the 1st document; specifically:
" The following should be considered when evaluating DoubleTalk Carrier-in-Carrier:
. DoubleTalk Carrier-in-Carrier can only be used for full duplex links where the transmitting earth
station is able to receive itself.
. DoubleTalk Carrier-in-Carrier can be used in both bandwidth limited and power limited situations
. For DoubleTalk Carrier-in-Carrier to provide savings in a power limited case, the original link
is either using a non-TPC FEC or if using TPC has a spectral efficiency of at least 1.5 bits/Hz
or better, i.e. QPSK, R=3/4 TPC or better
. The maximum savings is generally achieved when the original link is symmetric "

This tells me the technology is not universal, and cannot be expected to work in a majority of existing consumer grade installations. In otherwords, you pretty much have to buy their system from scratch. Not a bad thing, mind you - but something to consider.

No comment on the 2nd link, other than the fact that the link to tech specs outweighs the apparent attempt at free advertising.

//greg//

Mar 31^st, 2007 at 9:11am

The carrier in carrier technique does not cut bandwidth costs in half.

If you buy 1 MHz of bandwidth you get allocated a proportional amount of power (dBW).

To get the best capacity (in bit/s) out of that resource you need to use up all of the 1 MHz bandwidth and all of the power simultaneously. If you use less than 1 MHz you will get less bit/s. If you use less than the allocated power you will get less bit/s. So aim to operate a carrier (or multiple carriers) that fully utilise what you have paid for. You can put two equal height (half bandwidth) carriers side by side or put two full width, but half power, carriers on top of one another. If you choose the last option you need the special carrier-in-carrier modems to cancel out the mutual interference effect, by subtracting a delayed copy of the transmit signal from the received signal.

Turbo FEC code gives significant improvement over other FEC methods and is therefore the obvious choice, but you must adjust your FEC coding ratio and choice of modulation method to fully exploit the purchased resource. Larger dishes will result in lower bit/s cost.

Best regards, Eric.