Mocomtech CDM-QX - Manual (Page 189)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–3
The modem generating the ‘B’ carrier in this example should then be relocated in frequency to be
“on top” of Carrier ‘A’. At this point the demodulator-receiving carrier “A” will loose lock. The
carrier in carrier function must now be enabled in the Rx configuration menus. Once CnC is
turned on the demodulator will perform a search for a copy of the outbound modulators signal in
time and frequency within the composite signal received by the demodulator.
When the search algorithm has found the carrier, the modem front panel will report “
Search
Successful! ” It will also display an accurate value for the delay of the signal. The
demodulator will then reconfigure itself to match the delay value, cancel out the interfering
carrier (user’s outbound), then lock to and demodulate the desired carrier.
The modem is like all Comtech EF Data modems in that it has a full compliment of receive
monitored parameters. But for CnC operation a new parameter has been added. To access this
feature, change the menu from config Rx to monitor Rx, CnC and the following parameters will
be displayed. The normal BER, Eb/No and the new is the ratio between the interferer and the
desired carriers. The value display is the difference (in dB) of the interferer over the desired
carrier. This value is signed as the interferer (which the user side of the link has control over) can
be greater or less than the desired you are trying to receive. This is an invaluable piece of
information as with two carriers on top of each other it is otherwise impossible to accurately tell
if one side or the other should experience a fade. The CnC monitor menu will also report the
frequency offset between the two carriers.
Once this side of the link is configured and running properly, the other side of the link must be
configured so that the Rx frequency is the same as the Tx and CnC is turned on. At this point both
sides of the link should be locked and passing traffic normally.
Once the links have been configured such that the Eb/No values with good weather conditions are
such that there is an appropriate fade margin, record the ratio value so that between the Eb/No
value of the desired and the ratio value the user can determine if the link has degraded.
If the outbound carrier should go down the demodulator may drop sync but will relock to the
desired carrier. When the outbound carrier returns the demod may be able to relock rapidly if the
outage was brief or it may have to perform another search first. If the desired carrier should go
down the demodulator will continue to cancel the outbound but will be unlocked with respect to
the desired carrier. When the carrier returns the demod will relock.
While there are several parameters that can be configured when using CnC, it is recommend to
leave them set to the factory default settings. These include changing the minimum delay value
(in milli seconds) for the search function. The typical satellite delay will range from about 230 to
270 ms. If the minimum value is increase to say 200 ms the acquisition time will not be
significantly improved and if the unit should be tested at the IF level in the future it would not
lock as the delay would be in the micro seconds. The maximum delay can be decreased from 290
ms, but again the acquisition time will not be significantly improved. The other parameter is the
reacquisition delay value. This determines how long the demodulator will wait to perform another
search for the outbound carrier.