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CDM-625 Advanced Satellite Modem
®
DoubleTalk
Carrier-in-Carrier
The CDM-625 CnC module operates on the near-zero signal before the demodulator, and is
waveform agnostic. This means that no prior knowledge of the underlying modulation, FEC, or
any other waveform specific parameter is required in order to perform the signal suppression
operation. The only caveat to this is that the wavefom must be "sufficiently random".
Because acquiring the delay and frequency offset of the interfering carrier is fundamentally a
correlation operation, anything deterministic in the interfering carrier (within the correlation
window of the algorithm) will potentially produce false correlation peaks and result in incorrect
delays and/or frequency. Normally, this is not a problem, since energy dispersal techniques are
utilized in the vast majority of commercial and military modems. However, it is something that
must be kept in mind when troubleshooting a system that utilizes the DoubleTalk
®
Carrier
technique for signal suppression. One possible way to mitigate false peaks is to narrow
the correlation window. For example, if you know the delay to be around 240ms, set the
minimum search delay to 230ms and the maximum search delay to 250ms.
10.3 Application Requirements
The following conditions are necessary in order to operate DoubleTalk
•
Link must be full duplex.
•
A CDM-625 must be used at the end of the link where the cancellation needs to take
place.
•
The transponder is operated as Loopback. That is, each end of the link must be able to see
a copy of its own signal in the return (downlink) path from the satellite. The looped back
signal is then subtracted which leaves the signal from the distant end of the link.
DoubleTalk
•
The transponder needs to be "bent-pipe", meaning no on-board processing,
demodulation, regeneration can be employed. Demodulating/remodulating does not
preserve the linear combination of the forward and return signals and the resulting
reconstituted waveform prevents recovery of the original constituent signals.
Figure 10-3 shows a simplified conceptual block diagram of CnC processing. The two ends of
the link are denoted A and B and the uplink and downlink are shown.
®
Option
®
Carrier-in-Carrier
MN-0000036 (Ref MN/CDM625.IOM)
®
cannot be used in spot beam systems.
10–3
Revision 1
®
Carrier-in-
®
®
Carrier-in-Carrier
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