Comtech EF Data SLM-5650A Installation And Operation Manual page 469

G.2 Application Requirements
These conditions are necessary in order to operate DoubleTalk Carrier-in-Carrier:
•
Link must be full duplex:
•
A SLM-5650A must be used at the end of the link where the cancellation needs to take
place. Normally, this is both ends of the link.
•
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 downlink path from the satellite. The looped back
signal is then subtracted, which leaves the signal from the distant end of the link.
DoubleTalk Carrier-in-Carrier cannot be used in spot beam systems.
•
The transponder needs to be "bent-pipe" – meaning no on-board processing,
demodulation, regeneration can be employed. Demodulation/remodulation 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 G-1 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.
This performance is achieved through advanced signal processing algorithms that provide
superior cancellation while tracking and compensating for these common link impairments:
•
Time varying delay: In addition to the static delays of the electronics and the round-trip
delay associated with propagation to the satellite and back, there is a time-varying
component due to movement of the satellite. The CnC module tracks and compensates
for this variation.
•
Frequency offset and drift: Common sources are satellite Doppler shift, up and down
converter frequency uncertainties, and other drift associated with the electronics in the
SLM-5650A itself. The CnC module tracks and compensates for this frequency offset and
drift.
•
Atmospheric effects: Fading and scintillation can affect amplitude, phase, and spectral
composition of the signal and the degree to which it correlates with the original signal.
The CnC module tracks and compensates for these atmospheric related impairments.
•
Link Asymmetries: Various asymmetries in the forward and return link can produce
differences in the relative power of the two received signal components. These can be
both deterministic (static) or random (and time varying). An example of the former would
be the differences resulting from antenna size/gain variations between the two ends of
the link. An example of the latter would be transient power differences due to different
levels of atmospheric fading in the uplinks. CnC compensates for the asymmetries, up to
a certain extent.
Appendix H G–3
SLM-5650A Satellite Modem
Revision 10
MN-SLM-5650A