Comtech EF Data DMD-2050E Installation And Operation Manual page 107

DMD2050E Universal Satellite Modem
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 3-1 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 the following 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 DMD2050E 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.
MN-DMD2050E Revision 2
Theory of Operation
3–28