Specifications

DMD20/20LBST Universal Satellite Modem Theory of Operation
MN-DMD20-20LBST Revision 14 3–26
• Link must be full duplex.
• A Radyne DMD20 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 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 3-10 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:
1) 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.
2) 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
Radyne DMD20 itself. The CnC module tracks and compensates for this frequency offset and
drift.
3) 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.