Nd Cnc Example: Cnc Ratio With Asymmetric Links; Figure D-11. Asymmetric Link With The Same Data Rate But Different Antennas - Comtech EF Data CDM-Qx Installation And Operation Manual

CDM-Qx Satellite Modem
CnCPerformance Characterization
nd
D.8 2 CnC Example: CnC Ratio With Asymmetric Links
Networks with asymmetric antennas are common with a larger antenna at one site (hub)
and smaller ones at the other sites (remotes) and often have asymmetric data rates. In a
number of links even a significant rain fade is not a big factor in CnC performance. Some
links, particularly C-Band or X-Band, have insignificant rain fades and the key to
performance is setting both ends of the link to handle the asymmetry.
In asymmetric links, taking advantage of the available modulation and coding schemes is
another tool for building efficient CnC links. In these links the ideal CnC ratio is 0 dB,
but keeping the CnC ratio less than
7 dB, under all conditions, establishes links with margin. A link with a negative CnC
ratio is also acceptable because the interfering signal is below the desired signal.
An asymmetric C-Band link is shown in Figure D-11. It has equal symbol rate carriers
but the antenna at Site A is 4.5 meters antenna and Site B is 2.4 meters.
Asymmetric
Power

Figure D-11. Asymmetric Link with the Same Data Rate but Different Antennas

Conventional side-by-side carriers are transmitted by the link on the left, and CnC
carriers are deployed on the right. The conventional carriers are 8-PSK 2/3 TCM, and the
CnC carriers are QPSK 3/4 Turbo. Notice that the bandwidth to support the two
conventional carriers (BW1) is larger than the bandwidth for CnC (BW2), even though
the conventional link uses 8-PSK 2/3 while CnC is QPSK 3/4. The benefit of CnC
becomes apparent when it is realized that the bandwidth reduction possible with CnC is
also accompanied by a reduction in power compared to the conventional link.
BW1
Conventional Carriers
D–11
BW2
Asymmetric
Power
CnC Carriers
Revision 6
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