CDM-625 Advanced Satellite Modem
Revision 1
DoubleTalk
®
Carrier-in-Carrier
®
Option
MN-0000036 (Ref MN/CDM625.IOM)
10.7 2
nd
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 10-7
. It has equal symbol rate carriers but the
antenna at Site
A
is 4.5 meters antenna and Site
B
is 2.4 meters.
Figure 10-8. 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.
The link parameters and results are summarized in the following table:
Conventional Carriers
CnC Carriers
BW1
BW2
Asymmetric
Power
Asymmetric
Power
10–9
