Mocomtech CDM-QX - Manual (Page 198)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–12
In links without rain fade, the CnC ratio is constant and only the asymmetry of link due to
satellite footprint, different antenna sizes, different symbol rates, or modulation and code rates
alter the ratio.
9.4.7 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 0dB, but keeping the
CnC ratio less than 7dB, 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 9-13. 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 9-13. Asymmetric Link (Same Data Rate, 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 on the next page in Table 9-2. The link asymmetry
has increased the CnC ratio at Site ‘A’ to +5.3dB. Yet this results in a degradation of only 0.1dB.
This is a C-Band link so no additional change in signal level is expected due to rain fade. The CnC
ratio at Site ‘B’ is -5.3dB so no degradation is expected.
What is done if the CnC ratio is 10dB or more? In a C-Band link it is possible to tolerate the
additional impairment, but then the modem is operating with less margin. One possibility to reduce
the CnC ratio is to increase the amount of power transmitted from the remote site with the smaller
antenna. This is feasible in some instances where there are higher power satellite transponders. If
Conventional Carriers CnC Carriers
BW1 BW2
Asymmetric
Power
Asymmetric
Power