CDM-625 Advanced Satellite Modem
Revision 1
DoubleTalk
®
Carrier-in-Carrier
®
Option
MN-0000036 (Ref MN/CDM625.IOM)
10.5 CnC Carrier Frequency Offset
CnC carriers are normally placed directly on top of each other with the same center frequency for
both carriers. Normal operation is obtained when the center frequency of the two carriers is
within the normal acquisition range of the modem for standard and CnC carriers.
10.6 1
st
CnC Example: Adjacent Carriers, CnC Ratio and Rain Fade
As an example, a pair CnC carriers is flanked by two adjacent CnC pairs with a carrier spacing of
1.3 x Symbol Rate and the power level is the same for all carriers as shown in
Figure 10-6
:
Figure 10-6. CnC Example
In this scenario, the modulation is 8-PSK 3/4 Turbo with identical data rates.
Referring back to
Figure 10-6
, the degradation due to adjacent carrier spacing is negligible when
spacing is 1.3 x Symbol Rate and 0 dB is allocated for adjacent carrier degradation.
Initially the CnC ratio is 0 dB, and the desired and interfering carriers are operating at the same
power level. At one end of the link (Site
A
), a downlink fade of 4 dB is expected and an uplink
fade of 6 dB. The other end of the link (Site
B
) is allocated 2 dB for downlink and 3 dB for the
uplink:
Desired
Carrier(s)
Carrier
Spacing
Site
DL Fade
UL Fade
A
4 dB
6 dB
B
2 dB
3 dB
When a rain fade occurs at one site, the effect is felt at both sites as outlined in
Figure 10-7
,
which diagrams the worst case fade at Site
A
. The interfering carrier at Site
A
is attenuated twice,
once due to the uplink and the second time due to the downlink on the return path. The carrier
transmitted from Site
B
sees only the downlink fade when it is received at Site
A
. The resulting
power level changes at each site due to the rain fade and the resulting CnC ratio and Eb/No
degradation is summarized in the following table:
10–7
