Degradation Due To Carrier Spacing - Comtech EF Data CDM-Qx Installation And Operation Manual

CDM-Qx Satellite Modem
CnCPerformance Characterization
estimate its impact. The symbol rate ratio of CnC carriers and the allowable carrier offset
are discussed followed by some CnC examples.
The rules for CnC operation are summarized below:
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D.2

Degradation Due To Carrier Spacing

In satellite links, one of the impairments to estimate is the impact of carrier spacing on
performance and allocate the degradation to the link budget. Data was taken using the
CDM-Qx Modem, operating with Turbo coding, to measure Eb/No degradation with
decreasing carrier spacing to characterize performance in the presence of two equally
spaced like modulated adjacent carriers. This is done without CnC.
For testing, the modem is initially set up with noise to operate at a nominal or reference
Eb/No corresponding to a BER ≈ 10
modulated adjacent carrier is then added and the Eb/No degradation recorded. The test is
conducted with a single adjacent carrier as shown in Case A of Figure D-1, but this is
equivalent to two equally spaced adjacent carriers on either side of the desired carrier,
each 3 dB less than a single adjacent carrier as shown in Case B.
Adjacent
Carrier
Level
R = +3 dB
A
Figure D-1. Adjacent Carrier: A (As Tested) and B(As Plotted)
The results are plotted for two equally spaced adjacent carriers each at -3 dB, 0 dB, +3
dB, and +6 dB relative to the desired carrier to produce a family of operating curves.
Figure D-2
Both earth stations share the same footprint so each sees both carriers
CnC carriers are operated in pairs
One outbound with multiple return carriers is not allowed
Asymmetric data rates are allowed up to a symbol rate ratio of 3
Minimum symbol rate for CnC is 128 ksps
The CnC ratio is normally less than 10 dB
CnC operates with modems not modulators only or demodulators only.
Desired Adjacent
Carrier Carrier
Carrier
Spacing
through plot the results for the QPSK, 8-PSK and 16-QAM cases.
Figure D-4
-8
and with no adjacent carrier present. A like-
Adjacent
R = 0 dB
Carrier
B
D–2
Revision 6
MN/CDMQx.IOM
Desired Adjacent
Carrier Carrier
Carrier
Spacing