Comtech EF Data CDM-Qx, Installation And Operation Manual

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CDM-Qx Satellite Modem Revision 6
CnCPerformance Characterization MN/CDMQx.IOM
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:
•
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.
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
-8
and with no adjacent carrier present. A like-
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.
Carrier
Spacing
Adjacent
Carrier
Level
R = +3 dB
A
Adjacent
Carrier
Desired
Carrier
Carrier
Spacing
B
R = 0 dB
Adjacent
Carrier
Desired
Carrier
Adjacent
Carrier
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
through
Figure D-4
plot the results for the QPSK, 8-PSK and 16-QAM cases.
D–2