Comtech EF Data CDM-Qx, Installation And Operation Manual

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CDM-Qx Satellite Modem Revision 6
CnCPerformance Characterization MN/CDMQx.IOM
The following table contains the CDM-Qx configurations tested:
Modulation Forward Error
Correction
Reference Eb/No
At BER ≈ 10
-8
Symbol
Rate
Data Rate Rolloff
(
α
)
QPSK 3/4 Turbo 3.9 dB 1000 ksps 1500 kbps 20 and 35%
8-PSK 3/4 Turbo 6.3 dB 1000 ksps 2250 kbps 20 and 35%
16-QAM 3/4 Turbo 7.7 dB 1000 ksps 3000 kbps 20 and 35%
The results are plotted for Eb/No degradation versus relative carrier spacing where:
•
Eb/No degradation is the difference between the reference Eb/No and the Eb/No
read from the modem in the presence of the interfering adjacent carrier.
•
Relative Carrier Spacing is the distance between the centers of the desired and
adjacent carriers divided by the symbol rate.
There are two sets of adjacent carrier plots representing operation with 20% and 35%
rolloff (
α
). When
α
is 20%, the spectrum is narrower than it is for 35%. The effect of this
is noticed in the adjacent carrier plots. The 20% plots are displaced slightly to the left of
those for 35%. This makes it possible to space carriers slightly closer when the rolloff is
20%. The table below generalizes degradation (
≤
0.5 dB) for all modulation and coding
combinations when there are two adjacent carriers:
Carrier Spacing For
20% Rolloff Carrier Spacing For
35% Rolloff
Degradation
≤
0.5 dB
≥
1.1 x Symbol Rate
≥
1.2 x Symbol Rate
Some caution is required because carriers with 20% rolloff are more sensitive to
impairments and non-linearity in the link.
D–3