Mocomtech CDM-QX - Manual (Page 191)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–5
Figure 9-3. Adjacent Carrier: Case A (As Tested) and Case B (As Plotted)
The results are plotted for two equally spaced adjacent carriers each at -3dB, 0dB, +3dB, and
+6dB relative to the desired carrier to produce a family of operating curves.
Figure 9-4 through
Figure 9-6 plot the results for the QPSK, 8-PSK and 16-QAM cases.
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 3.9dB 1000 ksps 1500 kbps 20 and 35%
8-PSK 3/4 Turbo 6.3dB 1000 ksps 2250 kbps 20 and 35%
16-QAM 3/4 Turbo 7.7dB 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 (
≤ 0.5dB) for all modulation and coding combinations when there are two
adjacent carriers:
Degradation ≤ 0.5dB
Carrier Spacing for 20% Rolloff Carrier Spacing for 35% Rolloff
≥ 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.
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