Mocomtech CDM-QX - Manual (Page 199)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–13
the installation is a new one, a larger, though more expensive, antenna is possible at the remote site.
This simultaneously decreases the CnC ratio at the hub while increasing it at the remote site.
Another alternative is to reduce the modulation order and/or error correction code rate on the
receive side of the remote site. This decreases the power transmitted by the hub and reduces its CnC
ratio. The hub’s CnC ratio will decrease further if it is possible to increase the modulation order or
code rate at the hub. In the above example changing from QPSK 3/4 to QPSK 1/2 helps reduce the
power but requires additional 1.5 times more bandwidth. The 1.2dB Eb/No difference between rate
3/4 and 1/2 reduces the CnC ratio at the hub.
Table 9-2. Link Parameters / Results
Parameters Site ‘A’ Site ‘B’
Satellite EIRP (dBW) 37 37
Satellite BOo (dB) 6 6
Satellite BOi (dB) 10 10
Satellite SFD (dbW/m
2
) -78 -78
Satellite G/T (dB/K) 0 0
E/S Antenna (meters) 4.5 2.4
Data Rate (kbps) 192 192
Carrier Spacing Factor 1.3 1.3
Conventional Link 8PSK 2/3 TCM 8PSK 2/3 TCM
Occupied BW1 for 2 Carriers (kHz) 274.6 274.6
% of Transponder Power 0.55 0.16
CnC Link QPSK 3/4 Turbo QPSK 3/4 Turbo
Occupied BW2 for 2 Carriers (kHz) 166.4 166.4
% of Transponder Power 0.37 0.11
CnC Ratio (dB) +5.3 -5.3
Expected Eb/No Degradation (dB) -0.1 0.0
9.4.8 3 rd CnC Example: Asymmetric Link With Rain Fade
A reasonable question to ask about the previous example is the impact rain fade has on the link.
This example combines parts of the previous two examples using the same symbol rates and
asymmetric antennas and adds in fade at Site ‘A’, with the larger antenna and examines the CnC
ratio at both sites. The impact on both sites is also estimated due to a fade at Site ‘B’. Just as in
the earlier example, the larger fade (6dB up and 4dB down) occurs at Site ‘A’ (hub), and the
smaller fade (3dB up and 2dB down) is allocated to Site ‘B’ (remote).
Treating the fades in this way is convenient as a first order approximation, but other factors
influence performance such as noise increase and G/T degradation at the receive site.
From the previous examples there are some characteristics worth summarizing:
At the same symbol rate with the same modulation and code rate:
• The CnC ratio is highest at the site with the larger antenna (Site ‘A’);