Mocomtech CDM-QX - Manual (Page 200)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–14
• The CnC ratio for the site with the smaller antenna (Site ‘B’) is the same magnitude but
opposite sign: CnC(Site ‘B’) = -CnC(Site ‘A’).
For rain fade:
• The CnC ratio changes by the same amount as the uplink fade;
• Uplink fades at the near end decrease the CnC ratio;
• Uplink fades at the far end increase the CnC ratio.
The tables below summarize the Eb/No degradation at Site ‘A’ when a fade is introduced first at
Site ‘A’ and then at Site ‘B’. Next, the degradation at Site ‘B’ is evaluated when a fade appears
at Site ‘B’ and then at Site ‘A’. For this asymmetric case, the estimated CnC degradation is
0.1dB despite the significant fade.
Table 9-3. Eb/No Degradation For Asymmetric 4.5-Meter Antenna At Site ‘A’ and
2.4-Meter Antenna At Site ‘B’
Eb/No Degradation At Site ‘A’ Eb/No Degradation At Site ‘B’
Parameter
Due To Fade
at Site ‘A’ (dB)
Due To Fade
at Site ‘B’ (dB)
Parameter
Due To Fade at
Site ‘B’ (dB)
Due To Fade
at Site ‘A’ (dB)
CnC Ratio
at Site ‘A’
+5.3 +5.3
CnC Ratio
at Site ‘A’
–5.3 –5.3
Uplink Fade
at Site ‘A’
+6 –
Uplink Fade
at Site ‘B’
+3 –
Uplink Fade
at Site ‘B’
– +3
Uplink Fade
at Site ‘A’
– 6
Faded CnC
at Site ‘A’
–0.7 +8.3
Faded CnC
at Site ‘B’
–8.3 +0.7
Eb/No Degradation
(dB) at Site ‘A’
Per Fig. 9-12 QPSK
0.0 –0.1
Eb/No Degradation
(dB) at Site ‘B’
Per Fig. 9-12 QPSK
0.0 –0.1
9.5 Conclusion
There are several conclusions for operation with CnC:
• Operate adjacent carriers with sufficient spacing so there is no degradation;
• Adjust the modulation and code rate to alter the CnC ratio;
• Change the modulation and code rate to scale the symbol rate;
• Maximum CnC ratio is 7dB with plenty of margin;
• Maximum CnC ratio is 10dB with some degradation;
• Eb/No degradation is relatively tolerant to fades;
• QPSK is least sensitive to adjacent carrier and CnC ratio, followed by 8-PSK, then 16-QAM.