Mocomtech CDM-QX - Manual (Page 195)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–9
Figure 9-9. CnC Ratio For QPSK and 8-PSK
Figure 9-10. CnC Ratio For 16-QAM
Figure 9-9 plots the impact of CnC ratio on QPSK and 8-PSK constellations, while Figure 9-10
shows 16-QAM. In general, the higher the operating C/N of a carrier, the more sensitive it is to
degradation. QPSK is the least sensitive to CnC ratio followed by 8-PSK and 16-QAM.
9.4.4 Symbol Rate Ratio
CnC operation is restricted to a maximum symbol rate ratio ≤3. This is the ratio of the larger carrier
to the smaller one. Within these limits, the performance characterized applies. The limitation on the
symbol range still allows a wide range of data rates.
It is necessary to take into account the symbol rate ratio to properly estimate the CnC ratio, although
the modem does this automatically. For estimating the link parameters, the CnC ratio is adjusted by
10 Log (Symbol Rate Ratio). If the symbol rate ratio is 2.0 then the narrower carrier has a 3dB CnC
ratio when the CnC carriers have the same spectral density.