Mocomtech CDM-QX - Manual (Page 194)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–8
carrier. The table provided in Figure 9-7 summarizes which adjacent carrier plot to select for this
particular example, and which ones to use with Ratio ‘R’ between the composite adjacent and
composite desired carriers.
9.4.3 Carrier-in-Carrier ® Ratio (CnC Ratio)
The CnC Ratio represents the difference in power between the co-located interfering carrier and
the desired carrier (in dB):
CnC Ratio = Interferer Power – Desired Carrier
During CnC operation, the interfering carrier is removed by the CDM-Qx using a stored version
of the transmitted carrier to adaptively cancel it from the composite received signal. The desired
carrier remaining after the cancellation process is delivered to the demodulator and decoder to
recover the data.
When the CnC ratio increases, the level of the interferer rises relative to the desired carrier and
degradation grows. As the CnC ratio decreases, the desired carrier dominates and degradation
becomes negligible. Figure 9-8 represents two cases of the CnC Ratio when the interfering and
desired carriers are equal, and when the interferer is 6dB stronger than the desired carrier. This
representation is artificial because a real spectral plot displays only the composite power of the
combined carriers and is unable to distinguish two carriers, but it is instructive to describe the
underlying principle.
Figure 9-8. CnC Ratio
The following CDM-Qx configurations were tested for Eb/No degradation as a function of CnC ratio:
Modulation
Forward Error
Correction
Reference Eb/No
At BER≈10 -6
C/N
QPSK
21/44 Turbo 2.6dB 2.4dB
3/4 Turbo 3.7dB 5.5dB
7/8 Turbo 4.3dB 6.7dB
17/18 Turbo 6.5dB 9.3dB
8-PSK
2/3 TCM 5.3dB 7.9dB
3/4 Turbo 6.1dB 9.6dB
7/8 Turbo 7.1dB 11.3dB
17/18 Turbo 9.0dB 13.5dB
16-QAM
3/4 Turbo 7.2dB 12.0dB
7/8 Turbo 8.1dB 13.5dB
The Eb/No degradation is the difference between the reference Eb/No with no interfering carrier
present and the Eb/No reported by the modem at a given CnC ratio.
CnC Ratio = 0 dB
Equal Interfering And
Desired Carriers
A
CnC Ratio = 6 dB
Interferer >
Desired Carrier
B
+6 dB