Comtech EF Data CDM-625 - Manual (Page 56)

CDM-625 Advanced Satellite Modem Revision 15
Introduction MN-CDM625
1–20
1.4.5 DoubleTalk ® Carrier- in-Carrier ® (CnC)
Operating Mode
Requires the two links to share a common carrier frequency (Outbound and Inbound symbol rates do
not have to be equal)
Power Spectral Density Ratio and
CnC Ratio
• BSPK/QPSK/8PSK/8-QAM: –7 dB to +11 dB (ratio of power spectral density, outbound interferer
to desired inbound)
• 16-QAM: –7 dB to +7 dB (ratio of power spectral density, outbound interferer to desired inbound)
Note: With asymmetric carriers, the absolute power ratio (or CnC ratio) would be different,
depending on the ratio of the symbol rates.
Example:
Outbound interferer = 1 Msymbols/sec
Desired Inbound = 500 ksymbols/sec
Ratio of power spectral density = +7 dB
Absolute power ratio (CnC Ratio) = +7dB + (10
log
Outbound/desired symbol rate) = +10 dB
Maximum Symbol Rate Ratio 3:1 (TX:RX or RX:TX)
Inbound/Outbound frequency
uncertainty
Within the normal acquisition range of the demod, as follows:
• Below 64 ksymbols/sec: ±1 to ±(Rs/2) kHz, where Rs = symbol rate in ksymbols/sec
• Between 64 and 389 ksymbols/sec: ±1up to a max. of ±32kHz
• Above 389 ksymbols/sec: ±1 to ±(0.1Rs) kHz, up to a max. of ±200 kHz
Delay range 0-330 ms
Eb/No Degradation
(equal Inbound/Outbound power
spectral density)
• BPSK = 0.3dB
• QPSK = 0.3dB
• OQPSK = 0.3dB
• 8PSK = 0.5dB
• 8-QAM = 0.4dB
• 16-QAM = 0.6dB
For +10 dB power spectral density ratio (outbound interferer 10 dB higher than desired inbound) add
an additional 0.3 dB
Monitor Functions • Delay, in milliseconds
• Frequency offset (between outbound interferer and desired inbound). 100 Hz resolution
• CnC Power Ratio, in 0.1 dB (ratio of absolute power, outbound interferer to desired inbound)
• Power Spectral Density Ratio, in 0.1 dB
CnC Monitor Accuracy ±0.1 dB for symmetric symbol rate