CDM-625 Advanced Satellite Modem
Revision 1
DoubleTalk
®
Carrier-in-Carrier
®
Option
MN-0000036 (Ref MN/CDM625.IOM)
10–12
10.9 Summary of Specifications
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
From –7 dB to +11 dB (ratio of power spectral density, outbound interferer to desired
inbound)
Note:
With asymmetric carriers the absolute power ratio will be larger than this,
depending on the ratio of the symbol rates.
Example:
Outbound interferer = 1 Msymbols/sec Desired Inbound = 50 ksymbols/sec
Ratio of power spectral density = +10 dB
Absolute power ratio
(CnC Ratio)
= +10dB +(10 log Outbound/desired symbol rate) =
+23 dB
Worst case example:
Outbound interferer = 12.5 Msymbols/sec Desired Inbound = 18 ksymbols/sec
Ratio of power spectral density = +11 dB
Absolute power ratio
(CnC Ratio)
= +11dB +(10 log Outbound/desired symbol rate) =
+38.4 dB
Inbound/Outbound
frequency uncertainty
Within the normal acquisition range of the demod, as follows:
Below 32 ksymbols/sec: ±1 to ± (Rs) kHz, where Rs = symbol rate in ksymbols/sec
Between 32 and 389 ksymbols/sec: ± 1 up to a maximum of ± 32kHz
Above 389 ksymbols/sec: ±1 to ± (0.1Rs) kHz, up to a maximum 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
8-PSK = 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 ratio, in dB (ratio of absolute power, outbound interferer to desired inbound)
10.10 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
•
For equal symbol rates, maximum CnC ratio is 7 dB with plenty of margin
•
For equal symbol rates, maximum CnC ratio is 11 dB 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.
