DMD20/20LBST Universal Satellite Modem
Theory of Operation
MN-DMD20-20LBST Revision 14
3–44
3.15.14
DoubleTalk Carrier-in-Carrier 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
BSPK/QPSK/8-PSK/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 32 ksymbols/sec:
±1 to ± (Rs/2) kHz, where Rs = symbol rate in ksymbols/sec
Between 32 and 389 ksymbols/sec:
± 1up 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)
3.15.15
Carrier-in-Carrier Summary
Comtech EF Data’s DoubleTalk Carrier-in-Carrier can provide significant savings in operational
expenses. Considerations include:
•
DoubleTalk Carrier-in-Carrier can only be used for full duplex links where the transmitting
earth station is able to receive itself.
•
DoubleTalk Carrier-in-Carrier can be used in both bandwidth- and power-limited
situations.
•
The maximum savings is achieved when the original link is symmetric in data rate.
3.15.16
Glossary
Allocated Bandwidth
Bandwidth or Allocated Bandwidth or Occupied Bandwidth is the frequency space required by a
carrier on a transponder.
For example, a Duplex E1 (2.048 Mbps) Circuit with 8-PSK Modulation, FEC Rate 3/4 and 1.4
Spacing requires:
2.548 MHz = 2.048 / (3 * 0.75) * 1.4 * 2
For a 36 MHz transponder, 2.548 MHz corresponds to 7.078% Bandwidth Utilization.