Mocomtech CDM-QX - Manual (Page 190)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–4
9.4 CnC Performance Characterization
In a number of ways, CnC carriers behave similar to conventional carriers in satellite links. They
are both exposed to adjacent carriers, cross-polarization and rain fade, and exhibit impairments
when any of these become too great. In addition, CnC operates in an environment where:
• Carriers intentionally occupy the same spectral slot;
• Performance depends upon desired and co-located interfering carrier.
Several areas relating to CnC performance are discussed in the sections that follow, including:
• Adjacent carrier performance;
• Eb/No Degradation as a function of the CnC ratio;
• Symbol rate ratio;
• Carrier offset;
• The effects of rain fade and asymmetric antennas.
Nominally, these effects are treated independently so it is possible to add them together to
estimate the total degradation. Initially, degradation due to carrier spacing is examined to
characterize the adjacent carrier performance. Next, the CnC ratio is evaluated to estimate its
impact. The symbol rate ratio of CnC carriers and the allowable carrier offset are discussed
followed by some CnC examples.
The rules for CnC operation are summarized below:
• Both earth stations share the same footprint so each sees both carriers;
• CnC carriers are operated in pairs;
• One outbound with multiple return carriers is not allowed;
• Asymmetric data rates are allowed up to a symbol rate ratio of 3;
• Minimum symbol rate for CnC is 128 ksps;
• The CnC ratio is normally less than 10dB;
• CnC operates with modems not modulators only or demodulators only.
9.4.1 Degradation Due To Carrier Spacing
In satellite links, one of the impairments to estimate is the impact of carrier spacing on
performance and allocate the degradation to the link budget. Data was taken using the CDM-Qx
Modem, operating with Turbo coding, to measure Eb/No degradation with decreasing carrier
spacing to characterize performance in the presence of two equally spaced like modulated
adjacent carriers. This is done without CnC.
For testing, the modem is initially set up with noise to operate at a nominal or reference Eb/No
corresponding to a BER≈10
-8 and with no adjacent carrier present. A like-modulated adjacent
carrier is then added and the Eb/No degradation recorded. The test is conducted with a single
adjacent carrier as shown in Case ‘A’ of Figure 9-3, but this is equivalent to two equally spaced
adjacent carriers on either side of the desired carrier, each 3dB less than a single adjacent carrier
as shown in Case ‘B’.