Mocomtech CDM-QX operation manual Degradation Due To Carrier Spacing

CDM-Qx Satellite Modem	Revision 5
CnCPerformance Characterization	MN/CDMQx.IOM
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 10 dB
• CnC operates with modems not modulators only or demodulators only.

D.2 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 ≈ 10-8and 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 D-1, but this is equivalent to two equally spaced adjacent carriers on either side of the desired carrier, each 3 dB less than a single adjacent carrier as shown in Case B.

Adjacent

Carrier

Level Desired Adjacent

Carrier Carrier

R = +3 dB
A	Carrier
	Spacing

R = 0 dB Adjacent		Desired	Adjacent
	Carrier	Carrier	Carrier

	Carrier
B
	Spacing

Figure D-1. Adjacent Carrier: A (As Tested) and B(As Plotted)

D–2