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Appendix L
CDM-760 Advanced High-Speed Trunking Modem
Comtech EF Data provides an option for the CDM-760 to continuously optimize satellite
communication efficiency – Dynamic Predistortion (DPD).
L.1.1
What is Dynamic Predistortion (DPD)?
Dynamic Predistortion works in tandem with a Crest Factor Reduction (CFR) technique that is
dynamically applied to enhance performance. These technologies, collectively labeled and
referred to as DPD, create high signal integrity while operating the satellite transponder in the
higher efficiency nonlinear region. They work to provide a significant increase in link margin by
as much as 2 dB, and/or an increase in spectral efficiency by as much as 6%. These
improvements can translate to:
•
Reduced operating expense (OPEX) – e.g., Occupied Bandwidth & Transponder Power;
•
Reduced capital expenditure (CAPEX) – e.g., Block Up Converter/High-Power Amplifier
(BUC/HPA) size and/or antenna size;
•
Increased throughput without using additional transponder resources;
•
Increased link availability (margin) without using additional transponder resources;
•
A combination of any of the above to meet different objectives.
See Figure L-2. Predistortion works as an alternative to simply backing off a power amplifier by
employing an approximate inverse nonlinearity ahead of the power amplifier, at a low power
level to a digital baseband signal, such that the cascade operation is closer to ideal. Additionally,
when DPD is enabled, CFR precedes the predistorter to reduce the signal's magnitude range.
Working together, both preempt the degrading effects of the satellite transponder when
attempting to operate a link with highest spectrally efficiency.
Figure L-3 illustrates the benefit of DPD when applied to a 32-APSK constellation. All distortion
pictured is induced by driving the TWTA into its nonlinear region, i.e. additive noise is not
included in the plots. While maintaining the same output backoff a significant proportion of the
distortion is removed when DPD is applied. Here, the modulation error ratio (MER), a common
measure of constellation quality, significantly improves from 18.5 dB to 22.5 dB.
Figure L-3. Constellation – 32-APSK Improvement Example
Figure L-2. DPD Operation
(LEFT) Without DPD, MER=18.5 dB
(RIGHT) With DPD, MER=22.5 dB
L–2
Revision 4
MN-CDM760
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