Comtech EF Data CDM-760, Инструкция по установке и эксплуатации

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L–1
Appendix L. DYNAMIC
PREDISTORTION (DPD) OPTION
WARNING! BEFORE YOU ATTEMPT TO COMMISSION A SATELLITE LINK USING DYNAMIC
PREDISTORTION (DPD), YOU MUST ENSURE THAT THE LINK IS ROBUST ENOUGH FOR NORMAL
OPERATION. ONLY WHEN YOU HAVE DONE THIS – AND YOU RESOLVE
L.1 Overview
ALL SYSTEM ISSUES (E.G.,
ANTENNA-POINTING, CABLING, TERRESTRIAL INTERFERENCE, SATELLITE INTERFERENCE, ETC.) –
SHOULD YOU ATTEMPT THE USE OF DYNAMIC PREDISTORTION.
Figure L-1. Satellite Transponder
Space segment costs are typically the most significant operating expense for any satellite-based
service, having a direct impact on the viability and profitability of the service. When a single-
carrier utilizes an entire satellite transponder, e.g. 36-MHz or 72-MHz bandwidth, there is an
opportunity to capture additional link efficiency not available in a multi-carrier setting. This is
done by operating the transponder, specifically the traveling-wave-tube amplifier (TWTA)
diagrammed in Figure L-1, in the nonlinear saturation region.
The disadvantage to operating in this regime is that amplifiers suffer from nonlinear distortion –
commonly characterized by
amplitude-to-amplitude
modulation (AM-AM) conversion as well as
amplitude-to-phase
modulation (AM-PM). Amplifiers additionally incorporate short-term
memory arising from physical device phenomena that adds a filtering-like effect to the
transponder. Further, input and output multiplexing filters (IMUX and OMUX) add linear
distortion as signal bandwidths approach the limit of the transponder.
The in-band distortion produces degradation in bit error rates relative to that of an ideal
transponder. The traditional approach to mitigating these effects is to reduce the transponder
output power, i.e. increase output backoff. However, a reduction in the transponder output
power typically forces a lower modulation and coding order in the modem, decreasing the space
segment efficiency and further increases costs.