Appendix M. Transmit Spectrum Filtering Options; Overview; Observations And Recommendations - Comtech EF Data CDM-570A Installation And Operation Manual

SPECTRUM FILTERING OPTIONS
M.1

Overview

The CDM-570/570L Satellite Modem offered transmit filtering per Intelsat IESS-308.
The CDM-570A/570AL Satellite Modem now provides Transmit Alpha Filter Rolloff Factors (Tx
alpha, or simply Txα) of 0.05, 0.10, 0.15, 0.20, 0.25, or 0.35.
See "Section 1.4.1 Modulator" for detailed spectral density plots for all values of
Txα.
M.2

Observations and Recommendations

•
In a perfectly linear channel with tightly controlled RF carrier frequencies, there is no
penalty when using tighter rolloffs (smaller Txα). The benefit of this is that more carriers
can be fitted into a given amount of transponder bandwidth. You should, however,
consider that in many cases, transponders are power limited, not bandwidth limited,
leading to the conclusion that for power limited cases, tighter rolloffs do not provide an
obvious benefit.
If, on the other hand, the assumptions of linearity and/or frequency stability are not true,
then you should be aware of the potential degradations that can be caused by using
lower values of Txα.
•
As the rolloff decreases, the peak-to-average power ratio of the waveform increases, and
if an SSPA (or any other element in the link) is approaching compression, the carrier will
be more prone to spectral regrowth, and depending on the exact modulation type, this
may cause additional degradation due to compression of outer constellation points
(particularly with non-round constellations).
A White Paper that is to be published by Comtech EF Data will offer precise guidelines
on the amount of additional output backoff (OBO) that is required for these new rolloffs.
For now, Comtech EF Data recommends that, given a Txα of 0.20 or 0.15, you use an
additional OBO of at least 0.5 dB; for a Txα of 0.10 or 0.05, use an additional OBO of at
least 1.0 dB.
•
The recommended minimum carrier spacing equals(1 + Txα) x symbol rate,
assuming equal symbol rates for all carriers and zero frequency error between carriers.
Transmit Spectrum Filtering Options
CDM-570A/570A Satellite Modem with Optional Packet Processor
Appendix M. TRANSMIT
M–1
Revision 5
MN-CDM570A