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SPECTRUM FILTERING OPTIONS
S.1
Introduction
The CDM-625 Advanced Satellite Modem offered the choice of 0.35 or 0.25 for Transmit Alpha
Filter Rolloff Factor (Tx alpha, or simply Txα). The CDM-625A Advanced Satellite Modem now
provides the choices of 0.05, 0.10, 0.15, 0.20, 0.25, or 0.35.
See Chapter 1, Section 1.4.1 for detailed spectral density plots for all values of Txα.
S.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 after the initial release of the
CDM-625A 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.
Appendix S. TRANSMIT
S–1
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