Comtech EF Data CDM-625A, Installation And Operation Manual

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S–1
Appendix S. TRANSMIT
SPECTRUM FILTERING OPTIONS
S.1 Overview
The CDM-625 Advanced Satellite Modem offered the choices of 0.35 or 0.25 for Transmit Alpha
Filter Rolloff Factor (Tx Alpha, or simply Tx α ). 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 Sect. 1.4.1 Modulator (Chapter 1. INTRODUCTION) for detailed spectral density
plots for all values of Tx α .
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 Tx α ). 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.
•
The recommended minimum carrier spacing equals
(1 + Txα) x symbol rate
,
assuming equal symbol rates for all carriers and zero frequency error between carriers.