Comtech EF Data CDM-570A - Manual (Page 577)

M–1
Appendix M. TRANSMIT
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.