Comtech EF Data CDM-425, Инструкция по установке и эксплуатации

Страница: 729 / 744

CDM- Advanced Satellite Modem
Revision 0
Appendix N N–1 MN-CDM-425
Appendix N. TRANSMIT
SPECTRUM FILTERING OPTIONS
N.1 Overview
Chapter 1. INTRODUCTION
The CDM-425 Advanced Satellite Modem offers the choices of 0.05, 0.10, 0.15, 0.20, 0.25, or
0.35 for Transmit Alpha Filter Rolloff Factor (Tx Alpha, or simply
Tx
α
). See Chapter 1.
INTRODUCTION, Sect. 1.4.1 Modulator for detailed spectral density plots for all values of Tx
α.
N.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-425 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.