Carrier Spacing And Frequency Uncertainty; Figure A-1. Maximum Carrier Uncertainty, Fv, Expressed In Terms Of Symbol Rate, Sr - Comtech EF Data DST Operator's Manual

Digital Satellite Terminal System
Operation
A.1.2

Carrier Spacing And Frequency Uncertainty

When there is no carrier uncertainty, the carrier spacing is usually 1.3 * SR, where SR is
the symbol rate of the modem. More generally, the carrier uncertainty is:
The factor of 2 appears because the uncertainty is ±fv. The maximum carrier uncertainty
is illustrated in Figure A-1 where the ideal position of the carrier is shown in the center,
and its location due to uncertainty is displaced to extreme positions of +fv and -fv relative
to the ideal. Because the goal is to establish the minimum symbol rate for a given
frequency uncertainty fv is expressed in terms of symbol rate (SR) and Carrier Spacing
Factor (k).
The most efficient spectral utilization is obtained when the symbol rate is high enough
that a k * SR spacing factor is usable. This requires that fv is smaller than the carrier
spacing to prevent locking to the wrong carrier. Alternatively, if the carrier spacing is too
small compared to fv it is necessary to increase the spacing. For a typical link carrier
spacing is 1.3 * SR, when k = 1.3, and this is still acceptable as long as SR ≥ fv /(2 * 1.3).
When the uncertainty, fv, is higher, then k ≥ 1.3 is needed resulting in a wider spacing
between the carriers.
fv ≤ SR * 2 * k,
SR ≥ fv /(2 * k)
where,
fv = Total One Sided Frequency Variation or Uncertainty
SR = Symbol Rate
k = Carrier Spacing Factor, usually 1.3
Maximum Carrier Uncertainty, fv < 2 * k * SR
-15 -10 -5
Relative Frequency
Figure A-1. Maximum Carrier Uncertainty, fv, expressed
in terms of Symbol Rate, SR
or
k x SR k x SR
k * SR k * SR
fv_max
< 2 * k * SR
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A–2
Revision 1
MN/DST.IOM
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