Paradise P300 User Manual

Page 66

P300H

P300 Series Modem Installation and Operating Handbook

Page 66

Select FEC Type: [?]

1=None 2=Viterbi 3=Sequential 4=TCM Rate 2/3 5=Turbo

Change, Tx/Rx, Mod/Demod, FEC Type Menu

Select FEC Rate: [?]

1=1/2 Rate 2=3/4 Rate 3=7/8 Rate

Change, Tx/Rx, Mod/Demod, FEC Rate Menu (Vit/Seq)

OFFSET QPSK (OQPSK) PRIMER
When using Offset QPSK (OQPSK) modulation, the in phase and quadrature components of each symbol
do not change together (as with QPSK), but instead one component is delayed by half a symbol compared
to the other. Looking at a constellation (vector) diagram this effectively limits the instantaneous change of
phase to either +/-90 degrees, excluding the 180 degree phase change which takes the vector transition
through the zero amplitude point (equating to an instantaneous null in the RF envelope). By preventing the
carrier phase change moving through the null power point, large amplitude variations of the output carrier
are avoided. Consequently on links which exhibit some non-linearity (and therefore probably some AM to
PM conversion), there is reduced amplitude modulation (AM) to get converted to Phase Modulation (PM)
which would otherwise degrade the distant Eb/No as a phase noise effect.

6.6.11 Change, Tx/Rx, Modulator, FEC Menu

The operator is first prompted to select the Forward Error Correction (FEC) type, and the choices are
between:

NONE

(uncoded)

VITERBI

(if Viterbi FEC option fitted)

SEQUENTIAL

(if Sequential FEC option fitted)

TCM

(Trellis Code Modulation if the 8PSK/TCM feature is available, Rate 2/3 only)

Turbo

(Turbo Product Code FEC, if the Turbo option is available)

Pressing

YES

accepts the current FEC type, and where there is a choice of FEC rates leads onto the FEC

Code Rate selection screen. Note that 8PSK and TCM must be used together, and that for TCM only rate

is available.

For a review of the processing delay associated with the Sequential decoder, and how this is
minimised, please refer to "FaultMode = 3" in Appendix E.

If interworking with other manufacturers equipment it is worth reading the notes in the section “Interworking
different manufacturers equipment” on page 153 which covers primarily scrambling, but also the
requirements for Spectrum Invert with Viterbi/Sequential in BPSK and QPSK. If specifically interworking with
Comstream equipment with sequential FEC, see also Appendix E for “Fault Mode 11" (Comstream
compatible sequential mode).

For

Viterbi

and

Sequential

FEC the FEC rate selection screen is as follows:

The FEC code rate may be select as ½, ¾, or

Rate, again pressing

YES

will accept the current FEC rate

and return to the Change, Tx, Modulator menu. Most INTELSAT services use the more powerful ½ rate or
¾ rate FEC (increasing the data rate by factors of 2 and 4/3 respectively).

The operator is warned of unusual combinations of Modulation and FEC type/code rate when

SETUP-

CHECK

is requested.