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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.