Description
PSM-500/500L/500LT SCPC Satellite Modem
PSM-500/500L/500LT - Rev. 0.91
Page 1-16
error correction. For example, the common rate ½ means that for every data bit two bits are
transmitted, and in rate 5/6, 6 bits are transmitted for every 5 data bits. Better performance is
commonly considered higher coding gain at a given rate. There are always other factors to consider,
such as the latency (processing time) required, time to recover from a synch loss, signal acquisition
time, etc, etc.
FECs technology is often specifically adapted to a particular use. For example, it is common to use
special forms of Reed-Solomon, TPC and LDPC for video signals. These type signals and FECs
typically have a fairly high performance floor that are of little consequence for a wideband video
signal, but would be entirely unacceptable for most data information especially at low data rates.
Viterbi has been the standard high performance FEC used in satellite communications for
approximately 10 years. It has only been in the past few years that new technologies have emerged
which provide more coding gain with reasonable implementations. The standard PSM-500 FEC card
includes the circuitry for a Viterbi, TCM and Reed-Solomon Codec providing the PSM-500 with basic
functional capability for all standard operating parameters including 8PSK TCM and 16QAM. The
following are general descriptions of the characteristics of each of these functions.
The table below shows the currently available modes depending on modulation.
Note in Table A that the front panel selection number for each of the options is listed as the “Sel #”.
For example The Modulation modes show the selection numbers 0 through 6, and the FEC Type,
Option and Code Rate selection numbers are listed in the column to the right of each item. These
numbers can be referred to for front panel operation but are even more applicable to the SnIP Telnet
command line program named “
m500ctl
”. This program has specific commands that allow entering
either just the 3 digit FEC options or the full
M
odulation, FEC
T
ype,
O
ption,
C
ode rate and
R
eed-
Solomon mode (MTOCR) in a single entry. Refer to the SnIP documentation for more information.