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SLM-7650 Satellite Modem
Theory of Operation
4.3.2.2
Synchronous Scrambler/Descrambler
The Synchronous Scrambler does not multiply errors and is very suited for framed
data applications. The initial sequence of the frame may be used to start the
scrambler. Because there is no error multiplication, there is a 0.2 dB BER gain for the
synchronous scrambler. The synchronous scrambler is used in the following
applications:
Note:
4.3.3
Encoding/Decoding
Various FEC methods are available in the modem to enable operation in the noisy
satellite environment. These multiple FEC rates in combination with the various
modulation types enables the satellite operator to balance the power and bandwidth
availability of each transponder. These include convolutional encoding with Viterbi
decoding (CEVD) and Trellis encoding/decoding for 8PSK operation, and concatenation
of these with Reed-Solomon outer coding. Turbo Product Coding (TPC) is also available
as an option.
4.3.3.1
Convolutional/Viterbi (CEVD)
The combination of convolutional coding and Viterbi decoding has become an almost
universal standard for satellite communications. The modem complies with the Intelsat
IESS-308/309 standards for Viterbi decoding with a constraint length of seven. This is a
de facto standard, even in a closed network environment, which means almost guaranteed
inter-operability with other manufacturer's equipment. It provides very useful levels of
coding gain, and its short decoding delay and error-burst characteristics make it
particularly suitable for low data rate coded voice applications. It has a short constraint
length, fixed at 7, for all code rates. (The constraint length is defined as the number of
output symbols from the encoder that are affected by a single input bit.) By choosing
various coding rates (Rate 1/2, 3/4 or 7/8) the user can trade off coding gain for
bandwidth expansion. Rate 1/2 coding gives the best improvement in error rate, but
doubles the transmitted data rate, and hence doubles the occupied bandwidth of the
signal. Rate 7/8 coding, at the other extreme, provides the most modest improvement in
performance, but only expands the transmitted bandwidth by 14 %. A major advantage of
the Viterbi decoding method is that the performance is independent of data rate, and does
not display a pronounced threshold effect (i.e., does not fail rapidly below a certain value
of Eb/No). Because the method of convolutional coding used with Viterbi, the encoder
does not preserve the original data intact, and is called non-systematic.
•
IDR/Small carrier IESS-308
•
IDR IESS-308 with Reed-Solomon
•
IBS-309 Intelsat Business Service
•
IESS-310, 8PSK 2/3 with Reed-Solomon
•
Comtech CDM-550 framed operation
Compatibility issues do come up when dealing with scrambling. The scramblers
may be turned off if the modems are not compatible. An IF spectral inversion also
can be tried. Realize that the output IF signal will not look correct if the scrambler
is Off and no data is applied.
4–4
Revision 4
MN/SLM7650.IOM
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