Table of Contents
Advertisement
SLM-7650 Satellite Modem
Theory of Operation
The output of the vector rotation circuit is then sent to the FEC decoder or uncoded data
formatter. The output of the FEC decoder is the final output of the demodulator section.
The uncoded data formatter is used in uncoded operation only and bypasses the FEC
decoder entirely. Since in uncoded operation there is no FEC information to use for
determining lock, ambiguity resolution, and lock detect are performed differently than
when Viterbi or TPC FEC is used. Inversion ambiguity is resolved by:
1. Differentially encoding both the I and Q channels in the modulator.
2. Subsequently differentially decoding both the I and Q channels in the
In Offset QPSK modulation, the bit ordering ambiguity is resolved since the ordering is
inherent in the modulated carrier. By inserting the 1/2 symbol period delay in the I
channel data path and using the Q channel symbol interval as the reference, I occurs
before Q. Thus, the data is ordered I then Q into a single bit stream.
Carrier lock is determined by sensing the "quality" of the eye pattern. In OQPSK, both
eyes should be as wide as possible. In BPSK, the I channel eye should be wide open and
the Q channel eye should be closed. Special circuitry determines when the eye pattern on
either channel is not within a valid range, and declares lock accordingly.
The alternate descrambler provides OM-73 compatible descrambling of the decoded data
stream. The descrambler is the non-recursive counterpart to the OM-73 scrambler. The
scrambled data enters an 11-bit shift register, where taps 9 and 11 are exclusive ORed to
provide a synchronous reset to an 8-bit counter (adverse state detector). The following are
all exclusive NORed to form the descrambled output:
•
•
•
This function may be turned ON or OFF by the M&C. The output of the alternate
descrambler is the final output of the demodulator and is then sent to the baseband
interface section.
Carrier acquisition in the demodulator is a function symbol rate and modulation type.
Lower data rates and higher modulation types will require a longer acquisition time. For
very small carrier (<25 ksym/s), there is also the possibility that the demodulator will
lock to the wrong carrier if the sweep range encompasses like carriers within the sweep
range. The demodulator has available functionality that can shorten the acquisition time.
These are: Sweep Range, Sweep Center Frequency, and Reacquisition.
demodulator.
Terminal count of this counter
Inverse of the input data
Taps 9 and 11 of the shift register
4–16
Revision 4
MN/SLM7650.IOM
Advertisement
Table of Contents
