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Chapter 3. FUNCTIONAL
The modem has two fundamentally different types of interface – IF and Data:
•
The Data interface can be a bi-directional path, which connects with the
customer's equipment (assumed to be the DTE) and the modem (assumed to be
the DCE).
•
The IF interface provides a bi-directional link with the satellite via the uplink and
downlink equipment.
Transmit data is received by the terrestrial interface where line receivers convert the
clock and data signals to CMOS levels for further processing. A small FIFO follows the
terrestrial interface to facilitate the various clocking and framing options. If framing is
enabled, the transmit clock and data output from the FIFO pass through the framer, where
the overhead EDMAC data is added to the main data. Otherwise, the clock and data are
passed directly to the Forward Error Correction encoder.
In the FEC encoder, the data is scrambled, differentially encoded, and then convolutionally
encoded. Following the encoder, the data is fed to the transmit digital filters, which
perform spectral shaping on the data signals. The resultant I and Q signals are then fed to
the BPSK/QPSK/8-PSK/16-QAM modulator. The carrier is generated by a frequency
synthesizer, and the I and Q signals directly modulate this carrier to produce an IF output
signal.
The RX IF signal is translated and filtered at an intermediate frequency (IF) using the
coarse step synthesizer. This is mixed with a second synthesizer, resulting in the signal
being IF sampled with a high-speed analog to digital converter (A to D). The sampled IF
is then digitally split into an in-phase (I) and a quadrature (Q) component. An AGC
circuit keeps the desired signal level constant over a broad range of input levels. The I
and Q signals are then decimated to reduce the computation rate into the poly phase
matched filter.
Carrier and clock recovery is performed on the baseband I and Q signals after the
matched filter. The resultant demodulated signal is fed, in soft decision form, to the
selected FEC decoder (which can be Viterbi, TCM, Reed-Solomon, or Turbo if installed).
After decoding, the recovered clock and data pass to the de-framer (if EDMAC framing
DESCRIPTION
3–1
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