Mocomtech CIM-550 - Manual (Page 107)

81
Chapter 6.
Theory of Operation
Functional Description 81
Offset QPSK Operation 82
EIA-232 Operation 83
AUPC (Automatic Uplink Power Control) 88
6.1 F UNCTIONAL D ESCRIPTION
The CiM-550 has two fundamentally different types of interface - IF and data. The data
interface is 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 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
differentially encoded, scrambled, 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
QPSK/BPSK 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 received IF signal is first translated to a fixed IF frequency, using a frequency
synthesizer. An AGC circuit maintains the composite level within the IF bandwidth
constant over a limited range. Following this, the signal is sampled by a high-speed
(flash) A/D converter. All processing beyond this conversion is purely digital. The signal
is translated down to near zero frequency by a complex mix, and then is processed by a
digital Costas Loop, which performs the functions of Nyquist filtering, carrier recovery,
and bit-timing recovery. The resultant demodulated signal is fed, in soft decision form, to
the FEC decoder (Viterbi, Sequential or Turbo, and Reed-Solomon, if installed).