Mocomtech CDM-QX - Manual (Page 27)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
Introduction MN/CDMQx.IOM
1–3
1.2 Functional Description
The CDM-Qx/QxL Multi-Channel Satellite 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 is enabled) where the overhead
information is removed. Following this, the data passes to the Plesiochronous/Doppler buffer,
which has a programmable size, or alternatively bypasses the buffer. From here, the receive clock
and data signals are routed to the terrestrial interface, and are passed to the externally connected
DTE equipment.
Physically, a modem chassis is comprised of three main card assemblies:
• The IF Backplane card includes the frequency reference, power splitters, power summers,
the FSK link (L-Band version only) and the IF Loop back functions.
• The Digital Backplane card routes all the control signals, data path switching, Carrier-in-
Carrier
®
signals and power for all modules.
• The M&C controls all functions in the unit.
Within the chassis are four slots which allow any combination of modulators or demodulators to be
installed. If configured as a single modem, two plug-in cards comprising a modulator and
demodulator are required.
• A Modulator card contains the transmit interface circuits, the framer, the encoder or
encoders and the signal processing functions of modulation.