Mocomtech CDM-QX operation manual Functional Description

Chapter 3. FUNCTIONAL DESCRIPTION

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.

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