Dmd1050Ts Functional Block Diagram; Baseband Processing; Tx Baseband Processing - Comtech EF Data DMD1050TS Installation And Operation Manual

3.2

DMD1050TS Functional Block Diagram

Figure 3-5 shows the DMD1050TS Functional Blocks. The modem is shown in a typical
application with customer data, Tx/Rx RF equipment and an antenna.

3.2.1 Baseband Processing

The Baseband Processor does all of the functions required for an Intelsat Buisiness Service (IBS)
/ Intermediate Data Rate (IDR) Framing Unit, a Reed-Solomon (R-S) Codec. In addition, the
Baseband Processing Section provides for transmit clock selection and rate adaptation, as well
as a rate adapter and Plesiochronous/Doppler (PD) Buffer in the receive direction. A multiplexer
is also provided for the Serial Clock Transmission (SCT) Clock Source for Loop Timing
Applications. The transmit and receive paths can be configured independently under processor
control.
The DMD1050TS supports IBS & IDR compatible framing modes. Because the modem does not
have all supporting interfaces as stipulated by IESS308/309, it is not 100% compliant. IBS and
IDR framing modes are supported in a Closed Network Mode.

3.2.2 Tx Baseband Processing

The Tx Data and Clock enters the baseband processor, passes through a rate adapting First In
First Out (FIFO) and enters the framer processor. In Closed-Net mode, the data passes through
the framer unaltered. In IDR & IBS framing enabled mode, it adds the applicable framing, as
defined in IESS-308 and 309. The data is then sent to the R-S encoder.
The R-S encoder encodes the data into R-S blocks. The blocks are then interleaved and
synchronized to the frame pattern, as defined by the selected specification (IESS-308, IESS-309,
DVB, etc.). After R-S encoding, the composite data and clock are applied to the Baseband
Loopback Circuit.
3.2.2.1 Direct Sequence Spread Spectum (DSSS)
Because the DMD1050TS is ideal for systems with very small dishes, or on-the-move systems,
there is an increased interest in Spectrum Spreading. To meet this need, optional DSSS is
available for the DMD1050TS.
DSSS works by modulating the normal carrier with a pseudo-random PN sequence (chips) at a
much higher chip rate. The chip rate is determined by the carrier's symbol rate and a multiplying
(spreading) factor.
The DMD1050TS offers spreading in this Low Density Parity Check (LDPC) Modulation and Code
Rate (MODCOD):
•
HP-LDPC BPSK Rate 1/2
The spreading factors currently supported are 1 (Spreading Off), 2, 4, 8 and 16, up to a maximum
chip rate of 10 Mbps. You can select from among four built-in, pseudo-random PN sequences.
Thus, multiple spread carriers can occupy the same bandwidth in a Code-Division Multiple
Access (CDMA) manner.
Additionally, there is no loss in performance with spreading enabled on the DMD1050TS.
Theory of Operation
DMD1050TS Satellite Modem Board
3–6
Revision 1
MN-DMD1050TS