Comtech EF Data CDM-700 - Manual (Page 128)

CDM-700 High Speed Satellite Modem Revision 5
Forward Error Correction Options MN/CDM700.IOM
With the CDM-700, Comtech EF Data now provides one of the best FEC technologies currently
available. For high data rate operation the higher code rates are preferred to minimize the required
bandwidth. This offering provides the range of TPC code rates in combination with QPSK,
8-PSK, 16-QAM and 64-QAM modulation.
7.2.1 Range of Data Rates
The composite data rate range depends on the FAST option ordered with the CDM-700. The
options are arranged in ascending tiers by symbol rate and modulation type along with the
corresponding data rate range.
The Data Rate Range Table in the Chapter 10. Summary of Specifications summarizes the
composite data range by tiers. It also shows how the composite data rate is constructed from the
individual data ports on the data interface modules.
The data rate of each individual data port depends on the particular CDI-xx data interface
installed in the CDM-700. Additional information about the data rate range for each individual
port can be found in the chapters for each data interface module.
NOTE
For additional information on composite data rate, refer to the next paragraph (7.2.2).
7.2.2 Eb/No, Spectral Efficiency and Occupied Bandwidth
Configuration of this modem differs from conventional modems because the architecture uses an
internal statistical multiplexer to combine multiple data inputs together to a single stream. With
the multi-port interfaces, data rate determination is more consistent if the user sets the data rate
for each port on the interface. With the multiple ports, it is necessary to first determine the
composite data rate then include the framing overhead, code rate and modulation to arrive at the
symbol rate; from this, the occupied bandwidth is found. These are found as follows:
Composite Data Rate =
Σ Ports
Ports = The data rate for all enabled ports for Interface 1 (Intfc1) and Interface 2
(Intfc2), and depends upon the interfaces installed in Slot 1 and Slot 2. Disabled
ports have a data rate of zero. The data rate for each port is programmed under
the Config Intfc1 and Config Intfc2 menus.
Symbol Rate = 1.025 x Composite Data Rate / (m x CR), where
1.025 = the framing overhead
m = 2 for QPSK, 2 8PSK, 4 16-QAM, 5 32-QAM, 6 64-QAM
CR = code rate: 3/4, 7/8 (20/23 actual), 0.95 (17/18 actual)
Occupied Bandwidth = 1.19 x Symbol Rate, for 35% Rolloff
= 1.15 x Symbol Rate, for 25% Rolloff
= 1.12 x Symbol Rate, for 20% Rolloff (example only)
The occupied bandwidth is the bandwidth between -10 dB points of the power spectral
density.
7–2