Comtech EF Data CDM-625 - Manual (Page 456)

CDM-625 Advanced Satellite Modem Revision 15
Appendix B MN-CDM625
B–2
Example: In the above diagram, the (Co+No)/No measured is 4.6 dB. If Rate 1/2 QPSK is used,
this corresponds to an E
b
/N
0
of approximately 2.6 dB. The exact relationship used to derive the
table values is as follows:
E
b
/N
0
=10log
10
(10 (Co+No/No)/10) -1)-10log
10
(FEC Code Rate)-10log
10
(bits/symbol)
Where:
•
E
b
/N
0
and (Co+No)/ No are expressed in dB;
• bits/symbol = 1 for BPSK;
• bits/symbol = 2 for QPSK;
• bits/symbol = 3 for 8-PSK/8-QAM;
• bits/symbol = 4 for 16-QAM;
• FEC Code Rate for ‘uncoded’ = 1.
Note: Pay close attention to the sign of the middle term.
Chapter 12. ESC++ explains how the Eb/No performance degrades when ESC++ is
used, particularly at lower data rates, where the percentage overhead is high.