Page 351 - Satellite Communications, Fourth Edition

Error Control Coding 331

11.7 Link Parameters Affected by Coding
Where no error control coding is employed, the message will be referred
to as an uncoded message, and its parameters will be denoted by the sub-
script U. Figure 11.7a shows the arrangement for an uncoded message.
Where error control coding is employed, the message will be referred to
as a coded message, and its parameters will be denoted by the subscript
C. Figure 11.7b shows the arrangement for a coded message. For com-
parison purposes, the [C/N ] value is assumed to be the same for both
0
situations. The input bit rate to the modulator for the uncoded message
is R , and for the coded message is R . Since n code bits must be trans-
b
c
mitted for every k data bits, the bit rates are related as
R b
r c (11.10)
R c

[C/N ]
0

Modulator Tx Rx Demodulator

P e

R b R b
(a)

[C/N ]
0

Modulator Tx Rx Demodulator

R c R c

BER
Encoder Decoder
R b R b

(b)
Figure 11.7 Comparing links with and without FEC.