Page 350 - Satellite Communications, Fourth Edition

330 Chapter Eleven

11.6 Concatenated Codes
Codes designed to correct for burst errors can be combined with codes
designed to correct for random errors, a process known as concatenation.
Figure 11.6 shows the general form for concatenated codes. The input
data are fed into the encoder designed for burst error correction. This
is the outer encoder. The output from the outer encoder is fed into the
encoder designed for random error correction. This is the inner encoder.
The signal is then modulated and passed on for transmission. At the
receiver, the signal is demodulated. The inner decoder matches the
inner encoder and follows the demodulator. The output from the inner
decoder is fed into the outer decoder, which matches the outer encoder.
The term outer refers to the outermost encoding/decoding units in the
equipment chain, and the term inner refers to the innermost encod-
ing/decoding unit. In digital satellite television, the outer code is a R-S
code, and the inner code is a convolutional code. The inner decoder uti-
lizes Viterbi decoding.

Modulator Tx Rx Demodulator

Inner Inner
encoder decoder

Outer Outer
encoder decoder

R b R b
Figure 11.6 Concatenated coding (see Sec. 11.6).