Page 348 - Satellite Communications, Fourth Edition

328 Chapter Eleven

The CDV-10MIC is a single integrated circuit which implements all the functions
required for a constraint length 7, rate 1/2, and punctured 2/3 or 3/4 rate, convolu-
tional encoder, and Viterbi algorithm decoder. Important features of this chip are:
Full decoder and encoder implementation for rates 1/2, 2/3, and 3/4
Complies with INTELSAT IESS-308 and IESS-309 specifications
Extremely low implementation margin
No external components required for punctured code implementation
Operates at all information rates up to 10 Mbits/s. Higher speed versions are
under development
All synchronization circuits are included on chip. External connection of ambi-
guity state counter and ambiguity resolution inputs allows maximum application
flexibility
Advanced synchronization detectors enable very rapid synchronization. Rate, 3/4
block and phase synchronization in less than 8200 information bits (5500 trans-
mitted symbols).
Soft decision decoder inputs (3 bits, 8 levels)
Erasure inputs for implementing punctured codes at other rates
Path memory length options to optimize performance when implementing high-
rate punctured codes
Error-monitoring facilities included on chip
Synchronization detector outputs and control inputs to enable efficient synchro-
nization in higher-speed multiplexed structures.

Figure 11.4 Specifications for a single-chip Viterbi codec. (Courtesy of Signal Processors,
Ltd., Cambridge U.K.)

Decoding is a more difficult problem than encoding, and as the exam-
ple suggests, the search process could quickly become impracticable for
long messages. The Viterbi algorithm is used widely in practice for
decoding. An example of a commercially available codec is the CDV-
10MIC single-chip codec made by Signal Processors Limited, Cambridge,
U.K. The data sheet for this codec is shown in Fig. 11.4. The CDV-10MIC
utilizes Viterbi decoding. It has a constraint length of 7 and can be
adjusted for code rates of 1/2, 2/3, and 3/4 by means of what is termed
punctured coding. With punctured coding, the basic code is generated
at code rate 1/2, but by selectively discarding some of the output bits,
other rates can be achieved (Mead, 2000). The advantage is that a single
encoder can be used for different rates.

11.5 Interleaving
The idea behind interleaving is to change the order in which the bits
are encoded so that a burst of errors gets dispersed across a number
of codewords rather than being concentrated in one codeword.
Interleaving as applied in block codes will be used here to illustrate