Viterbi Decoding Theory; Figure 4-3. Demodulator Block Diagram - Comtech EF Data SDM-100A Installation And Operation Manual

SDM-100A Satellite Modem
RF
IF INPUT
50 TO 180 MHz
-55 TO -30 dBm
AGC
IF
LOOPBACK
PROCESSOR
RX DATA
RX CLOCK
RR

4.2.3 Viterbi Decoding Theory

The Viterbi decoder is used in open network applications, typically in IBS or IDR
communication systems. The Viterbi decoder operates in conjunction with the
convolutional encoder in the transmit modem. They correct transmission channel errors
in the received data stream.
Refer to Figure 4-4 for a block diagram of the Viterbi decoder.
The Viterbi decoder processes 3-bit quantized R0 and R1 parallel code bits, or symbols,
from the demodulator. The quantization is 3-bit soft decision in sign/magnitude format.
This is a representation of the data transmitted, corrupted by additive white Gaussian
noise. The decoder uses the code symbols produced by the encoder to determine which
symbols have been corrupted by the transmission channel, and it corrects as many as
possible.
The data signal passes through an ambiguity resolver, which compensates for the
potential 90° phase ambiguity inherent in a QPSK demodulator. If the decoder is
operating in 3/4 or 7/8 rate, the data signal is then "de-punctured." The "de-puncture"
pattern is the same as the puncture pattern used in the encoder.
Rev. 0
IF FILTER
RF
MPC
SYNTH
MICRO-
MPC
SYNCHRONOUS
DESCRAMBLER
MPC

Figure 4-3. Demodulator Block Diagram

ALIAS
FILTER
ALIAS
FILTER
90
0
BB
VCO
DDS
VITERBI
MPC
DDS
Theory of Operation
I
DIGITAL
A/D
NYQUIST
Q
DIGITAL
A/D
NYQUIST
DIGITAL
COSTAS
LOOP
SOFT DECISION
MAPPING
UNIQUE WORD
DETECTOR
DELAY
DIGITAL
CLOCK
LOOP
4–7