Digital Signal Processing Reference
In-Depth Information
TABLE 10.9
Input and Output Scheme for Voice Scrambler
Period 1
Period 2
Period 3
Period 4
Period 5
Period 6
Period 7
Period 8
Input
Sample 1
X
Sample 2
X
Sample 3
X
Sample 4
X
Output
X
X
X
X
Sample 1
Sample 2
Sample 3
Sample 4
Input
cosine
signal
Output
cosine
signal
CHANNEL
ENCODER
CHANNEL
DECODER
FIGURE 10.41.
Hard-decision decoding setup.
AWGN
Noise
Input
cosine
signal
Output
cosine
signal
CHANNEL
ENCODER
BPSK
MODULATOR
CHANNEL
DECODER
FIGURE 10.42.
Soft-decision decoding setup.
10.15 CONVOLUTIONAL ENCODING AND VITERBI DECODING
Channel coding schemes widely used in communication systems mostly consist of
the convolutional encoding and Viterbi decoding algorithms to reduce the bit errors
on noisy channels. This project implements a 3-output, 1-input, 2-shift register (3,1,2)
convolutional encoder used for channel encoding and a channel decoder employ-
ing soft-decision and basic Viterbi decoding techniques.
Soft Decision and Basic Viterbi Decoding
The system setups are used for soft decision and Viterbi decoding techniques. In
Figures 10.41 and 10.42, the channel encoder represents a (3,1,2) convolutional
encoding algorithm, and the channel decoder represents the Viterbi decoding
algorithm.
In the Viterbi decoding setup shown in Figure 10.41, a cosine signal is the input
to the channel encoder algorithm. The encoded output is stored in a buffer. The ele-
ments of this buffer provide the input to the channel decoder algorithm that decodes
it and returns the original cosine signal. Both the encoder and decoder outputs are
displayed within CCS.
In the soft decision decoding setup shown in Figure 10.42, a cosine signal is given
as input to the channel encoder algorithm. The binary output of the channel encoder
is modulated using the BPSK technique, whereby the 0 output of the channel
encoder is translated into
-
1 and the 1 output is translated into
+
1. Additive white
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