Communications Blockset

Viterbi Decoder

Decode convolutionally encoded data using the Viterbi algorithm

Library

Convolutional sublibrary of Channel Coding

Description

The Viterbi Decoder block decodes input symbols to produce binary output symbols. This block can process several symbols at a time for faster performance.

Input and Output Sizes

If the convolutional code uses an alphabet of 2ⁿ possible symbols, then this block's input vector length is L*n for some positive integer L. Similarly, if the decoded data uses an alphabet of 2^k possible output symbols, then this block's output vector length is L*k. The integer L is the number of frames that the block processes in each step.

The input can be either a sample-based vector with L = 1, or a frame-based column vector with any positive integer for L.

Input Values and Decision Types

The entries of the input vector are either bipolar, binary, or integer data, depending on the Decision type parameter.

Decision type Parameter	Possible Entries in Decoder Input	Interpretation of Values
Unquantized	Real numbers	+1: logical zero -1: logical one
Hard Decision	0, 1	0: logical zero 1: logical one
Soft Decision	Integers between 0 and 2b-1, where b is the Number of soft decision bits parameter	0: most confident decision for logical zero 2b-1: most confident decision for logical one Other values represent less confident decisions

To illustrate the soft decision situation more explicitly, the table below lists interpretations of values for 3-bit soft decisions.

Input Value	Interpretation
0	Most confident zero
1	Second most confident zero
2	Third most confident zero
3	Least confident zero
4	Least confident one
5	Third most confident one
6	Second most confident one
7	Most confident one

Operation Modes for Frame-Based Inputs

If the input signal is frame-based, then the block has three possible methods for transitioning between successive frames. The Operation mode parameter controls which method the block uses:

• In Continuous mode, the block saves its internal state metric at the end of each frame, for use with the next frame. Each traceback path is treated independently.
• In Truncated mode, the block treats each frame independently. The traceback path starts at the state with the best metric and always ends in the all-zeros state. This mode is appropriate when the corresponding Convolutional Encoder block has its Reset parameter set to On each frame.
• In Terminated mode, the block treats each frame independently, and the traceback path always starts and ends in the all-zeros state. This mode is appropriate when the uncoded message signal (that is, the input to the corresponding Convolutional Encoder block) has enough zeros at the end of each frame to fill all memory registers of the encoder. If the encoder has k input streams and constraint length vector constr (using the polynomial description), then "enough" means k*max(constr-1).

In the special case when the frame-based input signal contains only one symbol, the Continuous mode is most appropriate.

Traceback Depth and Decoding Delay

The Traceback depth parameter, D, influences the decoding delay. The decoding delay is the number of zero symbols that precede the first decoded symbol in the output.

• If the input signal is sample-based, then the decoding delay consists of D zero symbols
• If the input signal is frame-based and the Operation mode parameter is set to Continuous, then the decoding delay consists of D zero symbols
• If the Operation mode parameter is set to Truncated or Terminated, then there is no output delay and the Traceback depth parameter must be less than or equal to the number of symbols in each frame.

If the code rate is 1/2, then a typical Traceback depth value is about five times the constraint length of the code.

Reset Port

The reset port is usable only when the Operation mode parameter is set to Continuous. Checking the Reset input check box causes the block to have an additional input port, labeled Rst. When the Rst input is nonzero, the decoder returns to its initial state by configuring its internal memory as follows:

• Sets the all-zeros state metric to zero
• Sets all other state metrics to the maximum value
• Sets the traceback memory to zero

Using a reset port on this block is analogous to setting the Reset parameter in the Convolutional Encoder block to On nonzero Rst input.

Dialog Box

Trellis structure

MATLAB structure that contains the trellis description of the convolutional encoder. Use the same value here and in the corresponding Convolutional Encoder block.

Decision type

Unquantized, Hard Decision, or Soft Decision.

Number of soft decision bits

The number of soft decision bits used to represent each input. This field is active only when Decision type is set to Soft Decision.

Traceback depth

The number of trellis branches used to construct each traceback path.

Operation mode

Method for transitioning between successive input frames. For frame-based input, the choices are Continuous, Terminated, and Truncated. Sample-based input must use the Continuous mode.

Reset input

When you check this box, the decoder has a second input port labeled Rst. Providing a nonzero input value to this port causes the internal memory to be set to its initial state prior to processing the input data.

See Also

Convolutional Encoder, APP Decoder

References

[1]  Clark, George C. Jr. and J. Bibb Cain. Error-Correction Coding for Digital Communications. New York: Plenum Press, 1981.

[2]  Gitlin, Richard D., Jeremiah F. Hayes, and Stephen B. Weinstein. Data Communications Principles. New York: Plenum, 1992.

[3]  Heller, Jerrold A. and Irwin Mark Jacobs. "Viterbi Decoding for Satellite and Space Communication." IEEE Transactions on Communication Technology, vol. COM-19, October 1971. 835-848.

Unipolar to Bipolar Converter

Voltage-Controlled Oscillator