APP Decoder (Communications Blockset)

Decode a convolutional code using the a posteriori probability (APP) method

Library

Convolutional sublibrary of Channel Coding

Description

The APP Decoder block performs a posteriori probability (APP) decoding of a convolutional code. You can use this block to build a turbo decoder.

Inputs and Outputs

The input L(u) represents the sequence of log-likelihoods of encoder input bits, while the input L(c) represents the sequence of log-likelihoods of code bits. The outputs L(u) and L(c) are updated versions of these sequences, based on information about the encoder.

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

The inputs can be either:

Sample-based vectors having the same dimension and orientation, with Q = 1
Frame-based column vectors with any positive integer for Q

If you only need the input L(c) and output L(u), then you can attach a Simulink Ground block to the input L(u) and a Simulink Terminator block to the output L(c).

Specifying the Encoder

To define the convolutional encoder that produced the coded input, use the Trellis structure parameter. This parameter is a MATLAB structure whose format is described in the section, Trellis Description of a Convolutional Encoder," in the Communications Toolbox User's Guide. You can use this parameter field in two ways:

If you have a variable in the MATLAB workspace that contains the trellis structure, then enter its name as the Trellis structure parameter. This way is preferable because it causes Simulink to spend less time updating the diagram at the beginning of each simulation, compared to the usage in the next bulleted item.
If you want to specify the encoder using its constraint length, generator polynomials, and possibly feedback connection polynomials, then use a poly2trellis command within the Trellis structure field. For example, to use an encoder with a constraint length of 7, code generator polynomials of 171 and 133 (in octal numbers), and a feedback connection of 171 (in octal), set the Trellis structure parameter to

poly2trellis(7,[171 133],171)

To indicate how the encoder treats the trellis at the beginning and end of each frame, set the Termination method parameter to either Truncated or Terminated. The Truncated option indicates that the encoder resets to the all-zeros state at the beginning of each frame, while the Terminated option indicates that the encoder forces the trellis to end each frame in the all-zeros state. If you use the Convolutional Encoder block with the Reset parameter set to On each frame, then use the Truncated option in this block.

Specifying Details of the Algorithm

You can control part of the decoding algorithm using the Algorithm parameter. The True APP option implements a posteriori probability. To gain speed, both the Max* and Max options approximate expressions like

by other quantities. The Max option uses max{a_i} as the approximation, while the Max* option uses max{a_i} plus a correction term.

The Max* option enables the Scaling bits parameter in the mask. This parameter is the number of bits by which the block scales the data it processes internally. You can use this parameter to avoid losing precision during the computations. It is especially appropriate if your implementation uses fixed-point components. For more information about the Max* option, see the article by Viterbi in the References section below.

Dialog Box

Trellis structure: MATLAB structure that contains the trellis description of the convolutional encoder.
Termination method: Either Truncated or Terminated. This parameter indicates how the convolutional encoder treats the trellis at the beginning and end of frames.
Algorithm: Either True APP, Max*, or Max.
Number of scaling bits: An integer between 0 and 8 that indicates by how many bits the decoder scales data in order to avoid losing precision. This field is active only when Algorithm is set to Max*.

See Also

Viterbi Decoder, Convolutional Encoder; poly2trellis (Communications Toolbox)

References

[1] Benedetto, Sergio and Guido Montorsi. "Performance of Continuous and Blockwise Decoded Turbo Codes." IEEE Communications Letters, vol. 1, May 1997. 77-79.

[2] Benedetto, S., G. Montorsi, D. Divsalar, and F. Pollara. "A Soft-Input Soft-Output Maximum A Posterior (MAP) Module to Decode Parallel and Serial Concatenated Codes." JPL TMO Progress Report, vol. 42-127, November 1996. [This electronic journal is available at http://tmo.jpl.nasa.gov/tmo/progress_report/index.html.]

[3] Viterbi, Andrew J. "An Intuitive Justification and a Simplified Implementation of the MAP Decoder for Convolutional Codes." IEEE Journal on Selected Areas in Communications, vol. 16, February 1998. 260-264.

Algebraic Interleaver AWGN Channel