Communications Blockset

Parameters for Convolutional Coding

To process convolutional codes (including turbo codes), use the Convolutional Encoder, Viterbi Decoder, and/or APP Decoder blocks in the Convolutional sublibrary. If a mask parameter is required in both the encoder and the decoder, then use the same value in both blocks.

The blocks in the Convolutional sublibrary assume that you use one of two different representations of a convolutional encoder:

• If you design your encoder using a diagram with shift registers and modulo-2 adders, then you can compute the code generator polynomial matrix and subsequently use the poly2trellis function (in the Communications Toolbox) to generate the corresponding trellis structure mask parameter automatically. For an example, see Example: A Rate 2/3 Feedforward Encoder.
• If you design your encoder using a trellis diagram, then you can construct the trellis structure in MATLAB and use it as the mask parameter.

Details about these representations are in the sections Polynomial Description of a Convolutional Encoder and Trellis Description of a Convolutional Encoder in the Communications Toolbox User's Guide.

Using the Polynomial Description in Blocks

To use the polynomial description with the Convolutional Encoder, Viterbi Decoder, or APP Decoder blocks, you can use the utility function poly2trellis, from the Communications Toolbox. This function accepts a polynomial description and converts it into a trellis description. For example, the following command computes the trellis description of an encoder whose constraint length is 5 and whose generator polynomials are 35 and 31.

• trellis = poly2trellis(5,[35 31]);
  

To use this encoder with one of the convolutional coding blocks, simply place a poly2trellis command such as

• poly2trellis(5,[35 31]);
  

in the Trellis structure parameter field.

Convolutional Coding Features of the Blockset

Example: A Rate 2/3 Feedforward Encoder