Communications Blockset

Convolutional Encoder

Create a convolutional code from binary data

Library

Convolutional sublibrary of Channel Coding

Description

The Convolutional Encoder block encodes a sequence of binary input vectors to produce a sequence of binary output vectors. This block can process multiple symbols at a time.

Input and Output Sizes

If the encoder takes k input bit streams (that is, can receive 2^k possible input symbols), then this block's input vector length is L*k for some positive integer L. Similarly, if the encoder produces n output bit streams (that is, can produce 2ⁿ possible output symbols), then this block's output vector length is L*n.

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

Specifying the Encoder

To define the convolutional encoder, 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)

The encoder registers begin in the all-zeros state. You can configure the encoder so that it resets its registers to the all-zeros state during the course of the simulation. To do this, use one of these values of the Reset parameter:

• The value None indicates that the encoder never resets.
• The value On each frame indicates that the encoder resets at the beginning of each frame, before processing the next frame of input data
• The value On nonzero Rst input causes the block to have a second input port, labeled Rst. The signal at the Rst port is a scalar signal. When it is nonzero, the encoder resets before processing the data at the first input port.

Dialog Box

Trellis structure

MATLAB structure that contains the trellis description of the convolutional encoder.

Reset

Determines whether and under what circumstances the encoder resets to the all-zeros state before processing the input data. Choices are None, On each frame, and On nonzero Rst input. The last option causes the block to have a second input port, labeled Rst.

See Also

Viterbi Decoder, 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.

Convolutional Deinterleaver

Convolutional Interleaver