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VisualSim.actor.lib.comm
Class ViterbiDecoder

java.lang.Object
 VisualSim.kernel.util.NamedObj
     VisualSim.kernel.InstantiableNamedObj
         VisualSim.kernel.Entity
             VisualSim.kernel.ComponentEntity
                 VisualSim.actor.AtomicActor
                     VisualSim.actor.TypedAtomicActor
                         VisualSim.actor.lib.Transformer
                             VisualSim.actor.lib.comm.ViterbiDecoder

• All Implemented Interfaces:
• VisualSim.actor.Actor, VisualSim.kernel.util.Changeable, java.lang.Cloneable, VisualSim.kernel.util.Debuggable, VisualSim.kernel.util.DebugListener, VisualSim.kernel.util.Derivable, VisualSim.actor.Executable, VisualSim.kernel.util.Instantiable, VisualSim.kernel.util.ModelErrorHandler, VisualSim.kernel.util.MoMLExportable, VisualSim.kernel.util.Moveable, VisualSim.kernel.util.Nameable, java.io.Serializable, VisualSim.actor.TypedActor

• Direct Known Subclasses:
• TrellisDecoder

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• public class ViterbiDecoder
• extends VisualSim.actor.lib.Transformer

The Viterbi algorithm is an optimal way to decode convolutional and trellis codes. The code is specified jointly by the uncodedRate and polynomialArray parameters. To get a k/n code, set uncodedRate to k and give n integers in polynomialArray. See ConvolutionalCoder for details about the meaning of these parameters. On each firing, this actor will read n inputs and produce k outputs.

The decoder finds the most likely data sequence given noisy inputs by searching all possibilities and computing the distance between the codewords they produce and the observed noisy data. The sequence yielding the minimum distance is the decoded output.

There are two choices offered in this actor to compute the distance. If it the parameter softDecoding is set to be false, the input port will accept boolean tokens and compute the Hamming distance. If the parameter softDecoding is set to be true, the input port will accept double tokens and compute the Euclidean distance. The parameter constellation should be a double array of length 2. The first element specifies the amplitude of "false" input. The second element specifies the amplitude of "true" input. At this time, this actor can only handle binary antipodal constellations, but we expect to generalize this.

Soft decoding has lower probability of decoding error than hard decoding. But distance computation for hard decoding is easier, since it is based on bit operations. Moreover, hard decoding can be used when there is no direct observation of the noisy data, but only observations of a bit sequence that may have errors in it. With hard decoding, this actor serves the role of correcting errors. With soft decoding, it serves the role of reducing the likelyhood of errors.

There is some delay between the reading of input data and the production of decoded output data. That delay, which is called the trace-back depth or truncation depth of the decoder, is controlled by the delay parameter, which is required to be a positive integer. On the first delay firings of this actor, the outputs will be false. On each firing, the number of outputs produced is uncodedRate, so the output will have a prefix of delay*uncodedRate false-valued tokens before any decoded bits are produced. Larger values of delay generally reduce the probability of error. A good rule of thumb is to set delay to five times the highest order of all polynomials, provided that the convolutional code is a one that has good distance properties.

For more information on convolutional codes and Viterbi decoder, see the ConvolutionalCoder actor and Proakis, Digital Communications, Fourth Edition, McGraw-Hill, 2001, pp. 471-477 and pp. 482-485, or Barry, Lee and Messerschmitt, Digital Communication, Third Edition, Kluwer, 2004.

• Since:
• VisualSim3.0
• See Also:
• Serialized Form

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polynomialArray

public VisualSim.data.expr.Parameter polynomialArray

• An array of integers defining polynomials with binary coefficients. The coefficients indicate the presence (1) or absence (0) of a tap in the shift register. Each element of this array parameter should be a positive integer. The default value is {05, 07}.

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uncodedRate

public VisualSim.data.expr.Parameter uncodedRate

• Integer defining the number of bits produced at the output in each firing. It should be a positive integer. Its default value is 1.

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delay

public VisualSim.data.expr.Parameter delay

• Integer defining the trace back depth of the viterbi decoder. It should be a positive integer. Its default value is the integer 10.

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softDecoding

public VisualSim.data.expr.Parameter softDecoding

• Boolean defining the decoding mode. If it is true, the decoder will do soft decoding, and the input data type will be double; otherwise it will do hard decoding, and the input data type will be boolean. The default value is false.

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constellation

public VisualSim.data.expr.Parameter constellation

• The constellation for soft decoding. Inputs are expected to be symbols from this constellation with added noise. This parameter should be a double array of length 2. The first element defines the amplitude of "false" input. The second element defines the amplitude of "true" input.

ViterbiDecoder

public ViterbiDecoder(VisualSim.kernel.CompositeEntity container,
                     java.lang.String name)
              throws VisualSim.kernel.util.NameDuplicationException,
                     VisualSim.kernel.util.IllegalActionException

• Construct an actor with the given container and name. The output and trigger ports are also constructed.
• Parameters:
• container - The container.
• name - The name of this actor.
• Throws:
• VisualSim.kernel.util.IllegalActionException - If the entity cannot be contained by the proposed container.
• VisualSim.kernel.util.NameDuplicationException - If the container already has an actor with this name.

attributeChanged

public void attributeChanged(VisualSim.kernel.util.Attribute attribute)
                     throws VisualSim.kernel.util.IllegalActionException

• If the attribute being changed is softDecoding or trellisDecoding, set input port and constellation type to be complex if trellisDecoding is true; else if softDecoding is true, set them to double type; otherwise set the input port to type boolean. If the attribute being changed is uncodedRate or delay then verify it is a positive integer; if it is polynomialArray, then verify that each of its elements is a positive integer.
• Throws:
• VisualSim.kernel.util.IllegalActionException - If uncodedRate, or delay is non-positive, or any element of polynomialArray is non-positive.

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fire

public void fire()
         throws VisualSim.kernel.util.IllegalActionException

• Read n inputs and produce k outputs, where n is the number of integers in polynomialArray and k is the value of the uncodedRate parameter. The outputs are a decoded bit sequence, with a prefix of false-valued tokens produced on the first delay firings. The number of leading false outputs, therefore, is delay*uncodedRate. To decode, the actor searches iteratively of all possible input sequence and find the one that has the minimum distance to the observed inputs.
• Throws:
• VisualSim.kernel.util.IllegalActionException

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initialize

public void initialize()
               throws VisualSim.kernel.util.IllegalActionException

• Initialize the actor.
• Throws:
• VisualSim.kernel.util.IllegalActionException - If the parent class throws it.

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postfire

public boolean postfire()
                throws VisualSim.kernel.util.IllegalActionException

• Record the datum in buffers into their temporary versions.
• Throws:
• VisualSim.kernel.util.IllegalActionException - If the base class throws it

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