LMSAdaptive

VisualSim.simulators.sdf.lib
Class LMSAdaptive

• public class LMSAdaptive
• extends FIR

An adaptive filter using the Least-Mean Square (LMS) algorithm, also known as the stochastic gradient algorithm. The initial filter coefficients are given by the initialTaps parameter. The tap values can be observed on the tapValues output. The default initial taps initialTaps are {1, 0, 0, 0}. This actor supports decimation, but not interpolation.

When used correctly, this LMS adaptive filter will adapt to try to minimize the mean-squared error of the signal at its error input. In order for this to be possible, the output of the filter should be compared (subtracted from) some reference signal to produce an error signal. That error signal should be fed back to the error input.

The stepSize parameter determines the rate of adaptation. If its magnitude is too large, or if it has the wrong sign, then the adaptation algorithm will be unstable.

The errorDelay parameter must equal the total number of delays in the path from the output of the filter back to the error input. This ensures correct alignment of the adaptation algorithm. The number of delays must be greater than zero.

This actor is type polymorphic, supporting any data type that supports multiplication by a scalar (the stepSize) and addition.

The algorithm is simple. Prior to each invocation of the parent class (an FIR filter), which computes the output given the input, this actor updates the coefficients according to the following formula,

  newTapValue = oldTapValue + error * stepSize * tapData
 

where tapData is the contents of the delay line at the tap in question. This assumes that the decimation parameter is set to 1 (the default). If it has a value different from 1, the algorithm is slightly more involved. Similarly, this assumes that the errorDelay is 1.

• Since:
• VisualSim1.0

error

public VisualSim.actor.TypedIOPort error

• The error input port. The type of this port must match that of the input port.

errorDelay

public VisualSim.data.expr.Parameter errorDelay

• The number of samples of delay in the feedback loop that brings the error back. This has a type integer, and defaults to 1.

initialTaps

public VisualSim.data.expr.Parameter initialTaps

• The initial taps of the filter. This has a type of ArrayToken. By default, it contains the array {1.0, 0.0, 0.0, 0.0}, meaning that the output of the filter is initially the same as the input, and that the adaptive filter has four taps.

stepSize

public VisualSim.data.expr.Parameter stepSize

• The adaptation step size. This must have a type that can be multiplied by the input. It defaults to 0.01, a double.

tapValues

public VisualSim.actor.TypedIOPort tapValues

• The output of tap values. This has the same type as the initialTaps.

decimation

public VisualSim.data.expr.Parameter decimation

• Decimation is to reduce the sampling rate of the signal.The decimation ratio of the filter. This must contain an IntToken, and by default it has value one. No negative or Zero values.
  

decimationPhase

public VisualSim.data.expr.Parameter decimationPhase

• The decimation phase of the filter. This must contain an IntToken, and by default it has value zero.

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