Block LMS Filter
Compute output, error, and weights using LMS adaptive algorithm
Library
Filtering / Adaptive Filters
dspadpt3
Description
The Block LMS Filter block implements an adaptive least meansquare (LMS) filter,
where the adaptation of filter weights occurs once for every block of samples. The block
estimates the filter weights, or coefficients, needed to minimize the error,
e(n), between the output signal,
y(n), and the desired signal,
d(n). Connect the signal you want to filter to
the Input port. The input signal can be a scalar or a column vector. Connect the signal
you want to model to the Desired
port. The desired signal must have
the same data type, complexity, and dimensions as the input signal. The
Output
port outputs the filtered input signal. The
Error
port outputs the result of subtracting the output signal
from the desired signal.
The block calculates the filter weights using the Block LMS adaptive filter algorithm. This algorithm is defined by the following equations.
$$\begin{array}{c}n=kN+i\\ y(n)={w}^{T}(k1)u(n)\\ e(n)=d(n)y(n)\\ w(k)=w(k1)+f(u(n),\text{}e(n),\mu )\end{array}$$
The weight update function for the Block LMS adaptive filter algorithm is defined as
$$f(u(n),e(n),\mu )=\mu {\displaystyle \sum _{i=0}^{N1}{u}^{\ast}(kN+i)e(kN+i)}$$
The variables are as follows.
Variable  Description 

n  The current time index 
i  The iteration variable in each block, $$0\le i\le N1$$ 
k  The block number 
N  The block size 
u(n)  The vector of buffered input samples at step n 
w(n)  The vector of filtertap estimates at step n 
y(n)  The filtered output at step n 
e(n)  The estimation error at time n 
d(n)  The desired response at time n 
μ  The adaptation step size 
Use the Filter length parameter to specify the length of the filter weights vector.
The Block size parameter determines how many samples of the input signal are acquired before the filter weights are updated. The number of rows in the input must be an integer multiple of the Block size parameter.
The adaptation Stepsize (mu) parameter corresponds to µ in the equations. You can either specify a stepsize using the input port, Stepsize, or enter a value in the Block Parameters: Block LMS Filter dialog box.
Use the Leakage factor (0 to 1) parameter to specify the leakage factor, $$0<1\mu \alpha \le 1$$, in the leaky LMS algorithm shown below.
$$w(k)=(1\mu \alpha )w(k1)+f(u(n),e(n),\mu )$$
Enter the initial filter weights as a vector or a scalar in the Initial value of filter weights text box. When you enter a scalar, the block uses the scalar value to create a vector of filter weights. This vector has length equal to the filter length and all of its values are equal to the scalar value
When you select the Adapt port check box, an
Adapt
port appears on the block. When the input to this port is
greater than zero, the block continuously updates the filter weights. When the input to
this port is zero, the filter weights remain at their current values.
When you want to reset the value of the filter weights to their initial values, use the Reset input parameter. The block resets the filter weights whenever a reset event is detected at the Reset port. The reset signal rate must be the same rate as the data signal input.
From the Reset input list, select
None
to disable the Reset port. To enable the Reset port,
select one of the following from the Reset input list:
Rising edge
— Triggers a reset operation when the Reset input does one of the following:Rises from a negative value to a positive value or zero
Rises from zero to a positive value, where the rise is not a continuation of a rise from a negative value to zero (see the following figure).
Falling edge
— Triggers a reset operation when the Reset input does one of the following:Falls from a positive value to a negative value or zero
Falls from zero to a negative value, where the fall is not a continuation of a fall from a positive value to zero (see the following figure)
Either edge
— Triggers a reset operation when the Reset input is aRising edge
orFalling edge
(as described above)Nonzero sample
— Triggers a reset operation at each sample time that the Reset input is not zero
Select the Output filter weights check box to create a
Wts
port on the block. For each iteration, the block outputs the
current updated filter weights from this port.
Parameters
 Filter length
Enter the length of the FIR filter weights vector.
 Block size
Enter the number of samples to acquire before the filter weights are updated. The number of rows in the input must be an integer multiple of the Block size.
 Specify stepsize via
Select
Dialog
to enter a value for mu in the Block parameters: LMS Filter dialog box. SelectInput port
to specify mu using the Stepsize input port. Stepsize (mu)
Enter the stepsize. Tunable (Simulink).
 Leakage factor (0 to 1)
Enter the leakage factor, $$0<1\mu \alpha \le 1$$. Tunable (Simulink).
 Initial value of filter weights
Specify the initial values of the FIR filter weights.
 Adapt port
Select this check box to enable the Adapt input port.
 Reset port
Select this check box to enable the Reset input port.
 Output filter weights
Select this check box to export the filter weights from the Wts port.
References
Hayes, M. H. Statistical Digital Signal Processing and Modeling. New York: John Wiley & Sons, 1996.
Supported Data Types
Port  Supported Data Types 

Input 

Desired 

Stepsize 

Adapt 

Reset 

Output 

Error 

Wts 

See Also
Fast Block LMS Filter  DSP System Toolbox 
Kalman Adaptive Filter (Obsolete)  DSP System Toolbox 
LMS Filter  DSP System Toolbox 
RLS Filter  DSP System Toolbox 
See Noise Cancellation in Simulink Using Normalized LMS Adaptive Filter for related information.