Documentation

phased.TimeDelayLCMVBeamformer System object

Package: phased

Time delay LCMV beamformer

Description

The TimeDelayLCMVBeamformer object implements a time delay linear constraint minimum variance beamformer.

The BeamscanEstimator object calculates a beamscan spatial spectrum estimate for a uniform linear array.

To compute the beamformed signal:

  1. Define and set up your time delay LCMV beamformer. See Construction.

  2. Call step to perform the beamforming operation according to the properties of phased.TimeDelayLCMVBeamformer. The behavior of step is specific to each object in the toolbox.

Construction

H = phased.TimeDelayLCMVBeamformer creates a time delay linear constraint minimum variance (LCMV) beamformer System object™, H. The object performs time delay LCMV beamforming on the received signal.

H = phased.TimeDelayLCMVBeamformer(Name,Value) creates a time delay LCMV beamformer object, H, with each specified property Name set to the specified Value. You can specify additional name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).

Properties

SensorArray

Handle to sensor array

Specify the sensor array as a handle. The sensor array must be an array object in the phased package. The array cannot contain subarrays.

Default: phased.ULA with default property values

PropagationSpeed

Signal propagation speed

Specify the propagation speed of the signal, in meters per second, as a positive scalar.

Default: Speed of light

SampleRate

Signal sampling rate

Specify the signal sampling rate (in hertz) as a positive scalar.

Default: 1e6

FilterLength

FIR filter length

Specify the length of the FIR filter behind each sensor element in the array as a positive integer.

Default: 2

Constraint

Constraint matrix

Specify the constraint matrix used for time delay LCMV beamformer as an M-by-K matrix. Each column of the matrix is a constraint and M is the degrees of freedom of the beamformer. For a time delay LCMV beamformer, M is given by the product of the number of elements of the array and the value of the FilterLength property.

Default: [1; 1]

DesiredResponse

Desired response vector

Specify the desired response used for time delay LCMV beamformer as a column vector of length K, where K is the number of constraints in the Constraint property. Each element in the vector defines the desired response of the constraint specified in the corresponding column of the Constraint property.

Default: 1, which is equivalent to a distortionless response

DiagonalLoadingFactor

Diagonal loading factor

Specify the diagonal loading factor as a positive scalar. Diagonal loading is a technique used to achieve robust beamforming performance, especially when the sample support is small. This property is tunable.

Default: 0

TrainingInputPort

Add input to specify training data

To specify additional training data, set this property to true and use the corresponding input argument when you invoke step. To use the input signal as the training data, set this property to false.

Default: false

DirectionSource

Source of beamforming direction

Specify whether the beamforming direction comes from the Direction property of this object or from an input argument in step. Values of this property are:

'Property'The Direction property of this object specifies the beamforming direction.
'Input port'An input argument in each invocation of step specifies the beamforming direction.

Default: 'Property'

Direction

Beamforming direction

Specify the beamforming direction of the beamformer as a column vector of length 2. The direction is specified in the format of [AzimuthAngle; ElevationAngle] (in degrees). The azimuth angle should be between –180 and 180. The elevation angle should be between –90 and 90. This property applies when you set the DirectionSource property to 'Property'.

Default: [0; 0]

WeightsOutputPort

Output beamforming weights

To obtain the weights used in the beamformer, set this property to true and use the corresponding output argument when invoking step. If you do not want to obtain the weights, set this property to false.

Default: false

Methods

cloneCreate time delay LCMV beamformer object with same property values
getNumInputsNumber of expected inputs to step method
getNumOutputsNumber of outputs from step method
isLockedLocked status for input attributes and nontunable properties
releaseAllow property value and input characteristics changes
stepPerform time delay LCMV beamforming

Examples

Apply a time delay LCMV beamformer to an 11-element array. The incident angle of the signal is –50 degrees in azimuth and 30 degrees in elevation.

% Signal simulation
ha = phased.ULA('NumElements',11,'ElementSpacing',0.04);
ha.Element.FrequencyRange = [20 20000];
fs = 8e3; t = 0:1/fs:0.3;
x = chirp(t,0,1,500);
c = 340; % Wave propagation speed (m/s)
hc = phased.WidebandCollector('Sensor',ha,...
    'PropagationSpeed',c,'SampleRate',fs,'ModulatedInput',false);
incidentAngle = [-50; 30];
x = step(hc,x.',incidentAngle);
noise = 0.2*randn(size(x));
rx = x+noise;

% Beamforming
ha = phased.ULA('NumElements',11,'ElementSpacing',0.04);
hbf = phased.TimeDelayLCMVBeamformer('SensorArray',ha,...
    'PropagationSpeed',c,'SampleRate',fs,'FilterLength',5,...
    'Direction',incidentAngle);
hbf.Constraint = kron(eye(5),ones(11,1));
hbf.DesiredResponse = eye(5, 1);
y = step(hbf,rx);

% Plot
plot(t,rx(:,6),'r:',t,y);
xlabel('Time')
ylabel('Amplitude')
legend('Original','Beamformed');

Algorithms

The beamforming algorithm is the time-domain counterpart of the narrowband linear constraint minimum variance (LCMV) beamformer. The algorithm does the following:

  1. Steers the array to the beamforming direction.

  2. Applies an FIR filter to the output of each sensor to achieve the specified constraints. The filter is specific to each sensor.

References

[1] Frost, O. "An Algorithm For Linearly Constrained Adaptive Array Processing", Proceedings of the IEEE. Vol. 60, Number 8, August, 1972, pp. 926–935.

[2] Van Trees, H. Optimum Array Processing. New York: Wiley-Interscience, 2002.

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