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phased.MultipathChannel System object

Propagate signals in multipath channel

Description

The phased.MultipathChannel System object™ propagates a signal through a multipath channel. To run the object, you must provide characteristics for each path: time delay, gain, Doppler factor, reflection loss, and spreading loss.

For sonar applications, you can use the phased.IsoSpeedUnderwaterPaths System object to generate channel path characteristics. You can also supply these characteristics independently.

To model signal propagation through a multipath channel:

  1. Define and set up the propagator. You can set phased.MultipathChannel properties at construction time or leave them to their default values. See Construction. Some properties that you set at construction time can be changed later. These properties are tunable.

  2. To compute the propagated signal, call the step method of phased.MultipathChannel. The output of the step method depends on the properties of the phased.MultipathChannel System object. You can change tunable properties at any time.

Note

Instead of using the step method to perform the operation defined by the System object, you can call the object with arguments, as if it were a function. For example, y = step(obj,x) and y = obj(x) perform equivalent operations.

Construction

propagator = phased.MultipathChannel creates a signal propagator System object for a multipath underwater channel.

propagator = phased.MultipathChannel(Name,Value) creates a signal propagator System object with each specified property Name set to the specified Value. You can specify additional name and value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).

Properties

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Signal carrier frequency, specified as a positive real-valued scalar. Units are in Hz.

Example: 10000

Data Types: double

Signal sample rate, specified as a positive real-valued scalar. Units are in Hz. The System object uses this quantity to calculate the propagation delay in units of samples.

Example: 3e3

Data Types: double

Source of the maximum delay value, specified as 'Auto' or 'Property'. When you set this property to 'Auto', the channel automatically allocates enough memory to simulate the propagation delay. When you set this property to 'Property', you can specify the maximum delay by using the MaximumDelay property. Signals arriving after the maximum delay are ignored.

Maximum signal delay, specified as a positive scalar. Delays greater than this value are ignored. Units are in seconds.

Dependencies

To enable this property, set the MaximumDelaySource property to 'Property'.

Data Types: double

Interpolation method used to implement signal fractional delay and doppler time-dilation and compression, specified as 'Linear' or 'Oversample'. When this property is set to 'Linear', the input signal is linearly interpolated directly onto a uniform grid to propagate the signal. When this property is set to 'Oversample', the input signal is resampled to a higher rate before linear interpolation. For broadband signals, oversampling preserves spectral shape.

Data Types: char

Methods

resetReset state of System object
stepPropagate signal through multipath sound channel
Common to All System Objects
clone

Create System object with same property values

getNumInputs

Expected number of inputs to a System object

getNumOutputs

Expected number of outputs of a System object

isLocked

Check locked states of a System object (logical)

release

Allow System object property value changes

Examples

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Create a five-path underwater sound channel and compute the propagation path matrix, the Doppler factor, and the absorption loss. Assume that the source is stationary and the receiver is moving along the x-axis toward the source at 20 km/h. Assume the default one-way propagation.

Create the channel and specify the source and receiver locations and velocities.

numpaths = 5;
channel = phased.IsoSpeedUnderwaterPaths('ChannelDepth',200,'BottomLoss',10, ...
    'NumPathsSource','Property','NumPaths',numpaths);
tstep = 1;
srcpos = [0;0;-160];
rcvpos = [100;0;-50];
speed = -20*1000/3600;
srcvel = [0;0;0];
rcvvel = [speed;0;0];

Compute the path matrix, Doppler factor, and losses.

[pathmat,dop,absloss] = channel(srcpos,rcvpos,srcvel,rcvvel,tstep);

Create 500 samples of a 100 Hz signal. Assume all the paths have the same signal. Propagate the signals to the receiver.

fs = 1e3;
nsamp = 500;
propagator = phased.MultipathChannel('OperatingFrequency',10e3,'SampleRate',fs);
t = [0:(nsamp-1)]'/fs;
sig0 = sin(2*pi*100*t);
sig = repmat(sig0,1,numpaths);
propsig = propagator(sig,pathmat,dop,absloss);

Plot the real part of the coherent sum of the propagated signals.

plot(t*1000,real(sum(propsig,2)))
xlabel('Time (millisec)')

Create a seven-path underwater sound channel and display the propagation path matrix. Assume that the source is stationary and that the receiver is moving along the x-axis toward the source at 20 km/h. Assume two-way propagation.

speed = -20*1000/3600;
numpaths = 7;
csound = 1515.0;
channel = phased.IsoSpeedUnderwaterPaths('ChannelDepth',200, ...
    'PropagationSpeed',csound,'BottomLoss',10,'NumPathsSource','Property', ...
    'NumPaths',numpaths,'TwoWayPropagation',true);
tstep = 1;
srcpos = [0;0;-160];
tgtpos = [500;0;-50];
srcvel = [0;0;0];
tgtvel = [speed;0;0];

Obtain the path matrix, Doppler factor, loss, and target reflection and transmit angles.

[pathmat,dop,aloss,tgtangs,srcangs] = channel(srcpos,tgtpos,srcvel,tgtvel,tstep);

Create a 100 Hz signal with 500 samples. Assume that all the paths have the same signal but with different amplitudes. Then, propagate the signals to the target and back. You can use the angle information to calculate any angular dependence of the source and target responses. Each channel can have a different amplitude. This example uses a simple cosine model.

fs = 1e3;
nsamp = 500;
propagator = phased.MultipathChannel('OperatingFrequency',10e3,'SampleRate',fs);
t = [0:(nsamp-1)]'/fs;
ampsrc = cosd(srcangs(2,:));
amptgt = cosd(tgtangs(2,:));
sig0 = sin(2*pi*100*t);
sig = repmat(sig0,1,numpaths);
amptotal = ampsrc.^2.*amptgt;
sig = bsxfun(@times,amptotal,sig);

Because of the finite propagation delay, the first call to the propagator does not return the signal. Call propagator twice to obtain the returned signal.

propsig = propagator(sig,pathmat,dop,aloss);
propsig = propagator(sig,pathmat,dop,aloss);

Plot the real part of the coherent sum of the propagated signals. Compute the round trip time.

rng = rangeangle(srcpos,tgtpos);
tr = rng/csound;
plot((t+tr)*1000,real(sum(propsig,2)))
xlabel('Time (millisec)')

Create an underwater sound channel and plot the combined received signal. Automatically find the number of paths. Assume that the source is stationary and that the receiver is moving along the x-axis toward the source at 20 km/h. Assume the default one-way propagation.

speed = -20*1000/3600;
channel = phased.IsoSpeedUnderwaterPaths('ChannelDepth',200,'BottomLoss',5, ...
    'NumPathsSource','Auto','CoherenceTime',5);
tstep = 1;
srcpos = [0;0;-160];
rcvpos = [500;0;-50];
srcvel = [0;0;0];
rcvvel = [speed;0;0];

Compute the path matrix, Doppler factor, and losses. The propagator outputs 51 paths output but some paths can contain Nan values.

[pathmat,dop,absloss,rcvangs,srcangs] = channel(srcpos,rcvpos,srcvel,rcvvel,tstep);

Create of a 100 Hz signal with 500 samples. Assume that all the paths have the same signal. Use a phased.MultipathChannel System object to propagate the signals to the receiver. phased.MultipathChannel accepts as input all paths produced by phased.IsoSpeedUnderwaterPaths but ignores paths that have NaN values.

fs = 1e3;
nsamp = 500;
propagator = phased.MultipathChannel('OperatingFrequency',10e3,'SampleRate',fs);
t = [0:(nsamp-1)]'/fs;
sig0 = sin(2*pi*100*t);
numpaths = size(pathmat,2);
sig = repmat(sig0,1,numpaths);
propsig = propagator(sig,pathmat,dop,absloss);

Plot the real part of the coherent sum of the propagated signals.

plot(t*1000,real(sum(propsig,2)))
xlabel('Time (millisec)')

Compare the lengths of a propagated signal when the sonar is stationary differs compared to when the sonar is moving. In the second case, the sonar has a radial velocity of 25 m/s moving away from the target. Propagate the signal along a single path.

Define the system parameters: maximum unambiguous range, required range resolution, operating frequency, and propagation speed.

maxrange = 5000.0;
rngres = 10.0;
fc = 20.0e3;
csound = 1520.0;

Use a rectangular waveform for the transmitted signal.

prf = csound/(2*maxrange);
pulseWidth = 8*rngres/csound;
pulseBW = 1/pulseWidth;
fs = 80*pulseBW;
waveform = phased.RectangularWaveform('PulseWidth',pulseWidth,'PRF',prf, ...
    'SampleRate',fs);

Specify the sonar positions.

sonarplatform1 = phased.Platform('InitialPosition',[0;0;-60],'Velocity',[0;0;0]);
sonarplatform2 = phased.Platform('InitialPosition',[0;0;-60],'Velocity',[0;-25;0]);

Specify the target position.

targetplatform = phased.Platform('InitialPosition',[0;500;-60],'Velocity',[0;0;0]);

Define the underwater path and propagation channel objects. Assume one-way propagation.

paths = phased.IsoSpeedUnderwaterPaths('ChannelDepth',100, ...
    'CoherenceTime',0,'NumPathsSource','Property','NumPaths',1, ...
    'PropagationSpeed',csound);
propagator = phased.MultipathChannel('SampleRate',fs,'OperatingFrequency',fc);

Transmit a rectangular waveform.

wav = waveform();
nsamp = size(wav,1);
rxpulses = zeros(nsamp,2);
t = (0:nsamp-1)/fs;

Transmit and receive at the stationary sonar.

[pathmat,dop,aloss,tgtang,srcang] = paths(sonarplatform1.InitialPosition, ...
    targetplatform.InitialPosition,sonarplatform1.InitialVelocity, ...
    targetplatform.InitialVelocity,1/prf);
rxpulses(:,1) = propagator(wav,pathmat,dop,aloss);

Transmit and receive at the moving sonar.

[pathmat,dop,aloss,tgtang,srcang] = paths(sonarplatform2.InitialPosition, ...
    targetplatform.InitialPosition,sonarplatform2.Velocity, ...
    targetplatform.Velocity,1/prf);
rxpulses(:,2) = propagator(wav,pathmat,dop,aloss);

Plot the received pulses.

plot(abs(rxpulses))
xlim([490 650])
ylim([0 1.65e-3])
legend('Stationary sonar','Moving sonar')
xlabel('Received Sample Time (sec)')
ylabel('Integrated Received Pulses')

The signal received at the moving sonar has increased in duration compared to the stationary sonar.

References

[1] Urick, R.J. Principles of Underwater Sound, 3rd Edition. New York: Peninsula Publishing, 1996.

[2] Sherman, C.S. and J.Butler Transducers and Arrays for Underwater Sound. New York: Springer, 2007.

[3] Allen, J.B. and D. Berkely, “Image method for efficiently simulating small-room acoustics”, J. Acoust. Soc. Am, Vol 65, No. 4. April 1979.

Extended Capabilities

Introduced in R2017a

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