Reflected signal from moving bicyclist
returns the total reflected signal,
Y = reflect(
Y, from a bicyclist. The total
reflected signal is the sum of all reflected signals from the bicyclist scatterers.
X represents the incident signals at each scatterer.
ang defines the directions of the incident and reflected signals with
respect to the each scatterers.
The reflected signal strength depends on the value of the radar cross-section at the incident angle. This simplified model uses the same value for all scatterers.
Compute the backscattered radar signal from a bicyclist moving along the x-axis at 5 m/s away from a radar. Assume that the radar is located at the origin. The radar transmits an LFM signal at 24 GHz with a 300-MHz bandwidth. A signal is reflected at the moment the bicyclist starts to move and then one second later.
Initialize Bicyclist, Waveform, and Propagation Channel Objects
phased.FreeSpace objects. Assume a 300 MHz sampling frequency. The initial position of the bicyclist lies on the x-axis 30 meters from the radar.
bw = 300e6; fs = bw; fc = 24e9; radarpos = [0;0;0]; bpos = [30;0;0]; bicyclist = backscatterBicyclist( ... 'OperatingFrequency',fc,'NumWheelSpokes',15, ... 'InitialPosition',bpos,'Speed',5.0, ... 'InitialHeading',0.0); lfmwav = phased.LinearFMWaveform( ... 'SampleRate',fs, ... 'SweepBandwidth',bw); sig = lfmwav(); chan = phased.FreeSpace(... 'OperatingFrequency',fc,... 'SampleRate',fs,... 'TwoWayPropagation',true);
Plot Initial Bicyclist Position
move object function, obtain the initial scatterer positions, velocities and the orientation of the bicyclist. Plot the initial position of the bicyclist. The
dt argument of the
move object function determines that the next call to
move returns the bicyclist state of motion
dt seconds later.
dt = 1.0; [bpos,bvel,bax] = move(bicyclist,dt,0); plot(bicyclist)
Obtain First Reflected Signal
Propagate the signal to all scatterers and obtain the cumulative reflected return signal.
N = getNumScatterers(bicyclist); sigtrns = chan(repmat(sig,1,N),radarpos,bpos,[0;0;0],bvel); [rngs,ang] = rangeangle(radarpos,bpos,bax); y0 = reflect(bicyclist,sigtrns,ang);
Plot Bicyclist Position After Position Update
After the bicyclist has moved, obtain the scatterer positions and velocities and then move the bicycle along its trajectory for another second.
[bpos,bvel,bax] = move(bicyclist,dt,0); plot(bicyclist)
Obtain Second Reflected Signal
Propagate the signal to all scatterers at their new positions and obtain the cumulative reflected return signal.
sigtrns = chan(repmat(sig,1,N),radarpos,bpos,[0;0;0],bvel); [~,ang] = rangeangle(radarpos,bpos,bax); y1 = reflect(bicyclist,sigtrns,ang);
Match Filter Reflected Signals
Match filter the reflected signals and plot them together.
mfsig = getMatchedFilter(lfmwav); nsamp = length(mfsig); mf = phased.MatchedFilter('Coefficients',mfsig); ymf = mf([y0 y1]); fdelay = (nsamp-1)/fs; t = (0:size(ymf,1)-1)/fs - fdelay; c = physconst('LightSpeed'); plot(c*t/2,mag2db(abs(ymf))) ylim([-200 -50]) xlabel('Range (m)') ylabel('Magnitude (dB)') ax = axis; axis([0,100,ax(3),ax(4)]) grid legend('First pulse','Second pulse')
Compute the difference in range between the maxima of the two pulses.
[maxy,idx] = max(abs(ymf)); dpeaks = t(1,idx(2)) - t(1,idx(1)); drng = c*dpeaks/2
drng = 4.9965
The range difference is 5 m, as expected given the bicyclist speed.
bicyclist— Bicyclist target
Bicyclist, specified as a
X— Incident radar signals
Incident radar signals on each bicyclist scatterer, specified as a complex-valued M-by-N matrix. M is the number of samples in the signal. N is the number of point scatterers on the bicyclist and is determined partly from the number of spokes in each wheel, Nws. See Bicycle Scatterer Indices for the column representing the incident signal at each scatterer.
The size of the first dimension of the input matrix can vary to simulate a changing signal length. A size change can occur, for example, in the case of a pulse waveform with variable pulse repetition frequency.
Complex Number Support: Yes
ang— Directions of incident signals
Directions of incident signals on the bicyclist scatterers, specified as a
real-valued 2-by-N matrix. N equals the number of
X. Each column of
Ang specifies the
incident direction of the signal to a scatterer taking the form of an azimuth-elevation
pair, [AzimuthAngle;ElevationAngle]. Units are in degrees. See
Bicycle Scatterer Indices for the column
representing the incident direction at each scatterer.
Bicyclist scatterer indices define which columns in the scatterer position or
velocity matrices contain the position and velocity data for a specific scatterer. For
example, column 92 of
bpos specifies the 3-D position of one of the
scatterers on a pedal.
The wheel scatterers are equally divided between the wheels. You can determine the total
number of wheel scatterers, N, by subtracting 113 from the output of the
getNumScatterers function. The number of scatterers per wheel is
Nsw = N/2.
Bicyclist Scatterer Indices
|Bicyclist Component||Bicyclist Scatterer Index|
|Frame and rider||1 … 90|
|Pedals||91 … 99|
|Rider legs||100 … 113|
|Front wheel||114 … 114 + Nsw - 1|
|Rear wheel||114 + Nsw … 114 + N - 1|
The value of the radar cross-section (RCS) of a scatterer
generally depends upon the incident angle of the reflected radiation. The
backscatterBicyclist object uses a simplified RCS model: the RCS pattern of
an individual scatterer equals the total bicyclist pattern divided by the number of scatterers.
The value of the RCS is computed from the RCS pattern evaluated at an average over all
scatterers of the azimuth and elevation incident angles. Therefore, the RCS value is the same
for all scatterers. You can specify the RCS pattern using the
property of the
backscatterBicyclist object or use the default value.