Comparison of two Through the Wall Localization
Techniques with UWB Radar System
Xiao wei Zhao1 , Bruno Lescalier1 , Pierre Combeau2 , Omar Benahmed Daho1
Alain Gaugue1 , Jamal Khamlichi1 , Michel Ménard1
1
Laboratoire Informatique, Image et Interaction - (L3I), University of La Rochelle
Avenue Michel Crépeau 17042, La Rochelle Cedex 1 France
2
Xlim-SIC (Sinal Image Communication), UMR CNRS 6172
SP2MI, Bd Marie et Pierre Curie, BP 30179, 86962 FUTUROSCOPE CHASSENEUIL Cedex - France
1
[email protected]
2
[email protected]
Abstract—A comparison of two different through the wall
localization techniques with UWB radar system are presented
in this paper. The first technique is the trilateration technique.
This technique gives us a good resolution of localization, but
it has some difficulties for detecting several targets at the same
time. The second technique is the backprojection technique. This
technique gives us a good visual image, but the processing course
is complicated. Some simulation results and experiment results
of these two techniques will be discussed.
I. I NTRODUCTION
Detection and location system has developped for decades.
There are two types of system: active system and passive
system. The active system runs in centimeter and millimeter
wavelengths and even in X-ray, and the passive system runs
in millimeter and submillimeter wavelengths. For through
the wall detection, the passive system doesn’t always work
because its sensors are not sensitve enough. Some active
systems work well for through the wall detection, but some
of them have certain disadvantages, like resolution problems,
ionization problems, etc. Therefore a better through the wall
technology has been developped: ultra-wideband technology
(UWB).
UWB radar is one of the typical UWB devices, which is
widely used. UWB radar systems transmit signals across a
much wider frequency than conventional radar systems and
are usually very difficult to detect. The transmitted signal
is significant for its very low power spectrum, which is
lower than the allowed unintentional radiated emissions for
electronics. The most common technique for generating a
UWB signal is to emit pulses with very short durations (less
than 1 nanosecond). Through-wall detection is one of the
best applications of UWB radar, because the low frequency
has a good capacity to penetrate different kinds of materials,
by contrast, the high frequency has a good spatial resolution
but with low penetration capacity. According to the report
about utilization of frequency for through-wall systems by
the Federal Communication Commission (FCC), the frequency
operated is limited to below 960MHz or from 1.99GHz to
10.6GHz [1].
There are numerous localization techniques with throughwall sensing (TWS) UWB radar. These techniques can be
divided into roughly two cateories: methods based on antenna
processing and methods not based on antenna processing.
The first type of methods consist of both parametric and
non-parametric methods. The non-parametric method is an
application with the beams’ formation for localization ( for
example, CAPON method [2]). The parametric method uses
the signal modelling, but it is less robust than the model error
(for example, maximum likelihood method [3] or sub-space
method [4]).
The second type of methods can be divided into
the monopulse method and the noncoherent method. The
monopulse method has the possibility of estimating the target’s
angular position with a simple antenna configuration. The
noncoherent method is based on the envelope of the signals
and does not include phase information.
In this paper, we have chosen the noncoherent method based
on trilateration technique [5] and backprojection technique
[6][7] for detection and localization of a target in the presence
of a known uniform wall.
II. D ETECTION AND LOCALIZATION TECHNIQUES
A. Description of detection and localization problem
In our study, we search for some techniques for detecting
and locating targets behind a uniform wall. The detection
system used is a multistatic radar system based on UWB
technique. This system is established by one transmitting
antenna, fixed at (0cm, 0cm) and three receiving antennas,
fixed at (40cm, 0cm), (-40cm, 0cm) and (-80cm, 0cm). The
whole system is fixed close to a wall to detect and locate the
targets behind the wall. A copper oval cylinder with a major
axe of 16cm, a minor axe of 11cm, and a height of 180cm was
used as target 1. A rectangle cylinder with a long of 10cm,
a large of 5cm, and a height of 155cm was used as target 2.
The scene configuration is represented in Fig.1.
the target. In order to calculate the estimated propagation
distance for each sub-trajectory, we first have to know the
incidence angle and refraction angle, but since we don’t know
the target’s position, these two incidence angles cannot be
known. Obviously, the analytical method doesn’t work here, so
we propose the use of a digital method (convergence method).
There are different kinds of convergence methods, such as the
Newton Method which is a method for successively finding
better approximations to the zeroes (or roots) of a real-valued
function. We have chosen a convergence method ”Brent’s
method” [9] to achieve this, and to locate the target with
minimum error. This is a root-finding algorithm combining
the bisection method, the secant method and inverse quadratic
interpolation. It has the reliability of bisection but it can be as
quick as some of the less reliable methods. To find the target’s
position, we want to find a quantity:
When we detect and locate a target behind a wall, the
most difficult problem is that the existence of wall which
causes the change of signal propagation direction, so the signal
propagation time will also be raised. But in a real situation, we
don’t know the target’s position, nor the wall’s thickness, nor
the wall’s dielectric constant, so it is so difficult to calculate
the signal propagation time. Without this important factor, we
are unable to estimate the target’s position.
According to this problem and the configuration of the
detection scene, we simulated the scene Fig.1 with software
called RAdio Propagation SimulatOR (RAPSOR) [8] which
allows prediction of the multipath phenomena and their electromagnetic characteristics. This simulator is divided into four
main parts: inputs, outputs, the electromagnetic modelisation
based on Geometrical Optics (GO) laws extented to the
Uniform Theory of Diffraction (TUD) and the ray-tracing
algorithm for path determination. The simulation results will
be discussed in the final paper. These simuations will allow the
evaluation and comparison of the two localization techniques.
B. Trilateration technique
Trilateration technique is a method for determining the zone
or the point of intersection of N spheres (N ≥ 3) by giving
the center coordinates of these spheres [5].
In real conditions, during the propagation of signals, especially when the waves pass from one medium to another
(with different conductivity and permittivity), some physical
phenomena occur, such as reflection, refraction, diffraction and
change of propagation velocity. In our scenario, characteristics
of a wall (dielectric constant and thickness) are the main
factors that affect the propagation of signals (attenuation and
incidence angles, etc).
During the propagation trajectory, the signal passes through
three layers of medium: air-wall-air, so the trajectory is a
non-line-of-sight (NLOS) propagation between the radar and
r
(d0 −(lk10 +lk20
2
+lk30 +lk1j +lk2j
1
r
2
+lk3j ))2
1
j=1
(1)
and let q be a minimum. Where d0 is the signal propagation
distance from the transmitting antenna to the target and back
to jth receiving antenna; index of each trajectory l: k-target’s
different position, 1, 2, 3-subtrajectory number, j-receive antenna’s number. This trilateration technique is more detailed
in [10].
q=
Fig. 1. Measured scene (The thickness of the wall is 7cm and the distance
between radar and wall is 3cm)
∞
X
C. Backprojection technique
The standard backprojection technique [6] is based on the
well known time domain imaging method:Kirchhoff migration
[11]. It can be presented by an equation:
Ut (x, y) =
N
d0 + dn
1 X
Rn (
)
N n=1
v
(2)
where Ut (x, y) presents the vision scene in time domain,
Rn refers to the measured impulse responses at the Nth
receiver, and v is the propagation velocity of signal, d0 the
distance between the emitter and the target, and dn the distance
between the Nth receiver and the target.
This standard backprojection can present a vision scene with
all information reflected by the scatter target. In order to get
the result, we begin with dividing the whole vision scene into
pixels, then mesure the signal propagation time delay from
the emitter to the target and back to each receiver for each
pixel, note the amplitude of the received signal corresponding
to each pixel, and add them. With these processing steps, we
can get a complete information vision scene. But the effect of
the standard backprojection technique doesn’t work well (cf.
Fig.2), because it produces imprecise images. Next to the peak
of the target, there are some artifacts which are produced by
the summation. These artifacts decrease the spatial resolution
and provoke side lobe in the vision scene image.
So we brought in cross correlated backprojection technique
[6], it can be expressed by:
Ut (x, y) =
N
1 X
d0 + dn
d0 + dref
Rn (
)Rref (
)
N n=1
v
v
(3)
where Rref is the impulse response of the reference receiver
and dref is the distance between the reference receiver and
the target. It can clear the blurry image, but there are still
ambiguous points (cf. Fig.3).
Then the modified cross correlated backprojection technique
[6] was used in order to remove all the ambiguous intersection
points and reduce the noise level. This technique is expressed
by the following equation:
Ut (x, y) =
N
d0 + dn
d0 + dref 1
1 X
Rn (
) ∗ Rref 1 (
)∗
N n=1
v
v
(4)
d0 + dref 2
Rref 2 (
)
v
where the Rref 1 is the first impulse response of the reference receiver, and Rref 2 is the second impulse response of
the reference receiver.
Some results of simulation with these different backprojection techniques are shown (cf. Fig.2 - Fig.4), we can find the
difference effect among them. Obviously, the modified cross
correlated backprojection technique gives the better results.
Fig. 3. Simulation result (Target 1 only) by cross correlated backprojection
method (Upper: 2D; Bottom: 3D)
Fig. 2. Simulation result (Target 1 only) by standard backprojection method
(Upper: 2D; Bottom: 3D)
Fig. 4.
Simulation result (Target 1 only) by modified cross correlated
backprojection method (Upper: 2D; Bottom: 3D)
D. Detection and localization of multi-targets
1) With trilateration technique: to estimate the target’s
position, we have to know the signal propagation time delay
between the radar and the target. If we have two targets to
detect at the same time, every received signal by each receive
antenna has two targets echoes from the two targets, but we
can not know which echo corresponds to which target, so the
signal propagation time delay from the radar to each target
can not be chosen correctly. And if the number of targets
increases, the estimation process will be more complicated. So
the trilateration method works for only one target detection.
2) Wiht modified cross correlated backprojection: because
the modified cross correlated backprojection technique estimates the target’s position by reprensenting the vision scene
pixel by pixel, there is no problem of calculating the signal
propagation time delay between the radar and the target. We
can use this method to estimate 2 targets or more at the same
time. The simulation result is showed in Figure 5.
oscilloscope (Agilent 54855A, with a bandwidth of 6GHz)
used as the receiving part. The experiment was done in a
large hall, without parasite reflexion. A plaster wall with a
dielectric constant of 2.5 and a thickness of 7.4cm was used
as an obstacle between the radar and target. The experiment
scene is described in Figure 1. The results will be detailed in
the final paper.
IV. C ONCLUSION
The trilateration and the backprojection technique, both
give good results, but these techniques have their advantages
and disadvantages. The trilateration technique gives a good
resolution of localization, its signal processing time is not too
long, it is useful for tracing a target’s moving track, but we
can not use this method for detecting and locating two or
more targets at the same time. On the contrary, backprojection
can be used for multi-targets detection and localization, it
gives us a better complet visualization scene, but its signal
processing time is much longer than trilateration technique,
because it uses all the information of the signal, and it can
not give an enough good resolution. Concerning the precision
of localisation, these two methods have almost the same level.
R EFERENCES
Fig. 5. Simulation result (with the two targets) by modified cross correlated
backprojection method (Upper: 2D; Bottom: 3D)
III. E XPERIMENTS AND RESULTS
Our radar system is a multistatic radar system. This radar
system involves one transmitter and several receivers which
are separated by a considered distance to estimate the moving
target position (cf. Fig.1). In our system, a pulse module is
used as the transmitter, and an omnidirectional antenna is
mounted on its output. Its center emitting frequency is 4.7GHz
and its bandwidth is 3.2GHz. The pulse repetition frequency
is 600kHz. Three directional antennas (gain 7dB and field of
view +/- 45 [email protected]) are mounted on the inputs of an
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