See-Through-Wall Imaging Using Ultra Wideband
Short-Pulse Radar System
Yunqiang Yang, Aly E. Fathy
The ECE Department, University of Tennessee
Knoxville, TN 37996
Abstract: See-Through-Wall imaging radar is a unique application of Ultra Wideband
communication that can provide soldiers and law enforcement officers an enhanced
situation awareness. We have developed an Ultra-wideband high-resolution short pulse
imaging radar system operating around 10 GHz, where two essential considerations were
addressed, the effect of penetrating the walls, and the pulse fidelity through the UWB
components and antennas of the radar. Modeled and measured wall parameters, and the
effect of antenna types on signal fidelity will be discussed here in details.
Background
Ultra Wideband technology has been the subject of extensive research in recent years due
to its potential applications and unique capabilities. Meanwhile, there is a lot of interest
and attention on See-Through-Wall application because of current homeland security
issues, where extensive work has been performed in the field of short-pulse radar [1, 2].
As the primary advantages of UWB for short-range radar imaging include extremely fine
range resolution (theoretically sub-centimeter resolution), high power efficiency (because
of low transmit duty cycle), low probability of detection, low interference to legacy
systems, and its ability to detect moving or stationary targets [3].
We have developed a radar prototype, which utilizes an instantaneous 3dB bandwidth of
600 MHz and a center frequency of 10 GHz. In this paper, however, we emphasize two
essential and special design considerations of the system. First, the electromagnetic wave
propagation through walls made of typical building materials, and the pulse fidelity.
Walls were examined both theoretically and experimentally and pulse fidelity was
investigated in time domain.
Evaluation of Wall Materials
See-Through-Wall radar requires the ability to detect targets through materials such as
concrete, bricks, dry wall, and plywood. Relatively high-density materials like concrete
and bricks can result in considerable attenuation of electromagnetic waves, increasing
requirements for both radar power and signal processing. Fig. 1 shows the RF attenuation
in different types of walls as measured by our team in comparison to Currie, et al [4]. The
lines show that most walls, such as dry wall, plywood, and bricks are fairly transparent to
radar frequencies, thus making through-the-wall imaging possible. If radar signal must
penetrate concrete block, a practical operational frequency is about 3 GHz, and the usable
frequency range is no greater than approximately 10 GHz, as is one of the reasons the
system operates at 10 GHz center frequency. The other reason is that, despite the fact that
attenuation through materials is greater, the antennas and components are smaller than at
lower frequency, resulting in a more compact system.
0-7803-8883-6/05/$20.00 @2005 IEEE
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We also measured the insertion transfer function through wall slabs. Fig.2 shows the
experimental configuration, where the transmit and receive antennas are positioned on
line of sight, and transmission coefficients were measured with and without the presence
of wall slabs between antennas, using 85 IOC Vector Network Analyzer. The insertion
transfer function is defined as the ratio of such two transmission coefficients. An
Advanced Design System (ADS) model was developed to simulate this experiment,
shown in Fig.3. In the ADS model, wall slabs and free space paths were represented by
transmission-lines. The parameters of transmission line, such as dielectric constant and
loss tangent are adjusted according to the wall materials, i.e. when simulation matches
measurement. Fig.4 plots the insertion transfer functions versus frequency for drywall
slabs with different thickness, where we can see that the measured results match the
simulation very well. The simple ADS model will help us to take the wall effect into
account.
See-Thru-Wall Radar Prototype
The UWB radar system consists of four basic blocks: antennas, transceiver, data
acquisition, and imaging post processing, where the radar sends out UWB short pulse
signal, and receives echo retumed from targets. I/Q channel information are extracted
from echo signal, then digitized and saved for image processing. Computers are used for
user interface, and application programs for imaging processing.
The developed system utilizes a 1-18 GHz UWB double-ridge hom antenna for
transmission, and a 16-element antipodal Vivaldi [5] sub-array for reception, the system
is shown in Fig. 5. While, the developed antenna is part of a 16x16 synthetic-aperture
receive antenna. The radar operates at a center frequency of 10 GHz and has an
instantaneous 10dB bandwidth of 1 GHz. Off-the-shelf UWB equipments and
components were purchased for the experimental investigation.
Pulse Fidelity through Radar System
The image of the target behind walls is recovered based on the efficient transmission and
reception of pulse signal. The distortion of pulse signal will translate into the distortion of
images of targets. Therefore it is necessary to understand how pulse distortion is
generated and how the pulse fidelity can be preserved in our system.
There are several factors, which could possibly cause pulse distortion, changing timedomain pulse shape in other words. The first cause could be due to the imperfect UWB
performance of off-the shelf components. The second cause can occur due to the transmit
and receive antennas, since the intensity of radiated/received electromagnetic field varies
proportionally with the derivative of the antenna current in transmit, and the integral of
the current in reception.
Experimental measurements were carried out to investigate the pulse distortion through
components and antennas. The output of the pulser is up-converted to 10GHz through a
mixer, and then connected to an UWB antenna through an UWB amplifier. Receive and
transmit antennas are positioned for line-of-sight free-space transmission. On the
receiving end, instead of I/Q demodulation, we just simply down-converted the
modulated pulse signal. The pulser output and recovered down-converted pulse are both
hooked up to a Tektronix sampling oscilloscope and compared. Different transmit/receive
antenna pair were used, and the effect of antenna on pulse distortion was examined. Fig.
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6 shows the shape of recovered pulse, compared to the original one, after transmitting
through components and antennas. The comparison indicates that the pulse fidelity is best
preserved when using two identical UWB antennas for transmission and reception. Use
of different antennas can cause slight distortion, unless both have very wide bands.
Conclusion
An ultra wideband short-range radar has been developed and is based on SAR principles.
The developed radar was used to evaluate various wall materials, and study pulse signal
fidelity through the system. It was concluded that at 3 GHz, brick walls can be
penetrated, and systems can be operable up to 10 GHz, other wall materials would allow
operation to much higher frequencies. Our system even though it utilizes Vivaldi antenna
for reception and UWB hom antenna for transmission does not distort the signal quality.
Acknowledgements
The authors would like to thank Dr. Mongi A. Abidi of University of Tennessee,
Knoxville for his support in this work.
Reference:
[I] J. Taylor, Ed. Introduction to Ultra-wideband Radar Systems. Boca Raton, FL: CRC,
1995.
[2] Ultra-Wideband, Short-Pulse Electromagnetics 1, 2, 3 and 4, New York: Plenum,
1993, 1994, 1997, and 1999
[3] 1.1. Immoreeve and D.V. Fedotov, "Ultra wideband radar systems: Advantages and
disadvantages", in Proc. IEEE Ultra Wideband Systems and Technologies Conf,
Baltimore, MD, May 2002, pp.201-205.
[4] N.C. Currie, D.D. Ferris, and al, "New law enforcement application of millimeter
wave radar", SPIE Vol. 3066, pp2-10, 1997
[5] E.Ehud Gazit, "Improved design of the Vivaldi antenna," Proc. Inst. Elect. Eng., pt. H,
vol. 135, no. 2, pp. 89-92, Apr. 1988.
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Figure 5: Transmitter and Receiver Diagram
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Figure 6: Original and Recovered Pulse after transmission. (Red lines on top is original
pulse waveform, and green lines on bottom are recovered)
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