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8/13/2019 Advanced Microwave Imaging 2012
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26 September/October 2012
Digital Object Identifier 10.1109/MMM.2012.2205772
1527-3342/12/$31.002012IEEE
Date of publication: 13 September 2012
Sherif Sayed Ahmed ([email protected]), Andreas Schiessl, and Frank Gumbmann are withRohde & Schwarz GmbH & Co. KG, Munich, Germany. Marc Tiebout is with Infineon Technologies, Villach, Austria.
Sebastian Methfessel and Lorenz-Peter Schmidt are with the University of Erlangen-Nuremberg.
Due to the enormous advances made in
semiconductor technology over the
last few years, high integration densi-
ties with moderate costs are achiev-
able even in the millimeter-wave
(mm-wave) range and beyond, which encourage the
development of imaging systems with a high number
of channels. The mm-wave range lies between 30 and
300 GHz, with corresponding wavelengths between
10 and 1 mm. While imaging objects with signals
of a few millimeters in wavelength, many optically
opaque objects appear transparent, making mm-wave
FOCU
SED
ISSU
EFEATUR
E
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Sherif Sayed Ahmed, Andreas Schiessl,Frank Gumbmann, Marc Tiebout, Sebastian Methfessel,
and Lorenz-Peter Schmidt
imaging attractive for a wide variety of commercial
and scientific applications like nondestructive testing
(NDT), material characterization, security scanning,
and medical screening. The spatial resolution in later-
al and range directions as well as the image dynamic
range offered by an imaging system are considered
the main measures of performance. With the avail-
ability of more channels combined with the powerful
digital signal processing (DSP) capabilities of modern
computers, the performance of mm-wave imaging sys-tems is advancing rapidly.
The most commonly known imaging systems are
based on X-ray technology, which are applied in,
e.g., computed tomography (CT) for medical diag-
nostics [1], NDT applications [2], and luggage inspec-
tion at security checkpoints. These systems work in
a transmission setup. Furthermore, backscatter X-ray
systems, which work in a reflection setup, were inves-
tigated over the last years, especially for the screening
of passengers for concealed objects at airports [3]. On
the one hand, X-ray images have an inherent high lat-
eral resolution due to the extremely short wavelength
(m~102 nm 10 nm). But on the other hand, the energy
of the photons is high enough to ionize organic and
inorganic matter. Therefore, health aspects are critical
with respect to imaging of humans, especially in the
case of personnel screening at airports.
Another well-known imaging technology is the
ultrasonic inspection of materials for NDT applica-
tions [4] and screening of humans for medical diag-
nostics [5]. Depending on the medium of propagation,
a lateral resolution even in the submillimeter region
is achievable. However, for most ultrasonic devices, anappropriate coupling medium is required for an effi-
cient coupling of the ultrasonic wave in the respective
device under test (DUT).
In contrast, electromagnetic mm-waves offer a
contactless inspection of materials with nonionizing
radiation and a high spatial resolution. Since spa-
tial resolution and penetration depth are conflicting
parameters regarding the wavelength, e.g., the E-band
(6090 GHz with m=5 to 3.3 mm) is a good compro-
mise for NDT applications to detect flaws, material
inhomogeneities, and inclusions in dielectrics. A lat-
eral resolution of ~2 mm is sufficient for many applica-tions, e.g., the personnel screening at airport security
checkpoints. Furthermore, it is possible to exploit the
vectorial nature of electromagnetic waves and to carry
out polarimetric measurements [6]. This offers the
potential of classification of different scattering pro-
cesses [7] and thus an improved detection of anoma-
lies in the DUT is possible.
The mm-wave images can be generated by either a
passive or an active imaging approach. Passive imag-
ing systems detect the characteristic radiation of an
object and the reflected background radiation with-out the need of illuminating the DUT with additional
electromagnetic energy. Thus a passive mm-wave
image contains the information of the emissivity and
reflectivity of an object in the respective frequency
domain [8], [9]. Especially for outdoor applications,
this technique offers a high radiometric contrast with
respect to the emissivity of the imaged object due
to the low background radiation temperature (Tsky)
of the sky, i.e., in mm-wave range the clear sky has
Tsky1100 K. However, passive imaging systems suf-
fer from low radiometric contrast in indoor applica-
tions due to the high background temperature of the
environment. This can be solved by applying cooled
detectors to achieve a high radiometric sensitivity
[10] or by using a noise source as an illuminator [11].
Another drawback is the lack of depth information
concerning the investigated DUT. This results from
the fact that the detected signals can be understood
as thermal noise and thus the radiation is incoher-
ent. On the contrary, active imaging systems illumi-
nate the DUT and the reflected or transmitted field
can be detected coherently or incoherently. For many
applications, active imaging is necessary to achievean image with high dynamic range and radiometric
contrast. Regarding a transmission setup, the attenu-
ation and absorption through a dielectric specimen
can be mapped, while for a reflection setup, the
object reflectivity can be characterized. In the case
of spatially smooth objects relative to the applied
wavelength, the scattering process is dominated by
specular reflections [12]. Therefore, the visibility of
the DUT and the image quality depends on an appro-
priate illumination of the specimen and a proper
positioning of the antennas.
By applying a coherent broadband transmit andreceive signal or, equivalently, a time delay measurement
8/13/2019 Advanced Microwave Imaging 2012
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in active imaging, it is additionally possible to reconstruct
the spatial extend of the DUT along the range direction.
With a sufficiently large signal bandwidth, it is further-
more possible to analyze multiple reflections resulting
from a stratified dielectric medium. This information
can be used for instance to investigate delamination for
NDT applications [13] or to identify explosive sheets or
other concealed objects for personnel screening applica-
tions [14], [15]. The last named application requires a
reflection setup since the human body is not transparent
in the mm-wave region with the penetration depth ofhuman skin in the range of submillimeters. Due to the
high water content of the human skin, it behaves as a
strong reflector for mm-wave signals. Thus, the reflec-
tion imagery is dominated by the specular reflections,
making active imaging on large distance inappropriate.
Therefore, close-range imaging is necessary, which con-
sequently increases the complexity regarding the image
formation with respect to the conventional imaging
under far-field conditions.
Image FormationFor many NDT applications and especially for per-
sonnel screening, a reflection setup is necessary. To
accomplish a three-dimensional (3-D) reconstruction
of the DUT, a two-dimensional (2-D) aperture has to
be sampled with a broadband measurement signal at
each selected transmit-receive combination. The spa-
tial extension of the aperture determines the lateral
resolution dx,y, given approximately by
D
L ,x yx y
,,
.d m (1)
where Dx,y denotes the length of the aperture in thecorresponding direction, mthe wavelength, and Lthe
distance between object and aperture [16], [17]. The
resolution dzin range direction is determined approxi-
mately by the signal bandwidth Bof the measured RF
signal [16], [17], thus given by
B
c2
.z0
.d (2)
Accordingly, a large signal bandwidth B results
in an equivalent short pulse duration and hence in a
high range resolution. This is for example interesting
for monitoring delamination effects in NDT or thedetection of thin dielectric explosive sheets in person-
nel screening. In practice, the bandwidth is often lim-
ited by the employed semiconductor components, e.g.,
oscillators, mixers, and amplifiers.
Depending on the field of application, the spatial
sampling can be realized with mechanical scanning
techniques [18][20] or electronic sampling by switch-
ing between spatially distributed transmit and receive
antennas [21][23]. If real-time imaging is required,
electronic sampling with parallelized data acquisi-tion is necessary, which leads consequently to a higher
hardware complexity. A compromise between mea-
surement speed and technical complexity is a hybrid
concept with mechanical sampling in one spatial coor-
dinate and electronic sampling in the perpendicular
direction [24][26]. This is an appropriate approach
to inspect goods on a conveyor belt and offers also a
flexible choice of the imaging aperture (planar, cylin-
drical, etc.) with respect to the mechanical sampling
coordinate. Thus, an adaption of the imaging aperture
to the target geometry is possible, which results in an
improved target illumination [12].High lateral resolution results from a large aper-
ture dimension Das denoted in (1). This can be accom-
plished by hardware focusing with elliptic mirrors,
dielectric lenses, reflect arrays or antenna arrays with
hardware beamforming (HBF). No necessary image
formation has to be applied when the mm-wave image
is generated by focusing with mirrors and lenses.
However, these devices offer optimum resolution only
at the focal point [27]. Reflect arrays are planar devices
with a spatial distribution of adjustable reflective ele-
ments, which can be either continuous or binary mod-
ulated components [28]. If the spatial reflectivity over
the reflector can be electrically tuned, it is also pos-
sible to steer the resulting focal point in three dimen-
sions [21]. This approach is, however, limited by the
low bandwidth of the reflective elements of the reflect
array, which results in a poor range resolution. An
image with high dynamic range requires furthermore
a dense placement of these reflective elements which
is hardly achievable for large reflect arrays in the mm-
wave range.
Another approach that enables a flexible steering of
the focal point is by individual control of the transmitand receive antennas in the imaging array. The idea is
to weight the respective antenna elements by a proper
phase and magnitude factors to steer the electromag-
netic wave in the desired direction. This can be accom-
plished either with hardware- or digital-beamforming
(DBF), as illustrated in Figures 1 and 2, respectively.
HBF, however needs no post processing to focus the
image, requires essentially an exact knowledge about
the transfer functions of all transmit and receive anten-
nas, which have to be compensated by the respective
phase shifter and gain control. This requirement is
also hardly achievable for large imaging arrays andhence practically limits the system performance.
With the availability of morechannels combined with the DSPcapabilities of modern computers,the performance of mm-waveimaging systems is advancingrapidly.
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The most flexible approach is the DBF which is also
well known as aperture synthesis. The reflected sig-
nal is coherently detected at every receive antenna,
digitized and stored. After compensating for the influ-
ence of the transmit/receive transfer functions by a
proper calibration procedure, the data are weighted
by complex correction factors, in order to exclude the
free space transfer function, and coherently sum the
recorded reflections to form the focused radar image.In literature, this numerical procedure is named vari-
ously as DBF, aperture synthesis, back-propagation,
back-projection or migration technique [29][31]. In
contrast to HBF, several mm-wave images can be gen-
erated with the same raw data set. This is interesting
when different amplitude weighting is applied to the
raw data in order to generate images of different fea-
tures addressing, e.g., optimum spatial resolution or
enhanced image dynamic range.
This high level of flexibility made by the DBF comes
at the cost of the intensive signal processing involved,
which therefore often forms the bottleneck of thesystem performance. The image frame rate achieved
by a mm-wave imaging system is as well a consider-
able performance criterion for many applications. It is
determined by both, the measurement and the image
formation speed, which strongly depends on system
topology. In HBF systems, measurement time is pro-
portional to the number of scanned voxels and the
measurement time per voxel, which is connected to
the intermediate frequency (IF) bandwidth and the
switching speed of the system. In DBF systems with
parallel acquisition at the receivers, measurement time
is determined by the number of sequential transmitter
measurements, the RF bandwidth, the required unam-
biguous range, the transmitter switching speed, and
IF bandwidth. With mechanically scanning systems,
measurement time will be also limited by the achiev-
able scan speed while taking the required accuracy
of the antenna positioning into account. The achiev-
able image formation speed in a DBF system depends
mainly on the resolution of the image, the number of
collected measurements, and the complexity of the
underlying image formation algorithm. DSP units
thus govern the speed of image formation, whereasthey are continuously offering higher clocks and more
parallelization on their processing cores making DBF
solutions more applicable.
The sampling of the 2-D aperture can be accom-
plished by a monostatic or a multistatic arrangement
of the transmit and receive antennas. In a monostatic
setup, each antenna element in the imaging aperture
transmit and receive at the same position. The DUT is
sequentially illuminated from every antenna element.
The benefit of this approach is that only one transmit/
receive channel is required if the aperture is sampled
mechanically (see Figure 3). Electronic switchingbetween a higher number of transmit/receive elements
Figure 2.Hardware architecture of receive path for DBFimagers.
A/D A/D A/D A/D
Antenna Array
Fixed Gain
Digitalization
Image Formation Digital Signal Processing
Memory
Antenna Array
Phase Shifter
Variable Gain
Power Combiner
Digitalization A /D
Figure 1.Hardware architecture of receive path forhardware-beamforming imagers.
Electromagnetic mm-waves offer acontactless inspection of materials
with nonionizing radiation and ahigh spatial resolution.
Transmit/ReceiveAntenna
y
x
z
DUT
rA
r
Figure 3.Geometry definition for monostatic imaging. Thegreen lines show an example path for mechanical scanning.
8/13/2019 Advanced Microwave Imaging 2012
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is possible to improve the data acquisition speed, how-ever this leads to an enormous number of channels.
If the compensation of the free space attenuation is
neglected, the focusing in a monostatic arrangement
can be formulated as
c( ) ( )o r s r e, ,N
A
N
j R20
A
~=~
~
y v// (3)
where ( )s r ,A ~v denotes the received complex signal at
location rAv and angular frequency ~ , and ( )o rv is the
desired reflectivity distribution of the DUT. R r rA; ;= -v v
is the distance between the position rAv of the respec-
tive antenna element and the position rvof the desiredvoxel position. For DBF, there exist several concepts
for an efficient numerical implementation of the above
formula. The most popular approach is the reconstruc-
tion in the Fourier domain [32][34], which benefits
from the fast Fourier transformation (FFT). There are
also concepts for multilevel based reconstructions [35],
[36], which were adapted from the field of numerical
electromagnetics [37].
A multistatic arrangement samples the aperture
by spatially distributed multiple transmit and receive
antennas [22], [23], [25], [26]. The DUT is again sequen-
tially illuminated by the transmit antennas how-
ever the reflected electromagnetic field is coherently
detected by every receive antenna. Accordingly, the
total number of channels can be drastically reduced,
while collecting the same number of measurements
made by an equivalent monostatic array. In addition, a
multistatic approach offers the opportunity of a strong
parallelization of the data acquisition, on contrary to a
monostatic setup. This is beneficial for real-time imag-
ing applications. An efficient illumination is realized
by a proper positioning of the transmit and receive
antennas. With a multistatic array arrangement, the
reconstruction formula becomes
c( ) ( )o r s r r e, , ,N N N
T Rj R R( )
T R
T R0~=~
+
~
y v v/// (4)
where R r rT T; ;= -v v and R r rR R; ;= -v v are the distances
between the transmit antennas, and the receive anten-
nas relative to the position of the desired voxel, respec-
tively. For multistatic imaging, the data can be focused
with fast reconstruction methods in Fourier domain
[38], [39] or by multilevel concepts in space domain [35].
Space domain reconstruction is numerically expen-
sive, however does not suffer from any image degrada-
tion due to interpolation errors in Fourier domain.If the DUT is in the far-field of the array, the recon-
struction formulas (3) and (4) can be simplified by
assuming propagating plane waves. This leads to
reconstruction formulas which can be directly imple-
mented based on FFTs. For the applications of NDT
and personnel screening, the distance between the
imaged object and the imager is nearly equal to the
array dimensions. Therefore, the object is located in
the near field of the array and the far-field approxima-
tion does not apply. Consequently, the transmitted and
reflected signals have to be treated as spherical waves.
To generate a mm-wave image without ambiguities,
a dense array with an element spacing of half the min-
imum wavelength, concerning the transmit/receive
signals, should be realized. In multistatic imaging,
however, the dense array arrangement has to be real-
ized with either the transmit or the receive antennas
for each lateral direction. Therefore thinning of the
imaging array is possible without producing ambigui-
ties. A possible technique for thinning is the use of a
randomly populated array (see Figure 4) or aperiodic
element spacing [40]. These concepts are well known
from aperture synthesis in radio astronomy [41], butthey suffer from an increased sidelobe level which
results in a loss of dynamic range in the resultant mm-
wave image.
For multistatic imaging, the approach of an effec-
tive aperture [42][44] can be used to form a sparse
periodic array (SPA) design. This approach is valid
under far field conditions, where the resulting effec-
tive array factor AE(u, v) of the multistatic array is
equal to the multiplication of the transmit array factor
AT(u, v) with the receive array factor AR(u, v), where u
and vdescribe the direction cosines with respect to the
array. As the array factor is mathematically equal tothe Fourier transformation of the aperture, this leads
Figure 4.Geometry definition for multistatic imaging.
The distribution of the transmit and receive antennas areselected differently.
Transmit Antenna
Receive Antenna
y
x
z
DUT
rRrT
r
A multistatic arrangement samplesthe aperture by spatially distributedmultiple transmit and receiveantennas.
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to an effective aperture aE(x, y) of the multistatic arraywhich results from the 2-D convolution between the
transmit and receive aperture distributions aT(x, y) and
aR(x, y), respectively. Their mathematical dependences
are described in (5) and (6).
( ) ( ) ( )A u v A u v A u v, , ,E T R$= (5)
( ) ( ) ( )a x y a x ya x y, , ,E T R))= (6)
The main advantage of a SPA design is the reduction
of the total number of antenna elements with respect
to conventional dense arrays. This is achieved by keep-
ing a well sampled effective aperture, whereas the
physical apertures can be very sparse. As the target
distance L is similar to array dimensions, the target is
in the array near field, which produces residual ambi-
guities in the resulting mm-wave image. This effect
can be considerably reduced by introducing redun-
dant antenna elements [44] or by modifying the array
arrangement [25], [45].
Following the SPA design concept, a novel array
architecture was introduced in [22], which is capable of
compensating for the drawbacks of the near field oper-
ation. Figure 5 illustrates the array geometry, and Fig-
ure 6 shows the associated allocation of the effective
aperture. The system operates from 72 to 80 GHz and
covers an aperture of 50 cm times 50 cm, populated
with 16 antenna clusters. The total number of antennas
is 736 transmit and 736 receive antennas. Figures 7
and 8 show the point spread function (PSF) of the
focused beam for the transmitter (Tx) and receiver (Rx)
apertures, respectively. In spite of the strong ambi-
guities seen, the overall transmit-receive PSF shown in
Figure 9 is free of any ambiguities. The background
0.25 0.15 0.05 0 0.05 0.15 0.250.25
0.15
0.05
0
0.05
0.15
0.25
x(m)
y
(m)
Figure 5.Array geometry (red for Tx antenna lines, bluefor Rx ones) [22].
x(m)
y
(m)
0.5 0.3 0.1 0 0.1 0.3 0.5
0.5
0.3
0.1
0
0.1
0.3
0.5
123456789101112
13141516
Figure 6.Effective aperture of the multistatic array shownin Figure 5.
80
40
0
40
80
80 40 0 40 8060
40
20
0 dB
x(mm)
y
(mm)
Figure 7.Point spread function of the Tx array [22].
80
40
0
40
80
80 40 0 40 8060
40
20
0 dB
x(mm)
y
(mm)
Figure 8.Point spread function of the Rx array [22].
Thin film ceramic modules, LTCCmodules, and enhanced IC packagesintegrating antennas on their signalredistribution layers are all possibleoptions for medium-channel-countsystems.
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level is below -60 dB, which is essential for gener-
ating images of high dynamic range after focusing.
The lateral resolution is of 2 mm in both directions.
Figure 10 shows an image result of this system demon-
strating the high image quality produced.
Technology ChoicesThe technology choices mainly depend on the chosen
frequency bands (ranging from a few gigahertz to sev-
eral hundred gigahertz) and the number of channels
(ranging from a few ones to several thousands) presentin the system. Analog/RF front-end modules can be
built as waveguide modules, as microwave integrated
modules based on thin film ceramic technology, as
low-temperature cofired ceramic (LTCC) modules or
as an RF printed circuit board (PCB). Cost per channel
is decreasing in this list. For low-channel-count sys-
tems, the designer can rely on proven commercially
available modules, mostly available as connectorized
microwave integrated circuits or waveguide modules
at higher frequencies. With increasing channel count,the space consumed by the front ends becomes criti-
cal, and higher integration is necessary. This is best
achieved by developing dedicated multichannel Tx
and Rx front-end modules. In high-channel-count
systems, mature manufacturing processes that are
suitable for mass production with good reproduc-
ibility are vital for achieving reliable results. At high
frequencies, interface losses are not negligible and the
RF frequency generation have to move near to or into
the analog front end, as well as the front end has to
be placed as near as possible to the antennas to mini-
mize interface losses. For low-channel-count systems,low loss but space-consuming interconnect technolo-
gies, e.g., waveguides, can be used. High-channel-
count systems at high frequencies must integrate the
antenna into multichannel analog front-end mod-
ules. Thin film ceramic modules, LTCC modules, and
enhanced IC packages integrating antennas on their
signal redistribution layers are all possible options
for medium-channel-count systems or as submount
modules in high-channel-count systems. If the design
of monolithic integrated front ends can be afforded,
RF PCBs with chip-on-board technology, which allow
also for integration of multilayer planar antennas, are
suitable for frequencies up to 100 GHz. High-channel-
count systems at frequencies higher than 100 GHz
have not yet been realized. Such systems require even
higher integration levels of multichannel monolithic
microwave integrated circuits (MMICs), possibly with
included on-chip antennas.
The choice of semiconductor technology for mm-
wave imaging will be a never ending discussion
depending on the addressed system parameters and
the availability of manufacturing facilities. Since the
availability of deep-submicron CMOS technologieswith transit frequencies exceeding 200 GHz [46], three
main technology options exist to realize mm-wave
integrated circuits: 1) III-V technologies, 2) SiGe bipo-
lar (or BiCMOS), or 3) CMOS. All the three technology
classes, including III-V due to its large utilization in
mobile phones, are mature and can be used for pro-
duction with good reproducibility. Regarding the RF
performance, e.g., noise, output power and thermal
stability, III-V technologies still clearly outperform
silicon based technologies and should be the preferred
option for imaging systems with low number of chan-
nels, i.e., mechanically scanning ones. Integration den-sity capability of III-V technologies is obviously lower
80
40
0
40
80
80 40 0 40 8060
40
20
0 dB
x(mm)
(a)
(b)
y
(mm)
15 mm
2 mm
2 mm
Figure 9.(a) Overall transmit-receive PSF [22] and (b)
3-D rendering of the PSF, showing the resolution cell sizeand the surrounding sidelobes [22].
If the design of monolithic integratedfront ends can be afforded, RF PCBs
with chip-on-board technology, whichallow also for integration of multilayerplanar antennas, are suitable forfrequencies up to 100 GHz.
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than SiGe bipolar or CMOS,
but should be high enough
for first generation imag-
ing systems. On a long term
perspective, CMOS offers the
best capability of integrating
RF front ends, analog cir-
cuits, baseband processing,
analog-to-digital convert-ers (ADCs), digital-to-analog
converters (DACs), and DSP
units, all on one die. Integra-
tion of CMOS RF modules
is still, however, subject to
the challenges of solving
design difficulties to meet the
required performance at high
frequency and high band-
width, and to solve reliabil-
ity problems caused by hot
carrier degradation. Last butnot least, technology choice
will be determined by cost,
especially for large imaging
systems. With respect to the
total expected product vol-
ume, not only wafer produc-
tion costs must be taken into
account but also development
costs and the cost for a production mask set. For 65 nm
and 40 nm CMOS technology, the cost of the produc-
tion mask set is excessively high, which makes CMOS
not yet a feasible option. From todays point of view, a
pure bipolar process, which is already in use for mass
market 77-GHz automotive radar applications [47],
[48], gives the best cost effectiveness: mask set cost is
a fraction (less than a tenth) of a 40 nm CMOS mask
set, production cost is clearly lower than for the III-V
technologies, and a large design reuse from existing
automotive radar modules reduces development costs
and guarantees a short time-to-market. Next higher
integration levels are possible by using SiGe BiCMOS
technology, which includes a nowadays relatively
cheap 130 nm CMOS technology in order to integratemore digital and analog modules together.
The choice of the used antenna is of central impor-
tance for any imaging system. Transmit and receive
antennas must couple the electromagnetic wave to the
medium of propagation while following certain design
requirements to ensure proper operation. Furthermore,
image quality is highly influenced by the used signal
bandwidth which consequently must be supported
by the antennas. Antennas are often required to offer
high beamwidths as well as very stable phase centers.
The phase center describes a virtual point for a sphere
center where the phase front can be approximated tobe radiated from. The image formation algorithms rely
on the approximation of spherical phase fronts and
hence any deviation from this assumption within the
field of view will cause image degradation. Therefore,
phase centers should be stable over the beamwidth as
well as the bandwidth used, a criterion which is dif-
ficult to achieve with many types of antennas. Addi-
tionally, polarization purity becomes an issue when
polarimetric imaging is demanded. Typical antenna
types used in imaging systems include for instance
slotline, patch, waveguide, horn, and dipole antennas.
Tests with cavity backed circularly polarized spiral
antennas carried out in [49] showed positive aspects
of polarimetric imaging. In [50], a promising design
based on differential stripline feeds for realizing a
polarimetric imaging system was introduced. Last butnot least, antennas are required to be small in size . On
one hand, the size of the antenna structure must allow
for dense sampling of the wavefront at less than the
wavelength, and on the other hand miniature antenna
design offers a feasible integration with MMICs for
successful array integration.
QPASS SystemThe Quick Personnel Safe Screening system (QPASS)
was developed on the basis of multistatic DBF technol-
ogy to target the application of close-range personnel
screening at airports and critical infrastructure build-ings [51]. The imaging array operates from 70 to 80 GHz
Figure 10.Illustration of the imaging capability of the multistatic system using a U.S.Air Force (USAF) test chart made of a metal sheet and mounted in front of a bed of nailswith absorber in their background. The nails are fixed to a grid of 10 mm distance. Eachnail is of 5-mm diameter with an approximate radar cross-section of 50 dBsm. Due tothe high dynamic range of the image and the low sidelobe levels of the system, they are allclearly visible. The slots of the USAF chart are separable down to the 2 mm openings [22].
10 mm
2.5 mm
14 mm
(a) (b)
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with frequency-stepped continuous-wave technique.
The array design follows the same architecture as the
one in Figure 5, however extends to cover a two meters
times one meter aperture, as shown in Figure 11. Two
arrays of square aperture are stacked vertically, where
each includes 1536 Tx channels and 1536 Rx chan-
nels, making a total of 6144 RF channels. This ensures
proper illumination of the human body [52]. Although
being developed for a specific application, the system
architecture features a highly modular design offer-
ing a flexible platform to address further applications
[53], [54].
A basic unit, namely a cluster, integrates 96 Tx and
96 Rx channels in one housing, which is suited for flex-
ibly building imaging arrays of different geometries
and sizes. A dedicated digital back end unit, includ-
ing parallel analog to digital conversion and image
reconstruction kernels, has been developed. Four of
these units are integrated on a single PCB called an
IF-board in order to serve four clusters simultane-
ously. Four clusters, an IF-Board, a signal distributionboard, power supply, mechanics, and cooling parts
form together one unit. Four of these units are again
connected to a central board to form a complete array.
Then two of the arrays are connected to an industrial
PC (IPC) via fast PCI Express connection, resulting in
the complete imaging system.
The volume in front of the system is illuminated
sequentially by each of the Tx channels, and the com-
plex reflected signals are simultaneously and coher-
ently sampled by all Rx channels. These sampled data
are then processed, reflections are calculated, system
error correction is applied and the image is then recon-structed. The system block diagram of a single array is
shown in Figure 12.
Signal SourceDigital-beamforming relies on accurate phase mea-
surement for each Tx-Rx combination. Therefore, het-
erodyne reception is favorable, which hence requires
generation of coherent RF and local oscillator (LO)
signals. A dedicated synthesizer unit has been devel-
oped and optimized to generate the RF and LO signals
around 20 GHz in order to ease signal distribution to
all RF front ends. Direct digital synthesizers (DDSs)
are used to generate the signals, which are derived
from a highly stable oven-controlled crystal oscillator
(OCXO). DDSs are preferred here due to their ability to
switch frequencies very fast. Contrarily to free-running
oscillators, the DDSs can generate signals with a deter-
mined phase value, which is useful in many imaging
applications. After the DDSs, the frequency is multi-
plied by a factor of 256, and distributed to the clusters.
The choice of the used antennais of central importance for anyimaging system.
2 m
1 m
Cluster
47 Rx Antennas
47 Rx Antennas
94 Tx Antennas
Figure 11.Photograph of QPASS system (without cover)[55]. On the right, a cluster unit is shown [55].
Figure 12.System block diagram of a single array.
SignalSources
Distribution Network
fRF/4
(fRFfIF)/4
Synthesizer Control
Single Array Front End
1536IF Signals
AcquisitionHardware
A
D
DSP
Control
Unit
ImageProcessing
and
Visualization
MulticoreComputerFront-end
Control
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On each chip, the RF and LO signals are amplified,
quadrupled and distributed to four channels.
RF Front EndEach cluster contains 96 Tx and 96 Rx channels,
where four of them are used as internal reference
channels. The analog front ends are built of custom-
made four-channel receiver and transmitter chips,
which are connected to aperture-coupled patch-excited horn antennas. Those elements are embed-
ded in a RF multilayer PCB. The chips are mounted
in multilevel cavities, as the antennas differential
feed lines run on an inner layer of the PCB, and for
RF performance reasons, vias and longer bond wires
have been avoided, as shown in Figure 13. The horn
part of the antennas is integrated into the cluster
housing, which also carries two RF and two LO
input ports.
A custom chipset has been designed for this sys-
tem [56]. Both transmit (Figure 14) and receive (Fig-
ure 15) MMICs include four E-band channels and a
central RF or LO distribution with frequency quadru-
pling. The center frequency of operation is 75 GHz
Figure 13.Cut view of the multilayer PCB illustrating the integration of MMIC and the antenna structure inside thehousing of the cluster.
Fastening Screw Horn Antenna Cover
Patch Absorbing Material
Tx or Rx Chip CavityBond Wire
Heat Sink
ViaThermal ViasDifferential LineSlot
IF Part
RF Part
TempSensor T MUX
Analog Bus
RF Ch. 1
RF Ch. 2
RF Ch. 3
RF Ch. 4
PA
Gain On/Off
Buf
Enable Quadrupler
Buf Follower
RF
Figure 14.Block diagram of the four channels Tx SiGe Chip.
QPASS was developed on the basisof multistatic DBF technology totarget the application of close-rangepersonnel screening at airports andcritical infrastructure buildings.
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36 September/October 2012
with a bandwidth of approximately 10 GHz. Figures
16 and 17 show photos of the Tx and Rx SiGe chips,respectively.
The measured receiver conversion gain is 23 dB
with a SSB NF below 10 dB over a wide frequency
range from 70 to 82 GHz. The transmitter chip includes
4 output channels with an output power of more than
0dBm in a frequency range from 70 GHz to 86 GHz.
Both chips are supplied from a single 3.3 V supply
voltage and the power consumption per channel is
145 mW for Tx and 180 mW for Rx. The process used
for this chipset is a very cost-effective pure SiGe:C
bipolar technology similar to the one described in [57].
It is based on a double-polysilicon self-aligned transis-
tor concept with shallow and deep trench isolation. An
example transistor is shown in Figure 18. The SiGe:Cbase is deposited by selective epitaxy. A mono-crystal-
line emitter contact results in a small emitter resistance.
Different npn transistor types with cut-off frequencies
from 52 GHz to more than 200 GHz and collector-
emitter breakdown voltages at open base (BVCEO)
from 5 V to 1.8 V are available. In addition to npn and
pnp transistors, the process provides polysilicon resis-
tors with sheet resistances of 150 and 1,000 X/sq and
tantalium-nitride (TaN) thin film resistors with a sheet
resistance of 20 X/sq. A metalinsulatormetal (MIM)
capacitor with Al2O3 dielectric and a specific capaci-
tance of 1.4fF/nm is integrated in a four-layer copper
Analog Bus
IF 1
Buf
BufBuf Follower
Temp
Sensor
LO
LNA
RF Ch. 1
RF Ch. 2
RF Ch. 3
RF Ch. 4
IF 2 IF 3 IF 4
Figure 15.Block diagram of the four channels Rx SiGe chip [55].
Figure 16.Photograph of the Tx chip with the integratedfour RF channels (size 2.2 # 2 mm2) [56].
Figure 17.Photograph of Rx chip with the integrated fourRF channels (size 2.2#2 mm2) [56].
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September/October 2012 37
metallization. The use of an automotive-qualified bipo-
lar process was furthermore very advantageous due
to the reuse of 77-GHz mass market automotive radar
designs [58], [59], which enabled meeting design tar-
gets after just two design iterations.
AntennaThe planar antennas used in the system are optimized
to fulfill the requirements of the imaging application,together with the capability of integration with the
MMIC frontends in a 2-D array with high element
count. They offer a small footprint and a high band-width by using a differentially fed dipole, resonant
Base Emitter Collector
n+Poly-Si
p Mono-
SiGe: C(Base)
STI(Shallow Trench Iso)
Buried Layer
SiCp+-Poly
p-Isolation
p-Substrate
(a)
(b)
Collector Emitter Base
SiGe:C Base
Shallow Trench
Deep Trench
DT(Deep Trench Isolation)
Figure 18.Transmission electron microscopy image and a schematic of a cross section for a npn SiGe transistor [48], [56].(Printed with permission from Infineon Technologies AG, Munich, Germany.)
The radiated peak power isapproximately one milliwatt,
which is very low compared tocommunication devices.
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38 September/October 2012
aperture slots, and a patch element. Input matching and
beamshape are improved by a stacked cylindrical horn,
which also enhances isolation to neighboring elements
together with a via-ring cavity in the substrate [60], [61].
Polarization was purposely rotated by 45 in order to
reuse the same antenna on vertical as well as horizontal
antenna lines while keeping copolarized operation. The
internal layers of the PCB used to realize the antenna are
illustrated in Figure 13. Figures 19 and 20 show photos
of the integrated chip and the patch part of the antenna.
The simulation results of the antenna at 75 GHz for both
the copolarized and the cross-polarized components are
shown in Figure 21. The antenna has a wide beam withapproximately 8 dB gain and delivers high polarization
purity. The radiated peak power is approximately one
milliwatt, which is very low compared to communica-
tion devices, e.g., mobile phones.
Digital Back EndThe digital back end performs measurement acquisi-
tion, system control and monitoring, digitization of
IF signals, system error correction, and image recon-
struction. The IF signals are amplified and then digi-
tized by an eight-channel ADC chip at 50 MSa/s, as
shown on the left of Figure 22. The signals are furtherdown-converted digitally to
zero IF and subsequently fil-
tered. Conversion and DSP
are performed in parallel,
the system implements 2 x
1536 coherent digital receiver
chains, which is necessary to
achieve the short measure-
ment time. For each single
measurement, twelve sam-
ples are required to account
for the channel and filter set-
tling times [55]. The collected
reflection data are compared
to reference channels built
inside the system in order to
Figure 19.Chip integration in a multilayer PCB includingthe patch part of antennas shown on the right side [62].
Control and IF SignalsDifferential Line
Patch Antenna
Ground Contacts
Miled First Cavity
20-GHz
Input
Supply
Chip Mountedinto Miled
Second Cavity
3 mm
Three-WayWilkinson
Divider
Cavity
Thin-FilmResistors
Two-Way Wilkinson Divider
Figure 20.Photograph of the cluster without housing showing the signal distribution,chip integration, and the patch part of the antennas [61], [62].
Figure 21.Simulation of the radiation pattern of a single antenna showing the polarization purity and the beam quality. Onthe left, the copolarized component of the field is shown, and on the right the cross-polarized component.
Theta
(a) (b)
Theta
PhiPhix
dB
8
6
4
2
0
8
16
24
32
y
z
xy
t
Phi
z
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September/October 2012 39
Figure
22.Blockdiagramo
fthedigitalbac
kendusedintheQPASSsystem.
32x
32x
32x
32x
2x
48x
4x
32x
DataAcquisitio
n
Reconstruction
ReconstructionKernels
Cache
Cache
AGU
AGU
Cache
Cache
DDR3
1GB
DDR3
1GB
DDR3
1GB
DDR3
1GB
Legend
ADC
A
nalog-Digital-Converter
DDC
D
igitalDown
-Converter
HSSIH
igh-Speed
SerialInterface
AGU
A
ddress-Ge
nerating-Unit
IPC
I
ndustrial-PC
1536ReconstructionKernels
10.6
TOPS/s
AGU
AGU
Memory
Controller
M
emory
Co
ntroller
M
emory
Co
ntroller
Memory
Controller
Calculate
Reflections
DDC0
DDC1
DDC95
ej
ej
ej
ADC1
ADC0
ADC95
3072ADCs
at50MHz
138GS/s
32
High-Speed-
Interfacesat
36Gb/s
1.1
5Tb/s
32
High-Speed-
Interfaces
at10.9
Gb/s
349Gb/s
2PCIeat
32Gb/s
64Gb/s
IPC
DataCollection
IFSignals IFSignals
HSSI
HSSI
PCIExpress
Touchpanel
. . .
. . .
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40 September/October 2012
compensate for any thermal drifts and thus ensuring
high stability over long time of operation. Then the
image reconstruction takes place at each cluster unit in
a parallelized fashion, in order to minimize the trans-
ferred data rates inside the system. The digital back
end offers a fast PCI Express connection to the inte-
grated IPC, which is used to transfer the reconstructed
3-D images in magnitude and phase. The images can
then be prepared for direct display or used for further
image processing steps beforehand.
A cutting-edge realization of the digital back endhas been designed to deal with the huge data rates of
1.15 Tb/s collected by the system. The reconstruction
hardware needs to perform 10.6 Tera-operations-per-
second in order to deliver full image reconstruction in
approximately two seconds. Figure 22 illustrates the
signal flow within the digital back end and reveals
part of its inherent complexity.
The QPASS system is capable to produce 3-D
images of 30 dB dynamic range and 2 mm of lateral
resolution. Figure 23 illustrates an example image
of a person concealing two dielectric objects, which
demonstrates the system capability to address per-
sonnel screening applications. In Figure 24, another
image using colors is presented to demonstrate the
3-D content of the image. The color codes the range
information of each voxel, where red is close andblue is far relative to the imager surface. Figure 25
illustrates a detailed view of the pistol, and dem-
onstrates the high system resolution, allowing to
image features of a few millimeters in size. In the
application of personnel screening, privacy issues
can arise. Therefore, the 3-D images are further pro-
cessed with dedicated detection algorithms in order
to automatically and anonymously find concealed
objects of any potential hazards such as weapons or
explosives.
Moreover, the system is also capable to detect depth
variations down to 50 um [51], thanks to its exceptionalsignal phase stability. This corresponds to a phase
measurement accuracy of 5 in the reconstructed
image. Such a feature is attractive to many applications
addressing accurate 3-D modeling of surfaces, which
stands as a competitive solution to optical scanners.
With the flexible and modular design concept for
both the RF front ends as well as the digital back-
end, the system can be
reconfigured to adapt dif-
ferent imaging modes and
can be geometrically modi-
fied to cover various aper-
ture dimensions. The high
image dynamic range ensures
images of 30 dB free of any
artifacts, which also open the
possibility for image process-
ing techniques, including
super-resolution algorithms,
to enhance the imaging capa-
bility of the system specifi-
cally for certain applications.
Many algorithms for objectdetection and classification
are being either adapted or
newly developed to deal with
the rich 3-D image informa-
tion delivered by the system
in magnitude and phase.
Conclusion and OutlookMicrowave imaging systems
are exhibiting a continuous
improvement in their per-
formance combined with aremarkable increase in their
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1
y
(m)
x(m)
(a) (b)
0.5 0 0.5
Figure 23.Image of a person taken from 70 to 80 GHz [55]. Image shows the magnitude
information after being projected along range direction. Two concealed dielectric objects,liquid bag (up) and explosive simulant (down), are marked with red rectangles.
Active imaging ensures imageproduction with a high dynamicrange, which is required by manyapplications where objects are tobe found behind surfaces or inside
volumes.
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September/October 2012 41
complexity and level of integration. The advances in
semiconductor technology assist this development
on one side, and the increase in the computational
power of modern computers and DSP units sup-
ports the DBF techniques on the other side. Imag-
ing systems based on reflectors, mirrors, lenses, or
complex phased-array components are becoming
less attractive for many applications. Instead, soft-
ware derived technologies are coming to the frontierof the state-of-the-art solutions. These technologies
allow for an optimal image focusing at all range
distances and are not restricted to focal lengths.
The applicability of these techniques are moving to
cover the mm-wave range, and are even pushed to
reach the terahertz band.
Active imaging ensures image production with
a high dynamic range, which is required by many
applications where objects are to be found behind
surfaces or inside volumes. Multistatic array archi-
tectures for industrial and security applications
have been intensively investigated during the lastyears. Multistatic imaging allows for a huge reduc-
tion factor in the total number of needed channels,
and hence opens the opportunity for fully electronic
solutions to be realized. Many of the numerical com-
plications caused by multistatic imaging are nowa-
days affordable due to the available computational
capabilities.
As integration levels are getting higher, modu-
lar concepts with combined analog and digital units
are becoming reachable. Power consumption of the
involved devices is much reduced, thus allowing for
compact modular designs. Semiconductor technolo-
gies are offering various options for system realiza-
tion depending on cost and performance. In addition,
PCB manufacturing has been significantly enhanced
to be a cost-efficient carrier to MMICs and antennas
aside of each other. Frequency ranges up to 100 GHz
are currently realizable using these technologies, and
higher frequencies can be supported with submount
techniques.
The first steps towards a fully electronic
solution based on multistatic systems and
DBF technique have been made and proved tobe efficient and affordable. This is best demon-
strated by the QPASS system, which integrates
around 6,000 coherent RF channels realized on
SiGe technology and included as well an inte-
grated image reconstruction unit. Challenges
are still there to build even more advanced
imaging systems featuring full polarimet-
ric imaging, faster image reconstruction
units, and combined reflection-transmission
imaging. Polarimetric multistatic imag-
ing will increase the detection capabilities
by using methods based on ell ipsometryknown from optics.
(a) (b)
Figure 25.Photograph and mm-wave image of P99 pistol concealed
behind a thick pullover and a leather belt. Metal features inside the plasticgrip, e.g., the magazine, are clearly visible.
Figure 24.Image of a person concealing a P99 pistol onthe back. The reflectivity image is here multiplied by thecolored range information to visualize the 3-D content ofthe image. The range changes from red to blue as close to farfrom the imager, respectively.
The reconstruction hardware needsto perform 10.6 Tera-operations-per-second in order to deliverfull image reconstruction inapproximately two seconds.
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42 September/October 2012
In the near future, new imaging facilities based
on modern mm-wave technologies will bring new
opportunities to the humankind to take advantage of
simple hand-held up to professional large scale imag-
ers, serving their demands especially where x-ray or
ultrasonic methods are not feasible. New applications
assisted by tailored algorithms for image processing,
classification, and interpretation will come up one
after another. And at the same time, the system prices
will drop following the progress in semiconduc-tor market. This can make such systems applicable
for mass production and put them as an option for
everyday use.
AcknowledgmentThe authors would like to thank the German Federal
Ministry of Education and Research for funding part
of the presented activities.
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