Advanced Microwave Imaging 2012

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

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September/October 2012 27

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


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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.


5/18


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.


8/18


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)


9/18


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


10/18


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.


11/18


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].


12/18


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.


13/18


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


14/18


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

. . .

. . .


15/18


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.


16/18


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.


17/18


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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mm-wave imaging system, in Proc. EuSAR, 2012, pp. 4043.

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time digital-beamforming imager for personnel screening, in

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chipset for automotive LRR and SRR systems in SiGe BiCMOS

technology, IEEE Trans. Microwave Theory Tech., vol. 60, no. 3, pp.

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patch-excited horn antenna at millimeter-wave frequencies, in

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[61] S. Methfessel and L.-P. Schmidt, A waveguide to differential

stripline transition for planar mmWave antenna measurements,

in Proc. Millimetre Wave Days 6th ESA Workshop on Millimetre-Wave

Technology and Applications and 4th Global Symp. Millimeter-waves

GSMM, May 2325, 2011, Espoo, Finland, May 2011.

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front-end module with 2 x 96-channels for mm-wave imaging

systems, in Proc. European Microwave Conf., Oct. 29Nov. 1,

2012.

Documents

Advanced Microwave Imaging 2012