Advanced Microwave Imaging 2012

ED URE S U AT C FO E FE SU S Iue to the enormous advances made in semiconductor technology over the last few years, high integration densities with moderate costs are achievable even in the millimeter-wave (mm-wave) range and beyond, which encourage the D 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 Sherif Sayed Ahmed ([email protected]), Andreas Schiessl, and Frank Gumbmann are with Rohde & 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. Digital Object Identifier 10.1109/MMM.2012.2205772 Date of publication: 13 September 2012 26 1527-3342/12/$31.00©2012IEEE September/October 2012 The mm-wave images can be generated by either a passive or an active imaging approach. material characterization. These systems work in a transmission setup. 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]. and Lorenz-Peter Schmidt imaging attractive for a wide variety of commercial and scientific applications like nondestructive testing (NDT). Especially for outdoor applications. the E-band (60–90 GHz with m = 5 to 3. active imaging systems illuminate the DUT and the reflected or transmitted field can be detected coherently or incoherently.. However. Thus a passive mm-wave image contains the information of the emissivity and reflectivity of an object in the respective frequency domain [8]. Depending on the medium of propagation. Furthermore.g. the visibility of the DUT and the image quality depends on an appropriate illumination of the specimen and a proper positioning of the antennas. passive imaging systems suffer from low radiometric contrast in indoor applications due to the high background temperature of the environment. health aspects are critical with respect to imaging of humans.. Another drawback is the lack of depth information concerning the investigated DUT. i. and luggage inspection at security checkpoints. This offers the potential of classification of different scattering processes [7] and thus an improved detection of anomalies in the DUT is possible. e. For many applications. which are applied in.g. Sebastian Methfessel.. Furthermore. the performance of mm-wave imaging systems is advancing rapidly. NDT applications [2]. equivalently. Another well-known imaging technology is the ultrasonic inspection of materials for NDT applications [4] and screening of humans for medical diagnostics [5]. [9]. Frank Gumbmann. Therefore. the attenuation and absorption through a dielectric specimen can be mapped. This results from the fact that the detected signals can be understood as thermal noise and thus the radiation is incoherent. backscatter X-ray systems.Sherif Sayed Ahmed. it is possible to exploit the vectorial nature of electromagnetic waves and to carry out polarimetric measurements [6]. 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. the scattering process is dominated by specular reflections [12].3 mm) is a good compromise for NDT applications to detect flaws. for most ultrasonic devices. an appropriate coupling medium is required for an efficient coupling of the ultrasonic wave in the respective device under test (DUT). the personnel screening at airport security checkpoints. In contrast. Marc Tiebout. computed tomography (CT) for medical diagnostics [1]. In the case of spatially smooth objects relative to the applied wavelength. The most commonly known imaging systems are based on X-ray technology. and medical screening. and inclusions in dielectrics. Therefore. Regarding a transmission setup. The spatial resolution in lateral and range directions as well as the image dynamic range offered by an imaging system are considered the main measures of performance. electromagnetic mm-waves offer a contactless inspection of materials with nonionizing radiation and a high spatial resolution. material inhomogeneities. were investigated over the last years. security scanning. On the contrary. e.. active imaging is necessary to achieve an image with high dynamic range and radiometric contrast. e. A lateral resolution of ~2 mm is sufficient for many applications.e. But on the other hand. With the availability of more channels combined with the powerful digital signal processing (DSP) capabilities of modern computers. On the one hand. Passive imaging systems detect the characteristic radiation of an object and the reflected background radiation without the need of illuminating the DUT with additional electromagnetic energy. which work in a reflection setup. However. Since spatial resolution and penetration depth are conflicting parameters regarding the wavelength. X-ray images have an inherent high lateral resolution due to the extremely short wavelength (m ~ 10 –2 nm – 10 nm). By applying a coherent broadband transmit and receive signal or. especially in the case of personnel screening at airports. a lateral resolution even in the submillimeter region is achievable. Andreas Schiessl. the object reflectivity can be characterized. especially for the screening of passengers for concealed objects at airports [3].g. a time delay measurement September/October 2012 27 . the energy of the photons is high enough to ionize organic and inorganic matter. while for a reflection setup. in mm-wave range the clear sky has Tsky 1 100 K. This is an appropriate approach to inspect goods on a conveyor belt and offers also a flexible choice of the imaging aperture (planar. This can be accomplished either with hardware. D x. given approximately by d x. requires essentially an exact knowledge about the transfer functions of all transmit and receive antennas.y. [17].) with respect to the mechanical sampling coordinate. a reflection setup is necessary. Therefore. it is furthermore possible to analyze multiple reflections resulting from a stratified dielectric medium. the spatial sampling can be realized with mechanical scanning techniques [18]–[20] or electronic sampling by switching between spatially distributed transmit and receive antennas [21]–[23]. which leads consequently to a higher hardware complexity. However..g. making active imaging on large distance inappropriate. This is for example interesting for monitoring delamination effects in NDT or the detection of thin dielectric explosive sheets in person- nel screening. it is additionally possible to reconstruct the spatial extend of the DUT along the range direction. c0 . This can be accomplished by hardware focusing with elliptic mirrors. respectively. as illustrated in Figures 1 and 2. 28 September/October 2012 . which results in a poor range resolution. a two-dimensional (2-D) aperture has to be sampled with a broadband measurement signal at each selected transmit-receive combination. m L .or digital-beamforming (DBF). This requirement is also hardly achievable for large imaging arrays and hence practically limits the system performance. If real-time imaging is required. This approach is. y (1) where Dx. however needs no post processing to focus the image. In practice. the bandwidth is often limited by the employed semiconductor components. electronic sampling with parallelized data acquisition is necessary. The resolution dz in range direction is determined approximately by the signal bandwidth B of the measured RF signal [16]. which have to be compensated by the respective phase shifter and gain control. The idea is to weight the respective antenna elements by a proper phase and magnitude factors to steer the electromagnetic wave in the desired direction. it is also possible to steer the resulting focal point in three dimensions [21]. Reflect arrays are planar devices with a spatial distribution of adjustable reflective elements. in active imaging. oscillators. [15]. the reflection imagery is dominated by the specular reflections. cylindrical. Due to the high water content of the human skin. Thus. and amplifiers. which results in an improved target illumination [12]. y . an adaption of the imaging aperture to the target geometry is possible. HBF. To accomplish a three-dimensional (3-D) reconstruction of the DUT. 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 applications [14]. which consequently increases the complexity regarding the image formation with respect to the conventional imaging under far-field conditions. Depending on the field of application. If the spatial reflectivity over the reflector can be electrically tuned. however. 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 mmwave range. mixers. and L the distance between object and aperture [16]. With a sufficiently large signal bandwidth. limited by the low bandwidth of the reflective elements of the reflect array. A compromise between measurement speed and technical complexity is a hybrid concept with mechanical sampling in one spatial coordinate and electronic sampling in the perpendicular direction [24]–[26]. these devices offer optimum resolution only at the focal point [27]. e. which can be either continuous or binary modulated components [28].y denotes the length of the aperture in the corresponding direction. The last named application requires a reflection setup since the human body is not transparent in the mm-wave region with the penetration depth of human skin in the range of submillimeters. it behaves as a strong reflector for mm-wave signals. the performance of mm-wave imaging systems is advancing rapidly. High lateral resolution results from a large aperture dimension D as denoted in (1). 2B (2) Accordingly. m the wavelength. etc. a large signal bandwidth B results in an equivalent short pulse duration and hence in a high range resolution.With the availability of more channels combined with the DSP capabilities of modern computers. [17]. The spatial extension of the aperture determines the lateral resolution dx. close-range imaging is necessary. Thus. 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. dielectric lenses. Another approach that enables a flexible steering of the focal point is by individual control of the transmit and receive antennas in the imaging array. Image Formation For many NDT applications and especially for personnel screening. thus given by dz . In a monostatic setup. It is determined by both. measurement time is determined by the number of sequential transmitter measurements. The achievable image formation speed in a DBF system depends mainly on the resolution of the image. several mm-wave images can be generated with the same raw data set. the number of collected measurements. the required unambiguous range. which therefore often forms the bottleneck of the system performance. each antenna element in the imaging aperture transmit and receive at the same position. The image frame rate achieved by a mm-wave imaging system is as well a considerable performance criterion for many applications. the measurement and the image formation speed. in order to exclude the free space transfer function. Antenna Array Fixed Gain Digitalization A /D A /D A /D Memory A /D Image Formation Digital Signal Processing Figure 2. In literature. whereas they are continuously offering higher clocks and more parallelization on their processing cores making DBF solutions more applicable. optimum spatial resolution or enhanced image dynamic range. In HBF systems.The most flexible approach is the DBF which is also well known as aperture synthesis. The reflected signal is coherently detected at every receive antenna. This is interesting when different amplitude weighting is applied to the raw data in order to generate images of different features addressing. and coherently sum the recorded reflections to form the focused radar image. Geometry definition for monostatic imaging.. This high level of flexibility made by the DBF comes at the cost of the intensive signal processing involved. e. this numerical procedure is named variously as DBF. Antenna Array Phase Shifter Variable Gain Power Combiner Digitalization A /D Figure 1. Electronic switching between a higher number of transmit/receive elements Electromagnetic mm-waves offer a contactless inspection of materials with nonionizing radiation and a high spatial resolution. back-projection or migration technique [29]–[31]. The benefit of this approach is that only one transmit/ receive channel is required if the aperture is sampled mechanically (see Figure 3). In DBF systems with parallel acquisition at the receivers. the transmitter switching speed. The DUT is sequentially illuminated from every antenna element. and IF bandwidth. measurement time will be also limited by the achievable scan speed while taking the required accuracy of the antenna positioning into account. DSP units thus govern the speed of image formation. and the complexity of the underlying image formation algorithm. measurement time is proportional to the number of scanned voxels and the measurement time per voxel. The sampling of the 2-D aperture can be accomplished by a monostatic or a multistatic arrangement of the transmit and receive antennas. which is connected to the intermediate frequency (IF) bandwidth and the switching speed of the system. Hardware architecture of receive path for DBF imagers.g. which strongly depends on system topology. back-propagation. September/October 2012 29 . After compensating for the influence of the transmit/receive transfer functions by a proper calibration procedure. Hardware architecture of receive path for hardware-beamforming imagers. The green lines show an example path for mechanical scanning. the RF bandwidth. the data are weighted by complex correction factors. aperture synthesis. With mechanically scanning systems. digitized and stored. In contrast to HBF. Transmit/Receive Antenna y rA z r x DUT Figure 3. however this leads to an enormous number of channels. [39] or by multilevel concepts in space domain [35]. the approach of an effective aperture [42]–[44] can be used to form a sparse periodic array (SPA) design. v is the distance between the position rA of the respective antenna element and the position v r of the desired voxel position. The DUT is again sequentially illuminated by the transmit antennas however the reflected electromagnetic field is coherently detected by every receive antenna. The distribution of the transmit and receive antennas are selected differently. the reconstruction formula becomes o (y r) = / / / s ( v rT.A multistatic arrangement samples the aperture by spatially distributed multiple transmit and receive antennas. v rR . There are also concepts for multilevel based reconstructions [35]. Accordingly. ~ N~ N A made by an equivalent monostatic array. A possible technique for thinning is the use of a randomly populated array (see Figure 4) or aperiodic element spacing [40]. ~) e j c0 (R T + R R). In multistatic imaging. where u and v describe the direction cosines with respect to the array. the transmitted and reflected signals have to be treated as spherical waves. v). and o (v r) is the desired reflectivity distribution of the DUT. however. a dense array with an element spacing of half the minimum wavelength. [36]. Space domain reconstruction is numerically expensive. are the distances between the transmit antennas. In addition. With a multistatic array arrangement. and the receive antennas relative to the position of the desired voxel. which were adapted from the field of numerical electromagnetics [37]. If the compensation of the free space attenuation is neglected. and R R = . on contrary to a monostatic setup. this leads 30 September/October 2012 . while collecting the same number of measurements Transmit Antenna Receive Antenna rR rT y z r x DUT Figure 4. [25]. For the applications of NDT and personnel screening.v r. which benefits from the fast Fourier transformation (FFT). the total number of channels can be drastically reduced. the dense array arrangement has to be realized with either the transmit or the receive antennas for each lateral direction. [23]. This approach is valid under far field conditions. As the array factor is mathematically equal to the Fourier transformation of the aperture. where the resulting effective array factor AE (u. concerning the transmit/receive signals. v rA . v) of the multistatic array is equal to the multiplication of the transmit array factor AT (u. This is beneficial for real-time imaging applications. Therefore. ~) e j2 c0 R. These concepts are well known from aperture synthesis in radio astronomy [41]. For multistatic imaging. the object is located in the near field of the array and the far-field approximation does not apply. the data can be focused with fast reconstruction methods in Fourier domain [38]. a multistatic approach offers the opportunity of a strong parallelization of the data acquisition. v rT . Therefore thinning of the imaging array is possible without producing ambiguities. the reconstruction formulas (3) and (4) can be simplified by assuming propagating plane waves. v) with the receive array factor AR (u. where R T = . v rR.v r . This leads to reconstruction formulas which can be directly implemented based on FFTs. the focusing in a monostatic arrangement can be formulated as o (y r) = / / s ( v rA . but they suffer from an increased sidelobe level which results in a loss of dynamic range in the resultant mmwave image. Consequently. For multistatic imaging. is possible to improve the data acquisition speed. A multistatic arrangement samples the aperture by spatially distributed multiple transmit and receive antennas [22]. should be realized. however does not suffer from any image degradation due to interpolation errors in Fourier domain. The most popular approach is the reconstruction in the Fourier domain [32]–[34]. If the DUT is in the far-field of the array. respectively.v r . Geometry definition for multistatic imaging. [26]. ~) denotes the received complex signal at v location rA and angular frequency ~ . For DBF. the distance between the imaged object and the imager is nearly equal to the array dimensions. ~ N~ NT NR (4) (3) where s (v rA . To generate a mm-wave image without ambiguities. An efficient illumination is realized by a proper positioning of the transmit and receive antennas. there exist several concepts for an efficient numerical implementation of the above formula. R = . the target is in the array near field. and Figure 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. 0 dB 80 −0. v) $ A R (u.1 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 x (m) 0. can be considerably reduced by introducing redundant antenna elements [44] or by modifying the array arrangement [25].05 Thin film ceramic modules.0. and enhanced IC packages integrating antennas on their signal redistribution layers are all possible options for medium-channel-count systems. In spite of the strong ambiguities seen. The total number of antennas is 736 transmit and 736 receive antennas. populated with 16 antenna clusters. respectively. v) a E (x. whereas the physical apertures can be very sparse. which produces residual ambiguities in the resulting mm-wave image. Array geometry (red for Tx antenna lines.5 0 dB 80 40 y (mm) 0 –40 –80 –80 –40 0 x (mm) 40 80 –60 –40 –20 Figure 6.15 0. y) (5) (6) –80 –80 –40 0 x (mm) 40 80 –60 Figure 7. a novel array architecture was introduced in [22].15 −0. which is capable of compensating for the drawbacks of the near field operation. v) = A T (u. Their mathematical dependences are described in (5) and (6).15 40 0 0. Figure 8.3 −0.5 −0.25 0. y) ) ) a R (x.15 0.25 −0. Effective aperture of the multistatic array shown in Figure 5. Figure 5 illustrates the array geometry. September/October 2012 31 . respectively. This is achieved by keeping a well sampled effective aperture. y) = a T (x. Following the SPA design concept. y) of the multistatic array which results from the 2-D convolution between the transmit and receive aperture distributions aT (x.1 0.25 y (mm) 0 –40 −0.05 x (m) 0. y).1 −0.05 –20 –40 Figure 5. blue for Rx ones) [22]. the overall transmit-receive PSF shown in Figure 9 is free of any ambiguities. A E (u. [45]. LTCC modules.3 −0.25 −0.3 0. to an effective aperture aE (x. The background The main advantage of a SPA design is the reduction of the total number of antenna elements with respect to conventional dense arrays. Fig ures 7 and 8 show the point spread function (PSF) of the focused beam for the transmitter (Tx) and receiver (Rx) apertures. Point spread function of the Tx array [22].05 y (m) 0 −0. This effect 0. As the target distance L is similar to array dimensions.5 0.5 −0.3 0. Point spread function of the Rx array [22]. y) and aR (x.1 y (m) 0 −0. which is essential for generating images of high dynamic range after focusing.. If the design of monolithic integrated front ends can be afforded. which allow also for integration of multilayer planar antennas. III-V technologies still clearly outperform silicon based technologies and should be the preferred option for imaging systems with low number of channels. as microwave integrated modules based on thin film ceramic technology. Such systems require even higher integration levels of multichannel monolithic microwave integrated circuits (MMICs). interface losses are not negligible and the RF frequency generation have to move near to or into the analog front end. which allow also for integration of multilayer planar antennas. and higher integration is necessary. RF PCBs with chip-on-board technology. High-channelcount systems at high frequencies must integrate the antenna into multichannel analog front-end modules. possibly with included on-chip antennas. low loss but space-consuming interconnect technologies. High-channelcount systems at frequencies higher than 100 GHz have not yet been realized. Cost per channel is decreasing in this list. the space consumed by the front ends becomes critical. Analog/RF front-end modules can be 80 40 y (mm) 0 dB –20 0 –40 –80 –60 –80 –40 0 40 x (mm) (a) 80 –40 2 mm 15 mm 2 mm (b) Figure 9. 2) SiGe bipolar (or BiCMOS).g. as low-temperature cofired ceramic (LTCC) modules or as an RF printed circuit board (PCB).If the design of monolithic integrated front ends can be afforded. Since the availability of deep-submicron CMOS technologies with transit frequencies exceeding 200 GHz [46]. In high-channel-count systems. are suitable for frequencies up to 100 GHz. noise. or 3) CMOS. LTCC modules. Technology Choices The technology choices mainly depend on the chosen frequency bands (ranging from a few gigahertz to several hundred gigahertz) and the number of channels (ranging from a few ones to several thousands) present in the system. All the three technology classes.e. mostly available as connectorized microwave integrated circuits or waveguide modules at higher frequencies.g. output power and thermal stability. level is below -60 dB. e. (a) Overall transmit-receive PSF [22] and (b) 3-D rendering of the PSF. mature manufacturing processes that are suitable for mass production with good reproducibility are vital for achieving reliable results.. Figure 10 shows an image result of this system demonstrating the high image quality produced. Thin film ceramic modules. Regarding the RF performance. mechanically scanning ones.. The choice of semiconductor technology for mmwave imaging will be a never ending discussion depending on the addressed system parameters and the availability of manufacturing facilities. For low-channel-count systems. At high frequencies. With increasing channel count. built as waveguide modules. This is best achieved by developing dedicated multichannel Tx and Rx front-end modules. Integration density capability of III-V technologies is obviously lower 32 September/October 2012 . i. are mature and can be used for production with good reproducibility. showing the resolution cell size and the surrounding sidelobes [22]. three main technology options exist to realize mm-wave integrated circuits: 1) III-V technologies. can be used. including III-V due to its large utilization in mobile phones. as well as the front end has to be placed as near as possible to the antennas to minimize interface losses. the designer can rely on proven commercially available modules. For low-channel-count systems. RF PCBs with chip-on-board technology. are suitable for frequencies up to 100 GHz. waveguides. 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. The lateral resolution is of 2 mm in both directions. e. but should be high enough for first generation imag10 mm ing systems. antennas are required to be small in size . digital-to-analog converters (DACs). which makes CMOS phase centers should be stable over the beamwidth as not yet a feasible option. tionally. Last but (a) (b) not least. In [50]. Therefore. systems. CMOS offers the 2. horn. and a large design reuse from existing Tests with cavity backed circularly polarized spiral automotive radar modules reduces development costs antennas carried out in [49] showed positive aspects and guarantees a short time-to-market. the size of the antenna structure must allow The choice of the used antenna is of central imporfor dense sampling of the wavefront at less than the tance for any imaging system. Each total expected product volnail is of 5-mm diameter with an approximate radar cross-section of –50 dBsm. and DSP units. On a long term perspective. Next higher of polarimetric imaging. polarization purity becomes an issue when [48]. Typical antenna a fraction (less than a tenth) of a 40 nm CMOS mask types used in imaging systems include for instance set. On more digital and analog modules together. which includes a nowadays relatively polarimetric imaging system was introduced.5 mm best capability of integrating RF front ends. production cost is clearly lower than for the III-V slotline. requirements to ensure proper operation. and on the other hand miniature antenna antennas must couple the electromagnetic wave to the design offers a feasible integration with MMICs for medium of propagation while following certain design successful array integration. With respect to the Air Force (USAF) test chart made of a metal sheet and mounted in front of a bed of nails with absorber in their background. the cost of the producfield of view will cause image degradation. tion mask set is excessively high. Furthermore. one hand. a well as the bandwidth used. analog circuits. The nails are fixed to a grid of 10 mm distance. The slots of the USAF chart are separable down to the 2 mm openings [22]. they are all tion costs must be taken into clearly visible. technologies. not only wafer producthe high dynamic range of the image and the low sidelobe levels of the system. gives the best cost effectiveness: mask set cost is polarimetric imaging is demanded. The imaging array operates from 70 to 80 GHz September/October 2012 33 . and dipole antennas. and to solve reliabil14 mm ity problems caused by hot carrier degradation. image quality is highly influenced by the used signal bandwidth which consequently must be supported QPASS System by the antennas. Antennas are often required to offer The Quick Personnel Safe Screening system (QPASS) high beamwidths as well as very stable phase centers. especially for large imaging Figure 10. a criterion which is difpure bipolar process. Illustration of the imaging capability of the multistatic system using a U.than SiGe bipolar or CMOS. technology choice will be determined by cost. was developed on the basis of multistatic DBF technolThe phase center describes a virtual point for a sphere ogy to target the application of close-range personnel center where the phase front can be approximated to screening at airports and critical infrastructure buildbe radiated from. subject to the challenges of solving design difficulties to meet the required performance at high frequency and high bandwidth. Integration of CMOS RF modules is still. baseband processing. Last but cheap 130 nm CMOS technology in order to integrate not least. Due to ume. which is already in use for mass ficult to achieve with many types of antennas. waveguide. patch. all on one die. analog-to-digital converters (ADCs).S. The image formation algorithms rely ings [51]. account but also development on the approximation of spherical phase fronts and costs and the cost for a production mask set. Transmit and receive wavelength. a promising design integration levels are possible by using SiGe BiCMOS based on differential stripline feeds for realizing a technology. Addimarket 77-GHz automotive radar applications [47]. For 65 nm hence any deviation from this assumption within the and 40 nm CMOS technology. however. From today’s point of view. Photograph of QPASS system (without cover) [55]. Cluster 47 Rx Antennas 94 Tx Antennas 2m 47 Rx Antennas A basic unit. The array design follows the same architecture as the one in Figure 5. Although being developed for a specific application. which is useful in many imaging applications. making a total of 6144 RF channels. 34 September/October 2012 . a cluster unit is shown [55]. and the complex reflected signals are simultaneously and coherently sampled by all Rx channels. as shown in Figure 11. [54]. mechanics. Therefore. Four of these units are again connected to a central board to form a complete array. This ensures proper illumination of the human body [52]. however extends to cover a two meters times one meter aperture. The volume in front of the system is illuminated sequentially by each of the Tx channels. DDS’s are preferred here due to their ability to switch frequencies very fast. which hence requires generation of coherent RF and local oscillator (LO) signals. system error correction is applied and the image is then reconstructed. namely a cluster. a signal distribution board. These sampled data are then processed. Digital-beamforming relies on accurate phase measurement for each Tx-Rx combination. heterodyne reception is favorable. Four of these units are integrated on a single PCB called an “IF-board” in order to serve four clusters simultaneously. Two arrays of square aperture are stacked vertically. reflections are calculated. with frequency-stepped continuous-wave technique. A dedicated synthesizer unit has been developed and optimized to generate the RF and LO signals around 20 GHz in order to ease signal distribution to all RF front ends. 1m Signal Source Figure 11. A dedicated digital back end unit. resulting in the complete imaging system.The choice of the used antenna is of central importance for any imaging system. has been developed. an IF-Board. which is suited for flexibly building imaging arrays of different geometries and sizes. Direct digital synthesizers (DDS’s) are used to generate the signals. the DDS’s can generate signals with a determined phase value. the system architecture features a highly modular design offering a flexible platform to address further applications [53]. On the right. which are derived from a highly stable oven-controlled crystal oscillator (OCXO). Signal Sources Distribution Network Single Array Front End 1536 IF Signals Acquisition Hardware A D Image Processing and Visualization fRF/4 DSP Front-end Control (fRF–fIF)/4 Synthesizer Control Control Unit Multicore Computer Figure 12. power supply. System block diagram of a single array. and cooling parts form together one unit. Four clusters. the frequency is multiplied by a factor of 256. and distributed to the clusters. where each includes 1536 Tx channels and 1536 Rx channels. including parallel analog to digital conversion and image reconstruction kernels. Contrarily to free-running oscillators. Then two of the arrays are connected to an industrial PC (IPC) via fast PCI Express connection. The system block diagram of a single array is shown in Figure 12. integrates 96 Tx and 96 Rx channels in one housing. After the DDS’s. 1 RF Ch. which are connected to aperture-coupled patchexcited horn antennas. RF Front End Each cluster contains 96 Tx and 96 Rx channels. housing. 3 Buf Follower RF Ch. Block diagram of the four channels Tx SiGe Chip.On each chip. Temp Sensor T MUX Analog Bus RF Ch. The chips are mounted in multilevel cavities. 2 RF RF Ch. Those elements are embedded in a RF multilayer PCB. which also carries two RF and two LO input ports. 4 Buf Enable Quadrupler PA Gain On/Off Figure 14. and for RF performance reasons. vias and longer bond wires have been avoided. Cut view of the multilayer PCB illustrating the integration of MMIC and the antenna structure inside the housing of the cluster. as shown in Figure 13. as the antenna’s differential feed lines run on an inner layer of the PCB. the RF and LO signals are amplified. The horn part of the antennas is integrated into the cluster QPASS was developed on the basis of multistatic DBF technology to target the application of close-range personnel screening at airports and critical infrastructure buildings. Both transmit (Figure 14) and receive (Figure 15) MMICs include four E-band channels and a central RF or LO distribution with frequency quadrupling. The analog front ends are built of custommade four-channel receiver and transmitter chips. A custom chipset has been designed for this system [56]. where four of them are used as internal reference channels. The center frequency of operation is 75 GHz Fastening Screw Horn Antenna Cover Patch Absorbing Material Tx or Rx Chip RF Part Cavity Bond Wire Heat Sink IF Part Slot Differential Line Thermal Vias Via Figure 13. September/October 2012 35 . quadrupled and distributed to four channels. the process provides polysilicon resistors with sheet resistances of 150 and 1. The SiGe:C base is deposited by selective epitaxy. 2 LO Buf Follower Temp Sensor Buf RF Ch.8 V are available. Photograph of the Tx chip with the integrated four RF channels (size 2.000 X/sq and tantalium-nitride (TaN) thin film resistors with a sheet resistance of 20  X/sq. respectively. It is based on a double-polysilicon self-aligned transis- tor concept with shallow and deep trench isolation.4fF/nm2 is integrated in a four-layer copper Figure 16. The process used for this chipset is a very cost-effective pure SiGe:C bipolar technology similar to the one described in [57].3  V supply voltage and the power consumption per channel is 145 mW for Tx and 180 mW for Rx. Figures 16 and 17 show photos of the Tx and Rx SiGe chips. Different npn transistor types with cut-off frequencies from 52 GHz to more than 200 GHz and collectoremitter breakdown voltages at open base (BVCEO) from 5 V to 1. Figure 17. Both chips are supplied from a single 3. 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. 1 RF Ch. A metal–insulator–metal (MIM) capacitor with Al2O3 dielectric and a specific capacitance of 1.2 # 2 mm2) [56]. 3 RF Ch.RF Ch. In addition to npn and pnp transistors. 36 September/October 2012 . with a bandwidth of approximately 10 GHz. An example transistor is shown in Figure 18. Photograph of Rx chip with the integrated four RF channels (size 2. A mono-crystalline emitter contact results in a small emitter resistance.2#2 mm2) [56]. 4 Buf LNA IF 1 IF 2 IF 3 IF 4 Analog Bus Figure 15. Block diagram of the four channels Rx SiGe chip [55]. together with the capability of integration with the Base Emitter Collector n+ Poly-Si p+ -Poly SiC p MonoSiGe: C (Base) Buried Layer STI (Shallow Trench Iso) p– -Isolation DT (Deep Trench Isolation) p– -Substrate (a) Collector Emitter Base Shallow Trench SiGe:C Base Deep Trench (b) Figure 18.metallization.) September/October 2012 37 . (Printed with permission from Infineon Technologies AG. [59]. Germany. [56]. The radiated peak power is approximately one milliwatt. MMIC frontends in a 2-D array with high element count. The use of an automotive-qualified bipolar process was furthermore very advantageous due to the reuse of 77-GHz mass market automotive radar designs [58]. resonant Antenna The planar antennas used in the system are optimized to fulfill the requirements of the imaging application. Munich. which is very low compared to communication devices. They offer a small footprint and a high bandwidth by using a differentially fed dipole. which enabled meeting design targets after just two design iterations. Transmission electron microscopy image and a schematic of a cross section for a npn SiGe transistor [48]. and image reconaperture slots. The collected reflection data are compared to reference channels built Figure 20. inside the system in order to chip integration. The IF signals are amplified and then digibeamshape are improved by a stacked cylindrical horn. Figure 19. y′ Phi x′ y′ Phi x′ dB 8 6 4 2 0 Theta z′ z′ Theta The Th T he h eta –8 –16 –24 –32 (a) (b) Figure 21. the copolarized component of the field is shown. Conversion and DSP Wilkinson Thin-Film Divider are performed in parallel. Resistors the system implements 2 x 1536 coherent digital receiver Two-Way Wilkinson Divider chains. Digital Back End The digital back end performs measurement acquisition. Figures 19 and 20 show photos of the integrated chip and the patch part of the antenna. digitization of IF signals. mobile phones. Input matching and struction. [61]. twelve samples are required to account for the channel and filter set3 mm tling times [55]. Chip integration in a multilayer PCB including the patch part of antennas shown on the right side [62]. The internal layers of the PCB used to realize the antenna are illustrated in Figure 13. down-converted digitally to zero IF and subsequently filThree-Way Cavity tered. For each single measurement. system control and monitoring. 38 September/October 2012 . [62].. The radiated peak power is approximately one milliwatt. On the left. system error correction. The signals are further together with a via-ring cavity in the substrate [60]. which is very low compared to communication devices. tized by an eight-channel ADC chip at 50 MSa/s. as which also enhances isolation to neighboring elements shown on the left of Figure 22. e. which is necessary to achieve the short measurement time.g. Photograph of the cluster without housing showing the signal distribution. and the patch part of the antennas [61]. The antenna has a wide beam with approximately 8 dB gain and delivers high polarization purity.Differential Line Control and IF Signals Miled First Cavity Patch Antenna 20-GHz Input Supply Chip Mounted into Miled Second Cavity Ground Contacts 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. and a patch element. The simulation results of the antenna at 75 GHz for both the copolarized and the cross-polarized components are shown in Figure 21. Simulation of the radiation pattern of a single antenna showing the polarization purity and the beam quality. and on the right the cross-polarized component. ∑ Calculate Reflections Reconstruction Kernels DDC95 IF Signals ADC95 Memory Controller Cache AGU Cache 32 High-SpeedInterfaces at 36 Gb/s 1.6 TOPS/s Figure 22. .IF Signals HSSI HSSI PCI Express . . Block diagram of the digital back end used in the QPASS system. 39 .9 Gb/s 349 Gb/s Memory Controller Legend 2 PCIe at 32 Gb/s 64 Gb/s e jω 3072 ADCs at 50 MHz 138 GS/s 32x ADC → Analog-Digital-Converter DDC → Digital Down-Converter 32x HSSI → High-Speed Serial Interface AGU → Address-Generating-Unit DDR3 1 GB DDR3 1 GB IPC → Industrial-PC 1536 Reconstruction Kernels 10. . Touchpanel IPC September/October 2012 DDR3 1 GB DDR3 1 GB 32x Cache 48x AGU Cache AGU Reconstruction Memory Controller 4x Memory Controller 2x Data Collection 32x 32x Data Acquisition DDC0 ADC0 e jω DDC1 ADC1 e jω . .15 Tb/s AGU 32 High-SpeedInterfaces at 10. The images can then be prepared for direct display or used for further image processing steps beforehand. In Figure 24.6 Tera-operations-persecond in order to deliver full image reconstruction in approximately two seconds. where red is close and blue is far relative to the imager surface. The high image dynamic range ensures images of 30 dB free of any artifacts. another image using colors is presented to demonstrate the 3-D content of the image. which stands as a competitive solution to optical scanners. to enhance the imaging capability of the system specifically for certain applications. The reconstruction hardware needs to perform 10. Figure 25 illustrates a detailed view of the pistol. allowing to image features of a few millimeters in size.4 0. Two concealed dielectric objects. In the application of personnel screening.8 –1 0.5 0 x (m) (a) –0. which also open the possibility for image processing techniques.4 –0. Many algorithms for object detection and classification are being either adapted or newly developed to deal with the rich 3-D image information delivered by the system in magnitude and phase. Image shows the magnitude information after being projected along range direction. Therefore. Such a feature is attractive to many applications addressing accurate 3-D modeling of surfaces.15 Tb/s collected by the system. the 3-D images are further processed with dedicated detection algorithms in order to automatically and anonymously find concealed objects of any potential hazards such as weapons or explosives. The digital back end offers a fast PCI Express connection to the integrated IPC. With the flexible and modular design concept for both the RF front ends as well as the digital backend.5 The QPASS system is capable to produce 3-D images of 30 dB dynamic range and 2 mm of lateral resolution.6 –0. Moreover.6 0. Figure 23 illustrates an example image of a person concealing two dielectric objects.Active imaging ensures image production with a high dynamic range. the system is also capable to detect depth variations down to 50 um [51]. including super-resolution algorithms. in order to minimize the transferred data rates inside the system. A cutting-edge realization of the digital back end has been designed to deal with the huge data rates of 1. Image of a person taken from 70 to 80 GHz [55].8 0.2 0 y (m) –0. compensate for any thermal drifts and thus ensuring high stability over long time of operation. Conclusion and Outlook (b) Figure 23. Figure 22 illustrates the signal flow within the digital back end and reveals part of its inherent complexity. are marked with red rectangles. and demonstrates the high system resolution. t he system ca n be reconfigured to adapt different imaging modes and can be geometrically modified to cover various aperture dimensions. which demonstrates the system capability to address personnel screening applications. Microwave imaging systems are exhibiting a continuous improvement in their performance combined with a remarkable increase in their 40 September/October 2012 .2 –0. thanks to its exceptional signal phase stability. The color codes the range information of each voxel. privacy issues can arise. which is used to transfer the reconstructed 3-D images in magnitude and phase. This corresponds to a phase measurement accuracy of ±5 ° in the reconstructed image. Then the image reconstruction takes place at each cluster unit in a parallelized fashion. 0. which is required by many applications where objects are to be found behind surfaces or inside volumes. liquid bag (up) and explosive simulant (down). lenses. Multistatic array architectures for industrial and security applications have been intensively investigated during the last years. and the increase in the computational power of modern computers and DSP units supper-second in order to deliver ports the DBF techniques on the other side. The applicability of these techniques are moving to cover the mm-wave range. As integration levels are getting higher.. Multistatic imaging allows for a huge reduction factor in the total number of needed channels. which integrates around 6. mirrors. modular concepts with combined analog and digital units are becoming reachable. The reflectivity image is here multiplied by the are currently realizable using these technologies. Image of a person concealing a P99 pistol on the back.g.000 coherent RF channels realized on SiGe technology and included as well an integrated image reconstruction unit. known from optics. and colored range information to visualize the 3-D content of higher frequencies can be supported with submount the image. which is required by many applications where objects are to be found behind surfaces or inside volumes. faster image reconstruction units. In addition. are clearly visible. Power consumption of the involved devices is much reduced. from the imager. The range changes from red to blue as close to far techniques.complexity and level of integration. and combined reflection-transmission (a) (b) imaging. and are even pushed to reach the terahertz band. and hence opens the opportunity for fully electronic solutions to be realized. Active imaging ensures image production with a high dynamic range. thus allowing for compact modular designs. The first steps towards a fully electronic solution based on multistatic systems and DBF technique have been made and proved to be efficient and affordable.6 Tera-operationson one side. the magazine. complex phased-array components are becoming less attractive for many applications. Instead. The advances in The reconstruction hardware needs semiconductor technology assist this development to perform 10. September/October 2012 41 . These technologies allow for an optimal image focusing at all range distances and are not restricted to focal lengths. respectively. Polarimetric multistatic imaging will increase the detection capabilities Figure 25. Semiconductor technologies are offering various options for system realization depending on cost and performance. software derived technologies are coming to the frontier of the state-of-the-art solutions. Frequency ranges up to 100 GHz Figure 24. Challenges are still there to build even more advanced imaging systems featuring full polarimetric imaging. Imagfull image reconstruction in ing systems based on reflectors. e. Many of the numerical complications caused by multistatic imaging are nowadays affordable due to the available computational capabilities. Photograph and mm-wave image of P99 pistol concealed by using methods based on ellipsometry behind a thick pullover and a leather belt. This is best demonstrated by the QPASS system. PCB manufacturing has been significantly enhanced to be a cost-efficient carrier to MMICs and antennas aside of each other. Metal features inside the plastic grip. or approximately two seconds. -P. Schmidt. and W. Chattopadhyay. J. N. July 2003. IEEE MTT-S Int. M. “Passive millimeter-wave imaging. Shoucri. Lee. Dec. 3567– 3576. [27] P. pp. [24] D. N. 1978. Ulaby. IEEE. C. 56. Semenov. Schiessl. Corredoura. [16] M. no. A. Schmidt. 12–14. [18] K. Miller. vol. 6th European Radar Conf. Baharav. C. 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 imagers. McMakin. Schmidt. Introduction to Radar Systems. A. Schiessl and S. [29] M. A. H. pp. Gill.” in Proc. Fung. pp. 42 September/October 2012 . L. K. vol. Ahmed. “Digital beamforming in SAR systems.. I. vol. “Active millimeter-wave video rate imaging with a staring 120-element microbolometer array. D.” IEEE Trans. F. no. Gebert. classification. Mehdi. pp. Targonski. D. [14] S. S. Cloude and E. Krautkramer. “Distant detection of hidden objects with a THz imaging radar. [4] J.” Geophysics. M. and T. 2009. Wehner.” Proc. Jördens. Krieger. European Radar Conf. Ultrasonics. 2000. vol. Moffa. C. New applications assisted by tailored algorithms for image processing. L. Geosci. C.-P. A.” in Proc. Fischer. and H. R. Eddy Current. High-Resolution Radar. National Defense Industrial Association.49. pp. Nov. H. Ahmed.” in Proc. F. vol. “Illumination properties of multistatic planar arrays in near-field imaging applications. 2003. no. Pozar. 2. [12] S. vol. Ahmed. IEEE Int Geoscience Remote Sensing Symp (IGARSS). 1997. R. [31] R. 5410. 620–641. and L. 1990. 4th revised edition. and interpretation will come up one after another.” in Proc.” IEEE Trans Microwave Theory Tech. no. no.16th Annu. [3] W.4. 6211. Goldsmith. A. New York: McGrawHill. 652–653. Rutz. serving their demands especially where x-ray or ultrasonic methods are not feasible. Moore. Geosci. vol. G. 39–50. New York: McGraw-Hill. Jansenb. Huber. Polarimetric Radar Imaging. pp. Magnetic Particle. 3. 1. and G. Grossman. Skolnik. [25] F. 6th European Radar Conf. 498–518. Pottier. 49. vol. 2001. pp. no. And at the same time. Boca Raton. Nondestructive Testing: Radiography. “Penetrating 3-D imaging at 4. 6840. (EuRAD).12. 12. 49. no. 287–296. 245–248. R. Bottger. N.” in Proc.. von Aschen. B. This can make such systems applicable for mass production and put them as an option for everyday use.-P. vol. “Millimeter-wave imaging system for personnel screening: Scanning 107 points a second and using no moving parts. European Radar Conf. B. Koch. F. V. Wilk. J. pp. [17] R.” in Proc. Oct. [2] C. Sheen. Schmidt. “Advanced digital beamforming concepts for future SAR systems. “Passive millimeter-wave imaging and security. 1981. and P. I. Essentials Of Medical Imaging Series. Pottier. pp. 2009. and E. Hubers. and L. [15] H. H. no.-W.45.” in Proc. 2011. N. Gumbmann. 2010. Ahmed. J. “Applications of terahertz spectroscopy in the plastics industry. 2009. T. Wietzke.-P. Microwave Theory Tech. Bordoni. Reading. C. Vieweg. Antennas Propagat. Bryllert.-S. 3629–3638.10. pp. 1. [20] A. Z. 1735–1739. no. 1962. and A. Ultrasonic Testing of Materials.2. “Migration by Fourier transform. 2–9.. pp. Krautkramer and H. pp. MA: Addison-Wesley. [28] D. 2011 [26] X. 2004. [11] A. Boston: Artech House. Younis. Schlecht. Zhuge and A. Skalare. the system prices will drop following the progress in semiconductor market. vol. [21] P. [9] F. Geosci. Lee. Schiessl. Remote Sensing. C. S. pp. vol. S. A. 2000.” IEEE Microwave Mag. “Design of millimeter-wave microstrip reflectarrays. Patyuchenko... “Three-dimensional millimeter-wave imaging for concealed weapon detection. A. 2011. 1979. [13] S. 43.” IEEE Trans. New York: IEEE Press. Luukanen.” IEEE Trans. Dengler. SPIE Passive Millimeter-Wave Imaging Technology IX.” IEEE Trans. Geosci. and L. pp.” in Proc.” IEEE Trans. Remote Sensing.. 2007.” in Proc. Berlin: Springer-Verlag.” IEEE Trans. 1. Ostwald. 2007 and the 2007 15th Int. U. 2008.In the near future. Sept. London: CRC Press. pp. D. VA. decomposition theorems in radar polarimetry. Quasioptical Systems. 71. pp. [10] R. Schmidt. Lee and E. pp. Security Technology and Exhibition. R.” in Proc. no. SPIE Radar Sensor Technology VIII. Krumbholz. 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 imagers.” in Proc. Havlice and J. M. 34. Williamsburg. Yarovoy. European Radar Conf. pp. Siegel. 41. “Medical ultrasonic imaging: An overview of principles and instrumentation. Ahmed. Schmidt. Richter. 275–278. Appleby. Smirnov. 1–29. In the near future. [5] J. “A sparse aperture mimo-sar-based uwb imaging system for concealed weapon detection. Baukus. 1996 [8] L. 617–620. Apr. Yujiri. 23–48. 1998. Taber. 2008. 1581–1592. Moreira. 180–183. Liquid Penetrant. “A review of target. Hall. Microwave Workshop Wireless Sensing. Jan. G. Younis. A. vol. 2009.-P. 67. 2004. [6] J. and L. References [1] S. 59. Feb. Gaussian Beam Quasioptical Propagation and Applications. 509–518. T.-P. E. Acknowledgment The authors would like to thank the German Federal Ministry of Education and Research for funding part of the presented activities. Infrared and Millimeter-waves. Gumbmann and L. A. and M. [7] S. ASM International. Wiesbeck.. Syrigos. 4. D. Terahertz Electronics IRMMW-THz. and H. S. vol.” in Proc.” IEEE Trans. Remote Sensing. “X-ray imaging for on-the body contraband detection. pp. S. Computed Tomography. Taenzer. [23] S.and 25-m range using a submillimeter-wave radar. Oct. Microwave Theory Tech. Local Positioning and RFID. Liombart. “W-Band imaging of explosive substances. 9. A. A. [22] S. and P.45–48.. no. Stolt. July 2010. J. “Near field mm-wave imaging with multistatic sparse 2d-arrays. S. N. “Millimeter-wave imaging with optimized sparse periodic array for short range applications. M. O.. Cooper. 195–522. Bushong. [19] A. Schiessl. D. [30] G. Louis. F. E. 1–4. “High resolution permittivity reconstruction of one dimensional stratified dielectric media from broadband measurement data in the W-band. Conf. 29–32. (EuRAD). K. vol. SPIE Teraherz Photonics. 1995. Microwave Remote SensingActive and Passive-Microwave Remote Sensing and Radiometry. F. 2006. and A. Remote Sens. 2011. vol. pp. “Automatic detection of concealed dielectric objects for personnel imaging. 2771–2778. “A novel fully electronic active real-time imager based on a planar multistatic sparse array. Schiessl. vol. and J.. A.” in Proc. M. Gumbmann. D. Schinko. Hauck. Gebert. Future Security Conf. M. pp. R. pp. “A fast multilevel domain decomposition algorithm for radar imaging. Soumekh. 5. O’Donnell.-P. Lockwood and S. Evers.” IEEE Trans. Reuter. H. Schiessl. Rest. “Design of sparse MIMO arrays for short range imaging applications. Fourier Array Imaging. Meeting Bipolar/BiCMOS Circuits and Technology.” in Proc.. European Conf. Reininger. Seller. S. European Microwave Conf. Schmidt.. Dehlink. T. 1175–1184. Methfessel. Boag. M..” in Proc. 1994. pp. Platz. Schmidt.” in Proc.-P. May 2012. 60. no.-P. and T. 4. and D. Treml. K. Reiband. Napier. A. Kaeferboeck. Ahmed. [41] P. Pratti. Moline. 5. N. Antennas and Propagation EuCAP2012. Schreiter. 2012. vol. [50] K. Moreira. Wintermantel. R. Lopez-Sanchez and J. Schmidt.. Antennas and Propagation (EuCAP).. [55] S. Sheen. Martineau. Mag. and L. S. J. “Sparse linear array design for a short range imaging radar. Minichshofer. R. [39] N. 178–181. no.” in Proc. [52] S.53–56. Microwave Theory Tech. Birschkus. S. 5789. Methfessel. 6th European Conf. Schmidt. Knapp. 2001. Griffin. pp..176–179. 1983. Ahmed. “RX-TX analog front-end module with 2 x 96-channels for mm-wave imaging systems. “Illumination of Humans in Active Millimeter-Wave Multistatic Imaging. and L.. [57] J. “Low power wideband receiver and transmitter chipset for mm-wave imaging in SiGe bipolar technology. D. M. Solid-State Circuits. Freq. D. 2012. Aerosp. no. July 2003. June 17–Apr. Yin. Aerosp. 2009. “Optimizing the radiation pattern of sparse periodic linear arrays. 2012. Maurer.” in Proc. Schmidt. A. [49] D. H. pp. R.-P. vol. Ultrason. Salerno. pp. Knapp. Asia-Pacific Microwave Conf. pp. H. pp. Meister. “Hardware realization of a 2 m # 1 m fully electronic real-time mm-wave imaging system. pp. P. Aerosp. 13–14. [40] R. Lukashevich. Zielska. H. A. Mar. EuSAR. Collin and F. Schiessl. SPIE. 1–4. (EuRAD). Topak. Methfessel and L. Methfessel and L. Dixon. J. vol. and L. R. and L. S. J. D. “3-D radar imaging using range migration techniques”. 84–87. “Synthetic-aperture radar processing using fast factorized back-projection. Hellsten. Gianesello. Ekers. Tiebout. Majied. pp. Ahmed. R. C. Millimetre Wave Days 6th ESA Workshop on Millimetre-Wave Technology and Applications and 4th Global Symp. Mar. Steinbuch. 48. Bottner. “The very large array: design and performance of a modern synthesis radio telescope. 4th European Conf.” in Proc. A. J.. IEEE Radio Frequency Integrated Circuits Symp. A. Schmidt. L. F. S. Moeller. Schmidt. 60. 27. Fortuny-Guasch. no. 3529–3532. Thompson. Kolmhofer. 2008. “Millimeter-wave technology for automotive radar sensors in the 77 GHz frequency band. EuCAP2010.. 11. [46] A. F. G. 1969. Stenstrom. J. Ahmed. T. Thurn. Prague. pp. European Radar Conf.. K. L. 2012.. [43] G. Maire.” IEEE Trans. vol. [42] G. “An RCP packaged transceiver chipset for automotive LRR and SRR systems in SiGe BiCMOS technology. F. T. Knapp. Forstner. H. and E. Mar. Trivedi. and F. no. J. J. “A 77GHz 4-channel automotive radar transceiver in SiGe. [45] F. and G. R. Jager. 2011. Schnabel. Bock. 2008. Englewood Cliffs. Wandzura. F. “A waveguide to differential stripline transition for planar mmWave antenna measurements. June 1993. Kissinger. V. pp. Antennas Propag. pp. 17. D.” in Proc. V. 2011. G. Fischer. McMakin. B. Genghammer..-P. Wurzer. J. “Near field mm-wave imaging with multistatic sparse 2d-arrays. “SAR data focusing using seismic migration techniques.[32] M. and R. Jan. M. [37] R. 35. Espoo. Oct. [61] S. Schiessl. S. Krieger. 45. [33] C. EuSAR. E. Electron. Antenna Theory Part 1. Boguth. Juenemann. 1991 [34] J. and D. 6th European Radar Conf.” IEEE Trans. Shun-Meen. Cathelin. A. [60] S. [56] M. Syst. 728–737. 7–14. Ahmed. M. “Digital beamforming on receive: Techniques and optimization strategies for high-resolution wide-swath SAR imaging. [35] A. Electron. no. May 23–25. pp. “Circularly polarized millimeter-wave imaging for personnel screening”. 1755–1757. June 17–Apr. Weigel. 49. R. pp. Schmidt. A.. 117–126. 3. in Proc.-P. Foster. IMS 2012. “SiGe bipolar technology for automotive radar applications. Hasch. 1996. Barcelona. P. [59] S. Dec. pp.” IEEE Trans.-P. Waldschmidt. “Deep-submicron digital CMOS potentialities for millimeter-wave applications. 2005. 43. S. W. Mar. A. 2010.” IEEE Trans. Perndl.-P. 2012. Schmidt. “QPASS—Quick personnel automatic safe screening for security enhancement of passengers. Ghazinour.” IEEE J. IEEE. vol. vol. May 2011. Zucker. R. and L. F. Morgan.-P. H. [62] A. Ahmed and L.” IEEE Trans. 194–207. “High speeds in a single chip. S. Electron. 564–592.-P. R. Schafer. 1994. no. and L. 2009. H. S. P. vol. pp.” in Proc. 47. Antennas and Propagation. Apr. H. 778–794. S. [44] S. H. T. W. Schafer. no. P.1653–1656. Microwave Theory Tech. Cenanovic. S. L. “The fast multipole method for the wave equation: A pedestrian prescription. Y. Schiessl. Wuertele. M. [36] L. no. F. 2012.” in Proc. Stengl. Trotta. Lachner. R. Aufinger. 2004. 7–12. 7. Russer. Apr. [54] S. 3. 1295–1320. 40–43. “Design of a balanced-fed patch-excited horn antenna at millimeter-wave frequencies. Druml. C. U. S. 1497–1502. Rome. Ultrasonics Symp. H. New York: McGraw Hill.” in Proc. Syst. and S. and C. vol.. Pacheco. Aufinger. pp. Boguth. pp. W. Rest. P. Lockwood. [47] J. C.. 28–33. J. 1. 3. M.” in Proc. Zwick. 2009. R. M. vol. 845–860. Contr. Coifman. T. 71. Syst. Jammers. 39. Rokhlin. pp. pp. Schiessl. Belot. B. September/October 2012 43 . and A. Ostwald. vol. Tran. 10. M. M. no. May 2000. Samulak. M. vol.” in Proc. Rocca. K. Schmidt. C. Aaron. [58] H. Weigel. NJ: Prentice Hall. no. and J. R. E. [48] E. Cafforio.” in Proc. [51] S.” in Proc. 233–236. and L.-P. and R. Ulander. S. 760–776. pp. “A novel active realtime digital-beamforming imager for personnel screening.” in Proc.” Proc. pp. 2012. John. Ahmed et al. Gumbmann.” IEEE Antennas Propag. IEEE Trans. A. 666–671. 180–183. Li. Raynaud. Schmidt. 17.. Antennas Propag. pp.” IEEE Microwave Mag. 2009. Finland. D. Millimeter-waves GSMM. E. Starzer. O. “Optimizing sparse two-dimensional transducer arrays using an effective aperture approach. 1.” IEEE Trans. Meister. Lechelt. and A. [53] A. Sept.-C. S. Foster. RFIC IEEE Radio Frequency Integrated Circuits Symp. Wohlmuth. S. Juenemann. [38] F. Kasper. A. “Fully electronic active E-band personnel imager with 2 m 2 aperture. 2. vol. 2. 29–Nov. RFIC. Gumbmann. Li. Bock. pp. “Broadband polarimetric mmwave antennas with differential stripline feed. S.