ET4266 Introduction to Ultra-Wideband Systems

and AntennasHuman being imaging for concealed using UWB radars

and mm-waves

Eugenio Pasqua 1399136∗ Fernando Moinelo Delgado 1375377†

June 23, 2008

Abstract

The objective of this report is to investigate the current research onhuman being imaging for concealed weapon detection, with a particu-lar attention on UWB radars and mm-wave systems. Ultra-wideband(UWB) systems and antennas are a relatively new but mature tech-nology which still has a lot of potential. UWB systems use signalswith a ultra-wide bandwidth, which can be obtained both in the time-domain, by means of short pulses with instantaneous ultra-wide band-width, and in the frequency-domain, with the use of multiple carriers.UWB radars have a finer resolution than conventional systems, as theycan gather more information of the detection space with better qualitythan conventional radars. Although at the present days UWB imagingradars are mostly used in SAR for airborne applications, such tech-nology has characteristics which can potentially improve the currentstate-of-the-art imaging systems. Mm-waves radar imaging systemsuse the frequencies between 30 and 300 GHz, at which electromag-netic waves can easily penetrate the common clothing. At the momentmm-waves are one of the better available technologies for security ap-plications and are consequently described in detail.

∗E.Pasqua@student.tudelft.nl†F.MoineloDelgado@student.tudelft.nl

1

CONTENTS 2

Contents

1 Introduction 31.1 Overview of UWB technology . . . . . . . . . . . . . . . . . . 31.2 UWB radar . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Methods for CW Detection and Imaging 62.1 Introduction to CWD-I . . . . . . . . . . . . . . . . . . . . . 62.2 Imaging technologies and methods . . . . . . . . . . . . . . . 7

2.2.1 X-ray imager . . . . . . . . . . . . . . . . . . . . . . . 72.2.2 UWB and mm-wave imaging systems . . . . . . . . . 8

3 Conclusions 12

References 14

1 INTRODUCTION 3

1 Introduction

1.1 Overview of UWB technology

UWB technology is recently gaining high attention in the Telecommunica-tion field. More and more opinions indicates Ultra-wideband (UWB) tech-nology as “extremely simple and cheap to implement” and “very adaptive,using different frequencies as circumstance require”. The Federal Communi-cations Commission (FCC) has given the following definition of UWB: “anintentional radiator that, at any point in time, has a fractional bandwidthequal to or greater than 0.20 or has a bandwidth equal to or greater than500 MHz, regardless of the fractional bandwidth” [2], where the fractionalbandwidth is defined as

∆ffract = 2fup − flo

(fup + flo)

and the absolute bandwidth is

∆f = fup − flo

with fup and flo defined at the −10 dB level.FCC made also several regulations for UWB:

• frequency allocation: 3.1 to 10.6 GHz

• low radiated power (max −41.3 dBm)

• some roll off allowed at minimum and maximum frequencies

• stringent requirements for interference with GPS (at 1.5 GHz)

However, this formal definition defines an UWB signal only based on thebandwidth of the signal, completely ignoring the properties of the objectsinteracting with the signal. Many researchers prefer to define an UWBsignal as a signal whose spectrum is wide enough to cover the essentialspectrum (resonance frequencies) of the objects with which interacts. Anexample of an UWB “system” could be the human eyes: they work likepassive antennas, covering the essential spectrum of light. This spectrumbandwidth is wide enough to permit us to recognize the objects around us.

Ultra-wide bandwidth can be obtained with two different approaches,in the time-domain and in the frequency-domain (see fig. 1). In the time-domain approach or Impulse UWB (I-UWB), an UWB antenna transmitsvery short pulses (hundreds of picoseconds, fig. 2), which have an instanta-neous ultra-wide bandwidth. The pulses are transmitted without a carrier,which eliminates the need of expensive oscillators (mixers) at the receiver.Although the average transmitted power is low (orders of microwatts), ashort-duration pulse requires a high peak transmit power, and it can possibly

1 INTRODUCTION 4

Figure 1: UWB system types

interfere with the existing narrowband services. Anyway this requirementcan be relaxed with pulse compression.

In the frequency-domain or Multi Carrier UWB (MC-UWB), the ultra-wide bandwidth is obtained using multiple carrier frequencies. At any mo-ment the system works with a narrow bandwidth signal, whose frequencyvaries over time. This can be done essentially in two ways: the steppedfrequency approach, in which the frequency of the signal jumps from a fixedvalue to another, waits for a fixed dwell time and then increases again untilthe whole bandwidth is covered; and the swept frequency approach or Fre-quency Modulated Continuous Wave (FMCW), where the frequency slowlyvaries with time, covering the whole operational bandwidth. An approachanalogous to stepped frequency CW is the Spread Spectrum, where the fre-quency changes according to a Spread Code or sequence of frequencies sothat the signal is spread over a much wider bandwidth than is needed for theinformation being sent. With the appropriate spread codes is then possibleto avoid the frequencies with narrowband interferences, making the signalboth difficult to intercept and robust. Another different approach to UWBsystems are OFDM systems which use multiple carriers simultaneously con-figuring an instantaneously wide bandwidth. In both the time-domain andthe frequency-domain approach, UWB technologies has advantages in manyother ways.

1 INTRODUCTION 5

Figure 2: UWB pulses

1.2 UWB radar

Radar is one of the main application of UWB technologies since its earlyyears. The down-range resolution ∆R is indeed related to the operationalbandwidth B of the signal:

∆R =c

2B

with c being the speed of light in the medium. The cross-range resolution,or lateral resolution is given by

∆A =πr

2L= r · θB

where λ is the wavelenght of the radar signal, r the distance of the target,L is the antenna length and θB is the antenna beamwidth.

UWB radar has a very wide bandwidth and thus a very fine down-rangeresolution. The main advantages of a wider bandwidth consist in improvingthe accuracy of range measurements as it has higher resolution. In addition,it reduces the effects of passive interferences and improves the immunity tonarrowband radiation and noise.

Narrowband systems use sinusoidal signals as carriers for the informa-tion. The sinusoidal waveform is generated by oscillators that could besimple RLC circuits or expensive VCO. Also, sinusoidal waveforms keeptheir shape when linear systems are used, with changes only in amplitudeand phase. In the other hand UWB could be carrierless and so does not

2 METHODS FOR CW DETECTION AND IMAGING 6

Figure 3: UWB radar applications

require any mixer at the transmitter. Also, this implies that the changesof signal shape in UWB occurs during the detection and ranging processes.Antennas do not radiate in the range of frequencies closer to DC, so essen-tially the antenna is a high-pass filter that derivates the shape of the signal.If it is assumed that the UWB signal lenght is less that the size of the localscattering elements of the target, each discrete target reflects the signal andforms a pulse sequence called target image which can be used to identify thetarget.

UWB radars have many different applications, like synthetic apertureradars (SAR), stealth targets detection, but are also remote sensing at shortdistances for several other applications (fig. 3). Radar imaging is certainlyan interesting application of UWB radars, due to the essential increase ofthe quality and the quantity of the information which can be gathered.However, nowadays the use fo UWB signals for image formation is limitedto SAR systems mounted on airborne platforms.

2 Methods for CW Detection and Imaging

2.1 Introduction to CWD-I

Greater security at sensible location like airports, banks and many otherpublic buildings is needed because of the risks linked to terrorism, like hi-jacking or sabotage, but also smuggling incursions. A concealed weapondetection and imaging system is designed to find items considered to beweapons or in general objects that can be a threat to the public security.

2 METHODS FOR CW DETECTION AND IMAGING 7

Research in this field is addressed toward the development of an effective,non-invasive method to screen people for the presence of concealed weaponor contraband. Technical challenges are decreasing the false alarm rate andimproving the velocity of processing for real-time imaging systems.

The first technology used to check for concealed weapon and contrabandwas a metal detector. It was first employed in airports in the early ’70s in awalk-through portal security device for passenger screening. Metal detectorscould be active or passive types. An active detector produces a time-varyingmagnetic field that interacts with any electrically conductive or magnetizablematerial, generating eddy currents in it that are then detected. The moreconductive is the material, the stronger is the response of the object. Thus,such a system is only able to detect highly conductive materials (most ofwhich are metals), and unless it is provided with multiple detectors, it willnot be able to locate the object. On the other hand, a passive metal detector(like a gradiometer), is capable of detecting variation of the local magneticfield of the earth which are caused by the presence, in the detection space, offerrous materials. Though these systems were continually improved over theyears, metal detectors remains limited to the detection of metal objects, likehand gun or box cutters, and are useless against plastic or ceramic threats.Moreover they cannot discriminate between simple objects as coins, keys,metal bottons, etc. and actual threats, leading to a rather high number offalse alarms.

Metal detectors are in general detection-only systems, and they providepoor information. To obtain better performances and more information,some detection systems use an image-based detector: the system acquireimages of the detection space, and then compare the acquired image withan electronic database of images or signatures recorded for a large numberof possible threats, like handguns, knifes, etc. With this approach, thesystems is able to recognize the target, if its signature is already presentin the database. The problem is that in order to be sure to recognize anobject, the system needs to store all the different signatures. This means asystem of this kind would require a large information storage capability tocontain all the images and an image processing system, and would be muchmore expensive than a detection-only system.

2.2 Imaging technologies and methods

2.2.1 X-ray imager

An X-ray system uses low-power collimated x-rays to illuminate the sub-ject under control, performing a 2-D scan. An image of the body is thenreconstructed using the intensity of X-rays backscattered by the body viaCompton scattering. Such a system has a very good spatial resolution, whichpermits to identify possible threat items of every type of materials. X-rays

2 METHODS FOR CW DETECTION AND IMAGING 8

are a form of ionizing EM, and they can penetrate a few mm into a humanbody. This penetration capability is not enough to find items concealed intothe body cavities, or in general covered by heavy flesh. Moreover, the ion-izing radiation of such system poses serious health risks to the people beingscanned. This systems also have privacy issues. X-ray systems produce highclarity images which contain very detailed anatomical information. In mostcases, displaying anatomical details of a person is considered a violation ofthat person’s privacy.

2.2.2 UWB and mm-wave imaging systems

Among the emerging technologies, millimeter-wave (mmw) imaging systemsare one of the most inquired in security applications. Millimeter waves aregenerally between 30 GHz and 300 GHz, with wavelength ranging from 1to 10 mm. These wavelengths are quite longer than optical wavelengths,and they can penetrate many optical materials, such as common clothing.Moreover, millimeter waves are non-ionizing, so they don’t have any healthissues when used at low power, making them ideal for security screening ofpeople. They also have less privacy issues than x-rays (there are algorithmswhich can cover some anatomical details on the image). The fundamentalmechanism on which radars for human being imaging are based, is verysimple: the subject under control is illuminated by an incident field Eiradiated from the radar. The radiation is then scattered by the body, andthe scattered field Es is measured over the detection space and used to forma image of the body by means of a reconstruction algorithm.

The way EM waves scatters from a surface depends and their frequency,and on the reflectivity of the surface, which in turn is related to its physicalproperties. For concealed weapon detection systems, in order to detect anyitem concealed over a human body, it is necessary that such items have adetectable reflectivity contrast with the human body. At millimeter-wavefrequencies, metal objects like guns or knives have a high reflectivity con-trast with the body, while non-metal objects, like plastic or ceramic, havea smaller contrast. Mm-waves can easily penetrate clothing, but they donot penetrate deeply into the human body. Indeed, the human body highlyreflects frequencies higher than 15 GHz. This means that the sources ofthe scattered field are distributed over the body surface, and reconstructingthe 3-D distribution of the scattered field it is possible to reproduce theshape and the curvatures of the body, with anomalies which could indicatethe presence of objects with different physical properties concealed over thebody.

The quality of the 3-D images is characterized with respect to the pointscatterers by the size of the voxels (or volume pixels or cells). Each volumecell represents a distinct volume on a regular grid in the 3-D detection space,which can be filled with the measured field data. Voxel data usually have

2 METHODS FOR CW DETECTION AND IMAGING 9

Figure 4: Holographic system configuration

low resolution, as precise data are only available at the center of each cell.Anyway the data within each cell and their amount characterize the imageresolution. As the size of voxels is dependent on frequency, it is possible toincrease the volume which is covered by data and thus the resolution by usingwide-band signals, as well as a CW source swept over a large bandwidth.

In mm-wave systems, imaging of the detection space is typically achievedwith two different techniques, which are also suitable for lower microwave fre-quency bands. The first technique uses a focal-plane 2-D array of millimeter-wave antennas placed at the focal point of a large lens system. The scatteredfield is collected at the lens aperture, which focuses the RF energy into smallvolumes within its field of view. This makes possible to scan image voxelsin front of the lens, and to display a 3-D image in real-time. Anyway, such asystem also have some disadvantages, as a low resolution, a small apertureand a limited field of view, in addition to the high costs due to the 2-D array.

The second technique, called holographic imaging, is a means of 3-Dimaging of targets from data measured at different points of a 2-D aperture.With this technique the detection space can be rapidly scanned to effec-tively illuminate a target and the collected coherent returned signal can berecorded and mathematically reconstructed in a computer to form a focusedimage.

A simplified millimeter wave transceiver is shown in fig 4. A millimeter-wave signal is generated by a voltage controlled oscillator (VCO), whosefrequency is swept over the bandwidth of interest. A signal whose frequencyincreases or decreases with time is called a chirp signal (fig. 5). The chirpsignal is then transmitted using a wide-band antenna (e.g. a small pyra-midal horn antenna), reflected from a target and received by the receiveantenna. This antenna is typically of the same type of the transmitter, and

2 METHODS FOR CW DETECTION AND IMAGING 10

Figure 5: a) chirp signal; b) spectrum of a chirp signal; c) frequency variationwith time

it is positioned approximately in the same location of the transmit antenna.With such a configuration, also referred to as quasi-monostatic, the two ad-jacent antennas simulate a single transmit and receive antenna, with theadvantage of a best transmit to receive isolation compared to a monostaticconfiguration. The received signal is then divided into an in-phase compo-nent (with 0 phase shift) and a quadrature component (with a phase shiftof 90), which are coupled to the VCO. The complex signal is then sampledover a 2-D aperture using a 2-D scanner, and finally inputed to the imagereconstruction algorithm which form the image. The reconstruction algo-rithm in most cases involves some form of back propagation, back projectionor time-reversal procedures.

After the holographic image are reconstructed, they can be sent to anddisplayed in a video monitor where a trained operator can detect and identifyobjects concealed over the person under control or, in cases where displayingof images with detailed anatomical features may have privacy issues, an ad-ditional computer image recognition system can automatically identify thetarget, by comparing this image with an electronic database of all the pos-sible threats. The advantages of holographic imaging are higher resolution,the ability to mathematical focus at any single depth and a large aperturewhich allows a full-body field of view.

An example of such a system for CWD has being developed by theBattelle Pacific Northwest National Laboratory (PNNL) [7]. It consistsof a sequentially switched linear array of antennas driven by a wide-bandmillimeter-wave transceiver, both mounted on a fast mechanical scanner.During the operations, the array is electronically scanned over an horizontalaperture of 0.75 m, while being mechanically swept over a vertical apertureof 2 m. The system uses coherent illumination and detection (magnitudeand phase) of the scattered wavefront. The measured data are digitalizedusing an A/D converter and stored in a computer. After a full-aperture ofdata is collected, they are focused to form a full 3-D image, using an imagereconstruction algorithm developed at PNNL. The use of a wide bandwidth

2 METHODS FOR CW DETECTION AND IMAGING 11

Figure 6: Wideband (27-33 GHz) images of a man carrying two handguns,one checkbook and one leather wallet

is essential in this application, as it improves the down-range resolutionallowing the whole body to be in focus at the same time. The 3-D image isthen collapsed into a 2-D image, which can be displayed on screen.

Imaging results of this systems, in the bandwidth (27-33 MHz), are de-picted in the figure 6. The person under test was carrying two handguns,one tucked at the belt line under the man’s shirt, and a second small one inthe man’s left pants pocket, as well as several innocuous items. The imageswere taken from different angles. As it can be see in fig. 6 the objects thatwere concealed are quite evident. In the first image is clearly visible the gunconcealed under the belt. In the second image, the small handgun concealedin the left pocket shows up evidently. In the third image a paper checkbookinserted in the left back pocket is also visible. In the forth image, a leatherwallet on the man’s rigth back pocket is shown. Similar systems have alsobeen built at PNNL, operating at different bandwidths as from 22 to 47.5GHz, 40 to 60 GHz and 90 to 120 GHz.

Such systems could be farther improved in resolution using an UWBarray antenna to scan the detection space, although at the present days thisapplication of UWB radars is still in a development stage. UWB radarscould be used in holographic systems, for example, to achieve higher res-olution at much lower, more practical parameters, and with fewer arrayelements than conventional systems. The UWB radar arrays also have theability to focus received energy into the main lobe by adding elements. Thispractically eliminate the side-lobe effects that limit the sensitivity of conven-

3 CONCLUSIONS 12

tional systems. Moreover, only low transmit power is required due to highersensitivity and wider dynamic range in the receiver. Imaging the obtainedreturns from the steered UWB array is principally a matter of implementingthe appropriate processing software for a correlated merge of multiple 2-Dimages of the target object(s).

The systems described above, all use active illumination of target tomeasure the scattered returns. A passive system detects the natural radi-ations that all objects naturally emit and reflect over a broad spectrum offrequencies. The level of radiation emitted by an object is determined by anumber of factors such as the physical properties of the object and by itstemperature. The human body is an especially good emitter of millimeterwaves, with near to unit emissivities. By contrast, at millimeter wave fre-quencies metal objects have emissivities of about 0.2 or less, thus being verypoor emitters and excellent reflectors of radiations. Dielectrics objects suchas plastics and ceramics, have emission properties that are between those ofthe human body and metals.

Passive millimeter wave imaging systems typically use two basic mech-anism of detection. A first mechanism measures the temperature contrastcaused by emissions of the human body, which has a temperatures of about37 ℃, and the background radiation at ambient temperature (about 25 ℃)reflected by the concealed items. Metal object show up quite well with thistype of measurements, as they have a very high reflectivity. The secondmechanism utilizes the fact that objects concealed over the surface of a hu-man body will partially block the emission of the body itself, substituting itsown at lower temperatures. This mechanism is exploited for the detectionof non-metallic items.

3 Conclusions

In this paper an overview on current state of human being imaging for con-cealed weapon detection has been presented, with particular attention onultra-wideband (UWB) radars and mm-waves systems. Among the avail-able technologies, mm-wave are one of the most effective systems for the de-tection and identification of concealed threats. An example of this system,developed at the Pacific Northwest National Laboratory, has been presented.The images taken show the high resolution and high fidelity of the imagethat are available with this method. In spite of the good results showed,there still remains some important problems to be solved.

First of all, it is still an open issue which frequency band is better forconcealed weapon detection. The achievable resolution depends on the thebandwidth and the wavelength of radiations. The use of higher frequenciesallows to obtain finer resolution, but it requires high-frequency transmittersand receivers which at the the current state are more expensive than the

3 CONCLUSIONS 13

components for lower frequencies. Moreover, higher frequencies are alsocharacterized by high atmospheric attenuation, mainly due to the influenceof water vapour.

A second question regards the type of antenna which can be used. In theearly holographic imaging systems, a single transceiver was used to scan bothdimensions of the detection space. Such an embodiment would require quitea long time to mechanically scan the entire 2-D aperture and reconstructthe image, and it is thus impractical for CWD purposes. On the other side,with a 2-D array antenna, the sensors are electronically switched over theaperture. The scan is performed without the need of any moving part, thusallowing for real-time operations. The major drawback of such antennas isthe high cost of the antenna system. At the current state, a linear array ofantennas is a good compromise between the above solutions, as it can bescanned quickly to gather full-aperture image data.

Further improvements can be obtained using UWB systems. AlthoughUWB imaging radars have not still been widely tested for human beingimaging for concealed weapon detection, they are a very promising method,which could further improve the resolution and the accuracy of the systemsdeveloped since the present day.

REFERENCES 14

References

[1] N.G. Paulter, Guide to the technologies of concealed weaponimaging and detection NIJ Guide 602-00, 2001. Available:http://www.ojp.usdoj.gov/nij/pubs-sum/184432.htm

[2] FCC Ultra-Wideband Notice of Propose Rule Making (NPRM) FederalRegister, vol. 65, no.-l15, 14 June 2000; modified 2002 3

[3] A.G. Yarovoy, Ultra-Wideband Systems Proceedings of the 33rd Eu-ropean Microwave Conference - Munich 2003

[4] D.J. Taylor, Introduction to ultra-wideband radar systems CRC Press,1995.

[5] W. Steinway, G. Stilwell, H. Duvoisin, III, D.H. Fine, ConcealedObject Detection CyTerra Corporation, U.S. Patent 6 831 590 B1, Dec.14, 2004

[6] D. M. Sheen, D. L. McMakin, H. D. Collins, T. E. Hall, andR. H. Severtsen, Concealed explosive detection on personnel usinga wideband holographic millimeter-wave imaging system AEROSENSEaerospace/Defense Sensing and Controls, Orlando FL,USA, Proceedingsof the SPIE, Vol. 2755, 1996.

[7] D. M. Sheen, D. L. McMakin, and T. E. Hall, Three-DimensionalMillimeter-Wave Imaging for Concealed Weapon Detection IEEE Trans.on Microwave Theory & Tech., Sep. 2001, p. 1581-1592, vol. 49, No. 9,USA. 10

[8] J. Detlefsen,A. Dallinger, S. Schelkshorn, Approaches tomillimeter-wave imaging of humans In European Radar Conference Eu-RAD 2004, Amsterdam, The Netherlands, October 2004, pages 279-282.