Through-the-Wall Radar Life Detection and MonitoringVictor M. Lubecke, Olga Boric-Lubecke, Anders Host-Madsen, and Aly E. Fathy*,

University of Hawaii, Electrical Engineering Dept., Honolulu, HI 96822, USA

* University of Tennessee, Electrical and Computer Engineering Dept., Knoxville, TN 37996

Abstract - Technology that can be used to unobtrusivelydetect and monitor the presence of human subjects from adistance and through barriers can be a powerful tool for lawenforcement, military, and health monitoring applications. Tothis end, ultra-wide band radar has shown promise for real-timesubject imaging, and compact Doppler radar solutions havedemonstrated potential for providing non-invasive detection andmonitoring of cardiopulmonary activity for multiple subjects.These technologies work through walls and other obstructions,and can even leverage the presence of ambient radio signals toprovide a covert means to detect, isolate, and physiologicallymonitor multiple human subjects from a remote position.Practical applications ranging from counter-terrorism to healthmonitoring require systems that are accurate, affordable, andeasy to use. Current research efforts addressing these challengesthrough radio, signal processing, and sensor networking will bepresented.Index Terms - Through wall, STTW, UWB, Doppler, life

signs, passive radar.

I. INTRODUCTION

As our nation is confronted with new security challenges,including asymmetric battlefield threats abroad and defenseinfrastructure needs back home, enhanced battle-spaceawareness and effective warfighter protection are essential.Expeditionary warfighters are commonly faced with unknownenemy threats from behind opaque barriers, for which extremeprecautionary measures must be encumbered to minimize risk.Borders and perimeters must also be secured with minimumburden to personnel and maximum likelihood for detectingany intrusion. Battlefield search & rescue and triageassessments must also be conducted in a manner thatminimizes the chance of increased losses. Technology that canbe used to unobtrusively detect and monitor the presence ofhuman subjects from a distance and through barriers can be apowerful tool for meeting these challenges.Both basic Doppler and ultra-wide band (UWB) radar have

been investigated for see-through-the-wall (STTW)applications, for their particular barrier penetration advantagesin short range detection and localization. While othertechnologies like millimeter-wave and infra-red imaging havedemonstrated good resolution through clothing and packaging,barriers in STTW applications generally involve higherdensity materials like reinforced concrete, concrete blocks,sheetrock, brick, wood, plastic, tile, and fiberglass. Thesematerials prove problematic as barriers both in the degree to

which they absorb incident signals and manner in which theyscatter the signal.

Early efforts in Doppler radar were directed at assisting withlaw enforcement and disaster rescue by sensing the motion ofa subject attempting to hide, or buried under rubble [1].Ideally, respiration and even heart-beat related motion are ofinterest as they cannot be fully suppressed. In suchapplications, discerning the subject's motion of interest fromextraneous motion and stationary clutter is a significantchallenge, as is the isolation of multiple subjects [2,3]. Withthe advent of practical high speed sampling and fast pulsegeneration technology, UWB radar has become attractive forlow power through-wall personnel motion sensing, with aparticular interest in constructing a real-time image [4]. Thistechnology is also hampered by clutter and motion isolationissues, as well as persistent wide band hardware challenges.Wall loss, hardware, regulations, and probability of detectionalso affect transmit power levels, frequencies, and modulationchoices, ultimately limiting the effectiveness of any radar.To meet these challenges, cuffent Doppler and UWB STTW

radar research includes complex propagation channelmodeling based on real-world urban environment expectationsand multiple wall scenarios, advanced algorithms for real-timevital signs detection and high resolution imaging algorithms,antenna arrays and multiple receivers for increased resolutionand estimation accuracy, passive exploitation ofenvironmental signals, and compact radio integrationtechnology. Both a description of fundamental concepts andexamples of current research in UWB radar for imaging,Doppler radar for life-signs isolation and detection, andmultistatic radar techniques for passive personnel detectionare presented in the following sections.

II. ULTRA-WIDE BAND IMAGING RADAR

UWB radar is based on the comparison of echoes from shortduration pulse transmissions to detect small changes over timeresulting from target motion. In the simplest sense, the timesignatures (impulse response) of successive echoes from aroom containing a moving target would be identical, except atthe point in time associated with the signal returning from theposition of the moving target. Thus, differencing successivesignatures would produce an output that corresponded only tothe moving objects presence and range. The primary

1-4244-0688-9/07/$20.00 C 2007 IEEE 769

advantages of UWB for short-range radar imaging includeextremely fine range resolution (theoretically sub-centimeterresolution), high power efficiency because of low transmitduty cycle, potential for low probability of detection and lowinterference to legacy systems, and ability to detect moving orstationary targets. Security oriented products of this type likeRadar Vison by Time Domian are already available in theform of a briefcase sized system that can be held against awall or tripod mounted [5].The long term goal of military field imaging technology is

the rapid detection of enemies' maneuvering; throughacquiring relatively high-resolution images using advancedmultidimensional image processing, pattern recognitiontechniques, and fast data processing. An example blockdiagram of such a radar system, developed at the University ofTennessee, is shown in Fig. 1. The system consists of an RFT/R board, UWB antennas, a digital timing and control board,and an imaging processing software module [6].

Capturing the data is the most challenging task for a systemwith a 1 GHz bandwidth, as Nyquist's sampling theoremrequires a sampling rate of over 2 GS/s. In order to resolvedistances on the order of one centimeter, rates exceeding 10GS/s are required. Individual components at these rates areeither not yet offered or very expensive. Hence, a field-programmable gate-arrays (FPGA)-based system wasdeveloped here, to implement a hybrid sampling scheme usingreal-time and equivalent-time sampling techniques. Where, fora 10 MHz radar pulse repetition rate, the waveform isdigitized at a 100 MHz rate. 10 samples are taken during eachcycle, and then a 200 ps delay is placed on the ADC triggeringsignal for the next 10 samples. The total time to collect thedata comes out to 100 x 50 ns, i.e. 5 Fts [7]. In general, intensewideband signal processing would normally require an entirecustom VLSI or an ASIC implementation.

After taking redundancy into account, this system achievesa 2.4ms/frame rate, with an averaging factor of 8 to improveS/N ratio. FPGA-based equivalent time sampling and controlnetwork provides a low-cost and high-performance solutionfor a practical UWB radar system. For image formation,algorithms such as back projection methods can be used given

Fig. 1. Typical system block diagram for a UWB STTW radar.

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that wall effects are considered. A typical experimentalscenario and results are shown in Fig. 2, which is consistentwith results from extensive modeling based on FDTDalgorithms.

III. CONSTANT AND STEPPED FREQUENCY DOPPLER RADARThere is particular interest in Doppler radar for the detection

of stationary personnel, isolation of individual vital signs, andthe use of signals that are difficult to recognize upon intercept.Doppler radar transmits a signal toward a target region, andanalyzes phase changes in the echoes indicating target motion.This motion can be as minute as that caused by heart-beat andrespiration. Transmissions are generally single frequency, andthe degree of phase variation for a given displacement is thusalso proportional to the signal frequency. Doppler radar offersparticular advantages for STTW applications in that the lowinstantaneous receiver bandwidth and modest analog to digitalconversion speed requirements leads to promise for creatingcompact low-cost handheld systems that would be attractivefor combat personnel. The concept is illustrated in Fig. 3a.The transmitted RF signal, f( t), can penetrate a wide range ofbarrier materials to produce RF echoes from targets in the

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field of view. Echoes from moving targets, f( t+ (t)), possessdistinct phase variations related to target motion, (t),including that from human respiration and heart activity,which can be isolated and reported to the user. The radar unitcan contain one or more receivers, as needed to improve echoanalysis [8, 9].Frequency stepping can also provide a means for accurate

ranging without adding significant hardware complexity. Onesuch system known as "Radar Scope," developed by DARPA,is expected to be fielded to troops in Iraq as soon as thisspring. When placed against a foot-thick concrete wall, thehandheld battery operated system will give warfighters thecapability to sense through it and 50 feet into the roombeyond. The unit will reportedly sense within seconds, thepresence of even stationary subjects based on motionassociated with their breathing [10].There is of course, significant interest in moving beyond

basic motion detection, and building systems that can mitigateclutter and isolate specific motion of interest, including theheart-beat associated motion of multiple subjects. This isparticularly important in handheld systems used at somedistance from a wall, which is subject to detecting personnelon both sides including the radar operator. Multiple antennasystems utilizing advanced space-time processing known asMIMO have shown promise toward this end. A simplifiedblock diagram for one such system, developed at theUniversity of Hawaii, is illustrated in Fig. 3b.

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The signal processing of the MIMO Doppler signals needsto extract a number of different types of information from thesignals: the number of people present in the range of theinstrument, their position, existence of a dominating line ofsight component, individual heart-beat and respiration signals,and heart rate (and respiration rate) of individual subjects. Anumber of different algorithms have been developed for thispurpose. The most basic problem is to determine if there iseven a heartbeat present. The applications for this are obviousin search and rescue, but it is also crucial in medicalapplications to detect cardiac arrest. The approach is to modelthe heartbeat and Doppler signal, and then use a hypothesistest to determine the presence of this signal; the optimum testhere is to use a Generalized Likelihood Ratio Test (GLRT),which turns out to result in using a modified FFT of the signal.In [3] this was extended to multiple subjects, and it was shownthat it is possible to count 2, 1, or 0 subjects. This method canbe extended to multiple subjects, but it works best if there islittle multipath (Fig. 4).An alternative approach in [11] is to use blind source

separation (BSS). This is applicable in any multipath scenario.BSS uses characteristics of the heartbeat together withindependence of different people' s heartbeat (statisticalindependence of the heart rate variations) to extract signalswith these characteristics. The primary application is tomonitor the vital signs of multiple people, but it can also beused to determine the number of persons present. In [11] itwas demonstrated that it's possible two separate to respirationsignals, or two heartbeat signals. Once the heartbeat signalshave been separated, the individual heart rates can bemonitored as if JUSt a single person was present. A number ofdifferent methods for heart rate extraction were explored, andit was determined that in general autocorrelation gives the bestaccuracy.

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ACKNOWLEDGEMENT

The authors wish to acknowledge the assistance and supportof B.K. Park, and Y. Yang.

REFERENCES[1] K.M. Chen, D. Mirsa, H. Wang, H.R. Chuang, and E. Postow,

"An X-band microwave life detection system," IEEE Trans.Biomed. Eng., vol. 33, pp. 697-70, 1986

[2] V.M. Lubecke, 0. Boric-Lubecke, G. Awater, P.-W. Ong, P.L.Gammel, R.-H. Yan, and J.C. Lin, "Remote sensing of vitalsigns with telecommunications signals," World Congr. onMedical Physics and Biomed. Engineering, Chicago, IL, 2000.

[3] Q. Zhou, J. Liu, A. Host-Madsen, 0. Boric-Lubecke, and V.Lubecke, "Detection of Multiple Heartbeats Using DopplerRadar," IEEE ICASSP '06, Toulouse, France, v. 2, pp. 1160 -1163, May 2006.

[4] P. Withington, H. Fluhler, and S. Nag, "Enhancing homelandsecurity with advanced UWB sensors," Microwave Magazine,IEEE, vol. 4, no. 3, pp.51 - 58, 2003

[5] Website: http://www.timedomain.com/ (Radar Vision)[6] Y. Yang and A.E. Fathy, "See-through-wall imaging using ultra

wideband short-pulse radar system," IEEE Antennas andPropagation Soc. Intl. Symp. v. 3B, pp. 334 - 337, July 2005

IV.MULTISTATIC AND PASSIVE RADAR

Beyond the Doppler systems described, separate receiveunits, or "Nodes," can be scattered in the target vicinity tofurther enhance a Doppler STTW radar system, providingadditional information on RF echoes, and also negatingunintended user motion as the direct and echo signals theycompare would have a roughly equal and canceling sourcemotion contributions [12,13]. Such a system can be deployedfor passive unmanned sensing, exploiting transmissions in thevicinity like those from wireless telephones. This is illustratedin Fig. 5. In such a system, the scattered nodes can form acooperative ad-hoc network, comparing the telephone signalwith its target echoes and reporting to a local transceiver. Thetransceiver then reports to a remote monitoring site forcommand and control interpretation and decisions.

VII. ConclusionBoth UWB and CW/stepped frequency Doppler radar have

shown effectiveness detecting human presence in STTWapplications, with complementary advantages in performanceand form. Future systems seek to merge the advantages ofboth, and go beyond to isolate subjects, map structures, andexploit signals of opportunity.

[7] Y. Yang, S. Liu, J. Wang, A.E. Fathy, "FPGA-Based DataAcquisition and Beamforming System for UWB See-Through-Wall Imaging Radar," IEEE AP-S Intl. Symp. on Antennas andPropagation and USNC/URSI Nat. Radio Sci. Meeting, 2006.

[8] A. Droitcour, 0. Boric-Lubecke, V. Lubecke, J. Lin, G. Kovacs,"Range Correlation and I/Q performance benefits in single chipsilicon Doppler radars for non-contact cardiopulmonary signssensing," IEEE Trans. Microwave Theory Tech., Vol. 52, No. 3,pp. 838-848, March 2004.

[9] B. K. Park, 0. Boric-Lubecke, and V. Lubecke, "ArctangentDemodulation in Quadrature Doppler Radar Receiver Systemwith DC Offset Compensation," accepted for IEEE Trans. onMicrowave Theory Tech., 2007.

[10] Website: http://www.gizmag.com/go/5032/ (Radar Scope)

[11] N. Petrochilos, M. Reznik, A. H0st-Madsen, V. Lubecke, and0. Boric-Lubecke, "Blind separation of Human Heartbeats andRespiration by the use of a Doppler Radar Remote Sensing,"accepted for ICASSP'07, Honolulu, HI, April 2007.

[12] I. Mostafanezhad, B.-K. Park, V. Lubecke, and 0. Boric-Lubecke, "Performance and Limitations for Radar SensorNetworks," IEEE MTT-S Intl. Microwave Symp., Honolulu, HI,June 2007.

[13] V.M. Lubecke, 0. Boric-Lubecke, and E. Beck, "A CompactLow-Cost Add-On Module for Doppler Radar Sensing of VitalSigns Using a Wireless Communications Terminal," IEEE MTT-S Intl. Microwave Symp., Seattle, WA, June 2002.

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