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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 8, AUGUST 2015 2247

Complex Radar Cross Section Measurementsof the Human Body for Breath-Activity

Monitoring ApplicationsEmanuele Piuzzi, Member, IEEE, Paolo D’Atanasio, Stefano Pisa, Member, IEEE,

Erika Pittella, and Alessandro Zambotti

Abstract— An experimental setup for complex mono-staticradar cross section (RCS) measurements in the 1–10-GHz-frequency band, employing a suitably modified semianechoicchamber, is presented and characterized. The foreseen applica-tion is the measurement of the complex RCS of the human bodyduring respiratory activity, to ease the design and optimizationof ultrawideband (UWB) radar systems for breath-activitymonitoring. The proposed RCS test range is calibrated by meansof a readily available aluminum flat panel and its performanceis tested against canonical targets, evaluating uncertainty inmagnitude, and phase measurements. The setup is then employedto carry out investigations on the complex RCS of a volunteer,focusing on its changes resulting from breath activity. Applyingthe measured RCS patterns to a specifically developed model,the feasibility of the UWB radar approach for achieving acontinuous contact-less monitoring of breath activity in a subjectat rest is clearly demonstrated. Finally, experimental tests ofthe application of the proposed radar technique to real-worldscenarios are shown, and the safety of RCS measurementsand UWB radar monitoring, with reference to exposure ofthe monitored subject to the radiated electromagnetic fields, isevaluated.

Index Terms— Anechoic chambers, biomedical monitoring,radar cross sections (RCSs), radiation safety,ultrawideband (UWB) radar.

I. INTRODUCTION

CONTINUOUS monitoring of breath rate is of greatimportance for the diagnosis of many respiratory appa-

ratus pathologies, and for the vital monitoring during hospitalconfinement or home therapy [1]. Breath-activity monitoring isusually carried out by means of respiratory inductiveplethysmography (RIP) systems, employing a couple ofinductive bands placed around the thorax and the abdomen [1].Such systems are particularly uncomfortable and not alwaysapplicable to the patients. Using the electromagnetic radiationin the microwave region, on the other hand, it is potentially

Manuscript received August 8, 2014; revised December 5, 2014; acceptedDecember 23, 2014. Date of publication January 30, 2015; date of currentversion July 10, 2015. The Associate Editor coordinating the review processwas Dr. Mark Yeary.

E. Piuzzi, S. Pisa, and E. Pittella are with the Department of InformationEngineering, Electronics and Telecommunications, Sapienza Universityof Rome, Rome 00184, Italy (e-mail: [email protected];[email protected]; [email protected]).

P. D’Atanasio and A. Zambotti are with the Italian National Agency forNew Technologies, Energy and Sustainable Economic Development,Casaccia Research Centre, Rome 00123, Italy (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIM.2015.2390811

possible to monitor any physiological activity involving move-ments of parts of the body, without contact with the subjectunder observation. In particular, among the possible remotemonitoring solutions employing electromagnetic radiation,ultrawideband (UWB) radars are today proposed as valuablesubstitutes for RIP systems, thanks to their high movementresolution and to the extremely low energy spectral density ofthe radiated fields, making them suitable for use in complexenvironments, where interference with other apparatus canpose a problem [2]–[6].

A circuit-model suitable to design a UWB radar forbreath-activity monitoring has been proposed in [6]. Themodel, to be applied, requires knowledge of the radar crosssection (RCS) of the human body, together with its changesresulting from respiratory activity. In particular, because of theUWB nature of the transmitted radar signal, both the amplitudeand the phase of the backscattered field, as a function offrequency, must be assessed, to correctly reconstruct the signalreflected toward the receiving antenna. This means that thecomplex-valued RCS (amplitude and phase components) [7]must be determined in the frequency band of possibleinterest that, according to the Federal CommunicationsCommission (FCC) allocations for UWB medical imagingsystems, approximately ranges from 3 to 10 GHz [8].

Scalar RCS measurements are common practice anddifferent well-established techniques are available for thispurpose [9]–[11]. However, attention is mainly devoted toscalar (magnitude only) measurements and the usually con-sidered targets are those common for military and civil-ian radar applications, i.e., aircrafts and ships. Hence, thereis currently a lack of data on the RCS of the humanbody, with the exception of a few studies which, however,do not provide complete information on the complex RCSand on its changes resulting from breath activity [12]–[17].Among the experimental studies, an early investigation evi-dences scalar RCS values of the order of 1 m2, largelyfluctuating with frequency in the FCC-allocated band [12].Some more recent experimental studies are also available,which assess the effect of cardiopulmonary activity on theRCS of human subjects [13], [14]. In particular, in [13],measurements were performed using a frequency-modulatedcontinuous-wave radar. Due to system design limitations,complex RCS values in the frequency band of interest forUWB systems were not measured, but only RCS changesresulting from human heartbeat or respiration were detected.

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2248 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 8, AUGUST 2015

In [14], instead, a dual-frequency Doppler radar system wasused, once again allowing only detection of changes in theback-scattered field resulting from thorax movements inducedby cardiopulmonary activity. In addition to the cited exper-imental investigations, some recent numerical studies haveappeared [15]–[17], modeling the effect of body physiologicalmotion on computed RCS [15] or investigating RCS changesdue to body posture [16], [17]. Still, only scalar RCS valueswere computed and the related data were generally in goodagreement with experimental results in [12]. Altogether, thelimited data available are not statistically significant for estab-lishing a reference RCS for a robust UWB radar design and,above all, they lack information on the RCS phase and itsvariations with frequency.

To obtain reliable complex RCS values for a breathingsubject, in a previous paper the authors proposed a suitableindoor test range for mono-static RCS measurements, employ-ing a suitably modified semianechoic chamber originallydesigned for electromagnetic compatibility (EMC) testingapplications [18]. The test site and measurement procedurewere initially developed and tested for classic scalarRCS measurements. In this paper, the characterization isextended to cover full complex-valued RCS measurements,which are a mandatory requirement when dealing withUWB signals. Moreover, the test site is employed to recordcomplex frequency-dependent RCS values for a breathingvolunteer. The obtained measurement results are used as inputfor a specifically developed model of a UWB radar [6],to test a possible signal processing technique for extractingbreath-activity information from the received UWB signal. Theproposed technique is then successfully applied to performbreath-activity monitoring on a volunteer within a realisticnonanechoic environment. Some considerations on the inter-action between the radiated electromagnetic field and themonitored subject are finally drawn.

II. EXPERIMENTAL SETUP

In this section, the experimental setup for complexRCS measurements is presented and characterized. First, theindoor anechoic test range is introduced and the procedureadopted for site calibration is described. Then, the overallprocedure is verified on a canonical target, assessing themeasurement accuracy.

A. Indoor Anechoic Test Range

The employed RCS test range is built inside a semianechoicchamber originally designed for EMC tests, withexternal dimensions of the shielded enclosure equal to9.0 m × 6.0 m × 5.6 m. The internal side walls and ceilingare entirely covered by TDK IB-011 ferrite tiles to absorb theincident radiation up to 1 GHz. The operative range of thechamber is extended up to 18 GHz by means of UWB TDKIP-045C pyramidal absorbers installed on the central spotsof the side walls and the ceiling. The chamber, to becomesuitable for performing the RCS measurements, has beenturned to a fully anechoic one by appropriately disposing, onthe metallic floor, panels covered by ferrite tiles and pyramidal

absorbers. Moreover, some extra pyramidal absorbers havebeen placed close to the wall and corners behind thetarget, and their position has been optimized during a seriesof preliminary tests to minimize background reflections.In particular, appropriate positioning of extra absorbinglayers close to the corners proved essential in significantlyimproving overall performance. It is worth mentioning thatRCS measurements are usually carried out in fully anechoicchambers [9]–[11]. However, the analysis of the accuracyachievable adapting a standard semianechoic chamber withappropriately distributed extra absorbers, as presented inthe following sections, demonstrates the feasibility of suchapproach for many practical applications. This is an importantresult in view of the widespread availability of chambersoriginally developed for EMC testing. Moreover, extensionof traditional scalar RCS measurements to include alsocalibrated phase measurements appears a key point forall radar applications involving wideband signals. Indeed,complex measurements of the backscattered fields are standardpractice, both for allowing vector background subtraction andfor retrieving the time behavior of the reflected field for timegating purposes [19]. However, to the authors’ knowledgeno data are available in the literature about the phase of thecomplex RCS of a target, as defined in [7], nor is availableany analysis of the corresponding measurement uncertainty.

Since the foreseeable UWB radar systems for breath-activitymonitoring should employ a single antenna or a couple ofantennas closely spaced apart, the test site has been setup formono-static RCS measurements. Two different solutions havebeen experimented, employing a different transmit-receiveantenna. The first antenna is a commercial ETS-Lindgren 3117double-ridged guide horn, able to cover the 1–18-GHz fre-quency range, which would be a suitable candidate to act asUWB radar antenna in a clinical setting, where the antenna isattached to the ceiling of a hospital room, monitoring patientslying on a bed. The second examined antenna, instead, is amore compact custom-made Vivaldi-like antenna [20], ableto cover the 3–12-GHz frequency range, which would be asuitable candidate for a domestic setting, with the antennadirectly attached to the bed. A picture of the two antennas isshown in Fig. 1, while Table I summarizes the main antennacharacteristics. In the test site arrangement, the antennas arepositioned on a wooden tripod mount, allowing the modifi-cation of their height, which has been adjusted using a laserpointer to ensure the antennas were pointing toward the centerof the target. Measurements have always been performed withvertically polarized fields. Indeed, in view of the applicationto human body RCS measurements, the solution providing thehighest RCS is the one employing an electric field polarizedalong the main (vertical) body axis.

The canonical targets used for site calibration and validationare placed on a polystyrene column at two possible distancesfrom the antenna: 2 or 0.5 m. The two ranges have been chosento resemble the likely antenna-subject distance in a realisticUWB monitoring application, referring to clinical or homescenarios, respectively. It is worth mentioning, for the sake ofcompleteness that the employed site easily allows extensionof the range up to 3 m.

PIUZZI et al.: COMPLEX RCS MEASUREMENTS 2249

TABLE I

MAIN ANTENNA CHARACTERISTICS

Fig. 1. Picture of the employed antennas (a) ETS-Lindgren 3117double-ridged guide horn and (b) custom-made Vivaldi-like antenna.

Fig. 2. Picture of the RCS test site.

A picture of the test site, with the double-ridged hornantenna and the polystyrene column, is shown in Fig. 2.In particular, the extra pyramidal absorbers at the wall cornersbehind the column are clearly visible.

Reflection coefficient (S11 scattering parameter) complexmeasurements, required to derive the target mono-static RCS,have been performed using a Rohde & Schwarz ZVB20vector network analyzer (VNA). The VNA is placed out-side the chamber and connected to the antenna by meansof low-loss coaxial cables. The measurement system ispreliminarily calibrated at the antenna connector through

a short-open-load (SOL) procedure. The adopted SOLstandards are those of the Agilent 85032F kit. Dur-ing all measurements the VNA is setup to sweep the1–10-GHz frequency range, with a frequency step ofabout 3 MHz, an IF bandwidth of 10 kHz, a sourcepower of 13 dBm. Prior to each RCS measurement,the empty chamber (without target) background trace isacquired and subsequently subtracted from the target trace:this allows to compensate for unwanted reflections from theenvironment and the antenna itself [21]. The background traceis reacquired prior to each measurement (both for referenceand target measurements) to reduce drift effects. The use oftime gating to eliminate unwanted spurious reflections fromthe environment has also been tested, but it has not beenapplied because it did not provide any significant improvementin overall measurement accuracy. It is also worth mentioningthat more sophisticate techniques would be available to mit-igate the effect of background reflections, but they are mosteffective when tracking moving objects against a stationarybackground [22]. In the scenario of interest for this papersuch techniques might prove useful to detect RCS variationsresulting from breath activity, but not to measure the staticcomplex RCS of the human body. Finally, smoothing (movingaverage filter with a 5% aperture) is used to eliminate fastfading from the recorded traces.

B. Calibration Target

The RCS test range must be preliminarily calibrated throughmeasurements of a reference target having a well-characterizedRCS. The RCS of the actual target under investigationis then derived by comparison with the calibration targetresponse [21]. In particular, since complex RCS measurementsare to be performed, also the phase, along with the amplitude,of the reflected signal must be considered [7]. Therefore,starting from S11 measurements, the complex RCS of thetarget (σtgt) can be derived as

|σtgt| = |σref | |S11tgt − S11empty,tgt|2|S11ref − S11empty,ref |2 (1)

angle(σtgt) = angle(σref) + angle(S11tgt − S11empty,tgt)

− angle(S11ref − S11empty,ref) (2)


Fig. 3. (a) Computed magnitude and (b) phase of the complex RCS for the calibration target (flat panel).

where σref is the complex RCS of the calibration target,S11empty,tgt and S11empty,ref are the reflection coefficients mea-sured for the empty chamber, immediately prior to placing thetarget and the reference, respectively, on the support, S11tgt isthe reflection coefficient measured with the target under inves-tigation placed on the column at the chosen distance fromthe antenna, and, finally, S11ref is the reflection coefficientmeasured with the calibration target placed on the support,at the same distance.

The most commonly adopted calibration targets are metallicspheres [23], thanks to the simple alignment procedure theyallow and to the availability of analytical closed-form solutionsfor the scattered field [24]. The drawbacks of the sphere asa reference target, however, are the difficulty in realizinglarge-radius spheres and the resulting need to exploit tinyspheres, whose RCS tends to be relatively small, with possibleimpairments in measurement accuracy. For this reason, inthis paper, the adopted calibration target is chosen to be asimple flat aluminum panel, which proved to be a reliable andreadily available reference target for mono-static calibrations.The considered panel has transversal dimensionsof 9.6 cm (width) × 10.3 cm (height). The thicknessof the aluminum sheet equals 3 mm, which is enough toavoid unwanted bending of the panel. The main problemwith the flat panel is that accurate analytical solutions for itsRCS are available only in the form of physical optics (POs)calculations, which are valid only for panels having transversaldimensions equal to several wavelengths [25], [26]. It isimmediate to verify that the used panel has transversaldimensions ranging from 1/3 of the wavelength at 1 GHz toabout 3 wavelengths at 10 GHz, thus making the PO solutioninaccurate. On the other hand, use of a much larger panelwould make it impossible to perform far-field measurements,because the usually adopted far-field condition [27], requiringa distance R > 2D2/λ, where D is the transversal dimensionof the target and λ the wavelength, would be satisfied outsidethe chamber itself.

To overcome the problem of achieving an accurate solu-tion for the complex RCS of the flat panel, the commer-cial electromagnetic solver FEKO, based on the method of

moments (MoMs), has been employed to derive a referencesolution. In particular, the complex RCS has been obtainedstarting from the scattered field computed in the presence ofan impinging uniform plane wave. The magnitude of theback-scattered field, computed in the far field of the panel, hasbeen normalized considering the geometric attenuation factor,while its phase has been corrected for the optical path length.Therefore, the following formulas have been used:

|σref | = 4π R2 |Er |2|Ei |2 (3)

angle(σref ) = angle(Er ) − angle(Ei ) + β R (4)

where σref is the complex RCS of the calibration panel, Er isthe scattered field at distance d = R from the panel, chosenin the far field of the target, Ei is the incident field on thepanel (at distance d = 0), and β is the propagation constantin vacuum. All distances are computed from a referencepoint that is the intersection between the line representing theincidence direction and the target itself, i.e., the center of thefront face of the panel [7]. The same convention will be alsoused to define distances from the targets under investigationin the experimental setup.

The obtained complex mono-static RCS is plotted in Fig. 3.In particular, the magnitude and phase of the RCS computedusing FEKO are compared with the PO solution. Analysisof the two curves clearly evidences that the full wave MoMsolution shows small oscillations arising from resonances ofthe panel, oscillations not reproduced by the approximatedPO formula. Moreover, a shift of the MoM curves as comparedwith the PO ones can also be observed, resulting from the finitethickness of the sample (the PO solution, instead, refers to aninfinitely thin panel). To confirm the validity of the FEKOsolution, the panel has been also studied employing a customfinite-difference time-domain (FDTD) code [28], obtainingessentially the same results [18]. This gives confidence aboutusing the numerical solution of the flat panel as an accuratereference for complex test range calibrations. As a final note,it is important to stress that, considering the dimensions ofthe chosen panel, at the minimum distance considered in theexperiments, equal to 0.5 m, far-field conditions are roughly


Fig. 4. (a) Magnitude and (b) phase of the complex RCS for the verification target (cylinder): comparison between measured and reference values for thedouble-ridged horn antenna at 2 m.

satisfied only up to 4 GHz. However, numerical computationswith FEKO showed that the RCS computed at 0.5 m is almostcoincident with the far-field one, and only a minor loss ofaccuracy should be expected.

C. Test Range Validation

After performing the calibration, the site has been validated,separately for the two antennas employed, by measuring thecomplex RCS of a canonical cylindrical target, whosereference RCS has been once again computed making useof FEKO software. In particular, a metallic cylinder witha diameter of 12.8 cm and a height of 12 cm has beenconsidered.

The complex RCS, measured at a distance of 2 m, withthe double-ridged horn antenna, is compared in Fig. 4 withthe reference solution. It is important to emphasize thatthe reference solution has been computed considering thescattered field at a distance of 2 m from the cylinder, tobetter reflect the experimental condition. In any case, thedistance of 2 m satisfies the far-field condition up to 10 GHzand this means that the reference solution is indeed the far-field one. Fig. 4 shows that an optimum agreement betweenmeasured and reference RCS is achieved. In the frequencyregion of interest for UWB radar applications, namely,3–10 GHz, the maximum error on the magnitude of the mea-sured RCS is 1 dB, while the corresponding maximum error onthe angle is 5°. It is worth mentioning that a similar validationexperiment, carried out on a lossy cylinder, revealed the samelevel of accuracy on the measured RCS magnitude [18].

A further validation has been performed for theVivaldi-like antenna. The complex RCS measured at a distanceof 0.5 m with this antenna is compared in Fig. 5 with thepreviously computed reference solution. In this case, however,the distance of 0.5 m does not satisfy the far-field condition inthe examined frequency band. Notwithstanding this, analysisof Fig. 5 shows a good agreement between measured andreference RCS, with only a slight loss of accuracy as comparedwith measurements in the 2-m range. In particular, the loss ofaccuracy at high frequency probably stems from systematic

errors resulting from the violation of the far-field condition.In any case, in the frequency region of interest for UWB radarapplications, namely, 3–10 GHz, the maximum error on themagnitude of the measured RCS is 1 dB up to 9 GHz,increasing to 2 dB at the upper edge of the band. Similarlyto the double-ridged horn antenna, the maximum error on theangle is 6°.

Altogether, it is possible to conclude that the proposed RCStest range is suitable to measure the complex-valued RCS inthe explored distance and frequency ranges with a worst caseuncertainty generally less than 1 dB on the magnitude andabout 5° on the phase.

The experimentally derived uncertainty figure on the mea-sured RCS magnitude can be validated against the theoreticalone, obtainable through an uncertainty budget analysis per-formed according to [21]. To this end, the following mainuncertainty sources have been identified.

1) Pointing Error: The alignment between the antennaand the target has been accomplished by using a laserpointer, and it can be estimated that the maximumdifference in the pointing direction between calibrationand verification target is below 5°. According to [21],considering that the minimum −3 dB aperture in theconsidered frequency range is 40° for the double-ridgedantenna and about 50° for the Vivaldi-like one,this pointing error results in a worst case uncertaintyof 0.3 dB.

2) Cross-Polarization Error: Both the calibration and theverification targets are essentially non depolarizing in thespecific scenario (i.e., orthogonal incidence with point-ing direction at the center of the target) and, therefore,the only significant source of error can be the antennacross polarization. Since both antennas employed exhibita polarization isolation better than 20 dB, accordingto [21], the resulting worst case uncertainty can beestimated as 0.1 dB. This uncertainty figure affects bothcalibration and verification target measurements.

3) Nonlinearity: Nonlinearity error arises because of thedifference between reflection coefficient magnitude


Fig. 5. (a) Magnitude and (b) phase of the complex RCS for the verification target (cylinder): comparison between measured and reference values for theVivaldi-like antenna at 0.5 m.

measured for the calibration and the verification target.According to the ZVB20 VNA receiver specifications,nonlinearity error is better than 0.3 dB.

4) Range: The distance between the antenna and the targetshas been measured through a tape. It is possible toestimate that the maximum difference between the actualantenna-target distance for the calibration and verifi-cation targets is within a few millimeters. Accordingto [21], this gives rise to a worst case uncertainty that canbe up to 0.1 dB for the smallest tested range (i.e., 0.5 m).

5) Reference RCS: Based on the difference between thecomputed RCS magnitude for the flat panel with the twoemployed numerical techniques (i.e., MoM and FDTD),a worst case uncertainty of 0.2 dB can be estimated forthe reference RCS.

6) Background Subtraction: Even though each reflectioncoefficient measured with the target/reference in placeis corrected by subtracting the corresponding traceacquired on the empty chamber immediately before,this correction procedure adds one more uncertaintycontribution. Indeed, the mutual interaction between thetarget/reference and the support is not compensated and,above all, the inevitable drift in the experimental setup(network analyzer and chamber) hinders the correctionprocess. To assess this last uncertainty contribution,an analysis has been performed evaluating the variationin the measured RCS magnitude resulting from the useof old empty chamber traces, acquired up to 15 minbefore the measurement on the actual target. This timeperiod encompasses the longest interval that can bereasonably required to place the target in the chamber.The results of the analysis show a worst case uncertaintyof 0.5 dB. This uncertainty figure affects both calibrationand verification target measurements.

The above reported uncertainty contributions aresummarized in Table II, which also reports the overalluncertainty, obtained as a root-sum-square (RSS) uncertainty,following the procedure suggested in [21]. The final figureof 1.0 dB is perfectly compatible with the experimentally

verified uncertainty. It is worth mentioning that, alternativelyto using the RSS worst case uncertainty, the combineduncertainty might be computed [29], assuming a rectangulardistribution for the different uncertainty sources. Using suchan approach, a combined uncertainty of 0.5 dB is obtained,which might be expanded to 1.0 or 1.5 dB using coveragefactors of 2 (about 95% coverage probability for a Gaussiandistribution) or 3 (about 99.7% coverage probability for aGaussian distribution), respectively. The obtained agreementbetween theoretically computed and experimentally verifieduncertainty demonstrates that the considered uncertaintycontributions are those likely providing the dominant effects.It is worth mentioning that more complex formulations havebeen proposed in the literature for a statistical evaluationof random errors in RCS measurements [30]. However, thetype B approach here followed, according to [21] and [29],is sufficient to evaluate the order of magnitude of totaluncertainty, as a comparison benchmark for the experimentallyevaluated measurement bias.

A similar theoretical uncertainty evaluation for the RCSphase would be more intricate, but the good agreementobtained on the RCS magnitude gives confidence that alsothe experimentally derived uncertainty value for the phase isreliable. It is also important to evidence that errors arisingfrom the difference in the real antenna-target distance for thecalibration and investigated targets would show up as a lineartrend on the measured phase, that could be possibly detectedand corrected.

Altogether, the above reported validation study demonstratesthat use of a suitably modified semianechoic chamber forRCS measurements is perfectly acceptable, as long as a 1 dBuncertainty on the magnitude (and better than 10° on thephase) is the target value. The 1 dB figure is perfectly inline with common uncertainty and might be better improvedtrying to reduce the contribution arising from backgroundsubtraction: this could be achieved by enhancing the mechan-ical and thermal stability of the chamber.

As a final note, it must be stressed that the consideredvalidation target is small enough to ensure that the far-field


TABLE II

RCS MAGNITUDE UNCERTAINTY BUDGET

condition is already met at the 2-m range and almost met atthe 0.5-m range. Of course the final target, represented by thehuman body, will be much larger, thus violating the far-fieldassumption. This issue has already been investigated in [18],where a hollow metallic cylinder having an external diameterof 10.2 cm, a wall thickness of 2 mm, and a total heightof 34 cm, was considered. The obtained results showed thatthere was a critical issue related to the nonuniform illuminationof the target. The conclusion was that it is crucial to performRCS measurements in the near-field range at exactly theintended operating distance and employing the same antennato be adopted in the final radar system. Both these conditionshave been satisfied in the present work, considering the twodifferent antennas and the two corresponding ranges.

III. HUMAN BODY RCS MEASUREMENTS

After validating the RCS test site, a series ofRCS measurements on a female volunteer has been carriedout. For such measurements, the polystyrene column isreplaced by a wooden stool over which the human subjectstands. The stool height is chosen so as to bring the thoraxat the antenna level. It is understood that the change ofthe target support between calibration panel and humantarget will deteriorate the target–background interaction,thus impairing measurement accuracy, while the change inbackground reflections should not pose a problem since eachtarget acquisition is corrected employing the correspondingempty chamber trace. On the other hand, the dimensionof the human body, the antenna-target distance, and theantenna aperture are such that only the region of the bodyaround the thorax and, hence, distant from the woodenstool will be directly illuminated, thus limiting the effect oftarget–background interactions.

As an example of the obtained experimental results,Fig. 6 shows the measured complex RCS of the volunteer,considering the Vivaldi-like antenna in the 0.5-m range.In particular, two measurements are reported, correspondingto the end-inspiration and end-expiration breathing phases.The two traces clearly evidence that both the absolute valueand the phase of the RCS are greatly influenced by thesmall movements of the human thorax wall resulting fromrespiratory acts. It is interesting to note that respiratory actsinvolve a global change in the thorax morphology and notsimply a shift in the thorax wall: indeed the resulting effect isfar from a simple linear change in the RCS phase (that shouldbe expected from a mere target shift) but rather the whole RCSpattern is greatly altered. This implies that, assuming the useof a UWB radar, the probable effect on the reflected signalwill not be a simple time shift, but large signal distortionsmust be expected.

Analysis of Fig. 6 also clearly evidences that by measuringthe RCS of the human body at a given frequency, it wouldbe possible to monitor the breath activity simply followingthe changes in the magnitude (or phase) of the detectedRCS. However, such a monitoring technique would be verydemanding in terms of required instrumentation and, therefore,is not the most efficient solution to be employed. Instead, asalready evidenced in the introduction, use of a UWB radarwould provide a much more convenient and cost-effectivesolution. The following section will be devoted to highlightinghow the measured RCS patterns can be employed to studya possible UWB technique for breath-activity monitoring,allowing to identify a suitable signal processing to extractbreath-activity information from the acquired UWB signals.Section V will then demonstrate the practical applicabilityof the proposed UWB technique to a real-world monitoringscenario.


Fig. 6. (a) Magnitude and (b) phase of the complex RCS for a volunteer: comparison between end-inspiration and end-expiration breathing phases.

Fig. 7. Simulated voltages of a UWB radar for a breathing subject. (a) Source pulse. (b) Received voltages during breath activity.

IV. ANALYSIS OF A UWB RADAR FOR

BREATH-ACTIVITY MONITORING

To show a possible application of the measured complexRCS values for the design of UWB radars for breath-activitymonitoring, a series of real-time RCS acquisitions for abreathing female volunteer has been performed employingagain the Vivaldi-like antenna in the 0.5-m range. Com-plex RCS frequency spectra, similar to those reported inFig. 6, have been acquired at a rate of one spectrum persecond, starting from an end-expiration condition. Therefore,since the monitored volunteer exhibited a respiratory rate of10 breaths per minute, the first four acquisitions, covering atime interval of 3 s, spanned from the initial end-expirationcondition to the end-inspiration one, encompassing a half cycleof the respiratory activity. This is sufficient to assess thevariations in RCS to be expected during respiratory activityand, hence, to estimate the theoretical dynamic range at theradar output.

To evaluate the received signals for a possible UWB radar,the UWB radar model presented in [6] has been exploited,

considering a UWB source signal corresponding to the10th-order derivative of a Gaussian pulse with standarddeviation of 100 ps and a peak amplitude of 0.5 V(thus complying with the FCC emission mask for UWBradiation [8]), as shown in Fig. 7(a). The computed receivedsignals are reported in Fig. 7(b), which shows a zoomed ofthe received voltages around 4.7 ns, roughly representingthe center of the received pulses. It clearly appears that, asexpected, the four traces are not shifted in time, but ratherheavily distorted due to the dispersive behavior of the RCS.Therefore, a suitable algorithm must be used to extractinformation on the breath activity.

Just to cite an example, the most simple technique to derivea time signal proportional to breathing is voltage sampling ata fixed time instant, which is the usual choice in the so-calledrange-gating technique [31]. In particular, it is sufficient tochoose a suitable time instant, usually located in the centralportion of the received signal, and to sample voltage variationsat this fixed time delay. As the patient breaths, the receivedsignal is slightly shifted in time and altered and, therefore, the


Fig. 8. Simulated voltages of a UWB radar for a breathing subject: differencebetween end-inspiration and end-expiration received voltages.

voltage level at the fixed observation delay changesaccordingly. To better clarify this, Fig. 8 plots the differencebetween the last and the first pulse, corresponding to the differ-ence between the received voltages at end inspiration and endexpiration. The figure highlights that a point showing a highsensitivity is obtained at a delay of about 4.67 ns,corresponding to the center of the received pulses. Applyingthe fixed time instant sampling technique at this delay,the variation in the received voltage as the breath-activityprogresses from end-expiration to end-inspiration phases isabout 200 μV: this low-level signal can be detected usingthe integration technique usually adopted in range-gatingreceivers [31]. Analysis of Fig. 8 also evidences that thereare other time delays, not corresponding to the center of thereceived pulses, with a similar sensitivity.

V. EXAMPLES OF UWB BREATH-ACTIVITY

MONITORING IN REALISTIC SCENARIOS

To demonstrate the practical applicability of the proposedUWB technique to a real-world scenario, the breath activityof a volunteer has been monitored in an office (nonanechoic)environment. The subject was standing in front of the radarantenna, breathing normally. A piezoelectric respiratory belttransducer (MLT1132 by ADInstruments) was applied to thethorax to provide a reference trace for comparison.

The potential difficulties in a nonanechoic environmentstem from the presence of a relevant background clutter.However, the breath-activity detection technique proposed inthe previous section is based on changes in the receivedradar signal as the subject thorax moves. As long as thebackground clutter is stationary, its effect will not contributeto received voltage fluctuations and, therefore, will not hinderthe detection of the breath activity. This is the most typicalscenario to be encountered when monitoring a subject lyingin a bed, be it in hospital or home environment.

To test the effectiveness of the UWB technique, twodifferent UWB radar implementations are considered: an indi-rect time-domain reflectometry (TDR) approach, based on aVNA, and a commercial UWB radar solution. Some consider-ations on the safety of the proposed technique, with referenceto human exposure to the radiated fields, are also presented.

Fig. 9. Respiratory trace acquired on a volunteer: comparisonbetween indirect TDR UWB system and piezoelectric respiratory belttransducer.

A. Indirect TDR Approach

The first experimental setup employed to validate the pro-posed UWB radar is based on an indirect TDR technique.Such technique is implemented using a remotely controlledVNA (Agilent E8363C). The virtual instrument managingthe measurement procedure is developed within LabVIEWenvironment and allows to simulate arbitrary shaped sourcesignals and to visualize the corresponding target reflections [6].The desired UWB antenna is connected to the network ana-lyzer port, and the instrument measures the complex reflectioncoefficient at the input of the antenna due to the presence ofthe breathing subject thorax at a given distance. At the sametime, the virtual instrument generates the desired UWB sourcesignal, which is Fourier transformed and multiplied by themeasured reflection coefficient spectrum. The correspondingreceived signal is finally retrieved applying the inverse Fouriertransform. The process is continuously repeated, thus recover-ing changes in the received signal arising from breath activity.

In these realistic operating conditions, the received signalis strongly influenced by early time reflections due to theimpedance mismatch between the source and the antenna, pluslate-time reflections due to background clutter. To removethese reflections, the proposed TDR technique preliminarilyacquires the reflection coefficient of the antenna radiatingin free space and subtracts this calibration trace from themeasured spectra when the subject is present. In this way,a calibrated received signal is computed, influenced only bytarget reflections, from which the thorax movements can beextracted using the above presented algorithm (i.e., fixed timeinstant sampling technique).

Fig. 9 shows the result of application of the describedsetup, employing the Vivaldi-like antenna, to the monitoringof a volunteer placed 0.5 m far from the antenna. Theconsidered source signal is a monocycle (first derivative of aGaussian pulse) with a standard deviation of 200 ps. Tracesare acquired at a rate of 3.3 traces per second, correspondingto a sampling of the breath activity at 3.3 Hz. The fixedtime instant sampling technique is applied considering a timeinstant chosen at the center of the received pulses. Theresulting trace is directly compared with the reference one


Fig. 10. Respiratory trace acquired on a volunteer: comparison betweenNovelda UWB radar and piezoelectric respiratory belt transducer.

obtained with the respiratory belt. An optimum agreementcan be observed, confirming the validity of the proposedapproach. Obviously, the adopted setup is useful for validationpurposes, but not convenient for actual use in hospital or homeenvironment, due to the need of an expensive and large pieceof equipment like the network analyzer. For this reason, thefollowing section shows a further validation study, carried-outemploying a commercially available UWB radar system.

B. Novelda UWB Radar

Among the few commercially available UWB radarmodules, one particularly suited to test the proposedbreath-activity monitoring technique is the Novelda radar [32].

Novelda is constituted by a pulse generator and a samplerbased on the swept threshold coding technique, bothimplemented in standard CMOS technology. The radar isequipped with two 3.0–6.0-GHz bandwidth sinuous antennas,which in this paper have been replaced by two specularVivaldi-like antennas for better comparison with the indirectTDR system. The radar has been remotely controlled throughits USB interface using a MATLAB script which exploits thecapabilities offered by the Novelda NVA-R640 Developmentkit. The MATLAB code allows programming the radar andsaving the acquired traces. Software filters can be programmedin the radar firmware so as to remove static clutter from theacquired traces.

Fig. 10 shows the result of application of the described radarto the monitoring of the same volunteer, again placed 0.5-mfar from the antenna. Traces are acquired at a rate of25 traces per second, giving rise to a 25-Hz sampling fre-quency for breath activity. The fixed time instant samplingtechnique is once more applied considering a time instantchosen at the center of the received pulses. The resultingtrace is directly compared with the reference one obtainedwith the respiratory belt. Once again, an optimum agreementcan be observed, demonstrating the practical applicability ofthe approach with a compact and convenient radar module.

C. Safety Aspects

The proposed remote monitoring technique employselectromagnetic fields, which are radiated toward the

monitored subject. Therefore, questions might be raised aboutthe safety of the technique, in terms of human exposure tothe radiated fields. Among the different existing guidelines,defining safe limits concerning human exposure to electro-magnetic fields, one of the most renowned are those settledby the International Commission on Non-Ionizing RadiationProtection (ICNIRP) [33]. Such guidelines pose limits onthe incident electromagnetic field (reference levels) and onthe specific absorption rate (basic restrictions). In case ofpulsed exposures, such as those arising from UWB signals,restrictions on specific energy absorption are also imposed.

The first exposure scenario to be considered is the onerelated to RCS measurements and to the indirect TDR sys-tem. In both cases, the radiated field is generated by thenetwork analyzer and is a continuous wave signal withswept frequency. Therefore, the subject is exposed to anessentially sinusoidal field, whose frequency changes in therange between 1 and 10 GHz. The port output poweron the network analyzer is set to 13 dBm. Consideringthe maximum gain for the double-ridged horn and for theVivaldi-like antenna, effective isotropic radiated pow-ers (EIRP) of 25 and 22 dBm are obtained, respectively.Obviously, the above reported EIRP are largely overestimated,since they do not consider attenuation in the cables connectingthe antenna to the analyzer. Considering a standard operatingdistance of 2 m for the double-ridged horn antenna and of0.5 m for the Vivaldi-like one, the peak electromagnetic fieldimpinging on the subject is about 2 and 4 V/m for the twoconsidered scenarios, respectively. Both these figures are wellbelow the most stringent reference level considered in ICNIRPguidelines for general public exposure in the frequency rangeof interest, which equals about 40 V/m at 1 GHz. It is furtherworth mentioning that the reference level refers to the averagefield value over the human body area, which is obviously lowerthan the peak value above calculated.

The second exposure scenario, instead, corresponds to useof an actual UWB radar, like the Novelda one. A very detailedanalysis of such situation is available in [34], where it is shownthat, as long as the radar complies with the FCC emissionmask [8], in the foreseeable operating conditions humanexposure is at least four orders of magnitude below the relevantbasic restrictions.

VI. CONCLUSION

An experimental setup for mono-static complexRCS measurements in the 1–10-GHz frequency bandhas been validated and characterized. The test site allows toachieve realistic scattering data of human subjects for thepurpose of designing a UWB radar system for breath-activitymonitoring. The test range has been calibrated by means ofa simple aluminum flat panel and its performance has beentested against a canonical metallic target, showing a worstcase uncertainty within 1 dB for the magnitude and about5° for the angle of the measured complex RCS. The resultsdemonstrate the feasibility of employing suitably modifiedsemianechoic chambers for accurate RCS measurementstogether with the uncertainty levels reasonably achievable


on the measured RCS phase, usually neglected in classicscalar measurements. The setup has been employed to carryout investigations on the RCS of the human body and itsvariations resulting from breath activity. Traces acquired ona volunteer showed the potential of the complex RCS dataas a key input for the process of UWB medical radar design.In particular, the acquired RCS data allowed to identify asuitable processing of the acquired UWB reflected pulses toextrapolate a signal related to breath activity. Applicationof such processing in a realistic environment, using both anindirect TDR system and a commercial UWB radar module,confirmed the validity of the UWB radar approach for remotebreath-activity monitoring. Furthermore, it was demonstratedthat the employed power levels are such that the resultinghuman exposure remains well below the recognized safetylimits.

ACKNOWLEDGMENT

The authors would like to thank Dr. O. Testa for his supporton RCS simulations with FEKO software.

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Emanuele Piuzzi (M’09) received theM.S. (cum laude) and Ph.D. degrees in electronicsengineering from the Sapienza University of Rome,Rome, Italy, in 1997 and 2001, respectively.

He is currently an Assistant Professor of Electricaland Electronic Measurements with the Departmentof Information Engineering, Electronics andTelecommunications, Sapienza University of Rome.He has co-authored over 100 publications. Hiscurrent research interests include the measurementof complex permittivity of materials, time domain

reflectometry applications, biomedical instrumentation design, and evaluationof human exposure to electromagnetic fields.

Dr. Piuzzi is a member of the Italian Group of Electrical and ElectronicMeasurements and the Italian Electrotechnical Committee. He servesas a reviewer for several international journals, mainly in the field ofinstrumentation and measurement.


Paolo D’Atanasio was born in Rome, Italy, in 1959.He received the Full Degree (summa cum laude)in Physics from the Sapienza University of Rome,Rome, Italy, in 1986.

He joined the Italian National Agency for NewTechnologies, Energy and Sustainable EconomicDevelopment, Rome, as a Researcher, in 1988. From1986 to 1992, he studied the effects of ionizingradiations on polymeric materials, electronic com-ponents, and nuclear scintillators. Since 1993, hisresearch has focused on electromagnetic modeling

of high frequencies antennas and propagation in large domains, on exploitingparallel computers in computational electromagnetics and on electromagneticcompatibility (EMC) and electromagnetic interference (EMI) topics. Since2000, he is responsible of the Electromagnetic Compatibility Laboratory andsince 2011 he is Director of Research having also the responsibility of thequalification tests (EMC/EMI, vibration, and seismic). His current researchinterests include electromagnetic measurements of radar cross section, antennacharacterization, dielectric spectroscopy measurements, interaction betweenelectromagnetic fields, and biological systems.

Dr. D’Atanasio is a member of the Italian Society of Physics and the ItalianElectrotechnical Committee.

Stefano Pisa (M’91) was born in Rome, Italy,in 1957. He received the Electronics Engineeringand Ph.D. degrees from the Sapienza University ofRome, Rome, Italy, in 1985 and 1988, respectively.

He joined the Department of Electronic Engineer-ing, Sapienza University of Rome, as a Researcher,in 1989, where he has been an Associate Profes-sor since 2001. He has authored over 150 scien-tific papers and numerous invited presentations atinternational workshops and conferences. His cur-rent research interests include interaction between

electromagnetic fields and biological systems, therapeutic and diagnosticapplications of electromagnetic fields, and the modeling and design ofmicrowave circuits.

Dr. Pisa is currently a Consulting Member of the Scientific Committee onPhysics and Engineering of the International Commission on Non-IonizingRadiation Protection, and a member of the Advisory Group of the Dutchproject Electromagnetic Fields and Health. He serves as a reviewer fordifferent international journals.

Erika Pittella received the M.S. (cum laude) andPh.D. degrees in electronics engineering from theSapienza University of Rome, Rome, Italy, in 2006and 2011, respectively.

She is currently a Research Associate with theDepartment of Information Engineering, Electron-ics and Telecommunications, Sapienza Universityof Rome. Her main research activities are relatedto the modeling of ultrawideband radars for theremote monitoring of cardio-respiratory activity andthe design of sources, antennas, and receivers for

such systems. Her current research interests include dosimetric aspects of theinteraction between electromagnetic fields radiated by ultrawideband radarsystems and exposed subjects.

Alessandro Zambotti was born in Rome, Italy,in 1963. He received high school diploma in Indus-trial Electronics in 1982.

He was a System Technician of electro-opticalinfrared systems with Selenia-Alenia Company,Pomezia, Italy, from 1985 to 1993. In 1993, hejoined the Italian National Agency for New Tech-nologies, Energy and Sustainable Economic Devel-opment (ENEA), Rome, as a Technician, where heparticipated in the realization and setup of the Metro-logical Centre with the ENEA’s Trisaia Research

Centre. He was involved in the development of calibration procedures forthermocouples and instruments for electrical measurements. Since 2000, hehas been with the Electromagnetic Compatibility Laboratory, ENEA’s Casac-cia Research Centre, Rome, where he serves as the Laboratory Technician forelectromagnetic compatibility/electromagnetic interference measurements andtests according to MIL and IEC standards. He also collaborates in researchactivities on dielectric spectroscopy measurements, antenna, and radar crosssection measurements.

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