Rogier, Hendrik

Vallozzi, Luigi

Van Torre, Patrick

Verhaevert, Jo

English

Ghent University Academic Bibliography

Indoor Off-Body Wireless CommunicationUsing Static Zero-Elevation Beamformingon Front and Back Textile Antenna ArraysPatrick Van Torre∗†, Luigi Vallozzi∗, Hendrik Rogier∗, Jo Verhaevert†∗Department of Information Technology, Ghent University, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium†Department INWE, University College Ghent, Valentin Vaerwyckweg 1, B-9000 Ghent, Belgiumpatrick.vantorre@ugent.be, luigi.vallozzi@ugent.be, hendrik.rogier@intec.ugent.be, jo.verhaevert@hogent.beI. INTRODUCTIONOff-body wireless communication systems may be lifesaving for rescue workers operating in an indoor environment.A system including antennas and sensors integrated into theirgarment allows communication of vital measurement data toa base station in order to improve the safety and security ofthe rescue workers and the coordination of their interventionin general. The indoor propagation environment introducesspace and time varying channel behavior, including fading,shadowing and non-constant path loss. Specifically for off-body wireless communication links, the movements of theperson wearing the communication system will create alteringshadowing by the human body, as well as antenna reorienta-tion.As the reliability of the link is of paramount importance forthe monitoring of workers in an emergency situation, systemsconsisting of multiple textile antennas distributed over thebody of the wearer are used to improve the robustness of thelink. Such an enhancement is possible by employing transmitand/or receive diversity, using space-time coding and maximalratio combining, respectively.Transmit beamforming techniques are able to produce ahigher average signal-to-noise ratio at the receiver by concen-trating the available transmission power towards the receiver.Dynamic beamforming techniques, using channel estimates todetermine the optimal set of transmitted signals, theoreticallyoffer the best performance. However, their implementationfor off-body communication with a moving person requireshigh-rate channel feedback, increasing the complexity, costand power-consumption of battery-operated systems beyondacceptable limits.For this paper we present measurements for a static off-bodybeamforming system. Measurements have been described inliterature for outdoor-indoor scenarios [1] but not for indooroff-body beamforming. The off-body system includes twovertically oriented uniform linear arrays consisting of fourtextile patch antennas [2] at the front and back of the humanbody, drastically reducing the body’s shadowing effect.Previous measurements with only a single front antennaarray are described in [3]. The new measurement campaignhere documents the performance of a system deploying bothfront and back textile antenna arrays on the human bodyin similar conditions. Both arrays will be driven with thesame signals for equal patch numbers, resulting in a moreomnidirectional radiation pattern in the azimuth plane. Thisconfiguration significantly reduces the shadowing by the hu-man body.II. MEASUREMENT SETUP3/4 λ = 92 mmport 1port 2port 3port 4xyzLWFig. 1. Array of four textile patch antennas integrated into the firefighters’garment.Figure 1 displays the textile antenna array and its positionon the human body. A tip-truncated equilateral triangulartopology is used for each patch, which leads to low mutualcoupling while still providing sufficient gain in the broadsidedirection, i.e. away from the rescue worker. The antennapatches are driven in phase to produce a zero-elevation beam,concentrating the power towards a receiver situated on thesame floor. The performance of this beamforming array iscompared to a space-time coding system using the sameoff-body antenna configuration and the corresponding signal-to-noise and bit error characteristics are presented. In themeasurement campaign with one textile antenna array [3], theperformance is highly influenced by shadowing effects of thehuman body. Nevertheless, a higher average received signal-to-noise ratio is always obtained by means of beamforming.However, receiver diversity is essential, as the resultingreduction in signal variance is required to produce acceptablebit error characteristics in the demanding indoor propagationenvironment.The system shown in Fig. 2 realizes fourth-order receiverdiversity at the access point side using a horizontal array offour vertical dipoles. The dipole antennas are connected to themeasurement testbed which down converts and synchronouslysamples the signals.Fig. 2. Measurement equipment at the receiving side, including horizontalfour-dipole array.A floor plan of the measurement environment is displayedin Fig. 3. The Line-of-Sight (LoS) path is situated between thelabels A and B, whereas the Non Line-of-Sight (NLoS) pathis present between A and C. Along the LoS path the rescueworker walks towards or away from the transmitter, resultingin a constantly changing path loss. The NLoS path, marked“sideways”, is oriented approximately perpendicular to the linebetween the transmitter and receiver. A nearly constant pathloss results. The environment contains office equipment suchas metal PC cases and metal closets. The thickest walls inthe floor plan are solid brick, building-supporting walls. TheNLOS path is blocked from the receiver by two such walls.Other walls are also constructed in brick except for the thinnestwalls, which are plasterboard.III. ANALYSIS OF THE RECEIVED Eb/N0Eb/N0 refers to the ratio of the received energy per bitto the one-sided spectral noise density, a value that is equalfor space-time coding and beamforming for our experiments,under equal channel attenuation conditions. An overview ofthe average, minimum and maximum Eb/N0 received fordifferent situations is listed in Table I. Beamforming powergain is the difference between the average Eb/N0 in dB forbeamforming and space-time coding. Because of the concen-tration of power in low elevation angles this factor amounts toapproximately 3 dB for all measured cases. In [3], a slightlyhigher beamforming gain results for the LoS situations withthe beam directed towards the receiver because the power ismore concentrated in a narrower range of azimuth angles whenemploying only one antenna array. When, for the dual-arraysystem, the front array is directed towards the receiver, half ofthe power is radiated at the back array, away from the receiver.7.56m22.56 mdeskdeskdeskdeskdeskdeskdeskTo and/or from RxRx UnitRx antennasSidewaysA D CBFig. 3. Floor plan of the measurement environment.TABLE IEb/N0 FOR STC AND BEAMFORMING; BEAMFORMING POWER GAINSTC Beamforming Beamforming power gain[dB] [dB] [dB]NLoS, no receive diversitymin 34.1 19.3avg 44.1 47.0 2.9max 50.8 55.4NLoS, 4-th order receive diversitymin 41.6 43.9avg 51.0 54.4 3.4max 57.1 61.7LoS, no receive diversitymin 33.5 37.5avg 53.2 56.0 2.8max 59.9 62.3LoS, 4-th order receive diversitymin 49.7 48.5avg 59.4 62.3 2.9max 65.6 68.9A. Line-of-SightThe Eb/N0 for subsequent frames received along the LoSpath, is displayed in Fig. 4 and 5. Clearly, the shadowing by thehuman body is countered by the dual antenna array, installedfront and back. For the 2-array measurement considered in thispaper, the behavior of the Eb/N0 is dominated by changingpath loss and fading. Measurements in [3] show a large drop insignal level when the antenna array is oriented away from thereceiver, due to body shadowing combined with the radiationpattern of the single array. In comparison, the Eb/N0 varianceis much smaller for the dual-array system, which will resultin better bit error (BER) characteristics.0 20 40 60 80 10010203040506070frame nr.E b/N0 [dB]  Beam, dualSPT, dual Beam, singleFig. 4. Eb/N0 along the Line-of-Sight path, without receiver diversity (RX3)for beamforming and space-time coding.1) LoS without receiver diversity: Without receiver diver-sity, beamforming produces an Eb/N0 that is nearly alwayshigher than for space-time coding. The average Eb/N0 is2.8 dB higher for beamforming than for space-time coding.The minimum Eb/N0 values for space-time coding and beam-forming are 33.5 dB and 37.5 dB, respectively, for the dual-array system. For the single array configuration [3], thosevalues were 13.1 dB and 22.3 dB, respectively, when thebeam was oriented away from the receiver. Hence, for LoS,the signal minima are much less severe for the dual-arrayconfiguration.2) LoS with 4th order receiver diversity: Although thanksto the array gain the average Eb/N0 is approximately 6 dBhigher, similar results are obtained for the system with 4-thorder receiver diversity, for the beamforming as well as for thespace-time coding. Additional diversity gain limits the signaldips, with a minimum Eb/N0 that is 11.0 dB and 16.2 dBhigher than without receiver diversity, for beamforming andspace-time coding respectively.B. Non Line-of-Sight1) NLoS without receiver diversity: Fig. 6 displays thebehavior for NLoS without receiver diversity. For the NLoSenvironment, beamforming still produces a higher averageEb/N0 but due to the absence of diversity, serious fading is0 20 40 60 80 100303540455055606570frame nr.E b/N0 [dB]  Beam, dualSPT, dualBeam, singleFig. 5. Eb/N0 along the Line-of-Sight path, with receiver diversity (MRC)for beamforming and space-time coding.0 20 40 60 80 100102030405060frame nr.E b/N0 [dB]  Beam 0oSpace−timeFig. 6. Eb/N0 along the Non Line-of-Sight path, without receiver diversity(RX3) for beamforming and space-time coding.present on the signal. Clearly the space-time code is successfulin reducing the fading, indicating that even though the antennapatches in the array are closely spaced, a useful degreeof diversity is obtained. Several signal dips occur for thebeamforming, with a minimum Eb/N0 that is 14.8 dB lowerthan for the space-time coding (Table I). As further illustrated,these signal dips will significantly deteriorate the bit errorcharacteristics for beamforming.2) NLoS with 4th order receiver diversity: The NLoSperformance with 4-th receiver diversity is shown in Fig. 7.The average Eb/N0 is 3.4 dB higher for the beamformingthan for the space-time coding. This value is even higherthan for the LoS situation, which indicates that the NLoSsignal propagation over larger distances occurs mainly throughreflections and scattering at low elevation angles. Comparingto the NLoS situation without receiver diversity, the averageEb/N0 is 6.9 dB and 7.4 dB higher for space-time coding andbeamforming, respectively, because of the receiver array anddiversity gain combined.0 20 40 60 80 1004045505560frame nr.E b/N0 [dB]  Beam 0oSpace−timeFig. 7. Eb/N0 along the Non Line-of-Sight path, with receiver diversity(MRC) for beamforming and space-time coding.In the presence of receiver diversity, the performance ofbeamforming is much better because the signal dips are alsodrastically reduced. Now, the minimum Eb/N0 occurring forthe space-time coding is 2.3 dB lower than for beamforming.Although the combination of transmit and receiver diversityprovides 16-th order diversity for the space-time coding andonly 4-th order diversity for the beamforming, the increase indiversity gain offered by the space-time coding is smaller thanthe power gain produced by beamforming.IV. BIT ERROR CHARACTERISTICSThe bit error characteristics display the bit error rate (BER)obtained for beamforming and space-time coding, with andwithout diversity in LoS as well as NLoS conditions. To obtaina fair comparison of the BER produced by beamforming ver-sus space-time coding, we consider an equal total transmittedenergy per information bit Eb,tr for both scenarios. Therefore,we introduce the notion of normalized average Eb/N0, whichequals either the average Eb/N0 at the detector output (in thecase of STC), or the average Eb/N0 at the detector outputminus the beamforming power gain from Table I (in the caseof beamforming). In this way, displaying the BER curves asa function of the normalized Eb/N0 includes the power gainassociated with coherent beamforming.A. Line-of-SightAlong the LoS path a lower BER is obtained for the beam-forming without receiver diversity, because of the beamform-ing power gain. In case of receiver diversity, the beamformingpower gain is also visible but the space-time coding appearsto perform better for the lower BERs. Note that the curvefor the space-time coding probably bends down excessivelyfor the lower bit error rates, due to the limited number ofmeasurement points. Looking back at Fig. 5 there’s only oneframe where the Eb/N0 for beamforming is lower than forspace-time coding. Except for this single frame, beamformingalways performs better here. However this single frame has alarge impact on the lower end of the bit error curve.−5 0 5 1010−410−310−210−1100Eb/N0BER  RayleighSPT RX3Beam RX3SPT MRCBeam MRCFig. 8. BER as a function of the normalized average Eb/N0 per receiveantenna, recorded along the LoS path, for transmissions at equal total Eb,tr .B. Non Line-of-SightIn NLoS conditions without receiver diversity, the space-time code clearly outperforms the beamforming. The absenceof diversity results in a beamforming characteristic approach-ing the theoretical curve for Rayleigh fading. For the higherEb/N0 values, a lower BER is quickly obtained by the space-time coding, because of the transmit diversity gain exceedingthe beamforming power gain.In case of receiver diversity, the beamforming BER islower than the BER for space-time coding over the full rangedisplayed. For the beamforming, 4-th order diversity is presentwhereas the space-time code realizes 16-th order diversity. Theextra diversity gain achieved by space-time coding is minimalfor the BER range considered. Possibly the space-time code isbetter for very low bit error rates, but the number of measuredframes is too low to accurately determine this.−5 0 5 1010−410−310−210−1100Eb/N0BER  RayleighSTC RX3Beam RX3STC MRCBeam MRCFig. 9. BER as a function of the normalized average Eb/N0 per receiveantenna, recorded along the NLoS path, for transmissions at equal total Eb,tr .V. COMPARISON OF SINGLE- AND DUAL-ARRAY SYSTEMSAn earlier measurement campaign in similar propagationconditions with a single front antenna array is documentedin [3]. A comparison of different measurement campaignsshould be made carefully because the path traveled by therescue worker can never be exactly the same. However, somedifferences between single- and double-array systems areclearly visible. Note that, whereas in [3] the measurements forthe LoS path are split up in cases with the beam towards andaway from the receiver, this is not the case for the followingcomparison.Table II lists the transmit power needed per channel (4 trans-mitter output channels are used in all cases) to produce anaverage BER = 10−4 at the receiver for different antenna anddiversity configurations and for the LoS and NLoS measure-ment scenarios. In the table, MRC refers to 4-th order receiverTABLE IITRANSMIT POWER PER CHANNEL FOR A BER OF 10−4 .LoS NLoSBeam STC Beam STC[dBm] [dBm] [dBm] [dBm]1 front arrayRX3 −12.0 −20.9 −3.9 −8.2MRC −28.5 −27.7 −15.0 −16.42 arrays, front and backRX3 −33.2 −29.1 +5.0 −9.7MRC −43.7 −44.0 −18.5 −17.2diversity, whereas RX3 refers to reception at only antenna 3,situated in the middle of the receiving array in Fig. 2. Systemswith MRC require approximately 10 times less transmit power,compared to reception at RX3. In case of MRC, beamformingand space-time coding require nearly the same transmit power.Note that static beamforming can be realized using only powersplitters en is therefore much more low-cost than space-timecoding, which requires full transmitter chains for each channel.Without MRC, beamforming is often not efficient, due to theabsence of diversity in the link.The double array system requires significantly less transmitpower than the single array configuration for equal signalingand propagation conditions. The associated extra system gainfor the MRC cases is highest in LoS conditions, amounting to15.2 dB for beamforming and 16.3 dB for space-time coding.In NLoS conditions these values are 3.5 dB and 1.2 dB,respectively.The single array system with the array oriented towardsthe receiver in LoS conditions, performs better than the dual-array system, with an up to 5 dB better BER characteristic [3]and comparable average received signals. However, in realisticconditions the beam will be directed randomly as the rescueworker is performing his duties, and a much better averageperformance for the dual-array system results.In NLoS conditions with MRC, the required transmit poweris comparable for dual- and single-array systems, allow-ing for a few dB difference between results from differentmeasurement campaigns. Without MRC, the required poweris also comparable in case of space-time coding. However,beamforming without MRC requires 8.9 dB more power forthe dual-array system, compared to a single-array. This couldbe attributed to interference between scattered signals fromthe front and back beam, resulting in a larger signal varianceand hence a worse BER characteristic. Alamouti coding forthe two beams can improve the performance here, combiningthe benefits of beamforming and space-time coding.For the dual-array system in LoS conditions, beamformingwithout MRC requires 4.1 dB less transmit power than space-time coding, because of the effective concentration of powerin the direction of the receiver. However, for the single-arraysystem, beamforming requires 8.9 dB more transmit power dueto the shadowing by the human body, which is not eliminatedwith this configuration.VI. CONCLUSIONIn LoS propagation conditions, the dual-array system iseffective in eliminating the shadowing by the human body.This is confirmed by graphs displaying the evolution of thereceived Eb/N0 for subsequent frames, for single- and dual-array systems.Beamforming power gains of around 3 dB, compared tospace-time coding are realized in LoS as well as NLoSconditions, with or without receiver diversity. However, thevariance of the received signal is often higher for beamformingthan for space-time coding due to the total absence of diversityin the beamforming case. Without diversity at the transmitter,diversity should be implemented at the receiver to obtain a lowbit error rate at the receiver, without drastically increasing thetransmit power.Focusing on the required transmit power to obtain a givenbit error rate (BER = 10−4) at the MRC receiver, the beam-forming versus space-time coding performance is comparablefor dual- as well as single-array systems. Note, however, thatbeamforming can be implemented using simple power splitterswhereas space-time coding requires separate transmitter chainsand is hence a much more expensive solution. In NLoS therequired transmit power is only slightly higher for the single-array system, but for LoS conditions the dual-array systemrequires around 15 dB less power for an equal bit error rateat the MRC receiver. For off-body beamforming a dual-arrayfront and back antenna system is recommended, combinedwith receiver diversity.REFERENCES[1] C. Hermosilla, R. Valenzuela, L. Ahumada, and R. Feick, “Empiricalcomparison of MIMO and beamforming schemes for outdoor-indoorscenarios,” Wireless Communications, IEEE Transactions on, vol. 8,no. 3, pp. 1139 –1143, march 2009.[2] M. Scarpello, D. Vande Ginste, H. Rogier, “Design of a low-coststeerable textile array operating in varying relative humidity conditions”,Accepted in August 2011 for publication in Microwave and OpticalTechnology Letters, Wiley.[3] P. Van Torre P., M. Scarpello, L. Vallozzi, H. Rogier, M. Moeneclaey,D. Vande Ginste, J. Verhaevert, “Indoor Off-Body Wireless Communi-cation: Static Beamforming versus Space-Time Coding”, Accepted inOctober 2011 for publication in the International Journal of Antennasand Propagation – Special Issue on MIMO Antenna Design and ChannelModeling.

Indoor off-body wireless communication using static zero-elevation Beamforming on front and back textile antenna arrays

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Indoor off-body wireless communication using static zero-elevation Beamforming on front and back textile antenna arrays

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