Beach, MA

Neirynck, D

Nix, AR

Williams, C

English

In this paper, the results of an on-body, line-of-sight (LOS), multiple-input, multiple-output (MIMO) channel sounding investigation are reported. Transmission takes place between on-body antennas and a uniform linear array, positioned at close range and within line-of- sight of each other. Due to the on-body location of the antennas, the influence of actual radio channel can no longer be separated from the effects of user behaviour and body proximity. This paper therefore argues that the composite channel must be considered. Post-processing of the recorded channels shows that their capacities outperform those from equivalent Rayleigh channels. Taking into account the clear LOS in the experiment, this appears counter-intuitive to the well-known fact that uncorrelated scattering results in high capacity. An analysis of the results highlights some less familiar facts and leads to an overview of capacity generating mechanisms in MIMO systems. The paper adds an analysis of the sensitivity of the capacity to the properties of the channel matrix, which is used for a discussion of the practical use of MIMO in personal and body area network

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                          Neirynck, D., Williams, C., Nix, A. R., & Beach, M. A. (2006). Personal areanetworks with line-of-sight MIMO operation. IEEE Vehicular TechnologyConference, 6, 2859 - 2862. 10.1109/VETECS.2006.1683390Link to published version (if available):10.1109/VETECS.2006.1683390Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.Personal Area Networks with Line-of-Sight MIMOOperationDries Neirynck, Chris Williams, Andrew Nix, Mark BeachCentre for Communications ResearchMerchant Venturers BuildingUniversity of BristolBristol, BS8 1UB, UKdries.neirynck@bristol.ac.ukAbstract— In this paper, the results of an on-body, line-of-sight (LOS), multiple-input, multiple-output (MIMO) channelsounding investigation are reported. Transmission takes placebetween on-body antennas and a uniform linear array, positionedat close range and within line-of-sight of each other. Due to theon-body location of the antennas, the influence of actual radiochannel can no longer be separated from the effects of userbehaviour and body proximity. This paper therefore argues thatthe composite channel must be considered. Post-processing ofthe recorded channels shows that their capacities outperformthose from equivalent Rayleigh channels. Taking into accountthe clear LOS in the experiment, this appears counter-intuitiveto the well-known fact that uncorrelated scattering results in highcapacity. An analysis of the results highlights some less familiarfacts and leads to an overview of capacity generating mechanismsin MIMO systems. The paper adds an analysis of the sensitivityof the capacity to the properties of the channel matrix, which isused for a discussion of the practical use of MIMO in personaland body area networks.I. INTRODUCTIONSince [1], the potential capacity increases offered bymultiple-input, multiple-output (MIMO) antenna systems arewell-known and have been studied extensively, mainly foroutdoor and wireless local area network (WLAN) applications.It is well known that in these applications, uncorrelated scat-tering allows MIMO systems to extract a significant diversitygain. As a result, line-of-sight (LOS) and correlated channelsare usually assumed to have an adverse effect on MIMOperformance [2].In this paper, the results of a wideband MIMO measure-ment campaign in a body area network are reported. Themeasurements were made in 120 MHz bandwidth centredaround 5.2 GHz. The experiment highlights the difficultiesof measuring these channels due to the influence of the useron antenna characteristics. Evaluation of the capacity showsthat despite line-of-sight and gain imbalance, the measuredchannels exceed the capacity of equivalent Rayleigh fadingThe work reported in this paper has formed part of the Wireless En-ablers work area of the Core 3 Research Programme of the Virtual Cen-tre of Excellence in Mobile & Personal Communications, Mobile VCE,www.mobilevce.com, whose funding support, including that of EPSRC, isgratefully acknowledged. Fully detailed technical reports on this research areavailable to Industrial Members of Mobile VCE.channels. Analysis of these results emphasises some subtletiesin the literature. This paper adds an investigation of thesensitivity of the capacity to the properties of the channelmatrix, which is used to demonstrate the practical potentialfor MIMO in body and personal area networks (BAN andPAN).The measurement campaign on which this paper is based ispresented in section II. The results are analysed and discussedin the following section, while section V draws conclusionsfor the use of MIMO in BAN and PAN.II. CHANNEL MEASUREMENT CAMPAIGNThe results presented in this paper are based on measure-ments that took place in a small room (5 by 4 metre), partof the Wireless and Networks Research Lab on level 1 ofthe University of Bristol’s Merchant Venturers Building (UK).These measurements are a sub-set of the campaign previouslyreported in [3]. Apart from two people, the measurementequipment and some furniture, the room was empty. For themeasurements used in this paper, one of the two people presenthas two, transmitting, stacked patch antennas [4] mounted atchest height. These represent devices in a chest pocket orantennas integrated in the user’s clothing. The user is facingthe array and standing about 1 metre away from it. Thereceiving array is an eight plus two passive elements 5.2 GHzuniform linear array with half wavelength antenna spacing.For 2-by-2 MIMO system evaluation below, antenna elements1 and 7 are used.For channel measurements for cellular or WLAN scenarios,careful calibration of the antennas makes it possible to extractthe actual radio channel from the measured transmissionresponses. For BAN or PAN applications where some of theantennas are body-mounted, this is no longer possible. Theproximity of the body will change the antenna characteristics[5], while user actions [6], [7] and arbitrary antenna orientationwill lead to further unpredictable changes to the recordedtransmission response. Therefore, in this type of situations,it is necessary to extend the definition of the radio channel toa system level perspective that includes the antennas and usereffects.0-7803-9392-9/06/$20.00 (c) 2006 IEEE0 5 10 15 20 25 30 35 40 4500.10.20.30.40.50.60.70.80.91Wideband Capacity − 1 metre from array, facingCapacity, bits/s/HzProbability(capacity<abscissa)  Fixed TX Power and Noise LevelMIMO, measured dataSISO, measured dataSISO IID RayleighMIMO, IID RayleighMIMO, Power Imbalanced RayleighFig. 1. Wideband Capacity ComparisonIII. POST-PROCESSING RESULTSFig. 1 compares the wideband capacities of the measuredSISO and MIMO links with equivalent independent, iden-tically distributed (IID) Rayleigh channels. In order to dothis, the capacity has been averaged over the 120 MHzmeasurement bandwidth.The differences between the SISO capacities are due to again imbalance caused by body mounting, which results inantenna orientation and polarisation alignment differences. Inthe measured data, up to 10 dB power difference is observed.Therefore, fig. 1 also includes the capacity of an equivalentMIMO system with Rayleigh fading channels that have thesame power imbalance as observed in the measured channels.Despite the power imbalance and the clear line-of-sight,the measured channels outperform both the IID and powerimbalanced Rayleigh fading MIMO systems. This is due tothe deterministic effect of the LOS. Since there is no fading,the capacity will be constant over the measurement bandwidthand time period, while the Rayleigh wideband average is takenover a number of narrowband frequency bins that fade arbitrar-ily. For the measurements presented here, the measured datahas a capacity equal to that of the highest narrowband Rayleighfading channels over the whole measurement bandwidth.The latter is still surprising because it is well known thatuncorrelated channels result in high MIMO capacity. Sincethe system is operating in line-of-sight and short-range, it isunlikely that scatterers provide the necessary decorrelation.Indeed, analysis of the spatial correlation coefficients showsthat they are all high:abs(R) =1.0000 0.9726 0.9155 0.90820.9726 1.0000 0.9240 0.96660.9155 0.9240 1.0000 0.87940.9082 0.9666 0.8794 1.0000 (1)According to some papers, e.g. [2], this would imply thatthe MIMO capacity collapses to SISO case.IV. ANALYSIS AND DISCUSSIONAs the measurement results demonstrate, high correlationcoefficients don’t necessarily lead to low MIMO capacity. Inthe narrowband case, the capacity equation is given by [1]:C = log2[det[INR +ρNTHH†]]bps/Hz (2)Hence, the capacity is determined by the properties of HH†,which, for 2-by-2 MIMO can be extended as1:HH† =[h11 h12h21 h22] [h′11 h′21h′12 h′22]=[h11h′11 + h12h′12 h11h′21 + h12h′22h21h′11 + h22h′12 h21h′21 + h22h′22] (3)The entries on the diagonal will be real valued and equalto sum of the power of the respective channels. For a par-ticular location, this will be dependent on the path loss. Thevalue of the determinant, and hence the MIMO capacity, isdetermined by the sum of h21h′11 and h22h′12. The capacitywill be maximal when these off-diagonal values are zero.Since h21h′11 + h22h′12 is the inner product of the rows ofH , maximal capacity is achieved when channel matrix H isorthogonal [8].Note that hxh′y is proportional to the correlation coefficient.In most scenarios, the channel coefficients are out of thedesigner’s control and hence uncorrelated scattering is requiredto minimise the values of the off-diagonal entries. In pureLOS, however, the channel coefficients become deterministic.The phase difference between the channel coefficients is fixedand dependent on the operating frequency, array geometry andpositions [9]. Since the coefficients are complex valued, evenhighly correlated channel coefficients can add destructively,and result in orthogonal rows. In order for this to happen, asignificant phase difference between the channel coefficientsis required. This is possible when the spherical nature ofthe wavefronts has to be taken into account, i.e. for systemsoperating over a short distance or with significant antennaspacings [10]. In [11], this has been used to derive a designcriterion that maximises the channel capacity under LOSconditions.In order to get an appreciation of the sensitivity of thecapacity to orthogonality, consider the following normalised,LOS channel matrix:H =[e−jkd11 e−jkd12e−jkd21 e−jkd22]where dxy is the distance between transmit and receive anten-nas a and b and it is assumed that this distance is such that1In this paper, † will be used to indicate the Hermitian of a matrix, while′ is used to indicate the complex conjugate of a number0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20.50.550.60.650.70.750.80.850.90.951Magnitude Off−Diagonal HH†Capacity, Normalised to MaximumFig. 2. Relation between orthogonality and capacitythe path loss differences are negligible. As shown in [11], inthis case, HH† equals:HH† =[2 ejk(d21−d11) + ejk(d22−d12)ejk(d11−d21) + ejk(d12−d22) 2]where the magnitude of the off-diagonal sum varies between 0and 2 depending on the antenna spacing. Since the entries onthe diagonal are constant, the magnitude of the off-diagonalentry is a measure for the orthogonality of the channel matrix.The closer to zero, the closer to orthogonal the rows of thematrix are and the higher the capacity will be. The exactrelation between this value and the resulting capacity for asystem operating in the high SNR region is given in fig. 2. Dueto the logarithmic relation between the two, the magnitude ofthe off-diagonal entries has to rise up to 1.98 for the capacityto drop below 90 percent of its maximum value.Distance from Array (m) − Endfire DirectionDistance from Array (m) − Broadside DirectionCapacity over Area, Normalised to Maximum−5 −4 −3 −2 −1 0 1 2 3 4 500.511.522.533.544.55 0.50.550.60.650.70.750.80.850.90.951Fig. 3. Capacity in area in front of arrayTherefore, a MIMO system can still offer significant capac-ity increases in LOS conditions even if the conditions are notideal. Indeed, in the recorded channels, 2-by-2 MIMO achievesan 85 percent capacity increase compared to SISO systemsdespite 10 dB gain imbalance. In fig. 3, the capacity achievedby a 5.2 GHz MIMO system optimised for a separation of1 metre is shown over an area of 10-by-5 metres. As canbe seen, the MIMO system is able to extract a significantcapacity increase in most of this area. Therefore, it can beconcluded that despite the probable presence of a LOS, it isworth considering MIMO for PAN applications.V. CONCLUSIONSThis paper has reported on the results of a channel mea-surement campaign in a body area network. Difficulties to theproximity of the body have been discussed and it was proposedto extend the channel definition to include the influenceof the user. It was explained how, despite a clear line-of-sight, the capacity of the measured channels could outperformequivalent Rayleigh fading channels. Relevant references werehighlighted to show that orthogonality of the channel matrixis required to achieve high capacity and how, under LOSconditions, careful antenna placement can lead to such anorthogonal channel matrix. The paper added an analysis ofthe sensitivity of capacity to orthogonality. It was shown thatdespite significant deviations from orthogonality, MIMO canstill offer significant capacity gains. This knowledge was thenapplied to a personal area network scenario where it wasshown that this capacity increase can occur over a signifi-cantly big area to consider MIMO for personal area networkapplications.VI. ACKNOWLEDGMENTSThe authors are very grateful for the guidance from theIndustrial Members of the Wireless Enablers Steering Groupand would like to express particular thanks to Dr. DeanKitchener from Nortel Wireless Technology Laboratories UK.We would also like to thank Yannis Sarris for the manyuseful conversations on the topic of LOS MIMO.REFERENCES[1] G. Foschini and M. Gans, “On limits of wireless communications in afading environment when using multiple antennas,” Wireless PersonalCommunications, vol. 6, pp. 331–335, 1998.[2] S. Loyka and A. Kouki, “Correlation and MIMO communication archi-tecture (invited),” in 8th International Symposium on Microwave andOptical Technology, Montreal, Canada, June 2001.[3] D. Neirynck, C. Williams, A. Nix, and M. Beach, “Experimental capac-ity analysis for virtual antenna arrays in body and personal networks,”in International Workshop on Wireless Ad-hoc Networks, May 2005.[4] D. L. Paul, I. J. Craddock, C. J. Railton, P. N. Fletcher, and M. Dean,“FDTD analysis and design of probe-fed dual-polarized circular stackedpatch antenna,” Microwave and Optical Technology Letters, vol. 29,no. 4, pp. 223–226, May 2001.[5] J. Toftgard, S. Hornsleth, and J. Andersen, “Effects on portable antennasof the presence of a person,” IEEE Transactions on Antennas andPropagation, vol. 41, no. 6, pp. 739 – 746, June 1993.[6] D. Neirynck, C. Williams, A. Nix, and M. Beach, “Wideband channelcharacterisation for body and personal area networks,” in 2nd Interna-tional Workshop on Wearable and Implantable Body Sensor Networks,Apr. 2004.[7] A. Alomainy, A. S. Owadally, Y. Hao, C. G. Parini, Y. I. Nechayev,C. C. Constantinou, and P. S. Hall, “Body-centric wlans for futurewearable computers,” in 2nd International Workshop on Wearable andImplantable Body Sensor Networks, Apr. 2004.[8] D. Gesbert, H. Bo¨lcskei, D. A. Gore, and A. J. Paulraj, “OutdoorMIMO wireless channels: Models and performance prediction,” IEEETransactions on Communications, vol. 50, no. 12, pp. 1926–1934, Dec.2002.[9] P. Driessen and G. Foschini, “On the capacity formula for multiple input- multiple output wireless channels: A geometric interpretation,” IEEETransactions on Communications, vol. 47, no. 2, pp. 173–176, Feb. 1999.[10] J.-S. Jiang and M. A. Ingram, “Distributed source model for short-rangeMIMO,” in IEEE 58th Vehicular Technology Conference, VTC 2003-Fall, vol. 1, Oct. 2003, pp. 357–362.[11] I. Sarris and A. Nix, “Maximum MIMO capacity in line-of-sight,” inInternational Conference on Information, Communications and SignalProcessing, 2005, In Press.

Correlation and MIMO communication architecture (invited),”

Distributed source model for short-range MIMO,”

Effects on portable antennas of the presence of a person,”

Experimental capacity analysis for virtual antenna arrays in body and personal networks,”

FDTD analysis and design of probe-fed dual-polarized circular stacked patch antenna,”

Maximum MIMO capacity in line-of-sight,”

On limits of wireless communications in a fading environment when using multiple antennas,”

On the capacity formula for multiple input - multiple output wireless channels: A geometric interpretation,”

Outdoor MIMO wireless channels: Models and performance prediction,”

Wideband channel characterisation for body and personal area networks,”

Personal area networks with line-of-sight MIMO operation

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Personal area networks with line-of-sight MIMO operation

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