Some recent publications have extended the emulating
capabilities of reverberation chambers. While polarization
imbalance has been removed and Ricean-fading environments
are now properly emulated, these chambers are still limited to
isotropic nonline of sight (NLOS) scattering. By controlling the
power received, number of resolvable multipath components
(MPC), angular spread (AS), and angle of arrival (AoA), the emulation
of real-propagating environments with both isotropic and
nonisotropic scattering are demonstrated in this letter using a reverberation
chamber with several multiple-input–multiple-output
(MIMO) arrays.This work was supported in part by the Fundación Séneca,
the R&D unit of the Autonomous Region of Murcia (Spain) under project references
TIC-TEC 07/02-0005 and by the Spanish National R&D Programme
through TEC2007/63470/TCM

Martínez González, Antonio Manuel

Sánchez Hernández, David Agapito

Valenzuela Valdés, Juan Francisco

IEEE Antennas and Wireless Propagation Letters

Repositorio Digital de la Universidad Politécnica de Cartagena

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008 325Emulation of MIMO Nonisotropic FadingEnvironments With Reverberation ChambersJuan F. Valenzuela-Valdés, Antonio M. Martínez-González, and David A. Sánchez-Hernández, Senior Member, IEEEAbstract—Some recent publications have extended the emu-lating capabilities of reverberation chambers. While polarizationimbalance has been removed and Ricean-fading environmentsare now properly emulated, these chambers are still limited toisotropic nonline of sight (NLOS) scattering. By controlling thepower received, number of resolvable multipath components(MPC), angular spread (AS), and angle of arrival (AoA), the em-ulation of real-propagating environments with both isotropic andnonisotropic scattering are demonstrated in this letter using a re-verberation chamber with several multiple-input–multiple-output(MIMO) arrays.Index Terms—K-factor, q-factor, reverberation chamber.I. INTRODUCTIONW IRELESS channel behaviour has received much atten-tion in recent years, particularly multiple-input–mul-tiple-output (MIMO) techniques as a way to increase thechannel capacity. Channel modeling is normally divided intophysical and not physical models. Two essential parameters ofphysical model are the number of scatters (NS), closely relatedto the number of resolvable multipath components (MPC),and the angular spread (AS). In fact, it has been demonstratedthat MIMO capacity increases linearly with the AS for a lineararray [1]. System performance also depends on angle of arrival(AoA) and angle of departure (AoD), which have been widelystudied in the literature. Likewise, reverberation chambers areemployed to emulate the wireless channel behavior in order toavoid long and cumbersome measurement campaigns. A rever-beration chamber is typically a metallic enclosure wherein theelectromagnetic field distribution is stirred by several means inorder to obtain a field distribution that is statistically isotropicand homogeneous. The associated power density within thereverberation chamber typically follows that of an isotropicmultipath environment [2]. The emulating possibilities of thesechambers are typically limited to Rayleigh-fading scenarioswith isotropic scattering, some elevation angles and restrictedpolarization diversity. Some recent papers have demonstratedthe ability of reverberation chamber to emulate Ricean-fadingManuscript received November 6, 2007; revised April 10, 2008. First pub-lished July 17, 2008. This work was supported in part by the Fundación Séneca,the R&D unit of the Autonomous Region of Murcia (Spain) under project ref-erences TIC-TEC 07/02-0005 and by the Spanish National R&D Programmethrough TEC2007/63470/TCM.J. F. Valenzuela-Valdés is with the EMITE Ingeniería SLNE, E-30100 Es-pinardo, Murcia, Spain (e-mail: juan.valenzuela@emite-ingenieria.es).A. M. Martínez-González and D. A. Sánchez-Hernández are with theDepartamento de Tecnologías de la Información y Comunicaciones, Uni-versidad Politécnica de Cartagena, Cartagena E-30202, Spain (e-mail:david.sanchez@upct.es).Digital Object Identifier 10.1109/LAWP.2008.928488environments [3] and to eliminate typical polarization imbal-ance [4], and therefore reverberation chamber are acquiringwider acceptance as validating tools.In this letter, we have demonstrated that it is also possibleto emulate Rayleigh-fading scenarios with nonisotropic scat-tering using reverberation chambers. Seven different scenariosare employed with diverse AoA, AS, and MPC, and their asso-ciated correlation and cluster data properties are provided. De-spite the need for further research to accurately characterize thenovel emulation capabilities, the goal of this letter is to demon-strate that reverberation chambers can be used to emulate re-alistic propagation environments by providing some measuredquantitative data.II. EMULATING AND MEASURING TECHNIQUESThe measurements were carried out in our indoor laboratoryat the Technical University of Cartagena in Spain. The RC800reverberation chamber by Bluetest AB was used in a corner ofa 4 m 5 m 3 m room. The RC800 has dimensions of 0.8m 1 m 1.6 m. Three wall-mounted antennas, three polariza-tion stirring positions, one mechanical stirrer with 10 differentpositions, for each five different position of a second mechan-ical stirrer, a fixed platform position and 50-MHz frequency stir-ring were used. Measurements were performed at 1800 MHzand half-wave dipoles were used as MIMO array antennas.Seven different measuring scenarios were prepared. ScenarioA represents an empty reverberation chamber, providing thetypical isotropic scattering. In Scenario B, one absorber piece of24 cones is placed on the right-hand side wall of the chamber.Since the dipoles do not receive the expected multipath compo-nent of the absorbed side [5], a nonisotropic scattering scenariois obtained with modified AoA and reduced MPC compared toScenario A. In Scenario C, two absorbers are used instead ofone by adding a second absorber to the left-hand side, and amodified AoA histogram along with a further reduction of MPCis expected. Two and four PTFE 30–cm-high hollow cylindersof 5-cm-radius filled with lossy CENELEC Head SimulatingLiquid (HSL) at the frequency of interest are introduced in theempty chamber to provide for Scenarios D and E, respectively.The filled cylinders make the Q-factor of the chamber decrease[3], slightly increasing the K-factor but a change in the AoAhistogram and a further reduction in the MPC is also expected.The chamber door was left open in Scenario F, and the receivingantennas were located further apart outside the chamber justin front of the open door. In this way, nonisotropic scatteringis again obtained, modifying AoA and MPC. Finally, ScenarioG has the same configuration than Scenario F, but with the re-ceiving antennas placed in the lateral side outside the chamber,1536-1225/$25.00 © 2008 IEEE326 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008Fig. 1. Emulated fading scenarios with RC800. Left: Scenario C (door openfor view only). Right: Scenario F.TABLE IEVALUATED MIMO ARRAYSproviding a further modification of AoA, AS, and MPC com-pared to Scenario F. Since no absorbers or cylinders are em-ployed, an increment on the AS is expected for Scenarios F andG in comparison to the others.A photograph of some of these scenarios (C and F) is de-picted in Fig. 1. Likewise, in order to evaluate the correlationand cluster data properties in all these measuring scenarios,eight different MIMO arrays were employed using spatial-di-versity for each of the measuring scenarios. The employedMIMO arrays are listed in Table I for Scenario A asan example, where is the interelement spacing ,and and indicate the number of transmitting and receivingantennas, respectively.III. EVALUATION METHODSThe K-factor of the employed scenarios has been evaluatedby an estimation of the Q-factor using the cavity theory and byexperimental data analyses.A. Estimation of the Q-FactorThe Q-factor of a cavity can be estimated from the averagepower transmission level by [6](1)where is the volume of the cavity, is the speed of light, andis the frequency of operation. The K-factor can be calculatedfrom the Q-factor by [3](2)Fig. 2. Evaluation of received power level, K-factor, and Q-factor for all sce-narios.where is the directivity of the antenna under test, is thewavelength, and is distance between antennas. By combining(1) and (2) we obtain(3)The received power, Q-factor and K-factor evaluated using thismethod for all scenarios are plotted in Fig. 2. As it was expected,the power level decreases with increasing volume of lossy ma-terial. The Q-factor decreases progressively in Scenarios B toE respect to Scenario A due to the lossy material that was in-troduced inside the chamber. When the door is open and thenumber of modes consequently increased (Scenarios F and G),the situation can no longer be considered that of a cavity, andno Q-factor or associated K-factor is depicted. The values ofthe K-factor are very low, always below 0.02, thus, providing aRayleigh-fading-like environment for all emulated scenarios.B. Experimental Data AnalysesThe K-factor can also be calculated from measured S-param-eter in a reverberation chamber by [3](4)where is the measured parameter in the reverberationchamber for each antenna pair, is the radius of cluster data,and is the distance of centroid of cluster from the origin. Asan example, the scatter plots [3] of measured between theantenna pairs and for all scenarios are depictedin Figs. 3 and 4, respectively. In these figures, the x and y axisrepresent the real and imaginary part of , respectively. Ascatter plot would result in the data clustered in a circle and cen-tered about the origin for pure Rayleigh-fading environments.As the direct line of sight (LoS) energy became comparable tothe non-LoS energy, the cluster of data would move away fromthe origin, and the fading environment becomes a Rician one.Scatter plots are useful not only for identifying the LoS be-havior of the fading environments, but also to quantify the datadispersion through the K-factor. It can be observed from Figs. 3and 4 that decreases with increasing load of the chamber.Finally, the averaged K-factor obtained from data analyses ofVALENZUELA-VALDÉS et al.: MIMO NONISOTROPIC FADING ENVIRONMENTS 327Fig. 3. Scatter plots for all emulated scenarios.Fig. 4.   for all scenarios and transmitter antennas.the performed measurements taken is shown in Fig. 5, alongwith a comparison to K-factors estimated from the Q-factors.From this figure, one can observe that the two methods usedfor estimating the K-factor tend to provide similar results forall scenarios except for those wherein the chamber door isopen (F and G), where the cavity method cannot be applied.Reducing the Q-factor of the chamber with absorbing material(Scenarios B to E) did not change the size of the cavity, andconsequently the estimating method based on the Q-factorprovided good results. When the door was open, however, themeasured scenario could no longer be represented by a cavity,and a proper estimation of the K-factor was only obtained fromthe data taken from measurements.IV. MEASURED RESULTSThe correlation function in a MIMO system depends stronglyon the multipath parameters such as AS, MPC, or AoA [7], [8].The measured correlation coefficient between any two antennasin the array for all emulated scenarios and arrays is illustratedin Fig. 6.The results depicted in Fig. 6 show an increasing correlationcoefficient with decreasing MPC (NS), which has also been pre-viously reported [9]. As expected, the lowest correlation coef-ficient is obtained for Scenario A, i.e., for isotropic scattering.In Scenario F, the correlation is found to be similar to ScenariosFig. 5. K-factors for all scenarios.Fig. 6. Correlation coefficients for all scenarios.D and E until . While the higher correlationin Scenarios B and C respect to Scenario A is due to the use ofabsorbers, for Scenarios D and E, this is due to the presence ofthe HSL-full cylinders. Scenarios F and G are also highly cor-related despite the larger distance between transmit and receiveantennas. This is due to a considerable reduction in the MPCwhen the door is open. Consequently, a decrease in capacity isexpected for Scenarios F and G. In order to provide quantitativedata and to confirm the described relationship between correla-tion and multipath parameters, the number of resolvable MPC,AS, and AoA histograms were measured for all scenarios.In Fig. 7, the measured eigenvalues of the data matrix forall scenarios are depicted. In this figure, the number of MPCwas determined from the relative decrease between neighboringeigenvalues and confirmed by visual inspection of plots. Foreach MPC, the transfer function between all pairs of antennaswere estimated by multiple signal classification (MUSIC) high-resolution algorithm [10] with pseudoinverse. The number ofresolvable MPC ranged from 12 to 24, and decreases with as-cending scenario letter (A to G), as expected. The decrease inMPC for the scenarios with the open door (F and G) and no ab-sorbers or cylinders, however, would result in an increment ofAS. This is confirmed by Fig. 8, wherein the AoA is calculatedfrom the measured data again using the MUSIC high resolutionalgorithm [10]. The algorithm was modified to account for the328 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008Fig. 7. Measured eigenvalues of the data matrix for all scenarios.Fig. 8. Histograms of measured AoA using the high resolution MUSIC algo-rithm.fact that our receiving antennas are dipoles and therefore the ra-diation patterns are not constant as a function of the incidentangle.Fig. 8 clearly shows the effect of the diverse scenarios onthe AoA. The conventional enclosed chamber operation (Sce-nario A) shows a quasi-uniform angle distribution of energy,with power differences around 20 dB for diverse angles. Thisconfirms the quasi-isotropic behavior of the chamber, where theisotropicity could be improved by using more stirred positions.The addition of one absorber (Scenario B) displaces the AoAto the chamber side where there is no absorber, while addingtwo symmetric absorbers (Scenario C) makes a narrower AS, asexpected. The use of liquid-filled cylinders in Scenarios D andE at close presence of the antennas grants a different situation.The cylinders have to absorb energy, but due to the large differ-ence between the HSL permittivity and the air they also scattera large amount of the incident power. This can be observed inFig. 8 for Scenario D, where the AoA is displaced to the sidewhere the pair of cylinders is located, and for Scenario E, wheretwo different clusters are identified in the azimuth directionswhere the two pairs of cylinders are located. From Fig. 7, onecan also see that these scatters become relevant ones instead ofexisting ones, and MPC is not increased. Finally, Scenarios Fand G (door open), confirm the expected increase in the AS, aswell as the decrease in the received power due to the larger sep-aration between the transmit and receive arrays. Measured ASvalues defined as in [1] were 0.21, 0.18, 0.09, 0.20, 0.18, 0.23,and 0.27 for Scenarios A to G, respectively. In all modified sce-narios, a nonisotropic behavior is observed, with up to 50–dBpower differences for diverse angles.V. CONCLUSIONDiverse nonisotropic Rayleigh-fading environments havebeen emulated with a reverberation chamber. Different clusterdata, correlation properties and multipath parameters have beenobtained for diverse MIMO arrays. Despite the low K-factorsobtained with the chamber, the differences in AoA, NS, andAS provided different correlation properties. In addition, wehave shown that when the chamber door is open, an estimationof the emulated K-factor can only be ensured by processing themeasured data. Accurate characterization of the new environ-ments been emulated is foreseen, particularly for the sensitivityto deviations effects in the time domain.REFERENCES[1] A. S. Y. Poon, D. N. C. Tse, and R. W. Brodersen, “Impact of scatteringon the capacity, diversity, and propagation range of multiple-antennachannels,” IEEE Trans. Inf. Theory, vol. 52, no. 3, pp. 1087–1100, Mar.2006.[2] K. Rosengren and P.-S. Kildal, “Theoretical study of angular distribu-tion of plane waves in a small reverberation chamber for simulatingmultipath environment and testing mobile phones,” in Proc. IEEE An-tennas Propag. Soc. Int. Symp., 2001, vol. 3, pp. 358–361, vol 3.[3] C. L. Holloway, D. A. Hill, J. M. Ladbury, P. F. Wilson, G. Koepke,and J. Coder, “On the use of reverberation chambers to simulate a ri-cian radio environment for the testing of wireless devices,” IEEE Trans.Antennas Propag., vol. 54, no. 11, pp. 3167–3177, Nov. 2006.[4] P. S. Kildal and C. Carlsson, “Detection of a polarization imbalance inreverberation chambers and how to remove it by polarization stirringwhen measuring antenna efficiencies,” Microw. Opt. Technol. Lett., vol.34, no. 2, pp. 145–149, Jul. 2002.[5] M. Otterskog and K. Madesen, “On creating a non-isotropic propa-gation environment inside a scattered field chamber,” Microw. Opt.Technol. Lett., vol. 43, no. 3, pp. 192–195, Nov. 2005.[6] D. A. Hill, M. T. Ma, A. R. Ondrejka, B. F. Riddle, M. L. Crawford, andR. T. Johnk, “Aperture excitation of electrically large, Lossy cavities,”IEEE Trans. Electromagn. Compat., vol. 3, no. 3, pp. 169–178, Aug.1994.[7] P. D. Teal, T. D. Abahayapala, and R. A. Kennedy, “Spatial correlationfor general distributions of scatterers,” IEEE Signal Process. Lett., vol.9, no. 10, pp. 305–308, Oct. 2002.[8] K. Anim-Appiah, “Complex envelope correlations for non-isotropicscattering,” Electron. Lett., vol. 34, no. 9, pp. 918–919, Apr. 1998.[9] Z. Tang and A. S. 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Emulation of MIMO nonisotropic fading environments with reverberation chambers

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Emulation of MIMO nonisotropic fading environments with reverberation chambers

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