MIMO Channel Capact Measurement n the Presence of Spatal Clusters Usng a Fadng Emulator Tsutomu Sakata, Atsush Yamamoto, Koch Ogawa Panasonc Corporaton 6 Kadoma, Kadoma Ct, Osaka, 7-8, Japan Jun-ch Takada Toko Insttute of Technolog --, O-okaama, Meguro-ku, Toko, -8 Japan Abstract Ths paper presents a rado freuenc (RF) controlled spatal fadng emulator for the evaluaton of a handset MIMO antenna. The RF-controlled fadng emulator can control RF sgnals drectl n the spatal doman to produce an accurate rado propagaton channel model, whch ncludes the unform and non-unform angular power spectra (APS) n the horzontal plane. Fadng correlaton and MIMO channel capactes of a uas-dpole arra, n the case of up to three spatal clusters, are measured b usng the emulator. From agreement between the measured data and theoretcal values, the effectveness of the emulator was confrmed. Kewords- Fadng, multpath, cluster, handset, antenna, MIMO, channel capact I. INTRODUCTION Smple and approprate over the ar (OTA) testng for moble handsets wth a multple nput multple output (MIMO) sstem s ndspensable to the success of upcomng 3G-LTE and IMT-Advanced sstems. A handset MIMO s assumed to be used n multpath rado propagaton envronment, both n lne-of-sght (LOS) and non lne-of-sght (NLOS) stuatons. Thus, test eupment, representng a MIMO propagaton channel model, s needed for OTA testng. Some fadng smulators [-] can produce a fadng channel wth dela characterstcs, Doppler freuenc, etc. Although the fadng smulators are ualfed for the evaluaton of wreless crcuts, the smulators cannot be used for the test of a handset ncludng antennas, because of a lack of ablt to assess the radaton characterstcs of a handset antenna. A spatal fadng emulator [3-8] n an anechoc chamber s one of the solutons to drectl producng a rado multpath envronment. Ths paper presents an RF-controlled spatal fadng emulator, sutable for evaluatng a handset MIMO antenna. The RF-controlled fadng emulator has a smple structure because of the drect control of RF sgnals. In ths paper, the measurement procedure for MIMO antenna characterstcs s manl dscussed. In secton, an epermental setup and confguraton of the emulator are descrbed. A propagaton model for a handset MIMO antenna s presented n secton 3. Fnall performance evaluaton of a MIMO antenna usng the emulator s dscussed n secton 4. II. EXPRIMENTAL SETUP OF SPATIAL FADING EMULATOR Fg. shows photographs of the spatal fadng emulator n an anechoc chamber [3-8] and the scatterng unt, comprsng vertcall and horzontall polarzed antennas. The emulator contans 3 scatterng unts that are composed of two halfwavelength dpoles, crossng at rght angles, n order to represent a cross polarzaton power rato (XPR), as shown n Fg. (b). Ths arrangement enables two components of the rado sgnal to propagate from the same drecton. (a) (a) Setup n a rado anechoc chamber (b) (b) Scatterng unt Fgure. Photograph of the spatal fadng emulator.
Scatterng unt A Nakagam-Rce fadng can also be created n such a manner that the sgnal strength of a sngle scatterng unt s greater than that of the other unts [3]. As shown n Fg. (b), the 3 scatterng unts were located at evenl-spaced ntervals around a crcle 3 m n dameter. A handset antenna was set at the center of the scatterng unts. III. PROPAGATION MODEL FOR A HANDSET MIMO ANTENNA Fgure. (a) Epermental setup R ATT Phase Shfter D/A # #3 φ # # #3 r=.m #3 (b) Arrangement of the scatterng unts Power Dvder Epermental setup of the spatal fadng emulator. Fgs. (a) and (b) llustrate the epermental confguraton and the relatve poston between the antenna arra to be tested, shown b a red dot located at the center, and the emulator. The control crcut was comprsed of 6 sets of phase shfters and attenuators (two groups of 3 components for each polarzaton), whch var the phase and ampltude of the sgnals radated from the scatterng unts. The phase shfter and attenuator were operated n the rado freuenc (RF) band. Ths structure, usng RF-controlled phase shfter and attenuator, has an advantage n terms of smplfcaton of the emulator n comparson wth a dgtal phase shfter and attenuator, operated n a base band freuenc, snce dgtal phase shfter and attenuator reure both a freuenc downconverter and up-converter. The ampltude of the sgnals radated from the scatterng unts was vared usng the attenuators. A computer was used to calculate the values of phase shft of the sgnals emtted from the unts, n order to obtan a Ralegh-fadng channel, based on the Clarke s model. Recever Transmtter Computer A. MIMO channel model Fg. 3 shows the channel model of an M-b-N MIMO sstem, whch s an etended verson of the channel model for handset adaptve arras [9-]. M base staton (BS) antennas create a set of M uncorrelated waves, each of whch comprses Q c clusters surroundng around N handset antennas. The handset s assumed to be surrounded b Q c clusters, wth handset movng towards the azmuth drecton, φ. The angular power spectrum of the -th cluster s modeled b a Gaussan dstrbuton n the followng: ( φ m ) P ( φ ) = ep, () πσ σ where P and m are power and average drecton of angle of the cluster, respectvel. σ s the standard devaton of the angular power spectrum. As a result, the spatal power dstrbuton of the ncomng waves s defned as a mamum envelope wth respect to the cluster usng the followng euaton (see Fg. 8): { } ( φ) = ( φ) Ma. () In order to create the above dstrbuton usng the emulator, the radaton power of each scatterng unt s adjusted b the attenuator, as shown n Fg. (a). Channel response between the m-th BS antenna and the n- th handset antenna s calculated b followng euaton: Km λ h nm = En ( φ ) ep[ j{ kr + π t fd cos( φ φ ) + αm} ], = 4π r (3) where E n and f D are radaton component of the n-th handset antenna and the Doppler freuenc, respectvel. φ s the drecton of moton and φ s the drecton of the -th scatterng unt. α m s ntal phase of the sgnal radated from the -th scatterng unt. The waves radatng from each BS antenna are uncorrelated each other. For the nvestgaton of MIMO antennas, the waves from dfferent BS antenna are represented b dfferent sets of ntal phases, α m, of the waves. Wth regard to the XPR, the radaton powers of the vertcall and horzontall polarzed waves emtted from the - th scatterng unt are defned as follows:
_ v _ h XPR =, (4) + XPR = + XPR, () where s the total radaton power of the -th scatterng unt. were Hz and Hz, respectvel. In ths nvestgaton, the movng dstance of a handset was 6 wavelengths of the rado freuenc so that the number of snap shots was 3,. The handset MIMO antenna conssted of uas-dpole arra antennas wth a separaton of a half-wavelength, constructed wth a sngle dpole antenna beng placed at two dfferent locatons. Fg. 4 shows the arrangement of the handset dpole arra. As shown n Fg. 4, two arrangements were consdered. m= M T n= N R Cluster = = TABLE EXPERIMENTAL PARAMETERS Rado freuenc GHz Doppler freuenc Hz Samplng freuenc Hz Movng dstance 6 wavelengths Snap shot 3, Movng drecton degree Qc scatterer Fgure 3. MIMO channel model. Movng drecton K m R R φ p = d d R φ p =π/ R B. Evaluaton procedure of MIMO antenna In ths secton, a procedure of calculatng a MIMO channel capact s presented. The theoretcal open-loop Shannon capact, C s, of an n T n R MIMO sstem of the s-th snap shot s gven as follows: C γ H s = log I nr + HH [bts/sec./hz], (6) nt where A denotes the determnant of A, H s a channel matr comprsng an n T n R comple matr, and I nr ndcates the dentt matr of dmenson n T n R. γ s the nput sgnal to nose rato (SNR), defned as an SNR for each ncdent wave when an sotropc antenna s used for recevng the ncdent wave, whch allows the performance of antenna elements used n the MIMO arra to be ncluded n the measured results []. The average channel capact s gven b the followng euaton: C = [bts/sec./hz], (7) S C s S s= where S s the number of the snap shots. IV. PERFORMANCE EVALUATION OF MIMO ANTENNA In ths secton, the performance evaluaton of a MIMO antenna s presented, based on measured receved sgnals usng the spatal fadng emulator and a vector network analzer. The epermental parameters are lsted n Table. The rado freuenc was set at GHz. Doppler and samplng freuences (a) X algnment (b) Y algnment Fgure 4. Arrangements of the handset dpole arra. Frstl, MIMO antenna performance n the case wth one cluster was nvestgated. Fg. shows the epermental results of angular power spectrum n the horzontal plane, produced b the emulator. It s observed from Fg. that the emulator accuratel represented the angular power spectrum wth a Gaussan dstrbuton. Fadng correlaton between the two dpoles s shown n Fg. 6. The correlatons were defned as an absolute value of the comple fadng correlaton. ρ π ( φ) ep{ jkd cos( φ φ )} π = π π ( φ) dφ p dφ Theoretcal curves calculated from E. 8 are also plotted n Fg. 6. It s found from Fg. 6 that the measured correlatons were n good agreement wth the theoretcal values. The results ndcate that the fadng correlaton of the arra n the rado propagaton envronment wth one cluster can be measured b usng ths emulator. Two smbols of a whte crcle and a whte rectangle, drawn on the rght as, show the case where a unform angular power spectrum s assumed, whch s euvalent to a standard devaton σ of nfnt. The fgure shows that the two curves converge to the unform APS case. Fg. 7 shows MIMO and SISO channel capact of the two dpoles n the presence of the one cluster. It s found from Fg. 7 that the measured data are n good agreement wth the calculated results. Furthermore, t s observed from Fgs. 6 and 7 that ver hgh correlatons n the case of small σ degraded the MIMO channel capactes. (8)
Normalzed Power [db] - σ= σ= - σ=3 σ= - σ=7 σ= σ= - -8 - -6 6 8 Angle φ [degree] Fgure. Measured angular power spectrum wth one cluster and unform dstrbuton. Correlaton Coeffcent.8.6.4. Y algnment 4 6 8 Standard Devaton σ [degree] Fgure 6. Fadng correlaton between the two dpoles n the presence of one cluster. Channel Capact [bts/s/hz] m= deg. Gauss. Theor MIMO SISO X algnment SNR=3dB 4 6 8 Standard Devaton σ [degree] Normalzed power [db] - -4-6 ΔP -8 9 8 7 36 Angle [degree] Fgure 8. Angular power spectrum wth two clusters. MIMO antenna testng n the case of two and three clusters were then performed. Fg. 8 shows the angular power spectrum n the presence of two clusters., One cluster comng from the forward drecton, φ =, and the other from the backward drecton, φ = 8 degrees, were consdered, assumng a tpcal street mcro-cell propagaton envronment. ΔP s the power rato of the cluster from φ = to that of φ = 8 degrees. The three-cluster model was assumed to be a more complcated propagaton envronment, for eample crossng of the street. In ths nvestgaton, we consdered that the frst cluster came from the forward drecton of the handset and the other two, whose powers were smaller than that of the frst cluster b ΔP, arrved from φ = and 4 degrees, respectvel. In the followng eamnaton, all the standard devatons, σ, of the clusters were set at degrees. Fgs. 9(a) and 9(b) show fadng correlatons between the two dpoles n the presence of two and three clusters. Fg. shows the MIMO channel capact of the dpole arra wth the three clusters. As can be seen n Fgs. 9 and, the agreement between the measured data and the theoretcal curves s ver good. Ths fact ndcates that the emulator s effectve for the measurement of the fadng correlaton and MIMO channel capact n a multpath propagaton envronment wth two and three clusters. It s also found from Fg. 9(a) that the correlatons reman constant wth the varatons n ΔP. Wth regard to the three clusters, the correlatons become hgher as ΔP becomes larger, as shown n Fg. 9(b). These hgh correlatons result n reducton n the MIMO channel capactes, as can be seen n Fg.. It s concluded from the above nvestgatons that the RFcontrolled emulator can accuratel represent a propagaton model n the presence of clusters, and s thus a canddate as an eas and smple soluton for the evaluaton of handset MIMO antenna performance, especall wth respect to fadng correlaton and MIMO channel capact. Fgure 7. MIMO and SISO channel capactes of the two dpoles n the presence of one cluster.
Correlaton coeffcent Correlaton coeffcent.8.6.4. Theor 3 4 (a) Two clusters.8.6.4. Theor 3 4 (b) Three clusters Fgure 9. Fadng correlatons between the two dpoles n the presence of two and three clusters. Channel capact [bts/s/hz] SNR=3dB 3 4 Fgure. MIMO and SISO channel capactes of the two dpoles n the presence of three clusters. V. CONCLUSION An RF-controlled spatal fadng emulator for the evaluaton of a handset MIMO antenna has been presented. The RFcontrolled fadng emulator can control RF sgnals drectl n the spatal doman to produce an accurate rado propagaton channel model, whch ncludes both the unform and nonunform angular power spectra (APS) n the horzontal plane. Fadng correlaton and MIMO channel capact of a two halfwavelength dpole arra, n the case of up to three spatal clusters, were measured b usng the emulator. From agreement between the measured data and theoretcal values the effectveness of the emulator was successfull confrmed. REFERENCES [] http://www.elektrobt.com/what_we_delver/wreless_test _tools/products/eb_propsm_c8 [] http://www.azmuthsstems.com/acem.htm [3] H. Iwa, A. Yamamoto, T. Sakata, K. Ogawa, K. Sakaguch and K. Arak, Spatal fadng emulator for handset antennas, IEEE AP-S Conference, Vol. A, pp. 8-, Jul. [4] A. Yamamoto, T. Sakata, H. Iwa, K. Ogawa, J. Takada, K. Sakaguch and K. Arak, BER measurements on a handset adaptve antenna arra n the presence of cochannel nterference generated usng a spatal fadng emulator, IEEE AP-S Conference, Vol. 4A, pp. 6-9, Jul. [] A. Yamamoto, T. Sakata, H. Iwa, K. Ogawa, J. Takada, K. Sakaguch and K. Arak, BER Measurement on a Handset Adaptve Antenna Arra n a Ralegh-Fadng Channel b a Varable-XPR Spatal Fadng Emulator, IEEE AP-S Conference, pp. 469-47, Jul 6. [6] T. Sakata, A. Yamamoto, H. Iwa, K. Ogawa, J. Takada, K. Sakaguch, and K. Arak: "BER Evaluaton Sstem for a Handset Antenna n a Multpath Envronment Usng a Spatal Fadng Emulator," IEICE ISAP Conference, vol., pp.3-34, Aug.. [7] T. Sakata, A. Yamamoto, K. Ogawa, A Multpath Measurement Sstem wth a Varable Power Angular Spectrum for Handset MIMO Antennas, n Tech. Rep. of IEICE AP 8-, pp. 3 8, Apr. 8. (n Japanese). [8] T. Sakata, A. Yamamoto, K. Ogawa, An Evaluaton of the MIMO Transmsson Characterstcs n a Cluster Propagaton Envronment Usng a Spatal Fadng Emulator, n Tech. Rep. of IEICE AP 8-, Feb. 8. (n Japanese). [9] K. Ogawa, A. Yamamoto, and J. Takada, Multpath Performance of Handset Adaptve Arra Antennas n the Vcnt of a Human Operator, IEEE Trans. Antennas Propagat., vol. 3, no. 8, pp. 4 436, Aug.. [] K. Ogawa, H. Iwa, A. Yamamoto, and J. Takada: "Channel Capact of a Handset MIMO Antenna Influenced b the Effects of 3D Angular Spectrum, Polarzaton, and Operator," IEEE AP-S Conference, pp. 3-6, Jul 6. [] K. Ogawa, H. Iwa, and N. Hatakenaka : "3D-Radaton Measurements of a Handset Dverst Antenna Close To a Realstc Human Phantom n a PDA Stuaton," IEEE AP- S Conference, pp. 9-3, June 3.