Wideband and Ultra-Wideband Channel Measurement and Modeling for Broadband Wireless Systems

Transcription

1 Wideband and Utra-Wideband Channe Measurement and Modeing for Broadband Wireess Systems Jun-ichi TAKADA Department of Internationa Deveopment Engineering Graduate Schoo of Science and Engineering Tokyo Institute of Technoogy Meguro-ku, Tokyo , JAPAN Phone/Fax: , E-mai: Abstract The spatia domain information of the propagation is extremey important to take into account the antenna systems in the wireess broadband systems. This paper describes the concept and the modeing of the doube-directiona spatio-tempora channe for wideband and utra-wideband wireess systems. The research activities toward the estabishment of the channe mode in Tokyo Tech are aso described. Keywords: propagation channe, doubedirectiona channe, channe sounding, utra-wideband. 1. Introduction According to the increasing demand of the high speed wireess systems, a ot of new technoogies to enhance the data rate and the error performance have been proposed. Wireess channes are composed of antennas and propagation paths and they suffer from mutipath and co-channe interferences. Mutipe antenna systems such as a smart antenna and a MIMO transmission technoogy are utiizing the spatia domain diversity of the channes. The former technoogy is oriented to suppress the interference signa as we as to combine the desired signa to improve the transmission quaity. The atter technoogy is oriented to increase the channe capacity by virtuay estabishing the parae spatia channes. Their performances are very sensitive to the channe characteristics, and it is very important to estabish the versatie channe modes. Before the spatia domain has been focused, the fading fuctuation and the deay spread are modeed ony in the tempora domain. But these modes are just the projection of the spatio-tempora channe mode to the time domain, and are not directy appicabe to those techniques utiizing spatia domain. Many efforts have been made to estabish the spatia or spatio-tempora channe modes[1], [2], the standard modes that are accepted among everyone do not exist. The continuing comprehensive studies are therefore very important. The first haf of this paper presents the concept of wideband doube- directiona channe, as we as the activities of the sim- Fig. 1. norma vector DoAs DoDs DT veocity vector Ray-based channe response mode. uation, the measurement, and the modeing in Tokyo Tech. On the other hand, an utra-wideband (UWB) radio is now considered as a competey aternative approach to achieve the very high speed wireess ink, especiay for the persona area network within the distance of a few meters. It is a kind of a spreadspectrum system, but the spreading bandwidth is so wide (typicay more than 500 MHz) and the spectra density is so ow that it can share the same frequency band with other conventiona radio systems. Athough the ine-of-sight path is usuay dominant, the anti-mutipath technique such as rake receiver can be appied. The standard channe mode estabished in IEEE TG3a[3], it is just a cassica timedomain mode and the antennas are not separated by the spatia channes. Moreover, the distortion of the waveform due to the antennas is not negigibe. It is, therefore, necessary to distinguish the antennas and the spatia propagation paths to reaize the versatie channe modes. In the atter haf of this paper, the measurement approaches of the antenna and the doube-directiona propagation paths for UWB systems in Tokyo Tech are described. 2. Wideband Channe 2.1. Concept of Channe Modeing To separate the effects of antennas and propagation in the wireess channe, the vector compex antenna directivity and the doube-directiona ray response are separatey considered in this paper. Figure 1 shows the schematic view of the antennas and the ray paths. The channe response can be written CM

2 in the form of (Channe Response) = rays where antenna directivity is expressed as a vector (poarization) compex directivity incuding phase rotation due to position offset, and the ray response is expressed as a dyadic (vector vs. vector) compex path gain between and origins. By using the matrix form, the array antenna directivity can be expressed in the equivaent compex baseband as E(Ω) = e θ1 (Ω) e θ2 (Ω). e θm (Ω) e ϕ1 (Ω) e ϕ2 (Ω). e ϕm (Ω) [ ˆθ(Ω) ˆϕ(Ω) ], (2) where Ω = (θ, ϕ). Even in the wideband system, the frequency characteristics of the antenna can be usuay negigiby sma compared with those of the propagation channe, so that E(Ω) is assumed not to be a function of the frequency. In the same way, the propagation ray paths mode can be expressed as Γ(t, τ, Ω R, Ω T )= [ ˆθ R (Ω R ) ˆϕ R (Ω R ) L =1 ] T [ γ θθ γ ϕθ γ θϕ γ ϕϕ ][ ˆθ T (Ω T ) ˆϕ T (Ω T ) δ(ω R Ω R )δ(ω T Ω T ) δ(τ τ )exp(j2πf d t) exp( jk(ω R ) v R t)exp(+jk(ω T ) v T t), (3) where T and R indicate the transmitter and the receiver, respectivey, f d is the Dopper frequency due to motion of scatterer, k(ω) is the wavenumber vector directed to Ω, and v is the moving veocity of the antenna. Even in the wideband system, the frequency characteristics of the path gain can be usuay negigiby sma compared within the bandwidth. Therefore, ony the time deay due to the propagation path is taken into account. By using the antenna and the ray-response of the propagation environment, the MIMO channe response mode can be expressed as H(t, τ) = Ω T Note that the antenna response is deterministic and unchanged, whie the propagation response dynamicay changes in various manners. The microscopic ( antenna directivity) change is due to the Dopper shift which is expressed (ray response) in the ast two exponentias. As far as the matrix ( antenna directivity), composed of γ αβ is unchanged, the channe can be (1) assumed to be wide-sense stationary. This is often termed by the short-term fading. On the other hand, more macroscopic change is due to the birth and the death of the ray, i.e. the matrix composed of γ αβ is graduay changing according to the change of the surrounding environment. This is often termed by the ong-term fading or the shadowing. Ω R E R (Ω R ) Γ(t, τ, Ω R, Ω T ) E H T (Ω T ) ] dω R dω T (4) 2.2. Doube-Directiona Wideband Channe Sounding Themeasurementoftheray-response(3)intherea environment is caed the doube-directiona channe sounding. The instrument is caed the channe sounder. At east two commercia products are known to measure the doube-directiona channe response[4], [5]. Both channe sounders have the foowing common features: 1. Singe / architecture is depoyed for the reduction of hardware compexity and the ease of the caibration. Antennas are switched at both and sides (time division mutipex). Therefore, the rea-time measurement is not possibe. Usuay, the data processing is done after coming back from the fied campaign. 2. Superresoution agorithms are used to extract the ray parameters, such as directions of departure and arriva, time deay of arriva, and signa magnitude. Now, space-aternating generaized expectation maximization (SAGE) agorithm and its modification are commony used[6], [7]. It is based on the maximum ikeihood estimate, and there is no restriction about the waveform or the array geometry. Contrary to subspace-based methods such as MUSIC and ESPRIT, SAGE utiizes the transmit waveform, and therefore, the preprocessing for the suppression of the coherence is not required. Aternativey, we have been deveoping a rea-time MIMO channe sounder which utiizes the foowing new technoogies[8], [9]: 1. MIMO / architecture is depoyed. Each transmits the mutitone signa with sight offset of the frequency. At side, signas can be separated in the frequency domain. As we, a nove comprehensive caibration system has been proposed to eiminate the deviation of mutipe /. The measurement time is drasticay reduced since no antenna switching is required. 2. Instead of SAGE, muti-dimensiona unitary ES- PRIT combined with the spatia smoothing technique has been depoyed. It is advantageous that ESPRIT 1478

3 directy outputs the estimated parameters, in contrast to SAGE or MUSIC in which the spectra search is necessary. Rea-time impementation of the ES- PRIT is possibe. Now we are in the process to get the radio icense at 5 GHz band. incidence pane θ θ θ diffraction pane unfoded pane ht X1 X1 X2 X2 hr 2.3. Ray-Tracing Simuation The propagation prediction by using ray-optica approaches is getting very popuar, and many researchers impemented their own ray-tracing toos. In many cases, however, the vaidation of the simuation by comparing with the fied test resuts had been very difficut as the sounding data were not usuay avaiabe. We have aso impemented severa different raytracing simuators. One is 2D-3D hybrid approach for the microceuar environment[10], [11], [12]. Figures 2 show the concept of 2D-3D hybrid method. Since the antennas at both ends are beow the roof top eve, and the propagation over the rooftop is negigibe. Therefore, ray-aunching is executed in horizonta pane to consider the specuar refection and the edge diffraction. In the ray-aunching method, a ot of rays are aunched from the source point toward a the directions. When a ray is arriva to a surface of some specific buiding, it is refected according to the refection aw. When it is arriva to an edge of some buiding, the edge is considered as a new secondary source to aunch the ray again toward a the directions. At the receiving point, a capture circe is considered to check the arriva of the ray, since the rays are discretize in the anguar domain. Therefore, the size of the circe is varying according to the ength of the ray. After the determination of 2D rays, 3D ray-path formuation is done by considering the height of the buidings and checking the existence of refection/diffraction points on the surfaces of the buidings. The ground refection can aso be considered by taking the image of point with respect to the ground. Some ray acceeration techniques are used, such as back-face cuing and voume bounding. In the present impementation, Fresne refection coefficient and Lubbers empiricay corrected UTD diffraction coefficient are used. To vaidate the simuator, we have made severa fied tests. Among them, we compare the azimuthdeay spectrum obtained by the simuation and the wideband sounder measurement with a directiona antenna in a microceuar environment in the residentia area in Yokosuka[13]. Figure 3 shows the map of the test site. It is a residentia area with wooden houses and concrete was. The specification of the sounder is shown in Tabe I. It is not a super resoution sounder, but a standard PN correation type sounder connected to a rotating paraboic antenna. Fig deg 0 deg ht Concept of 2D-3D hybrid ray-tracing. North X1 Average Buiding Height: 8 m height 4.4 m,height2.7 m 20 m Fig. 3. Map of the test site; Yokosuka Highand, Japan. Figures 4 and 5 compares the azimuth-deay spectra obtained by the fied test and the simuation, respectivey. Both resuts are in agreement with respect to the dominant signas, but some non-specuar components are observed ony in experiment. Therefore, the simuator can accuratey predict the strong arriva waves. Contrary, the signas with ong deay are difficut to predict, as the simuator ony considers the specuar refection. Some diffuse effects sha be integrated in near future[14]. 3. Utra-Wideband Channe To consider the utra-wideband (UWB) channes, the frequency characteristics of the antennas as we as the scattering process can not be assumed as constant over the bandwidth. Additiona discussion is TABLE I Specification of the sounder. frequency bandwidth deay resoution antenna antenna beamwidth 8.45 GHz 100 MHz 20 ns vertica hafwave dipoe V-po 50 cm paraboa 4 X2 hr CM

4 Path gain [db] Azimuth ange [deg] Deay [ns] Fig. 4. Azimuth deay spectrum; experiment. Path gain [db] Fig. 5. necessary Azimuth ange [deg] 360 Deay [ns] Azimuth deay spectrum; simuation Extension of Channe Mode to UWB The concept of the channe mode, which consists of the antennas and the propagation as presented in Eq. (1), is vaid. However, in the UWB systems some extensions are necessary to consider the frequency characteristics of the antennas and the propagation oss. The antenna directivity (2) can be extended to e θ1 (f, Ω) e ϕ1 (f, Ω) e θ2 (f, Ω) e ϕ2 (f, Ω) [ ] ˆθ(Ω) E(f, Ω) =... ˆϕ(Ω) e θm (f, Ω) e ϕm (f, Ω) (5) On the other hand, the ray path propagation (3) sha be more carefuy treated, as the deay time τ is the Fourier pair of f. Therefore, the Fourier transform of Eq. (3) sha be considered as Γ(t, f, Ω R, Ω T )= [ ˆθ R (Ω R ) ˆϕ R (Ω R ) L =1 ] T [ γ θθ (f) γ θϕ (f) γ ϕθ (f) γ ϕϕ (f) δ(ω R Ω R )δ(ω T Ω T )exp( j2πfτ ) exp(j2πf d t) ( exp j 2πf ) c ˆr R v R t ][ ˆθ T (Ω T ) ˆϕ T (Ω T ) ( exp +j 2πf ) c ˆr T v T t.(6) ] Fig. 6. Eectric fied directivity of dipoe antenna. It is noted that γ αβ (f) incudes the frequency dispersion effect which may resut in the time dispersion as we Antenna as a Part of UWB Channe Figure 6 shows the eectric fied directivity of the dipoe antenna. It is cear that radiation pattern changes according to the frequency. The frequency transfer function of this antenna toward θ =90 and θ =60 are compared in Figs. 7 and 8. It is cear from these figures that the directive transfer function may vary very drasticay even for a same antenna. Athough it is easy get the directive transfer function of the antenna via the numerica simuation, it is more difficut via the measurements. By now we consider two aternative measurement methods. One method[15] is just to use two identica antennas to point in the same directions to each other. The antenna transfer function is just a square root of the measure transfer function divided by the free space transfer function. Another way is the use of compex 3-antenna method[16]. It measures the compex antenna factor which can be easiy transformed into the antenna transfer function Doube-Directiona Utra Wideband Channe Sounding It is not so compicated to extend the doubedirectiona channe sounding to appy for the UWB. From Eq. (6), it is easiy understood that the directions of departure and arriva, as we as the deay time of arriva are constant for each of the ray paths over the UWB frequency. However, the compex ampitude of the ray, i.e. γ αβ (f), is a function of the frequency. Therefore, instead of estimating a singe compex ampitude, the ampitudes of subbands are estimated separatey. In this manner, we extended the SAGE agorithm for UWB sounding[17]. 1480

5 Transimitter Antenna ( Dipoe ) Cabe (20m) Network Anayzer ( Agient 8720ES ) Port1 Port2 Measurement of s 21 Preamp (30 db) Cabe (0.5m) Receiver Antenna ( Dipoe ) Cabe (6m) X-Y Positioner (DEVICE D3425AV1/O-D) and Controer (DEVICE DX3165AV1/O-C) Setting position GPIB GPIB Fig. 7. Magnitude of transfer function of dipoe antenna. Note PC Contro and data acquisition Signa Processing Fig. 9. UWB singe-directiona channe sounder using vector network anayzer. 6.67m Wa (b) Path #4 (h: 0.70m) Scanning Pane Wa (a) Path #1, m Foor to ceiing : 2.6m Path #2 Wa (d) Path #6 (h:1.58m) Wa (c) Path #5 Fig. 8. Phase of transfer function of dipoe antenna. 7.67m Fig. 10. Foor pan of the measurement environment and resut of ray path identification. Figure 9 shows the measurement setup. Due to the imitation of the faciity, the singe-directiona measurement was done by negecting the directivity of the fixed antenna. A measurement test has been done in an empty room shown in Fig. 10 together with the ray paths estimated by the sounding. The measurement parameters are isted in Tabe II. The rays estimated by the sounding can be we described by the specuar refections. By now, we sti have some error in the ampitude estimation, and some improvement is necessary. 4. Concusion This paper presented our approaches for wideband and UWB channe measurement and modeing approaches. The most important point of our approaches is that the antennas and the propagation paths sha be separatey modeed. For this purpose, antenna modes sha be described by the poarized directive transfer function of a over the surface. Propagation modes sha be described by the sum of ray paths. Each ray path has the doube-directiona information, i.e. directions of departure and arriva, as we as the deay time of arriva, and the dyadic path gain that expresses the poarization-dependent atten- uation and depoarization. Acknowedgement The author acknowedges Dr. Houtao Zhu, Mr. Kei Sakaguchi, Mr. Sathaporn Promwong, Mr. Katsuyuki Haneda and a the other contributors of this work in Tokyo Institute of Technoogy. The author aso acknowkedes Prof. Prayoot Akkaraekthain, Prof. Vech Vivek and Prof. Teerasiapa Duwamipata of King Mongkut s Institute of Technoogy North Bangkok to provide the opportunity to summarize the work. References [1] R. Erte, P. Cardieri, K.W. Sowerby, T.S. Rappaport and J.H. Reed, Overview of Spatia Channe Modes for Antenna Array Communication Systems, IEEE Persona Commun., vo. 5, no. 1, pp , Feb [2] L.M. Correia (ed.), Wireess Fexibe Personaised Communications COST 259: European Cooperation in Mobie Radio Research, Wiey, [3] J. Foerster, Channe Modeing Sub-committee Report Fina, IEEE P /368r5-SG3a, Nov [4] MEDAV RUSK Sounder, [5] Eektrobit PropSound, [6] B.H. Feury, M. Tschudin, R. Heddergott, D. Dahhaus, and K.I. Pedersen, Channe Parameter Estimation in CM

6 TABLE II Measurement parameters for UWB channe sounding. Frequency The number of sampes in frequency domain Bandwidth of each subband The number of sampes in spatia domain Estimated components Antennas Poarization of the wave 3.1 [GHz] 10.6 [GHz] 751 points ( f =10[MHz]) 100 [MHz] in horizonta pane with interva of 4.8 [cm] (ess than haf waveength of 3.1 [GHz]) Azimuth φ, eevationθ, time deay τ, compex ampitudes at a the subbands Biconica antennas Vertica Vertica Mobie Radio Environments Using the SAGE Agorithm, IEEE J. Seect. Areas Commun., Vo. 17, No. 3, pp , Mar [7] B.H. Feury, X. Yin, K.G. Rohbrandt, P. Jourdan, and A. Stucki, Performance of a High-Resoution Scheme for Joint Estimation of Deay and Bidirection Dispersion in the Radio Channe, in Proc. 55th IEEE Veh. Tech. Conf. (VTC 2002/Spring), May [8] K. Sakaguchi, J. Takada, and K. Araki, A Nove Architecture for MIMO Spatio-Tempora Channe Sounder, IEICE Trans. Eectron., vo. E-86C, no. 3, pp , Mar [9] K. Sakaguchi, K. Kuroda, J. Takada, and K. Araki, Comprehensive Caibration for MIMO System, 5th Int. Symp. Wireess Persona Mutimedia Commun. (WPMC 02), pp , Oct [10] H. Zhu, J. Takada, K. Araki, and T. Kobayashi, A Random-Phase-Assisted Ray-Tracing Technique for Wireess Channe-Modeing, ACES J., vo. 15, no. 3, pp , Nov / (errata) vo. 16, no. 1, pp , Mar [11] H. Zhu, J. Takada, K. Araki, and T. Kobayashi, Verification of a Two-Dimensiona/Three-Dimensiona Hybrid Ray-Tracing Method for Spatiotempora Channe Modeing, Radio Sci., vo. 36, no. 1, pp , Jan.-Feb [12] H. Zhu, J. Takada, K. Araki, and T. Kobayashi, A Ray- Tracing-Based Characterization and Verification of the Spatio-Tempora Channe Mode for Future Wideband Wireess Systems, IEICE Trans. Commun., vo. E84-B, no. 3, pp , Mar [13] J. Takada, J. Fu, H. Zhu and T. Kobayashi, Spatio- Tempora Channe Characterization in a Suburban Non Line-of-Sight Microceuar Environment, IEEE J. Seected Areas in Commun., vo. 20, no. 3, pp , Apr [14] H. Budiarto, K. Horihata, K. Haneda, and J. Takada, Mutipath Characteristics of Non-specuar Wave Scattering from Buiding Surface Roughness, 14th Int. Symp. Persona, Indoor and Mobie Radio Commun. (PIMRC2003), TM14-1, Sep [15] S. Promwong, W. Hachitani, J. Takada, and P. Tangtisanon, Experimenta Evauation of the Free Space Transmission Characteristics of Utra Wideband Antennas, Tech. Rep. IEICE, AP2003-**/MW2003- **/OPE2003-**/SAT2003-**, Juy [16] S. Ishigami, H. Iida and T. Iwasaki, Measurements of Compex Antenna Factor by the Near-Fied 3-Antenna Method, IEEE Trans. Eectromag. Compat., vo. 38, no. 3, pp , Aug [17] K. Haneda, and J. Takada, An Appication of SAGE Agorithm for UWB Propagation Channe Estimation, 2003 IEEE Conf. Utra-Wideband Syst. Tech. (UWBST2003), Nov (to be presented). Jun-ichi TAKADA was born in Tokyo, Japan, in He received B.E.,M.E.andD.E.degreesfromTokyo Institute of Technoogy in 1987, 1989, and 1992, respectivey. From 1992 to 1994, he has been a Research Associate at Chiba University, Japan. From 1994, he has been an Associate Professor of Internationa Cooperation Center for Science and Technoogy (INCOCSAT), Tokyo Institute of Technoogy, Japan. There, he served as a coordinator of Core University Academic Cooperation Program sponsored by Japan Society for Promotion of Science (JSPS) for six years to estabish and acceerate the research cooperation with South East Asian Universities incuding Thaiand. From 1997 he aso participated in the project of Research Center for Communication and Information Technoogies (ReCCIT), King Mongkut s Institute of Technoogy Ladkrabang, which was sponsored by Japan Internationa Cooperation Agency (JICA) as a short-term expert. He has visited Thaiand more than thirty-five times. From 2001, he has been with the Department of Internationa Deveopment Engineering, Tokyo Institute of Technoogy, as an Associate Professor, where he tries to transfer his experiences on the academic cooperation to the students. His current research interests are wireess channe characterization and modeing, utra-wideband radio, and appied radio instrumentation and measurements. He is a member of IEEE, IEICE Japan, and ECTI Thaiand. His group is participating in COST 273 Towards Mobie Broadband Mutimedia Communications, a cooperation program sponsored by European Union. 1482