A Novel Architecture for MIMO Spatio-Temporal Channel Sounder

Transcription

1 436 IEICE TRANS. ELECTRON., VOL.E85 C, NO.3 MARCH 2002 PAPER Specal Issue on Sgnals, Systems and Electroncs Technology A Novel Archtecture for MIMO Spato-Temporal Channel Sounder Ke SAKAGUCHI a), Jun-ch TAKADA, and Kyomch ARAKI, Regular Members SUMMARY Implementaton of Mult-Input Mult-Output (MIMO) channel sounder s consdered, takng hardware cost and realtme measurement nto account. A remarkable dfference between MIMO and conventonal Sngle-Input Mult- Output (SIMO) channel soundng s that the MIMO sounder needs some knd of multplexng to dstngush transmttng antennas. We compared three types of multplexng TDM, FDM, and CDM for the soundng purpose, then we chose FDM based technque to acheve cost effectveness and realtme measurement. In the framework of FDM, we have proposed an algorthm to estmate MIMO channel parameters. Furthermore the proposed algorthm was mplemented nto the hardware, and the valdty of the proposed algorthm was evaluated through measurements n an anechoc chamber. key words: moble propagaton, channel sounder, MIMO channel response, channel parameter estmaton, hardware mplementaton 1. Introducton MIMO communcaton system s consdered to play a key role n the 4th generaton moble communcaton system that wll realze bt rate as hgh as 100 Mbps (e.g., [1]). Ths s because the MIMO communcaton system can ncrease the channel capacty wthout expandng the requred frequency bandwdth (e.g., [2]). Snce the performance of MIMO communcaton system depends on the drectonal as well as the temporal behavor of the channel, feld measurement data of MIMO channel s strongly requred to develop and evaluate the MIMO communcaton systems. There are two usages of the feld measurement data. One s a raw measurement data, and the other s a parametrc data. The MIMO channel s consdered to be parameterzed by drecton of arrval (DOA), drecton of departure (DOD), and tme of arrval (TOA). The raw measurement data s only devoted to the performance evaluaton [3], but parametrc data s for both development and performance evaluaton snce t s ndependent on the confgurable parameters n the communcaton system such as antenna drectvty, array Manuscrpt receved August 3, Manuscrpt revsed October 30, The authors are wth the Department of Electrcal and Electronc Engneerng, Tokyo Insttute of Technology, Tokyo, Japan. The author s wth the Department of Internatonal Development Engneerng, Tokyo Insttute of Technology, Tokyo, Japan. a) E-mal: ke@moble.ss.ttech.ac.jp confguraton, and sgnal bandwdth. Therefore t s very mportant to measure the MIMO channel parameters to develop the MIMO communcaton systems. In addton to that, f these channel parameters can be measured smultaneously, not only the spatal and temporal spreadng of the channel, but also the mutual relatonshp between the spatal and temporal characterstcs of the channel can be analyzed. Thus, we need such a knd of channel parameter measurement system whch we call MIMO spato-temporal channel sounder. The requrements for the MIMO spato-temporal channel sounder are as follows: Smultaneous estmaton of DOA, DOD, and TOA Realtme measurement wth respect to the coherent tme that s a functon of Doppler frequency whch s not only caused by the movement of the moble termnal but also the movement of scatterers such as surroundng vehcles Large dynamc range of measurement system Cost effectve hardware Recently such a channel sounder s proposed n [4], [5]. But none of them satsfy all of the above requrements. It s because they are just an extenson of conventonal SIMO channel sounder [6], [7], n other words t s a sequental SIMO channel soundng. In ths paper, we propose a full MIMO channel sounder. A remarkable dfference between MIMO and SIMO channel soundng s that the MIMO sounder needs some knd of multplexng to dstngush transmttng antennas. In [4], the frst prototype of MIMO channel sounder usng tme dvson multplexng (TDM) technque, namely swtchng and synthetc aperture array antenna, was proposed. Snce t used the synthetc aperture array antenna, the envronment must be statc durng the measurement, or all the effects caused by movng objects need to be elmnated by usng Doppler flter. In other words, t s far from realtme measurement. In ths paper, we systematcally compared three types of multplexng technque, TDM, FDM (frequency dvson multplexng), and CDM (code dvson multplexng), from the soundng pont of vew. Then we found that the FDM based technque can acheve all of the above requrements. In the FDM based technque we have proposed: New transmttng sgnal confguraton (we call t mult-tone FDM)

2 SAKAGUCHI et al.: MIMO SPATIO-TEMPORAL CHANNEL SOUNDER 437 New parameter estmaton algorthm (extenson of multdmensonal superresoluton algorthms [8] [10]) Furthermore the proposed algorthm was mplemented nto the hardware, and the valdty of the proposed algorthm was evaluated through measurements n an anechoc chamber. Ths paper s organzed as follows. Secton 2 provdes the formulaton of MIMO channel response vector. In Sect. 3 measurement technque of MIMO channel s dscussed and then we propose a new technque. The proposed technque s mplemented nto the hardware n Sect. 4. The proposed algorthm and mplemented hardware are valdated through the measurement n Sect. 5. Fnally, Sect. 6 gves a concluson of ths paper. Mathematcal notatons used n ths paper are as follows: X T : transposton : Kronecker product : Hadamard product Fnally the operator vec{ } maps a matrx to a vector by stackng the columns of the matrx. 2. MIMO Channel Response Typcal envronment for MIMO channel soundng s llustrated n Fg. 1. Consder a m s -element transmttng array antenna at the moble staton MS (Tx), and a m r - element recevng array antenna at the base staton BS (Rx). The channel s a superposton of multpath components. Each path s departed from the transmttng array wth an azmuth angle θ s and s arrvng at the recevng array wth an azmuth angle θ r, where s an ndex of multpath components. Between Tx and Rx, each path has a delay tme τ and complex ampltude γ that s a functon of scatterng and propagaton coeffcents. A m r m s channel matrx H C m r m s at the center frequency of f c can be expressed as H = γ (t)e j2πf cτ a r (θ r )(a s (θ s )) T, (1) where a s (θ) and a r (θ) are transmttng and recevng array response vectors, for the plane wave mpngng from the azmuth angle θ, respectvely. It should be noted that the complex ampltude γ (t) s tme varant due to the Doppler frequency. For convenence to treat a multdmensonal problem, the channel matrx H C m r m s s reformulated to a m r m s dmensonal vector h C m r m s by usng vec{ } operator as h =vec{h} = γ (t)e j2πf cτ a r (θ r ) a s (θ s ). (2) As the next step, frequency response vector a f (τ) s ntroduced for the wdeband measurement. Fnally the three dmensonal channel response vector h C m r m s m f s formulated as h = γ (t)a r (θ r ) a s (θ s ) a f (τ ). (3) In ths formulaton, the MIMO channel parameters are DOA (θ r), DOD (θs ), and TOA (τ ). Ths equaton has a smple form that s easly extendable to nclude elevaton angle estmaton. If we consder an unform lnear, rectangular, or crcular array (ULA, URA, or UCA) antenna and unform m f frequency sample ponts, Eq. (3) can be consdered as multdmensonal harmonc retreval problem. Therefore the parameter sets {θ r,θs,τ } can be smultaneously estmated by usng multdmensonal superresoluton algorthms. In ths paper we employed 3-D Untary ESPRIT [9], [10]. If we consder the real channel envronment, t s mpossble to dstngush all of the multpath components even by usng the superresoluton algorthms due to the fnteness n the sgnal to nose rato, antenna aperture, and sgnal bandwdth. Snce the measurement data s devoted to the development of the MIMO communcaton system, we do not need the MIMO channel sounder wth nfnte resoluton. The data measured wth fnte resoluton that satsfes requrements for development of the MIMO communcaton systems s enough. On the other hand, we have to desgn the channel sounder to acheve the best possble performance under the gven physcal restrctons. Such an analyss was done n [11]. BS URA Fg. 1 normal vector DOAs MS DODs velocty vector UCA MIMO channel soundng envronment. 3. Measurement Technque of MIMO Channel Agan recall Eq. (1) as a channel response matrx. In the pont frequency measurement, the receved sgnal vector y s descrbed as y(t) =Hs(t)+n(t) C m r, (4) where s(t) C m s s a transmttng sgnal vector and n(t) C m r s a nose vector. In the wdeband measurement, H s characterzed wth another parameter,

3 438 IEICE TRANS. ELECTRON., VOL.E85 C, NO.3 MARCH 2002 delay τ, and s convolved wth transmttng sgnal sequence s. Anyway, the observable sgnal s only the superposton of contrbutons from all transmttng antennas. Therefore some knd of multplexng technque s needed to mplement the MIMO channel sounder. By usng an analogy wth the mult-user communcaton scenaro, three types of multplexng are conceved, namely TDM, FDM, and CDM. Comparson of these multplexng technques for soundng purpose s shown as follows n terms of realtme measurement, hardware cost, and major drawbacks. TDM CDM Realtme measurement poor Measurement perod s m s tmes baseband sgnal perod plus guard nterval for excess delay and swtchng. Hardware cost excellent Only one transmtter channel s needed. Major Drawback Absolute tme synchronzaton between transmtter and recever s requred. Realtme measurement excellent Measurement perod does not depend on m s. Hardware cost poor It needs m s transmtter channels. Major Drawback Dynamc range of the system s lmted by m s due to the cross-correlaton between dfferent codes. FDM To accomplsh the wdeband measurement by usng FDM technque, we ntroduced a new transmttng sgnal confguraton as llustrated n Fg. 2. In the case of two Tx antennas, frstly a multtone sgnal wth tone separaton of F s prepared. Then ths sgnal and frequency shfted replca of the sgnal are multplexed through the transmsson from dfferent antennas. The frequency shft f should be a fracton of F to keep an orthogonalty between all of the tones. Smaller f needs larger perod of Dscrete Fourer Transform (DFT) to separate multplexed sgnals n the recever sde. Therefore, f = F /m s s the most effectve way for multplexng n the case of m s transmttng antennas. We call ths technque as mult-tone FDM (MTFDM). Realtme measurement good Measurement perod s m s tmes baseband sgnal perod. Hardware cost good It requres m s local oscllators, but one baseband sgnal generator. F f Fg. 2 f f Tx antenna 1 Tx antenna 2 Mult-tone FDM for MIMO soundng. Major Drawback Some modfcaton s needed for the data model descrbed n Sect. 2, snce the frequency sample ponts n each transmttng antenna are dfferent. We successfully solved the drawback n FDM technque as follows. The newly ntroduced concept s a FDM response vector a FDM (τ ) C m s defned as a FDM (τ ) = [1,e j2π f τ,, e j2π(m s 1) f τ ] T. (5) By usng ths vector, the transmttng array response vector s rewrtten as a (φ s )=a s (θ s ) a FDM (τ ) C m s, (6) where φ s s a functon of θs and τ. Ths formulaton s a natural way to descrbe that the frequency sample ponts for each transmttng antenna are shfted by the nteger multple of f. Fnally the channel response vector for FDM based MIMO system s wrtten as h = γ (t)a r (θ r ) a s(φ s ) a f (τ ). (7) Equaton (7) can be consdered agan as a multdmensonal harmonc retreval problem, so that the parameter sets {θ r,φs,τ } can be smultaneously estmated n the same way descrbe n Sect. 2. It means θ s s calculated from the estmated parameters, φ s and τ. Thus, the channel parameters can be estmated even by usng the dfferent frequency sample ponts n each transmttng antenna. If the above soluton s consdered, there s no weakness n the FDM based technque, whle at the same tme, t can acheve both cost effectveness and realtme measurement. Therefore we choose ths technque for the hardware mplementaton. 4. Hardware Implementaton Based on the dscusson n Sect. 3, we mplemented the FDM based MIMO channel sounder. It s a modfed verson of the SIMO channel sounder proposed n [12], [13]. For smplcty to confrm the algorthm proposed n Sect. 3, we employed 2 elements lnear patch

4 SAKAGUCHI et al.: MIMO SPATIO-TEMPORAL CHANNEL SOUNDER 439 Rx Tx Fg. 3 Transmtter block dagram. 4[m] Fg. 6 Measurement setup n the anechoc chamber. -30 Fg. 4 Recever block dagram. Tx Antenna 1 Tx Antenna 2 processor, DFT at the rate of 125 [khz]s performed to separate the multplexed sgnals. Fnally the 3-D Untary ESPRIT algorthm s appled to the extracted mult-tone sgnals. Power [dbm] Fg. 5 Table Frequency [MHz] Frequency spectrum of transmttng sgnals. Confguraton of mult-tone sgnal. # of tones 20 tone separaton 500 [khz] bandwdth 9.5 [MHz] array antenna both n Tx and Rx. Block dagrams of transmtter and recever are shown n Fg. 3 and Fg. 4 respectvely. In ths paper, we concentrate on the MTFDM confguraton, whereas the detaled hardware setup s descrbed n [12], [13]. Baseband mult-tone sgnal s generated by usng Arbtrary Waveform Generator (AWG). Confguraton of the mult-tone sgnal s descrbed n Table 1. To mplement the proposed MTFDM, two IF oscllators were ntroduced. We employed 880 [MHz]and [MHz] for IF. We used a 125 [khz]shft n frequency nstead of 250 [khz]to avod the effect of DC offset. Fnally, these sgnals are up-converted to 5.85 [GHz]band and transmtted from each antenna. Transmttng sgnal spectrum s shown n Fg. 5. In the recever sde, we employed the low-if archtecture, where IF=5 [MHz]. Then these downconverted sgnals are sampled by usng 20 Msps A/D converter wth 12 bt resoluton. In the dgtal sgnal 5. Measurement Example Measurement experment was conducted n an anechoc chamber to valdate the FDM based algorthm proposed n Sect. 3 and the FDM based hardware mplemented n Sect. 4. Measurement setup s llustrated n Fg. 6. In ths stuaton, we have assumed that the anechoc chamber s perfectly confgured, so that only a drect wave exsts between Tx and Rx. The most smple envronment, wth the drect path only, s enough to confrm the FDM based algorthm, snce the valdaton n the multpath envronment was done n e.g. [5]by usng TDM based algorthm. The resoluton of these two algorthms s consdered to be the same theoretcally. Both array antennas were located on the rotators separated by a dstance of 4 [m]. Both rotators were turned around at 15 [deg]ntervals, Rx rotator angles: { 30, 15, 0, 15, 30 [deg]}, Tx rotator angles: { 15, 0, 15 [deg]}, and measurements were conducted for each par of angles. Ths measurement sequence was repeated 5 tmes to assure the repeatablty. Calbraton of the hardware was performed by the smple backto-back calbraton. Throughout the measurements, we took 30 tmes of snapshot, and the path gan, E[ γ 2 ], to nose, σ 2, rato was about 25 [db]. All of the measurement results wth respect to the DOA and DOD are shown n Fg. 7. If the estmated results are on the every 15 [deg]grd, t means good performance of the measurement s acheved n terms of DOA, DOD, and ndrectly TOA through Eq. (6). From ths fgure, we could confrm the valdaton of the proposed algorthm and mplemented hardware. It s noted that the slght degradaton of the estmates s due to the set up and calbraton error durng the measurements. 6. Concludng Remarks MIMO channel soundng s an attractve way to mea-

5 440 IEICE TRANS. ELECTRON., VOL.E85 C, NO.3 MARCH 2002 Estmated DOD [deg] Fg Estmated DOA [deg] Expermental results n the anechoc chamber. sure the propagaton mechansm n the moble cellular envronment. In ths paper, an archtecture for MIMO spato-temporal channel sounder was thoroughly nvestgated. After the consderable dscussons about technques of multplexng to dstngush the transmttng antennas, the FDM based archtecture was chosen to acheve cost effectveness and realtme measurement. In the framework of FDM, we have proposed a new transmttng sgnal confguraton and a new algorthm to estmate the MIMO channel parameters DOA, DOD, and TOA smultaneously. We confrmed the valdty of the FDM based archtecture through the measurement n an anechoc chamber. To accomplsh the development of MIMO channel soundng system, further nvestgaton for followng tems wll be requred. Frst one s an optmzaton of MIMO array confguraton, and the other s a novel calbraton for MIMO soundng system. [5] M. Stenbauer, A.F. Molsh, and E. Bonek, The doubledrectonal rado channel, IEEE Antennas & Propag. Mag., vol.43, no.4, pp.51 63, Aug [6] R.S. Thomä, D. Hampcke, A. Rchter, G. Sommerkorn, A. Schneder, U. Trautwen, and W. Wrntzer, Identfcaton of tme-varant drectonal moble rado channels, IEEE Trans. Instrum. & Meas., vol.49, no.2, pp , Aprl [7] J. Takada, K. Sakaguch, X. Zhu, K. Arak, M. Hrose, and M. Myake, A superresoluton spatal-temporal channel sounder for future mcrowave moble communcaton system development, Proc IEEE Asa-Pacfc Conf. Crcut and Systems, pp , Nov [8] A. Rchter, D. Hampcke, G. Sommerkorn, and R.S. Thomä, Jont estmaton of DoD, Tme-Delay, and DoA for hghresoluton channel soundng, Proc IEEE Vehcular Technology Conf., vol.2, pp , May [9] M. Haardt and J.A. Nossek, Smultaneous schur decomposson of deveral nonsymmetrc matrces to acheve automatc parng n multdmensonal harmonc retreval problem, IEEE Trans. Sgnal Processng, vol.46, no.1, pp , Jan [10] K. Sakaguch, J. Takada, and K. Arak, Multpath parameter estmaton by usng 3D Untary ESPRIT, IEICE Techncal Report, AP98-35, June [11] K. Sakaguch, J. Takada, and K. Arak, MIMO spatotemporal channel sounder, IEICE Techncal Report, AP , RCS , Oct [12] C. Kenmoch, K. Sakaguch, M. Suda, K. Fukuch, J. Takada, and K. Arak, The development of spatotemporal channel sounder by usng 3-D Untary ESPRIT, IEICE Techncal Report, AP2000-2, Aprl [13] K. Sakaguch, K. Kuroda, J. Takada, and K. Arak, The development of spato-temporal channel sounder usng 3-D Untary ESPRIT algorthm, submtted to IEICE Trans. Communcatons. Acknowledgement Ths work s partly supported by the Assocaton of Rado Industres and Busnesses (ARIB), and Scentfc Grand n Ad from Japan Socety for Promoton of Scence (# and # ). References [1] S. Kurosak, Y. Asa, T. Sugyama, and M. Umehra, 100 Mbt/s SDM-COFDM over MIMO channel for broadband moble communcatons, IEICE Techncal Report, AP , RCS , Oct [2] J.B. Andersen, Antenna arrays n moble communcatons: Gan, dversty, and channel capacty, IEEE Antennas & Propag. Mag., vol.42, no.2, pp.12 16, Aprl [3] H. Fuj, T. Abe, S. Tomsato, and T. Matsumoto, Performance evaluaton of a space-tme turbo equalzer n frequency selectve MIMO channels by usng feld measurement data, IEICE Techncal Report, AP , RCS , Oct [4] M. Stenbauer, D. Hampcke, G. Sommerkorn, A. Schneder, A.F. Molsch, R. Thomä, and E. Bonek, Array measurement of the double-drectonal moble rado channel, Proc IEEE Vehcular Technology Conf., vol.3, pp , May Ke Sakaguch was born n Osaka, Japan, on November He receved the B.E. degree n electrcal and computer engneerng from Nagoya Insttute of Technology, Japan, n 1996, and the M.E. degree n nformaton processng from Tokyo Insttute of Technology, Tokyo, n From 2000 he s a Research Assocate at Tokyo Insttute of Technology. Hs research nterests are n moble propagaton, array sgnal processng, and software defned rado. He s a member of IEEE.

6 SAKAGUCHI et al.: MIMO SPATIO-TEMPORAL CHANNEL SOUNDER 441 Jun-ch Takada was born n Tokyo n l964. He receved B.E., M.E. and D.E. degrees from Tokyo Insttute of Technology, n 1987, 1989 and 1992, respectvely. In he was a Research Assocate n Chba Unversty, Japan. From 1994 he s an Assocate Professor at Tokyo Insttute of Technology. He receved the Excellent Paper Award and Young Engneer Award from IEICE Japan n 1993 and 1994, respectvely. Hs current research nterests are array sgnal processng, moble communcaton and numercal smulaton of waves. He s a member of ITEJ, IEEE, SIAM, AGU and ACES. Kyomch Arak was born n Nagasak on January 7, He receved the B.S. degree n electrcal engneerng from Satama Unversty, n 1971, and the M.S. and Ph.D. degrees n physcal electroncs both from Tokyo Insttute of Technology, n 1973 and 1978, respectvely. In , and , he was a Research Assocate at Tokyo Insttute of Technology, and n he was an Assocate Professor at Satama Unversty. In and he was a vstng research scholar at Unversty of Texas, Austn and Unversty or Illnos, Urbana, respectvely. He s currently a Professor at Tokyo Insttute of Technology. Dr. Arak s a member of IEEE and Informaton Socety of Japan. Hs research nterests are nformaton securty, codng theory, communcaton theory, crcut theory, electromagnetc theory, mcrowave crcuts etc.