EMOS Platform: Real-Time Capacity Estimation of MIMO Channels in the UMTS-TDD Band

Transcription

1 EMOS Platform: Real-Tme Capacty Estmaton of MIMO Channels n the UMTS-TDD Band Raul de Lacerda #1, Leonardo Sampao Cardoso #2, Raymond Knopp #3, Davd Gesbert #4, Mérouane Debbah 5 # Communcaton Department, Eurecom Insttute 2229, Rt des Crḙtes - B.P. 193, Sopha Antpols, France 1 raul.de-lacerda@eurecom.fr 2 leonardo.sampao@eurecom.fr 3 raymond.knopp@eurecom.fr 4 davd.gesbert@eurecom.fr Supelec 91190, Gf-sur-Yvette, France 5 merouane.debbah@supelec.fr Abstract Ths work presents some ntal results concernng the MIMO channel capacty of real wreless channels n the UMTS-TDD band usng the Eurecom MIMO Openar Sounder (EMOS). Ths paper descrbes the necessary steps to estmate n real-tme the wreless MIMO envronment, offerng the possblty to dentfy relable MIMO channels as well as nstantaneous channel capacty. In partcular, the problems related to addtve and phase-shft nose are solved by employng OFDMA technology. Fnally, based on measurements, we analyze the mpact of polarzaton on the capacty performance. I. INTRODUCTION Durng the last years, many studes were developed to nvestgate the capacty offered by Multple-Input Multple-Output (MIMO) systems [1], [2], [3]. By explotng the multpath propagaton channel, multple antenna systems were shown to sgnfcantly ncrease the performance of sngle antenna systems, also known as Sngle-Input Sngle-Output (SISO) systems. As a consequence, sngle user MIMO systems have ganed more and more attenton. In order to analyze the MIMO gans, Eurecom Insttute has developed a MIMO platform called Eurecom MIMO Openar Sounder (EMOS) whch employs 4 transmt antennas and 2 receve antennas. The man dea behnd EMOS s to carry out real MIMO channel measurements on a real-tme bass, unlke [4]. EMOS s capable of provdng real tme measurements wth polarzaton and adjustable antenna spacng. Ths work presents some ntal results concernng the MIMO channel capacty of real wreless channels n the UMTS-TDD band usng Eurecom MIMO Openar Sounder (EMOS). Ths paper descrbes the necessary steps to estmate n real-tme the wreless MIMO envronment, offerng the possblty to dentfy relable MIMO channels as well as the nstantaneous channel capacty. Fnally, based on the measurements, we analyze the mpact of polarzaton on the capacty performance. In secton II, we present the EMOS platform, showng all of ts man characterstcs and parameters. Next, n secton III, we present the procedures developed to estmate the channel. In secton IV, ntal measurements results usng the EMOS are presented. Fnally, some conclusons and perspectves are drawn n secton V. Throughout ths paper, we use lower case letters to represent scalars and bold lower case letters to represent vectors. The superscrpts denote the hermtan of the matrx argument. I n s the dentty matrx of sze n n. E{ } s the expectaton operator. X = (x j ) j=1,...,n2 =1,...,n 1 s the n 1 n 2 matrx whose (, j)-element s the scalar x j. II. EURECOM MIMO OPENAIR SOUNDER (EMOS) EMOS s a real-tme platform able to carry out real transmssons usng the UMTS-TDD band. Based on the OpenAr system developed at Eurecom [5], [6], EMOS s able to operate n real-tme dealng wth real RF sgnals. It s developed wth the purpose to create an deal archtecture for expermentng wth real wreless envronments as well as analytcal results valdaton. (a) Base Staton server. Fg. 1. Base-staton antenna confguraton. (b) Powerwave Antenna. The platform conssts of a base-staton that sends a sgnalng frame contnuously, and some (one or more) termnals that

2 TABLE I POWERWAVE ANTENNA (PART NO ) Parameter Frequency range (MHz) Frequency band (MHz) ( ) ( ) ( ) ( ) Electrcal downtlt 0 o to 8 o Number of elements 4 TABLE II PANORAMA ANTENNA (PART NO. TCLIP-DE3G) B. Transmt Frame Although orgnally based on the UMTS-TDD standard, recent developments have pushed the Eurecom s team to use an OFDMA based sgnalng. Hence, at the base-staton, four data sequences are transmtted n parallel,.e., each TX chan has a ts own sequence. To smplfy the processng complexty at the recever, four frequency-orthogonal sequences were employed. Frame (64 OFDM symbols) Paramater Frequency range (MHz) ( ) ( ) Frequency band (GSM850) (GSM900) (GSM1800) (GSM1900) (3G UMTS) DATA (Not used) 7 OFDM symbols Guard nterval 8 OFDM symbols receve the frames to estmate the channel. For the basestaton (see Fg. 1(a)), an ordnary server PC s employed wth four PLATON Cards 1 [7], where each card s connected to a power amplfer whch feeds an antenna. As far as the termnals are concerned, an ordnary laptop computer s used along wth Eurecom s dual-rf CardBus/PCMCIA card [8], whch allow to employ two antennas for two-way real-tme expermentaton. (a) Dual-RF CardBus/PCMCIA Card. Fg. 2. A. Antenna Settngs Termnal antenna confguraton. (b) Panorama Antennas. The antenna employed at the base-staton s the Powerwave part no (see Fg. 1(b)). It s a 3G broadband antenna composed of four elements whch are arranged n two crosspolarzed pars. The man parameters concernng the basestaton antenna are lsted n Table I. The antennas employed at the termnal are the Panorama Antennas, part no. TCLIP-DE3G (see Fg. 2(b)). It s bascally a 3G antenna wth a clp mount for laptop computers. The man parameters concernng the termnal antenna are lsted n Table II. 1 The PLATON cards were orgnally bult as an UMTS-TDD testbed and nclude much more functonaltes than requred for EMOS. SYNC (1 OFDM symbol) Fg. 3. Frame Structure. Estmaton plots (1 OFDM symbol each) The transmt frame s llustrated n Fg. 3. Because of the constant varaton of the wreless channel and n order to take nto account the coherence tme of the channel, we consdered a small frame (the frame duraton s approxmately of 2.5ms). For a relable estmaton, we dvded the frame n 64 packets, where each packet represents an OFDM symbol composed only by 320 symbols (256 useful symbols and 64 symbols of cyclc prefx, whch gve 64 symbols for each TX chan). For processng purposes, the frame s consttuted by 4 dfferent knds of data: The frst part of the frame s composed of one sngle OFDM symbol. Ths symbol has a specal structure that permts the termnal to easly synchronze wth the basestaton. The second part of the frame s composed of useful data. For the moment, t s not used. The thrd part s composed of zeros. Ths part s used to estmate the nose characterstcs. The fourth and last part of the frame, s composed of a sequence of OFDM plot symbols. C. Recever Processng At the recever, the termnal does the frame synchronzaton procedure and suppresses the phase-shft nose generated by the dual-rf CardBus/PCMCIA card. After that, t estmates the MIMO channel for channel capacty analsys. 1) Frame Synchronzaton: The frst step of the recever processng s the frame synchronzaton. At ths step, the recever stores the receve data n a memory wth twce the sze of a frame. After that, the recever does a correlaton analyss between the receved data and the OFDM symbol dedcated to the synchronzaton purpose. Then, the synchronzaton s decded based on the poston of the maxmum value resultng from that correlaton. It s mportant to notce that for the moment, ths synchronzaton procedure s employed for each frame, wth the

3 objectve to guarantee that no error due to synchronzaton wll occur durng the channel estmaton and/or capacty analyss. 2) Phase-Shft Nose Suppresson: The second step of the recever processng s the phase-shft nose suppresson. Generated by the RF crcut, the phase-shft nose was observed to have a slow varaton characterstc. For ths reason, to guarantee a good phase-shft suppresson for the receved sgnal at each receve antenna, we mtgate the phase shft for each OFDM plot symbol of each frame. Assumng that s the receved OFDM plot symbol vector (1 320) of the kth OFDM plot symbol of the receved frame, where 17 k 64. We model the phase-shft nose as beng constant for each OFDM symbol and dfferent for dfferent OFDM symbols, whch turns out to be a good model for the Eurecom s dual-rf CardBus/PCMCIA. For the noseshft suppresson, frst the rato between the phase-shft of the frst OFDM plot symbol (k = 17) and all the other plots are estmated by the followng equaton: υ (k) = y (17) [a] 320 [a]. (1) a=1 After that, we multply each vector by the respectve normalzed and estmated phase-offset υ (k), whch gve us a constant phase-shft for each frame new = υ(k) old. (2) 3) Channel Estmaton: The next step s the most mportant one and t concerns the estmaton of the MIMO channel. To dmnsh the effects of the whte nose, each OFDM plot s used to estmate the MIMO channel and all estmatons of one frame are averaged. As a consequence, a relable MIMO channel estmaton s obtaned per frame. Consder x as beng the transmtted sgnal, H the MIMO channel matrx, n the addtve whte gaussan nose and y the receved sgnal, the system model can be represented n frequency by the followng equaton [f] = H (k) [f] [f] + n (k) [f] (3) where represents the frame ndex, k represents the ndex of an OFDM symbol of a frame, f represents the dscrete and normalzed frequency generated by the OFDM sgnalng, [f], [f] and n (k) [f] are respectvely the receved vector (N r 1), the transmtted symbol vector (N t 1) and the AWGN vector (N t 1), and H (k) [f] s the channel matrx (N r N t ). By usng the OFDM sgnalng propertes, the MIMO channel matrx estmated by each transmtted OFDM plot s gven by H (k) [f] (rx,tx) = [f] (tx) = H(k) [f] (rx,tx) [f] (tx) + n (k) = H (k) [f] (rx,tx) + n(k) where rx and tx represents respectvely the receve and the transmt antennas. To mtgate the nose of the channel estmaton procedure, we average all the 48 channel estmatons of one frame. Assumng that the channel s constant durng the transmsson of one frame (the expected coherence tme of our measurements s around 10ms), we have that E( (k) H [f] (rx,tx) ) = E [ [f] rx,tx + n(k) [f] rx H (k) PILOT [f] tx [ [ ] (k) n H [f] (rx,tx) = E H (k) [f] (rx,tx) + E = H (k) [f] (rx,tx) and for hgh SNR, we have 48 =1 ] n (k) ] (4) (5) (6) (7) (8) (9) H [f] = H (k) [f] (10) 4) MIMO Capacty Analyss: The last step of the recever processng s the MIMO capacty estmaton. Based on the classcal results avalable n the lterature [1], [2], we calculate the capacty by analyzng the estmated MIMO channel matrx. The result gves us an estmated capacty per frequency C [f] = log 2 [det (I 2 + ρ 4 H [f] H )] [f] (11) where ρ s the sgnal-to-nose rato (SNR) for each recever chan. As one can see, for the capacty analyss, the constant phase-shft of each frame (υ (k) ) does not affect the capacty because t dsappears durng the calculaton of the capacty. For the analyss presented n ths paper, two dfferent capacty results are consdered: 1) Assumng a gven SNR, whch means that the columns of the channel are normalzed; 2) Assumng the real SNR, whch means that after the channel normalzaton we analyze the capacty for the estmated SNR. III. MEASUREMENT As descrbed before, the analyss conducted n ths paper are based on the transmsson from the base-staton wth four transmt antennas to the termnal wth two receve antennas. On the analyss, along wth 4x2 MIMO archtecture, we also show the capacty performance obtaned when 2x2 and 1x1 antenna combnaton s consdered. The man rado characterstcs adopted by EMOS for ths measurement are lsted n the Table III

4 A. Envronment TABLE III MEASUREMENT CHARACTERISTICS Parameter Center frequency MHz Bandwdth 5 MHz Base-Staton Tx Power 34 dbm Number of Tx Antennas 4 Number of Rx Antennas 2 For the measurement, an outdoor scenaro very close to Eurecom Insttute s consdered, whch s characterzed by a sem-urban hlly envronment, composed by short buldngs and vegetaton (see Fg. 4). The base-staton antenna s stuated n one of the hghest buldngs of the regon and has a drect vew of the envronment. The outdoor measurements were conducted n a parkng very close to the buldngs that we see n the fgure. IV. SOME RESULTS In ths secton we present some measurements performed n an outdoor envronment. The purpose of these results s to show the mpact of the transmt archtecture (number of antennas and/or polarzaton). Furthermore, we can evaluate the gans offered by the use of MIMO structures n a usual realstc envronment. For ths reason, we analyze the achevable capacty when we employ not only a 4 2 MIMO, but also assumng other antenna combnatons: 1 1 MIMO, 2 2 MIMO wth co-polarzed antennas at the transmtter, 2 2 MIMO wth cross-polarzed antennas at the transmtter. The results presented here represent only an llustratve measurement campagn made wth the EMOS platform and performed at the route shown n Fg Fg. 6. Route where the measurements where performed. B. Polarzaton Fg. 4. Vew from the Base-Staton. For the capacty evaluaton, two dfferent knds of polarzatons are consdered. The goal s to analyze the mpact of the use of co-polarzed antennas wth space dversty and cross-polarzed co-located antennas at the transmtter. The consdered transmt structures are shown n Fg. 5. For the case where we have 4 transmt antennas, t s consdered the two pars of co-polarzed/cross-polarzed antennas. TX Antenna elements (a) Co-polarzed confguraton. Fg. 5. TX Antenna elements (b) Cross-polarzed confguraton. Transmt antenna polarzatons. At the recever, the MIMO channel s estmated and the nstantaneous capacty s derved as descrbed before. In Fg. 7, an estmated channel between the transmt antenna 1 and the recever antenna 1 s shown (for each par of transmt-receve antennas an estmated matrx lke the one presented n the fgure s obtaned). As t can be noted, a route wth constant channel characterstcs and wth a good SNR ( 30dB) was chosen. The measurement was performed at a storage rate of a frame at each 0.1s and wth a receve antenna space equal to λ/2. The plot shown n Fg. 8 shows the nstantaneous capacty acheved by each frame over the measurement run. Ths result s obtaned by the capacty average among all consdered frequences. As t can be noted, the average behavor of the capacty for all antenna combnatons follows the sgnal fluctuaton due to the path loss and fast fadng. The capacty CDF shown n Fg. 9 assumes a constant receve SNR of 10dB and an average of the obtaned capacty among dfferent frequences. Furthermore, we also plotted the achevable capactes when..d. MIMO channels randomly generated are consdered.

5 Cumulatve dstrbuton Emprcal CDF 4x2 2x2cross 2x2co 1x1 4x2 d 2x2 d 1x1 d Mutual nformaton [bts/s/hz] Fg Estmated channel over measurement (TX1 - RX1). 4x2 2x2cross 2x2co 1x Fg. 9. Mutual nformaton [bts/s/hz] Capacty for a gven SNR (10dB). ndeed greater than the SISO one. However, the gan s less than the..d. case. Moreover, cross-polarzed antennas were shown to nearly double the capacty n comparson of the SISO case. Many developments and studes are envsoned for the contnuaton of the EMOS project, ncludng the evaluaton of the mpact of several scenaro characterstcs on the capacty and the effect of multple users. Further developments nclude the enablng of multple user channels, uplnk-downlnk operaton as well as the evaluatons of algorthms that effcently explot the gans provded by MIMO Fg. 8. Snapshots over tme Capacty over measurement (actual frame SNR). As t can be seen, the real envronment performs worse than the..d. MIMO channels. It can also be seen that the use of MIMO ncreases the capacty when we compare t wth the SISO case. It s mportant to note that the gan offered by the use of 2 2 MIMO s really mportant, almost doublng the SISO capacty. In the other hand, ncreasng the number of antennas n ths envronment for more than 2 antennas at the transmtter does not yeld n a further ncrease. Another mportant concluson of the presented result s that we see an mportant gan when a cross-polarzaton antenna s used as compared wth the obtaned capacty when co-polarzed antennas are used at the transmtter. V. CONCLUSIONS AND PERSPECTIVES In ths paper, the EMOS platform, developed at the Eurecom Insttute was presented. A descrpton of the platform and all the procedures adopted for channel estmaton and capacty calculaton was detaled. To llustrate the EMOS, some channel estmaton and capacty results were also presented. As expected, the MIMO performance was shown to be ACKNOWLEDGMENT The authors wth to thank Dr. Maxme Gullaud and Dr. Helmut Hofstetter for the ntal development of the EMOS and for the nsghtful dscussons. REFERENCES [1] I. Telatar, Capacty of mult-antenna gaussan channels, AT&T Techncal Memorandum, June [2] G. F. Foschn and M. J. Gans, On lmts of wreless communcatons n a fadng envronment when usng multple antennas, Wreless Personal Communcatons, pp. 6: , August [3] H. Bolcske and A. J. Paulraj, Space-frequency coded broadband OFDM systems, Wreless Communcatons and Networkng Conference, vol. 1, pp. 1 6, September [4] A. Molsch, M. Stenbauer, M. Toeltsch, E. Bonek, and R. Thom, Capacty of MIMO systems based on measured wreless channels, IEEE Journal on Selected Areas n Communcatons, vol. 20, pp , September [5] (2006) The OpenAr Interface webste. [Onlne]. Avalable: [6] M. Wetterwald, C. Bonnet, H. Callewaert, L. Gauther, R. Knopp, P. Mayan, A. Menoun Hayar, and D. Nussbaum, A UMTS-TDD software rado platform. Chapter of Reconfgurable Moble Rado Systems :A Snapshot of Key Aspects Related to Reconfgurablty n Wreless Systems by Vver, Gullaume (Ed), Apr [7] C. Bonnet, L. Gauther, P. A. Humblet, R. Knopp, A. Menoun Hayar, Y. Moret, A. Nordo, D. Nussbaum, and M. Wetterwald, An all-ip software rado archtecture under RTLnux, Annales des télécommuncatons Volume 57, n7-8, jullet-août 2002, [8] (2005) Eurecom Dual-RF CardBus/PCMCIA Rado Equpment. [Onlne]. Avalable: MIMO I.doc