COMPARISON OF RAY TRACING SIMULATIONS AND MILLIMETER WAVE CHANNEL SOUNDING MEASUREMENTS

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1 COMPARISON OF RAY TRACING SIMULATIONS AND MILLIMETER WAVE CHANNEL SOUNDING MEASUREMENTS Behnam Neekzad, Kamran Sayrafian-Pour*, Julio Perez, John S. Baras Universiy of Maryland *Naional Insiue of Sandard & Technology Swiss Federal Insiue of Technology College Park, MD, USA Gaihersburg, MD, USA Zurich, Swizerland ABSTRACT Temporal-Angular channel sounding measuremens of an indoor millimeer wave channel (6 GHz is analyzed o deermine he locaion of wo dimensional clusers of arrivals a he receiver. The measuremen scenarios are also emulaed by a ray racing ool. The resuls are similarly analyzed o verify possible agreemens and deermine he effeciveness of such ools in predicing cluser locaions as well as ray arrival saisics wihin clusers in millimeer wave indoor channel. I. INTRODUCTION Millimeer wave echnology is becoming increasingly imporan in many miliary and commercial applicaions. Remoe sensing, radio asronomy, passive imaging, radar and high daa rae communicaion are among hese applicaions. To esablish a high daa rae communicaion link beween wo nodes, accepable Signal o Noise Raio (SNR a he receiver is required. However, due o he high propagaion pah loss and signal aenuaion ha is associaed wih he millimeer wave signal, i is exremely valuable o have knowledge of how millimeer wave signals propagae in he environmen. Such informaion along wih appropriae smar anenna echnology could enable a sysem o maximize he qualiy of he received signal, and herefore, achieve higher hroughpu. Indoor environmens creae challenging mulipah propagaion scenarios wih significan ime and angular spread. Join emporal-angular clusering phenomenon have been observed a frequencies commonly used in wireless neworks and -D saisical channel models have been proposed based on such phenomenon [,]. However, a millimeer wave frequencies, and o he bes of our knowledge, very few repors have been made for such clusering behavior. The clusering of arrival rays can have a significan impac on channel capaciy. Un-clusered models end o overesimae he capaciy if he mulipah componens are indeed clusered []. Considering angular (i.e. spaial aspecs of indoor channel is exremely imporan for sysems employing muliple anennas. Millimeer wave wireless communicaion sysems can grealy benefi from his knowledge as he physical size of array anennas makes hem amenable for pracical implemenaion [3]. Knowledge of cluser locaions is especially useful for such sysems as i could be exploied by various spaial diversiy combining or beamforming algorihms o enhance he sysem s performance []. The objecive of his paper is o sudy he -D clusering phenomenon of millimeer wave propagaion (6 GHz a indoor environmens and paricularly verify he usefulness of Research suppored by Naional Aeronauics and Space Adminisraion under award No NCC /7/$5. 7 IEEE. ray-racing o predic he cluser locaions and some of heir corresponding saisics. The res of his paper is organized as follows. Measuremen seup and he corresponding environmens will be described in secion. In secion 3, we will ouline he simulaion mehodology. Resuls are provided in secion 4 and finally, conclusions and fuure work will be discussed in secion 5. II. MEASUREMENT SETUP AND ENVIRONMENTS Field measuremens have been conduced in [4] for boh residenial and office environmens. The residenial environmen is a Line of Sigh (LOS scenario as shown in Fig.. The room where he measuremens ook place has no furniure and a LOS pah exiss beween he ransmier and receiver. The floor, ceiling and he door are made of wood. The surface of he walls and ceiling is covered wih wallpaper. There are also hree large windows (plane glass on he wo inersecing walls. The heigh of he ceiling and he window is.47m and.m respecively. The ransmier and receiver are locaed.m above he floor..48m Door (Widh:.75m, Heigh:.m.95m RX Window # Wall side 3m.785m.48m.64m.m.64m 6.85m TX Window #.64m.965m.97m Figure : Layou of he residenial room The office environmen is a Non-Line of Sigh (NLOS scenario as shown in Fig.. The room is made of seel wall, seel ceiling and seel floor. The floor and he ceiling are covered wih carpe and plaser board, respecively. Glass windows exis on one side of he office. To obain he angular componen of he received millimeer wave signal, a direcional anenna wih a narrow angular resoluion of 5 degrees was used a he receiver. The anenna was posiioned o face he ransmier iniially, and hen mechanically roaed in seps of 5 unil full 36 was obained. A Vecor Nework Analyzer (VNA was used as he signal generaor and receiver. All measuremens were conduced in he frequency domain using he VNA. The receiver anenna used in [4] is a pyramid horn anenna wih dbi gain (see Fig. 3. The 3dB beamwidh of he anenna in horizonal direcion is 5. A he ransmier, in addiion o Window #3 3.57m

2 an omni-direcional anenna, various pyramid horn anennas wih, 6 and dbi gains were used for he measuremens. These are equivalen o 6, 3, and 5 3-dB beamwidhs. Calibraion was performed in an anechoic chamber wih one meer reference disance o remove he anenna effecs. 4 m he same ime provide a geomeric inerpreaion of he resuls. 6.5 m 4.5 m Meeing room.5 m 5 m 6 m m 7 m 7.5 m 7. m.5 m m Wall ( m Locker room 7.5 m Figure : Layou of he office.5 m During he measuremens, he VNA was se o ransmi 4 (8 for he office coninuous waves ones uniformly disribued over he frequency range 6-64 GHz which resuls in frequency seps of 7.5 MHz (3.75 MHz for he office. This frequency resoluion gives maximum excess delay of abou 33.3 ns (66.7 ns for he office. The 3 GHz bandwidh resuls in a emporal resoluion of.5 ns. An example of he raw daa produced by his seup is shown in Fig. 4. This is a 3-D plo of he received power wih respec o ime and azimuh angle. In order o reduce he effec of non-saionariy in he channel and improve he signal-o-noise raio, 8 measuremens were aken for each angle and he resuls were averaged. Figure 4: Example of raw daa produced by he measuremen The parameers of he simulaion were se o closely mach boh he measuremen environmens and he process. The precise geomery of boh residenial and office environmens were enered ino he WiSE building modeler and in his way, he 3-D layous of he measuremen environmens were consruced as seen in Figures 5 and 6. Here, we have only focused on he room where he measuremen was conduced and ignored he res of he building layou. The assumpion is ha he signals ha leave he room and bounce back inside are oo weak o be considered. This is a valid assumpion due o he usually high aenuaion of he walls for millimeer wave signals. Window # Window # Window #3 (a (b Figure 3: Sample pyramidal horn anenna paern a he receiver (a H-plane (b E-plane III. SIMULATION METHODOLOGY In his paper, we have used WiSE o emulae he millimeer wave channel sounding experimen described in he previous secion. Wireless Sysem Engineering (WiSE is a sophisicaed ray-racing ool ha has been developed and verified by Bell Laboraories [5]. The main difficuly in simulaing an indoor RF channel is he srong dependence of he received signal on he layou of he building and all oher obsacles inside (i.e. mulipah channel. In paricular, all walls, windows and oher objecs ha affec he propagaion of RF waves will direcly impac he signal srengh and more imporanly he direcions from which RF signal is received. Using ray-racing will give us he opporuniy o mimic he condiions of an indoor channel o he exend possible and a Figure 5: Simulaed residenial layou.5 m Figure 6: Simulaed office layou.5 m.5 m To reduce he complexiy of he model represening he office environmen, each row of ables wih deskop compuers has been replaced by a wall-ype obsacle of appropriae heigh and aenuaion. This was done o recreae he NLOS condiion beween he receiver and ransmier. The radio characerisics of he walls (i.e. dielecric properies were also chosen o approximaely mach he

3 consrucion maerials of all objecs in he measuremen environmens. 3-D Anenna paern files were also creaed o mach he gain paern of all he anennas used in he experimen. Finally, he measuremen process was also recreaed in WiSE by roaing he direcional anenna a he receiver wih 5 seps unil full 36 coverage was obained. For each direcion of he receiver anenna, he frequency range of 6-64 GHz was swep by a coninuous one in seps of 7.5.MHz (3.75 MHz for he office in order o generae he ransfer funcion of he indoor channel. The complex wo-dimensional impulse response of he channel was produced by using he same windowing funcion as in he VNA and hen aking he inverse Fourier Transform of he daa. IV. CLUSTER IDENTIFICATION METHODOLOGY Kernel Densiy Esimaion (KDE is a nonparameric Probabiliy Densiy Funcion (PDF esimaion approach ha can be applied o a given se of measured daa [6,7]. KDE is a robus, simple and convenien mehod o esimae he PDF of a random variable given is sample realizaion. We have used he wo dimensional version of his echnique in order o esimae he emporal-angular disribuion funcion of he ray arrivals a he receiver. Having his disribuion funcion and using an appropriae hreshold, all -D clusers (i.e. emporalangular can be easily idenified. Le (, θ denoe he random variable ha represens he ime and angle of arrival of a ray a he receiver. If f (, θ is he -D PDF of hese arrivals, hen he mulivariae kernel densiy esimae of his funcion (i.e. f ˆ(, θ can be expressed by: N (, θ ( i, θi fˆ(, θ = K( Nh h h h θ i= Where h and h θ are he emporal and angular bandwidhs respecively; ( i, θ i i =,,..., N are sample realizaions of he random variable (, θ (i.e. arrivals wih he unknown densiy funcion f (, θ, N is he number of samples, and K( x, y is he kernel funcion. Here, we have wo ses of samples which have been obained by measuremen and simulaion. The kernel funcion is ofen seleced o be a PDF ha is symmeric boh in ime and angle. We used he -D Normal disribuion o be he kernel funcion as wrien below: K( x, y = exp( ( x, y π Therefore, he kernel densiy esimae of f (, a will be: fˆ(, θ = Nh h N θ i= (, θ ( i, θi exp( π h h Using his approach, we processed he ime-angle impulse response of he channel obained by boh measuremen and θ θ simulaion. In he nex secion, we provide he resuls of his analysis and discuss heir geomeric relevance. V. RESULTS One dimensional emporal clusering phenomenon has been observed in indoor channels and wideband saisical channel models have been proposed accordingly [8]. Here, we also observe ha arrivals come in few groups (i.e. clusers scaered in differen coordinaes hroughou he ime-angle space. These -D clusers are idenified by using he KDE and an appropriae hreshold represening he noise floor of he receiver. Figures 7 and 8 display he clusers obained by processing he measuremen and simulaion daa when an omni-direcional anenna is used in he residenial environmen. In general, srong resemblance in cluser shape and locaion is observed beween he resuls derived from measuremen and simulaion Azimuh (Degree Figure 7: -D clusers idenified from he measuremen daa (Residenial LOS environmen Azimuh (Degrees Figure 8: -D clusers idenified from he simulaion daa (Residenial LOS environmen A comprehensive geomeric inerpreaion of hese resuls can be provided by considering he main pahs ha he RF signal ravels beween he ransmier and receiver. The rayracing simulaion displays 9 clusers of arrivals a he receiver. The coordinae of hese clusers are lised in Table. Each cluser formaion is he resul of a unique pah beween he ransmier and receiver. These pahs which include direc LOS and various single-refleced and double refleced signals have been graphically idenified in Figure 9. Clusers 3

4 hrough 7 are also presen in he measuremen daa wih he excepion ha he clusers 6 and 7 have been merged and form a bigger cluser. Clusers number 8 and 9 which are he resuls of he reflecion from he wall behind he receiver and ransmier respecively, are no presen in he measuremen daa. This is due o he fac ha he receiver and ransmier are posiioned a he same heigh and he measuremen equipmen (e.g. covered by elecromagneic absorbers in case of he ransmier blocks he pah of he refleced signals from he back-wall. Such equipmen does no physically exis in he simulaion; and herefore, hese clusers form as a resul of he reflecion from boh walls. Cluser simulaed daa. The scaering effec of he furniure and oher office equipmen in his environmen, which canno be accuraely modeled by he ray-racing ool, causes some differences in he shape and in some cases locaion of he clusers. Again, in general, 4 clusers are observed ha are he resuls of he LOS and single-refleced pahs. The clusers due o he reflecion from he meal wall and he direc pah beween he ransmier and receiver have merged and form a bigger cluser in he measured daa. As menioned before, his is caused by he scaering effec ha all he furniure in he Cluser # Table : Mached Cluser cener s coordinaes Cluser Arrival Angel (Degrees Cluser Arrival Time (nsec Exis in Experimen Exis in Simulaion -5 5 Cluser Cluser 3 5 6, , 7.5 (merged (a No No Cluser Cluser 5 (b Percenage of Occurance Simulaion Experimen Cluser Relaive Arrival Angle (Degrees Cluser Figure : Disribuion of he relaive angle of arrivals (c Figure 9: Pahs resuling o clusers (a,,3 (b 4,5 & (c 6,7 The plos indicaing he clusers for he case when direcional anennas wih differen beamwidhs were used a he ransmier also showed srong resemblance beween he simulaed and measured daa. These plos have been omied for breviy. We have also invesigaed he disribuion of rays wihin a cluser in boh measured and simulaed daa. Two ypes of disribuion can be obained: relaive ray angle and ime of arrival. The erm relaive indicaes he angular or emporal displacemen wih respec o he cluser coordinae. Figure displays he disribuion of he relaive angle of arrivals for boh measured and simulaed daa. Similarly, Figure shows he disribuion of he relaive ime of arrivals for boh measured and simulaed daa. A close mach for ray disribuion is observed in boh figures. The daa from he NLOS office environmen was also processed for cluser idenificaion. Figures and 3 demonsrae he achieved clusers for he measured and Percenage of occurrence Simulaion Experimen.5.5 Relaive ray arrival ime (nsec Figure : Disribuion of he relaive ime of arrivals office generaes. The oher wo clusers occur because of he single reflecion from he wall wih glass windows and he wall behind he receiver. Looking a he ime coordinae of he laer cluser, a mismach is observed beween he resul of he simulaion and measuremen. A physical explanaion for his discrepancy can be given by noicing he velociy change of RF waves as i goes hrough differen maerials ha exis on is pah beween he ransmier and receiver [9]. In general, he exac coordinae, shape and size of he clusers depends on he deails of he propagaion environmens; 4

5 however, as seen in his sudy, he locaions of he main clusers (i.e. high densiy can be reasonably approximaed by he ray-racing ool. The clusers in figures and 3 are due o he use of an omni-direcional anenna a he ransmier. If a direcional anenna is used insead, he resemblance of cluser s shape and locaion obained hrough measuremen and simulaion is increased. The RF energy ransmission in ha case is more focused oward cerain direcion and hen, he impac of he scaering phenomenon on he ray arrivals is reduced. Furher resuls demonsraing his poin have been omied for breviy Azimuh (Degrees Figure : -D clusers idenified from he experimen (Office NLOS environmen, Omni TX Azimuh (Degrees Figure 3: -D clusers idenified from he simulaion (Office NLOS environmen, Omni TX VI. CONCLUSION In his paper we have capured he influence of geomery on he clusering phenomenon of a millimeer wave channel hrough he use of a ray racing ool. We have shown ha for indoor environmens ray-racing could be an effecive ool o predic he locaion of he clusers around a receiver. For scaer-free environmens and LOS scenarios rayracing seems o provide a good mach for cluser locaion and inra-cluser arrival saisics. For environmens wih heavy scaering, NLOS scenarios and direcional anennas a he receiver and ransmier, ray-racing predicion of he clusers sill seems o be reasonably close o he resul of empirical measuremen. More sudies are required o furher validae he above asserions and invesigae furher deails. Knowledge of he spaial disribuion of RF energy around he receiver in conjuncion wih array anennas and appropriae signal processing algorihms can lead o sysems ha provide superior performance by aking advanage of clusers locaion and heir corresponding characerisics. Also, saisical models ha conain boh he angular and emporal componens of he millimeer wave indoor channel are needed o ensure more efficien design of such sysems. ACKNOWLEDGEMENT The auhors would like o express heir graiude o Dr. Hirokazu Sawada, Dr. Yozo Shoji and Dr. Hiroyo Ogawa from he Naional Insiue of Informaion and Communicaions Technology (NICT in Japan for providing he measuremen resuls. Also, special hanks o Dr. Hirokazu Sawada for providing addiional deails of he measuremen environmen ha were required o se up he ray racing simulaions. REFERENCES [] Q. H. Spencer, B. D. Jeffs, M. A. Jensen, A. L. Swindlehurs, Modeling he saisical ime and angle of arrival characerisics of an indoor mulipah channel, IEEE Journal on Seleced Areas in Communicaions, Vol. 8, Issue 3, Pages: , March [] C. Chong, C. Tan, D. Laurenson, S. McLaughlin, M. A. Beach, A. R. Nix, A New Saisical Wideband Spaio-Temporal Channel Model for 5-GHz Band WLAN Sysems, IEEE Journal on Seleced Areas in Communicaions, Vol., No., Feb. 3 [3] K. Huang, Z. Wang, Millimeer Wave Circular Polarized Beam-Seering Anenna Array for Gigabi Wireless Communicaions, IEEE Trans. on Anennas and Propagaion, Vol. 54, Feb. 6. [4] H. Sawada, Y. Shoji, H. Ogawa, Angle of Arrival Measuremen in Home and Office Environmens, Naional Insiue of Informaion and Communicaions Technology (NICT, Japan, doc# IEEE c, IEEE c [5] S. J. Forune, D. M. Gay, B. W. Kernighan, O. Landron, R. A. Valenzuela, M. H. Wrigh, WISE design of indoor wireless sysems: pracical compuaion and opimizaion, IEEE Compuaional Science and Engineering, Vol., Issue:, Pages: 58 68, Spring 995 [6] D. W. Sco, Mulivariae Densiy Esimaion: Theory, Pracice and Visualizaion New York: Wiley, 99. [7] A. W. Bowman, A. Azzalini, Applied Smoohing Techniques for Daa Analysis, Oxford Universiy Press, 997 [8] A. Saleh, R. A. Valenzuela, A Saisical Model for Indoor Mulipah Propagaion, IEEE Journal on Seleced Areas in Communicaions, Vol. 5, Issue, Pages: 8-37, Feb. 987 [9] M. A. Hussein, Characerizaion of Ulra Wideband Communicaion Channels, Ph.D. Disseraion, Virginia Polyechnic Insiue and Sae Universiy, March 3 [] K. H. Li, M. A. Ingram, A. V. Nguyen, Impac of clusering in saisical indoor propagaion models on link capaciy, IEEE Trans. on Communicaions, Vol. 5, Page(s: 5-53, April [] B. Neekzad, K. Sayrafian-Pour, J. S. Baras, Energy efficien millimeer wave radio link esablishmen for low probabiliy of inercep wih smar anenna, 5h Army Science Conference, Orlando, FL, Nov. 6 5