ing approximation of the ring structures was appropriate. Oursimulation results showed that slits could be introduced tothe ring structures without distorting the frequency responseof the resonator. We also found two sets of resonance peakswhich can be understood as being due to the existence of twodifferent coupling mechanisms: one through the electric field,and the other through the magnetic field. For infinitely thinconductors, the two sets of peaks could be observed for bothedge- and side-coupled ring resonators. It was also demon-strated that one could accurately determine the dielectricconstant of the substrate by fitting the FDTD simulationresults of a ring resonator response to the experimentallyobserved S -parameter spectrum.21

ACKNOWLEDGMENT

The authors thank Steven Perini and Michael Lanagan forproviding the experimental data of Figure 10.

REFERENCES

1. M.E. Mayercik, Resonant microstrip rings and dielectric materialŽ .testing, Microwaves Apr. 1991 , 95]102.

2. P.A. Barnard and J.M. Gantray, Measurement of dielectric con-stant using a microstrip ring resonator, IEEE Trans Microwave

Ž .Theory Tech 39 1991 , 592]595.3. H.A. Wheeler, Transmission line properties of parallel strips

separated by a dielectric shield, IEEE Trans Microwave TheoryŽ .Tech MTT-13 1965 , 172]185.

4. M.V. Schneider, Microstrip lines for microwave integrated cir-Ž .cuits, Bell Syst Tech J 48 1969 , 1421]1444.

5. E.O. Hammerstad, Equations for microstrip circuit design, ProcEuropean Microwave Conf, Hamburg, Germany, Sept. 1975, pp.268]272.

6. I.J. Bahl and D.K. Trivedi, A designer’s guide to microstrip line,Ž .Microwaves May 1977 , 174]181.

7. K. Chang, Microwave ring circuits and antennas, Wiley, NewYork, 1996.

8. I. Wolff and N. Knoppik, Microstrip ring resonator and disper-Ž .sion measurements on microstrip lines, Electron Lett 7 1971 ,

779]781.9. R.P. Owens, Curvature effect in microstrip ring resonators, Elec-

Ž .tron Lett 12, 1976 , 356]357.10. S.G. Pintzos and R. Pregla, A simple method for computing the

resonant frequencies of microstrip ring resonators, IEEE TransŽ .Microwave Theory Tech MTT-26 1978 , 809]813.

11. K. Chang, T.S. Martin, F. Wang, and J.L. Klein, On the study ofmicrostrip ring and varactor-tuned ring circuits, IEEE Trans

Ž .Microwave Theory Tech MTT-35 1987 , 1288]1295.12. A. Taflove, Computational electrodynamics. The finite-difference

time-domain method, Artech House, Boston, MA, London, Eng-land, 1995.

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14. C.A. Harber, Handbook of thick film hybrid microelectronics,McGraw-Hill, New York, 1974.

15. G. Mur, Absorbing boundary conditions for the finite-differenceapproximation of the time-domain electromagnetic-field equa-

Ž .tions, IEEE Trans Electromag Compat EMC-23 1981 , 377]382.16. E.O. Brigham, The fast Fourier transform and its applications,

Prentice-Hall, Englewood Cliffs, NJ, 1988.17. G.K. Gopalakrishnan and K. Chang, Study of slits in microstrip

ring resonators for microwave and optoelectronic applications,Ž .Microwave Opt Technol Lett 5 1992 , 76]79.

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1015.

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Ž .crowave Theory Tech 44 1996 , 953]956.

Q 2000 John Wiley & Sons, Inc.

URBAN CHANNEL PROPAGATIONMODELING USING THE SHOOTINGAND BOUNCING RAY TECHNIQUEI. Y. Kelly,1 H. Ling,1 and W. J. Vogel11 Department of Electrical and Computer EngineeringUniversity of Texas at AustinAustin, Texas 78712-1084

Recei ed 7 October 1999

ABSTRACT: A study of urban channel characteristics is carried outusing the shooting and bouncing ray technique. Propagation characteris-tics are generated from the simulation, and are compared to measure-ment results for the downtown area of Austin, TX, at 1.9 GHz. Antennadi ersity studies are also carried out using the simulation. Q 2000 JohnWiley & Sons, Inc. Microwave Opt Technol Lett 24: 396]399, 2000.

Key words: propagation; channel modeling; SBR; CPATCH

1. INTRODUCTION

As the demand for wireless personal communications in-creases, so does the need for accurate propagation channelmodels in complex environments. In this work, we carry out astudy to characterize the urban propagation channel by usingray tracing. We consider a ray tracer based on the shooting

Ž . w xand bouncing ray SBR technique 1]3 . The particularscenario addressed has a base-station antenna height belowthe level of the average urban building, and falls into themicrocell category. Propagation characteristics such as slow-fade and fast-fade distributions are examined and comparedto measurement results for the downtown area of Austin, TX.The application of the simulation is demonstrated by carryingout spatial and polarization diversity studies.

This paper is organized as follows. In Section 2, we de-scribe the simulation methodology, and present results of theurban channel characteristics simulated using the SBR-based

w xcode CPATCH 3 on a CAD model of downtown Austin, TX.In Section 3, the simulation results are compared againstmeasurement data. Both the slow-fade and fast-fade charac-teristics are examined. In Section 4, an antenna diversitystudy is carried out using the simulator. Spatial diversity andpolarization diversity schemes are investigated to determinethe diversity gains of the different scenarios.

2. RAY-TRACING SIMULATION

The SBR-based code CPATCH is used to carry out urbanchannel simulation. CPATCH is designed for the calculationof antenna coupling in complex environments. It performsthese calculations by shooting a dense grid of geometric opticrays from the transmitting antenna, and tracing the rays asthey multiply reflect among the surfaces of a CAD descrip-tion of the environment. After the ray tracing is carried out,the received signal at the receiving antenna is computed by

Contract grant sponsor: Texas Higher Education Coordinating Boardunder the Texas Advanced Technology Program

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 24, No. 6, March 20 2000396

summing up the contributions of each ray via a ray-tubew xintegration scheme 1, 4 .

The simulation is carried out using a CAD model of thedowntown area of Austin, TX, that came from an architect’s

Ž .rendition Fig. 1 . The part of Austin on which the simula-tions were run is focused around Congress Avenue. A trans-mitting antenna is placed 18 m above the ground plane at theintersection of Congress Avenue and 7th Street. A grid

Žconsisting of 70 blocks}ten cross streets from north to.south, 11th]2nd Street , and seven down streets with Congress

Avenue at the center}is used to approximate a cell witha diameter of about 1 km. All calculations are made at1.91 GHz, and both transmitting and receiving antennas weredipoles.

The strength of the received signal is simulated at multi-ple-receiver locations. The computed power signal alongCongress Avenue is shown in Figure 2. Because CongressAvenue contains the transmitting antenna, it has a direct

Ž .line-of-sight LOS receiver path. The received signal is calcu-lated every 1 m, and is normalized with respect to the

Ž .transmitted power to indicate propagation loss Fig. 2 . Byusing a smoothing window of 40 wavelengths, a slow orlong-term fade version of this power signal can be obtainedŽ .Fig. 2 . The resulting slow-fade curve is not completelysmooth, but has dips along its path. The dips correspond tothe location of the streets that cross Congress, indicatingsome channeling of power into the non-LOS cross streets.

3. COMPARISON OF SIMULATION AND MEASUREMENT

A measurement campaign was carried out in January 1997.The transmitting antenna was placed on Congress Avenueand 7th Street at a height of about 18 m, and data werecollected by a moving vehicle on the street level on the same70 blocks. Using both measurement data and ray-tracingdata, power plots of the entire 70-block area were generatedŽ . Ž . Ž .Fig. 3 . In Figure 3 a and b , the power plots for thesimulations are compared to the measurement data. A fairlygood agreement is observed between the two plots. As ex-pected, the main LOS streets, Congress and 7th Street, havethe highest signal strengths. We also observe a channelingpattern as power is being channeled into the side streets, butit gets lower as the receiver location gets farther from the

Figure 2 Power plot of Congress Avenue with smoothed signal 25db lower

transmitter location. We note that there is a smaller dynamicrange in the simulated data. There are several possible rea-sons for this result. First, all surfaces were assumed perfectlyconducting. Thus, rays travel farther, and contribute more tothe more distant receiver locations. Second, the CAD modeldoes not contain all of the buildings located in downtownAustin. In fact, the farther from the transmitter location, thefewer buildings the model has. Even with these inaccuracies,the slow fade predicted by the simulation agrees fairly wellwith the measurements.

Ž .Probability distribution functions pdfs were used to com-pare the fast-fade component of both simulated and mea-sured data. It is commonly believed that the Rician distribu-

Ž .tion applies for a street with a dominant line-of-sight LOSw xcomponent plus multipath components 5]7 , with a pdf

described by

2 2Ž .y y y q a yaŽ . Ž .pdf y s exp I 1Rician 02 2 2ž / ž /b 2b b

Figure 1 CAD model of Austin, TX

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 24, No. 6, March 20 2000 397

Ž .Figure 3 a Simulated data power plot for downtown Austin, TX.Ž .b Measurement data power plot for downtown Austin, TX

where I is the modified Bessel function. In the above0expression, the LOS component is taken into account by theparameter a, which is proportional to the amplitude of theLOS signal. The b parameter is a measure of the standarddeviation of the signal. For signals without an LOS compo-nent, the above expression reduces to the Rayleigh distribu-

Ž .tion i.e., a s 0 :

2y yyŽ . Ž .pdf y s exp . 2Rayleigh 2 2ž /b 2b

Both the measured and simulated data were fitted to theŽ .Rician distribution in 1 using data from a 25 m moving

window. Figure 4 shows the results for the a parameter formeasurement data. We found that the main LOS streetsŽ .Congress and 7th contained the highest a parameter, andthus are highly Rician, as are the portions of the side streetsclosest to these streets. However, a dropped to approximatelyzero on the streets farthest from the transmitter location.Thus, the farther from the transmitter, the closer to theRayleigh distribution the pdf becomes. The simulated resultsare also indicated in Figure 4 at a few selected locations. As

Figure 4 Rician a parameter for measured data

we can see, these results are consistent with the measureddata. The b parameter, which is a measure of the standarddeviation of the signal, is fairly constant throughout.

4. DIVERSITY STUDY

In this section, an antenna diversity study is carried out usingthe simulator. Both spatial diversity and polarization diversityschemes are investigated to determine the diversity gains ofthe different scenarios. The main goal of using antennadiversity is to minimize deep fades while keeping the meanpower levels high. For the spatial diversity study, two trans-mitting dipoles are spaced 3 m apart. A single dipole receiverat a 458 slant is used to emulate the position of a handsetbeing held to the ear of a user. The signals from the twospatially diverse channels are combined to form a singlesignal of stronger strength. We should point out that, al-though the uplink case is usually of more practical interest asopposed to the downlink scenario depicted in the simulation,the two results should be identical since the principle ofreciprocity can be invoked.

The transmitters are again placed on Congress Avenueand 7th Street at a height of 18 m. The receiver is run alongCongress Avenue. The combined signal obtained from thetwo spatially diverse antennas is higher than that of the singleantenna. Furthermore, the deep nulls in the single-antennacase are mostly mitigated. The cumulative distribution func-

Ž .tions cdfs for the case with and the case without spatialŽ .diversity are tabulated, and are shown in Figure 5 a . There is

an approximate 6 dB gain when two spatially diverse anten-nas are used as compared to using a single antenna.

Next, a polarization diversity study is carried out. Thescenarios considered here are similar to those used in the

w xmeasurement campaign reported in 8 . In the first case, apair of cross dipoles is used as the transmitter, one beingvertically polarized and the other being horizontally polar-ized. The receiving antenna is again a single dipole at a 458slant. When the signal in each channel is monitored sepa-rately, it is observed that the signal strength in the horizontalchannel is much lower than that in the vertical channel.Therefore, adding a horizontally polarized transmitter doesnot add much diversity gain, as can be seen in the comparison

Ž .using the cdf in Figure 5 b .

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 24, No. 6, March 20 2000398

Ž .Figure 5 a Diversity gain comparison of spatial diversity versusŽ .single dipole. b Diversity gain of vertical and horizontal diversityŽ .versus dipole. c Diversity gain for 458 polarized transmitters versus

dipole

In the second case, we consider two orthogonal transmit-ting dipoles oriented 458 with respect to the vertical. Again,the cdf is used as a basis for comparison. As seen in Fig-

Ž .ure 5 c , the result of this test is better than that of thevertically and horizontally polarized transmitters. The diver-sity gain is about 3 dB. Overall, spatial diversity appears togive a much bigger diversity gain than what is achievableusing polarization diversity in this urban setting.

5. CONCLUSION

A study of channel propagation characteristics in downtownAustin was carried out using the shooting and bouncing raytechnique. General characteristics were extracted from simu-lation data. Both the large-scale power coverage and fast-fadecharacteristics from the simulation compared well with thoseobtained from measurement data at 1.9 GHz. Studies ofspatial diversity and polarization diversity were carried outusing simulation. Spatial diversity was found to have a largerdiversity gain than polarization diversity.

ACKNOWLEDGMENT

The authors would like to thank DEMACO, Inc., for the useof the CPATCH code.

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8. X. Tian, H. Ling, and W. Vogel, Investigation of base-stationpolarization diversity in personal communication systems, 1998USNCrURSI Nat Radio Sci Meeting Dig, Atlanta, GA, p. 145.

Q 2000 John Wiley & Sons, Inc.

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 24, No. 6, March 20 2000 399