Propagation Factors Controlling Mean Filed Strength on Urban Streets

7/31/2019 Propagation Factors Controlling Mean Filed Strength on Urban Streets

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822 IEEE TRANSACTIONS ON ANTENNASAND PROPAGATION, VOL. P - 3 2 , NO. 8, AUGUST 1984Propagation Factors Controlling MeanField Strength on Urban Streets

Fumo m G W , ENIOR MEMBER, IEEE,U S U M U YOSMDA, MEMBER, IEEE, TSU TO M U T D U C H I ,AN D MASAHIRO TJMEHIRA

Abstract-Calculation of mean field strength for urban mobile radio hasbeen made on a ray-theoretical basis assuming an ideal city structure withworm building heights. The result shows that building height, streetwidth, add street orientation as well as mobile station antenna height arecontrolling prop agation param eters in addition t o the ordinary factors.Th e mijor theoretical characteristics agree approximately with experimen-tal data including conventional empirical predictions. This suggests a wayof theoretically predicting mean field strength in an urban area.

P I. INTRODUCTIONREDICTION OF mean signal strength for arbitrary propaga-tion onditions is essential for designingmobile adiosystem. Prediction for irregular terrain can be made using ground-wave propagation heories,withstatistical errainfactorscom-binedasnecessary.However,prediction inurban areasmustrely at present on a purelympirical metho d [1 .

More precise prediction fo r urban areas is n eeded, particularlywhen the size of radio zones in a cellular mobile radio has to bedecreased [2]. For this purpose, it k ne.cessary to provide a cleartheoreticalbasis or hepropagation actorswhichdeterminemean field strength.

Studies on propagation structures in urban areas revealed that"ray-theoretical" waves play an important ole,and suggestedthat majorropagationharacteristics might beontrolledby such principal waves [3]. This may ead to a possibility ofpredictingmeanfieldstrength by mean s of ray-theoreticalap -proach, to the exte nt that ray components are dominant.This paper analyzes the controlling propagation factors basedon he geometricaloptics ssuming implewo-raymodel.Comparison with experiment resultsin fajrly good agreement.11. PRINCIPLE OF PREDICTION BASED O N GEOMETRICALOPTICS

Propagation in urban areas shows complicated features due tovarious propagation modes caused by com plicated radio environ-ments. However, detailed analyses of mu ltipath waves disclosedthe following aspects as t o the propagation structures on urbanstreets [3].Therear ea f i t e number of mult ipath waves arriving at a

receiving pointrom discrete irections. ome of them resubject to simple geometrical optics, such as diffraction and/orreflectionbybuildings. In many cases stableprincipalcompo-nents ex ist, thoug h the strengths may ary along a street.This suggest a plausible propagation model consisting of ray-theoretical waves plus noway-theoretical waves that are inter-preted as scattered waves from nearby building s. If ray-theoreti-cal waves are dom inant, as observed in the detailed experim ents,

Manuscript received June 9, 1983; revised December 5 , 1983.F. Ikegami, S. Yoshida, andT. Takeuchiare witb the Department ofElectron ics, Kyoto University, Kyoto 606, Japan.M. Umehira is withYokosuka ElectricalCommunication Laboratory,Nippon Telephone and Telegraph Public Corporation, Yokosuka 238-03,Japan.

* UILDINGSFig. 1. A multipatb propagation model.some m ajor propagation characteristics might be determined byconsidering ray-theoretical components.

T h i s propagationmod el is drawn in Fig. 1 assumingplanewaves. The available power P(x ) received by an isotropic antennais given by

where E is field strength, 8 is phase, and h is wavelength. A meanpow er received by a vehicle moving over a distance I is given asfollows:

Th esecond erm of (2 ) , the ntegrand of which is oscillatorydue to interference of each two of multipath waves, tends to dis-appearwhen E is sufficiently argecompared with theperiodsof fluctuation. Then the first term remains,o then

where pi is a mean power of he ith multipathwave over a distance0018-926X/84/0800-0822$01.000 1984 IEEE


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IKEGhMI etal . : FIELD STRENGTH ON URBAN STREETS 8231. Consequently, the mean signal strength is obtained by the rootme an square of the field strengths of com pone nt waves.

Here in he above calculation,atten tion should be paid inchoosingan averaging distance 1. As spatial fading in generalconta ins luctu ation s of various spatial eriods,mean signalstrength is dependenton a sample length 1 over which signalstreng th is averaged. So , a fading sample length should be chosenaccording to the requirement of th e analysis

Fading onurban treets is considered to be composed oflong-term ndhort-term fading. It is generally understoodtha t he orm er is caused by variation of building diffractionloss (shadowing) and he atter by nterference of multipathwaves.As the present study concerns ong-term fading, samplelength must be take n sufficiently long comp ared with short-termfading periods and small compared wi th the sizes of buildings.In t h i s paper, which treats radio frequencies above V HF , samplelength will be taken as about 10- 50 m considering the wave-lengths nd building sizes together with the requirementsofth e analysis.In the mod el of Fig. 1, the power of each wave is assumedconst ant over an averaging section. Som e of the arriving waves,scattered waves for nstance,may have multipathstructure inthemselves producing Rayleigh-distributed short-termading,but it is a ppropriate t o assume tha t the mea n power s almostconstant over the section.As th e mean received signal power is given by t he pow er sumof he arrivingwaves, a small num ber of strong waves mainlycontributed to the mean power; then the error caused by neglect-ing weak waves is relatively small. Let he ota l power umsof he acco unte d and he disregarded waves be C P, and Z P d ,respectively, then the error s given by

XPa + p dA = 10 log = 10 log (1 + 6) [dB]ZPawhere 6 = ZPd/ZPa. For example, if the disregarded power isless than the accounted, the errors within 3 dB.

According to the referenced detailed experiments [ 3 ] , direct-diffrac ted an d single-reflected waves are observed to be the prin-cipal waves in ma nycases, because multiple diffraction and eflec-tion result in large attenu ation .

Considering the above, the following assumption wibe takenfor an ideal city structure as shown in Fig. 2, consisting of regu-larly aid-out blocks withuniform building height. A direct-diffracted ray (@ in Fig. 2 ) and a ray single-reflected by a buildingacross th e street (0 in Fig. 2) comprise the principal dominan tcomponents that exis t at almost all receiving points. Other ray-theoretical waves andonray-theo retical waves depictedydotted ines are all neglected by assuming tha t the power sumof hose waves is small comparedwith the power sum of theabove two principal ays.

The ieldstrengthsof he tw o principal aysca nbe easilycalculated if propagation path p rofdes for both rays e available,with mean field stren gth evaluated by (4).III. APPROXIMATE EQUATION O F MEAN FIELD STRENGTHFig. 3 shows a geometry of two principal rayswhich determinemean field strength on a street based on the assumptionsescribedin the previous chapter. In this igure,an angle between the

IFig. 2. Ideal city structure and geometrical optical rays.

n V

III

P l a n

Cross sectionantenna

W

I - - - W - - - - IFig. 3 . Geometry of two principal rays.

incident wave and a street, denoted by a, will for conveniencebe referred to as street angle (0< P


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824 IEEE TRANSACTIONS O N ANTENNAS AND PROPAGATION, VOL.AP-32, NO. 8 , AUGUST 1984H= 2Om f = 395.425MHzh,= 2.5m @= 900W= 30 m L =,4dB

It - . -. E J0 5 10 1 505 30

w (m)Fig. 4. Examples of relative field strength variation across a street.

TABLECHARACTERISTICS OF THE M I 0 WAVE SOURCES UTILIZED IN THE TESTS-

Mean field strength is obtained asE =d.qTq

. *

Examples of relative field strength variation across a street arecalculated s hown n Fig. 4 with L , [dB] as aparameter.Reflection loss, being largely dependent on radio and buildingparameters, akes values of the order of 4 - 1 0 dB on averageaccording to the experiments of VHF and U H F bands [4]. SinceFig. 4 indicates thatmean field trength s lmost onstanta c r o s astreet orprob able values of L,, mean field strengthon astreetmayapprox imately be representedby thata t hecenter of a st reet , given by

E + (0.225/2) 1 +7 o a I ( f f -lr; (w= W / 2 ) . ( 9 )Replacing th e wavelength by radio frequ ency , he above equ a-tion is rewritten in dB as

- 20 log (ff - h,) - 10 log (sin a) - 10logf [dB], (10)where W, , h, are in mete r s andf isn MHz.

It is no ted that mean field strength at the center of a streetisapproximatelydescribedasa unction of the ndependen tpropagation parameters, W , H , h,, a, andf, the effect of each isqu ite simply understo od regardless of reflection loss.

In th e ollowing sections, m ean field strength ill be tentativelycalculated with a constant reflection loss L, = 6 d B , consideringtha t L, doe s not directly affect each param eter.

IV. COMPARISON WITH FIELD EXPERIMENTSA . Outline ofExperiments

In order to compare he heoretical predictions described inSection 111 with experiment, measurements of mean field strengthwere made in the campu s of Ky oto University and on representa-tive streets in Kyo to City in the 200, 400, an d 600 MHz bands,radiated from different sites, as hown in Table I an d Fig. 5 .On the university campus, spatial distribution of mean fieldstrength were measured along and across a street, a nd w ith he ightabove the ground.B,Along a Street

Fig. 6 shows a comparison of the measured (running averageof +5 m) and calculated (every 2 m j mean field strengths alongcourse A on the universitycampus. Thecalculation, based inprinciple on the above theory, was made in practice by a com-puter aided method applicable to the actual building conditions[5]. Satisfactory greem ent is seen betw een alculation ndexperim ent as a whole.C Across a Sh-eet

As shown in Fig. 7, a measured field strength across courseA (thin line) displays interference of the diffracted and reflectedwaves, togetherwith he andomscattered waves. Th edottedcurve shows a running average of +1 m around a receiving point,and scomparedwith he heoretical curve (thick ine)with


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er al . : FIELD STRENGTH ON URBAN STREETS 825

Fig. 5. Map of test area in Kyoto City.


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826 IEEE TRANSACTIONS ON ANTENNAS ANDPROPAGATION, VOL. AP-32, NO. 8, AUGUST 1984

Courae A400 lulz- thsorstical- mmured

1 10 100 rn XKIDiatance (m i

Fig. 6. Comparisonof measured and calculated mean ield strengthon courseA .

- - w e d...". :ming average- theoreticalCOUESE A400 Hliz

J 60

zol07 8 9 10 ll 12 13 14 15 16 17 18, , I I I I I I . I I .

Distance from the building In1Fig. 7. Variation of field strength across a street.

appreciable greement. As stated in Section 111, meanieldstrength is almost constantacross a street.D. eight Gain

A measuring antenna was raised up to ab ou t 10 m above theground at representative points on he campus. An example isshown in Fig. 8. The running average (kl m) denoted by a dottedline shows a endency similar to he theoretical curve (thickline).E. Difference at BothSides of a Street

Mean field stxength is almost constant across a street as men-tioned above. Confiiation was made in urban parts of KyotoCity, com paring the values at both side s f streets, Measurem entswere made along outer car lanes nearest to both sidewalks. Thereceiving points app roximately correspond to w = W/4 nd 3W/4,respectively, inFig. 3.Fig. 9 shows hedifference of mean ield trengths 50-mmean ) at both sides of a street versus street angle. No significantdifference is seen in average, except that the plots tend to scat-ter at small street angles.Differencesofmean ieldstrengthsat w = W/4 nd 3W/4are calculated as show n in Fig. 10, for various combinations ofstreet width (1 0 - 50 m every 10m) and building height (10 -50 rn every 10 m). Although hedifference is dependentonstreet width and building height, the w hole endency is similarto the measured.

112I w u r s c A40 0 MHz

Fig. 8. Height-gain haracteristics.

f = 600 MHz+ : meaaured

0 - t *t- 1 5 1 - . - v . . , . r0 30 60 90

Street angle (deg)Fig. 9. Differenceofmean ield strengths atboth sides ofa street versus street.angle.

E Dependence on Street WidthIt is comm only experienced that mean field strength is high

on a w ide street. In Fig. 11, the mean values and standard devia-tions of he measuredmean ieldstrengthsnormalizedby thefree-s pace value are plo tted against street width classified every10 m , referred to the mean value of W < 0 m. Though the meas-ured values are widely scattere d, the m ean values agree with thetheoretical curve.G. Dependence on Street Angle

I t is also acommonexperience hatmean ieldstrength ishigh when incident wave direction is nearly parallel t o a street[ l ] [ 6 ] . Fig. 12 shows the mea sured relative mean field stre ngthnormalized by the free-space value versus street angle, compared


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JKEGAMI e t a l . : FIELD STRENGTH ON U R B A N STREETS 827f = 60 0 )3Hrelevation angle : 6(deg)H - 101.50m (every 1Om)W = 101.5Om (every 10m)

H-50m, W-lOm

0 30 60 90Street angle (deg)

Fig. 10. Theoretical differen cesofmean field strengthsat o = W/4 and 3W/4 (E (W/4) - E(3 W/4)).

20-mE

C

E4J

f = 4 0 0 MHz: measured (mean f standard deviation)- theoretical T

u

% oa1 Streetidthmeter)

Fig. 11. Mean field strength versus streetwidth.

2 0 rEm

0L1wY.a

- theoretical: 40 0 MHz measured(mean 5 standard deviation)

(mean 2 standard deviation)

- 1 0 L * . . . I0 30 60 90

STREET ANGLE (deg)Fig. 12. Mean field strength versus street angle.

with he heoretical curve. The increasing tende ncywithde-creasing street angle appears to show aBe em ent.V.COMPARISON WITH TH E CONVENTIONAL EM PIRICALMETHODA Height Gain

The height-gain curvesgiven in the empirical method [ I ] iscompared in Fig. 13 with he heoretical curves calculated orH = 1 2 , 1 5 , an d 20 m.As seen in the igure , dense urbanapproximatelycorre-sponds to H = 20 m: and moderate urban to H = 12 m. Theheight-gain dependence on city size in the empirical method isphysically interpreted as the variation ofdiffraction loss withmean building height.B. Dependence onPropagation Distance

Fig. 1 4 shows the mean and the variation range of mean fieldstrengths versus propagation distance measured at 400 MHz inKyoto City, in comparison with the predicted curve. The attenua-tion rate of he empirical curve for suburban areas is well co-incidentwith th at of the measured values (approximated byd - l e 4 ) , while that of the heoretical curves ( d - l ) for variousstreet widths deviates a t longer distances.The disagreement of the theoretical curve seems to be du e tothe assumptionofuniform building height. For omp licatedbuilding heightdistributions,moreattenuationmaybe apt tooccur at longer distances because of increased probability ofmulti-edge d iffraction.C Dependence on Radio Frequency

Comparison of radiorequency ependence etweenhetheoreticaland empirical meth ods isshown in Fig. 15, whichgives a relative value referred to 200 MHz. The ttenuatio nrate of the heory (-10 dB/decade) differs from he empiricalprediction of -6.16 dB/decade. However, the discrepancy is notmore than 2 - dB a t frequencies below 1 GHz.

VI . DISCUSSIONIt has been found hat an appro xima te heory can describealmost all major characteristics of me an field strength on urbanstreet. This means that variations in mean field strength can be

interpreted as those, resulting from diffraction loss, as usuallyterme d by shadowing. In a sense, the presen t study has con-firmed thevalidity of this concept.However, it is surprising that a theo ry based on the assump-tionsof a simple two-raymodelan duniform building heightagrees with exp eriments p erformed in actual, com plicatednviron-ments. W hat this means will be considered in the following.At first,it should be recalled that disregard of wave power sumless than the accou nted power sum (6 G 1 ) produces a relativelysmall error (G3 dB). This is in contrast with multipath fading,in which disregard of a comparable wave componentmay re-sult in a gross mistreatment of fading. Mean field strength isinsensitive to neglect of weak wave co mp one nts on acco un t ofth e power sum law . The results of the analysis seem to indicatethat the typical two-ray configuration can represent the principalmultipath.

Next, consider theassu mp tion of uniform building height.The effect of a variety of building heights in actual city condi-tions wouldbe smoothedout in average forsufficiently largeamount of data. The clear results obtained arebecause all param-


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828 IEEE TRANSACTIONS ON ANTENNASAND PROPAGATION,VOL.AP-32,NO . 8, AUGUST 1984

10 -9 -8 -

7-2 6'-P.r lP); -E; - -

3 -

2 -

height :reference antenna

4 m

t h e o r e t i c a l

e m p i r i c a lp r e d i c t i o n.......d e n s e u r b a n-___.e n s e u r b a n. a b o v e 40 0 MHz)

' ( b e l o w 40 0 MHz)-.- m o d e r a t e u r b a n( 4 0 0 MHz)II0 I 1-20 -10 0 10 20

H e i g h t g a i n r e f e r r e d to a n te n n ah e i g h t of 4 m ( d B )Fig. 13. Theoreticalheight gain compared with the emp irical prediction [l].

lo o 40 0 MHz H= 8m

---- theoretical----- : empirical2 0

2 . 3 4 5 6 7 8 9 1 0 2 0DISTAUCE PBoI(DAHSUTTBR ( k id

Fig. 14. Mean field strengthversuspropagation distance.

eters are mu tually indepen dent of each other as given by (10).I t shouldbenoted,however, hat (10) doesno t give th eab-solute value f or heactualnon unifo m building heights.Th eabsolute value should be calculated by ta king m ultiedg e diffrac-tion loss and/or more than two mdtipath aves into account.

What is most mportant in the results of he present studyis that the mean field strength is approx imately determin ed bydiffracted and reflected fields subject only to geometrical optics.This m eans tha t a mean field strength could be predicted theoreti-cally if the propagation loss is calculated for each principal ray-theoretical wave.

The above discussion natu rally implies that there is a limita-

1 Frequency (GHz)

-10.1-4+Irl; i - h e o r e t i c a l--- : e m p i r i c a lam -20Fig. 15. Mean field strengthversusradio requency.

tion in the validity of the present metho d. Being based on theassumption hat ray- theoretical components are dominant, thismethod would be no more valid w hen ray-theoretical waves arelargely attenuated by deepdiffraction, whichmay akeplaceat higher requenciesand orhigher buildings and or owercell antennaheights.Though heexact imitingconditionsar eno t yet very clear, further exploration of he imits is beyondthe scope of hi s study.

VII. CONCLUSIONA theoretical calculation imited to .diffractedandreflected

fields can approximately describe themajorcharacteristicsofmean ieldstrengthsonurbanstreets,withcontrolling factorsof building height, mob ile station antenna height, street widthand street angle in addition to the ordinary propagation param-eters.The theor y also gives the height-gain curves in he con -ventional empirical method a lear physical meaning.

The results of the stud y suggest a way to predict mean fieldstrength on a theoretical basis for arbitrary radio and city struc-ture conditions. A prototyp e comp uter-aided prediction systembased on this p rinciple haseen developed, and appearso show thefeasibility of this method [51 .

APPENDIXReduction of (6) a nd (7 )appropriate for ordinary urban mobileadio.In Fig. 3 , the ollowingar eassumed orsimplicityan dar e

1) The roof of a diffracting building is well within the l ineof sight of a transmitting antenna.2) A diffracting building is substituted by an infinitely longknife edge transverse to the wave propagation direction.

3) The ground reflection is ignored.The path profile s then transformed t o Fig. 16.A diffraction field behind a knife edge is given by the well-

known Fresnel's equation as approximated byE + 0 . 2 2 5 E o / ~ 0 1 )

fo r u> 1 with an error less than 1 dB [7] . 'v a l u e s ,i f O s O a n d d , S w w , 2 W - w , a n d H .The Fresnel's parameter u takes he ollowingapproximate

u1 6,a ( ~hr )d- , fo r diffracted wave (12)


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e t ~ l . : IELD STRENGTH ON URBAN STREETS 829

l----( Pransmitter

v2 =a ( ~h,)Jsin (a/x(2W- w), for reflected wave.(13)E , and E2 are given by

E1 + 0.225E,/v,, E2 k 0.225E,/vzL,ing i n (6) an d (7).

ACKNOWLEDGMENTThe authors are indebted to the many students who workedn this study during their graduate courses.

REFERENCES[ l ] Y. Okumura, E. Ohmori, T. Kawano, andK. Fukuda, Field strengthand its variability in VHF and UH F land mobile service, Rev. Elec.Comm. Lab., vol. 16 , pp . 825-873, Sept.-Oct. 1968.[2] V.H. MacDonald, The cellularconcept, BeliSyst. Tech. J. , vol. 58 ,no. 1 , pp. 15-42, Jan. 1979.[3] F. Ikegam i and S. Yoshida, Analysis of multipath propagation structurein urban mobile radio environments, I E E E Trans. Antennas Propa-[4] S . Mitobe and S. Ito, Measurement of VHF reflection from precastconcrete walls, NHK Tech. Rep., pp. 103-109, Mar. 1974 (inJapanese).[5] F. Jkegami and S . Yoshida, Feasibility of predicting mean fieldstrength for urban mo bile radio by aid of building data bases, in Conf.

Rec. ZEEE Znt. Con f. C om mu n., (ICCSS), paper A2.6,Boston, MA,June 20-23, 1983, pp . 68-76.[6] W. C. Jakes, Jr. Ed., MicrowaveMobile Communicutions. NewYork: Wiley, 1974, sec. 2 .2 .7 .[7] C . R. Burrows and S.S. Attwood, RudioWavePropagation. NewYork: Academic, 1949, p. 464.

gut., V O ~ .AP-28, 110. 4 , pp . 531-537, July 1980.

submarine cable systems, satellite communication systems, and mobile com-munication systems as the Director of the Transmission Svstem DevelopmentDivision at Yokosuka ECL. Since 1975, he has been a Professor at KyotoUniversity, Faculty of Engineering, Kyoto, Japan.Dr. Ikegam i is a member of the Institute of Electronics and CommunicationEngineers of Japan and the Institute of Television Engineers of Japan.

Susumu Yoshida(78) was born in Hyogo Prefec-ture, Japan, on November 26, 1948. He received theB.S., M.S., and Ph.D. degrees in electrical engi-neering from Kyoto University, Kyoto, Japan, in1971, 1973, and 1978, respectively.In 1973 he joined the Faculty of Engineering,Kyoto University, Kyoto, Japan, as a ResearchAssociate. Since 1979 he has been an AssociateProfessor at Kyoto University. From 1971 to 1976,he was engaged in the research of automata theoryfrom a stochastic point of view, transmission codetheory, and computer networks. Since then he hasDr. Yoshida is a member of the Institute of Electronics and Com municationbeen engaged in the research of mobile communication systems.Engineers of Japan and the Information Processing Society of Japan.

Tsutoma Takeuchi was born in Kyoto City, Japan,on November 13th, 1953. He received the B.S. andM.S. degrees in electronics engineering from KyotoUniversity, Kyoto, Japan, in 1976 and 1978, respec-1978 to 1982, he was worked for theComm unication Laboratory, N ippon Tel-egraph and Telephone Public Corporation, Tokyo,Japan, where he worked on the development ofTDMA equipments for satellite communicationssystem. In 1982, he joined the Faculty of Engineer-ing, Kyoto University, Kyoto, Japan. Since 1982, he

Mr. Takeuchi is a memb er of the Institute of Electronics and Com municationhas been engaged in the research on mobile communication systems.Engineers of Japan.

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Propagation Factors Controlling Mean Filed Strength on Urban Streets

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