base station height

8/8/2019 Base Station Height

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25 2 IEEER A N S A C T I O N S O N V E H I C U L A RE C H N O L O G Y,O L .T-29 ,0 .2 , MAY 19

[ 141 T. Nagatsu et 0 1 . . Base station radio equipmentfor 800 MHz band landmobile telephone sy stem,Re v. E . C . L . .NTT. Japan. vol. 25. 1977.

[ I S ] S. Kozono and K. Watanabe, Influence of environmental buildingsonUHF landmobile adiopropagation, fEEE Trans.Commun.. vol.

[ I61 W. Magnus e t a l . ,Formulas and Theorems fo r the Special Functions ofCOM-25, O c t . 1977.

Mathematical Physics. Springer-Verlag. 1 9 6 6 .p. 242.

Mr . Hata is a member of he Institute of Electronics and CommunicatEngineersof Japan

Masaharu Hata was born in Yamaguchi. Japan, onAutumn I, 1950. He receivedhe B.S.E .E. andM.S.E.Edegrees rom Kyushu University. Fukuoka.Japa n, in 1973 and 1975. respectively.

Since 975 he has beenwith theElectoricalCommunication Laboratories, Nippon TelegraphndTelephone Public Corporation ( N I T ) . From 1975 to1978 he w as engaged i n the d evelopment of an 8 0 0 -MHz and mobile elephone ystem.His urrentwork is on the research of digitalmobile adiocommunication systems.

Takayoshi Nagatsu was born in Toyama. Japan.March 12, 1946. He receivedhe B.S. E.E. M.S.E.E. degreesro manazawaniversity.Kanazawa. Japan, i n 1968 and 1970. respectivel

Since 970 heaseenwith theElectoricaCommunication Laboratories. Nippon TelegrapTelephoneublicorporation (NTT). He wconcerned with work on the developm ent of an8MH zband mobile elephonesystem rom1970 o1976.and on radiopropagation of satellitecommunication systems from 1977 to 1978. His curr

Mr. Nagatsu is a memberof the Institute of Electronics and Communicawork is on the design of a mobile telephone system.

Engineers of Japan.

Studies of Base-Station Antenna Height Effects on MobileRadio

WILLIAM c. Y. EE . MEMBER. EEE

Absmcr-As is well known , a ase-station antenna height gain actor of 6dB/octave has been predicted theoretically for signal path l o s s over flatground and has been e r if d by measured data. However, he 6-dB/octaverule for antenna height effect cannot be used to predict signal strength forterrain contours if the terrain is not flat. A model has been d eveloped forwaves propagating over a nonflat ground which allows the antenna heighteffect to be predicted in differen t types of actual terrain contours. In themodel, theactual terrain profile is classified as on e of two different kinds ofgeneral terrain types. The elative received power due to the actual terrainpath contour s predicte d by consider ing the reflection points of the wavesalong the path. Experimental dataave been used to verify he theoreticallyestimated results and they show good agreement.

AI . INTRODUCTION

S IS WELL KNOWN, a base-station antenna height gainfactor of 6 dB/octave (i.e., doub ling he anten na height

increases signal level by 6 dB) has been predicted theoreticallyfo rpat h loss over flatg round [ l ] - [3 ] . Themeasurements[4], 5] inlat suburban nd rban areas have generallyagreed with this fact. It is observed from he measured datacollected in hilly areas,however, the6-dB /octav e rule foranten na height effect c anno t be used to predict signal strength

Manuscript receivedM ay 5 , 197 9; rev ised Octobe r 31 , 1 9 7 9 . Por-t ionsof hispaperwerepresentedat heSympos iumonMicrowaveMobile Communications, Denver, CO, March22, 1 9 7 8 .

Th e auth or was with Bell Laboratories, nc. ,Whippany,NJ .He isno wwith TTDefenseCom munica tions Division, Nutley, NJ 07 11 0.Te l e p h o n e (201) 284-3373.

for terrain conto ur if the terrain is not flat. Since the mo[6]- [8] used toobtain 6 dB/octave for th eantennaheigheffect a t the base stat ion is generally workable for errest

propagations, we have only to check if this model canbapplied t o a mobile radio environm ent.

11. DESCRIPTIONO F AN EXISTING MODEL FOR FLATERRAIN

In this ection an existingmodel [8 ], 191 used for lterrestrial propagation is exam ined for mobile radio rec eptAssume that a base-station transmitter and a mobile receare separated by a large distance d, and he terrain betwethe wo sites is flat as shown n Fig.1. Three possible kiof waves may occu r at he mobile receiver: a direct wavreflected wave, anda surface wave. The esult ant receisignal power [8 ], [9 ] is then

reflected surfacewave wave

where

P, transmitting power into the an tenna,gl gain of the base antenna,

0018-9545/80/0500-0252$00.75 0 980 IEEE


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2.5EE: BASE-STATION ANTENNA HEIGHT EFFECTS

P

RECEIVED POWER

Pr P, 0, a, [&It. I

+ P @ J A S +I a

ASSUME :REFLECTIONOEFF. p - I

PHASEDIFF.4 r h h

A + =/ 3 ( r l - r o l z A a iAd

9 P ( y - ) , 9 ,2

Fig. 1. Anexist ingmodelon lat errain.

height of the base-station antenna,gain of the mobile antenn a,height of th e mobile antenna ,distance betweenhe base station ndhe mobileantenna,wavelength.reflection coefficient of the ground,the erm due to heeff ect of the surface wave [8] ,[9 ] which can be neglected in the mobileadioenvironment,phase difference betweenhe direct wave andhereflected wave (=47~h h2 Ad).

For mobile adio comm unications, he grazing angle$, asshown in Fig. 1 is always small and Ad B h l h a .Hence it isreasonable t o assume that

A 4 1

and

p 2 - 1 . (2 )

Under this assumption,(1) becomes

or, by taking a loga rithm on both sides, we obtain

1 The erm is small n he requency ange above 30 MH z and canbe neglec ted above 300 MHz. I t is becau se he earth is not a per fec tconductor, some energy is t ransmitted into the ground and is absorb edpropor t iona l to the f requency.

Both 6 , and h 2 indicatea6-dB/octaveantenna height gaireceived power increases by6dB as theanten na height doubled.

Comparing the m easureme nt data of mobile ra dio propagtion [4] with (4), we have found hat he erm 61 of (4) ifitted, ut 62 is shown iscrepantly. Okumuras ata [4]show that only a 3-dBgain is observed by doubling the mob

ante nna height in urba n and subu rban areas. Ott and Plitk[ lo] did n ot observe the noticeable 3-dB/o ctave ntennaheight gain at hemob ile. Hence the terrestrial propaga timodel of (4) can only be used to evaluate the base-statioantenna height effect over flat ground.

111. A THEORETICALMODEL FOR NONFLAT TERRAI

Since natural terrain is n ot always flat, a mod el has to used to cover anonflat errainsituation. Hence the existinmodel used in S ection I1 has to be m odified in this section.

Assume thata base-station transm itteranda mobile rceiver are separated by a large distanced . Between the tranmitter and the receiver, a terrain combining level ground wihilly slopes (th e lop e angle 6 is much less than 1 rad) present in our model, as shown in Fig. 2. There are two typof terrain to be considered:

type A terrain: he mobile ante nna is ona slope high

typeBerrain:he mobile ante nna is on a lowerlatthan the base station,

ground level.

The parameters d, h l , and h 2 are already shown in Fig.The parameters D and H shown in Fig. 2 are as follows:

D the length of the flat ground,H the heightdifferencebetween the flat grou ndand he

mobile.

In both ypes A and B, one wave s alwaysreflected on hslope of he hill (Condition 1). Consistent with Snells Lawthe potat which a reflection ofa wave occurs is alwaclose to the top of the hilly slop e where the mobile unittype A or the base-station antennaof ype B is located.Asecond reflected wave: however,ma y or may notexist,de -pending on the length of flat ground and the antenna hei(Condition 2 ) . Here we have to distinguish betweenheanten na height an d the effective anten na height. T he antenheight, h l or h2 show n in Fig. 2 , is the actual heightabovethe local ground. The ffective anten na height h l or h

defined in this paper is the height referenced to the exten dground planewhere the reflection point occurs as shoin Fig. 3. In the past, the effective anten na height was defidifferently by differentuthors [4 ] , [ 9 ] . The effectivanten na height stated in hispaperhas a relation uniqueassociated with he reflectionmechan ism. Consider the flowing two conditions.

Condition 1 : Only one reflection wave can be genera ted.For type A :

d h lD < D I =

h i + h 2 f H(see Appendix).


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254 IEEE T R A N S A C T I O N S O N V E H I C U L A R T E C H N O L O G Y, V O L . V T- 2 9 ,N 0 . 2 , MA Y 1980

THEMOBILEANTENNA

BASE-STATIONA N T E N N A

TH E MOBILEANTENNA

Fig. 2. Two types of nonflat errain.

For t ype B :

dh2D < D 1 =

h l + h 2 + H(see Appendix).

The mobile receiver is assumed to receiver only two waves,one direct wave and on e reflected wave, as shown n Fig. 3fo rbot h ype s. In type A, an effective anten na heighthl (shown n Fig. 3(a)) replaces theantenna height hl at hebase station based on the model show n in Fig.1 :

DHh l s h l +-

d - D

In type B, an effective anten na heighth 2 eplaces the antennaheight h2 at the mob ile unit shown in Fig. 3(b)):

DHh 2 = h 2 +-

d - D

We can apply the path loss formula over a flat ground shownin (3) to the above two types of terrain contour toield

( a ) TYPE At

P, = P, Dh>z --d

d z 01 01

Fig. 3. Condi t ion 1 : one reflected wave can occur.

in which h l an d h 2 are effective an tenna heights.Condition 2 : A s econ d reflected wave can occu r.

For type A :

D > D l = dh1

H + h 1 +h2

For t ype B :

The Appendix explains (10) an,

(1 1

~ 1). Under the conditio1of (10) for ype A or (1 1) for ype B, the mob ile receivemay receive three waves, on e direct wave and wo reflectwaves, as shown in Fig.4 . The received power at the mobiunit can be estimated.

A . Basedon the Existing Model, Describedn Section II

Over a non flat terrain thre e possible waves, on e direct atwo reflected waves received, can be sum med up as follows:

\ 2

- - Ifree-spacetransmissionformula

where th e parametersP, , g l and g2 are shown in Fig. 1 and

p l , p 2 reflection oefficients of hegrounddependonthe angle of incidence when the angleof incidence is very small, p + -1 independent of hepolarization [3]);


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LEE: BASE-STATIONTENNA EFFECTS 255

As (14) ndicates, if a second reflected wave is generated ((10for type A and (1 1) for typeB), the received power is inverselyproportional o d2 . t is th e same as f it is received fromfree-space transmission. Also, the antenna height is outof th epicture, as shown n (14). However, the measured data 4]show that the received signal obtained from he mobile uniis always less thanhatro mhe free-space propagation.

Besides, the effect of changing the base-station antennheighttronglyppears in the measured data. Hence theresult f rom (14) cann ot be applied t o mobile radio propaga

h k ~ f H\

1. - tion over aonflaterrain.\

DI d - D l

\ B. A New Approach\

Since the existingmodel cannot be applied to mobileradiopropagation over non flat erra in, especially when twreflected waves are expected,a new app roac h is describedas follows. Three waves, onedirect and wo reflected, resummed up n a more general form than in (lo), such as

P, = p 0 ao - a l e i A 1 -ct2eJ A 2 I2 (15)/

// r .- whereh l + H

/ 0, d - D l/

/

// TYPE B Po = free-space transmissionormula

IMAGE

A h2Fig. 4 . Condit ion 2: tw oeflection waves canccur. =Prglgz

-4nd)?

(16 )

A , , A? phase differencesbetween the reflected and hedirect paths as the waves propagaterom thetransmitting antenna to thereceiving antenn a.

For d 3 h l .(H + h 2 ) , the grazing angle (angle of incidence)is very small, and he phase differencesA , and A 2 are alsosmall. Reflection oefficients p I and p2 then become -1 ,and the following approxim ations can be made:

sin Ai 2 Ai

Let a a ] , 2 be additional attenuation factors due to landto-mobile ropagation co mp are d o land-to-land free-spacpropagation. is theattenuationofa direct wave. The woreflected waves, one which has a reflection point close tthe mobile contributes mostly to specular reflection with a

attenuation a1 and heother which has its reflection poinaway from the mobile contributes mostly to diffuse scatering with its attenuation a2 [ l l ] . Since the diffuse scattering has little directivity, th e energy received by the diffusscattering is muc h smaller than hat by the specular reflection, we may assume tha t

where i = 1 , 2. Substituting these approximations into(1 2), Also from the assumption of (2),a % a ] . hen (15) becomeswe obtain

I 1 - e iA212 = 32 c o s ( A , - A , )

- 2 (COS A1 + CO S A,)2 1 + 2AIA2.

Substituting (13) into (12), we obtain

(13) where h l an d h 2 are the effective antenna heights whichcan be related t o the anten na heightsh l an d h2 as illustratedin (5) or ( 6 ) . Equations (18) and ( 9 ) are identical. It meansthat, if there are two reflected waves, only on e reflectedwave whose reflection poin t is closer to th e mobile contr ibutemost ignificantly.Theangential plane for this reflectionpoint near the mobile is used to e stimate the effective antenn

(14) height as shown in Fig.5 for each of two terrain type s, typeAand type B.


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25 6 IEEERANSACTIONS ON VEHICULARECHNOLOGY,OL . V T- 2 9 , N0.2, MAY 1980

F O R TYPE A

Fig. 6. Effect on antenna height gain factor.

Fig. 5 . Estimate of effective antenna heights.change of antennaheight is

IV. PREDICTION O F ANTENNA HEIGHT EFF ECT FROMTHE THEORETICAL MODEL

The signal change due to ant enn a height change does notdepend upon the actual antenna height above the local groundlevel though heactualante nna height is easy to visualize.Rather, tdependson he effective antenna height deter-minedby the terrain contour between the mobile and hebase. There is no linearelationship etween the ctua lanten na height a nd he effective antenna height. Doublingth eactua l base-station antenna height, therefore, does notnecessarily change the signal levels by6 dB. Four examplesshown in Fig. 6 (assume th at the slope angle0 is small, sayless than 10") will demon strate this fact. Th e effective antennaheights o f two antennas are first obtained based on the loca-tion of thebase station as shown in Fig. 6.

I ) Type A Terrain, D = 0: The power increased due to th echange of anten na height is

where h l " is the increased effective antenna height at hebase.

2 ) Typ e A Terrain, D # 0: The power increased due to thechange of antennaheight is

3) Type B Terrain, D = 0: The power increased du e to the

4 ) Type B Terrain, D # 0: The power increased due to thchange of antenna height is

Fromhe above examples we have dem onstra tedhatdoubling the base-station an tenna height may not necessariincrease t he signal level by 6 dB, sometimes mo re sometimless, dependen t upon the effective antenna height at the givsite. Hence the 6-dB/octave antenna height rule cannot bapplied in the hilly area.

V. COMPARISON OF THEO RYWITH EXPERIMENTALRESULTS

Data at Whippany,NJ , Area

Since Caples [SI made theuburban (Whippany areameasurements with base antenna elevations at60,80,and100 ft above ground and frequency a t 8 21 MHz, we comparthe results pred icted fro m the model with Caples' unpublishemeasured data. Though it is very hard to find aparticularconto ur of the terrain to ma tch all of our models, we founsome contours of type A. The six pointsmarked inalpha-betical orderon Fig. 7 are the sites used fo r omparison.For demonstration, Fig. 8 shows a contour in the Whippanarea whichhas met a condition of having three waves, i.e.D > d h l / ( H + h l + h2). The signal received by the mobil


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LEE: BASE-STATION A NTE NNA H E I G H T EFFECTS 25 7

400 '

300'

8s-

2200'

100'

0'

I

Fig. 7 . Sites used in the Whippany, NJ , area.

I I I I I I

12000' 10000' 80001 6 0 0 0 ' 40001 2000' 0

+ DISTANCE

Fig. 8 . A terrain contour of site C in Whippany area.


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258 I E E E TRANSACTIONS ON V E H I C U L A R T E C H N O L O G Y, V O L . V T - 2 9 ,N 0 . 2 , MA Y 19

A PAM kEKj*(T(hl+M PREDICTED MEASURED

A P

ANT HEIGHThi RESULTS Ld0 1 RESULTS (dB)

F i g . 9. A list of comparisons betw een predicted nd measured e-sults (Whippany area).

unit from eachbase-station anten na height h l is based onits effective heighth l ' n which for

Hence, by applying (16), we may calculate the relative gainsreceived from he base-station ant enna with threedifferentantenna heights andcomp are those with he measured data

as follows:pr(h1 = 80 ft ) Pr(h1' = 360 ft )

A P = -

pr (h l = 60 ft)- Pr(hl ' = 340 ft )

I 1.06- .5 dB (predicted)1.5 dB (measured)

Pr(hl = 100 ft ) Pr(h l ' = 380 ft )A P =

-Pr(h 1 = 60 ft) - r (h l ' = 340 ft)

= 1.1 1- 1 dB (predicted)1.7 dB (measured).

Fig. 9 is a list of comparisonbetween predictedand meas-uredesults fo r six locationsn the Whippany area. Thepredicted results agree well wit h the measured ones.

Data in the Camden-Philadelphia Area

Kelly [12] has done somemeasurements n the Camden-Philadelphia area. Fig. 10 indicates the sites n tha t area nwhich t he measured data were collected. The data used here

, 2 M I L E S ,F i g . 10. Sitesused inCamden-Philadelphia area.

I I I I I 1 I I 1 1

I 2 3 4 3 6 7 8 9 Y l L l

Fig. 1 1 . Measured data from Camden-Philadelphia area.


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L E E : BASE-STATION ANTE NNA HEIGHT EFFECTS 25 9

8 -

eIn

7 -

WK 6 -

v); -4 -

3 -

0 WHIPPANYREA

X CAMDEN.HILADELPHIA

I I I I I I

1 2 3 4 5 6 7 8 9 1 0 1 1

PREDICTEDN dB

Fig. 12 . Indication of errors inprediction.

were those ecorded rom wo base-station anten nasatdif-ferent heights, 136 ft and 23 4 ft above sea level, respectively.To compare he data with he prediction, we picked data nthedirectionof 50N and 60"N, as shown in Fig. 11.Theterrain profile in these two directions fits our model as can beseen in Fig. 1 1.

We picked hreedatapoints A, B,C, in thedirectionof50"N and four points D, E, F, G, in the direction of 60"N.The ratio of two receiving powers obtained from two differentantennaheights, AP in decibels, foreach ocation is listedin Fig. 11. The differences ofA P between the predicted and

meas uremen t results re mall. The reason for using onlytw o slopes to repre sent those points is tha t the general terrainis flat as one slope angle shows in the figure. The reader shouldnot be confusedwith he drawingwhich uses twodifferentscales on x and y axes, respectively.

VI. ERRORS I N PREDICTION

Since there aredifferences betwee n hepredicted valuesand the measured ones, we have to kno w the deviation of thedifferences. First, we plot hepointswithpredicted valuesa t he x axis and he measured values at he y axis, shownin Fig. 12 . Th e 45' line is the line ofpredictionwithouterror. The dotted points are from the Whippany area and the

cross points are fromhe Camden-Philadelphia rea. Mostof hemare close to he line ofpredictionwithouterror.The mean value of all the data is right on the lineof predic-tionwithouterror.The variation of hepredicted value is0.8 dB from the measured one.

VII. CONCLUSIONS AND SUMMARY

We summarizeom eheoretical resultsn Se cti on 111.The model we use here can explain a lot of phenom ena sinceth eground is not always flat.Froma hilly area we have

specified two types of terrain profile t ha t the received wavewould be affected by the errain. From he ypes of terrainwe have found the following.

1) The relative received power canonly be predictedbyusing the effective base-station antenna height which is measured from the plane on wh ch theeflection point occurs.

2) Only o ne direct wave and one reflec ted wave (if two ,

th eone closest to hemob ile is used) areused to predictthe relative received power.3) The experim ental data shown here agree well with the

prediction.4 ) Doubling theantenna heightmay not necessarily in-

crease the signal level by6dB over nonflatground,some-times m ore ometimes less, dependent ponhe effectiveantenna height.

APPENDIX

CONDITION OF HAVING A REFLECTED WAVE FROMA FLAT GROUND

According to Snell's Law, the ncide nt angle equals thereflected angle. At a siteof reflection D l (see Fig. 2)

h1 h , + H- fo rype AD~ 0 - 0 ;

or

an d

or

Therefore when D > D l , there always exists a reflected wave.

ACKNOWLEDGMENT

Theautho r would like to han k Bell Laborato ries, Inc.,for etting him publish his paper. The stimulation from thispaper had led theautho r o urther develop a new mobileradio propagation model before he leftBell Lab oratories.

REFERENCESP. David nd J . Voge. Propagation of Waves. Oxford,England:Pergamon, 1969, p. 59 .Propagation of the National Defense Research Committee. Radio WavePropagation. New York: Academic, 1949, p. 386.W . C . Jakes, J r. , ef a l . . MicrowaveMobileCommunication. NewYork: Wiley. 1974. p. 8 3 .


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base station height

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