A comparison between theoretical propagation models andmeasurement data to distinguish urban, suburban and open areas in

João Pessoa, Brazil

R. M. S. Cruz1, F. C. da Costa1, P. F. Braga1, G. Fontgalland2, M. A. Barbosa de Melo2, R. R. M. Do Valle2

1Universidade Federal do Rio Grande do Norte, Natal – RN, 2Universidade Federal de Campina Grande,Campina Grande – PB, Brazil

Abstract ⎯ The theoretical propagation models founded inliterature are compared with measured data with the goal ofcharacterizing areas in João Pessoa, Brazil. The complexbehavior of the fading and multipath phenomena in mobilecommunication systems are generally simplified bymathematical models. The restrictions and simplifications overthe probabilistic behavior of the signal impose limitations to theprediction models. In order to increase their analysis limitations,an accurate measurement procedure is done to detail theenvironment characteristics. This paper presents theaccomplishment of measurement data using AgilentTechnologies’ E7474A Drive Test System in three specific areasof Joao Pessoa city (Brazil) and vicinity. The measured resultscompared with theoretical ones obtained from predictionmodels show us the best way to classify the above regions asurban, suburban or open areas.

Key words ⎯ Drive test, fading, measurement, multipath,prediction models, propagation, signal behavior.

I. INTRODUCTION

Several propagation models have been developed over thelast decades for mobile communications network planning. Itis clear that an essential knowledge of the propagationmodels is necessary to provide an efficient coverage design.First, to know the model limitations and second, how close toa real environment it can predict [1]. The received signalfluctuates due to various factors such as: obstruction fromobjects in the radio path, when the mobile moves into aspecific area, and multipath propagation produced byreflection, refraction and diffraction, which can be combinedconstructively or destructively.

These phenomena are often express as a ratio signal-to-noise (S/N) at the receiver and are known as signal fading. Aconsequence of this signal level below the receiver sensitivityis to render the communication impossible or interrupted [2].

In order to minimize the multiple fluctuations of the signal,most of the models are based on three signal behaviorsdescribed in this section and shown in Fig. 1. The first ray(R1) leaves from the base station but suffers reflections beforearriving at the mobile: this case occurs principal due to theproximity to the buildings, trees and high constructions. Thesecond one (R2) is directly incident in the mobile, leavingfrom the base station, without suffering reflections or

diffractions, which commonly occur in open or suburbanareas.

The last ray (R3) represents the cases where the signalsuffers at least one diffraction, which generally occur in areaswhere there is a larger density of high buildings. Fig. 1 alsoshows the displacement of the mobile in a radio environmentcharacterized by the presence of houses, buildings and otherobstacles.

Fig. 1. Graphical representation of the three-ray model.

Nowadays, the communication systems engineer has at hisfingertips many models to predict the behavior of radiocommunication systems in the presence of fading. However,the questions that arise are unavoidable: Which model shouldbe used? How can us classify a case as urban, suburban oropen area with the help of propagation models? Up to thepresent, many researchers have used the Rayleigh distributionassuming that there is no worse case than that. Thisassumption has been proved to be wrong since there are caseswhere the Rayleigh predictions are too ideal [2].

Among the various classical prediction models welldescribed in literature, we selected Friis equation, theOkumura-Hata, JRC, Diffraction Screens and London modelsto analyze the signal behavior in the mobile radioenvironments chose in our study. Friis Equation was usedonly as a reference in order to observe the experimental databehavior.

This paper presents a perform of practical measurementsand the collection of experimental data using the AgilentTechnologies’ E7474A Drive Test System in order to analyzethe multipath signal phenomenon in Joao Pessoa city andvicinity. The results will permit us to identify these regions inurban, suburban or open areas. Besides, they will also provethe prediction models efficiency and validity.

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II. PREDICTION MODELS

A. The Okumura-Hata Model

The Okumura et al. model is a fully empirical methoddeveloped from a series of measurements made in Japan atseveral frequencies representative of mobile communicationssystems. Curves were fitted to measured values as a functionof a number of basic propagation parameters such as the typeof environment, the terrain irregularity and the antennaheights [3]. The following elements are taken into account:terrain features and environment types (urban, suburban andopen areas) which are well described in [2].

Hata [4] made use of [3] model theory and equations forbasic propagation in order to develop computerized coveragecalculation applications. He also gives expressions for themore commonly used correction factors to obtain a series ofexpressions to calculate the basic propagation loss, Lb, (lossbetween isotropic antennas) for urban, suburban and openareas. The Okumura-Hata model expression for the basicpropagation loss is the following:

Lb (dB) = 69.55 + 26.16 log(f) – 13.82 log(ht) – a(hm)+ (44.9 – 6.55 log(ht)) log(d) (1)

Where f is in megahertz, the antennas heights (ht and hm) arein meters, and the distance between the transmitter andreceiver (d) is in kilometers. These losses correspond to aflat urban area. For a mobile antenna height (hm) of 1.5m, themodel consider a (hm) = 0 [1].

In suburban areas, Okumura-Hata model gives the path lossLb – Lps, [8] where

4.528

log2 2 −⎥⎦

⎤⎢⎣

⎡−=

fLps

(2)

and in open areas as Lb – Lpo, where

94.40)log(33.18)(log78.4 2 −+−= ffLpo (3)

B. Modified JRC Model

The modified JRC (UK’s Joint Radio Committee of PowerIndustries) [9, 10] model uses a different approach to that ofthe Okumura-Hata model for dealing with terrain irregularityeffects. In this model the detailed terrain profiles are used andin order to compute the losses due to the terrain obstacles,diffraction models are applied. As regards the determinationof the losses due to elements in the neighborhood of themobile (that cannot be read on a terrain database), the methodprovides expressions for the excess losses in urban areas. Forthis model, a reference loss is defined [1]:

Lfs (dB) = 32.4 + 20 log f (MHz) + 20 log d (km) (4)

C. Friis Free-Space Equation

The received power strength can also be observed andcalculated using Friis Free-Space Equation, as follows:

2)/4( λπ r

GGPP tr

tr = (5)

Where Pr is the power passed from the lossless antenna to thereceiver, Pt is the transmitted power and Gt and Gr are thegains of the transmitter and receiver antennas, respectively.For isotropic antennas Gt = Gr = 1 [5].

E. The Diffracting Screens Model

Walfisch and Bertoni [8, 9] modeled the rows of the citybuildings as a series of absorbing diffracting screens with afinal diffraction down to street level gave an overallpropagation model for the case of an elevated fixed antennaabove the building roofline to a location at street level.Maciel, Bertoni and Xia [10] proposed an approach similar tothat of Walfisch and Bertoni, and applied a diffractionfunction to the fixed-site antenna for the case of the fixed-siteantenna below the average rooftop level. The resultingexpression for path propagation Lds based on the models ofMaciel, Bertoni, Xia and Walfisch [10, 8, 9], is written interms of the free space loss F and excess losses Le2 due todiffraction along the rooftops, and a final diffraction Le1

down below rooftop level. The average signal is

⎥⎦

⎤⎢⎣

⎡ +−−−−=

Hb

dHbLLFL eeds 17

17log18

2

21(6)

The expressions for F, Le1 and Le2 are found in [5].

F. Ibrahim and Parsons Method – The London Model

Ibrahim and Parsons [11,12] extracted the empiricalbehaviors from measured data of propagation with regard tosuch factors as land-usage factor, degree of urbanization andvarying terrain height for the mobile. The expression makeuse of the same parameters that are available on land-usemaps of London, England, and the method can be applied toother cities for which similar land-use maps exist. The “bestfit” model based on measurements in London is

⎥⎥⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢⎢⎢

⎣

⎡

−+−+

⎥⎦

⎤⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡ +++⎥⎦

⎤⎢⎣

⎡ +−

++−−

−=

5.5087.037.0265.0)log(

...156

100log5.1440

156

100log86

...40

log2640

)log(8)7.0log(20

UHLd

ff

ffHH

L

mb

ip

(7)

where Lip is Ibrahim and Parsons propagation, in dB; L is theland-use factor, percentage of grid covered by buildings; H isthe height difference between grid containing the fixed siteand grid containing the mobile, in meters; U is theurbanization factor, percentage of buildings in grid taller thanthree levels;d is the range, in kilometers (not beyond radio horizon) [5].

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III. THE MEASUREMENT SET UP

Agilent Technologies’ E7474A Drive Test System was usedin the measurement procedure to obtain RF coverage and theservice performance for wireless communications networks.This device provided, efficiently, the systems evaluationoperating in TDMA frequency band (890 to 900MHz) of alocal operator. The software of measurement, based onWindows platform, automatically collected and stored thedata of the receiver in a PC laptop, while the covering area isrouted. The distances were registered with the aid of a GPSsystem incorporated to the drive test [13]. The displacementin the prediction places were made using a vehicle in whichwas installed the Agilent Technologies drive test whosereceiving antenna was connected externally. The trajectorywas specified starting 6 meters from the bottom of the localradio station installation up to 4200 meters away. Thesoftware provided with the drive test was programmable totake the samples of field at each 5 seconds. The parameters ofdistance, the geographic coordinates of each point and themobile received power were recorded in the PC laptop for thegraphical representation.

IV. RESULTS

The classical models were compared with the measurementresults for three base stations used in our study: Jacumã city,Bancários and Bessa quarters.

A. Jacumã area site

The first measurement took place in Jacumã city, situatedin the south coast of Paraíba (Brazil). In Jacumã base station,a unidirectional antenna is installed in a tower of 60m.Leaving the base of the tower in a random direction, up to4km of distance, the experimental data were recorded in thePC laptop [13]. Jacumã is characterized for having a littlerough relief, the presence of creeping vegetation and fewhouses. Thus, according to Okumura-Hata model [3,4], it canbe considered as a quasi-open area in relation to the othersites, in Joao Pessoa city.

As suggested by Okumura-Hata model [3,4], we can useFriis model [5] to describe the power received level. It shouldbe pointed out here that in Friis equation [5] the distancefrom the source (antenna transmitter) was taken along thevirtual line from the source up to 4Km away (hypotenuse of atriangle), and the measurement were made from a line placedat the bottom of the antenna up to 4Km away (adjacentcateto). It explains the great difference between the results inthe first 1000 meters, as shown in Fig. 2. JRC model [1]details the terrain profile in order to compute the losses due tothe terrain obstacles and diffraction. Then, the Okumura-Hata[3,4] and JRC [1] models present a good approximation to thepropagation path-loss, as intended (Fig. 3). Analyzing theLondon model [5] and the Diffracting Screens method [5], wecan conclude that they are not appropriated to predict thepath-loss in Jacumã city (Fig. 3). It is clearly seen becauseboth methods are applicable to big metropolitan cities. Theytakes into account the building heights, the separationbetween rows of buildings, the distance from mobile to

building on the street, a land-use factor, the height differencebetween grid containing the fixed antenna, the gridcontaining the mobile and the urbanization factor. Theseparameters are not significant for a quasi-open area asJacumã city.

Fig. 2. Jacumã site – Received Power – experimental data and Friis.

Fig. 3. Jacumã – Propagation Path Loss.

B. Bancários quarter site

The second measurement was carried through in Bancáriosquarter, whose base station has 40m of height [13]. Thisquarter is characterized as a residential set, of little roughrelief and constructions of medium heights (no higher than15m). Leaving from the bottom of the tower in a rectilineartrajectory, we could observed that there were no significantobstacles in the way of the main lobe of the antenna.

The differences between the two attenuation curves(measured data and Friis equation [5]) occur due the fact that(5) is applied to flat terrains and the site is characterized forbeing a residential set. The signal suffers reflections, notconsidered in (5), that present significant contribution to thereceived signal (Fig. 4). Although there are variations in thereceived power values, the curves behavior stands the same.It is assumed here a constant presence of fading in the signal.The fluctuation in the signal level can then be predicted in anarrow band.

Once again, all five models are shown in Fig. 5 and therespective propagation losses used in the study. It is observedthrough these data that the Okumura-Hata [3,4] has a goodmodeling closed to the tower, but diverges after few metersaway. The results for the Diffracting Screens model [5] areclose to Friis equation [5] and both are not in agreement with

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the measured data. It can be easily understood since theDiffracting Screens method is modeled according to theresulting expression for path propagation Lds based on themodels of Maciel, Bertoni, Xia and Walfisch [10,8,9], writtenin terms of the free space loss F [5].

Besides, it follows the rows of the buildings as a series ofabsorbing diffracting screens of uniform height. Then, forthis site, we can use the QE factor [5], since the fixed-siteantenna is above the rooftop levels of the buildings. Aswritten before, this site is characterized for being a residentialset, with small height buildings not taller than three or fourlevels. The use of these two models should take into accounta compensation factor to prove the validity of this method ina region like Bancários.

The JRC model [1] presents the better approximation to theenvironment in question. It can be accepted since JRC modelis more accurate than the Okumura-Hata model because ittakes into account a detailed study about the terrain profilesand applies diffraction models to compute the losses due tothe terrain obstacles.

Because this site didn’t present good results to be classifiedas an urban area according to Okumura-Hata [3,4], Londonand Diffracting Screens models [5], and even because it’s atipical residential set, without higher edifications, it can beclassified as a suburban area of João Pessoa city.

Fig. 4. Bancários site – Received Power – experimental data and Friis.

Fig. 5. Bancários – Propagation Loss.

C. Bessa quarter site

The third measurements took place in Bessa quarter. Thebase station is surrounded by buildings, many of them, taller

than the tower. However, they are not high neither numerousenough to classify Bessa as an urban area. Besides that, thereare some sectors with few houses, like as an open area. Then,this quarter was also considered as a suburban area of JoãoPessoa city. In Fig. 6, the measured data are compared withFriis space loss equation [5]. In almost all of the pointsmeasured, the buildings hid the antenna transmitter. In thiscase, the direct ray between the base station and the receiverdoes not exist. Then, we can consider the received signal as asignal completely spread and composed of only multipathcomponents.

In Fig. 7 we have the propagation path loss according tothe five models used in this study. Although there are manybuildings in Bessa quarter (some of them higher than the basestation tower), this amount is still a minimum if comparedwith the several constructions of London, for example. Thus,the London model [5] is not coherently applied to this regionand Bessa quarter cannot be considered as an urban area, likeLondon city. On the other hand, the Diffracting Screensmodel [5] had a better approximation than that of Okumura-Hata model [3,4] to the measured data. We can admit thisbecause the base station used as reference in themeasurements is surrounded of buildings, many of them,taller than the proper tower. It possible due the calculationswith the factor QL [5] used to fixed-site antenna in which it isbelow the rooftop levels of the buildings. Once again, theJRC model showed the best approximation.

Fig. 6. Bessa – Received Power. – experimental data and Friis

Fig. 8. Bessa – Propagation Loss

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V. CONCLUSIONS

The results obtained by measured data using AgilentTechnologies’ E7474A Drive Test System shows that theprocedure adopted can be expanded to others sites to classifythem as urban or suburban areas. For few houses andbuildings the Friis equation can be used to predict the powerlevel and the loss from a base station with goodapproximation. JRC model had the best approximations inrelation to measured data. The other models didn’t have agood approximation because their characteristics are notapplied to the regions in study (Jacumã, Bancários e Bessasites). The objective of using these models was, therefore, toobserve their behavior in environments different from thosein which they are appropriately applied and give us morebases to identify the kind of each region.

When the number of buildings and houses begins toincrease, the model should consider the multiple reflectionsand diffractions phenomena. Besides, a more detailed studyof the terrain features should be done, in order to betterclassify the regions in urban, suburban or open areas. Thisplays an important hole in the choice of the prediction model.

The trends of this work are to cover all area of João Pessoacity. In order to better represent the regions studied, a fitcriterion may be done and used in the theoretical models.

ACKNOWLEDGMENTS

The authors express their gratefulness to CEFET - PB fortheir support and to the Vitae Foundation for the yieldedequipment.

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