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Radio-frequency behavior in the GSM band at Porto

J. P. CarmoPolytechnic Institute of Bragana

Campus de Santa Apolnia

5301-854 Bragana, Portugal

jcarmo@ieee.org

Abstract This paper compares the behavior of a mobile link

in an urban environment with the predicted values, obtained

by three theoretical models. The measurements were taken

during a test-drive made in the main business and commercial

zone of Porto, named Centro. The measured data at 900 MHz

and 1800 MHz were compared with model predictions.

Globally, all of the three models agree very well with

propagation conditions of the environment.

I. INTRODUCTIONIn the last decade, the mobile communications had a

significant social and technological growing. This leadsoperators to offer services with a excellent quality of service(QoS), and a consequently traffic increasing due to userssolicitation. This result in system saturation, if nothing ismade to deal with it. Consequently we assist to a degradationof QoS. To maintain normal QoS, the operators are in thenecessity to increase theirs capacity. This increasing isachieved by two ways: i) increasing the service area, and ii)increasing the capacity, maintaining fixed the service area.For both types of expansion, the propagation characteristics

are aspects of specialised studies. Due to dynamic behaviourof the radio channel and users mobility, difficultiesconcerned with channel characterisation arises. Phenomenon,whose analysis little time ago, had a purely academic value,became in the concern of operators. In this paper, it ispresented the results of the work, after using the three wellknown prediction models in the literature, theirsconfrontation of measured with predicted data, andconclusions.

II. MEASUREMENT PROCEDURESThe measurements were made in collaboration with a

portuguese mobile operator. It was used a mobilestation (MS), a GPS receiver coupled to a laptop runningTEMS from ERICSON. This last one integrates all thecomponents in the setup. The complete test-drive pathillustrated in the Figure 1 was reconstructed in MATLABenvironment from the GPSs coordinates. Each collectedsample is a temporal mean, which was obtained in a6 through 7 seconds window. The MS operating at thefrequency of 900 MHz and moving with speeds of 60 km/h,

travels 300 through 350 wavelengths [m], per each second.

In order to keep the power above the -85 dBm, the MS madesystematically handovers. This can be observed in thecollected data from the complete test-drive shown inFigure 2. The changes on traces reflects the instants when thehandovers were made. According to the operator, the-85 dBm is the minimum power that assures a call with anacceptable subjective quality.

Figure 1. Complete test-drive path.

III. THEORYIn general terms, the following equations applies to is to

denote the path loss, due to its simplicity and fastimplementation:

PL(d)=A+Blog10(d) [dB] (1)

The power at MS is

pR(d)=pTd-ngn(,)gmax [W] (2)

where the following quantities are:

- d: distance [m] from fixed station (FS);- pR: power [W] at MS;- pT: power [W] injected at the feeder of the FS antenna;

- : feeder-loss at FS (transmission lines, wave guides);- n: propagation factor (normally > 2);

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- gn(,): FS antenna normalised gain, considering the

azimute and elevation ;- gmax: maximum gain of FS antenna.

Figure 2. Power of the signal [dBm] at MS.

From equation (2), the ratiopR/pT in dB is

PL(d)=10log10(pR/pT)=PR-PT (3)

resulting in

PL(d)=10log10[pTgn(,)gmax]-10nlog10(d) (4)

Path loss given by equation (1), is a median value notexceeding 50% of time or 50% of locations under study [5].One additional termX0 due to Rayleigh fast-fading [4,6] canbe considered. X0 is the standard deviation with values

around 4dB [6]. The calculation ofX0 is out of scope of thispaper. The frequency dependence, given by Clog10(fMHZ),where fMHZ is the frequency in MHz is also important. Thefinal expression for path loss is

PL(d)=A+Blog10(d)+Clog10(fMHZ)+X0 +G (5)

Resulting forA and B parameters, respectively,

A=10log10[pTgn(,)gmax] and B=-n/10 (6)

Effective gain G [3] due to MS antenna height is

G=20log10(hre/href) (7)

where

- hre: MS antenna height [m];- href: MS antenna reference height [m].

Once defined the general form of path loss model, let'slist the three models used to characterised the propagationbehaviour ofCentro:

1) Linear regressionH(d)=Areg+Breglog10(d) [7,8];

2) Hata-COST-231 model [9-12];

3) Okumura curves based model [5,13].

1) The first model is based on regression lines (RL) usingminimum square criterion, obtaining Areg and Bregparameters, resulting in several logarithmic linesPL(d)=Areg+Breglog10(d) that reasonably describes the

propagation behaviour of the zone under study. Thesemodels are very easiest to compute and integrated incommercial informatic tools for prevision.

2) Hata-COST 231 model is well described in theliterature, not being described on this paper.

3) Okumura based model was obtained from curves inFigure 41(c) and Figure 41(d) in [13], to 900 MHz and1500 MHz, respectively. The value of 1500 MHz was themost close to the 1800 MHz found on that Figure. The goalwas to get the general expression

PL=A(fMHZ,hte)+B(fMHZ,hte)log10(dkm)+G(hre) (8)

where G(hre) is the gain defined on equation (7). In ourstudy the MS antenna height was hre=1.6 m, and likerecommended on [13], href=1.5 m. Equation (8) wasinterpolated from the two following equations

PL900 MHZ=A1(hte)+B1(hte)log10(dkm) (9)

PL1500 MHZ=A2(hte)+Br2(hte)log10(dkm)r (10)

The functions A1, A2, B1 and B2 were obtained fromFigure 41 in [13] taking in account that in Porto the antennaheights to every FS not exceed 70 m and distances of interestto the study not exceed 20 km. These functions wereobtained from Tables 1 and 2, to 900 MHz and 1500 MHz.A(fMHZ,hte) and B(fMHZ,hte) were obtained after interpolating

the two pairs of functions [A1(hte), B1(hte)] and[A2(hte),B2(hte)], respectively, giving

A(fMHZ,hte)=-116.6-0.0117fMHZ+0.1hter (11)

teMHz

MHzteMHz

hf

fhfB

+

+

+=

600

9000025.00625.0

600

900441.0525.37),( L

(12)

Table 1: Values from Figure 41(c) in [13] for 900 MHz.

hte (m) dkm (km) Field strength

(dBV/m)Path loss

(dB)

1 74 -124

30 10 38 -16020 28 -170

1 76 -122

50 10 42 -157

20 32 -167

1 79 -120

70 10 46 -153

20 36 -163

Path loss values on Tables 1 and 2 were obtained fromequation (4) in Hata [9].

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Table 2: Values from Figure 41(d) in [13] for 1500 MHz.

hte (m) dkm (km) Field strength

(dBV/m)Path loss

(dB)

1 71 -131

30 10 35 -16820 25 -178

1 74 -129

50 10 39 -164

20 29 -174

1 76 -127

70 10 42 -161

20 32 -171

IV. RESULTSResult comparison was made taking in account the power

measured at MS and the power predicted by three models aswell as the MS position in relation to FS in the zone understudy. As shown in Figure 1, this rectangular area understudy is delimited by

- left corner at (410950 N,080647 W);

- right corner at (410834N,080615 W).The perfect knowledge of radiation pattern of the

antennas at fixed stations was a mandatory requirement tothe best accuracy of the precisions. The three FS were:

1) FS 2828 at (4109 39 N,080636 W), with oneomni-direcional antenna, down-tilted mechanically 3 to theNorth, located near "Hospital Santa Maria";

2) FS H271 at (410926N,080632 W), with threesymmetric sectors, located in "Faria Guimares";

3) FS H2026 at (410845N,083626W), withthree sector symmetric sectors, located in "Batalha".

Table 3: RL obtained for the path loss.

FS Regression Lines (RL)

2828 PL(d)=-34.92-34.06log10(d)

H271 PL(d)=33.00-58.27log10(d)

H2026 PL(d)=389.97-194.1log10(d)

Three FS PL(d)=-33.41-29.09log10(d)

At Table 3, all RL were obtained from regression ofpower at MS during test-drive. Path loss and power previsionwere obtained for each position i of MS on the test-drive [5]by following formulas in accordance with Figures 3 and 4,respectively,

PLobserved(di)=Pobserved-EIRPjk-Gn(jk,jk) (13)

Ppredicted(di)=EIRPjk+Gn(jk,jk)+PLpredicted(di) (14)

where

- PLobserved(di): real path loss [dB];- PLpredicted(di): predicted path loss [dB];- di: distance between antennas with MS at point i [m];

- Pobserved(di): power of the received signal [dBm];- Ppredicted(di): predicted value for power of the received

signal [dBm];

- EIRPjk: powerEIRP[dBm] radiated by antenna kfrom

FSj - already includes maximum gaingmax;- Gn(jk,jk): normalised radiation pattern of antenna k

on FSj;

- jkand jk: vertical and horizontal angles fromantenna kin FSj with respect to MS.

FS j

Antenna k

)(dP iobserved

id

),(GEIRPPjkjknjktransmited +=

jk

MS at position ijk

transmitediobservediobservedP)(dP)(dPL =

[ ] jktransmited EIRPPmax =

Figure 3. Methodology used on observed path loss.

FS j

Antena k

)(dPLP)(dP ipredictedtransmitedipredicted +=

id jk

MS at position ijk

)(dPL ipredicted

),(GEIRPPjkjknjktransmited +=

[ ]jktransmited

EIRPPmax =

Figure 4. Methodology used on power prevision.

FS B

1d 2d 3d4d5d 6d

AB

C

B

A

C

1d

2d 3d

4d

5d

6d

FS A FS C

Figure 5. Method used to combining all FS.

Table 3 shows one RL for each of the three FS and onefourth RL taking in account all three FS. In the analysis, itwas taken in account the combination of all FS. That

combination was made by sorting increasingly the distances

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from FS, no matter what FS refers the pair measure-estimation, as illustrated in Figure 5. The Figure 6 comparesthe previsions maiden at test-drive path inside the area ofinterest (Centro), in conjunction with power measured on

those points. A note must be done in order to clarify thateach predicted value on the graph is the biggest of theprevisions from three FS. Table 4 was constructed taking inaccount that for each position i of the MS in the test-drive

had one absolute error i given by

i=|Ppredicted(i)-Pobserved(i)| (15)

where

- Pobserved(i): power of the signal received [dBm];

- Ppredicted(i): prevision to the power of the receivedsignal [dBm].

The mean absolute error [dBm] and the standarddeviation [dB] are respectively,

N

N

i

i== 1

( )2

1

1

==

N

N

i

i (16)

where N are the total samples acquired on Centro (not allacquired during the complete test-drive).

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Traveled distance inside Baixa (km)

MS

receivedandobservedp

ower(dBm)

Okumura based predicted power

RL predicted power

Hata-COST231 predicted power

MS observed power

-100

-90

-80

-70

-60

-50

-40

-30

-20

Figure 6. Predicted power versus observed power at test-drive path inside

Centro.

V. CONLUSIONSIs expected that first three RL at Table 3 are in

accordance with local propagation conditions near its FS,due to. the accordance of the fourth RL with propagationconditions of the overall Centro. That conclusion is obtainedif we look the low values of the absolute error and a standarddeviation, of 0.16 dBm and 1.56 dB, respectively. Thepropagation factor n=2.91 is a good confirmation of the fact.

As expected in RL model, the Okumura and Hata-COST231 models also agree very well with local conditionsof propagation in terms of attenuation.

We obtained low values of absolute errors and standarddeviations, of 0.15 dBm and 0.14 dBm, respectively, toOkumura based model; 1.42 dB and 1.40 dBm, respectively,to Hata-COST231 model.

To conclude all the three models are very suited topredict the behavior of the path loss at Porto's Centro.

Our results were confirmed after applying our parametersat the prediction tools of a portuguese mobile operator.

Table 4: Mean absolute error and standard deviation.

FS Model [dBm] [dB]RL 17.69 0.99

2828 Hata 24.69 1.16

Okumura 21.68 1.04

RL 10.24 0.89

H271 Hata 9.48 0.84

Okumura 9.44 0.75

RL 7.56 0.72

H2026 Hata 17.62 1.12

Okumura 16.80 1.11

RL 0.16 1.56

Three FS Hata 0.14 1.40

Okumura 0.15 1.42

A - REFERENCES

[1] V. Garg, J. Wilkes, Principles and applications of GSM, PrenticeHall, 1999.

[2] R. Macario, Cellular radio: principles and design, Second edition,Macmillan press, 1997.

[3] W. Lee, Mobile Communications Engineering - Theory andapplications, Second edition, McGraw-Hill, 1998.

[4] T. Rappaport, Wireless communications priciples and practice,Prentice Hall, 1996.

[5] J. Parsons, "The mobile radio propagation channel", Penteh Press,1992.

[6] V. Garg, IS-95 CDMA and cdma2000 Cellular/PCS systemsimplementation, Prentice Hall, 2000.

[7] H. Xia, H. Bertoni, L. Maciel, A. Lindsay-Stewart, R. Rowe, Radiopropagation characteristics for line-of-sight microcellular andpersonal communications, IEEE Trans. Anten. and Prop., Vol. 41,No. 10, pp. 1439-1447, Oct. 1993.

[8] J. Hernando, Comments on A simplified analytical model forpredicting path loss in urban and suburban environments, IEEETrans. Veh. Technol., Vol. 48, No. 5, page 1740, Sep. 1999.

[9] M. Hata, Empirical formula for propagation loss in land mobileradio service, IEEE Trans. Veh. Technol., Vol. 29, No. 3, pp.317-325, Aug. 1980.

[10] K. Siwiak, Radiowave propagation and antennas for personalcommunications, Artech House, 1995.

[11] J. Doble, Introduction to radio propagation for fixed and mobilecommunications, Artech-house, 1996.

[12] Digital Mobile radio towards future generation systems COST 231Final Report, 1999.

[13] Y. Okumura, E. Ohmori, T. Kawano, K. Fukuda, Field strength andits variability in VHF and UHF land-mobile radio service, TokioElec. Commun. Lab., Vol. 16, pp. 825-873, Sep.-Oct. 1968.

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VITAE

J. P. Carmo graduated in 1993 and received his MSc degree in 2002, both in Electrical

Engineering and Computers from the University of Porto, Portugal. In 2007, he obtainedthe PhD. Since 1999, he is a lecturer at the Polytechnic Institute of Bragana, Portugal.