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EFFECT OF MULTIPATH FADING ON MILLIMETER WAVEPROPAGATION-A FIELD STUDYAdcl A. Ali and Mohammed A. Alhaider

Electrical Engineering Department, College of Engineering,King Saud University, Riyadh, Saudi Arabia

ABSTRACT

A field study on wave propagation covering a wide range of theelectromagnetic spectrum was actively running for four years in thecity of Riyadh, Saudi Arabia. The region can be considered a typicalarid climate where the rate of evaporation is higher than the rate ofprecipitation. The study involves the operation and continuous, com-puterized monitoring of two microwave radiolinks operating at 12 GHz.three millimitric wave links with radio frequency of 40 GHz, and aninfrared link with 0.88 \.lrn wavelength. A meteorological station isoperated and monitored as well.

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?

attenuation I)y ra infa l l . A major problem with tile short hop system isdue to th e noise accumulated in th e tandem repeaters. However, digi-tal modulation with regeneration a t each repeater will solve theproblem. Here the l imiting design factor may not be rainfall attenu-at ion, rather , equipment re l iabi l i ty will set th e l imi t on system per-formance.

In Saudi Arabia and similar areas with l i t t l e ra infa l l , muchlonger hops may be feasible. Besides rainfall , propagation of mill i -metric waves is marked by many other ptlenomena such as bandwidth deco-herence [3J, and depolarization or polarizat ion rotat ion. Since allof th e above phenomena are weather dependent, their effect on propaga-t ion should be evaluated for th e part icular transmission climate.

Unfortunately, almost all the data reported in the published

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the earth's surface could be clear of sand. Hence we may exppctthat radio links will not be affected by sand storms. However, duststorms comprising much smaller particles (less than 0.01 mm in radius)may be found at as high as l1undred meters or more. This will reduceth e visibil i ty and affect the propagation in millimetric wave band[6-12J. Multipath fading due to temperature and pressure gradients isalso expected to affect the propagation in Saudi Arabia. Our findingson the effect of rain sand/dust storms on propagation in arid land arereported elsewhere [13-16J, however, the effect of multipath fading onpropagation is reported here.II . EXPERIMENTAL SYSTEM DESCRIPTION

A block diagram of th e experimental system is shown in Figure 1.The system comprises: (1 ) a set of five transmission links operating

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4

I l l . MEASURED ATTENUATION DUE TO MULTIPATH FADINGFading of signal level over l ine-of - s igh t l inks strongly depends

on th e hop length, frequency, terra in and climate. For short hops theprobabi 1 i ty of occurence of deep fades becoilies di meni shi ng1y slila 11.However, since an extended hop length a t 10 to 20 kill is possible forregions with rare rain ac t iv i t i e s , clear weather fading ca n effect thelink re l iabi l i ty in a s imilar way as rain and sand storms fade eventswere observed during the course of measurements with fade durationsranging from a fraction of a minute to several hours. Fading of thereceived signal for periods of minutes have been observed on themill imeter wave 1 ink. The fade was repeated for hours with occasionalenhancement, or sc in t i l l a t ion , or with power fading. The shape ofth is fade indicates th e possibi l i ty of multipath propagation due toatmospheric layer with strong refract ion gradient or due to ground

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3) Scintillation occurred simultaneously wi tt l nlultipath fade.Siynal enhancement and power fading are relatively rat'e.

4) The probaoility of Illultipath propayation is higher in winter(November to February) than in summer (March to June). thismay be explained by the fact that the winter is characterizedby relatively moderate temperature (of the order of 15"C) andhigh relative humidity (50 to 80'1).summer is hot (20 to 48C) but dry.

On the other hand,

5) For 2% and 1% of the time. multipath fades may exceed 2.8 dBand 4.8 dB respectively.

6) The distribution of multipath fade may be described byRayleigh distribution.

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b

b = 112; 1/4; 1/8 for gul f coast or similar hot humid area, normal interior temprature mountainous or very dry, respectively.

For the 14 Km links operating at 40 GHz an occurence factorof ' 0.03 in calculated using the above expression with a = 2 and b0.25. Such values of a,b are the proper values for the path, and themeaured occurence factor (0.033) is fairly close to the calculated0.03. Hence i t seems that the formula (1) , which is well in use formicrowave frequencies is sti l l holding accurately at extremely highfrequency band.

b) Amplitude and duration statist icsi) 40-GHz links

The probability Y(F) that fade depth exceeds F dB, duringfading is well approximated by an exponential or a normal distribu

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I

I t is also interest ing to note that the experienced fade overthe 14-Km l inks can be approximated by a Rayleigh dis t r ibut ion in theform:

P(F) = 0.033 x IO-F/lO ( 4)Fade duration is essent ia l ly of exponential dis t r ibu t ion ,

however, small fade has a normal dis t r ibut ion of fade durat ion. Theprobabili ty Y(t) tha t fade duration exceeds t minutes is given by:

Y(t) (5 )

where To' the average fade duration is an inverse 1 i near function offade depth in the form:

T = 5 + 37o F .;; 15 dB (6 )

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Y(t) " eXIJ(-t/1U) ( 8)From the measurements of multipath, i t has been observed that

multilJath propagation occured during the period from midnight to noontime, IJrobably non uniform distribution of temperature and relativehumidity occured in this time. Scintillation occured simultaneouslywith multipath fading. Signal enhancement and power fading were relat ively rare. It is also observed that the probabili ty of multipathpropagation is higher in winter than in summer. This may be explainedby the fact that winter is characterized by relatively JTk)derate temperature (in the order of 15C) and high relative humidity (50-80'1,).On the other hand, summer is hot (35-48C) but dry.

IV. CONCLUSION

A field study aimed at studying the propagation of mill imeteric

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i i) Even at the short h o ~ of U.75 km operating at near infraredfrequency, multipath fading was experineced. Fade amplitudeand duration statistics are given for both the 4U-GHz and theinfrared links.

Although reliability and outage analysis are not included in thispaper, such analysis can be easily pursued based on the complete sta-t istics of rain attenuation, sand storms and llIultipath fading. It canalso be verified that the three factors viz. sand storm, rain andmultipath fading may play equa1ly important roles in determining thelink reliablity for the long hops in arid climate.ACKNOWLEDGEMENT

This work is supported by grant No.AR-5/29 from King AbdulazizCity for Science and Technology (KACST).

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1U

LIST OF CITED REFERENCES

1. Goldhirsh, J . , Robinson, R.L., "Attenuation and Space DiversityStat is t ics Calculated frOIll Radar Reflectivity Data of Rain", IEEETransaction on Antennas and Propagation, Vol.AP-30, No.6, Nov.1982.

2. Mori, Y., Higuti, 1., Morita, K., "Estimation of Effect ofFrequency Band Diversity on Earth-Satell i te Links in Microwaveand r>1illimeter Wave Bands", Electronic and Communications inJapan, Vo1.61-B, No.10, pp.5S-61, 1978.

3. Neves, J . , Watson, P.A., "Cross-Polarization, DifferentialAttenuation and Differential Phase Shift Measured on a 36.5 GHzTerrestrial Link", URSI Commission F Symposium, Canada,Preprints, pp.5.1.1 to 5.1.6, 1980.

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9. Gibbins, C.J. and Pike, r ~ . G . , "Millimetre, Infrctred and OpticalPropation, Studies on a 500 m Range", 5th Intl . Conf. on Antennasand Propag., PL2, rCAP-87, York, U.K., pp.50-53, 1987.

10. Goldhirsh, J . , "A Parameter Review and Assessment of Attenuationand Backscatter Properties Associated with Duststorms over DesertRegi ons in th e Frequency range of 2 to 10 GHz", IEEE Trans. onAntennas and Propagation, Vol. AP-30, No.6 , pp. 1122-1127,Nov. 1982.

11. Ghobria1, S.L and Sharief, S.N., " ~ l i c r o w a v e Attenuation andGross Polarization in Duststorms", IEEE Trans. on Antennas andPropagation, Vol.AP-35, No.4, pp.418-425, 1987.

12. Bashir, S.O. and ~ l c E w a n , N.J., "Crosspolarization and GainReduction Due to Sand or Dust on rlicrowave Reflector Antennas,"

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12

16. M.A. Alhaider and A.A. Ali, "Experimental Studies on rlillirneterwave and Infrared Propagation in Arid Land: The Effect of SandStorms", 6th International Con renee on Antennas and PropagationrCAP 89, Coventry, U.K., 4-7 April, 1989.

17. Barnett, \LT., "Multipath Propagation at 4, 6 and 11 GHz", HSTJ,51,2, pp.321-361, Feb. 1972.

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T.RHWS.WO.p_

AGe,lFYOUADU '4

'MS" C a b ~ s

Rt'ceivt'(S14 km+-~ O k m

141m+-

0.750 km+-t

,--

T Mim i t t t ' ( SD 34.925 GHz39..434 GHz

[] 9.306 GHz:

' 012 t.7S5GHZ

"\'J 0." pm

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Table 1. Parameters of the Link Systellls

LinkParameterTransmitter frequency(GHz)Transmitted power (dBm)Transmi t terPolarizationAntenna type.Antenna size, 1IAntenna gain (dB)beamwidth (0)Recei ver type

Radome

BKM HUK/HUP

12.4653 39.306/39.434+20 +23x-tal DRO & ImpatVertical VerticalParabola Cassegrian23 I 5/8 9"35 353.0 2.5SH SH

No Kapton(0.005")

NEC IR

38.925 0.880 vm

+10 +14.77Impatt Ga At As

Vertical RandomCassegri a Fresne 1 1ens

14" 6" x 6"401.6 0.17 divergenSH SiliconAvalanchediodeNo Glass

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Tlme o-r--measurementMonth x 103 (min)February 40.32r ~ a r c h 28.8April 43.2May 28.8June 28.8November 43.2

Table 2: Number of events and type of multipath fading on the40 GHz, 14 km link, in Riyadh, during 1987.-Ter1OOof Averagefade temperatureoccurence

ON 16ON 14MD 22.5

DN/MD 29.5DN/NS 25DN/MN 16

AveragerelativetyurTlDer-or -ITilriUtes -oTT.faee re-nT-s--du-r-rn g-wnic1fexceeds 1 to 6 dBIwi I l -W-nJ),- - ---- l-fota-'luillbe-r

__L s 0 ~ t i 1 1 _ a _ ~ ~ ~ l e _ n h ~ c e r n e n t r_ f ade_ of _ J ~ i _ n u t e s 46.5 1730 17

735 73S590 suo 10%740500

1342 213 1:'55December 44.64 MD 14

78.040.033.026.050.068.0 29026 I1116 [

~ . , . - ---_ ... --'-r fade depth (dB)

ON day period from dawn to moonMD day period from midnight to dawn.NS day period from noon to sunset