Indian Journal of Radio & Space Physics Vol. 26, June 1997, pp.154-158 .. f ;,,':- A " .••.- .. { . Radio w~v!l ~~Itn;:~t~~::cOo~i!:ai:::~nkation' at -" ~~~~'S~~~~Ahmad' Radio & Atmosplied~£misi"ij·)%tional.p~icai..Laooratory, New Delhi 110012 .B.eret¥~....19.9b;..rev.isOO received 20 February 1997 ~d strength measurements at 150 MHz were conducted as a function of distance for various base st:ation antenn.a ?eights ~ the so~th I~dian coastal zones and the observed results are compared with dlfT~rentpre.dICtIontechniques to identify suitableprediction method for estimating path loss. These are carried ou~ ill ~rban, sub-urban, q.ua~i-open~nd open regions for various base station ~n.n~. heights. These studies WIllbe useful for designing mobile communication systems in these region~ : ...../'j ..-."'" '~-' 1 Introduction The tremendous growth in cellular and personal communication systems in the country is imposing severe constraints on the limited frequency spectrum. Efficient planning of these systems requires the knowledge of the environment and the intervening terrain. This knowledge and the associated propagation studies will be useful for frequency planning, in deciding transmitter powers, in choosing base station antenna heights, etc. Depending on the antenna height and transmitter powers the base station is usually classified as cellular (up to several killometres) or microcellular (up to 1 km) [Refs 1,2]. Antenna heights for typical cellular base stations range from 40 to 93 m above ground level, while for microcellular these are about 3-5 m (Ref. 3). Transmitter powers in macrocells range from 1 to lOW, whereas in microcells they range from 0.1 to 1 W. In mobile communications, prediction of path loss assumes greater significance in view of the constantly changing environmental conditions. To identify a suitable prediction technique, field strength measurements were conducted in the south Indian coastal zones and compared with different prediction techniques 4.9. Though this types of comparison was attempted by earlier workers 10.1 1, their conclusions cannot be adopted to our environment. Rossi and Levy 1 2 investigated ray launching methods and geometrical theory of " - .Rresent addressQlf62, Old Rajinder Nagar, New Delhi 110 060 " diffraction to compute the field strength in the urban areas of France. In fact, this is the first time that these types of studies are reported from this region of the world. 2 Experimental details The field strength measurements were conducted radially from the transmitter with an effective radiated power of 5 W up to distance of 30 km in different regions, viz. urban, sub-urban, quasi-open (with trees etc.) and completely open area at 150 MHz. The transmitters are operated by the Department of Telecommunications. Transmitting antenna is omnidirectional monopole antenna and a yagi receiving antenna has been used. The sensitivity of field intensity meter is 10 /lV/m up to 1000 MHz. The measurements were carried out for a period of 45 days for base station antenna heights of 16, 30 and 40 m. The measurements were taken at every I km for a duration of 10 min using a field intensity meter and in this way the whole stretch was completed. This was repeated for various base station antenna heights in a particular environment region on the same day. The same procedure was repeated in other regions. However, continuous monitoring of the signal strength during the motion of the vehicle was not carried out. Sampling rate was one measurement per minute. The average field strength value of 100 samples at a given distance was converted into a path loss value. In the present study where the measurements were conducted, the urban region consists of
Transcript
Indian Journal of Radio & Space PhysicsVol. 26, June 1997, pp.154-158
Radio & Atmosplied~£misi"ij·)%tional.p~icai..Laooratory, New Delhi 110012
.B.eret¥~....19.9b;..rev.isOO received 20 February 1997
~d strength measurements at 150 MHz were conducted as a function of distance for various basest:ation antenn.a ?eights ~ the so~th I~dian coastal zones and the observed results are compared withdlfT~rentpre.dICtIontechniques to identify suitable prediction method for estimating path loss. These arecarried ou~ ill ~rban, sub-urban, q.ua~i-open~nd open regions for various base station ~n.n~. heights.These studies WIllbe useful for designing mobile communication systems in these region~ : ...../'j
..-."'" '~-'
1 IntroductionThe tremendous growth in cellular and personal
communication systems in the country is imposingsevere constraints on the limited frequencyspectrum. Efficient planning of these systemsrequires the knowledge of the environment and theintervening terrain. This knowledge and theassociated propagation studies will be useful forfrequency planning, in deciding transmitter powers,in choosing base station antenna heights, etc.Depending on the antenna height and transmitterpowers the base station is usually classified ascellular (up to several killometres) or microcellular(up to 1 km) [Refs 1,2]. Antenna heights for typicalcellular base stations range from 40 to 93 m aboveground level, while for microcellular these are about3-5 m (Ref. 3). Transmitter powers in macrocellsrange from 1 to lOW, whereas in microcells theyrange from 0.1 to 1 W.
In mobile communications, prediction of pathloss assumes greater significance in view of theconstantly changing environmental conditions. Toidentify a suitable prediction technique, fieldstrength measurements were conducted in the southIndian coastal zones and compared with differentprediction techniques 4.9. Though this types ofcomparison was attempted by earlier workers 10.1 1 ,
their conclusions cannot be adopted to ourenvironment. Rossi and Levy 1
2 investigated raylaunching methods and geometrical theory of
"- .Rresent addressQlf62, Old Rajinder Nagar,New Delhi 110 060 "
diffraction to compute the field strength in the urbanareas of France. In fact, this is the first time thatthese types of studies are reported from this region ofthe world.
2 Experimental details
The field strength measurements were conductedradially from the transmitter with an effectiveradiated power of 5 W up to distance of 30 km indifferent regions, viz. urban, sub-urban, quasi-open(with trees etc.) and completely open area at 150MHz. The transmitters are operated by theDepartment of Telecommunications. Transmittingantenna is omnidirectional monopole antenna and ayagi receiving antenna has been used. Thesensitivity of field intensity meter is 10 /lV/m up to1000 MHz. The measurements were carried out fora period of 45 days for base station antenna heightsof 16, 30 and 40 m. The measurements were takenat every I km for a duration of 10 min using a fieldintensity meter and in this way the whole stretchwas completed. This was repeated for various basestation antenna heights in a particular environmentregion on the same day. The same procedure wasrepeated in other regions. However, continuousmonitoring of the signal strength during the motionof the vehicle was not carried out. Sampling ratewas one measurement per minute. The average fieldstrength value of 100 samples at a given distancewas converted into a path loss value.
In the present study where the measurementswere conducted, the urban region consists of
PRASAD et al.: MOBILE COMMUNICATIONS AT VHF IN SOUTH INDIAN COASTAL ZONES 155
buildings of up to 2 storeys but with some openspace between them with street widths of the orderof 30 feet. The region does not consist of any skyscrapper, etc. Sub-urban region is characterized bylow density single storey dwelling units with sometrees. Quasi-open area consists of terrain partly withagricultural lands and partly woods and trees. Sinceit is a coastal area, a lot of greeneries are presentthroughout the year and some of the region iscrossed by rivulets. Open region consists of flatopen areas with agricultural rice fields and aresparsely populated.
3 ResultsThe field strength values are converted into path
losses relative to free space and are shown in Fig. 1.Figure 1 shows the variation of path loss withdistance up to 30 km for urban, sub-urban, quasi-open and open areas with base station antennaheights (hb) of 16, 30 and 40 m. The path lossincreases with distance and decreases with increasein antenna height. It is well known in radio wavepropagation that increase in antenna height increasesthe LOS horizon distance which, in turn, results inincrease in signal strength. This is observed asdecrease in path loss. Also the path loss is highestin urban areas followed by sub-urban, quasi-openand open areas. In urban region multiple reflections
Fig. I-Variation of observed path loss with distance fordifferent base station antenna height in urban, sub-urban, quasi-open and open regions
from cluster of buildings reduce the signal strength.Hence the path loss is highest in urban areasfollowed by sub-urban, quasi-open and open areas.At a distance of 10 km, path loss decreases by 6 dBwhen hb is changed from 16 to 30 m, and decreasesby 8 dB when the hb changes from 16 to 40 m. At adistance of 20 km, the decrease in path loss is 7 dB,when hb is changed from 16 to 30 m and it decreasesby 9.5 dB when antenna height is changed from 16to 40 m. This seems to agree more or less with therule of 6 dB/octave, i.e. when the antenna height isdoubled, path loss decreases by 6 dB (Ref. 13).Since the terrain is without any obstructions this isbeing obeyed. Generally, the rule of 6 dB/octave isobeyed in those terrains which are flat, i.e. withoutobstacles. When hb increases, the decrease in pathloss becomes larger at distances beyond 10 km. Atany given distance, the path loss decreases by 6.5dB from urban to sub-urban, by 8 dB from sub-urban to quasi-open, by 13.5 dB from quasi-open toopen areas. Also at a distance of 10 km in the quasi-open area, variation of path loss with the height ofreceiving antenna has been studied. It was observedthat an increase in the height of receiving antenna by0.9 m decreased the path loss by 1.5 dB and that by1.8 m decreased the path loss by 2.5 dB. Apart fromthis, tilting the receiving antenna by 6" increased thepath loss by 1.5 dB and l' tilt increased the path lossby 3.0 dB. A tilt of 2' resulted an increase in pathloss of5 dB.
4 Propagation modellingAll wave propagation models are divided into 3
major categories: empirical, semi-empirical anddeterministic. The low computation time is the realadvantage of the first two, whereas the approximateconsideration of the environment into the modelsleadmg to large prediction errors is the majordrawback of these models. Deterministic modelsgive an accurate prediction by incorporating largedata base of the environment. Therefore, the choiceof the coverage prediction model depends on thepropagation environment and the extension of thecoverage area.
In mobile communications, propagation takesplace through multiple diffraction, reflection andscattering from an extremely large number ofobjects. Since it is very difficult to locate scatterersdeterministically, characterization of the signalwithin the coverage zone is done statistically.Recently, it was shown that deterministic wave
156 INDIAN J RADIO & SPACE PHYSICS, ruNE 1997
propagation models incorporating a building database might be feasible in a microcellularenvironment and can provide an accuratedetermination of path 10SSl ,15. The two importantparameters that the communication engineersmainly concerned are (i) link budget and (ii) timedispersion. The link budget can be deduced from thegains of transmitting and receiving antennae, andfrom amount of received power which, in turn,depends on the path loss between the two terminals.Multipath propagation gives rise to time dispersionin which the wave travels along different paths inreaching the receiver due to different propagationmechanisms like reflection, scattering anddiffraction. This manifests as delay and this timedispersiveness of the channel decides the maximumdata rate to be transmitted without requiringequalization. For example, if a digital signal has asymbol duration more than 10 times the r.m.s. delayspread, then the equalizer is not required for bit errorrates better than 10.3• The delay spread is inverselyproportional to coherence bandwidth. This impliesthat if the symbol duration is shorter than the delayspread, we have both inter-symbol interference andfrequency selective fading. When the symbolduration is longer than the delay spread, we have flatfading and no inter-symbol interference. A bit errorrate of 10.3 denotes the threshold of reliability oflink. Bit error rates greater than this contribute tooutage. Then equalizers are required to bring theerror rate to less than 10.3. The estimation of timedelay is necessary for accurate determination ofvehicle location in the area of navigational services.
4.1 Comparison with prediction methodsA comparison of path losses predicted by Hata4
,
Egli6, and Blomquist and Ladell9 methods withthose of observed values is shown in Fig. 2 forhb=16, 30 and 40 m for open region. Hata's methodgave good agreement with the observed values forall the antenna heights. The agreement is good at allthe distances with a deviation of around 1 dB. Forhb=16 m, Blomquist and Ladell's method deviatesby 10 dB at 10 km distance and the deviationincreases at larger distances. The deviation alsoincreases with increasing hb' Egli's methodoverestimates the path losses by as much as 20 dB at10 km for hb=16 m and the deviation increases withdistance. The trend is the same for hb=30 and 40 m.
Hata's method4 and Ibrahim and Parsons8
methods are employed for predicting path losses in
Fig. 2-Comparison of observed path losses with the predictedpath losses in the open region
the urban region and the results are shown in Fig. 3.Here Hata's method shows reasonable agreement.Hata's method underestimates the path loss by 4 dBat 10km and by 5 dB at 20 km distance. The trend isthe same for hb=30 and 40 m. Hata's predictioncurves follow the trend of observed lossesthroughout the range of distances. Ibrahim andParsons' method underestimates the path loss by 10dB at 6 km and the curve steeply increases andoverestimates the path loss after 16 km. At 30 km,the deviation is 5 dB. For hb=30 m, the deviation is6 dB at a distance of 10 km and steeply increases asin the case of 16 m height and overestimates thepath loss beyond 18 km with a deviation of 7 dB at30 km distance. The trend is the same for hb=40 malso. Here the empirical model of Ibrahim andParsons is used. The land usage factor used in theirmethod varied from 50% to 35% at differentdistances from the transmitter, and the mobilerelative spot height varied from 7 to 10 m. The landusage factor has been arrived by taking the total areadivided b) the area occupied by the buildings at
PRASAD et al.: MOBILE COMMUNICATIONS AT VHF IN SOUTH INDIAN COASTAL ZONES 157
m"0
...-,-,-,-'"
//
/I
II
I
150 MHz ( URBAN)
50
45
• 40wU<I
"-if) 35wwa:u,
30oI--
w>I--<I.JWa:(f)
(f)
o-'
10 14 18 22DISTANCE, km
26 30 34
Fig. 3-Comparison of observed path losses with the predictedpath losses in the urban region
different points of the propagation path. In the sub-urban region for hb=16 m, Hata's methodunderestimates the path loss by 6 dB at 10 km andthe deviation increases to 10 dB at 30 km. Thecomparison is shown in Fig. 4. The correspondingdeviations for hb=30 and 40 m at 10 and 30 kmdistances are 7 dB in both the cases. Comparison ofITU-R predicted values with that of observed valuesin the sub-urban region showed that agreement isnot good for any antenna height. At a distance of 10Ian, the deviation is 15 dB and remains the sameeven at 30 km distance for hb=16 m. These aredepicted in Fig. 5. The study showed that themethods which incorporate environmentalcorrection factors like Hata showed betteragreement than other methods which depend onfrequency, distance, base station antenna height, etc.
5 Summary and conclusionsSome field strength measurements were
conducted in south Indian coastal zones at 150 MHzin urban, sub-urban, quasi-open and open regionsfor base station antenna heights of 16, 30 and 40 m.A comparison of observed values of path loss withthose of lTU-R7
, Hata", Blomquist and Lade119,
50
45
40
35
'""0.30
wU<ro, 25(J)
ww~ 200I--
W>~.JWa:(J)(J)
0.J5
01 I2 6
150 MHz(SUBURBAN)
------
14 18 22DISTANCE, km
2610 30 34
Fig. 4-Comparison of observed path losses with the predictedpath losses in the sub-urban region
Fig. 5-Comparison of observed path loss with that of JTU-Rprediction method in the sub-urban region
Egli6, Ibrahim and Parsons" methods has beencarried out. In the open region Hata's method givesgood agreement with the observed values for allbase station antenna heights. In the urban regionalso Hata's prediction curves follow the observedpath losses with a deviation of 5 dB. The agreementof lTU-R's method with the observed values is notgood. In the sub-urban region Hata's methoddeviates by 6 to 10 dB, but in all regions Hata'scurves follow the trend of observed path losses.
158 INDIAN J RADIO & SPACE PHYSICS, JUNE 1997
The decrease in path loss with increase in basestation antenna height is significant when hb ischanged from 16 to 30 m rather than from 30 to 40m, but more or less obeys the rule of 6 dB/octave. Inthe field of mobile radio wave propagation, thestandard Bractice is to employ the concept ofdB/octave 6, and hence it is followed here. Theobserved values. showed that the path lossdecreases by 6.5 dB from urban to sub-urban, by 8dB from sub-urban to quasi-open, and by 13.5 dBfrom quasi-open to open areas.
AcknowledgementsThe authors are grateful to the Department of
Telecommunication, A P Circle, for helping them toconduct these measurements and especially to Mr MV Bhaskar Rao, CGM, for his keen interest in theseexperiments.
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