Character is at Ion of Modile Radio Signals in Rural Areas

8/9/2019 Character is at Ion of Modile Radio Signals in Rural Areas

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CHARACTERISATION OF MOBILE RADIO SIGNALS IN RU RAL AREAS

S Mockford, A M D Jurkmani and J D Parsons

Th e University of Liverpool, U K

Lit&u!%

An extensive series of propagation field trialshas been undertaken within two rural aress of NUEngland.data w e r a small a rea shows that the Ric ian PDFconsisten tly provide. a more accurat e model tha n theRnyleigh. Nak ng mi and Ueibull distributiona. Thisfinding has been observed in both discrete terrainenvironme nts (towns. villages. woodland etc.) and overaggregate rural routes.of the measured fas t -fading s ignal h m e a lso beencomputed and compared with theoretical veluas obtainedby assum ing a Rayleigh diatributed signal.

Analyais of the wa su re d signal envelope

The second order atatiatica

1. l t u & w h

For many years there has been an exten8ivedevelopment of land mobile radio comuni catio n

aystems.further sccelereted. especially in the mobiletelephone field. The "roll-out" of the UK 900 HHzcellular systems, for exanple. is now well underwaywith a11 the mnjor cities and the motorway networkbeing covered.these eystems will include mnny more rural areas wherethe call size will be larger. to reduce installationand maintenan ce costs. Coverage is likely to be themnin problem in ru ral areas and it is essential thatsystems planners have reliable prop aptio n models.Thus, it is important that the statistics of thesignal received within a rural mobile redioenvironme nt are known. At present, these signalstatistics are ill -defined and therefore acomprehensive characterisation of the received signalvariability has been undertaken.

Within recent years the developmen t has

The next phase of devel opment for

Clarke's model [ l ] for the multipath fading hasbeen fou nd reliable for the urban mobile radiochannel. This model predicts that the statistica ldietributi on of the f ield strength values shou ldfollow a Rayleigh distribution. This is obtained by

central limit theorem arguments, based on thes u pe r po s it i on o f a l arg e d e r f e qu al ma g ni t ud ecomponents from different reflecting and diffractingobstacles. in the immediate vicinity of the mobile .In practice, this 'ideal' situation does not alwaysexist. In a typical rural area, for example. thenumber of scat terers may be quite sm all, end themagnitude. of the in dividual scattered components canvary. with line-of-sight paths being common. Theeffect of both of these conditions would be to causethe fast-fadi ng sigaal statistics to be non-Rayleigh.In this situati on, if system planners were to use theRayleigh model. they would over-estimate the severityof the signal fadin g, and the resultant design wouldbe based upon pessimistic modelling. As a result, thetransmitte r power may be unnecessarily high and thiscould lead to interfere nce problems. For this reason.an assessment is made in this paper. of whetherClarke's model is an adequate representation of thenarrowband channel in rural aress.

Other characteristics of the short-term fadingthat are inves tigated in this work, are the level

crossing rate (LCR) and average fade durat ion (AFD).The LCR and AFD. which are of particular interest tosystem planners, have not previously been investigatedin rural areas.

2 . vwo series of field trials were conducted forthis work, with continu ous transmissions from twooperational callular radio base stations beingmonitored. Theae base stations were located at PoleHeights in the Delame re ares of Cheshire, and atBillinge Hill in Lancsshire.De lv er e reglona are topographically simple,relstively flat rural areas.

Both the Billinge and

Ench survey route wss chosen to be representstiveof a 'normal highway' with, typically. an undulatingterrain profile and e highly variable local topogrsphyelong the route. It wes particularly important thateach survey route passed through different ruralenvironmenta. such as towns, villages. hamlets,woodland, etc. so that, the data collecte d in thefield trials would provide extensive informetion

pertaining to discrete rural terr ain aettinga.both test aress, ro utes with radial endcircumfer ential direction s relative to the barestation were selected.

In

A single receiver with a dynamic range of 75dBwas used to measure the envelope of the signalreceived on a vehicular-mounted X/ 2 dipole antenna.The continuous fading signal envelope waa samp ledapproximately 18 times per wavelength at 9 0 0 ~ ~ .measureme nt equipment was installed in a Rover 2600motor car. which was used as the mobile laboratory. Asaloon car was chosen so that all the measureme ntswould be rep resentativ e of a typica l cellular radioinstallation.

3.

- TheA typical recording of the fast-fading signal

envelope is shown in Figure 1. where the transmissiondistance is approximately 11.5ka. This is datacollected aa the mobile travels along the A 5 1 road inthe direction of th e Delanere transm itter, and passes

through the village of Vic arscross (rad ial route).There is an obvious change in the statisti cs of thereceived signal for the relatively 'open-aspect'areas. either side of the village section. A strongdirect componen t may be recei ved in these particula rlocations and this will cause the envelope statisticsto differ from those in surroundin g areas. Obviously.it would be misleading simply to use the completa 2kmsection of data to estim ate the characteristics of thefast-fadi ng. since this route obviously coversdifferent types of terrsin, in each of which there maybe a different distribut ion. Consequen tly, it wasdecided that a furth er detailed investigation shouldbe made to annlyse the variations of the signnlstatistics along numerous terrain routes. As aresult. diagrams such as Figure 2 were produced.shows the important feature s of the surveys made inthe Delamere area.been plotted on these diagrams:

(1) Lower diagram:

This

The following characteristics have

This sho ws the ground height and mean signal

strength along the route, on the same axis. Theground height data, which was derived from a terr aindatabase. shows the profile over which the nobiletravelled. The mean s ignal strengths. which werecalculate d from the survey data, were averag ed over

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150 metres of linear travel. This particularaveraging distance vas used because the fast-fa dingsignal statistics were found to be stationary overthis distance. A further factor was the need to avoidexcessive computationa l analysis, as the surveyscovered large distances.

It should be pointed out that a signal threshold

of -90dBm has been identified in this diagram.signifies the threshold below which the signalstrength receiver s cannot accurately measure the deepfades in the received signal. Therefore , whenever themean signal falls below this level, some deviatio nfrom Rayleigh statisti cs must be expecte d.

(ii) Top diagram:

This

This shows the standard deviation of thefast-fadi ng envelope, expressed in decib els, plottedagainst distance. The theoretica l standard deviationfor a Rayleigh distributed signal is 5 . 5 7 6 8 I21. andthis magnitude is indicated by a broken line. Thisdiagram can theref ore be used as an indication of thesignal variation.

It is evident that there are large sections ofthese aggregate terrain routen where the signalstatistics do not conform to the Rayleighdistribut ion. Furthermore. it is apparent that thetopography of the area immediately surro unding themobile has a prominent influence on the signal

variabili ty. In many of the relatively open areas.between to wns and villages, the Rayleigh model docsnot appear to be a good approxi mation to the receivedsignal statisti cs, whilst in the built-up areas thestatistic s tend to conform more readily to Rayleigh.

Close inspection of both diagrams, reveals anoticeable correla tion between the mean signal valuesand the signal variability. For example, in locationswhere there is a si gnificant increase in the meansignal strength, there is a corresponding decrease inthe standard deviation of the fast-fading envelope,end the measured CDF does not conform to Rayleighstatistic s. This is an expected, because whenever adominant signal component is received, a reduction inthe signal fading should be accompanied by an increasein the local mean signa l value.

The results in Figure 2 show that, as expected.thare is some correlatio n between the mean si gnal andthe ground height along the route. Obviously, whenthe mobile is situat ed on higher ground , there will bea higher probability that a line-of-s ight path exists.

as the number of obstructions between the transmitterand receive r reduces.

4.

It has been concluded, in the previous section.that Rayleigh statistic s cannot be used to descri beaccuratel y the small-scale signal variation s in manydiscrete rural environments. This conclusion meantthat a further investi gation should be made to assessthe suitability of other known distributions, such asRice, Nakagami and Yeibull, in dascribi ng thesmall-sca le signal variations in rural areas.

In order to investigate which of thesediatribution functions, best dascribes theexperimental results, the PDFs wasur ed for numerouadata files were modelled by each of the fourdiatributions.theoretical distribution functiqns can be varied,giving different Chi-squared ( x ) quantities, theywere chosen to give the minimum possible value in eachof the four cases.

this method are shown in Figure 3. the measured and theoretical probability distribut ionsare plotted on the same axis, for the fourdistributions considered. Also, the appropriate

Since the parameters of the

Results which were obtained using

In this diagram.

pirameter values, which gave the 'b est-fit' or minimumx -statistic, are given or each PDF.

Results simila r to those illustrated in Figure 3 were obtained for each type of homogeneous terrain.for various transmitter-receiver separation distances,from lkm upto 15km. To assess the overall performanceof each distri bution function in these areas, the

respectiv e parameter values for the variouspropagation distances have been averaged. In order toestimate the overall accuracy of the four PDFs in eachterrain environme nt, the observed values of x 2 for theappropria te experimen ts should be added, and also, thenumber of degrees of freedom from each individualexperiment should be added.

In all tha typ ical rural areas considered, theR p i a n d i st r ib u t on gives the least overallx -statistic. & , and thus tha best fit.indicates that the Rice PDF is consistent ly the bestmodel in all environments considered.

1This result

Another important conclusion that can be draw nfrom these results. is that for a11 terraine n v i r o m e n s, the Rayleigh distribut ion has thehighest & quantity. Therefore, the Rayleighdistribution is the least accurate of the fourdistributions considered. in modelling the statisticsof the East-fading envelope.

5

The probabil ity density function of the Rice

distribution is

where r is the signal envelope, r is the magnitudeof a stgady ljne-of-sight or specufarly reflectedcomponent. a is the variance of the randomcomponent s. "If system plannera uere to use the RicePDF to model the algnal stati stics within rura l areas.then they need to use some empirical values of theRice parameters. The optimum parameter values for theRician distr ibutions are presented in table 1.

5. Orde r Statis-

Statistics such as the Level crossing rate (LCR)and Average fade duration (AFD) are of interest todesigners of mobile cowuni catio n networks.system utilises digital modulation , for example. thechannel quality deteriorates rapidly vhen the receivedsignal level fades below a noise-related threshold.

Inthis situation, these statistics are particularly

useful in predicting bit-error-ratea and error-burstdurations.

m e n a

Tha LCR and AFD were determined for those areasin which the signal fading was most pronounced, andalso where it was at a minimum. Figure 4 showstypical LCR and AFD results with the theoreticalresults foe a Rayleigh distributed signal.

The measured LCR curves are slwaya considerablyless than the theoretical values. Indeed, there is alarge deviation from theoreti cal for the data secti onswere the fading is most prominent.between the ncasured and theoretical tCRs should be ofsome stgnif icancs to system planners.situations . the Rayleigh distrib ution may be used tow d e l the 'worst-caae' sicuation in a ruralenvironme nt. Hovever. the results illustrated here,indicate that this method would ofte n produceexcessive ly pessimist ic results.example, the measured crossing rate is less than 0.02at a fade depth of -25dB. whereas the theor etical

Rayleigh value is 0.15.

This discrepancy

In some

In Figure l r , for

The measured AFD curves also deviatesignifica ntly from the theore tical Rayleigh results .



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Also shown in Figure 6 are the CDFs of theaverage duration of fades at threshold levels of-20dB.at fade depth -1 0dB is also illustrated.been included as they may provide parameters thatcould be of some use to system planners. It isapparent, for exnmple. that at a fade depth of -20dB.

In the case of the PDF plot, at a fade depth of -10dB.the Deak orobabilitv occu rs when the fade duration is

(b)

-10dB and OdB and the PDF of the fade durationsThese have

901 of the durations are less than OK equal to 0.11. (C)

The experimental LCR values. measured overtypical rural routes. are always less than thoseforecast theoretically. This is because , withina rural environment. the signal fading is seldomas severe as thatdistributed slgnal.

The experimental AFD results are not in good

agreement with theoretical Rayleigh resulta.

predicted for a Rayleigh

. .

approximately 0.121.

From the analysis of the signal variability. thefollowing conclusions can be drawn

(a) The use of the Rayleigh distribution to model thefast-fa ding signal variations within ruralareas, can often produce inaccurate results.The Ricisn distribution provides an accuratedescription of the small-s cale aignal

BEBeRWCES

111 Clarke, R.H.: A statistical theory of mobileradio reception, Bell Sys. Tech. J.. 1968, 47.pp.957-1000.

( 2 1 Lorenz . R.W.: Theoretic al distributi onfunctions of mlt ipa th fading processes In themobile radio environment, and determination oftheir parametaro by measurements. Technia herBericht 455 TBr 66.Deutsch en Bundespost (in German).

Forschungainstitut de r

vsriability rithin rural areas.

Figure I

I Vicarscrms village., I0 m . 1 5 0 IOM)

IO-

Figure 2

A typical recordingof the tast-fodingsignal in rural route

Received signalcharacteristics for atypical rural route



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0.1* os

-30 -20 -10 0 I0

Figure 3 Theoretical and Experimental probability distributions of the fast-fading signal in rural areas

fadeDeplh(de d m

ean).

!lo-1 (c)5

pDf*-10dB-2 -1 0 1

LOG10 LOG10

N d K e d fad0 D u r kd k e d fadc

DudionFigure 4 Second-order statistics of the fast-fading signal in rural oreos


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Character is at Ion of Modile Radio Signals in Rural Areas