1.9 ghz measurement-based analysis of diversity power versus the number of rake receiver tines at...

8/6/2019 1.9 GHz Measurement-based Analysis of Diversity Power Versus the Number of RAKE Receiver Tines at Various Syst

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1.9 GHz Measurement-based Analysis of Diversity Powerversus the Num ber of RAKE Receiver Tinesat Various System BandwidthsGregory T. Martin and Michael Faulkner

Mobile Communications and Signal Processing GroupDepartment of Electrical and Electronic EngineeringVictoria University of TechnologyPO Box 14428 MCMC MelbourneVICTORIA 8001, AustraliaTel: +613 96884454 Fax: +613 9688 4908email: [email protected]

ABSTRACTBased on wid e band propagation measurem ents in three dif-ferent environments, this paper examines the relationship be-tween the number of RAKE receiver fugers or tines andthe proportion oft he to tal incident time-dispersed signal pow-er utilised in the receiver, for various system bandw idths.Wide band propag ation measurement data with a resolutionof 40 nS is filtered to produce chan nel sounding power delayprofiles corresponding to system RF bandw idths of 1.25,2.5,5 8 , and 16 MHz. The number and power levels of all resolv-able rays are found, and the agg regate power in the strongestn ays, where n anges from 1 to 10, is calculated as a fractionof the total profile power. Results show how much receivedpower is discarded as a result of using a limited num ber oftines, providing a guide for choosing he optimum number oftines for wid e band systems. A simple empirical model givingthe number of tines is included.

INTRODUCTIONIn free space, only a tiny fraction of a com mun ications systemradiated power is intercepted by th e receiver antenna, with therest being lost in other directions. Typical mobile PCS radiosystems operate in a cluttered urban environment where prop-agation cond itions are far remov ed from the free space case,and where m ulti-path propagation is the norm . Indeed, with-out multi-path effec ts, cellular mobile com mun ication sys-tems cou ld not function. Often no line-of-sight path exists,and the receiver depen ds upon the mu lti-path signal consist-ing of time-dispersed rays arriving via reflection , refractionand diflkaction mechan isms.Wide band CDMA systems usingRAKE eceivers are able toexploit multi-path propagation to benefit from path diversity.Using more than one path, or ray, increases the signal poweravailable to the receiver, and with suitable comb ining, im-proves the received signal-to-noise power ratio, reduces fad-ing, and eases the power con trol problem.Third generation mob ile systems either under way or pro-posed, include CDMA technology with RF bandwidths rang-ing from 1.25 MHz to 20 MHz. Re ql] . In the USA, the IS-95standard (1.25 MH z) is well under way, and in Europe exam -

ples of wide band CDlMA research include the RACE Link-CDMA project (8MHit) which resulted in a wo rking test-bed,ReQ21, and the RACE,CoD iT project, subsequently furtherdeveloped into a mobile test-bed. RAKE receivers sufferfrom high complexity.,which increases with the num ber oftines utilised. How does the designer choose the best numberof tines, bearin g in mind increa sing comp lexity and cost, andthe trend to diminishing returns, as the nu mber is increased?By examining the utilisation of the av ailable signal power asthe number of tines is increased, this pape r studies one aspectof this question.

CHANNEL DATAChannel SounderA wide band sounder is used to m easure impulse responses ofthe radio channel at 11890MH z, using a swept time delaycross correla tor methaid. The chan nel soun der resolution de-pends on he RF bandw idth, the chipping rate, and the slidingcorrelator filter. Here the chipping rate is 25 M Hz, and theresolution is 40 nS, equ ivalent to a path length difference of12 metre. Om nidirectional vertically polarised antenn as areused, generally with a (discone s he transmitting antenna anda halfwave anten na for the receiver. Mo re detail may be foundin Ref.[3].Measurement EnvironmentsAdelaide. Adelaide has a population of over one m illion, andis the capital of the state of South A ustralia. Measurementsused here were done ir n North Adelaide, a suburban area w ithpredominantly single storey buildings. Mea surements werecentred on the Ho tel A,delaide. To the n orth the terrain is flat,with a mixtu re of singlle storey suburban housing, and low risecommercial buildings and shops. To the south the land fallsaway gen tly to parklands of the Torrens River, with the down-town city area beyond the river. Locating the transmitter onthe flat roof of the hotlel, six storeys up, gave unobstructedview s in all directions.Melbourne. Melbourne is the capital of the state of Victoria,and with a pop ulation lof over three m illion is the secon d larg-est city in Australia. Measurements were done three kilome-

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tres from the centre, in North Fitzroy, an inner suburban areawith residential terrace housing, com prising a m ixture of on eand two storey dw ellings.A two storey terrace house located in De lbridge Street nearthe top of a slight hill, was used as a base station site, with thetransmitter antenna on the upper front balcony. The balconyantenna was be low the prevailing roof lines, and is classed asa low antenna.Sydney. Sydney is the capital of the state of New SouthWales, and is the largest city in Australia. Measurementswere don e in the main city area with the transmitter overlook-ing the city from the 23rd. floor of the University of Technol-ogy Sydn ey, a prominent high building to the south of the citycentre.

CITY

Receiver. In all cases, the receiver was ca rried in a car, witha half wave omni-directionalroof mounted antenna. The carwas station ary for each measurement, with lo cations scatteredthrough out the measured area, rather than follow ing an equi-spaced m easurement route.A su mm ary of statistics of the propagation data app ears inTable 1.

LOCATION

Table 1:LargestRangeused

defined by this filtering process is twice the low pass filtercutoff frequency. All bandw idths referred to in this paper areRF system bandwidths.Figure 1 shows part of a PD P before addition al filtering, andwith filtering, for system band widths of 1.25 MHz and 8M H Z .

% probability < value(nS)rms delay spread mean delay

S Y D N E Y f il e s y d b l 8

25%metres)

Y-40 , ,

4

Filter 1 25 M H zFilter 8 M H z__

-0 1 2 3

50% 90% Max 25% 50% 90% Max

Figure 3xcess Delay (US)

Melbourne Delbridge Street balcony 650 42 1 1 1 297 581 33 112 274 494I IAdelaideSydney

Hotel Adelaide roof 2500 44 130 349 2126 36 92 356 1141University of Tech. Sydney 1800 123 430 875 1452 138 378 1195 2106

METHODFilteringBy filtering data collected using a wide band channe l sounder,the end result is equivalent to repeating the measurementsseveral times using a sounde r with reduced bandwidths. Achannel imp ulse response measured with infinite bandwidthwould show every possible ray arriving at the receiver. As thebandwidth is restricted, the peaks of the impulse responsebroaden, and the resolvable rays each consist of the three di-mensional vector addition of an increasing number of closelyspaced paths. These resultant peaks are now less determinis-tic, and su ffer fast fading. The voltage magnitud e of the meas-ured wide band impulse response, padded with zero samp lesbefore and after, is filtered in the time doma in by a low passthird order Butterworth digital filter, using a m aths softwarepackage. The time delay introduced by the filtering process isremoved for cosm etic reasons, so the outpu t is time alignedwith the un filterd signal. After filtering, the m agnitudes aresquared to obta in the pow er delay profile (F'DP), and the peakamplitude isnormalised to 0 &.The system RF bandwidth as

Tine Pow er and Total Pow erImpulse respon se excess delay is measured between -1 USand+25 US elative to the strongest peak. Dynam ic range of themeasurement i.e. the pow er difference between the peak pow-er and the noise floor, is typically between 30 and 40 & .Datawith insufficient dynamic range is discarded. All k peaks inthe pow er delay profiles are identified.A peak is defined as asamp le preceded by two increasing samples, and follow ed bytwo dec reasing samples. After the PDP has been reduced to alist of peak values, the total profile power P t d s calculatedas the sum of all peak powers. The strongestn peaks are iden-tified, and the cumulative s u m of these peak pow ers is calcu-lated for va lues of n between 1and 10.Th e sum of peak pow-ers in the stron gest n peaks is called the tine po wer P nwheren is the num ber of tines, and results are given in terms of theratio of tine pow er to total power, P d . user de f ie dthreshold is applied to all peak powers, and any power valuesbelow the threshold are set to z ero.The value of the threshold used is -25 dE3.

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if 4 ?hreshold then Pi = 0

I0 , 1 , I

k

1total PDP power ptotd= Cpi

the three different measurem ent environments is plotted in a

ntine power P, = Cp, whereP, are the strongest peaks1 in descending order

pn4 o t d

receiver power utilised is the ratio p =-sedRESULTSResults for tw o of the three different environments are plottedin Figures 2 to 5, as cumulative distributions at two differentbandwidths, 1.25 MHz and 8 MHz.The ve rtical axis gives theprobability that the power ratio P d i s reater than the P dvalue on the horizontal axis. A family of curves for n= l ton=10 is shown. The n=3 curve, for example, is the curve forthree tines, using the combined po wer of the three strong estrays.

MELBOURNE 1.25 MHrwmdeebalcony

100 ,

MELBOURNE 8 MHzDelbridge bakony

0.0 0.2 0.4 0.6 0.8 1.0P O W W R ~ S Om m w Figure 3

SYDNEY 1.25 MH zUniversityof Technology

0.0 02 0.4 0.6 0.8 1oFigure 4owerRatio Pn/Pta!ai

SYDNEY 8 MHrUniwm of Tednr~logy

0.0 02 0.4 0.6 0.8 1 0PowerRat~oPnlP(0tal Figure 5

To llustrate the effect of bandwidth more clearly, the data

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MELBOURNE\ \ Delbridge Balcony1.25 M M 2.5MM SYDNEYUniversity of Technologyc a o3 -1C05c0sw2 -2%m

1 2 3 4 5 6 7 8 9 10Number ofTines Used (n) Figure 6

ADELAIDEHotel Adelaide roof.5 MHzE 0.00-m

-0.5IC0$ -1.0mc)m8 -1.5CIc8 -2.0B0-5 -2.5(L:$g -3.0I8 -3.5EP -4.0 ~ I I I I I I I 1

1 2 4 5 6 7 a 9 IONumber of Tines Used (n) ~i~~~ 7

DISCUSSIONThe number of variables involved and wide variation s in thechannel impu lse response, complicates the presentation of re-sults in a mean ingful way. E nvironmental factors are very ev -ident, with the results for the Sydney measurements beingnoticeably different, with a m uch higher number of rays withsignificant power, compared w ith the less dispersive Mel-bourne and Adelaide data. Figures 6,7a nd 8, based on the

-2mUm32 -3P22'-4.-

IP -7y I I I I I I1 2 3 4 5 6 7 8 9 10

Number of TinesUsed (n) Figure 895% cumu lative distribution level, give the best in sight intothe effect of bandwid th . P d v a l u e s presented here will onlybe wor se (i.e. less) in 5% of cases. A similar presentation isused by Allpress in Req41, which de scribes meas ureme ntswith a variable chipping rate sounder over short sections ofthree streets in central Bristol, UK . Chip rate was set at 1.25MH z, 2.5 MHz, 5MHz, 10MHz and 20 MHz, with the meas-urements repeated at each rate. A graph is presented based onthe average of measured data, showing the perce ntage of totalmulti-path power used, as a function of combined diversityorder, and show s that a maximum of three tines utilises 80%or more of the total power even at the highest chip rate. Ov er-all conclusio ns in RefC41 based on the soundin g work andstudies using a CDM A testbed, suggest that with chip ratesbelow 5 MH z there is little advantage in using multi-path di-versity, and that to benefit fiom m ulti-path dive rsity, chiprates should be 10 MHz or greater.In propagation environm entsused in this paper, it appe ars thatthere is no b enefit in using multi-path diversity with narrowband 1.25 MHz CDMA systems. To benefit from diversity,bandwidths of at least 4 MH z are indicated. This may not bethe case in some American urban environments, where veryhigh rms delay spreads of the order of lOuS have been report-ed Fef .51, and where multipath diversity may benefit evennarrow band 1.25MHz CDMA systems. How ever, eventhough urban Sydney look s very sim ilar to a typical high riseAmerican city, the Australian measurements used in this pa-per have not produced r m s delay spreads greater than app rox-imately 2uS.Tabulating the -1dB evel from Figures 6 ,7 , and 8 to give thenumber of t ines required to keep P d g r e a t e r han -1dB (o r80% of total profile power) in 95% of cases, gives Table 2.

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Table 2:Num ber of tines required to give pUed -1 dl3 in 95% of cases 1

1

Melbourne a = 1Adelaide a = 1Sydney a = 2

TINENUMBER MODEL

1.25MHz 2.5MHz 4 M H z 8 M H z 1 6 M H z

4~

1 2 2 3

2 3 4 6

A very sim ple empirical model results fiom the ab ove table,and serves as guideline for the num ber of tines which shouldbe considered for internal diversity in a CD MA RAKE re-ceiver.Where a s an environment factor, with a = 1 or medi-um dispersion environments, such as suburban areas, anda = 2 for higher dispersion chan nels, such as downtownhigh-rise city areas with a high base station an tenna, andBw,, is the system bandwidth in MHz, the number oftines Nti,, (rounded to the nea rest integer) is given by:

Applying this gives the results shown in Table 3:Table 3:

number of tines fiom Simple Model3 ims

Systemsin Japan, ACTS Mo bile Telecomm unicationsSum- [41S.A.Allpress, 660ptimising Rate and Di-mit, Granada, Spain, Nov emb er 1996. versity Order for Mobile Cellular DS-CDMA Systems, PhD[2] S.C.Swales et al, A Rigorou s Evalua tion of CDMA thesis, University of Bristol,UK. December 1993.Techniques for Future European Personal CommunicationsSystems,LINK Final Projec t Report, University OfU.K. February 1996.

[5]Jeffrey Jorgensen, Ivica Kostan ic and William Foose, Ap-plication ofc ha nn el Sounding o CDMA PCS Design at 1900MH z, IEEE 47th. VTC, Vo l3, pp .1937-1941, May 1997.

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1.9 ghz measurement-based analysis of diversity power versus the number of rake receiver tines at...

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