capacity analysis for uwb

7/31/2019 Capacity Analysis for Uwb

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CAPACITY ANALYSIS FOR A POWER CONTROLLED UWB INDOOR SYSTEMRomeo Giul iano ' and Franco M azzenga'

'RadioLahs Consorzio Universita Industria, Via del Politecnico 1 , 00133, Rome, Italy;'University ofR om e Tor Vergata, Via del Politecnico 1, 00133, Rome, Italy;email: romeo.eiiiliano(~.uniroma2.itn ~ azzt .n ea~ , i n r .u n i romd ~ . i t ,

ABSTRACTUltra WideBand (UWB) is a potential key technology forthe development of future short-range indoor radiocommunication systems providing very high hit ratesservices, low power consumption and location capabilities.In this paper we provide a semi-analytic approach toevaluate the system outage probability for a master-slavenetwork based on UWB direct sequence code divisionmultiple access scheme. The proposed approach for systemanalysis is quite flexible and allows to easily includerealistic path loss mode ls, power control and multiple accessinterference. The technique has been va lidated by com paringthe sem i-analytic data with those from classical Monte Carlosimulation and obtaining results in ve q good agreem ent.The UW B coverage-capacity tradeoff is also investigated.I. INTRODUCTIONUltra Wide Band (UWB) technology is loosely defined asany wireless transmission scheme with bandwidthoccupancy of more than 25% of the center frequency, ormore than 1.5 GHz [I]. UWB has been identified as apotential key technology to enable the development ofdynamic networks for future short-range indoor radiocommunication systems providing very high bit ratesservices, low power consumption and location capabilities.UWB signals are generated using suh-nanosecond pulsesspreading energy over very large frequency hand. For thisreason UWB spectrum cannot he allocated exclusively andUWB hand have to overlap with many other narrowbandsystems. Thus to guarantee existing systems from UWBemissions the Federal Communications Commission (FCC)an d also the European Telecommunications StandardizationInstitute (ETSI) have practically restricted the UWBoperating hands in the 3.1-10.6 GHz frequency range andregulated UWB power emission defining frequency-powermasks for each UWB applicatioddevice [2]. Depending onthe spectral characteristics of the considered UWB signals,the frequency-power mask limits the maximum UWBterminal transmission power and this should be accountedfor in the calculation of the UWB system capacity. Ingeneral the evaluation of UWB system capacity is animportant research topic and som e results based on classicaltechniques of the information theory have been presented in[3]-[5] for the pulse position modulation UWB operatingover the additive white Gaussian noise (AWGN) channel.Differe ntly from [3]-[5], in this paper w e provide a semi-analytic approach to evalua te the outage probability and then

the network capacity intended as the maximum number ofusers in the system that can he served with a fixed outagelevel. An UWB master-slave network based on UW B DirectSequence-Code Division Multiple Access (DS-CDMA) isconsidered. The outage probability is first obtained in aclosed form equation including unknown randomparameters that in general depend on the network geometryand on path-loss. The statistics of the random param eters arethen obtained by very fast simulation procedure and are usedin the theoretical calculation of the outage probability, Theproposed semi-analytical approach is very flexible andallows to easily include arbitrary network geometries,realistic path loss models, power control and multiple accessinterference from other UWB devices connected to the othermasters in the area. This technique considerably extends theapproach in [ 6 ] which is only applicable to hexagonal andregularly spaced cells. In indoor environment theassumption of regular hexagonal geom etry is not applicablesince masters c an he arbitrarily located over the service area.In this paper only the upstream communication directionwith ideal power control is considered. The paper isorganized as follows. In Section 11, the UWB systemarchitecture and reference scenario are described and aclosed form expression for the outage probability isobtained. In Section 111, theoretical results are obtained andvalidated by comparing them with Monte Carlo simulatedresults. Besides UWB coverage-capacity analysis isintroduced. Finally, conclusions are drawn in Section IV.This work was developed within the IST ULTRAWAVESEuropea n research project.11. UW B OUTAGE PROBABILITYA master-slave UWB system architecture is considered andit is illustrated in Fig.1. The U W B slaves, all transmitting atthe same hit rate, are located over a rectangular service areain accordance to a uniform distribution. The UWB networkincludes N,,, masters regularly spaced on a rectangular grid(see Fig.1). The extension of the proposed approach toirregular geometries is straightforward. Time divisionduplex transmissions are considered and it is furtherassumed that masters and slaves are synchronized so toavoid interference between upstream and downstreamtransmissions. Finally ideal power co ntrol is considered.

0-7803-X523-3/04/$20.0002004 IEEE. 724


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......... .......................s . -. *yn,a * i* M,*wvN-=,j ~ '."'l"-.b...................IC--- -~.---- . ,. . . .

e * .I . . :.Is - p I r +

. . . . . . :". ">-af: n : . . . . . . . . .. ' . ' .a . B

1 ' . 1. ....... .I__.................-5 I I tb I S$7.8Figure 1 - UW B system scenario.

Assuming a reference master in the area _ the outageurobabilitv is defined a s:

where C/ I is the signal to thermal noise plus interferenceratio as measured at the reference master receiver and po isthe target signal to noise ratio which is commonly selectedto account for the degradations due to fast fading effects.The Cil is:~ ( 2 )I I,,,, +I=., + 7where C is the received power level, I;", is the internalinterference due to users connected to the reference UWBmaster, I, is the interference due to U WB slaves connectedto other masters and 7 s the thermal noise power which canalso include the interference power due to narrowbandsystems with bands overlapping the UWB band. Assumingthat slaves served by the reference master are (quasi)-synchronized, lints:

-

( 3 )where a s the fraction of interfering power due to (possible)loss of orthogonality due to multipath, among the N+l UW Bslaves served by the same reference master. By definition0 5 a 5 1 and in the following we consider the worst caseinterference assuming a I . It is not difficult to extend thefollowing formulation to the a


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conditional p.d.fs of the external interference I,, ar erequired . Except in very few cases , due to the complexity ofthe path loss formulation describing the propagationcharacteristics of a realistic indoor environment, it can bevery difficult to evaluate in closed fonn the statistics of therandom variables appearing in (8). For this reason we resortto a fast snapshot based M onte Ca rlo simulation whose maingoal is to evaluate the histograms of the random variablesappearing in the outage probability in (14). Starting from thesimulated histograms a particle approximations of the p.d.fsare then derived and used in (14) to calculate fin",, .g. weapproximate:

m = 0, 1,2, ...,M ( 1 5 )where q,,,re the frequencies of the random variables

obtained after normalization of thecorresponding Monte Carlo histograms with bins' centersnIn,.he particle approximation in (IS) is very usefulallowing to transform integrals into sums. The accuracy ofthe approximation is related to the selection of nln#hat ingeneral are not uniformly spaced. In order to simplifycalculations a unique set for n,,s considered disregardingm.111. RESULTSTo obtain the results reported in this Section the followingone slope propagation model is considered

L(di = mm{MCL, L ,d" + o s w ] . (dB) (16)where MC L indicates the minimum coupling loss, y is thepath loss exponent, w is a normal random Gaussian variablewith zero mean and unit variance and q s the standarddeviation of the shadowing ( 0 5 4 dB is considered). In th efree space propagation case y=2 and us=O dB. The MC L in(16) is calculated assuming free space propagation fordistance d d o .g. MCL+rrdd k 2 where h is the wavelengthassociated to the UWB center frequency; d e l m wasconsidered. The constant L o in (16) is evaluated in order toensure continuity in the path loss model at d = do . Finally,the bins' centers of the histograms used for the particleapproximation .in (15) were nl =ITo+IA [dB], withf l o = - 1 2 0 d B m a n d A = 1 d B , w i th I =O , 1,..., 130.Validation of ih e proposed approachThe proposed calculation procedure was validated with theresults obtained from standard Monte Carlo snapshotsimulation. In Fig.2 we plot the outage probability as afunction of the overall number e.g. NXlmes,f active UWBslaves in the service area. Different values of the marginm,=C/rlpo have been considered.

Lfi.,(xlm) E C K ~ , J ( X - ~ , ) .i=n

I, C W ki=,F ig u re 2 - System outage probability vs number of slaves inthe area; different margin m,, N,,,=lS; refe renc e maste r inthe center of the service area.The curves in Fig.2 have been obtained with UWB slavesactivity fac torp = I and referring to the master in the centerof the service area. The p,,, in (14) are calculated only onceand are used in (13) to evaluate the outage probability fordifferent values of the activity factor p . From (14) Pnrnrapidly tends to unity as n and/or m increase. In general itwas observed that a good approximation of the Po,,,n (13)can be obtained assuming /?,,.,=I when 1 -fin),,


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Figure 3 - System outage probability as function of thenumber of UWB slaves in the area; different values ofN,,,.In Fi g 3 we plot the outage probability referred to the centralmaster for different values of the path loss exponent, y. Inthe ideal power control conditions, capacity increases withthe path loss exponent and this is due to the reduction of theexternal interference power due to other slaves notconnected to the reference master.

N-=>s F 4 d b e . b n h - . , o a srb

F ig u re 5 - System outage probability as function of thenumbe r of UW B slaves in the area for different values of thepath loss exponent y.

N.*ill **=.**........... ........... ........._..2 ..........................: i.....e *-_,_-_-.....'mi ,/...... -., .- F,

. . . . . . . . . . . . .

Figure 4 - System outage probability as function of thenumber of UW B slaves in the area for three different APs inthe area.UW B Coverage-Capacity analysisIn Fig.6 we plot the capacity of the UWB system as afunction of the received signal power threshold C for twovalues of the outage probability. Data in Fig.6 refer to thecentral master (#AP=8)an d to the peripheral master (#AP=l).Furthermore we assumed pp-15 dB and the UW B systembandwidth, Wgfi,~rg=GHz, noise figure, F= 5 dBcorresponding lo a hit rate of 50 Mb/s and the reference&/No =3 dB and 7 -74 dBm. In every case os O dB.From Fig.6 it can be observed that as C increases the systemcapacity is independent of C. This is confirmed in (7) thatbecomes practically independent of C when m, s very large(infinite m the limit).

m-

F ig u re 6 - UWB System capacity vs the received powerlevel C; different values of system outage probability andpath loss exponents; #AP=8 ontinuous lines, #Ap= Idashedlines.

Since the UWB transmission power is limited by the FC Cmasks, the minimum value of C, indicated with C,,,, abovewhich capacity saturates is an important parameter toevaluate the coverage area corresponding to the maximumcapacity. In fact, indicating with P r the maximum UWBtransmitter power, C,,&P, the minimum allowable pathloss L,,, is :L,," = -, (17)C,,"Given the propagation model, L,,n can be related to thecoverage radius of the selected AP. As expected, theincrease in the outage probability requirements can lead to alower value for C,,, and then to an increase in the APcoverage radius R. In Table 1 we indicate the coverageradius of the AP for different system outage probabilityrequirements in the simple case of free space propagationwith os=O dB. Data refer to the central master and

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P F -5.6 dBm. The calculation of R allows to evaluate thepercentage of covered service area. If no coverage holes arepresent the UWB system is optimally designed and weobtain the maximum capacity. Obviously in the presence ofcoverage holes we need to reduce the threshold level Cbelow C,,,,, thus leading to an increase of the maximumtolerable path loss and to a corresponding reduction in theoverall system capacity. The increase in the number of theserving masters in the area may help in reducing the Crequirements. The value of P F -5.6 dBm used to obtain thedata in Table 1 is indicated in the graph plotted in Fig.7where the maximum UWB transmission power is plotted a sa function of the UWB signal center frequency.Table 1 -Coverage radius of the central AP for different

R[m] (y=2) I 39.8 I 22.4 I 7.1Nrim.,@P,,,,=l%,y=2, cr..OdB I 110 I 150 I 165Data in Fig.7 were obtained through computer calculationby varying the position of the UW B signal center frequency.Fo r each center frequency the power spectral density of theUWB signa l was recalculated and positioned inside theFCC frequency-power mask in order to evaluate themaximum transmission power obtainable withouttrespassing the mask limits. The data in Fig.7 refer to th ecase of a bipolar pulse amplitude modulated UWB signalwith baseband Gaussian pulses [7]. Those data also accountsfor the frequency dependent distortion effects due to theantenna o n the transmitted UWB signal IS].

O m I bRb., u s c m , a sI I

Figure 7 Maximum UW B transmission power P7 s afunction of the UW B center frequency,&.

IV . CONCLUSIONSA semi-analytical approach for the calculation of UWBsystem outage probability has been presented. The

calculation procedure allows to easily include arbitrarynetwork geometries, realistic path loss models, powercontrol and multiple access interference. The effectivenessof the proposed technique was tested by comparing theresults with those obtained through classical Monte Carlosimulation. A very good agreement was evidenced. TheUWB system coverage-capacity analysis was discussed interms of the minimum threshold level C,,,, required to obtainthe system capacity and the maximum allowable path loss(e.g. coverage) accounting for the FCC limitations on theUW B transmission power.REFERENCES[I ] J. Foerster, E. Green, S , Somayazulu, D. Leeper (INTELArchitecture Labs), U ltra-Wideband Technology for Shortor Medium-Range Wireless Communications, IntelTechnology Journal Znd Quarter, 2001.[2] Federal Com munications Commission (FC C), Revisionof Part 15 of the Commissions Rules R egarding Ultra-Wideband Transmission Systems, First Report & Order,ET Docket 98-153, FCC 0248; Adopted: February 14,2002; Release d April 22, 2002.[3] M. Z. Win and R.A. Scholtz, Impulse radio: Ho w itworks, lEE E Commun. Lett.,Vol. 2, 1998.[4] L. Zao, A. M. Haim o vich , Cap ac it y o f M - q P P MUltrawideband Communications over AWGN Channel,IEE E ICC 2001,pp.1191-1195.[5 ] 0.Wintzell, D. K. Zigangirov et al. On the Capacity ofa Pulse-Position-Hopped CDM A System, lEEE Trans, onInformation Theory, Vol. 47, No. 6, September 2001.[6 ] K. S. Gilhousen, 1. M. Jacobs e t al. On the Capacity of aCellular CDMA System, IEEE Trans. on VeichularTechnology, Vol. 40, No.2 , May 1991, pp.303-312.[7] M. Hamalainen, V. Hovinen et al. On the UWBcoexistence with GSMYOO, UMTSMiCDMA and GPS,IEE E Journal on Selected areas in Communications, V01.20,No.9, December2002, pp.1712-1721.[SI R. Giuliano, F. Mazzenga, F. Vatalaro: On theInterference between UMTS and UWB Systems, IEEEUWB Systems and Technologies (UWBST03), Sept. 2003,Virginia (US).

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capacity analysis for uwb

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