Angular-spectral antenna effects in ultra wideband communications links

Transcription

1 ULTRA WIDEBAND SYSTEMS TECHNOLOGIES AND APPLICATIONS SPECIAL SECTION Angulr-spectrl ntenn effects in ultr widend communictions links W.Q. Mlik, D.J. Edwrds nd C.J. Stevens Astrct: In trditionl pproches to ntenn chrcteristion, spectrl nd ngulr dispersion re modelled seprtely. In the pper, the dependence of the power rdited from ultr widend (UWB) ntenns jointly on frequency nd direction is estlished experimentlly. It is demonstrted tht oth omni-directionl nd directionl ntenns typiclly exhiit higher directivity with frequency, with the former experiencing seven-fold increse with respect to the lowest frequency in the FCC UWB frequency rnge ( GHz). The consequences of this ehviour re highlighted nd the effect on the communictions link is quntified. The effective ville ndwidth of system is found to e highly sensitive to the ngulr disposition of the ntenn t ech end of the link. It is shown tht the ndwidth is severely limited in some directions, nd creful orienttion of the ntenns is required to chieve full UWB opertion for rnge of ntenn designs. The 1 db ndwidth of verticlly polrised UWB ntenn cn e s little s 2 GHz in the equtoril plne when the joint dispersion is considered. The distortion cused to UWB signl is investigted nd significnt vrition in the rdited signl wveform with ngle is demonstrted. 1 Introduction Ultr widend (UWB) communictions technology provides unprecedented dt rtes over short distnce supporting high cpcity links nd lrge user densities [1]. With other fvourle chrcteristics such s unlicensed spectrum usge, low power consumption, nd little multiuser interference, it holds gret promise for some next genertion communictions pplictions. UWB systems re chrcterised y their lrge frctionl or solute ndwidths [2]. The current Federl Communictions Commission (FCC) regultions in the US provide for 7.5 GHz nd for unlicensed indoor wireless communictions [3]. This ndwidth fr exceeds the coherence ndwidth of the indoor rdio chnnel, resulting in frequency-selective ehviour [, 5]. The ntenn is n integrl prt of wireless system nd well-designed ntenn is criticl for system performnce. UWB ntenn design requires creful modelling of its performnce s prt of the communictions link. Mny types of UWB ntenns hve een proposed in literture, nd designs such s the iconicl, owtie, d-dot, log-spirl, Vivldi, TEM horn nd ridged circulr horn re mong the most populr [ 11]. Trditionl ntenn chrcteristion is centred round fundmentl ntenn prmeters relted to the input chrcteristics nd rdition ptterns [12]. While this is usully sufficient for nrrownd ntenns, severl issues rise in the cse of UWB ntenns tht merit deeper nlysis. The end-to-end signl distortion introduced y the r IEE, 2 IEE Proceedings online no. 25 doi:1.19/ip-com:25 Pper first received 31st Jnury nd in finl revised form 29th August 25 The uthors re with Deprtment of Engineering Science, University of Oxford, Prks Rod, Oxford OX1 3PJ, UK E-mil: wsim.mlik@eng.ox.c.uk trnsmitter nd receiver UWB ntenns is effectively filtering opertion such tht the ntenn response is convolved with the signl wveform. The physicl size nd shpe of the ntenn lso ffects its time-frequency response. For exmple, log-periodic ntenn disperses UWB signl s the low frequency signl components re lunched erlier thn the high frequency components, where the connection to the trnsmission line is t the rod end [13]. In such cse, the phse centre of the ntenn trnsltes with frequency resulting in temporl distortion [1]. This form of distortion cn e descried in terms of the group dely through time-domin trnsient response nlysis [15, 1]. Aprt from these, the frequency dependence of the rdited power s defined y the Friis trnsmission eqution [17] for freespce propgtion lso cuses wveform distortion owing to non-uniform power flux density cross the UWB nd. After trnsmission nd reception through the ntenns, the UWB signl power spectrum slope is determined y the ntenn comintion [1, 19]. This cn e interpreted s differentition or integrtion opertion in the time domin [2], introducing phse shifts nd wveform distortion in the signl [21], nd is thus highly detrimentl to the performnce of oth singlend nd multi-nd UWB communictions links [22]. The chrcteristion of frequency dependence of vrious ntenn prmeters is thus of prmount importnce for UWB system design. Besides spectrl nd temporl dispersion, ngle is nother domin where the ehviour of n ntenn cn hve significnt impct. This is prticulrly importnt for indoor communictions systems such s wireless personl re networks, where dense multipth propgtion gives rise to wide rnge of ngles of rrivl [23 25]. In the sence of the theoreticl isotropic rditor, rodcst trnsmitter using n omni-directionl ntenn is expected to rdite the signl in ll directions within the zimuthl plne with symmetric IEE Proc.-Commun., Vol. 153, No. 1, Ferury 2 99

2 gin nd group dely such tht the wvefronts hve neruniform intensity nd plnr shpes. In prctice, however, this condition is sometimes violted y UWB ntenns, cusing look-ngle dispersion [1]. Insted of nlysing the frequency nd look-ngle distortion seprtely, this pper jointly investigtes the ngulr nd spectrl dispersive effects of UWB ntenns. The vrition of ntenn rdition ptterns with frequency is first estlished with the simple exmple of dipole ntenn, nd is then evluted experimentlly using discone [2, 27] nd Vivldi [2 31] ntenns tht represent the generl cses of omni-directionl nd directionl UWB ntenns respectively. To quntify the rdition pttern vrition with frequency, the ntenn em re nd directivity re clculted s functions of frequency. The effect of this ehviour on the ville system ndwidth is quntified in terms of the effective ngulr ndwidth, nd the distortion cused to generic UWB wveform is evluted. While the detils of the dispersive ehviour will vry from ntenn to ntenn, the results of this pper illustrte the phenomenon nd highlight its consequences without loss of generlity. 2 Dipole rdition ptterns The dipole is inherently nrrownd ntenn ut is representtive of mny other, more complex ntenn shpes [12]. Also, its field equtions re nlyticlly trctle nd well understood. On the sis of its compct, simple nd economicl design, the dipole hs een suggested for use in short-rnge UWB trnsmission [32]. For verticlly oriented dipole, the elevtion plne fr-field rdition pttern cn e expressed in terms of the ngle of elevtion y nd frequency f s F ðy; f Þ¼ cos kl cos 2 kl sin y 2 cos y ð1þ where l is its length, k ¼ 2p/l is the wvenumer nd l is the wvelength. For.1 m long dipole, the power rdition pttern vries significntly within the FCC-llocted UWB nd ( GHz). This is shown in Fig. 1, where the normlised elevtion plne rdition ptterns t four equidistnt frequencies within tht nd re plotted. An increse in operting frequency leds to higher directivity nd n increse in the numer of loes. The directions of the loes lso chnge with frequency, incresing in elevtion. This oservtion points to the importnt prolem of frequency-dependent ntenn rdition in UWB communictions. The following Sections proe the existence of similr phenomen in some specilised UWB ntenns nd evlute their performnce for the trnsmission nd reception of signl comprising lrge frequency content. 3 Antenn mesurements The discone is populr choice of ntenn for omnidirectionl, linerly polrised systems. With conveniently mnufctured design nd ndwidth exceeding decde [27, 33], it is populr for conducting UWB chnnel mesurements in the lortory [3, 35]. The rdition pttern of the discone is similr to electriclly smll dipole, iconicl, nd owtie ntenns [3]. For directionl propgtion, perture ntenns such s the Vivldi nd vrious horn designs cn e used. The Vivldi ntenn, f = 3.1 GHz f =.1 GHz n ellipticlly tpered slotline structure, hs een shown to yield good rodnd performnce [31]. A pir of identicl Vivldi ntenns ws mnufctured using photolithogrphic process, while the discones were mde y mchining. Ech discone ntenn, mde from luminium, consisted of hollow cone nd lrge circulr ground plne, with its geometry descried y the cone length l ¼.5 m, cone hlf-ngle c ¼ 51, nd ground plne dimeter d ¼.2m,sshowninFig.2. The lnced ntipodl Vivldi ntenn, shown in Fig. 2, hd dimensions of d ¼. m nd l ¼.1 m. An ellipticlly tpered slotline structure, it hd triplnr form with two lyers of dielectric sndwiched etween three lyers of duroid sustrte. Both pirs of ntenns were extensively chrcterised in n nechoic chmer. A vector network nlyser (VNA) wsusedtomesurethes 11 ( f ) prmeter of the scttering mtrix S. The VNA ws clirted prior to the mesurement to remove the ttenution, dely nd phse distortion of the cles nd connectors. The S 11 ( f ) is used to derive the reflection coefficient G( f ), which yields the ntenn return loss in logrithmic scle often used to chrcterise the rdition performnce of n ntenn over given frequency nd. Figure 3 shows the reflection loss of the four ntenns. For the discone ntenns, the reflection loss lies well elow 1 db for the entire UWB nd, i.e. f l ¼ 3.1 to f h ¼ 1. GHz. This is lso true for the Vivldi except for smll rnge of frequencies ner 9 GHz, owing to which the rdition efficiency of these Vivldi ntenns is higher in the lower portion of the UWB nd. Furthermore, the corresponding ntenns hve similr G( f ), which confirms tht the ntenns of ech type re identicl. The power rdition ptterns were mesured in the nechoic chmer using the setup shown in the pln view in Fig.. A set of n f ¼ 1 discrete equidistnt frequencies within the UWB nd were mesured with continuouswve signl genertor nd power metre, providing frequency resolution of D f ¼ 5 MHz. Both ntenns were plced on electromgnetic sorent locks to prevent ny f = 5. GHz f = 1. GHz Fig. 1 Normlised elevtion plne field rdition ptterns of verticlly polrised dipole ntenn t vrious frequencies, with the circulr xis signifying the vrition long the elevtion 33 1 IEE Proc.-Commun., Vol. 153, No. 1, Ferury 2

3 nechoic chmer l Ψ receive ntenn 2.5 m trnsmit ntenn d to power meter to signl genertor Fig. 2 Construction of UWB ntenns Discone ntenn with lrge circulr ground plne Blnced ntipodl Vivldi ntenn reflection loss, db reflection loss, db d Discone 1 Discone Fig. 3 Mesured reflection loss Discone Vivldi ntenns ground reflections, nd were seprted y distnce of r ¼ 2.5 m. The trnsmitted power ws P t ¼ 15 dbm, nd low-noise mplifier with 3 db gin ws connected to the receiving ntenn. The trnsmitting ntenn ws fixed t one position while the receiving ntenn ws plced on computer-controlled turntle nd rotted in the elevtion, y, nd zimuth, j, plnesfrom1 to 31 with ngulr resolution Dy ¼ Dj ¼ 11. Consider the rottion of the receiving ntenn y n ngle r while the trnsmitter is sttionry t t ¼ 1, with Vivldi 1 Vivldi 2 l Fig. Antenn rdition pttern mesurement in n nechoic chmer the ntenn seprtion r kept constnt. According to the Friis eqution [17], the received power is function of the product of the ntenn gins P r ð; f Þ¼G t ð t ; f ÞG r ð r ; f Þ ð2þ where G t ( t, f )ndg r ( r, f ) re the gins of the trnsmitting nd receiving ntenns respectively, l is the wvelength corresponding to f, nd the trnsmitted power nd freespce loss terms hve een neglected. With one sttionry ntenn, the received power cn e represented y P r ð; f Þ¼Gð; f ÞGð; f Þ ð3þ where G t (, f ) ¼ G r (, f ) ¼ G(, f ) for the identicl ntenns. It is strightforwrd to see tht p Gð; f Þ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P r ð; f Þ ðþ so tht the gin of single ntenn-under-test cn e derived from the received power s Gð; f Þ¼ P rð; f Þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð5þ P r ð; f Þ This procedure is used to otin the multi-frequency rdition ptterns, P(y, j, f), of the ntenn in the elevtion nd zimuth plnes. Normlistion with the highest received signl intensity removes the effect of the rditorsensor seprtion nd provides the rdition ptterns used susequently, i.e. Pðy; j; f Þ P n ðy; j; f Þ¼ ðþ supfpðy; j; f Þg y;j UWB ntenn rdition ptterns To develop n insight into the rdition chrcteristics of these UWB ntenns, the power rdition ptterns of the discone nd Vivldi ntenns re studied in this Section. The discone is linerly polrised, circulrly symmetric nd omni-directionl in the zimuthl plne. The fr-field elevtion pttern of thin discone resemles tht of monopole, ut for lrge vlues of the cone hlf-ngle c, the presence nd numer of ny sideloes depends on c. The electricl length of the cone k l ¼ 2p/l determines the cutoff frequency nd the elevtion plne em pttern. The ground plne introduces slnt to the IEE Proc.-Commun., Vol. 153, No. 1, Ferury 2 11

4 elevtion pttern depending on the electricl dimeter k d ¼ 2p/d nd frequency f. The Vivldi ntenn is stripline-to-freespce trnsformer with very rodnd input mtch. Its rdition pttern, however, is expected to vry considerly with frequency s the phse centre shifts within the structure. The dvntge of the design is tht mximum gin is mintined in the oresight direction. The lower cutoff frequency of the Vivldi is determined y the flre ngle, such tht the perture is hlf wvelength wide t tht frequency. Figure 5 shows the normlised, co-polr power rdition ptterns of the discone nd Vivldi ntenns in the elevtion nd zimuth plnes cross the UWB nd. According to the definition of the ngles in the Figure, the discone elevtion oresight is nominlly t 791, nd for the Vivldi it is t 1. Keeping in view the rnge of the rdited power, the imges re thresholded to lower level of 15 db. From Figure 5, the discone ntenn hs n elevtionplne rdition pttern tht evolves with frequency. There is very low power when the two discone ntenns re perpendiculr to ech other, s expected. Towrds the lower end of the nd, lrge, well-defined minloe is oserved close to the equtoril direction. Considerle rdition is lso present t wide rnge of ngles owing to the flow of current to the edges of the ground plne, since de2l t low f. With incresing frequency, the oresight pttern contrcts nd trnsltes, while the ckplne propgtion vnishes. The most noticele chnges close to the GHz, eyond which the em diverges, nd the minloe continues to nrrow. For lol, the ntenn strts ehving like multiple-wvelength monopole insted of qurter-wve monopole, resulting in nrrower ems nd n increse in the numer of sideloes nd nulls [12]. The sideloes re produced y the reflection of the wve from the discontinuity t the fr end of the finite cone s explined in [37], where the reltionship of ntenn dimensions with emwidth, em tilt nd sideloe level is discussed for finite, lrge-ngle conicl ntenn without ground plne. The discone s zimuth rdition pttern in Fig. 5 is uniform with the elevtion ngle s expected for n omni-directionl ntenn, ut there is considerle vrition with f owing to the non-uniform return loss nd ntenn efficiency. The Vivldi ntenn s elevtion pttern in Fig. 5c nd zimuth pttern in Fig. 5d oth show strong oresight gin. power power db elevtion ngle, deg zimuth ngle, deg power db power db elevtion ngle, deg c zimuth ngle, deg d db Fig. 5 Evolution of UWB ntenn rdition ptterns with frequency Discone, elevtion plne Discone, zimuth plne c Vivldi, elevtion plne d Vivldi, zimuth plne 12 IEE Proc.-Commun., Vol. 153, No. 1, Ferury 2

5 At the lower end of the spectrum there is pprecile ckplne propgtion, especilly in the cse of the elevtion pttern which is nerly omni-directionl up to the centre frequency. The desired ngulr discrimintion is chieved only t high frequencies. The elevtion minloe nrrows with incresing f nd the sideloes vnish. The zimuth pttern hs more ripples thn the elevtion pttern, indicting greter lekge through sideloes, which decy t lower frequencies for most ngles. Thus the elevtion rdition pttern of the discone ntenn is strongly dependent on frequency. The ntenn s frequency properties t one look-ngle re strkly different from those t nother. Alterntively, the frequency components re rdited non-uniformly in different directions. In typicl indoor wireless ppliction, terminl using the discone ntenn in verticlly polrised configurtion held y user sitting down would experience different frequency response, nd thus performnce, thn while stnding. Also, it is common to plce the trnsmitting nd receiving ntenns in the horizontl plne. In such sitution, signl with only suset of the frequencies will e lunched in the ntenn plne, while wider rnge of frequencies might e ville t steeper ngle. All of these effects will cuse wveform distortion nd power loss in single-nd UWB systems, while in multi-nd systems, the ntenn efficiency in given su-nd will vry sustntilly with the look-ngle. 5 Rdition properties The ntenn emwidth nd directivity cn e used to quntify the ngulr-spectrl vrition of UWB ntenns s oserved in the previous Section. The minloe emwidth cn e evluted in terms of the em solid ngle O A ðf Þ¼ ¼ Z 2p Z p Z 2p Z p P n ðy; j; f ÞdO P n ðy; j; f Þ sin ydydj ð7þ Figure shows the em solid ngle of the discone nd Vivldi ntenns. Both hve lrge em solid ngle t low frequencies, lying in the region of 2 3 sr, which flls exponentilly with frequency in nerly identicl fshion for the two ntenns reching.1 sr t f h. Even though the em solid ngle of the Vivldi remins smller, the discone lso shows smll O A for lrge f, ehving like directionl ntenn. This oservtion is confirmed y the vrition of the directivity, defined s Dðf Þ¼ p ðþ O A ðf Þ For the discone, the directivity strts with 3 dbi t f l nd increses with frequency to 23 dbi t the upper end of the UWB spectrum, s shown in Fig.. From this result, it is cler tht the discone does not ehve s constnt-gin ntenn over the UWB frequency rnge. Indeed, the discone is even more directive thn the Vivldi in the midnd region. The incresed directivity of the discone in the upper nd hs the positive consequence of extending the link s long s the trnsmitting nd receiving ntenns re oriented long the min em. However, this vrition in directivity cn lso hve significnt impct on the trnsmitted power. The system designer must scertin tht the trnsmitted signl power t ny frequency nd ngle does not exceed the EIRP regultory limits s result of vrile directivity. em solid ngle, sr directivity, dbi The ove results estlish tht omni-directionl UWB ntenns do not exhiit constnt gin over the UWB nd. Wveform distortion The effect of ntenns on UWB system performnce cn e quntified y defining suitle figure of merit for the qulity of the communictions link. We define ngulr ndwidth, B n (y, j), s the n db ndwidth t ngulr coordintes (y, j), where n is suitle threshold commonly tken s 3 or 1 compred to the pek of the rdition pttern. This is the effective ndwidth perceived y terminl in the direction specified y (y, j), nd is direct mesure of the ntenn ngulr-spectrl dispersion. Figure 7 shows the elevtion plne ngulr ndwidth B n ðy; jþj j ¼ constnt for the discone nd Vivldi, with the full system ndwidth B mx ¼ 7.5 GHz lso mrked for reference. Most noticely, there is only very smll window of y to pproch the full B 1 (y, j), close to y ¼ 51.Thisngle is significntly displced from the equtoril plne, y ¼ 91, usully deemed idel for such omni-directionl ntenns, t which the hlf-power ndwidth B 1 (y, j) islessthn 2 GHz. The Vivldi ntenn retins third of its mximum B 1 (y, j) t ll vlues of y, which mounts to prtil spectrl lekge in undesired directions. Figure illustrtes the mesured ngulr trnsfer functions of the discone ntenn, given y p jhðy; j; f Þj ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P n ðy; j; f Þ ð9þ t y ¼ {1, 31, 1, 91}. The lest ttenution nd reltive uniformity in the trnsfer function is oserved t y ¼ 1, which grees with the conclusions drwn from Fig. 7, nd therefore the lest spectrl distortion will e cused to the signl rdited t tht ngle. The edge reflections in finite ntenn cuse rdition of multiple trnsients whose seprtion vries with the look ngle [3]. This cn e overcome with the use of tpered resistive [39] or cpcitive [] loding. Edge reflection nlysis, however, is not included in this tretment. The response of the ntenn to signl, s(t), is filtering opertion given y Rðy; j; f Þ¼SðfÞHðy; j; f Þ ð1þ Discone Vivldi Discone Vivldi Fig. Vrition of em solid ngle nd directivity of UWB ntenns with frequency Bem solid ngle Directivity IEE Proc.-Commun., Vol. 153, No. 1, Ferury 2 13

6 ndwidth, GHz ndwidth, GHz db db 1 db reference 3 db db 1 db reference elevtion ngle, deg Fig. 7 Effective ngulr ndwidth of discone nd Vivldi ntenns in the elevtion plne The reference line indictes the full 7.5 GHz ndwidth of UWB signl occupying the FCC UWB nd, nd y is defined s in the sphericl coordinte system Discone Vivldi where h(y, j, t) represents the directionl impulse response of the ntenn, relted to the ntenn trnsfer function in (9) through the Fourier trnsform, nd x is the integrtion vrile. Figure 9 shows the originl monocycle s(t) with its 3 db ndwidth stisfying the FCC UWB spectrl msks, while Fig. 9 through d show the distorted wveforms r(y, j, t). It is seen tht discone ntenn lters the pulse shpe depending on the elevtion ngle, nd the wveform t y ¼ 1 hs greter similrity to the trnsmitted (or templte) wveform thn those t 31 nd 91, owing to fltter ntenn response nd wider effective ndwidth t 1 s discussed erlier. mplitude, V mplitude, V power, db 1 1 θ = θ = 3 θ = θ = time, ns c 2 time, ns Fig. 9 Wveform distortion y discone ntenn t vrious elevtions The mplitude is pek-normlised, nd y is defined s in the sphericl coordinte system Originl wveform y ¼ 31 c y ¼ 1 d y ¼ 91 d Fig. Frequency trnsfer function of the discone ntenn t vrious ngles of elevtion, with y defined s in the sphericl coordinte system where S( f ) is the Fourier trnsform of s(t) ndr( f )isthe spectrum of the distorted signl. The effect of ntenn dispersion on UWB signl is illustrted with the help of Gussin monopulse [1] commonly used in impulse rdio systems [2]. The monopulse, lso known s Gussin doulet, is the first derivtive of Gussin pulse with single time-domin zero-crossing nd the functionl form t t2 sðtþ ¼S e 2t t 2 ð11þ where t signifies the time decy constnt nd S the mplitude. The output wveform is given y rðy; j; tþ ¼ Z 1 1 sðt xþhðy; j; zþdx; ð12þ The ngulr-spectrl effects of other UWB ntenns re expected to result in signl dispersion in fshion similr to tht nlysed ove. Also, other UWB wveforms would experience ntenn distortion, nd oth full-nd nd multi-nd systems would e ffected. This includes the orthogonl frequency division multiplexing (OFDM) [2] or direct-sequence [3] modulted signls used in multi-crrier UWB implementtions []. In communictions link, the signl rdited y UWB trnsmitter is filtered y the ntenn sed on its direction-of-deprture. Multipth components of this ntenn-filtered signl, fter trnsmission, my undergo comintion of vrious propgtion mechnisms, ltering its frequency content [5]. In the indoor propgtion chnnel, lrge numer of multipths rech the receiver [, 3] with wide rnge of directions-of-rrivl (DOAs) [2], cusing the pulse shpe of ech multipth rrivl to e individully ltered depending on its DOA [, 7], nd consequently presenting highly distorted composite signl to the susequent receiver components. Antenn distortion cn thus degrde the signl-to-noise rtio (SNR) t the receiver considerly. In UWB signls tht lredy operte t low SNR, this cn drsticlly increse the outge proility nd symol error rte, reduce the cpcity, nd limit the coverge rnge. 1 IEE Proc.-Commun., Vol. 153, No. 1, Ferury 2

7 7 Conclusion It hs een shown tht the ngulr rdition properties of UWB ntenns vry sustntilly over the frequency nd. For the verticlly polrised, omni-directionl discone ntenn, the low frequency components of UWB signl re rdited lmost isotropiclly, midnd frequencies re rdited in well-defined oresight pttern in the elevtion plne, while high frequencies experience em splitting nd slnting. With the directionl Vivldi ntenn, there is strong low- nd medium-frequency lekge off the oresight while the high frequency components chieve the desired directivity. As reciprocl phenomenon, the frequency content of the signl rdited y these UWB ntenns is function of the look ngle. This cn result in unpredictle system performnce in typicl short-rnge wireless network, in which the loction of the moile terminls is generlly unknown nd vrying. This chrcteristion provides evidence for the existence of ngulrspectrl signl dispersion resulting from ntenns, nd it is estlished tht the ntenn chrcteristics vry jointly with frequency nd direction. The oresight emwidth of the discone ntenn flls exponentilly with frequency, the em solid ngle decreses from 3.3 sr to.1 sr cross the FCC UWB nd, nd its directivity increses from 3 dbi to 23 dbi. A comprison of the vrition of the discone nd Vivldi em solid ngle nd directivity proves tht n omni-directionl ntenn does not provide constnt gin cross the nd. The effective ngulr ndwidth of the ntenns is found to e sensitive to direction. The 1 db ndwidth of verticlly polrised discone ntenns is less thn 2 GHz in the horizontl plne, severely ffecting the opertion of full-nd UWB system. Pulse shpe distortion with look-ngle is lso oserved s consequence of ndwidth vrition owing to ntenn dispersion. These results estlish the vriility of ntenn rdition properties over the UWB frequency rnge nd underline the significnce of this effect in system design nd modelling. Acknowledgments This work ws supported y the Engineering nd Physicl Sciences Reserch Council, UK, under grnt GR/T2179/1. The uthors re grteful to Dr. Ben Allen nd Dr. Dominic O Brien for their vlule comments nd suggestions. 9 References 1 Win, M.Z., nd Scholtz, R.A.: Chrteriztion of ultr-widend wireless indoor chnnels: communiction-theoretic view, IEEE J. Sel. Ares Commun., 22, 2, (9), pp Win, M.Z., nd Scholtz, R.A.: Impulse rdio: how it works, IEEE Commun. Lett., 199, 2, (2), pp Revision of Prt 15 of the Commission s rules regrding ultrwidend trnsmission systems: First report nd order, Federl Communictions Commission, Wshington, DC, USA FCC 2, 1 Fe. 22 Hshemi, H.: The indoor rdio propgtion chnnel, Proc. IEEE, 1993, 1 5 Mlik, W.Q., Edwrds, D.J., nd Stevens, C.J.: Optiml system design considertions for the ultr-widend multipth chnnel. Proc. IEEE Veh. Technol. Conf., Dlls, TX, USA, Sep. 25 Schelkunoff, S.A.: Electromgnetic wves (Vn Nostrnd, Boston, USA, 193) 7 Frr, E.G.: Optimiztion of the feed impednce of impulse rditing ntenns, prt II: TEM horns nd lens IRAs, Sensor nd Simultion Note 3, 1995, s listed t no. 55 on ilio.html Johnk, R.T., nd Ondrejk, A.R.: Time-domin clirtions of d-dot sensors. NIST Technicl Note (NISTTN 1392), Fe Gison, P.J.: The Vivldi eril. Proc. Eur. Microwve Conf., Brighton, UK, Sep Andrews, J.R.: UWB signl sources, ntenns & propgtion. Appliction Note AN-1, Aug Powell, J.: Antenn design for ultr widend rdio. Msters thesis, Msschusetts Institute of Technology, 2 12 Krus, J.D.: Antenns (McGrw-Hill, New York, USA, 19, 2nd edn.) 13 Hrmuth, H.F., nd Sho, D.-R.: Antenns for non-sinusoidl wves - 1: Rditors, IEEE Trns. Electromgn. Compt., 193, EMC-25 1 Schntz, H.G.: Dispersion nd UWB ntenns. Proc. IEEE Int. Workshop Ultr Widend Sys. joint with Conf. Ultr Widend Sys. Tech., Kyoto, Jpn, My 2 15 Sorgel, W., nd Wieseck, W.: Influence of the ntenns on the ultrwidend trnsmissions, EURASIP J. Appl. Signl Process., 25, 25, pp Sorgel, W., Wldschmidt, C., nd Wieseck, W.: Antenn chrcteriztion for ultr widend communictions. Proc. Int. Workshop Ultr-Widend Sys., Oulu, Finlnd, June Friis, H.T.: A note on simple trnsmission formul, Proc. IRE Wves Electrons, 19, 3, pp Schntz, H.G.: Introduction to ultr-widend ntenns. Proc. IEEE Ultr-Widend Sys. Tech., Reston, VA, USA, Nov Mlik, W.Q., Edwrds, D.J., nd Stevens, C.J.: The impct of physicl lyer frontend chrcteristics on ultr-widend rdio. Proc. Int. Conf. Telecom., Cpe Town, South Afric, My 25 2 Hrmuth, H.F.: Antenns nd wveguides for nonsinusoidl wves (Acdemic, New York, 19) 21 Kunisch, J., nd Pmp, J.: Considertions regrding the correltion etween UWB ntenn trnsmit nd receive responses. Proc. URSI Int. Symp. Electromgn. Theory, Pis, Itly, My 2 22 Hussin, M.G.M.: Antenn ptterns of nonsinusoidl wves with the time vrition of Gussin pulse - Prt 1, IEEE Trns. Electromgn. Compt., 19, 3, (), pp Mlik, W.Q., Stevens, C.J., nd Edwrds, D.J.: Synthetic perture nlysis of multipth propgtion in the ultr-widend communictions chnnel. Proc. IEEE Workshop Sig. Proc. Adv. Wireless Commun., New York, USA, June 25 2 Spencer,Q.H.,Jeffs,B.D.,Jensen,M.A.,ndSwindlehurst,A.L.: Modeling the sttisticl time nd ngle of rrivl chrcteristics of n indoor multipth chnnel, IEEE J. Sel. Ares Commun., 2, 1,(3), pp Chong, C.-C., Tn, C.-M., Lurenson, D.I., McLughlin, S., Bech, M.A., nd Nix, A.R.: A new sttisticl widend sptio-temporl chnnel model for 5 GHz nd WLAN systems, IEEE J. Sel. Ares Commun., 23, 21, (2), pp King, R.W.P.: Theory of liner ntenns (Hrvrd Press, Cmridge, MA, USA, 195) 27 Sndler, S.S., nd King, R.W.P.: Compct conicl ntenns for wide-nd coverge, IEEE Trns. Antenns Propg., 199, 2, (3), pp Lngley, J.D.S., Hll, P.S., nd Newhm, P.: Novel ultrwidendwidth Vivldi ntenn with low crosspolristion, Electron. Lett., 1993, 29, (23), pp Fourikis, N., Liouts, N., nd Shuley, N.V.: Prmetric study of the co- nd cross-polristion chrcteristics of tpered plnr nd ntipodl slotline ntenns, IEE Proc., Microw. Antenns Propg., 1993, 1, (1), pp Lngley, J.D.S., Hll, P.S., nd Newhm, P.: Blnced ntipodl Vivldi ntenn for wide ndwidth phsed rrys, IEE Proc., Microw. Antenns Propg., 199, 13, (2), pp Guillnton, E., Duvignc, J.Y., Pichot, C., nd Cshmn, J.: A new design tpered slot ntenn for ultr-widend pplictions, Microw. Opt. Tech. Lett., 199, 19, (), pp Allen, B.: Antenn nd propgtion design chllenges for ultr widend wireless systems. Proc. Loughorough Ant. Propg. Conf., Loughorough, UK, Apr Liu, G., nd Grimes, C.A.: Sphericl-coordinte FDTD nlysis of conicl ntenns mounted ove finite ground plnes, Microw. Opt. Tech. Lett., 1999, 23 3 Ghssemzdeh, S.S., Jn, R., Rice, C.W., Turin, W., nd Trokh, V.: Mesurement nd modeling of n ultr-wide ndwidth indoor chnnel, IEEE Trns. Commun., 2, 52, (1), pp Sulonen, K., Suvikunns, P., Vuokko, L., Kivinen, J., nd Vinikinen, P.: Comprison of MIMO ntenn configurtions in picocell nd microcell environments, IEEE J. Sel. Ares Commun., 23, 21, (5), pp Blnis, C.A.: Antenn theory: nlysis nd design ( John Wiley & Sons, New York, USA, 1997) 37 Adchi, S., Kouyoumjin, R.G., nd Sickle, R.G.V.: The finite conicl ntenn, IRE Trns. Antenns Propg., 1959, 7, (5), pp Mloney, J.G., Smith, G.S., nd Scott, R.: Accurte computtion of the rdition from simple ntenns using the finite-difference time-domin method, IEEE Trns. Antenns Propg., 199, 3, (7), pp Wu, T.T., nd King, R.W.P.: The cylindricl ntenn with nonreflecting resistive loding, IEEE Trns. Antenns Propg., 195, AP-13, (3), pp Ro, B.L.J., Ferris, J.E., nd Zimmermn, W.E.: Brodnd chrcteristics of cylindricl ntenns with exponentilly tpered cpcitive loding, IEEE Trns. Antenns Propg., 199, AP-17, (2), pp IEE Proc.-Commun., Vol. 153, No. 1, Ferury 2 15

8 1 Conroy, J.T., LoCicero, J.L., nd Ucci, D.R.: Communiction techniques using monopulse wveforms. Proc. IEEE Conf. Mil. Comm., Atlntic City, NJ, USA, Oct. 1999, Vol. 2 2 Btr, A., et l.: Multind OFDM physicl lyer proposl for IEEE 2.15 Tsk Group 3, IEEE P2.15-3/2r-TG3, July 23 3 Runkle, P., McCorkle, J., Miller, T., nd Welorn, M.: DS-CDMA: the modultion technology of choice for UWB communictions. Proc. IEEE Ultr-Widend Sys. Tech., Reston, VA, USA, Nov. 23 Aiello, G.R., nd Rogerson, G.D.: Ultr-widend wireless systems, IEEE Microw. Mg., 23,, (2), pp Molisch, A.F.: Ultrwidend propgtion chnnels - theory, mesurement, nd modeling, IEEE Trns. Veh. Technol., 25, 5, (5), pp Crmer,R.J.-M.,Scholtz,R.A., nd Win, M.Z.: Evlution of n ultr-wide-nd propgtion chnnel, IEEE Trns. Antenns Propg., 22, 5, (5), pp Wilson, R.D., nd Scholtz, R.A.: Templte estimtion in ultrwidend rdio. Proc. Asilomr Conf. Signls Systems Computers, Pcific Grove, CA, USA, Nov IEE Proc.-Commun., Vol. 153, No. 1, Ferury 2