Baldauf, Michael, A.

Fügen, Thomas

Knörzer, Sandra

Wiesbeck, Werner

KITopen

Channel Analysis for an OFDM-MISO TrainCommunications System Using Different AntennasSandra Kno¨rzer, Michael A. Baldauf, Thomas Fu¨gen and Werner WiesbeckInstitut fu¨r Ho¨chstfrequenztechnik und Elektronik, Universita¨t Karlsruhe (TH)Kaiserstr. 12, 76131 Karlsruhe, GermanyE-mail: Sandra.Knoerzer@ihe.uka.deAbstract— For the design of an OFDM train communi-cations system it is essential to characterise and considerthe channel parameters. The transmission channel of ahigh-speed train scenario is frequency selective as well astime variant. Thus, delay spread and Doppler spread areinvestigated as crucial parameters for the OFDM systemperformance. Using a ray-tracing tool realistic impulseresponses of the transmission channels are simulated. Theinvestigated system includes three base stations operatingin common frequency mode along a railway track andone mobile station situated on a high-speed train. Forthe mobile station different antenna patterns are includedin the simulation model. The results are compared andassessed with respect to delay spread, Doppler spread andreceive power. When using directional antennas a distinctreduction in Doppler spread is achieved.Index terms — OFDM, high-speed trains, wave propa-gation, ray-tracing, Doppler shift, Doppler spread, direc-tional antenna.I. INTRODUCTIONA communications system based on Orthogonal Fre-quency Division Multiplexing (OFDM) [1] is a promi-sing approach to provide a high data rate in a mobileenvironment [2]. The multi-path propagation of a trans-mitted signal leads to delay times that may cause inter-symbol interference (ISI). This ISI can be avoided if theguard interval of an OFDM symbol is longer than thesignificant part of the impulse response of the transmittedsignal [3], [4]. Moreover, moving mobile stations causeDoppler shift and Doppler spread within an OFDMspectrum, which can result in inter-carrier interference(ICI) [5], [6]. To diminish the ICI the subcarrier spacingof the OFDM system has to be chosen large enough. Asthe constraints against ISI and ICI reduce the useful datarate, a trade off has to be made between allowed ISI, ICIand the data rate.Thus, to design a high-speed train communications sy-stem the channel properties need to be considered care-fully. E.g. with a high Doppler spread of the channel thesubcarrier distance of the system has to be large. If theDoppler spread can be diminished, e.g. by a directionalantenna, the subcarrier distance can be reduced. Thisresults in a higher number of subcarriers within the samebandwidth and therefore a higher data rate of the system(assuming a constant bit error rate).In this paper a channel model for high-speed trainsis used to analyse the characteristic channel parame-ters, especially the reduction of the Doppler spread. InSect. II the details of the model are explained. Twotypical environments are introduced as scenarios. Thewave propagation is simulated using a 3D ray-tracingtool (cf. Sect. III). In Sect. IV the channel parametersfor two different antennas at a mobile station and forthree base stations in common-frequency mode (MultipleInput Single Output, MISO) are presented.II. SIMULATION SCENARIOThe environment of high-speed trains is representedby two typical scenarios. The first scenario is callednoise barrier scenario and includes a concrete floor, twopairs of metallic railway tracks, several pairs of metallicpylons and a concrete noise barrier on each side of thetrack. The second scenario is the vegetation scenario inwhich the noise barriers are replaced by trees on bothsides of the track. The trees are modelled by rectangularboxes with scattering wave propagation characteristics.The distance to the railway tracks, the length, width andheight of the boxes are statistically generated (Gaussiandistribution).Fig. 1 shows a schematic 2D drawing of the basicsimulation environment without noise barriers or trees.The overall length of the scenario is 2000 m. Along therailway track every 65 m a pair of pylons is placed facingeach other. The train model complies with the Germanhigh-speed train Inter-City Express (ICE) with a lengthof 205 m. On top of the train the mobile station antennais situated. The distance from the rear part of the train1-4244-0264-6/07/$25.00 ©2007 IEEE 809Authorized licensed use limited to: IEEE Xplore. Downloaded on April 23, 2009 at 04:22 from IEEE Xplore.  Restrictions apply.EVA-STAR (Elektronisches Volltextarchiv – Scientific Articles Repository) http://digbib.ubka.uni-karlsruhe.de/volltexte/1000011407 First published in:  2000 m1500 m1000 m800 m500 m0 mBSBS PylonBS65 mICE100 m MSFig. 1. Basic scenario: On top of a concrete floor, two pairs of railway tracks and every 65 m a pair of metallic pylons (vertical bars)are placed. Three base stations (BS) are situated at 500 m, 1000 m and 1500 m. The high-speed train (ICE) moves from 800 m to 1000 m(measured at the rear part of the train). The mobile station antenna (MS) is positioned at the top of the train with a distance of 100 m to therear part of the train.to the antenna position measures 100 m.The presented OFDM communications system is inten-ded to work in common-frequency mode. State-of-the-art train traffic control systems are able to preciselylocate the train position which can then be used todetermine and switch the relevant base stations. Only thebase stations next to the train send synchronised OFDMsignals, e.g. the nearest three base stations. Hence, inthe modelled scenario three vertically polarised omnidi-rectional antennas are placed as base station antennas at500 m, 1000 m and 1500 m.In the following, first an omnidirectional and then adirectional antenna are integrated as antenna for themobile station in the channel model. Both antennas arevertically polarised. As directional antenna pattern asin(x)/x characteristic is considered. This pattern typeoccurs as array factor in linear antenna arrays withuniform amplitude distribution [7]. The pattern data inazimuth and elevation (cf. to Fig. 2) is included in the 3Dray-tracing simulation and considered when calculatingthe channel parameters. In the simulation its main lobeis adjusted into the direction of travel with an antennagain of 11.2 dBi. All channel parameters given in thispaper represent the channel including the base stationand mobile station antenna patterns.(a) (b)Fig. 2. Antenna pattern of the directional antenna in (a) azimuthand (b) elevation.III. WAVE PROPAGATION MODELFor the wave propagation a 3D ray-tracing tool basedon the theory of geometrical optics (GO) and the Uni-form Theory of Diffraction (UTD) is used. The modelincludes modified Fresnel reflection coefficients for thereflection as well as diffraction based on the UTD.Furthermore, scattering from vegetation is added to thissimulation tool. The background and implementation ofthis wave propagation phenomenon in the ray-tracingtool is presented in [8], [9], [10] and [11].The major advantage of a 3D ray-tracing tool is its abilityof wide-band analysis of the channel. The frequencyselectivity (delay spread) and time variance (Dopplerspread) of the channel can be determined. Here, espe-cially the Doppler spread matters as this parameterinfluences the ICI of an OFDM system.So far, there is no frequency allocated for a high-speedtrain communications system. All simulations presentedin this paper are carried out in the 5 GHz ISM band whe-re the IEEE 802.11a wireless LAN standard is operating.IV. SIMULATION RESULTSIn [12] simulations with directional antennas at themobile station and one base station show that a severereduction but also an increase of the Doppler spreadcompared to an omnidirectional antenna is possible. Inthe following, the influence of the antenna at the mobilestation in a common frequency network (represented bythree base stations) is investigated.The simulation scenario is described in Section II. Thetrain is assumed to be a perfect electric conductor (PEC)and to move uniformly with 400 km/h along the railwaytracks. The movement of the train through the scenariois simulated by a sequence of snapshots. Each snapshotrepresents another position of the train in the scenarioand therefore another moment in time. The distancebetween two sequential train positions measures 1 mwhich corresponds to 9 ms difference in the time domain.In the first snapshot the rear part of the train is located810Authorized licensed use limited to: IEEE Xplore. Downloaded on April 23, 2009 at 04:22 from IEEE Xplore.  Restrictions apply.at 800 m, in the last snapshot the rear part of the trainis positioned at 1000 m (cf. to Fig. 1).For the scenarios considered in this paper the Dopplerfrequency of the incident paths at the mobile station fDisfD =vrλ(1)with vr = vICE · cosϕ.Where λ is the wavelength and ϕ denotes the angle ofarrival (AoA) between the direction of incidence of thepropagation path and the direction of travel. vICE and vrrepresent the absolute velocity of the ICE and its relativevelocity with respect to the direction of the incidentpath, respectively. Since the base station antennas andthe mobile station antenna are positioned approximatelyat the same height, the propagation paths vary in a verysmall angular range in elevation. Due to this reason, theelevation component is not further discussed.For an OFDM communications system a low Dopplerspread and low delay spread is desirable. Bearing in mindthe two main directions of the incident paths and thedependency of the Doppler shift on the AoA, directionalantennas can e.g. attenuate paths from the backwarddirection and amplify paths with frontal incidence suchthat the Doppler spread is reduced.In communications systems antennas with nearly omni-directional patterns are widely used at the mobile stationdue to its variable position. Thus, an omnidirectionalantenna is used as a reference antenna for the channelcharacterisation and compared to the behaviour of adirectional mobile antenna.In Figs. 3 to 6 the delay spread and Doppler spreadversus snapshot time for both scenarios are presented.The light green line and dark blue line represent theresults with an omnidirectional antenna and a directionalantenna, respectively.In Fig. 3 the delay spread values of the directionalantenna are distinctly lower than those of the om-nidirectional antenna. Except for snapshot times ofapproximately 0.9 s. In this region the curve of the om-nidirectional antenna experiences a deep dip. The delayspread values fall below those of the directional antennacurve. This is due to the fact that the mobile stationpasses at this snapshot time the base station situatedat 1000 m. For the omnidirectional antenna case thedirect path is very strong at this position and experiencesonly a short delay. Thus, the delay spread declinesfor these snapshot times. However, the notch can notbe recognised for the directional antenna because theantenna pattern with its pattern lobes and nulls weightsthe paths depending on their AoA at the mobile station.When investigating the propagation paths in the snaps-hots for both scenarios it can be seen that the dominantpaths arrive within two main angular ranges at themobile station (see also [12]). There are paths arrivingfrontally and paths that result from backward incidence.As derived in equation (1), the Doppler shift of eachpath depends on its AoA. The frontal incidence yieldsmaximum positive Doppler shift, the rays from the backmaximum negative Doppler shift. With a train velocityof 400 km/h and 5.2 GHz transmit frequency, this leadsto a theoretical maximum value of the Doppler spreadof 4000 Hz.The Doppler spread shown in Fig. 5 reduces considera-bly when using a directional antenna. As already seenfor the delay spread, the Doppler spread values of bothantennas reach the same order of magnitude when theomnidirectional antenna curve reaches a low point. Thisis again due to passing the second base station at 1000 m.Figs. 4 and 6 show the delay spread and Doppler spreadcurves versus snapshot time for the vegetation scenario.In this case, the delay spread for both antennas rangeswithin the same order of magnitude. Nevertheless, theDoppler spread curves exhibit a significant improvement,i.e. lower values, for the directional antenna. Analogue tothe noise barrier scenario the notches for the delay spreadand Doppler spread values appear in the omnidirectionalantenna curves.As mentioned before, some propagation paths are am-plified by a directional pattern, others are attenuated.The question arises if the positive effect of reducing theDoppler spread results in a decreased receive power.In Figs. 7 and 8 the receive power of the omnidirectionaland the directional antenna are plotted for the noisebarrier and the vegetation scenario, respectively. Thetransmit power is 0 dB. It is clearly shown that thedirectional pattern increases the receive power becauseof the antenna gain. In the first part of the simulation(up to approx. 0.6 s) the increase in receive power isapprox. 10 dB compared to the omnidirectional antennawhich corresponds to the antenna gain. I.e. the mainpropagation paths are within the main lobe of the antennapattern. When the mobile station is very close to thesecond base station the receive power is in the sameorder of magnitude as the omnidirectional antenna. Afterpassing the second base station the receive power for thedirectional antenna rises again with a difference betweenthe curves of approx. 5 dB. Therefore, even if some pathsare attenuated by the pattern the overall receive power811Authorized licensed use limited to: IEEE Xplore. Downloaded on April 23, 2009 at 04:22 from IEEE Xplore.  Restrictions apply.0 0.3 0.6 0.9 1.2 1.5 1.82004006008001000120014001600Snapshot time in sDelay spread in ns    omni antenna  dir. antennaFig. 3. Delay spread, noise barrier scenario0 0.3 0.6 0.9 1.2 1.5 1.82004006008001000120014001600Snapshot time in sDelay spread in ns    omni antenna  dir. antennaFig. 4. Delay spread, vegetation scenario0 0.3 0.6 0.9 1.2 1.5 1.8050010001500200025003000350040004500Snapshot time in sDoppler spread in Hz    omni antenna  dir. antennaFig. 5. Doppler spread, noise barrier scenario0 0.3 0.6 0.9 1.2 1.5 1.8050010001500200025003000350040004500Snapshot time in sDoppler spread in Hz    omni antenna  dir. antennaFig. 6. Doppler spread, vegetation scenario0 0.3 0.6 0.9 1.2 1.5 1.8−90−85−80−75−70−65−60Snapshot time in sReceive power in dB    omni antenna  dir. antennaFig. 7. Receive power normalised to transmit power, noisebarrier scenario0 0.3 0.6 0.9 1.2 1.5 1.8−90−85−80−75−70−65−60Snapshot time in sReceive power in dB    omni antenna  dir. antennaFig. 8. Receive power normalised to transmit power, vegetationscenario812Authorized licensed use limited to: IEEE Xplore. Downloaded on April 23, 2009 at 04:22 from IEEE Xplore.  Restrictions apply.TABLE I200m SIMULATION DISTANCE, THREE BASE STATIONStime-average delay spread στ time-average Doppler spread σfD median receive power PR,mednoise barrier vegetation noise barrier vegetation noise barrier vegetationomnidirectionalantenna1077.5 ns 719.7 ns 2889.0Hz 2574.7Hz −79.8 dB −84.3 dBdirectionalantenna641.5 ns 525.3 ns 570.2Hz 356.4Hz −74.1 dB −77.8 dBis still higher (except a very small time span) comparedto the omnidirectional antenna.To compare the channel parameters quantitatively,Table I lists the time-average delay spread, time-averageDoppler spread and median receive power for bothscenarios and antennas. On the one hand, the vegetationscenario is more suitable for communications as thetime-average delay spread is lower. This is due to thefact that there are less propagation paths with multipleinteractions in the vegetation scenario. Also the Dopplerspread is diminished for the vegetation scenario com-pared to the noise barrier scenario. On the other hand,the median receive power is smaller compared to thenoise barrier scenario when using the same antenna.Nevertheless, the median receive power of the directionalantenna for the vegetation scenario is higher than thevalue of the omnidirectional antenna for the noise barrierscenario. Therefore, the use of the directional antennaimproves the median receive power even for the adversescenario.In total, a clear advantage in system performance canbe derived for the directional antenna. The time-averageDoppler spread reduces to a fraction of the value forthe omnidirectional antenna whereas the receive powerinceases. For an OFDM communications system minimalDoppler spread is aspired. If the system parameters areoptimised for the channel the OFDM system allows anincreased data rate when the Doppler spread decreasesand the demanded bit error rate remains constant.V. CONCLUSIONA channel model for high-speed train communicationsyields the relevant parameters for two typical scenarios.In the model a synchronised common-frequency networkwith three base stations and one mobile station (MISO)is simulated. This paper shows for a realistic scenariothat the Doppler spread using a directional antenna isreduced and the receive power is increased compared tothe omnidirectional antenna. These results imply that thechannel impulse responses including the antennas can beoptimised to reduce the ISI and particularly the ICI inOFDM communications systems. Therefore, assuminga fixed bit error rate the system performance can beimproved considerably by smart utilisation of directionalantennas at the mobile station.REFERENCES[1] S.B. Weinstein and P.M. Ebert, “Data transmission byfrequency-division multiplexing using the discrete fourier trans-form,” IEEE Transactions on Communications, vol. 19, no. 5,pp. 628–634, Oct. 1971.[2] M. Russell and G.L. Stuber, “Interchannel interference analysisof OFDM in a mobile environment,” IEEE 42th VehicularTechnology Conference, vol. 2, pp. 820–824, July 1995.[3] M. Engels, Wireless OFDM systems: How to make them work?,Boston Kluwer Academic Publishers, 2002.[4] A. Peled and A. Ruiz, “Frequency domain data transmissionusing reduced computational complexity algorithms,” IEEEInternational Conference on Acoustics, Speech, and SignalProcessing, vol. 5, pp. 964–967, Apr. 1980.[5] S. Vogeler, L. Broetje, P. Klenner, V. Kuehn, and K.D. Kammey-er, “Intercarrier interference suppression for OFDM transmissi-on at very high velocities,” 9th International OFDM Workshop(InOWo), pp. 81–85, Sept. 2004.[6] S. Vogeler, P. Klenner, and K.D. Kammeyer, “Mulitcarriertransmission for scenarios with high Doppler influence,” 10thInternational OFDM Workshop (InOWo), Aug. 2005.[7] C.A. Balanis, Antenna Theory, Analysis and Design, 2ndedition, Chapter 6 and 7, John Wiley & Sons, Inc., 1997.[8] V. Degli-Esposti, “A diffuse scattering model for urban pro-pagation prediction,” IEEE Transactions on Antennas andPropagation, vol. 49, no. 7, pp. 1111–1113, July 2001.[9] S. Kno¨rzer, J. Maurer, S. Vogeler, K.-D. Kammeyer, andW. Wiesbeck, “Channel modeling for a high-speed train OFDMcommunication link supporting high data rates,” InternationalConference on ITS Telecommunications, pp. 333–336, June2005.[10] J. Maurer, S. Kno¨rzer, and W. Wiesbeck, “Ray tracing in richscattering environments for mobile-to-mobile links,” Internatio-nal Conference on Electromagnetics in Advanced Applications,pp. 1073–1076, Sept. 2005.[11] T. Fu¨gen, J. Maurer, T. Kayser, and W. Wiesbeck, “Capability of3-D ray tracing for defining parameter sets for the specificationof future mobile communications systems,” IEEE Transactionson Antennas and Propagation, vol. 54, no. 11, pp. 3125–3137,Nov. 2006.[12] S. Kno¨rzer, M.A. Baldauf, J. Maurer, and W. Wiesbeck,“OFDM for multimedia applications in high-speed trains: Chan-nel model including different antenna types,” 3rd InternationalWorkshop on Intelligent Transportation, pp. 77–82, Mar. 2006.813Authorized licensed use limited to: IEEE Xplore. Downloaded on April 23, 2009 at 04:22 from IEEE Xplore.  Restrictions apply.

Channel Analysis for an OFDM-MISO Train Communications System Using  Different Antennas

https://publikationen.bibliothek.kit.edu/1000011407/802337

Channel Analysis for an OFDM-MISO Train Communications System Using Different Antennas

Abstract

Similar works

Full text

Available Versions

KITopen