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Multi-Carr ier Spread-Spectrum and Related TopicsTransm itReceive-Antenna Diversity Techniques forOFDM Systems

A R M I N D A M M A N N ,TEFAN KAISERInstitute of Communications and Navigation, German Aerospace Center (DLR), P.O.Box 1 16, D-82230 Wessling, Germany{Armin.Dammann, Stefan. Kaiser) @DLR.de

Abstract. In this paper, we investigate different antennadivers ityconcepts,which can be easily applied to orthogonalfrequency division multiplexing (OFDM) systems. The focus is on standard compatibility, i.e. these schemes can beimplemented to already existing OFDM systems without changing the standards.The introduced diversity techniques reapplied exemplarily to the DVB-T system. Bit error performance investigations were done by simulation for differentDVB-T and diversity parameter sets.

1 INTRODUCTIONFuture mobile wireless systems are required to pro-vide high data rate services in a spectral efficient man-ner due to the high costs of bandwidth resources, e.g.

w 400,000,000 UMHz for UMTS in Germany. In termsof power efficiency - especially for mobiles - and electro-magnetic pollution it is required t o keep the isotropic radi-ated power as low as possible. Particularly the electromag-netic radiation charge becomes more and more importantfor the acceptance of wireless systems i n society. Wirelesssystems have to operate in different environments. So amobile is expected to work reliably in scenarios like rural,urban, indoor, ou tdoor, etc.Mobile communication systems mainly suffer fromtime-varying multipath fading with extremely differentmultipath intensity profiles [l]. For systems, which haveto work in multipath environments, an improvement in er-ror perform ance may becom e very difficult. Already aslight improvement in the bit error rate can necessitate ahuge amount of additional transmitter power, which con-tradicts the aforementioned item of an economically use oftransmission power. It is an enormous challenge to designwireless communication systems, which are capable to dealwith these vary ing scenarios.Orthogonal frequency division multiplexing OFDM)[2] is a suitable technique for broadband transmission inmultipath fading environments and is implemented in newbroadcast standards like digital audio broadcasting (DAB)[3] or terrestrial digital video broadcasting (DVB -T) [4] aswell as wireless local area network (WLAN) standards [ 5 ]such as HIPERLANl2 or IEEE 802.1 la.Because of the poor error performance of OFDM in

multipath environments, it is necessary for wireless com-munications system s to use techniques like interleaving andchannel coding in addition to OFDM. hese techniquesadd redundancy and diversity in time and frequency di-rection. For many scattering environments, spatial diver-sity is another effective way to improve the error perfor-mance of wireless radio systems. In [6, 73 space-time-coding is proposed in order to get the benefits of chan-nel coding in combination with spatial (antenna) diver-sity. Unfortunately space -time-coding is not suitable forextending existing systems, because this would make nonstandard conform able modifications necessary. T hereforefor standardized systems additional spatial diversity tech-niques can only be implemented, if this modifications keepthe systems standard compatible. In [8] such techniques forthe transmitter sid e are proposed.

In this paper we will investigate standard conformableantenna diversity techniques, which are well suited for theextension of existing standardized OFD M systems. Sec-tion 2 introduces diversity techniques for both transmitterand receiver. A t this, the main id ea is to increase the fre-quency selectivity of the resulting channel transfer functionby specific cyclic delays at the transmitter andor receiverantennas. Th e transmitter sided delay diversity is also in-vestigated in combination with receiver sided maxim um ra-tio combining. It is shown in Section 3 how the mentioneddiversity techniques are applicable to the DVB-T systemin order to improve the bit error performance in multipathenvironments. In Section 4 the DVB-T system and trans-mission parameters as well as the used channel models aredescribed. Finally, simulation results for the bit error ratesare presented for various DVB-T system parameter sets in

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Figure 1: OFDM transmitter with DD

Figure 2: OFDM receiver with D Dindoor and outdoor environments.

2 SPATIALANTENNA DIVERSITY W I T HOFDMIn this section we will introduce Delay Diversity (DD),Cyclic Delay Diversity (CDD), Phase Diversity (PD) and

Maximum Ratio Combining (MRC), hich can easily beapplied to existing OFDM system standards with little ef-forts. A combination of this diversity techniques is eas-i ly possibte. Thus, we can even with hindsight improveOFDM ystems and find an optimal tradeoff between com-plexityand performance.2.1 DELAY IVERSITY

Thissection will briefly introduce delay diversity (DD),which was described in [S]. Figure I shows the block dia-gramof an N-transmitter-antennaOFDM ystem with DD.The OFDM modulated signal is transmitted over N anten-nas, whereas the particular signals only differ in an antennaspecific deiay. Before shifting, an additional cyclic prefixas guard interval may be inserted. The functional blockUC tands for upconversion from the baseband into theRF-band. Note, that in case of DD n = I , . . . N - I,denote simple time shifts.Because of linearity, i t is also possible to implementDD at the receiver side. The appropriate block diagram ofa M-antenna receiver with DD is shown i n Figure 2. Thereceived signals are downconverted ( DC ) into the base-band and shifted in time direction accordins to d ,,,, rn =1. . , 31 1. After the superposition from these signals

is built, the guard interval section is removed and the re-sulting signal is finally transformed into frequency domair(IOFDM).To avoid intersymbol interference I S ) the time delay:S,, 6 must hold the condition6,,6,,, 5 rg-rmu, n = l , . , . , N - l ,

m = , . . . ,Ad-1, (1:where rg is the guard interval length and rmaXenotesthe multipath channel delay spread. For tight dimensionedguard intervals, where rg is only slightly larger than rmmaxEquation 1 strongly restricts the choice of the time delays& S In the next section cyclic delay diversity is intro-duced, which overcomes this problem.2.2 CYC LIC DELAY DIVERSITY

Figure 3 illustrates the difference between DD andCDD in the time domain and shows the transmission of2 consecutive OFDM symbols with their cyclic prefixes asguard intervals. For clarity, the lStubcarrier is plotted as asine wave. The reference signal is undelayed and transmit-ted (resp. received) for both DD and CDD. In the case ofDD it can be seen, that the DD signal is a simple copy of thereference signal, but delayed by 6. It's also observable, thatOFDM ymbols of the DD signal pmly overiap the guardinterval of the subsequentOFDM ymbol in the referencesignal at 6. The result is the above mentioned restriction inthe choice of 6 see Equation 1). In the case of CDD onecan see, that there is no overlapping of CDD OFDM ym-bols with the reference signal OFDM symbols, whereas thet ime signals of DD and CDD in the time section used forOFDM demodulationare totally equal. This makes the per-formanceof CDD equal to DD while there is no IS1 n caseof DD, i.e. Equation 1 holds. Concurrently, no IS1 can oc-cur with CDD due to a too large dimensioned cyclic delay.The OFDM ymbols of the CDD signal can be generatedfrom the reference signal OFDM symbols ust by applyinga cyclic time shift of dCY to the reference signals' OFDMsymbols and subsequent insertion of the cyclic prefix.Figure 4 shows the block diagram of an N-transmitter-antennaOFDM ystem with CDD. The OFDM modulated

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Transmit/Receive-Antenna Diversity Techniques for O FDM Systems

V

Figure 4: OFDM transmitter with CDD.

IOFDM

VFigure 5: OFDM rec eiver wirh CDD.

signal is transmitted over N antennas, whereas the partic-ular signals only differ in an ant enna specific cyclic shift.After cyclic sh ifting, the guard interval is inserted. Again,the functional block UC performs upconversion of thesignals from the baseband into RF-band. Again becauseof linearity CDD can be implemented at the receiver. Fig-ure 5 shows t he principle block diagram of a CD D receiver.Because of simp le time shifts for DD resp. cyclic timeshifts for CDD, these techniques are implementable withonly slig ht additional complexity as long as the delays d;,and ~ 5 : ~ re multiples of the system sampling time. Thenext section introduces phase diversity (PD). which is donein the frequency domain and allows implicit the choice ofarbitrary delays.

2 3 P H A S E DIVERSITYThe equivalence between PD and CDD is a pmp eny ofthe Discrete Fourier Transformation (DFT) and can directlybe seen from the length A IDF T definition

. K - 1

CDDiignal (2)K-1- -where e, k, s(e) and S k ) denote the discrete time,frequency and the complex-valued signals in time- and

I , Subcarrier OFDM SymbolFigure 6 : Indoor charnel snapshot or a ingle antenna system

frequency-domain respectively with -f,k = 0 . . .A 1.As it can be seen from Equation 2, a cyclic delay bCY in thetime domain corresponds to a phase factor of e-3 inthe frequency domain. PD is not only restricted to linearlyincremented phases. It's also possible to choose phase fac-tors e j b k ) ,where d ( k ) is an arbitrary function of the dis-crete frequency k. Equation 2 also sho ws that the operationfor PD ha s to be done before OFDM modulation. So for anM-antenna PD system, hf OFDM transformations have tobe calculated. T herefore the implementatio n of PD is morecomplex compared to CDD.

CDD and PD are independent of the existence of acyclic prefix (guard interval) and are capable to increasethe channel frequency selectivity without increasing theovemll channel delay spread because these operations aredone befom guard interval insertion and are restricted to theOFDM ymbol itself. The effect can be seen, i f we have alook on the overall channel transfer function

l * k 6 c Y

. N - 1where Hn,,(k, e denotes the channel transfer functionfrom the nth transmitter antenna to the mch eceiver an-tenna and S i Y stands for the transmitter antenna specificcyclic delay qy= 0). Figure 6 shows a snapshot ofI H z ( l E , )I? = IHa,m(k, )I? for a single antenna sys-tem over the number of processed OFDM symbols andthe first 5 12 of overall 2048 subcaniers. I H T , ? de-notes the squared absolute value of the resulting channeltransfer function at receiver antenna rn. The OFDM sym-bol duration is Tu = 224 ps. This yields a subcarrierspacing of Afc = 1/Tcr = 4464 Nz In contrast. Fig-ure 7 shows a snapshot of IHF(k,L')l? = IHo,,,,(k, e +e-J*ka:yHl ,m(k, )I? with hfY = 10 (samples) and I< =2048 for a 2-transmit-antenna-system.

For Figures 6 and 7, the Indoor Commercial Chan-nel B channel model, which is briefly described in Sec-tion 4.1, has been used. It can obviously be seen, that an

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-. ,6-40.---:I---S*r* # OFDMSymbol

Figure 7: Indoor channel snapshot or a C D D system.

Q

V I RCFigure 8: OFDM receiver with MRC.

additional transmitter antenn a with CD D increases the fre-quency selectivity, i.e. decreases th e coheren ce bandwidth.i n Section 4.3 simulations will show, that a lower coher-ence bandwidth lead to a better error performance for theconsidered DVB-T system in multipath environment. Inorder to achieve any diversity effects, i.e. to get construc-tive and destructive interference within the OFDM signalbandwidth B , the inserted (cyclic) delay s Y have to fulfill

2.4 MAX I MUM RATIO COMBI NI NG

4)

Another well known standard conformable diversitytechnique is MRC. Figure 8 shows the block diagramof a classical M-bran ch MRC -OFD M receiver. Afterdownconversion into the baseband, the hf time-signals areOFDM demodulated and combined by subcarriers, usingthe MRC scheme, which optimizes the signal-to-noise-ratio (SNR) for each subcarrier and is described e.g. in[ I 71.

3 APPLICATIONO THE DVB-T SYSTEMThe above mentioned techniques can be standardconformably applied to OFDM based WLAN standards(HIPERLANR, IEEE 802.1 l a [ 5 ] ) , broadcasting systems(DAB [3], DVB-T [4]) or Multicarrier-CDMA systems [ 9 ] .In this section we will apply CDD and MRC. introducedin Section 2, exemplarily to the DV B-T system. DVB -Tis basically a coded OFDM system containing an outershortened Reed-Solomon code concatenated with an inner(punctured ) convolutional code. Th e investigations in thispape r restrict to the inner system of DVB-T for non hier-archical transmission parameter sets, i.e. outer coding anddecoding are not considered.

3.1 TRANSMITTERFor the implementation of C DD at the DVB-T transmit-

ter, only a second signal path after the OFDM-modulationhas to be added. Figure 9 shows the transmitter sided in-ner part with transmitter CDD. After channel coding andinterleaving, the bit-stream is mapped to complex-valuedQAM-symbols. The functional block Frame Adaption isresponsible for QAM-symbol interleaving, pilot insertionand transmission parameter signaling ( T P S ) . Th e resultingsymbol-stream is OFDM -modulated. Finally. the signal issplitted, upconverted and transmitted directly on the onehand and cyclic shifted on the other hand. It is importantto note that signal splitting does not increase the overalltransmission power....

I -- YFrame

Adaption4[ Pilot g,wi nalsFigure 9 Inner non hierarchical DVB-T transmitter part with

CDD.

3 2 RECEIVERAs mentioned above, we use a MRC receiver for oursimulations. Figure 10 shows a block diagram of theDV B-T MRC receiver implementation. After down con-version and guard interval removal, the received signalis OFDM-demodulated and equalized using zero forcing.For our investigations we assume perfect knowledge ofthe channel state information CSI). Both complex-valuedsymbo l-streams are combined and QA M demodulated withsoft-out values before symbol- and bit-deinterleaving is

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Transmit/Receive-AntennaDiversity Techniques for OFDM Systems

Signal ConstellationCode Rate

V

II 4-QAM IQQAM 64-QAM

Figure 10: Non hierarchical DVB-TMRC receiver.done. Finally, the bit stream is soft-decision-maximum-likelihood (SDM L) decoded.

Data-Rate [MB itkjfor 8-MHZchannels

4 SIMULATIONS

6.03 12.06 16.09 16.59

In this section we will present simulation results forDVB-T with antenna diversity in indoor and outdoor sce-narios. The simulation s were done for Doppler frequen-cies, which follow from typical pedestrian m ovement in in-door environment and m obile vehicle movement in outdoorparking-lot scenarios.

4.1 C H A N N E L MO D E L STable 1 shows the main properties of the wide sensestationary uncorrelated scattering (WSSUS) channel mod-els, which were used for simulations. For the individualscatterers Rayfeigh fading is assum ed.

Table I : Main channel model properties.I indoor outdoor II Number ofTam I t 7 I 10 II max. Channel Delay r,,, 75011s 1 5 ~ s_ _I

max. Doppler Frequency f D m J x ii 10 HZ i 50H zDoppler Specmum Form rectangular JakesThe mo bile radio channel models are described in [lo].For the indoor scenario the Indoor Commercial ChannelB channel model is used with a maximum Doppler fre-quency of 10 Hz, which correspond to a mobile velocity

of 1.25: ( 4 . 5 p ) for operation in the 2.4 GHz band. Forthe outdoor scenario the Outdoor Residential - High An-tenna (Channel B) channel model is used. Th e maximumDoppler frequency for the outdoo r scenario is50Hz, hichresults in a mobile velocity of 6 . 2 5 7 (22 ,SF) . Antennadiversity simulations were done for pairwise uncorrelatedWSSUS channels Hn,,,(k, t .4.2 DVB -T SYSTEM PARAMETERS

For 8 MHz channels the DVB-T standard defines somebasic parameters. In the 2k-mode (DFT-length I = 2048)

the OFDM symbol duration is Tu = 224 ps. The num-ber of used carriers is L = 1705. This yields a subcar-rier spacing of 1/Tu = 4464 Hz and a spacing betweenthe spectrum edge carriers of L )/To = 7.61 MHz.For the 8k-mode (A = 8192), the basic parameters areTU = 896 p s and L = 6817. Subcarrier spacing and edgecarrier spacing will follow equivalently. Beside these ba-sic parameters, there are s ome additional transmission pa-rameters regarding m odulation, guard interval length andchannel coding. For 2k-systems and the o utdoor channel aguard interval length of rg = 1/8 . 2 2 4 ps = 28 p s haveto be chosen because of the maxim um outdoor channel de-lay of rmax 15 p s . Therefore, a guard interval length of1/32 is enough for 8k-systems. Table 2 summarizes themain DVB-T sim ulation parameters.Table 2: DVB-T system param eters o r simulations.

Simulation I 1 1 1 3 1 4 1I DVB-TMcde II 8k I 2k

= 28psGuard Interval

4.3 RESULTSThe Monte Car10 simulation s are restricted to the in-ner DVB-T system as it is described in Section 3. It isimportant to note, that the overall transmitted power is keptequal for all simulation runs. i.e.. the transm itted power pertransmit antenna decreases with increasing num ber of an-tennas. The signal power at the receiver antennas has notbeen normalized. This means that the overall received sig-nal power increases as the number of receive antennas in-creases. For simulations we will restrict to the use of atmost 2 antennas at the transmitter respectively the receiverside. For the BER vs. S N R imulation s of the 2TX-antennaCDD systems a cyclic delay of 6 Y = 10 . 1.1 p s

2 & = 0.13 ps. see Equation 4 is chosen.Figure 11 shows the bit enor rate vs. the SNR for theindoor channel w ith d ifferent diversity techniques, appliedto the DVB-T system in non hierarchical 8k-m ode with 4-QAh4-modulation and code rate 112. Note that the SNRhere equals E , /No (the signal energy div ided by no isepower spectral density). A single-antenna-system is givenas a reference, i.e., fo r this system no spatial diversity is im-plemented. As it can be seen from Figure 1 1, the receiver-MRC system outperforms the single-antenna system about7.5 dB in SNR at a BER of 2 . The reason therefore isthe Yd receiver antenna, which provides the receiver with

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Figure I I BER YS. SNR for 8k-mode. 4-QAM. code rate IR in-door

Figure 12: BER vs. SNRfor 8k-mode.4-QAM, code rate IR, out-door.additional signal power. Fu rthermore the two propagationpaths (channel transfer functions) are uncorrelate d. Sosub-carriers. which are in a deep fade for receiver-antenna 1may have good channel properties for antenna 2. W ith ad-ditional transmitter-CDD, a further gain of about 1.6 dBcan be achieved using a cyclic delay of d = 1.1 ps. Note,that the transmitter-CDD gain decreases with the use ofMRC. As shown in Section 2, CDD increases the chan-nel selectivity and therefore decreases the Occurrence ofpacket errors after demodulation. This packet errors mayappear also before decoding in spite of interlea ving due toextremely wide deep fades, which particularly occur in en-vironme nts with small channel delay sprea ds, e.g. indoorscenarios.

Figure 12 shows the bit error performance for aDVB-T system in non hierarchical 8k-mode with 4-QAM-modulation and code rate 1/2 for the outdoor environment.Again, most can begained, if MRC is used at the receiver.In general the error performance is better for all systemcombinations compared to the indoor scenario. Note, thatthe outdoor channel provides a higher maximum channeldefay and therefore a higher frequency selectivity, whichexplains the better e m r performance compared to the in-door channel. It is also interesting to see. that the gains

Figure 13: Delay Diversity G ain vs. Delay at E R = ? .8k-mode,4-QAM, code rate IR. f orberween the different system combinations for the outdoorchannel become smaller compared to indoor channels. Sothe lower the maximum channel delay spread rmaxhehigher is the achievable gain for additional use of CDD.

Figure 13 shows the transmitter-CDD gain for single-antenna and 2-antenna MRC-receivers in indoor and out-door environm ents vs. the cyclic delay at the Yd transmitantenna of a CDD transmitter. As the results, showed inFigure 1 1 and 12, signify, there is a saturation effect i nterms of the cyclic diversity delay. It can be observed,that a cyclic delay of 6 > 1.S p s results in no further im-provement. Figure 13 also shows, that the achievable gainis much higher for indoor scenarios compared to outdoorscenarios due to the extremely different maximum channeldelays of T~~ = 15 ps for outdoor and rmax ?SO nsfor indoor scena rios. Different from the indoor results, theoutdoor delay diversity gain shows local minima at about6 = 0.55 ps. A reason is the structure of tap delay modelfor the outdoor channel. For d = 0.55 ps, relatively pow-erful paths of the outdoor channel model come close toeach other. This tight adjacent echos yield a reduced fre-quency selectivity of the overall channel transfer functionIH, (t', k)I2 m = 1 ~

The BER and delay diversity gain results for 8k-systems with 16-QAM and two different code rates are il-lustrated in Figures 14-15. Table 2 shows the main param-eter sets. First, it can be observed that the a chievable delaydiversity gains for the 16-QAM system remain in the samedimensions as the respective 4-QAM results, shown in Fig-ure 13, if the code rate is equal. The influence of the choic eof the DVB-T code rate can be seen if we compare Fig-ure 14 with 15. It can be seen that for a higher code rate theachievable CDD gain s slightly increase.

Figure 16shows the simulation results for a 2k-systemusing 64-QA M and code rate 112. As Table 2 shows, thedata rate is comparable to that of the 8k-system with 16-QAM and code rate Y 3 . Nevertheless, the 8k-system out-performs the 2k-system in case of the 2TN2RX antennaconfiguration at about I .7 d B .4 dB) for the indoo r (out-door) channel. Figure 16 a) also shows the BER perfor-53b ETT


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Transmithleceive-AntennaDiversity Techniques for OFDM Systems

(a)BER vs . SN R

(b) Delay Diversity Gain vs. Delay at BER = 2 .Figure 14: 8k-mode, I6-QAM code ra te IR.

mance of the I TW lR X 2k-system in an AWGN channel.A S pointed out before, the diversity techniques, con-sidered in this paper, increase the frequency selectivity ofthe channel. Of course, the BER for real systems de-pends on the performance of the channel estimation algo-rithm. For our investigations we assumed perfect knowl-edge of the channel state information (CSI), so this effectcan not be observed in the simulation results. If the ad-ditional (cyclic) delay is chosen properly, no significant

degradation of the BER performance is expected, becausethe channel estimation has to be designed to work reli-ably even in environments with high delay spread (out-door). The maximum overall (effective) delay spread forthe indoor channel simulations is about rg = rn1 6=0.75 ps + 1 .5 ps = 2.25 ps, which is by far less than thedelay spread rmax 15 p s of the outd oor channel. With6 = 1.5 ps the effective delay spread increment for theoutdoor channel is about 10 .

As stated in Section 2.2, the performance of DD andCDD is equal as long as Equation holds, i.e. there is noIS1 due to a too short guard interval (see Figure 3). The

(a)BER vs.SNR

I I I0.5 I 1.56 P

b)Delay Diversity Gain vs. Delay at BER = 2 . lov4

Figure IS: k-mode. I6-QAM code rate 2/3.guard interval lengths for all simulations are chosen largeenough to avoid ISI, even in the case of DD. The simu-lations presented above take into account the intercanierinterference (ICI), caused by Doppler spread. A rule ofthum b is that the ICI du e to Doppler spread of the channelcan be neglected, if that Doppler spread is below 10% ofthe subcarrier spacing. If we have a look on Table 1, wecan see that this rule holds for all of the simulations. How-ever, if we would choose higher Doppler spreads, a higherdegradation of the BER performance of 8k-systems com-pared to their 2k counterparts can be expected. Simulationresults on furth er 2k-system parameter set s can be found i n[ I l l .

5 C ON C L U S I ON SIn this paper delay diversity, cyclic delay diversity,phase diversity and maximum ratio combining have beenpresented. The (conditional) equivalence between cyclicdelay diversity and phase diversity respectively delay di-

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(a )BER vs.SNR

(b) Delay Divmity Gain vs. Delay at BER = 2 .Figure 16: 2k-mode, 64-QAM, code rate IR.

versity have been shown. It is the purpose of the proposedtechniques to increase the delay spread resp. the frequencyselectivity of the resulting channel transfer function by spe-cific cyclic deiays at the transmitter and/or eceiver anten-nas. The presented antenna diversity schem es can easilybe implemented in existing OFDM ystems w ithout chang-ing the standards or the receivers. The main advantage ofcyclic delay diversity and phase diversity compared to de-lay diversity is that no additional IS1 can occur at all dueto th e additional implementation of that techniques. It wasshown exemplarily, how this techniques are applicable tothe DVB-T ystem. imulations showed, that the highe r thedelay spread of the channel, the better is the bi t error per-formance. Therefore, the lower the channel delay spread ofthe channel model fo r single-antenna systems, the higheris the achievable gain for additional implementation of the

REFERENCESA. Darnmann, S. Kaiser

proposed diversity techniques.

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dio Broadcasting Systems; Digital Audio Broadcasting(DAB) to mobile, portable and fired receivers. April 2000.EN300401 V1.3.1.[4] European Telecommunications Standard Institute ETSI.Digiral Kdeo Broadcasting (DVB); Framing structure,channel coding and modulation fo r digital terrestrial tele-vision. July 1999. EN 300744 V1.2.1.[ 5 ] R. van Nee, G Awater. M. Morikura, H. Takanashi, M.Webster, and K.W. Halford. New high-rate wireless LANstandards. IEEE Communications Magazine. pages 82-88,

December 1999.[6] V. Tarokh, N. Seshad ri, and A. R. Calderbank. Space-timecodes for high data rate wireless communication: Perfor-mance criterion and code construction. IEEE Tmnsactions

on Information Theory. Vol. 4 4 , No. 2, pages 144-764,March 1998.[7] S.M. Alamouti. A simple transmit diversity technique forwireless communications. IEEE Journal on Selecred Areasin Communications. Vol. 16, No. 8.pages 1451-1458, Oc-tober 1998.181 S. Kaiser. Spa tial transmit diversity techniques for broad-band OFDM ystems. I n Proc. of IEEE Global Telecom-munications Conference (GLOBECOM 2000).pages 1824-

1828, November 200 0.[9] S.Kaiser. OFDM code division multiplexing with unequalerror protection and flexible data rate adaption. In Proc.of IEEE Global Telecommunications Conference (GLOBE-COM 2001), pages 861-865.November 2001.

[lo] Joint Technical Com mittee on W ireless Access. Final Re-port on RF Channel Characterization. September 1993.JTC(AIRfl3.09.23-238R2.

[ I I ] A. Dammann and S. Kaiser. Standa rd conformable antennadiversity techniques for OFDM and its application to theDVB-T system. In Pmc . of IEEE Global Telecommunica-tions Conference (GLOBECOM 2001 , pages 31 00-3 105,November 200 I .

itluniiscripr received u n April 5 , 200253s

spaital diversity

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