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Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

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Synch.Synch.Comparison of Hybrid <strong>Multi</strong>ple Access Schemes 1233.4.2 UWB Transmission SchemesA UWB transmission scheme for a multi-user environment is illustrated in Figure 3-12,where for each user a given time-hopping pattern, i.e. PN code, is assigned. The transmitteris quite simple. It does not include any amplifier or any IF generation. The signal of thetransmitted data after pulse position modulation according to the user’s PN code is emitteddirectly at the Tx antenna. A critical point of the transmitter is the antenna, which mayact as a filter.The receiver components are similar to the transmitter. A rake receiver as in a conventionalDS-CDMA system might be required to cope with multi-path propagation.The baseb<strong>and</strong> signal processing extracts the modulated signal <strong>and</strong> controls both signalacquisition <strong>and</strong> tracking.The main application fields of UWB could be short range (e.g. indoor) multi-usercommunications, radar systems, <strong>and</strong> location determination/positioning. UWB may havea potential application in the automotive industry.3.5 Comparison of Hybrid <strong>Multi</strong>ple Access SchemesA multitude of performance comparisons have been carried out between <strong>MC</strong>-CDMA <strong>and</strong>DS-CDMA as well as between the multi-carrier multiple access schemes <strong>MC</strong>-CDMA,<strong>MC</strong>-DS-CDMA, SS-<strong>MC</strong>-MA, <strong>OFDM</strong>A, <strong>and</strong> <strong>MC</strong>-TDMA. It has been shown that <strong>MC</strong>-CDMA can significantly outperform DS-CDMA with respect to BER performance <strong>and</strong>b<strong>and</strong>width efficiency in the synchronous downlink [8, 13, 14]. The reason for the betterperformance with <strong>MC</strong>-CDMA is that it can avoid ISI <strong>and</strong> ICI, allowing an efficient <strong>and</strong>simple user signal separation. The results of these comparisons are the motivation toconsider <strong>MC</strong>-CDMA as a potential c<strong>and</strong>idate for future mobile radio systems, whichshould outperform 3G systems based on DS-CDMA.Datauser 0DatauserK − 1Mod.Mod.Prog.delayPN codeuser 0...Prog.delayPulsegenerat.Pulsegenerat.AntennaTS TransmitterAntennaAntennaK Correlatorsor Rakes. . .K−1 0Pulsegenerat.Pulsegenerat.Program.delayBaseb<strong>and</strong>processingPN codeuser 0...Datauser 0DatauserK − 1PN codeuser K − 1TS TransmitterProgram.delayBS ReceiverPN codeuser K − 1Figure 3-12<strong>Multi</strong>-user UWB transmission scheme

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