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

Introduction 7Table 4Advantages and drawbacks of different multiple access schemesMultiple access scheme Advantages DrawbacksFDMA – Low transmit power– Robust to multi-path– Easy frequency planning– Low delay– Low peak data rate– Loss due to guard bands– Sensitive to narrowbandinterferenceTDMA – High peak data rate– High multiplexing gain inthe case of bursty traffic– High transmit power– Sensitive to multi-path– Difficult frequency planningCDMA – Low transmit power– Robust to multi-path– Easy frequency planning– High scalability– Low delay– Low peak data rate– Limited capacity per sectordue to multiple access interferencesignal is offered by the auto-correlation properties of the spreading codes [47]. Theso-called rake receiver should contain multiple correlators, each matched to a differentresolvable path in the received composite signal [42]. The performance of a DS-CDMAsystem will strongly depend on the number of active users, the channel characteristics,and the number of arms employed in the rake. The system capacity is limited by selfinterferenceand multiple access interference, which results from the imperfect auto- andcross-correlation properties of spreading codes. Therefore, it will be difficult for a DS-CDMA receiver to make full use of the received signal energy scattered in the timedomain and hence to handle full load conditions [42].The technique of multi-carrier transmission has received wide interest, especially forhigh data rate broadcast applications. The history of orthogonal multi-carrier transmissiondates back to the mid-1960s, when Chang published his papers on the synthesis of bandlimitedsignals for multi-channel transmission [8, 9]. He introduced the basic principleof transmitting data simultaneously through a band-limited channel without interferencebetween sub-channels (without inter-channel interference, ICI) and without interferencebetween consecutive transmitted symbols (without inter-symbol interference, ISI)inthetime domain. Later, Saltzberg performed further analyses [43]. A major contribution tomulti-carrier transmission was presented in 1971 by Weinstein and Ebert [51] who usedthe Fourier transform for baseband processing instead of a bank of sub-carrier oscillators.To combat ICI and ISI, they introduced the well-known guard time between the transmittedsymbols with raised cosine windowing.The main advantages of multi-carrier transmission are its robustness in frequencyselective fading channels and, in particular, the reduced signal processing complexityby equalization in the frequency domain.The basic principle of multi-carrier modulation relies on the transmission of data bydividing a high-rate data stream into several low rate sub-streams. These sub-streams aremodulated on different sub-carriers [4, 7, 12]. By using a large number of sub-carriers,