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

{{48 Fundamentalsdata symbolsspreading codeT c01•L-1sub-carrier f 0sub-carrier f 1sub-carrier f Nc −1•spread data symbols01•L-1{T sFigure 1-18MC-CDMA signal generation for one userMC-CDMA transmits a data symbol of a user simultaneously on several narrowbandsub-channels. These sub-channels are multiplied by the chips of the user-specific spreadingcode, as illustrated in Figure 1-18. Multi-carrier modulation is realized by using thelow complex OFDM operation. Since the fading on the narrowband sub-channels canbe considered flat, simple equalization with one complex-valued multiplication per subchannelcan be realized. MC-CDMA offers a flexible system design, since the spreadingcode length does not have to be chosen equal to the number of sub-carriers, allowingadjustable receiver complexities. This flexibility is described in detail in Chapter 2.MC-DS-CDMA serial-to-parallel converts the high rate data symbols into parallel lowrate sub-streams before spreading the data symbols on each sub-channel with a userspecificspreading code in the time direction, which corresponds to direct sequencespreading on each sub-channel. The same spreading codes can be applied on the differentsub-channels. The principle of MC-DS-CDMA is illustrated in Figure 1-19.MC-DS-CDMA systems have been proposed with different multi-carrier modulationschemes, also without OFDM, such that within the description of MC-DS-CDMA thegeneral term multi-carrier symbol instead of OFDM symbol is used. The MC-DS-CDMAschemes can be subdivided into schemes with broadband sub-channels and schemes withdata symbolsT cserialtoparallelconverterspreading code0 1 • L-1sub-carrier f 0sub-carrier f 1sub-carrier f Nc −1•spread data symbols0 1 • L-1{T sFigure 1-19MC-DS-CDMA signal generation for one user