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

MC-DS-CDMA 97The user-specific delay relative to the first arriving signal is given by τ (k) .Ifallusersaresynchronized, then τ (k) = 0forallK users.As soon as each sub-channel can be considered as narrowband, i.e. the sub-channelbandwidth is smaller than the coherence bandwidth (f ) c , the fading per sub-channel isfrequency nonselective and low complex detection techniques compared to broadband subchannelscan be realized. Narrowband sub-channels are achieved by choosing a sufficientlylarge number of sub-carriers relative to the bandwidth B. A rough approximation for theminimum number of sub-carriers is given byN c ≥ τ max B. (2.105)The overall transmission bandwidth is given by B and τ max is the maximum delay of themobile radio channel.2.2.4 SpreadingSince MC-DS-CDMA is of interest for the asynchronous uplink, spreading codes such asPN or Gold codes described in Section 2.1.4.1 are of interest for this scenario. As for asynchronoussingle-carrier DS-CDMA systems, good auto- and cross-correlation propertiesare required. In the case of a synchronous downlink, orthogonal codes are preferable.2.2.5 Detection TechniquesMC-DS-CDMA systems with broadband sub-channels can be split into N c classical broadbandDS-CDMA systems. Thus, single- and multi-user detection techniques known forDS-CDMA can be applied in each data stream, which are described in Section 1.3.1.2 andin Reference [53]. Analyses for MC-DS-CDMA systems with broadband sub-channelsand pre-rake diversity combining techniques have been carried out in References [22]and [38].MC-DS-CDMA is typically applied for asynchronous uplink scenarios. The assignedsingle-user detector for MC-DS-CDMA with narrowband sub-channels can be realizedby a spreading code correlator on each sub-channel. Figure 2-26 shows the single-userdetector with N c correlators.f 0c (k) (t)dty(t)f Nc −1c (k) (t)P/SdtFigure 2-26MC-DS-CDMA correlation detector