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

Diversity Techniques for Multi-Carrier Transmission 3136.3.1.5 Discontinuous Doppler Diversity (DDD)The introduction of ICI with time-variant phase diversity/Doppler diversity can be avoidedby applying discontinuous Doppler diversity (DDD). The principle of DDD is to quantizethe time-variant phase offset m,n (t) such that it is constant during one OFDM symbol butchanges between adjacent OFDM symbols [9]. This guarantees the same time diversity asintroduced with TPD but avoids completely the ICI. The increased time variance of thesuperimposed channel at the receiver antenna has to be taken into account in the systemdesign in the same way as with TPD. This holds especially for the pilot grid design usedfor channel estimation.6.3.1.6 Sub-Carrier DiversityWith sub-carrier diversity (SCD), the sub-carriers used for OFDM are clustered in Msmaller blocks and each block is transmitted over a separate antenna [6]. The principleof sub-carrier diversity is shown in Figure 6-13.After serial-to-parallel (S/P) conversion, each OFDM block processes N c /M complexvalueddata symbols out of a sequence of N c .EachoftheM OFDM blocks maps itsN c /M data symbols on to its exclusively assigned set of sub-carriers. The sub-carriers ofone block should be spread over the entire transmission bandwidth in order to increasethe frequency diversity per block, i.e. the sub-carriers of the individual blocks should beinterleaved.The advantage of sub-carrier diversity is that the peak-to-average power ratio per transmitantenna is reduced compared to a single antenna implementation since there are fewersub-channels per transmit antenna.6.3.2 Receive Diversity6.3.2.1 Maximum Ratio Combining (MRC)The signals at the output of the L receive antennas are combined linearly so that theSNR is maximized. The optimum weighting coefficient is the conjugate complex of theassigned channel coefficient as illustrated in Figure 6-14.transmitterOFDMset 0OFDMset 101receiverS/PIOFDMOFDMset M − 1Figure 6-13M − 1Sub-carrier diversity