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

Diversity Techniques for Multi-Carrier Transmission 309transmitterOFDM01d 1d M − 1M − 1receiverIOFDMFigure 6-8Delay diversitydelayed signals. Delay diversity can be considered a simple form of STTCs. Increasedfrequency selectivity can enable a better exploitation of diversity, which results in animproved system performance. With delay diversity, the multi-carrier modulated signalitself is identical on all M transmit antennas and differs only in an antenna-specific delayδ m ,m= 1,...,M − 1 [20]. The block diagram of an OFDM system with spatial transmitdiversity applying delay diversity is shown in Figure 6-8.In order to achieve frequency selective fading within the transmission bandwidth B,the delay has to fulfill the conditionδ m ≥ 1 B . (6.8)To increase the frequency diversity by multiple transmit antennas, the delay of the differentantennas should be chosen asδ m ≥ km , k ≥ 1, (6.9)Bwhere k is a constant factor introduced for the system design. The factor has to be chosenlarge enough (k ≥ 1) in order to guarantee a diversity gain. A factor of k = 2seemstobesufficient to achieve promising performance improvements in most scenarios. This resultis verified by the simulation results presented in Section 6.3.1.2.The disadvantage of delay diversity is that the additional delays δ m ,m= 1,...,M − 1,increase the total delay spread at the receiver antenna and require an extension of the guardinterval duration by the maximum δ m ,m= 1,...,M − 1, which reduces the spectralefficiency of the system. This disadvantage can be overcome by phase diversity presentedin the next section.6.3.1.2 Phase DiversityPhase diversity (PD) transmits signals on M antennas with different phase shifts, where m,n ,m= 1,...,M − 1, n = 0,...,N c − 1, is an antenna- and sub-carrier-specific phaseoffset [14, 15]. The phase shift is efficiently realized by a phase rotation before OFDM,i.e. before the IFFT. The block diagram of an OFDM system with spatial transmit diversityapplying phase diversity is shown in Figure 6-9.