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

RF Issues 197receivedsignalto detection× × ×feed back correctionfilterFFTextract phaseof pilots−• • •×Σ×phaseamplitudenormalizationchannelestimation• • • + ×e −jΦfeed forward correctionFigure 4-59Common phase error correctionThe common phase error is evaluated and corrected before detection and further processing.The phase of the pilot symbols is extracted after the FFT and the channel phaseobtained from the previous OFDM symbol is subtracted on each pilot carrier. Afterweighting each pilot phase with the amplitude of the previous estimate, all remainingphase errors are added and normalized, which results in the common phase error estimate.This estimate is used in the data stream for common phase error correction as wellas to correct the pilots used for channel estimation. Since with an increasing number ofsub-carriers N c the common phase error between adjacent OFDM symbols becomes moreuncorrelated, it might be necessary to estimate and correct the common phase error withineach OFDM symbol, i.e. pilot symbols have to be transmitted in each OFDM symbol.4.7.1.4 Analysis of the Effects of Phase NoiseOFDM systems are more sensitive to phase noise than single-carrier systems due to theN c times longer OFDM symbol duration and ICI [67, 68]. In OFDM systems, differentialmodulation schemes are more robust against phase noise than non-differential modulatedschemes. Moreover, the OFDM sensitivity to phase noise increases as high ordermodulation is applied [11, 84].In Figure 4-60, the degradation due to phase noise is shown for a coded DVB-Ttransmission since this is a system with high numbers of sub-carriers [74]. The chosenmodulation scheme is 64-QAM. OFDM is performed with 2k FFT and the channelfading is Ricean. The performance for a receiver with and without common phase errorcorrection is presented, where it can be observed that common phase error correction cansignificantly improve the system performance. Results of the effects of phase noise onMC-CDMA systems can be found in References [82] and [86].4.7.2 Non-linearitiesMulti-carrier modulated systems using OFDM are more sensitive to high power amplifier(HPA) non-linearities than single-carrier modulated systems [80]. The OFDM signal