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

Synchronization 147FFTr(k)estimatedfrequencyerrorarctang(.)Time domain processing2 nd OFDM Ref. Symbol( . )*Delay N c1 st OFDM Ref. SymbolFigure 4-12 Moose maximum likelihood frequency estimator (M = N c )4.2.4 Time SynchronizationAs we have explained before, the main objective of time synchronization for OFDMsystems is to know when a received OFDM symbol starts. By using the guard time, thetiming requirements can be relaxed. A time offset not exceeding the guard time gives riseto a phase rotation of the sub-carriers. This phase rotation is larger on the edge of thefrequency band. If a timing error is small enough to keep the channel impulse responsewithin the guard time, the orthogonality is maintained and a symbol timing delay canbe viewed as a phase shift introduced by the channel. This phase shift can be estimatedby the channel estimator (see Section 4.3) and corrected by the channel equalizer (seeSection 4.5). However, if a time shift is larger than the guard time, ISI and ICI occur andsignal orthogonality is lost.Basically the task of the time synchronization is to estimate the two main functions: FFTwindow positioning (OFDM symbol/frame synchronization) and sampling rate estimationfor A/D conversion control. The operation of time synchronization can be carried out intwo steps: coarse and fine symbol timing.4.2.4.1 Coarse Symbol TimingDifferent methods, depending on the transmission signal characteristics, can be used forcoarse timing estimation [24, 25, 78]. Basically, the power at baseband can be monitoredprior to FFT processing and, for instance, the dips resulting from null symbols (seeFigure 4-9) might be used to control a ‘flywheel’-type state transition algorithm, as knownfrom traditional frame synchronization [43].Null symbol detectionA null symbol, containing no power, is transmitted, for instance, in DAB at the beginningof each OFDM frame (see Figure 4-13). By performing simple power detection at thereceiver side before the FFT operation, the beginning of the frame can be detected; i.e.the receiver locates the null symbol by searching for a dip in the power of the receivedsignal. This can be achieved, for instance, by using a flywheel algorithm to guard against