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

Synchronization 151A first simple solution is to use the method described above to evaluate the properposition of the window and to readjust it dynamically. However, this method generatesa phase discontinuity between symbols where a readjustment of the FFT window occurs.This phase discontinuity requires additional filtering or interpolation after FFT operation.A second method, although using a similar strategy, is to evaluate the shift of the FFTwindow that is proportional to the frequency offset of the clock oscillator. The shift can beused to control the oscillator with better accuracy. This method allows a fine adjustmentof the FFT window without the drawback of phase discontinuity from one symbol to theother.4.2.5 Frequency SynchronizationAnother fundamental function of an OFDM receiver is the carrier frequency synchronization.Frequency offsets are introduced by differences in oscillator frequencies in thetransmitter and receiver, Doppler shifts, and phase noise. As we have seen earlier, thefrequency offset leads to a reduction of the signal amplitude since the sinc functionsare shifted and no longer sampled at the peak and to a loss of orthogonality betweensub-carriers. This loss introduces ICI, which results in a degradation of the global systemperformance [60, 75, 76].In the previous sections we have seen that in order to avoid severe SNR degradation,the frequency synchronization accuracy should be better than 2 % (see Section 4.2.2.3).Note that a multi-carrier system is much more sensitive to a frequency offset than asingle-carrier system [67].As shown in Figure 4-8, the frequency error in an OFDM system is often correctedby a tracking loop with a frequency detector to estimate the frequency offset. Dependingon the characteristics of the transmitted signal (pilot-based or not) several algorithms forfrequency detection and synchronization can be applied:– algorithms based on the analysis of special synchronization symbols embedded inthe OFDM frame [8, 55, 60, 62, 63, 81];– algorithms based on the analysis of the received data at the output of the FFT(non-pilot-aided) [12]; and– algorithms based on the analysis of guard time redundancy [13, 38, 78].Like time synchronization, frequency synchronization can be performed in two steps:coarse and fine frequency synchronization.4.2.5.1 Coarse Frequency SynchronizationWe assume that the frequency offset is greater than half of the sub-carrier spacing, i.e.f error = 2z +φ , (4.38)T s πT swhere the first term of the above equation represents the frequency offset, which is a multipleof the sub-carrier spacing where z is an integer and the second term is the additionalfrequency offset being a fraction of the sub-carrier spacing, i.e. φ is smaller than π.