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

154 Implementation IssuesDeframingFFTr(k)Pilots and referencesCoarsecarrierfrequencyestimationChannelestimationCommonphase errorFine frequencysynchronizationFigure 4-19Frequency synchronization using reference symbolsThe phase ambiguity limits the maximum frequency offset value. The phase offset canbe estimated by using pilot/reference aided algorithms [81]. Furthermore, as explained inSection 4.2.5.1, for fine frequency synchronization some other reference data (i.e. CAZACsequences) can be used. Here, the correlation process in the frequency domain can bedone over a limited number of sub-carrier frequencies (e.g. ±N 2 sub-carriers).As shown in Figure 4-19, channel estimation (see Section 4.3) can additionally delivera common phase error estimation (see Section 4.7.1.3), which can be exploited for finefrequency synchronization.4.2.6 Automatic Gain Control (AGC)In order to maximize the input signal dynamic and to avoid saturation, the variation ofthe received signal field strength before FFT operation or before A/D conversion can beadjusted by an AGC function [14, 81]. Two kinds of AGC can be implemented:– Controlling the time domain signal before A/D conversion. First, in the digital domain,the average received power is computed by filtering. Then, the output signal is convertedto analogue (e.g. by a sigma-delta modulator), which controls the signalattenuation before the A/D conversion.– Controlling the time domain signal before FFT. In the frequency domain the outputof the FFT signal is analyzed and the result is used to control the signal before theFFT.4.3 Channel EstimationWhen applying receivers with coherent detection in fading channels, instantaneous informationabout the channel state is required and has to be estimated by the receiver. Thebasic principle of pilot symbol aided channel estimation is to multiplex reference symbols,so-called pilot symbols, into the data stream. The receiver estimates the channelstate information based on the received, known pilot symbols. The pilot symbols canbe scattered in the time and/or frequency directions in OFDM frames (see Figure 4-9).