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

140 Implementation IssuesTo summarize, in the next sections we make the following assumptions:– The terminal stations are frequency/time-synchronized to the base station.– The Doppler variation is slow enough to be considered constant during one OFDMsymbol of duration T s ′.– The guard interval duration T g is larger than the channel impulse response.4.2.1 GeneralThe synchronization algorithms employed for multi-carrier demodulation are based eitheron the analysis of the received signal (non-pilot-aided, i.e. blind synchronization) [12, 13,38] or on the processing of special dedicated data time and/or frequency multiplexed withthe transmitted data, i.e. pilot-aided synchronization [13, 24, 25, 60, 81]. For instance, innon-pilot-aided synchronization some of these algorithms exploit the intrinsic redundancypresent in the guard time (cyclic extension) of each OFDM symbol. Maximum likelihoodestimation of parameters can also be applied, exploiting the guard time redundancy [78]or using some dedicated transmitted reference symbols [60].As shown in Figure 4-8, there are three main synchronization tasks around the FFT:(a) timing recovery, (b) carrier frequency recovery, and (c) carrier phase recovery. In thispart, we concentrate on the first two items, since the carrier phase recovery is closelyrelated to the channel estimation (see Section 4.3). Hence, the two main synchronizationparameters that have to be estimated are: (a) time positioning of the FFT window includingthe sampling rate adjustment that can be controlled in a two-stage process, coarse- andfine-timing control, and (b) the possible large frequency difference between the receiverand transmitter local oscillators that has to be corrected to a very high accuracy.As known from DAB [16], DVB-T [19], and other standards, the transmission is usuallyperformed in a frame-by-frame basis. An example of an OFDM frame is depicted inFigure 4-9, where each frame consists of a so-called null symbol (without signal power)transmitted at the frame beginning, followed by some known reference symbols anddata symbols. Furthermore, within data symbols some reference pilots are scattered intime and frequency. The null symbol may serve two important purposes: interferenceand noise estimation, and coarse timing control. The coarse timing control may use theTimeOFDM frameFrequencyNull symbolReference symbols(e.g. CAZAC/M)Figure 4-9Pilots scatteredwithin OFDM symbolsExample of an OFDM frameData