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

168 Implementation Issues4.3.6 Time Domain Channel EstimationChannel estimation for OFDM systems can be applied in the time domain, as in singlecarriersystems. The basic principle is to insert PN sequences periodically in the timedomain between OFDM symbols where the channel impulse response is obtained bycorrelation in the time domain [91, 92]. In OFDM systems, the signal used for timedomain channel estimation can be generated in the transmitter in the frequency domain.The insertion of equal-spaced pilot symbols in the frequency domain results in an OFDMsymbol that can be considered as split into several time slots, each with a pilot. Assumingthat the impulse response of each time slot is identical, the channel response in the timedomain can be obtained by averaging the impulse responses of the time slots [58, 91].The cost of time domain channel estimation compared to frequency domain channelestimation is its higher complexity. Time domain channel estimation can be combinedwith time domain channel equalization [9].4.3.7 Decision Directed Channel EstimationUnder the assumption that the channel is quasi-stationary over two OFDM symbols, thedecisions from the previous OFDM symbol can be used for data detection in the currentOFDM symbol. These schemes can outperform differentially modulated schemesand are efficient if FEC coding is included in the reliable reconstruction of the transmittedsequence [56]. The principle of decision directed channel estimation including FECdecoding is illustrated in Figure 4-29.In the first estimation step (start), reference symbols have to be used for the initialchannel estimation. These reference symbols are the symbols S n,0 ,n= 0,...,N c − 1,of the initial OFDM symbol. The initial estimation of the channel transfer function isgiven byHˆn,0 = R n,0= H n,0 + N n,0. (4.69)S n,0 S n,0The channel can be oversampled in the frequency and time direction with respectto filtering, since all re-encoded data symbols can be used as pilot symbols. Thus, thechannel transfer function can be low-pass filtered in the frequency and/or time directionFFTequalizerG n, i = f(Hˆn, i−1 )symboldemapperchanneldecoderCSIR n, i−1delay2D filteringHˆR= n, i−1n, i−1Sˆn, i−1startSˆn, i−1referencesymbolsymbolmapperchannelencoderFigure 4-29Decision directed channel estimation