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

148 Implementation IssuesTx OFDM frameNo power detected =start of an OFDM frameNull symbol = no Tx powerPower detectedReceived powerFigure 4-13Coarse time synchronization based on null symbol detectionoccasional failures to detect the null symbol once in lock [43]. The basic function ofthis algorithm is that, when the receiver is out-of-lock, it searches continuously for thenull symbols, whereas when in-lock it searches for the symbol only at the expected nullsymbols. The null symbol detection gives only coarse timing information.Two identical half reference symbolsIn Reference [81] a timing synchronization is proposed that searches for a training symbolwith two identical halves in the time domain, which can be sent at the beginning of anOFDM frame (see Figure 4-14). At the receiver side, these two identical time domainsequences may only be phase shifted φ = πT s f error due to the carrier frequency offset.The two halves of the training symbol are made identical by transmitting a PN sequenceon the even frequencies, while zeros are used on the odd frequencies. Let there be Mcomplex-valued samples in each half of the training symbol. The function for estimatingFFTr(k)metric |M(d)| 2estimatedtiming offsettiming error|M(d)| 2Time domain processing1/2 OFDM ref. symb.( . )*Delay N c /21/2 OFDM ref. symb.PowerestimationFigure 4-14Time synchronization based on two identical half reference symbols