(A) decision-directed initial frame synchronization algorithm for OFDM systems

Abstract

Orthogonal frequency division multiplexing (OFDM) is a bandwidth efficient signalling scheme for digital communications ranging from modems to digital audio broadcast (DAB). The main difference between frequency division multiplexing (FDM) and OFDM is that in OFDM the spectra of the individual carriers mutually overlap, resulting in optimum spectrum efficiency. Nevertheless, the OFDM carriers exhibit orthogonality on a symbol interval if synthesized such that they are spaced in frequency exactly at the reciprocal of the symbol interval. The synthesis can be accomplished perfectly utilizing the discrete Fourier transform (DFT) which provides full digital implementation. One of the principal advantages of OFDM is its utility for transmission at nearly optimum performance in unequalized channels and multipath channels. Intersymbol interference (ISI) and interchannel interference (ICI) can be entirely eliminated by inserting a guard interval between symbols, whose length is made equal to or greater than the time spread of the channel. One of the disadvantages of OFDM is that the performance of the system is very sensitive to the synchronism of the receiver. Sensitivity to a time offset caused by the uncertainty in the arrival time of the OFDM symbol is much higher in multicarrier systems than in single carrier systems. Furthermore the demodulation of a signal with an offset in the carrier frequency occurred by the mismatch of the oscillators in the transmitter and receiver can cause a high bit error rate due to ICI. If the frame synchronization of the OFDM signal is not achieved correctly, the phase rotation corresponding to each subcarrier would occur and data recovery becomes impossible consequently. In this study, an initial frame synchronization algorithm for OFDM systems has been developed. First, the performance of the guard interval detector is analyzed by deriving the probability that the detector fails to search the beginning point of the useful OFDM frame...