TIME AND FREQUENCY SYNCHRONIZATION IN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM) SYSTEMS

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) has attracted tremendous attention in the last few years particularly in applications that require high data rates to be supported under fading channels. OFDM has an inherent resistance to multi path components and hence offers critical simplifications in its corresponding single carrier based systems. However, the price of these benefits is paid in the synchronization operations that introduce considerable degradation in performance if not countered appropriately. Timing Synchronization requires the precise location of FFT window to be determined. In addition, the orthogonality of the carriers is frequently disturbed by the fading channel. If this orthogonality is not restored, it introduces Inter Carrier Interference (ICI) and performance is severely compromised. This work analyses some of the algorithms currently being used to achieve timing and frequency synchronization. These algorithms are extended to high mobility cases where Doppler shift in frequency is another irritant that brings additional fading and degradation. The additional fading produced by the Doppler shift results in a residual error even after a fine frequency estimation scheme. This work proposes a PLL based recovery method which removes this proportional phase shift iteratively for all samples in an OFDM symbol. We determine the imaginary component that remains after a QPSK data point is raised to a power of 4 and this imaginary component is the correction provided to a loop filter that generates the necessary phase correction for the next sample. We propose a new system level diagram that removes the fine frequency estimation procedure in Minn and Bhargava’s estimator and replaces it with our phase recovery scheme. Both systems have been tested under severely degraded channels and our system is found to outperform the one that employs fine frequency estimation. The proposed system, therefore, offers possibilities to support high data rates with a low error probability in a burst mode and at the same time lowers the computational complexity in the overall system.