A Frame Synchronization and Frequency Offset Estimation Algorithm for OFDM System and its Analysis
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A Frame Synchronization and Frequency Offset Estimation Algorithm for OFDM System and its Analysis Ch. Nanda Kishore1 and V. Umapathi Reddy1, 2 1 Hellosoft 2 IIIT,
India Pvt Ltd, 82 703, Road No 12, Banjara Hills, 500 034 Hyderabad, AP, India Hyderabad, India
Received 13 July 2005; Revised 12 December 2005; Accepted 19 January 2006 Recommended for Publication by Hyung-Myung Kim Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for transmitting data at very high rates over time dispersive radio channels. In an OFDM system, frame synchronization and frequency offset estimation are extremely important for maintaining orthogonality among the subcarriers. In this paper, for a preamble having two identical halves in time, a timing metric is proposed for OFDM frame synchronization. The timing metric is analyzed and its mean values at the preamble boundary and in its neighborhood are evaluated, for AWGN and for frequency selective channels with specified mean power profile of the channel taps, and the variance expression is derived for AWGN case. Since the derivation of the variance expression for frequency selective channel case is tedious, we used simulations to estimate the same. Based on the theoretical value of the mean and estimate of the variance, we suggest a threshold for detection of the preamble boundary and evaluating the probability of false and correct detections. We also suggest a method for a threshold selection and the preamble boundary detection in practical applications. A simple and computationally efficient method for estimating fractional and integer frequency offset, using the same preamble, is also described. Simulations are used to corroborate the results of the analysis. The proposed method of frame synchronization and frequency offset estimation is applied to the downlink synchronization in OFDM mode of wireless metropolitan area network (WMAN) standard IEEE 802.16-2004, and its performance is studied through simulations. Copyright © 2006 Ch. N. Kishore and V. U. Reddy. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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INTRODUCTION
Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation scheme in which high rate data stream is split into a number of parallel low rate data streams, each of which modulates a separate subcarrier. Recently, OFDM has been adopted as a modulation technique in wireless metropolitan area network (WMAN) standard [1]. In OFDM system, timing and frequency synchronization are crucial for the retrieval of information (see [2]). If any of these tasks is not performed with sufficient accuracy, the orthogonality among the subcarriers is lost, and the communication system suffers from intersymbol interference (ISI) and intercarrier interference (ICI). Several techniques have been proposed recently for OFDM synchronization. Those suggested in [3–9] use certain stru
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