A Technique for Dominant Path Delay Estimation in an OFDM System and Its Application to Frame Synchronization in OFDM Mode of WMAN

Hindawi Publishing Corporation EURASIP Journal on Wireless Communications and Networking Volume 2006, Article ID 86147, Pages 1–8 DOI 10.1155/WCN/2006/86147 A Technique for Dominant Path Delay Estimation in an OFDM System and Its Application to Frame Synchronization in OFDM Mode of WMAN Ch. Nanda Kishore and V. U. Reddy Hellosoft India Pvt Ltd., 8-2-703, Road no. 12, Banjara Hills, Hyderabad 500 034, India Received 15 February 2006; Revised 22 June 2006; Accepted 10 October 2006 Recommended for Publication by Sangarapillai Lambotharan 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. Recently, several techniques have been proposed for frame synchronization in OFDM system. In multipath environment, the transmitted signal arrives at the receiver through direct and multiple delayed paths. In some cases, it is possible that power of the signal arriving through delayed path may be larger than that of the direct path (earliest path if there is no direct path). In those cases, estimate of the frame boundary may be shifted by a quantity equal to the delay of the dominant path. In such cases, there will be intersymbol interference (ISI) in the demodulated symbols unless the frame boundary estimate is preadvanced such that it dwells in the ISI-free portion of cyclic prefix or at the symbol boundary. In this paper, we propose a method for estimating the shift in the frame boundary estimate using a preamble having two identical halves. We assume that frame boundary and frequency offset estimation have been performed prior to the estimation of the shift. We also examine the quality of the frequency offset estimate when the frame boundary estimate is shifted from the desired value. The proposed method is applied to downlink synchronization in OFDM mode of WMAN (IEEE 802.16-2004). We use simulations to illustrate the usefulness of the method and also to support our assertions. 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. 1. INTRODUCTION Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation scheme for transmitting data at very high rates over multipath radio channels. 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]). Several techniques have been recently proposed for OFDM frame and frequency synchronization. In multipath environment, the transmitted signal arrives at the receiver through multiple paths and the corresponding signals are delayed with respect to direct path or earliest path if there is no direct path. In some cases, power of the signal arriving through a delayed path is larger than that of the signal from the earliest path (we refer to the corresponding delayed path as the dominant path). In those cases, es- timate of the frame boundary may be shifted by a quantity equal to the delay of the dominant path (the delay is measured with respect to the arrival time of the earliest path). In [3], Yang et al. proposed a method that uses pilot subcarriers throughout the frame for estimating the shift in the frame boundary. However, their approach requires huge memory to store around 10 estimates of channel impulse responses and also the received samples corresponding to 10 OFDM symbols, thereby introducing a delay of 10 OFDM symbols in the demodulation process. To correct the error in the boundary estimate due to channel dispersion, the authors in [4] suggest a method which is computationally expensive due to matrix operations involved. For finding the channel impulse response, their method takes around LM complex multiplications and additions where M is the length of the repeated training symbol segment and L is the length of the cyclic prefix (CP). Moreover, both methods [3, 4] find the channel impulse response using the received OFDM symbols starting 2 EURASIP Journal on Wireless Communications and Networking from the preadvanced boundary, where preadvancement is chosen by an arbitrary amount, and find the first significant channel tap by testing the tap power in a window of L using a threshold. This preadvancement may degrade the estimation performance in the cases where the preadvanced frame boundary falls in the interference portion of CP. In this paper, we propose a method for estimating the shift in the frame boundary using a preamble having two identical halves. This is the same as the first symbol of the preamble considered in [5] and the second symbol of the preamble specified for downlink mode of WMAN-OFDM. Our method requires 2M complex multiplications for finding the channel impulse response and a memory to store samples of two received OFDM symbols. We assume that frame boundary and frequency offset estimation have been performed prior to the shift estimation. We examine the quality of the frequency estimate when the frame boundary estimate is shifted from the desired value. Though the method is based on the preamble cited above, it is not restricted to the way the frame boundary and frequency offset are estimated. The proposed method integrates very well with our synchronization technique [6], and for this reason, we give the steps of the algorithm of [6] for the sake of continuity. The method is applied to downlink synchronization in OFDM mode of WMAN (IEEE 802.16-2004). We use simulations to illustrate the usefulness of the method and also to support our assertions. The rest of the paper is organized as follows. Section 2 gives briefly the background of the frame synchronization technique [6]. Quality of frequency offset estimate as given by the algorithm of [6], when the frame boundary estimate is shifted from the desired symbol boundary, is examined in Section 3 and simulation results to support our assertions in this regard are also given in this section. The proposed method of estimating the shift in the frame boundary and simulation results to illustrate its effectiveness in the frame synchronization are presented in Section 4. Performance of the method when applied to downlink synchronization in WMAN-OFDM mode is demonstrated through simulations in Section 5. Finally, Section 6 concludes the paper. M CP M Figure 1: Preamble, preceded by CP, considered for the proposed method. samples of the OFDM symbol are given by r(n) = e j[(2π n/N)+θ0 ] 1 υ l=0 h(l)x(n  l) + η(n), (2) where θ0 is an initial arbitrary carrier phase and η(n) is a zero mean circularly symmetric complex w (...truncated)