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

Abstract

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.