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. 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.