Method and apparatus for recovering timing information in orthogonal frequency division multiplexing (OFDM) systems

Abstract

An improved OFDM receiver is disclosed that repositions peaks in an OFDM frame to a desired position away from the frame boundary to reduce the probability of timing ambiguity. Each OFDM frame is divided into at least two windows during an acquisition mode to identify the index within each window having the maximum correlation. The improved timing acquisition of the present invention permits the Fast Fourier Transform (FFT) operation to operate on the correctly aligned symbol for improved accuracy. In addition, the present invention provides improved mechanisms for declaring when timing is acquired or when timing has been lost. In one implementation, the peaks are shifted from the frame boundary to the center of the OFDM frame, thereby removing the ambiguity of whether a given peak is associated with a previous or subsequent frame. A timing FSM processes a number of timing estimates to determine when the timing information has been acquired and shifts the OFDM signal, as necessary, to maintain the peak in the desired position during a tracking mode. The timing FSM determines that timing acquisition is completed if a timing estimate does not shift by more than a guard interval for a predefined timing acquisition length. After a predefined inter-mode settling period, the timing FSM will transition to a tracking mode. If the differences between the estimated times and the desired position of the OFDM frame are consistently greater than the length of the guard interval, then a loss of tracking is achieved and the timing FSM will return to an acquisition mode.

Dependencies are inferred from the claim text.