I. Introduction

OFDM (Orthogonal Frequency Division Multiplexing) technique is considered as the one that has most potential for next generation communications and has been widely used in digital audio broadcasting (DAB), digital video broadcasting (DVB), and broadband wireless local area networks (IEEE 802.11a) [1], [2]. It can effectively mitigate the effects of multipath propagation on the frequency selective fading channels as long as the channel length is shorter than or equal to the length of the cyclic prefix (CP). In this case, the signals can be easily detected by a set of parallel per-subcarrier signal detectors on flat fading channels [3]. However, when the channel length is longer than the length of CP, orthogonality of each subcarrier will be destroyed and inter-block interference (IBI) occurs, rendering conventional algorithms [1], [3], [4] inapplicable. Different approaches [5]–[7] have been proposed to detect the signals of OFDM systems wherein the channel length is longer than that of CP. In [5], [6], a time domain equalizer is inserted to reduce the channel to a filter model with an order shorter than or equal to the length of CP. The existing per-subcarrier detection algorithm [3] can thus be applied to detect the signals. This time domain equalization can also be transferred to a frequency domain operation as in [7]. However, accurate channel length estimation, which is difficult in practice, is required in these algorithms.