I. Introduction
Orthogonal Frequency Division Multiplexing (OFDM) is a promising technique for high speed data transmission in a frequency-selective fading environment. It was adopted as a standard for European digital video broadcasting terrestrial transmission (DVB-T) [1] and wireless local area network (W-LAN) [2]. To achieve high data rate as well as good performance, coherent detection is commonly used in most existing OFDM systems [3]. The coherent detection relies on the knowledge of the channel state information (CSI). One simple approach to obtain CSI is to send some pilot symbols at the transmitter. In recent years, many works have been presented on the questions of pilot design and channel estimation in the pilot symbol assisted OFDM (PSA-OFDM) system. The locations of putting these pilot symbols along the data stream, i.e. the pilot pattern, were examined in [4]–[7]. The least square (LS) or minimum mean square error (MMSE) method for OFDM channel estimation on the other hand could also be found in [8]–[11].