| Orthogonal Frequency Division Multiplexing (OFDM) has become a very hot topic in wireless industry. It is also a widely known modulation scheme in which a serial data stream is split into parallel streams that modulate a group of orthogonal sub-carriers. One primary reason for the popularity of OFDM is its ability to provide good performance in multipath channels with significant time delay spreads by converting this inter-symbol interference (ISI) channel into an ISI-free channel. As a result, it has been used in a lot of wireless communication systems such as DVB-T, IEEE 802.11a, IEEE 802.16d/e, and the upcoming 3GPP LTE.However, when the channel is time-variant, it would destroy the orthogonality of received sub-carriers and introduce inter-carrier interference (ICI), especially when the terminal speed is high. This would degrade the system performance considerately. Therefore, to have an acceptable reception quality for the applications that experience high delay and Doppler spread, there is a requirement of accurate channel estimation and a requirement for ICI mitigation within one OFDM symbol. This is also a key of this paper.In this paper, we introduce the fundamentals of OFDM and wireless mobile communications firstly. At the same time, we recall a lot of methods, which focus on channel estimation using different algorithms. For the purpose of channel estimation in time variant channels, we provises several innovative methods of ICI mitigation.This paper make an analysis of ICI which result from Doppler, and then present 4 algorithms from the angle of time-domain, frequency-domain, employing CP and employing pilot. The corresponding theorical analysis and computer simulation are also provided in this paper. From this analysis, we can find that these algorithms have low complexcity and high performance. As a result, they are easily used in hardware implementation, which has both theory and engineer significance with a bright future. |
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