Analysis Of Time-Varying Channel And ICI Cancellation Method In OFDM DTV Systems

Orthogonal Frequency Division Multiplex (OFDM) has been adopted by many wireless communication systems as a key modulation technique to combat frequency selective fading channel, especially those with wide bandwidth and high data rate, such as high-definition digital television (HDTV) systems. Recently, wideband communication under a high mobility scenario has been drawing more and more attention as an increasing number of video services are embeded in mobile user terminals such as cellular phone or vehicle. Unfortunately OFDM technique is vulnerable in such high mobility transmission environment, which destroys the mutual orthogonality of sub-carriers and thus causes severe inter-carrier interference. In fact, How to eliminate ICI is considered as one of the most critical issue to fit OFDM technique in a time-varying channel scenario.This paper researches on the ICI cancellation technique. We particularly focused on ICI incited by time-selective fading channels, i.e. time varying channels. First, the problem is analyzed using a mathematical model, in which the mathematical expression of ICI is derived and thoroughly analyzed. Some conclusions on the statistic characteristic of time-varying channels and ICI are made.Some non-iterative ICI cancellation techniques are described and researched on the basis of the mathematical model. Moreover, Their performances and computational complexity are analyzed in order to evaluate the algorithms. In the research of ICI cancellation techniques, we find that the non-iterative methods, especially the frequency domain MMSE equalizer costs too much computational complexity although they have sufficient performances. In order to cut down the complexity, some iterative ICI cancellation methods are discussed. Analysis depicts that there's dramatic decline in computational complexity of these equalizers with reasonable deterioration of the performance.Moreover, a novel MMSE iterative ICI cancellation method is proposed. This equalizer takes into consider of the ICI caused by adjacent sub-carriers and reduced the chance to make a wrong hard decision. It has only slightly increased complexity comparing to the conventional one, yet the performance, according to simulation results, is improved in time-varying channel scenarios.After giving the new method, we discussed how to obtain channel impulse response (CIR) estimation in a time varying channel as well as other related parameters. Particularly, we proposed a time-domain convolution-based method, to convert time domain channel estimation to the frequency domain transmission matrix. It is shown by analysis that this method is more simplified than the traditional one and can be used in any system where time domain time-varying CIR needs to be transformed to frequency domain response.