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Mobile OFDM communications

Abstract

Orthogonal frequency division multiplexing (OFDM) has recently received increased attention due to its capability of supporting high-data-rate communications in frequency-selective fading environments with a comparatively simple receiver structure. However, the time variations (high mobility) of the channel during one OFDM symbol destroy the orthogonality of the different subcarriers and generate power leakage among the subcarriers, known as Inter-Carrier Interference (ICI), which will cause degradation in system performance. Therefore, our research emphasis on Doppler sensitivity is especially meaningful given the popularity that fixed wireless/low mobility OFDM systems have experienced (e.g. 802.11a). In this dissertation, we first analyze the theoretical performance degradation of OFDM system due to Doppler spreading by developing a two-dimensional Gram- Charlier representation for the bivariate pdf of the ICI, after the bivariate probability density function (pdf) of the ICI is shown to be a weighted Gaussian mixture. Secondly, in order to suppress ICI in OFDM systems, we describe an MMSE based OFDM detection according to the estimated channel information through a simple and efficient polynomial surface channel estimation technique. Thirdly, we consider filtered multitone (FMT) modulation over frequency-selective time-varying fading channels. Both ICI and ISI are quantified under several different fading channels conditions. An analysis of the C/I ratio of the FMT system is compared with the conventional OFDM system in order to demonstrate the underlying trade-offs between the spectral efficiency and system performance. Finally, a per-channel tap-delay-line equalizer is proposed at the FMT receiver to mitigate the ISI. A closed -form expression of the upper bound for its C/I ratio is derived for the per-channel equalized FMT system and used to evaluate the system performance. Performance comparisons are made with an un-equalized FMT system as well as a standard OFDM system

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