Summary: | 博士 === 國立交通大學 === 電子工程學系 電子研究所 === 101 === In high mobility scenarios, orthogonal frequency division multiplexing (OFDM) systems experience temporal channel variations within one symbol time to a degree re- sulting in that the orthogonality among subcarriers is destroyed by inter-carrier interference (ICI) and significant performance degradation may follow, if ICI is left untreated. This dissertation is concerned with the challenging problems caused by high mobility to advanced mobile communication systems that adopt orthogonal frequency-division multiple access (OFDMA) technologies where standard specifications and concerns about complexity demand low-cost methods with deployment readiness and decent performance. In this dissertation, comprehensive frameworks are provided to develop effective approaches for dealing with time-varying channels.
To fully understand the problems that fast channel variations may cause to OFDMA systems, fundamental properties and modeling issues of doubly selective fading chan- nels are studied. A simple ICI indicator is devised to show the relative severity of ICI on subcarriers in OFDMA symbols, paving the way for efficient ICI cancellation strategies. Furthermore, a thorough analysis of the ICI indicator is provided to reveal the reasons why it works. It is shown that its probability density function (PDF) is determined by the moving speed meanwhile is insensitive to other factors such as chan- nel power delay profiles and Doppler power spectra. As a result, the applicability of it to indicate channel variations is quite wide-range. Some possible applications of the ICI indicator are also discussed; in particular, its PDF provides valuable information, such as the maximum Doppler spread or the channel fading rate.
Equipped with the understanding of mechanism of ICI generation, two approaches to deal with the ICI issue are developed. In the first approach, with the help of the ICI indicator, a per-subcarrier adaptive (PSA) framework which can work with a variety of existing ICI cancellation methods is proposed to greatly reduce computational complexity while maintaining performance. Novel zero forcing (ZF) and minimum mean-square error (MMSE) equalizers based on PSA processing and perturbation- based (PB) approximation are introduced. The proposed equalizers strike a good balance between performance and implementation cost (up to 80 % savings); therefore they are especially suitable for OFDMA downlink receivers.
The other approach to the ICI issue is, knowing it is a result of channel variation, to recognize the possibility of using it to gain diversity. Bit-interleaved coded modulation with OFDM (BICM-OFDM) is an attractive approach to achieve time and frequency diversity. Remarkable diversity gain can be obtained when the channel is doubly selective fading. In this dissertation, the asymptotic diversity orders of BICM-OFDM systems in doubly selective fading channels for both single-input-single-output (SISO) and multiple-input-multiple-output (MIMO) cases are derived. In addition, the system bit-error rate (BER) behavior in practical situations with moderate signal-to-noise ratios (SNRs) is also investigated. In the SISO case, the diversity order depends on the rank of the channel correlation matrix. Therefore, the channel variations induced by fast fading contributes to improving diversity. In the MIMO case, the diversity order can be further increased when factors like cyclic delays or phase rolls are introduced.
For both approaches, ample simulation evidences are provided to verify theoretical analysis and performance claims. Possible directions on related topics are also outlined for further exploration.
|