BICM-OFDM 系統在時變通道下之干擾消除和多樣性分析

Abstract

在高速移動的環境下,正交分頻多工 (OFDM) 系統將遭遇通道在一個符碼的時 間內產生一定程度以上變化,其影響使得子載波間彼此產生干擾 (ICI) 進而導致正 交特性被破壞。倘使沒有妥善處理子載波間干擾,將會造成系統效能嚴重低落。 本論文著眼於移動式無線通訊系統,在採取正交分頻多重存取 (OFDMA) 的先 進技術之下,因高速移動而將面臨上述的問題。考慮在其規格限制下,如何設計出 適合實現之低複雜度方法,且具有足夠的效能。本論文從多方面的考量來發展有效 率地處理時變通道之方法。 為了充分了解時變通道對正交分頻多重存取系統所造成的影響,首先必須對於 時間以及頻率方面皆會有選擇性的雙選擇性衰落 (doubly-selective fading) 通道的基 本性質以及模型建立等問題進行研究。透過推導以及觀察,我們發現了一個簡單易 得且可用來度量出各子載波所承受之干擾程度的指標 (ICI indicator),此指標將為隨 後所提出的高效率子載波間干擾消除策略做鋪陳。此外,我們對於子載波間干擾指 標也進行了詳盡的分析從而闡述其可行性。從結果中可知其機率分布和移動速度密 切相關,並且對於其他因素例如通道功率延遲以及都普勒功率頻譜則較不敏感。因 此子載波間干擾指標可以廣泛地適用於不同的通道環境下。我們也討論了其他可能 的應用,例如,其機率分布可提供最大的都普勒頻率或是通道衰落的速率等極為有 價值的資訊。 在了解子載波間干擾的生成機制後,我們發展了兩種方法來處理子載波間干 擾。第一種方法是藉由子載波間干擾指標的輔助,提出了基於各個子載波適應性 (PSA) 處理架構。PSA 可結合許多目前現有的子載波間干擾消除方法並且在不犧 牲效能的情況下大幅地降低計算複雜度。搭配 PSA 以及 擾動近似 (PB),我們也設計了新穎的迫零 (ZF) 和最小均方誤差 (MMSE) 等化器。而這些提出的等化器在 效能和實現成本 (可節省 80% 計算量) 上取得了不錯的平衡,也因此特別適合用於 OFDMA 下行鏈路接受器。 從另外一方面來看子載波間干擾的問題,我們了解子載波間干擾是源自於通道 的變化,進而認知到妥善運用此變化來獲取多樣性的可能性。結合了位元交織編碼 調變 (BICM), BICM-OFDM 是一個非常有效的方式來獲得時間和頻率多樣性。 再 者,雙選擇性衰落通道可以提供顯著的多樣性。故本論文中,我們推導了在單輸入 單輸出 (SISO) 和多輸入多輸出 (MIMO) 下漸進最大多樣性分析。我們進一步研究 了在實際的狀況中,信噪比 (SNR) 非無限大時系統錯誤率曲線的行為。我們發現在 SISO 情況下,多樣性多寡取決於通道相關矩陣的秩,因此因快速衰落所引起的通道 變化將有助於增進多樣性。在 MIMO 的情況下,藉由使用循環延遲 (CDD) 以及相 位混合 (PRD) 的技術,可更進一步提昇多樣性。 對於以上兩種方法,我們都提供了充足的模擬數據來驗證理論分析的正確性以 及其對於效能上的改進效果。最後,我們也考慮了未來可能繼續探索的相關主題和 方向。

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.