正交分頻多工系統之參數盲蔽估測

Blind Parametric Estimation in OFDM Systems

林子喬(Tzu-Chiao Lin)

指導教授 ： 馮世邁

國立臺灣大學/電機資訊學院/電信工程學研究所/博士(2016年)

https://doi.org/10.6342/NTU201600822

摘要

在這篇博士論文中，我們致力於研究正交分頻多工系統之參數盲蔽估測。內容主要分兩個部分。在第一個部分，我們探討了載波頻率偏移和實虛部非協調的問題。在第二個部分，我們研究了雙向中繼站傳輸網路的通道響應之估測。

載波頻率偏移和實虛部非協調是低成本通訊架構中常見的問題。載波頻率偏移會嚴重地破壞正交分頻多工系統當中的正交性，造成錯誤率大幅提升。而實虛部非協調的存在也會影響到載波頻率偏移的估測。因此在論文的第一部分，我們會探討有關載波頻率偏移和實虛部非協調的盲蔽估測問題。

首先，我們提出了一個利用循環前綴的演算法來盲蔽估測正交分頻多工系統中的載波頻率偏移和實虛部非協調問題。類似於其他循環前綴的方法，我們的方法複雜度也很低。在載波頻率偏移估測的部分，我們的方法能有效地對抗時間不同步造成的影響。此外我們還算出了理論上的錯誤率及克雷曼-勞氏邊界。在實虛部非協調估測的部分，我們的方法不需要知道真實的載波頻率偏移量。因此結合上述兩種方法，我們可以解決載波頻率偏移和實虛部非協調同時存在的問題。首先我們先估測被載波頻率偏移影響的實虛部非協調參數。接著將實虛部非協調作適當的補償。最後我們可以估測補償實虛部非協調影響後的載波頻率偏移量。

另一種估測載波頻率偏移和實虛部非協調參數的方法是利用振幅相等的星座圖。其中有個複雜度很低且很有效率的作法叫做頻率分析。頻率分析法相當簡單，但是在許多通訊系統中會有虛擬載波的存在，而虛擬載波會造成頻率分析法的錯誤率大幅提升。因此我們根據頻率分析法的理論基礎來探討虛擬載波造成的影響並提出了適當的修正。並且利用弦波近似和拋物線近似來降低這方法的複雜度。

在過去十年間，中繼站傳輸網路的研究逐漸盛行了起來。其中雙向中繼站傳輸網路因為傳輸率夠高而受到較大的關注。在這篇論文的第二部分，我們研究了放大轉發雙向中繼站傳輸網路的通道響應之估測。並將正交分頻多工系統應用於此類網路。該網路中的通道以兩段式步驟來做估測。首先我們利用能量降低的做法估測自我干擾的通道響應。其次利用子空間的作法估測真正發射端及接收端之間的通道響應。此外我們還算出了理論上的錯誤率及近似的克雷曼-勞氏邊界。

關鍵字

正交分頻多工系統 ； 盲蔽估測 ； 載波頻率偏移 ； 實虛部非協調 ； 放大轉發雙向中繼站傳輸網路 ； 通道響應

並列摘要

In this dissertation, we study blind algorithms for parametric estimation in orthogonal frequency division multiplexing (OFDM) systems. Our works include two parts. In the first part, we discuss the problem of carrier frequency offset (CFO) and in-phase and quadrature-phase (I/Q) imbalance in OFDM systems. In the second part, we study the identification of channels for OFDM-based amplify-and-forward two-way relay network (AF-TWRN) systems.

CFO and I/Q imbalance are two common front-end imperfections in low-cost communication devices. It is known that CFO is a crucial problem in OFDM systems. It can destroy the orthogonality between subcarriers and cause significant degradation in system performance. Moreover, the existence of the I/Q imbalance usually reduces the accuracy of CFO estimation. In the first part of the thesis, we study blind estimations for CFO and I/Q imbalance.

Firstly, we propose algorithms based on cyclic prefix (CP) for blind estimations of CFO and I/Q imbalance in OFDM transmission over multipath channels. Like other CP-based methods, the proposed methods enjoy low complexity, and closed form formulas are derived. For CFO estimator, the proposed method is very robust to the symbol timing synchronization error. In addition, we also carry out the theoretical mean square error (MSE) analysis and derive the Cramer-Rao bound (CRB). For I/Q parameter estimator, the proposed method does not need to know the CFO value. Hence, by combining these two estimators, we can solve the joint estimation problem of CFO and I/Q imbalance in two stages. First the I/Q parameter is blindly estimated in the presence of CFO and then the CFO can be blindly estimated after compensating the I/Q effect.

Another popular technique for blind estimation of CFO and I/Q imbalance is by exploiting constant modulus (CM) constellations. An effective low cost method, called frequency analysis (FA) method was proposed. The FA methods are simple and have closed form solutions. However, in many existing standards for communications, virtual carriers (VC) are introduced for the purpose of system design. When there are VCs, the FA methods degrade significantly. We propose the algorithms based on FA for blind estimation of CFO in the presence of VCs. The cost functions can be well approximated by sinusoidal and parabolic functions and thus the closed form formulas can still be exploited.

In the last decade, research on wireless relay networks became popular. In particular, the TWRN has drawn a lot of attention because its overall communication rate is approximately twice of that achieved in the one-way relay network (OWRN). In the second part of the thesis, we propose a blind channel estimation algorithm for AF-TWRN. The OFDM modulation is adopted for frequency selective channel. The proposed method can be also applied to zero-padded OFDM systems. The two cascaded channels are estimated in two steps. First the cascaded channel causing the self-interference is estimated using a proposed power reduction method. Then the other cascaded channel from source to destination is estimated by subspace method. Closed form formulas for channel estimates are derived. In addition, we also carry out the theoretical MSE analysis and derive the approximated CRBs.

並列關鍵字

OFDM ； blind estimation ； CFO ； I/Q imbalance ； AF-TWRN ； channel identification

參考文獻

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