臺灣博碩士論文加值系統

通道效應，例如多路徑與時變衰竭將會降低無線通訊系統表現。其中，多路徑衰竭與都普勒效應在多載波系統中造成符號間干擾與子載波干擾。如不加以妥善處理，上述干擾將會嚴重影響聯結可靠度。與疊加性通道雜訊不同的是，符號干擾以及子載波干擾不能隨著信號雜訊比的增加而消除。因此，通道等化與干擾消除乃回復系統表現必要之道。

吾人提出一個新穎的頻域干擾消除器，予以抑制通道干擾(符號與子載波)、共同通道干擾，與重疊系統干擾。不同於以往的技術，此消除器演算法不需要時域過度取樣或者使用一個以上的接收天線。上述特點可以經由利用空載波(系指在標準多載波系統中必備的)與一個類似廣義旁瓣消除器來達成。此一方法可以達到比前人更佳的位元錯誤率與更簡易的計算複雜度。

接下來，吾人針對正交分頻多工系統提出一個迭代干擾消除演算法，用於抑制雙重衰變通道引發子載波干擾、共同通道干擾，以及重疊系統干擾。類似於上述技術，此方法不需要任何時域過度取樣或是使用一個以上的接收天線。使用空載波來接續迭代抑制干擾造成的有害影響，此法藉著擴展廣義旁瓣消除器的零空間，以取得更佳的干擾估計。模擬指出吾人提出的技術，零空間擴張干擾消除，優於前人所發表的單方衰變干擾抑制演算法。零空間擴張干擾消除的計算複雜度亦將被討論。

Channel effects such as multipath and time-varying fading have been known to hamper performance of wireless communication systems. In particular, multipath fading and Doppler effects causes intersymbol interference (ISI) and intercarrier interference (ICI) in the case of multicarrier systems. If not handled properly, these interferences can severely degrade link reliability. Unlike additive channel noise, BER degradation from ISI and ICI cannot be prevented by simply increasing the SNR. Hence, channel equalization and interference cancellation is often necessary in order to mitigate these detrimental effects.

A new frequency domain interference canceler is supposed to suppress channel induced interference such as ICI and ISI, co-channel interference, and overlaid systems interference. Unlike earlier schemes, this proposed algorithm requires no temporal oversampling nor the use of more than one receive antenna. All the above is achieved by exploiting null subcarriers (a.k.a. virtual / unused / unmodulated subcarriers) inherent in standard multicarrier systems, and by a generalized sidelobe cancellation (GSC) like scheme. This proposed method can offer superior bit error rate over earlier methods as well as simpler computations over another GSC like scheme.

Next, an iterative interference cancellation algorithm is proposed for OFDM based systems that can mitigate doubly-fading channel induced ICI, co-channel interference, and overlaid systems interference. Similar to the proposed scheme above, the present technique does not require any temporal oversampling nor the use of more than one receive antenna. This is achieved by properly exploiting the null subcarriers (NSCs) inherent in standard multicarrier systems to successively and iteratively suppress the adverse effects of the interference by expanding the nullspace of a generalized sidelobe cancellation like scheme to allow for better estimate of the interference. Simulation results show that the proposed technique, called Interference Cancellation via Subspace Expansion, or ICVISE, can outperform previously proposed technique which can only cancel interference caused by frequency-selective fading channels. The computational complexity of ICVISE and other related algorithms will also be illustrated and discussed.

摘要 ..................................................................................................................................i
Abstract ..........................................................................................................................iii
Acknowledgement .......................................................................................................vi
Contents .........................................................................................................................vii
Abbreviations and Notations .....................................................................................viii
List of Figures .................................................................................................................x
List of Tables .................................................................................................................xii
Chapter 1 Introduction ......................................................................................................1
1.1 Research Motivation ……...………………….....................................................1
1.2 Thesis Organization ……….................................................................................3
1.3 Publications .......................................................................................................3
Chapter 1 Background ......................................................................................................5
2.1 OFDM System Model……...………………….....................................................5
2.1.1 OFDM Standards with Null-Sub-Carriers..........................................................7
2.2 Doubly-Selective Fading Channel..........................................................................8
2.2.1 Characterization of Multipath Channel...................................................8
2.2.2 Doubly-Selective Channel Model for OFDM Systems....................................9
2.3 Previous Work..........................................................................10
2.3.1 Time Domain Approach……………………...................................................11
2.3.2 Frequency Domain Approach……………………........................................11
2.3.3 Iterative Methods…………………………………....................................12
Chapter 3 Interference Cancellation for OFDM Systems with Insufficient CP……...…..14
3.1 Proposed NSC-based Equalizer……….....................................................14
3.2 Results and Comparisons.…………………............................................................17
3.2.1 Computational Complexity............................................................................17
3.2.2 Monte-Carlo Simulations………………………………...............................18
Chapter 4 Equalization of Doubly-Selective Channels for OFDM-Based Systems……….24
4.1 Motivation and Previous Work…….….....................................................24
4.2 Methodology……...…………………............................................................27
4.2.1 System Model...................................................................................27
4.2.2 Proposed Scheme…………………………...........................................29
4.3 Simulation Results………………….…............................................................34
4.3.1 Computational Complexity..............................................................................35
4.3.2 Monte-Carlo Simulations…………………………...........................................37

Chapter 5 Conclusions and Future Works………………....……………………………..41
5.1 Conclusions…………………..………............................................................41
5.2 Future Works………….……………............................................................42
Bibliography....................................................................................................................43

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