Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/6181
標題: 直序展頻分碼多重接取與正交分頻分碼多重接取之訊號處理:干擾抑制,時序回復與空時訊號處理
Signal Processing for DS/CDMA and OFDM-CDMA Systems: Interference Suppression, Timing Recovery and Space-Time Processing
作者: 陳俊達
Chen, Jun-Da
關鍵字: CDMA, OFDM, Interference, Timing Recovery, Space-Time Processing
分碼多重接取, 正交分頻多重接取,干擾, 時序回復, 空時訊號處理
出版社: 電機工程學系所
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摘要: 分碼多重接取可以比分時多重接取及分頻多重接取提供更大的系統容量。不像另外兩種多重接取技術,分碼多重接取的容量是受限於多重接取干擾和遠近效應,而不是雜訊。在多使用者的環境下,展頻碼的正交性會由於多重路徑延遲而嚴重的失真,以致於系統容量會受限於多重接取干擾。因此,為了改善整個系統的效能,干擾抑制是很重要的技術之ㄧ。展頻通訊要求傳送端和接收端的展頻碼同步。即使兩者間只有些許的不同步,接收的能量也會不足而造成系統效能的降低。此外,利用天線陣列可以增加連結品質和頻譜效益,而進ㄧ步應用於多傳輸率的系統中。未來通訊的另一項主題是正交分頻分碼多重接取之技術;此技術我們針對多載波分碼多重接取與多載波直序展頻分碼多重接取兩種方法來探討。上述提及的問題將會在此論文中討論。 首先,我們考慮多重接取干擾及訊符干擾的抑制,同時包含了資料偵測。此適應性濾波器的價值函數是根據解展頻的結果及資料的輸出來組成。它是一種在訊符層級來更新權重的簡單架構。此架構利用權重的更新來讓解展頻的輸出能收斂到期望結果。因此,此接收機的計算複雜度會比傳統的碼切片層級接收機來的低。其次是介紹以廣義旁波瓣消除器為基本的盲蔽式接收機,此接收機是將信號空間分隔為限制子空間及正交子空間。一般而言,廣義旁波瓣消除器是在空間上被用來消除旁波瓣以突顯期望信號的陣列裝置。我們延伸其在空間上的概念到時間上並應用在盲蔽式適應性濾波器。此濾波器是利用廣義旁波瓣消除的概念來消除多重接取干擾,來將問題轉換到非受限的最小化。 在直接序列的展頻系統中,傳統的延遲鎖定迴路被用來作為碼同步之用。然而,延遲鎖定迴路的設計並沒有考慮到通道衰減的存在。在我們所提出的接收機架構裡,適應性濾波器被用來估計期望信號及抑制多重路徑和多重接取干擾的效應。同時,我們使用適應性演算法來利用上述相同的錯誤信號來估計碼延遲。除了上述方法外,我們還應用了決定回授的差分檢測技術於接收機來完成非同調檢測。最後,以訊符為基本的適應性天線接收機將被應用在多頻取樣的分碼多重接取系統中。當智慧型天線應用於分碼多重接取系統中,天線權重的更新方程式會與展頻碼向量有關,而我們提出的架構是在時域上調整解展頻碼的值。我們將此架構稱為維納碼濾波器,其動作與維納濾波器類似。維納濾波器在本質上具有對抗干擾的特性。而維納碼濾波器不只具有在時域上來對抗干擾的能力,同時也有解展頻的功能。我們可將其視為維納濾波器及解展頻的結合。 正交分頻分碼多重接取是結合了正交分頻多工與分碼多重接取優點的一種技術。在未來的高資料傳輸率的無線通訊系統,正交分頻分碼多重接取比起直序展頻分碼多重接取來的有吸引力。它可以被分成兩類。一種是將所要傳送的元件經由許多的子載波同時的傳輸;另ㄧ種是經由窄頻的直序波形來並列傳送的技術。在本論文中,前面的系統稱為多載波分碼多重接取,而後者為多載波直序分碼多重接取。我們將考慮當此兩種架構經由衰減通道時,幾種不同的適應性接收機架構。
Code-division multiple access (CDMA) has been claimed to offer large capacity improvements over time-division multiple access (TDMA) and frequency-division multiple access (FDMA) in cellular radio systems. Unlike the other two multiple access techniques, the capacity of CDMA tends to be limited by multiple access interference (MAI) and the near-far problem rather than noise. In a multiuser environment, orthogonality among spreading codes is severely distorted due to multipath delay spread, such that the system capacity will be limited by the incurred MAI. Effective interference reduction will increase system capacity, which means interference suppression techniques are necessary in improving the overall system performance. Spread spectrum communication needs to synchronize the spreading waveforms in transmitting and receiving ends. If the two waveforms are out of synchronization by a little chip time, insufficient signal energy will reach the receiver, the performance of data demodulation is thus degraded. Besides, the use of antenna arrays for diversity techniques can enhance the link quality and spectral efficiency, and further apply to multirate schemes. Another challenge is the combination of orthogonal frequency division multiplexing (OFDM) and CDMA: the OFDM-CDMA technique; we classify it to multicarrier code-division multiple access (MC-CDMA) and multicarrier direct-sequence code-division multiple access (MC-DS-CDMA). The above mentioned issues will be discussed in detail in this thesis. Firstly, we consider the problem of simultaneous MAI-rejection, intersymbol interference (ISI) suppression and data detection. This adaptive receiver is based on cost functions evaluated at the code despreader''s and data detector''s output. It is a bit-rate adaptive receiver and is simple and is designed in such a way that the despreader output statistic would converge to desired property. The filter weights are updated only once every bit interval. Therefore, the computational complexity for our receivers is lower than that of the conventional chip-decision methods. Secondly, the blind generalized sidelobe canceller (GSC) -based receiver is introduced by dividing the signal space into two subspaces, a constraint subspace and an orthogonal subspace. In general, the GSC is used to cancel the sidelobe to highlight the array manifold of signal of interest in spatial domain. We extend the spatial concept of the GSC Filter to temporal domain and then refer to blind adaptive filter which can be converted to an unconstrained minimization problem by using GSC to eliminate MAI. In direct-sequence (DS) spread spectrum (SS) systems, the conventional delay lock loop (DLL) has been considered for pseudo-noise (PN) code tracking, even though the DLL in not designed for operation when channel fading is present. In our proposed receiver, the adaptive filter is used to estimate the desired signal and suppress the multipath and MAI effects based on the received signals. Simultaneously, we use the adaptive algorithm to estimate the code delay through the same error signal. In addition to above process, the noncoherent receiver employs the decision-feedback differential detection to recover the MDPSK signal. At last, the symbol-based adaptive antenna receivers are employed in multirate CDMA systems. The weight updated equations of the adaptive antennas are related to the spreading code vector when smart antennas are used in CDMA systems, and we adjust the despreading code in time domain. We call this new scheme Wiener code filter, which behaves like the Wiener filter. Wiener filter in the adaptive antenna that has the ability to be against interference in nature. Wiener code filter of adaptive antenna not only has the ability to against interferences in time domain but also has the function of despreader in CDMA multiuser environment. We can regard it as the structure of combining the Wiener filter and despreader. The OFDM-CDMA, which combines CDMA with OFDM, has become an attractive alternative to DS-CDMA for future high-data-rate wireless communication systems. OFDM-CDMA systems can be categorized mainly into two types. One conveys all elements of each transmittted sequence over a number of subcarriers simultaneously, and the other is a scheme of parallel transmission of narrowband DS waveforms. In this thesis, the former system is specifically refered to as MC-CDMA and the latter one as MC-DS-CDMA. We will concentrate on the construction of adaptive equalization for MC-CDMA and MC-DS-CDMA communication systems over a dispersive channel.
URI: http://hdl.handle.net/11455/6181
其他識別: U0005-0606200614244600
文章連結: http://www.airitilibrary.com/Publication/alDetailedMesh1?DocID=U0005-0606200614244600
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