空間塊狀編碼正交分頻多工系統之效能分析

Performance Analysis of Spatially Block Coded OFDM Systems

江炳宏(Ping-Hung Chiang)

指導教授 ： 李學智

共同指導教授 ： 林丁丙(Ding-Bing Lin)

國立臺灣大學/電機資訊學院/電信工程學研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.01989

摘要

近年來，人們對於高效能第四代寬頻無線行動通訊系統的需求己與日俱增，因此同時使用多根天線在基地台與用戶端也變得不可或缺。多天線技術能提供網際網路及多媒體服務所需之高傳輸容量並且大大地增加無線傳輸的範圍與可靠度。在衰落環境下，具有多根天線在發射機與接收機的多輸入多輸出系統能提供多樣性。藉由使用多天線，多重空間通道可被建立，而且這些通道不太可能同時遭受嚴重衰落。
由於具有抵抗多路徑衰落以及能提供極高資料傳輸率的能力，正交分頻多工己被納入多項無線標準中，如數位音訊廣播(DAB)、陸地數位視訊廣播(DVB-T)、IEEE 802.11a無線區域網路(WLAN)以及IEEE 802.16無線大都會網路(WMAM)。採用正交分頻多工而不採用單載波技術的原因為其用於處理高通道延遲擴散問題的等化器具有較低的複雜度。透過將可用頻寬分割成許多窄頻且遭受近似平坦衰落的子載波，它可減低頻率選擇性衰落的效應。藉由結合正分頻多工與多輸入多輸出技術，通道響應變成一個矩陣。因為每一個子載波可被獨立地等化，時空等化器的複雜度可被避免。基於上述，多輸入多輸出正交分頻分工，即多輸入多輸出與正交分類多工的結合，己被採納為第四代空中介面之一。
為了對這最有希望的第四代空中介面-多輸入多輸出正交分頻分工-有更深入的了解，我們研究它在多路徑時變瑞雷衰落通道的同調與非同調系統架構。針對Alamouti技術，我們分析、討論並比較兩輸入多輸出正交分頻多工系統的同調與非同調偵測機制，更進一步說明如何將上述概念推廣至具有三及四根傳輸天線的多輸入多輸出正交分頻分多工系統。藉由針對系統實作和位元錯誤率效能的同調與非同調系統架構比較，我們得到並總結一些重要且有助於系統設計的論點。
本篇論文之主要貢獻總結如下：在第三章中，對於Alamouti分集系統，我們推廣由Vielmon等人所提出之零仰制、決策回授及共最大可能性偵測器至其兩輸入多輸出型式並且推導出其對應之位元錯誤率表示式。就零仰制偵測器於劇烈變化通道下，我們明確地證明了由傳送分集所帶來之分集好處不再存在。對於決策回授偵測器，我們所推導之位元錯誤率表示式包含了錯誤蔓延的效應。在第四章中，我們以階層的方式討論非同調序列偵測及其特例並將之應用於單輸入多輸出和兩輸入多輸出正交分頻多工系統。就非同調兩輸入多輸出正交分頻多工系統，我們提出了三種新的系統架構。我們也作了一些有關差分編碼方向的討論，這些討論對於設計非同調正交分頻多工系統是非常重要的。

關鍵字

天線分集 ； 最大可能性偵測 ； 多輸入多輸出 ； 非同調最大可能性序列偵測 ； 正交分頻多工 ； 效能分析 ； 塊狀時空碼。

並列摘要

Recently, the demand for high performance fourth-generation (4G) broadband wireless mobile communication systems has increased remarkably and hence using multiple antennas at both base station and subscriber ends becomes a must. Multiantenna technologies can provide high transmission capacities required by Internet and multimedia services and also dramatically increase the range and reliability of wireless transmission. In a fading environment, a multiple-input- multiple-output (MIMO) system having multiple antennas at the transmitter and receiver can provide diversity. By employing multiple antennas, multiple spatial channels are created and it is unlikely all channels will fade simultaneously.
With its natural resistance to multipath fading and its ability to support extremely high data rates, orthogonal frequency division multiplexing (OFDM) has been adopted in several wireless standards such as digital audio broadcasting (DAB), terrestrial digital video broadcasting (DVB-T), IEEE 802.11a wireless local area network (WLAN), and IEEE 802.16 wireless metropolitan area networks (WMAN). Rather than a single carrier solution, OFDM is chosen as a result that its equalizer dealing with the problem of high channel delay spread has lower complexity. It reduces the effect of frequency-selective fading by dividing the transmission bandwidth into many narrowband subcarriers, each of which exhibits approximately flat fading. By combining OFDM with MIMO techniques, the channel response becomes a matrix. Since each subcarrier can be equalized independently, the complexity of space-time equalizers is avoided. Thereupon, MIMO OFDM, a combination of MIMO and OFDM, is adopted as one of the 4G air interfaces.
To gain more insights of MIMO OFDM, the most promising 4G air interface, we study its coherent and noncoherent system structures in multipath time-varying Rayleigh fading channels. Based on the Alamouti scheme, we analyze, discuss, and compare two-input-multiple-output (2IMO) OFDM systems with coherent and noncoherent detections. The extensions to MIMO OFDM systems with three and more transmit antennas are also provided. By comparing coherent and noncoherent system structures based on implementation issues and bit-error-rate (BER) performances, we obtain and summarize some important points which are useful for system design.
The major contributions of this dissertation are summarized as follows. In Chapter 3, for Alamouti diversity systems, we generalize the zero-forcing (ZF), decision-feedback (DF), and joint ML (JML) detectors, proposed by Vielmon et al., to their 2IMO versions and derive the corresponding BER expressions. For the ZF detector, we prove explicitly that, in extremely varying channels, the diversity advantage from transmit diversity vanishes. For the DF detector, we derive its BER expression which takes the effect of error propagation into account. In Chapter 4, we interpret the NSD and its special cases hierarchically and apply them to noncoherent SIMO and 2IMO OFDM systems. For the noncoherent 2IMO OFDM systems, we propose three novel system structures. We also give some remarks on differential encoding directions, which are crucial in designing noncoherent OFDM systems.

並列關鍵字

Antenna diversity ； maximum-likelihood (ML) detection ； multiple-input-multiple-output (MIMO) ； noncoherent ML sequence detection (NSD) ； orthogonal frequency division multiplexing (OFDM) ； performance analysis ； space-time block coding (STBC).

參考文獻

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