可變式結構的最大事後機率子載波干擾等化器之設計與模擬

Abstract

隨著無線通訊科技的演進，在我們的日常生活中出現了越來越多的行動通訊應用。而為了要滿足現代人們對於數據資料和多媒體應用的大量需求，也促使了能支援更快速傳輸速率的通訊技術產生。對比於傳統的通訊傳輸，正交分頻多工傳輸技術就被認為是相當有前景的一項技術。正交分頻多工傳輸技術主要優點在於它的高頻譜使用效率以及能夠更有效地對抗多種路徑衰減效應，但在高速移動環境中，由於各個子載波間正交性遭到破壞，子載波間的干擾會使得錯誤率提高，造成系統效能降低。為了要消除和彌補如此的通道失真，這個領域的研究向來是一項相當熱門的議題。 從先前的文獻探討得知在通道等化技術上主要分為時間域和頻率域兩大類型。而在這篇論文中，我們著重在頻率域等化方法的討論。在頻率域等化方法裡，最常見的就是圍繞著通道頻率響應矩陣去做估計、處理。先前文獻探討裡提出的子載波干擾指標比比傳統等化方式更能藉由此指標更精確地估計出每個子載波上受到干擾的程度。先前的文獻中雖在軟體模擬上的運算複雜度和系統效能取得更好的平衡點，但卻未討論到硬體實作的層面。 而本篇論文將利用此子載波指標搭配最大事後機率演算法上並加以改良成適合用於硬體實現的方法與討論。本篇論文的章節架構為: 第一章背景介紹及研究動機。第二章系統模型及子載波衡量指標的介紹。第三章介紹傳統最大事後機率演算法以及化簡版本。第四章為改良最大事後機率子載波干擾等化器的硬體架構介紹。第五章為軟體及硬體模擬結果及討論。第六章為本篇論文摘要及結論。

In the high mobility scenario, the wireless communication systems that adopts Orthogonal frequency-division multiplexing (OFDM) transmission scheme will experience rapid channel variations such as high-speed railway. It is considered that will induce inter-carrier interference (ICI) and BER performance degradation due to the orthogonality between subcarriers is destroyed in OFDM transmission systems. In order to eliminate the distortions, the research topics in the field of channel compensations are attractive for many years. The channel equalization techniques can be classified into time-domain and frequency-domain methods. The goal of this thesis is design a low complexity ICI cancellation scheme in frequency-domain, which is easier to be implemented in practical. The most common technique to deal with the distortion causing by the ICI is focus on the channel frequency response (CFR) matrix. It is known that the MAP receiver based on BCJR algorithm offers optimal BER performance but the computational complexity is too high to implement in practical systems. From empirical observations, a measure which is called ICI indicator was introduced to indicate the ICI level on each subcarrier [1], which offers more precise ICI measurements than conventional methods. Armed with the ICI indicator, the modified BCJR algorithm can achieve better balance between computational complexity and BER performance. However the previous works haven’t mentioned the explorations on hardware. The goal of this thesis will modify BCJR algorithm with the help of the ICI indicator to make it more appropriate to implement on hardware. We can show some advantages on hardware through the contributions of this thesis. Discussions and simulation results both on software and hardware will also be provided in the thesis. This thesis is organized as follows. Chapter1will introduce background and motivations. Chapter2 gives an introduction of OFDM-based system model over doubly selective fading channels. Besides, a measurement which is called ICI indicator is also provided in chapter2. Chapter 3 will give an introduction of BCJR algorithm and discussion of simplified versions. MAP ICI equalizer based on simplified BCJR and incorporate with ICI indicator is also provided. Chapter 4 gives hardware design units of variable-width trellis MAP ICI equalizer. Chapter5 will give simulation results and discussions both on software and hardware. Finally, the conclusion of this thesis will be provided in Chapter6.