為了支援在無線通信下可靠的傳輸，混合式自動重送請求的方法已被廣泛應用在許多無線通信系統中。雖然混合式自動重送請求的方法可以藉著重傳的機制來增加傳輸的可靠性，但傳輸的效率卻會被降低。因此，在本篇博士論文中，我們考慮網路編碼的技術。網路編碼合併是一個能改善傳輸效率的有效方法，並且被廣泛的研究和利用在許多無線通信系統中，例如雙向中繼傳輸系統，廣播／群播系統和合作網絡。在這些系統當中，網路編碼合併通常在位元層級下執行，因此，為了做資料偵測，使用硬式決策和合併是最簡單的方法。然而，在編碼傳輸的系統下，例如混合式自動重送請求傳輸系統，使用軟式決策和合併是非常理想的方法。因此，為了改善無線混合式自動重送請求串流傳輸系統的傳輸效率，在無線串流傳輸系統下，我們提出一個基於網路編碼之混合式自動重送請求協定。
首先，我們提出一個基於網路編碼分集的方法，在符元層級下，合併多個符元產生一個分集符元。延伸至封包層級，我們提出一個產生基於網路編碼分集封包的方法，藉著有彈性地合併多個錯誤解碼的資料封包，產生一個分集封包。有鑑於此，我們提出一個基於網路編碼之混合式自動重送請求協定的傳輸和重傳機制，此機制可以有效率地傳送資料封包和重傳分集封包，並且不需要花費額外的功率資源或頻寬資源。根據收到的多個資料封包和一個或者是多個分集封包，接收端可以聯合解碼出所有相關的資料封包。
除此之外，從星座映射的角度切入，我們提出兩個方法，都可以更進一步地改善基於網路編碼之混合式自動重送請求協定的接收效能。一個名為雙映射法，另一個則名為基於閾值部份累加對數相似比值法。首先，為了最小化未編碼之位元錯誤率，一個星座映射最佳化的問題被提出來討論。緊接著，我們提出雙映射法，此法中，原始的資料封包和網路編碼合併的分集封包分別使用不同的星座映射，並且為了避免對位元錯誤率之效能產生影響，我們只考慮使用格雷映射。我們提出兩項理論分析，一項是位元錯誤率近似值之運算，另一項是位元錯誤率上限值之運算。觀察模擬和分析的結果，提出的雙映射法優於傳統的單映射法，也就是原始的資料封包和合併的分集封包皆使用相同的星座映射。
另外，基於雙映射法，從一組具有相關性的符元中，一個提取高質量軟式訊息的問題也被提出來討論。因此，為了改善軟式訊息提取的質量，我們提出一個新的軟式訊息提取法，並且命名為基於閾值部份累加對數相似比值提取法。相較於傳統的對數相似比值提取法，我們提出的方法有較低的計算複雜度和很好的偵測效能。分別針對16-QAM和64-QAM，我們提出方法的複雜度可以大幅度地降低95.7%和98.24%。兩種通道編碼的方式，迴旋碼和渦輪碼皆被使用來做系統效能的評估。不論是哪一種編碼方式，我們提出的基於網路編碼之混合式自動重送請求協定和兩個方法都可以明顯地降低為了做分集重傳所耗費的資源需求量，進而大幅度地改善整體的系統吞吐量。當使用迴旋碼時，分別在16-QAM和64-QAM下，整體的系統最大吞吐量增加了24%和28%；當使用渦輪碼時，分別在16-QAM和64-QAM下，改善的最大增益甚至達到36%和41%。

To support reliable transmission in wireless communications, the hybrid automatic-repeat-request (HARQ) approach is widely adopted in many wireless communication systems. Although the HARQ approach can increase the transmission reliability by the retransmission mechanism, the transmission efficiency is decreased. Therefore, network coding (NC) technique is considered in this dissertation. The NC combining is an effective method to improve the transmission efficiency, and is widely investigated for using in many wireless communication systems, such as two-way relaying systems, broadcast and multicast systems, and cooperative networks. In these systems, since the NC combining is generally performed at the bit-level, using hard decision and combining is the simplest approach for data detection. However, for a coded transmission system, e.g., an HARQ transmission system, using soft decision and combining is highly desirable. Therefore, in order to improve the transmission efficiency of wireless HARQ streaming transmission system, we propose an NC-based HARQ protocol in the wireless streaming transmission system. We first propose an NC-based diversity (NCD) scheme which combine multiple symbols to obtain a diversity symbol at the symbol-level. Extended to the packet-level, an NC-based diversity packet generation is proposed, where the diversity packet can be generated by flexibly combining multiple erroneously decoded data packets. Accordingly, a transmission and retransmission mechanism of the NC-based HARQ protocol is proposed, which can effectively transmit the data packets and retransmit the diversity packets without the need for extra power or bandwidth resources. The receiver can jointly decode all the corresponding data packets based on the reception of the data packets and one or multiple diversity packets.
In addition, the two approaches are investigated from the perspective of constellation mapping, which can further improve the reception performance of the proposed NC-based HARQ protocol. One is the two-mapping (TM) approach, and the other is the threshold-based partial accumulated (TPA) log-likelihood ratio (LLR) approach. First, the problem of constellation mapping optimization for uncoded bit error rate (BER) minimization is investigated. The TM approach is then proposed, where the different constellation mappings are respectively used for the original data packets and the NC-combined diversity packets, and constraint on only using gray mappings to prevent an impact on BER performance. We provide two theoretical analyses, an approximation of the BER and an upper bound of the BER. The simulation and theoretical results demonstrate that the proposed TM approach is better than the conventional single mapping (SM) approach, where the constellation mapping is the same used for the original data packets and the combined diversity packets.
Second, based on the proposed TM approach, the problem of extracting high quality of soft information from a set of correlated symbols is studied. Therefore, a novel soft information extraction scheme, named as TPA LLR extraction, for improving the quality of soft information extraction is proposed. When comparing with the traditional LLR extraction schemes, the proposed scheme has lower computation complexity and good detection performance. The complexity of the proposed scheme is significantly reduced to 95.7% and 98.24% respectively for 16-QAM and 64-QAM. Two channel coding schemes, convolutional code and turbo code, are considered in the system performance evaluation. No matter in which coding scheme, the proposed NC-based HARQ protocol and the two proposed approaches can significantly reduce the amount of resources required for diversity retransmission, thus greatly improving the overall system throughput. When considering the convolutional code, the overall system throughput is greatly improved, with a maximum gain of 24% and 28% for 16-QAM and 64-QAM, respectively; when considering the turbo code, the maximum improvement gain even reaches to 36% and 41% respectively for 16-QAM and 64-QAM.

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