協同式無線網路下中繼選取技術之效能分析

Performance Analysis of Relay Selection in Cooperative Wireless Networks

吳念恩(Nien-En Wu)

指導教授 ： 李學智

國立臺灣大學/電機資訊學院/電信工程學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.02747

摘要

為了達到無線網路下高傳輸率與高服務品質的需求，協同式通訊被視為大有可為的新興技術。在具有多中繼節點的協同式網路下，中繼選取技術擁有許多吸引人的優勢，例如可放寬節點間所需的協調、高頻譜使用效率，以及完整的空間上多樣性。此論文探討關於中繼選取技術的兩個重要議題：“中繼選取技術在不完美通訊環境下的效能如何?”以及“實際上如何實現中繼選取技術?”。更明確地說，我們將從理論效能分析與實務上實現機制的角度來探討中繼選取技術。
在雙跳躍解碼前送協同式網路下，投機式轉傳(Opportunistic relaying)與選擇式合作(Selection cooperation) 為最主要的兩種中繼選取技術。在投機式轉傳技術中，中繼選取是基於雙跳躍的中繼通道路徑並且是在傳送端開始傳送前完成。而在選擇式合作技術中，中繼選取是基於第二次跳躍的通道路徑並且是在傳送端開始傳送之後完成。在現實的無線網路中，同頻干擾(Co-channel interference) 與過時通道資訊(Outdated channel state information) 可能會發生且造成效能上的衰減。在此論文的第一部分，我們提出一個用於廣泛地探討同頻干擾與過時通道資訊對於中繼選取效能的影響之分析架構，其衡量效能的準則為符元錯誤率(Symbol error probability) 或中斷機率(Outage probability)。在同頻干擾影響下，我們討論投機式轉傳架構下兩種中繼選取的方式，分別為基於訊號雜訊比與基於訊號干擾雜訊比。而在過時通道資訊下，我們比較投機式轉傳與選擇式合作這兩種技術。理論結果證明以可達到的多樣性階級而言，相較於同頻干擾，中繼選取較易受到過時通道資訊的影響。最後，在同頻干擾與(或) 過時通道資訊的情況下，我們探討三種特別受到矚目的無線中繼轉傳之應用：感知中繼網路、安全協同式網路與漸進式中繼網路。
在此論文的第二部分，為了在無集中式實體的情況下於實務上實現中繼選取，我們分析一個分散式基於分割的(Decentralized splitting-based)選取機制。在分散式基於分割的中繼選取架構中，每個中繼點根據它的通道增益與一個預先定義的門檻值來局部地決定是否要傳送旗標訊號。依照此方法，擁有較差通道增益的中繼點將被排除，而不用依賴集中式實體來尋找最佳中繼點。我們藉由探討該預先定義門檻值的影響，從實用性與效率的角度來討論這個分散式基於分割的方法，對應的效能評量標準為平均搜尋的時間。我們推導保證能在有限搜尋時間內尋找到最佳中繼點的門檻值設計準則，也提供可使得平均搜尋時間漸進性與中繼點數目無關的門檻值設計準則。

關鍵字

協同式通訊 ； 中繼選取 ； 效能分析

並列摘要

Cooperative communication is a promising technique proposed to meet the increasingly high rate demands and quality of service requirements in wireless networks. In multi-relay cooperative networks, relay selection offers several attractive benefits such as relaxation of inter-node coordination, high spectral efficiency, and full spatial diversity. In this dissertation, we address two fundamental issues on relay selection, namely,“how relay selection performs in imperfect communication environments?”and “how to select the best relay in practice?”. More specifically, we investigate relay selection from the perspectives of theoretical performance analysis and practical implementation mechanism.
Opportunistic relaying (OR) and selection cooperation (SC) are two major relay selection schemes for dual-hop decode-and-forward cooperative networks. In OR, the relay selection is performed based on the metrics of the dual-hop relaying paths before the source transmissions. In SC, on the other hand, the best relay is selected by using the metrics of the second-hop paths after the source transmissions. In practical wireless networks, co-channel interference (CCI) and outdated channel state information (CSI) may occur and cause performance degradation. In the first part of this dissertation, we provide an analytical framework for comprehensively studying the effect of CCI and outdated CSI on the performance of relay selection. Important performance metrics such as symbol error probability or outage probability of relay selection are examined under CCI or outdated CSI. Specifically, in the presence of CCI, we study OR with signal-to-noise ratio-based and signal-to-
interference-plus-noise ratio-based selection criteria. In the scenarios with outdated CSI, we analytically compare OR and SC. The theoretical results show that relay selection is more susceptible to outdated CSI than CCI in terms of the achievable diversity order. Finally, three wireless relaying applications of particular interest, specifically, cognitive relaying networks, secure cooperative networks, and incremental relaying networks are investigated under CCI and/or outdated CSI.
In the second part, we discuss and analyze a decentralized splitting-based (DSB) selection mechanism for practical implementation of relay selection without the existence of a centralized entity. Particularly, in the DSB relay selection scheme, each relay decides locally whether or not to transmit according to its own metric and a pre-defined threshold. In this way, relays with poorer metrics are excluded and the best relay is determined without the need of a centralized entity. We study the DSB method from perspectives of utility and efficiency by examining the effect of the pre-defined threshold with average searching time as the relevant performance metric. We derive the criteria of the threshold, which guarantee the determination of the best relay within finite searching time. The threshold criterion under which the average searching time is asymptotically irrelevant to the number of relays is
also provided.

並列關鍵字

Cooperative communication ； Relay selection ； Performance analysis

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