在無線網路中，合作式通訊利用分散式的節點，有效的增加空間分集的特性，以減輕通道衰弱的影響。利用這些中繼站的幫助，使得資料在傳輸的過程中更為可靠，相較於傳統的多輸入多輸出的系統，也能夠將低硬體設計的複雜度。因此，選擇一個最佳的中繼點將成為重要的課題。
而近來雙向中繼網路受到大家的注目。傳統的單向中繼網路，兩個用戶完成一次交換訊息需要四次通道的使用，使得頻寬效益不高。而雙向中繼網路只需要兩次通道的使用，有效的改善頻寬效益。
在此篇論文中，我們在雙向解碼後轉送中繼網路的系統提出了選擇中繼點的方法。一開始，我們先針對雙向中繼解碼後轉送網路的系統去分析整體平均字符錯誤率，再根據我們分析並化簡過後的錯誤率結果，提出我們選擇中繼點的方法，有基於通道增益以及距離的方法。模擬結果將顯現出，我們的方法優於幾個常見的選擇中繼點的方法。此外，我們也找出雙向中繼解碼後轉送網路的系統中繼點最佳的位置。

In this thesis, we propose a single relay selection scheme based on symbol-error-rate (SER) performance analysis for two-way decode-and-forward (DF) relay networks. The proposed scheme aims at selecting the best relay that minimizes the overall average end-to-end SER of the system, where the multiple-access phase, along with the broadcasting phase, is considered. By contrast, the previous works only took the broadcasting phase to form their selection criteria. We first derive a closed-form average end-to-end SER of the system. Then the SER expression is further approximated in the high SNR regime to form a computation-affordable relay selection criterion. The theoretical result of our SER analysis is verified by Monte-Carlo simulations. Moreover, simulation results show that the proposed relay selection scheme achieves significant performance improvement over the random selection scheme and performs very close to the optimal selection scheme in the sense of minimum SER.

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