多用戶雙向傳輸合作式中繼系統之中繼點選擇設計

Abstract

在本篇論文中，我們探討了在多用戶雙向傳輸合作式中繼系統裡中繼點的選擇設計。在研究中，我們考慮了一個更符合實際狀況的系統，包含了多個用戶、多個中繼點，以及一個終端點；然而，在絕大部份現有的研究中，作者考慮的都是一個用戶以及一個終端點。另一方面，在大部份現有的研究中，正交的通道時常被假設以用來避免干擾的產生。不過，如此的假設並不這麼符合實際狀況並且導致較差的頻帶效率。因此，在本篇論文中，我們在處理中繼點選擇問題的同時也將多用戶干擾納入考量。我們考慮在展頻系統(CDMA)中，展頻碼(spreading code)彼此不正交的狀況。由於訊號對干擾及雜訊比(SINR)在通訊系統中是通訊品質的指標，因此我們在處理中繼點選擇的問題時，將致力於最大化系統中所收到的較差的訊號對干擾及雜訊比做為選擇的標準。除此之外，為了消除干擾，我們同時考慮在每個中繼點做線性濾波器的設計。推導結果顯示，線性濾波器類似於最小均方差估測(MMSE detector)。最後，我們模擬了提出的架構在幾個不同的參數下的效應，例如考慮不同的用戶數目、中繼點數目，以及展頻碼長度。另外，我們也比較了所提出的中繼點選擇方法和任意的中繼點選擇方法之間效能的優劣，模擬結果顯示我們提出的方法的確可以得到選擇的增益，位元錯誤率(BER)要低於任意的中繼點選擇方法。

In this thesis, we study a relay selection (RS) problem in multi-user two-way cooperative relaying systems which multiuser interference is involved. Different from the most research activities, we investigate a more practical scenario which consists of multiple users, multiple relays and a single destination. Regarding the sources and the destination as the mobile handsets and base station, respectively, this scenario is more similar to communication systems in reality compared with those consisting of only one source and one destination. On the other hand, we take multiuser interference into account, however, it is not the case for the most works. Channel orthogonality is assumed frequently in many studies to avoid interference. Nevertheless, such assumption is not so realistic and degrades the bandwidth efficiency. To be closer to reality or more bandwidth efficient, taking the multiuser interference into consideration is necessary. In our work, we consider a code division multiple access (CDMA) system with non-orthogonal spreading sequences. Since signal to interference-plus-noise ratio (SINR) is a benchmark of communication quality, RS based on maximizing the SINR of worse link is performed in our study. Besides, aiming at mitigating the interference, we consider the design of linear filter at each relay as well. The result shows that the linear filter is similar to minimum mean-square error (MMSE) detectors. Furthermore, we simulate the proposed scheme with several different parameters such as the number of users and relays, and the length of spreading sequences. Also, we compare the proposed relay selection method with random relay selection approach, and the result shows that our proposed method has better performance in terms of the bit error rate.