無線雙向中繼傳輸之前送策略研究

Abstract

本篇論文介紹在無線雙向中繼傳輸下的前送策略。在雙向中繼傳輸中，中繼站同時收到由兩端點站傳來的訊號，中繼站接著將收到的混合訊號廣播給兩端點站。利用自身傳送的資料，端點站可以自混合訊號中擷取出預期收到的資料。 在本篇論文中，我們首先簡介中繼傳輸模型以及兩種基本的前送策略：放大前送和解碼前送。接著我們著重於一種以放大前送為基礎而發展出來的前送策略，這種前送策略我們稱之為摺疊前送。我們討論在使用二元調變的折疊前送系統中，中繼站的運作機制以及端點站的資料偵測規則。我們在二維訊號平面進行錯誤率分析，並且將摺疊前送的字元錯誤率表示式由雙重積分化簡為數個單一積分的總和。模擬結果顯示在未編碼的系統中，當通道為對稱狀態時，摺疊-前送可以提供比放大前送高1-1.5 dB的效能增益。所謂的通道對稱是指兩個端點站與中繼站的鏈結有相同的通道增益。當通道狀態不對稱時，摺疊前送所能提供的效能增益會隨之減少。 接下來我們考慮高次調變的摺疊前送系統。在高次調變的系統中，摺疊前送的運算必須避免解碼模糊的問題。解碼模糊的問題是指端點站利用自身傳送的資料也無法將對方端點站的資料辨別出來。我們提出使用適當的摺疊閾值來避免解碼模糊問題。模擬結果顯示在高次調變的系統中，前送策略的選擇，包括是否使用摺疊前送以及該選擇何種摺疊閾值等，必須根據通道狀態、端點站的調變型態及訊號強度來進行調整。

This thesis introduces the forwarding strategies for wireless bidirectional relay transmission. In bidirectional relay transmission, a relay receives simultaneously arrived signals from two terminals and then forwards the mixture of signals to the terminals. With the help of self data, each terminal can extract desired data from the received mixing signal. In this thesis, we first give a brief overview on relay transmission models and the two basic forwarding strategies: amplify-and-forward (AF) and decode-and-forward (DF). Then we focus on an AF-based forwarding strategy, which we termed fold-and-forward (FF). We discuss the relay operation and the corresponding detection rule at the terminals in a FF system with BPSK modulation. We also give error analysis in a two-dimensional signal plane, which simplifies the symbol error probability expression of FF system from a double integration into a sum of single integrations. Simulation results show that in an uncoded system, FF provides 1-1.5 dB gain over AF in high signal-to-noise ratio (SNR) region when the channels are symmetric. “The channels are symmetric” means that the two terminal-to-relay links have the same channel gain. The performance gain of FF system degrades when the channels become asymmetric. Then we consider FF system with higher order modulation. In higher order modulation system, the design of FF operation must avoid a decoding ambiguity problem, which means that the terminal cannot distinguish data from the other terminal with the help of self data. We address the use of a proper folding threshold to avoid the ambiguity problem. Simulation results show that in high order modulation system, the choice of forwarding strategy, including whether to use FF or not and what folding threshold should be selected, depends on the channel conditions, modulation type and the signal strength of the terminals.