應用於中繼通道之正交振幅調變訊號之放大前送策略研究與設計

Abstract

隨著通訊產業的發展，各種應用對於高速且穩定的資訊傳輸的需求也日益增加。 當透過無線通道傳輸訊息時，多天線傳輸系統(MIMO system)是已知能夠有效增加訊號之多樣性(Diversity)進而提高系統效能的一種可行方式，但多天線的通訊設備其複雜度必然高於單天線之設備，因此如何使用單天線之設備達成相近之效果成為了一個值得研究的課題。 合作式通訊( Cooperative communication)是一種利用中繼節點(Relay node)來中繼網路內傳輸之資料，以增加數個單天線通訊設備之間訊號傳輸之多樣性之方法。但使用中繼節點也同時會造成頻譜效率(Spectrum efficiency)的降低。近年來有許多對於合作式通訊之相關研究，證明了將網路編碼(Network coding)之概念應用到中繼網路(Relay network)架構中，可有效的減少因訊號的中繼機制而增加的額外傳輸時間。但是為維持傳輸之錯誤率，中繼節點必需加大其傳輸功率。運用參考資料[19]與[20]的基本概念，此篇論文進一步延伸現有技術與改善傳輸效能，文中將提出以減少可能出現的符碼(Symbol)數量之概念應用於放大前送(Amplify-and-forward)系統之方法，藉以降低中繼節點所需之傳輸功率。 此方法是利用在完美的通道下，經過網路編碼處理過的正交振幅調變訊號對複數座標原點具有對稱性且其絕對值仍可被接收端(Destination node)解碼的特性來減少所有可能出現的符碼之數量，但是當介於兩個傳送端(Source node)與中繼節點(Relay node)之間的兩通道(Channel)間具有相位差(Phase difference)時，則有可能出現無法將原始訊號還原之情形，本論文嚐試同時在兩傳送端送出訊號之相位上加上一微小的角度，以補償此現象，模擬結果顯示，此方法可一定程度的補償因通道相位差而造成的效能降低。 本論文也將提出能夠延伸應用至多於三個傳送端之中繼網路的網路編碼架構；當一個經過網路編碼的訊號中，僅包含一個未知符碼時，接收端即可利用已知之資訊對此訊號進行解碼，利用此特性可以使用目前已存在之雙向中繼通道(2-way relay channel)上之網路編碼技術，針對多個傳送端所傳輸之訊號進行網路編碼以及傳送順序的排序，本論文並將針對傳送端之數量與此方法之效能的關系進行分析。

As telecommunication technologies advance rapidly, there are increasing demands in applications that require high speed and stable data transmissions. While transmitting signals through wireless channels, the technique of multiple-input and multiple-output (MIMO) transmission is considered as an effective way to increase the capacity and reliability of general communication systems. However, the complexity of a multi-antenna device is higher than a single-antenna device. Hence, how to achieve similar effect to multi antennae with single-antenna devices is a topic worth investigation. A cooperative communication system can utilize relayed transmission to achieve cooperation diversity by a group of single-antenna devices, at the cost of reduced spectrum efficiency. It has been shown that by applying network coding to relay networks, one can reduce delays in data forwarding processes. However, it demands more transmission power than without network coding, in order to maintain the minimum distance of the resulted data constellation. In this thesis, we will propose a method that reduces the number of candidates of the resulted network-coded constellation for transmitted m-QAM data obeying AF protocol. The proposed method is based on the property that the constellation of network coded QAM signal will be symmetric to the origin, if the channels are perfect. And the absolute value of a signal point in the constellation still can be decoded by the other source in the 2-way relay channel. However, if the two channels from the two sources to the relay have phase difference, decoding performance based on the minimum distance will degrade. This thesis tries to shift the phase of the two signals transmitted by the sources by a small amount so as to compensate the effect of the phase difference between channels. Simulation results shows that, the proposed method can reduce the phase effect, and we will optimize this method for the 4-QAM. We will also propose a network coding scheme that generalizes two-way relay to multi-way relay channels which involves more than three source nodes. Since a destination node can decode a network coded signal if and only if the signal contains only one unknown data symbol, one can use the existing network coding schemes for 2-way relay channels to encode the symbols in the case that more than three sources are involved. We will also analyze the relationship between the amount of sources and the performance while applying this scheme.