為了於衰落通道下提供強健的無線數據傳輸，各種中繼節點的使用機制已被廣泛提出。某些中繼節點所使用的篩選機制，若中繼節點所接收到的訊息其可靠度被視為太低，中繼節點將選擇不將其後傳。一些研究人員稱此種機制為選擇性解碼與後傳機制。此種機制的使用將可以抑制錯誤漫延並提高分集增益於合作式通訊網路。在本論文中，主要的研究將於此機制下完成。進一步的說，我們系統所採用的中繼節點機制是篩選式的中繼節點（中繼節點進行篩選任務）。它採用對數似然比（LLR）或訊號雜訊比(SNR)的形式來計算其所接收到每一個位元（從來源端）之可靠度。若被篩選位元的對數似然比或訊號雜訊比其大小低於預先所設定之臨界值，那麼它將被刪除（即不後送到目的端）。合作式中繼網路中的另一個主要探討的議題為分集合併機制。採用適當的分集合併方式將可主宰於目的端的解碼效能。先前對於具有篩選機制之中繼節點的研究工作均於目的端使用最大比例合併機制以合併其所接收之分集訊號。但最大比例合併機制對於具有中繼節點的合作式通訊網路而言並非為最佳的合併機制，因為中繼節點偶爾會做出錯誤的決策。因此在本研究中，於目的端並不採用如先前研究工作所廣泛使用的最大比例合併機制，而是推導出最佳的分集合併權重。當傳輸通道設定為瑞利衰落，我們在不同場景條件下推導系統所對應之封閉形式位元錯誤率（BER）表示式。這些場景是依據每一個端點可獲得不同程度之通道狀態訊息（CSI）與目的端是否裝置有能量偵測器(ED)來給與區別。藉由使用這些封閉形式的位元錯誤率表示式，可以有效的優化系統參數，取得系統最小的位元錯誤率。這些系統參數包含了篩選臨界點、分集合併權重與功率分配比例。觀察得到的系統效能模擬結果與透過系統之封閉形式位元錯誤率計算所得之理論值可發現其非常匹配與接近，此外，也可發現因為系統只需要統計性的通道狀態訊息，所以使用低複雜度的計算即可快速得到所需之系統最佳功率分配比例。將所提出之系統與其他中繼節點具有篩選機制的系統相比較其效能，可發現我們提出之系統架構於位元錯誤率效能與通道狀態訊息要求量方面均具有較好的表現。

To provide robust wireless data transmission over fading channels, various schemes which involve the use of relays have been proposed. In some of those schemes, the relay chooses not to forward the received message if its reliability is deemed as too low.
Some researchers refer to such schemes as selective decode-and-forward. It can be used to suppress the error propagation and hence improve the diversity gain in the cooperative network. Our work in this dissertation falls into such a category. More specifically speaking, the relay in our system is a censorial relay (a relay that performs censorial task). It evaluates the reliability, in terms of log likelihood ratio (LLR) or signal-to-noise ratio (SNR), of a received data bit (from the source). If its LLR or SNR magnitude is below some preset threshold, then it is censored (i.e. not sent to the destination).
The diversity combining schemes are another issue in the cooperative relaying
network. Diversity combining strategy can affect the result of decoding in the destination.
Previous works that address the performance of censorial relays with LLR or SNR
thresholds often adopt maximum ratio combining (MRC) for signal combining at the
destination. However, it should be noted that MRC is not optimal for relay-assisted cooperative diversity systems, because the relay can make wrong decisions sometimes.
Hence, instead of adopting MRC at the destination, an optimal diversity combining weights is sought in this research. When the channel is Rayleigh faded, closed-form bit error rate (BER) expressions for the proposed system are derived for several scenarios.
Those scenarios are differentiated by the availability of an energy detector (ED) and the various degrees of knowledge regarding the channel state information (CSI). Aided by those closed-form BER expressions, the system parameters can be efficiently optimized
to achieve the minimum BER. More specifically speaking, these system parameters
include censoring-thresholds, weighted combining factors and power allocation index.
Simulation results are observed to closely match theoretical values, as computed by the
afore-mentioned closed-form BER expressions. It is observed that the incorporation
of power allocation into the proposed censor-and-relaying cooperative communication
system greatly improves the BER performance. Moreover, the power allocation task
can be carried out fast with low computing complexity, because the proposed scheme
only require the statistical CSI. As compared to some existing relay-assisted systems
in which censoring is incorporated, the performance of our system is better in terms of
BER and also in terms of the requirement on the knowledge about the CSI.

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