Device-to-device (D2D) 通訊概念的提出是用於未來有效改善蜂巢網路的負載。然而，一般的D2D通訊，從發送端裝置至接收端裝置會建立一個直接的通訊鏈路，這一段直接通訊鏈路將會受到兩裝置間的距離和鏈路品質的影響而造成通訊品質上的限制。在本篇論文中，提出了一種用於彌補直接的D2D通訊的不足，是為中繼輔助D2D通訊，此裝置對裝置通訊的方法能增加LTE-A網路的系統容量。我們旨在設計一個對於中繼輔助D2D通訊之中繼裝置的選擇策略，藉由此策略能夠增進D2D通訊的效能。此外，我們也提出了基於跨層分析的中繼輔助D2D通訊策略，其共同考量到裝置的剩餘電池時間還有利用排隊理論分析之端對端通訊之延遲。我們也建立了另一個用於延遲評估的端對端通訊模型，以排隊理論與自適應調變與編碼特性的結合進行端對端的延遲分析。模擬分析結果能夠確實顯示出所提出的包含剩餘電量與端對端延遲之中繼選擇策略能夠有效的增強整個系統的效能。

Device-to-device (D2D) communications have been proposed as an effective way for traffic offloading in futuristic cellular systems. However, using only D2D communication, which establishes a direct link between a source and destination, limits advantages brought in by D2D communications due to a long separation distance or poor link quality between the source and destination user equipments (UEs). In this thesis, relay-assisted D2D communication is proposed as a supplement to direct D2D communications for enhancing traffic offloading capacity of LTE-A systems. We aim to design a relay UE selection strategy for relay-assisted D2D communications, which can improve the performance of relay-assisted D2D communications significantly. We also propose a cross-layer relay selection scheme that considers the criteria jointly, relay UE (RUE) remaining battery time, and end-to-end transmission delay on relay-assisted D2D path. We establish an end-to-end delay estimation model based on queuing theory combining adaptive modulation and coding (AMC) for relay-assisted D2D. Simulation results validate the overall performance of the proposed relay selection scheme within RUE remaining battery and end-to-end transmission delay.

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