D2D通訊現今被視為一項很重要的技術，他能夠讓裝置不需透過基地台便可直接地與其他裝置進行傳輸。在這篇論文裡，D2D通訊於蜂窩網路的架構下，使用蜂窩裝置上行鏈路的頻率資源，直接地與其他用戶進行通訊。特別的是，我們考慮了兩種D2D模式，分別是D2D直接通訊模式與D2D加入中繼站通訊模式。D2D加入中繼站後的傳輸模式在D2D用戶間距離太遠時或D2D之間的傳輸通道不甚理想時能有效改善D2D通訊的性能。針對能夠最大化整體系統的輸出然而同時保證D2D用戶及蜂窩式用戶的訊躁比，我們利用在D2D直接通訊模式與D2D加入中繼站通訊模式之間的模式選擇來確保能得到最大的系統輸出。然而，D2D與蜂窩式用戶之間由於使用共同通道所造成的干擾是個嚴重的問題。因此，我們又提出了一個功率控制演算法來試圖減緩此干擾問題。再者，如何適當地分配及選擇被複用的蜂窩用戶通道以及控制傳輸功率來減緩共同通道所造成的干擾是個挑戰。為了簡化此問題，我們先劃分出可被D2D所複用的蜂窩用戶候選者，再將我們的問題分成兩個子問題，分別是D2D直接通訊模式與D2D加入中繼站通訊模式來進行個別分析。此功率演算法，透過凸函數的優化特性所計算出的最佳功率來最佳化我們的問題。最後的結果顯示透過適當的模式選擇以及功率控制，D2D通訊可以在不對蜂窩網路造成嚴重干擾的前提下有效地改善系統總輸出。

Device to device (D2D) communications are viewed as a vital technology nowadays, which allow direct transmission between two cellular devices without through base stations. In this thesis, D2D communications nested in a cellular network, transmitting data to each other directly using the uplink frequency resource of cellular users. In particularly, we considered two kinds of modes, direct D2D mode and relay assisted D2D mode, respectively. Relay assisted transmission could efficiently enhance the performance of D2D communications when D2D users are too far away from each other or the quality of D2D channel is not good enough. Aiming at maximizing the overall system throughput while guaranteeing the signal-to-noise-and-interference ratio (SINR) of both D2D pair and cellular links, we utilize
a mode selection strategy between direct D2D mode and relay assisted mode to ensure the maximal system throughput. However, co-channel interference between the D2D links and the cellular links is a severe problem; thus, we proposed a power control algorithm to mitigate
the interference. Furthermore, how to properly assign the reusing channel and control the transmit power to mitigate the co-channel interference between the D2D links and the cellular links is a main challenge. To simply our problems, we identified candidate cellular users whose frequency resource can be reused by D2D pair and then decomposed our problem into two subproblems, two modes respectively. Finally, we proposed a power control scheme using the characteristic of convex function to optimize our problems. The results show that by proper mode selection and power control, D2D communications can effectively improve the total throughput without generating harmful interference to cellular networks.

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