近年來，聯邦式學習(Federated Learning) 因為人工智慧(Artificial Intelligence) 日 益受到矚目，資料頓時成相當重要的資源，然而資料的儲存、傳輸、運算，對於大 多的應用服務提供公司(Application Service Provider) 來說，儼然成為了相當大的壓 力，由於，行動邊緣運算(Mobile Edge Computing) 的出現，以及日益普及，聯邦式 學習逐漸因其，去中心化的概念，以及保護隱私權特性，成為了當紅的熱門技術。 直觀地說，擁有更多的資料參與，有機會擁有更高的準確率，然而，越多的使 用者提供資料，要給付更高的費用(Incentive Payment)，並且，每個使用者裝置擁 有的運算及傳輸資源是異質的，會造成完成時間(Completion Time) 的掌握更加困 難。 因此，在本論文中，我們提出了一個最佳化問題，名為CoTENSOR， 同時考量 給付的費用以及完成時間。於此問題中，我們不僅討論了每個使用者裝置的不同費 用要求，異質的傳輸運算能力，以及為了確保產生足夠準確的結果，增加了資料量 總和的限制，並且設想做為中心伺服器的基地台擁有多個通道，能使多個使用者裝 置同時上傳。 接著，我們證明了CoTENSOR的難度，並且提出了一個近似演算法，名為 CoACT，此演算法同時考量了給付的費用以及完成時間，並且維持了準確率。 CoACT首先產生了所有可能的組合，依此產生出每個使用者裝置相對應的虛擬費用 (Pseudo Training Cost)，並利用動態規劃(Dynamic Programming) 的想法找尋最小的 虛擬費用，找出組成最小費用的使用者裝置集合，並且利用近似的技術降低其時間 複雜度。 實驗結果顯示，我們的演算法(CoACT) 不僅優於其他傳統方法(例如：隨機選取 裝置、選取最便宜的裝置、找尋增加時間最小的裝置)大約40%，也確保了準確率 的水準，並且在達到目標準確率上亦有相當好的表現，以及由於演算法的特性，可 以執行在平行的環境，我們也為此作了實驗。

Federated Learning (FL) has emerged in recent years to decentralize the model training with user devices. FL not only preserves data privacy but also ensures the model accuracy. To optimize the performance of FL, many critical issues, such as completion time minimization and device selection have been explored in the literature. However, none of previous work jointly considers the above two issues. In addition, few related studies consider multiple available channels. Therefore, in this thesis, we consider a more real and complex scenario, multiple channels are available to be allocated, which means it is available for multiple devices to upload at the same time. Moreover, channel allocation and uploading scheduling for the user devices in mobile networks also influence the completion time in each communication round of FL. To optimize all the issues above jointly, we first formulate a new optimization problem, named CoTENSOR, and prove its NP-hardness. Subsequently, a (3+e)-approximation algorithm, named CoACT, is proposed to find a near-optimal solution. Specifically, CoACT presents a new notion, pseudo training cost (detailed later), to evaluate the selection of each device so as to acquire an adequate set of devices to collaboratively train a global model. It also compacts the upload scheduling for those selected devices over multiple channels to jointly reduce total cost including the incentive payment (given to users in exchange for their computing and data) and completion time (of a global training round). We conduct an experimental evaluation with CIFAR10, which is a large-scale and public released image dataset and build and train a simple deep neural network model based on FL framework. Simulation results manifest CoACT can reduce the total cost by 40% compared with the traditional approaches and ensure test accuracy and train loss.

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