在本文中，我們研究了使用蜂窩網絡(Cellar Network)的用戶所遇到的特殊問題，像是遠端用戶的資料在近端用戶執行連續干擾消除（SIC）時會有的危險。由於這個問題沒有完善的在文獻中被討論，我們研究一個NOMA系統模型用來顯現用戶之間的主要洩漏問題並指出功率分配係數的差異會影響網路中其中一個用戶的保密優化程度。本研究還特別關注公平指數的概念，以確保此系統模型的體驗對其用戶具有相同獲最佳的公平性。這是通過使用加權概念來找出最佳的功率分配係數給用戶的。此外我們還研討了針對下行鏈路NOMA通信系統物理層安全性所潛在的研究挑戰。

In this thesis, we study one of the special case issues encountered by a user pair in the cellular network i.e potential that the far user information is at risk when the near user performs successive interference cancellation(SIC).As this problem scenario is not well studied in the literature,we discuss a NOMA system model in order to emphasis on the major leakage issue between the user pair and indicate the difference in power allocation coefficient that leads to secrecy optimization for one of the users in the network.The study also focuses on fairness index concept in order to ensure that the system model experiences equal or optimal fairness for its users.This is done by finding out the optimal power allocation coefficient for the users using the concept of the weighted sum.Furthermore, we discuss the key features of Physical layer security for downlink NOMA communication system with their potential research challenges.

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