在兩階層式行動網路中，如何適當的將一個微型基地台使用者分配至一個微型基地台下以使達成干擾消除且最大化可達到的資料速率是一個很重要的研究。在本論文中，我們證明了解決此一問題為完全非多項式問題 (NP complete problem)。傳統上為解決此一問題，線性規劃 (linear programming) 常被採用以找出最佳解，然而線性規劃的方法將會比較耗時。因此，我們提出了一個智慧式微型基地台選擇演算法 (smart femto base station selection algorithm) ，在微型基地台使用最大傳送功率的前提下，透過適當的目標微型基地台之選擇達成干擾消除之目的。當微型基地台皆使用最大傳送功率卻無法找出干擾消除之解時，我們提出一個下行功率控制演算法 (downlink power control algorithm) 可使每個微型基地台根據所服務的微型基地台使用者調整傳送功率，藉此找出一個無干擾且可達到最大資料速率的拓樸。從模擬結果可以發現，使用智慧式微型基地台選擇演算法或是下行功率控制演算法可以較低的時間複雜度即可找出干擾消除且資料速率較高之拓樸。
再者，微型基地台的訊號也有可能會穿過牆壁對巨型基地台使用者造成同頻干擾。為了兼顧提高室內傳輸速率且不影響巨型基地台的服務品質 (quality of service) 的前提下，我們提出一個微型基地台傳送功率調整演算法 (femto base station adjustment algorithm) 使微型基地台可根據附近的巨型基地台使用者之相對位置調整其傳送功率。由模擬結果可以發現微型基地台傳送功率調整演算法可透過傳送功率之控制減少對巨型基地台使用者之干擾，進而大幅降低巨型基地台使用者之失效率 (outage probability)。同時，傳送功率調整演算法不僅可保證巨型基地台使用者的服務品質，也不會因為調整傳送功率而大幅減低微型基地台使用者的資料速率。; How to associate a femto user (FU) to an appropriate femto base station (FBS) to avoid interference and maximize the achievable data rate (ADR) of FUs have attracted much more attention in 2-tier mobile networks. The evidence provided in this dissertation proves it as an NP-complete problem. To maximize the ADR of the FUs, the methods for assigning FUs to neighboring FBSs are examined to determine the required transmit power level adjustments for the FBSs to mitigate the downlink (DL) interference among the FBSs and the macro base station (MBS). Traditionally, the linear programming (LP) approach is adopted to obtain the optimal solution for this problem but requires substantially more time to obtain the solution. To improve the drawback, we propose an easy but smarter scheme, named smart FBS selection algorithm (SFSA), to distribute FUs to non-interfered FBSs as well as to obtain the maximal ADR for each FU. If the SFSA fails to solve this problem, a DL power-control algorithm (DPCA) is employed to find a solution that causes the least amount of interference. The simulation results show that the SFSA and DPCA require only a minor amount of computation time to obtain a feasible solution.
Then, this dissertation investigates how to adjust the transmit power of each FBS to mitigate interference problems between the FBSs and mobile users (MUs). A common baseline of deploying the FBS to increase the indoor access bandwidth requires that the FBS operation will not affect outdoor MUs operation with their quality-of-service (QoS) requirements. To tackle this technical problem, an FBS transmit power adjustment (FTPA) algorithm is proposed to adjust the FBS transmit power (FTP) to avoid unwanted co-channel interference (CCI) with the neighboring MUs in downlink transmission. FTPA reduces the FTP to serve its FUs according to the QoS requirements of the nearest neighboring MUs to the FBS so that the MU QoS requirement is guaranteed. Simulation results demonstrate that FTPA can achieve a low MU outage probability as well as serve FUs without violating the MU QoS requirements.
