臺灣大學: 電信工程學研究所吳靜雄; 李揚漢陳彥蒼Chen, Yen-TsangYen-TsangChen2013-03-272018-07-052013-03-272018-07-052011http://ntur.lib.ntu.edu.tw//handle/246246/253001由於具有高頻譜利用率並且能有效解决寬頻無線通信系统中的蜂巢細胞內干擾問題，正交分頻多工 (Orthogonal Frequency Division Multiplexing, OFDM) 技術已經被廣泛認為必將應用於未來的寬頻無線通信系统中，包括後3G (beyond 3rd Generation) 長期演進系统 (Long Term Evolution, LTE) 等。在以正交分頻多重存取 (Orthogonal Frequency Division Multiple Access, OFDMA) 為多重存取方式構建的蜂窩移動通信網路中，也可以做到頻率復用系數 (reuse factor) 為1，即整個系统覆蓋範圍内的所有蜂巢細胞 (cell) 可以使用相同的頻帶為本蜂巢細胞内的用户提供服務。在OFDM系统中，各子載波 (subcarrier) 之間的正交性有嚴格的要求。雖然由於載波頻率和相位的偏移等因素會造成子載波間的干擾，但是可以在實體層採用先進的無線信號處理演算法使這種干擾降到最低。因此，一般認為OFDMA系统中的蜂巢細胞内干擾很小，而影響系统性能的主要干擾來自蜂巢細胞之間干擾 (Inter-Cell Interference, ICI)。 通道估測在OFDM系統中是一個非常重要的任務，前導訊號 (pilot) 通常安插在資料子載波之間，用來做為無線通道增益的估測，前導訊號的特性和安插的位置對於最小平方通道估測方法的均方差有很大的影響。為了使相鄰的蜂巢細胞能使用相同的頻帶為本蜂巢細胞内的用户提供服務，且能有效的降低蜂巢細胞間的干擾，由於Zadoff-Chu序列的交相關函數擁有很好的正交特性，因此在本篇論中我們採用Zadoff-Chu序列作為前導訊號的設計，相鄰的蜂巢細胞採用不同且互相正交的Zadoff-Chu序列作為前導訊號可以有效降低蜂巢細胞間的干擾且提升蜂巢細胞內的用戶和基地台之間上行和下行的傳輸效能。 此外，我們考慮到如果用戶處在多個基地台覆蓋範圍的交界處，在上行傳輸時可能會受到其他蜂巢細胞內的用戶嚴重干擾，導致本蜂巢細胞的基地台無法解調出用戶的訊號，過多的上行傳輸錯誤將會使系統服務品質下降。這時我們提出一個換手 (handover) 決策機制，利用Zadoff-Chu序列的交相關函數來測得各基地台所受到的干擾功率，服務基地台可提供相鄰基地台所受到干擾量的資訊給用戶，來讓用戶選擇與干擾量較小的基地台連線。Orthogonal frequency division multiplexing (OFDM) has been widely considered to be adopted in the future wideband wireless communication systems including the Long-Term Evolution (LTE) because it has high spectral efficiency and can avoid intra-cell interference. The cellular mobile communication network based on orthogonal frequency division multiplexing access (OFDMA) also can achieve frequency reuse of one, which means that all cells in the system can use the same frequency band to serve each own users. In OFDM systems the orthogonality among subcarriers is critically demanded. Although some factors like carrier frequency and phase shift will result in inter-carrier interference, it can be depressed through advanced wireless signal processing algorithm in the physical layer. Therefore, it is thought that intra-cell interference is very small in OFDM systems. The major interference which affects the system performance is the inter-cell interference (ICI). Channel estimation is an important issue in OFDM systems. Pilot sequences are often multiplexed into the data to estimate the wireless channel gain. Both pilot sequences and their placement can affect the mean square error (MSE) of the least squares (LS) channel estimation. In order to make neighboring cells use the same frequency band to serve its own users and in the meanwhile mitigate the inter-cell interference, we choose Zadoff-Chu sequences as pilot design since its cross-correlation has good orthogonal property. Therefore, assigning neighboring cells different and orthogonal Zadoff-Chu sequences can mitigate the ICI efficiently and improve the downlink and uplink transmission performance between the base station and the users. Besides, we consider that the user located at the crossover area of multiple base station coverage can be seriously interfered with the interferers in other cells in the uplink; as a result, the serving base station can not demodulate the user’s signal correctly and excessive uplink transmission errors will result poor Quality of Service (QoS). To solve this problem, we propose a handover decision scheme, where we use the cross-correlation property of Zadoff-Chu sequences to estimate interference power seen by each neighboring base station. Then the serving base station will provide interference information of each neighboring base station to the user. The user can select the target base station which has least interference as the serving base station.3311541 bytesapplication/pdfen-US蜂巢細胞之間干擾Zadoff-Chu序列前導訊號通道估測換手inter-cell interference (ICI)Zadoff-Chu sequencepilotchannel estimatehandoverthesishttp://ntur.lib.ntu.edu.tw/bitstream/246246/253001/1/ntu-100-R97942119-1.pdf