在無線細胞系統中，同頻干擾效應對於系統效能來說是一個限制的因素,區域性點對多點傳輸系統 Local Multipoint Distribution System (LMDS) 這種固定的無線傳輸系統也不例外。在 LMDS 中 下鏈通常是較高容量的,而且較窄的用戶端天線的波束也是排拒同頻干擾的一項因素，所以下鏈的同頻干擾是較嚴重的。在第二章中將有較詳細的說明。 在Orthogonal Frequency Division Multiplexing (OFDM)系統中，資料同時傳輸於平行的次載波，每一個次載波佔據可用頻寬中的一小部分。因此OFDM最基本的原理就是將高速率的資料分成為一小較低速率的資料。在無線通訊領域中，有許多系統已將OFDM納入系統標準中，如：Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB) …等。這是因為OFDM有非常高的頻譜效率及對多重路徑效應有較好的容忍力。 OFDM/FDMA能夠依據使用者需求不同的資料速率而給予不同數目的次載波。在OFDM/FDMA中，每個使用者經由預先決定的次序來使用一組次載波。在非滿載的情況下，利用不同細胞間使用不同的次載波使用次序將可降低細胞間的同頻干擾。這是本篇論文的主旨。 In wireless cellular systems, co-channel interference is the limiting factor to system performance. Fixed wireless systems such as Local Multipoint Distribution Service (LMDS) is no exception to this rule. In LMDS, because the downlink is usually higher capacity and the narrow beam of subscriber’s antenna is an interference-limiting factor, the downlink interference is more serious. Consequently, improve of co-channel interference on the downlink direction of LMDS is the subject of this thesis. In OFDM systems, data is transmitted simultaneously on parallel sub-carriers, each occupying a small fraction of the available bandwidth. The basic principle of OFDM is to split a high-rate data into a number of lower rate system. Multi-user OFDM is a promising new modulation technique for wireless communications. It includes many of the advantages of broadcast OFDM that is used for Digital Audio Broadcasting (DAB) and for Digital Video Broadcasting (DVB) in Europe and Australia. OFDM was selected for these systems primarily because of its high spectral efficiency and multi-path tolerance. OFDM/FDMA is one of the multi-user OFDM multiple access. In OFDM/FDMA, the number of sub-carriers for a specific user can be varied, according to the required data rate. In the simplest OFDM/FDMA system, each user occupy fixed group of sub-carriers. We consider parts of the sub-carrier be used. Then we can improve system by using sub-carriers allocation. In this thesis, we proposed a simple sub-carriers allocation method compared to the convention and random OFDM/FDMA.
