| Summary: | 碩士 === 國立中央大學 === 電機工程研究所 === 91 ===
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.
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