由於LMDS乃是使用28GHz之雙向數位式細胞系統，同頻干擾(Co-channel Interference) 對於系統性能是一個限制因素，尤其LMDS的下傳鏈路(Downstream)通常有較高的容量，因此在下傳鏈路的同頻干擾比上傳鏈路(Upstream)嚴重。正交分頻多工(Orthogonal Frequency Division Multiplexing; OFDM)是一套有效率的方法，可以傳輸高速率資料及對抗碼際干擾(Inter-symbol Interference)和多路徑傳輸的衰減。本論文主要是探討如何提昇以OFDM為基礎之區域多點分配服務系統的性能，我們將模擬LMDS的寬頻通道、架構OFDM傳輸系統，利用多重輸入多重輸出(Multi-Input-Multi-Output; MIMO)架構來估測用戶端(Subscriber)與基地台(Base station) 與細胞間干擾之干擾台(Interfering Stations)之通道頻率響應，並 以估測的通道頻率響應來實現通道等化(Channel Equalization)。如此一來，細胞間的干擾可以有效地降低。本論文也會討論利用調適性調變(Adaptive modulation)技術，以提昇系統的效能。 LMDS usually operates at millimeter-wave frequencies about 28 GHz and it is a two-way digital cellular system, co-channel interference is the limiting factor of system performance. In the downstream, the channel capacity is higher than that in the upstream, and the co-channel interference of the downstream is more serious than that of the upstream. In order to reduce the co-channel interference, we apply the orthogonal frequency division multiplexing (OFDM) to the LMDS system to simulate the OFDM-Based LMDS. OFDM is an efficient and suitable technique to combat ISI in wideband channels. Since each OFDM sub-carriers has a very narrow spectrum. It can transmit high data rate and combat the inter-symbol interference and the multipath fading. In this thesis, we will improve the performance of OFDM-Based LMDS system by using MIMO structured channel estimation. In order to promote systematic efficiency, this thesis will adopt adaptive modulation techniques too.
