| 摘要: | 網際網路的豐富資源以及多媒體服務的大受歡迎，刺激了寬頻網路的發展，建置寬頻網路深入到每個家庭，整合通訊、電腦及資訊，建立網路桃花源是世人的理想；使用者莫不期待高品質、高可靠度以及高速的通訊系統來有效率的擷取網際網路資源。在許多解決方案中，儘管已經有將高速傳輸實現在雙絞線上的非對稱數位用戶迴路(ADSL)，或是擴充現有有線電視網路的混合光纖同軸電纜(HFC)等有線方案，但是在有線基礎建設不足的區域或是對於新加入競爭的業者而言，無線的解決方案具備了建置時程短以及成本低廉之優勢，因此仍然獲得了網路以及服務提供業者的青睞。寬頻無線接取網路，區域多點分配系統 Local Multipoint Distribution System (LMDS) 這種固定式的無線傳輸系統為一工作於28G Hz之雙向數位式細胞式系統；因為影像傳輸需極寬之頻寬，使得系統需密集的重複使用頻率，因此同頻干擾成為影響系統效能的關鍵因素，又由於用戶使用高指向性之天線，此種干擾造成信號衰減之區域，端視干擾台之位置。 為確實分析在微波頻段之通道效應，本論文首先討論降雨衰減與植被去極化之影響，結果顯示提出之優質的細胞建置規劃可獲致較佳之系統效能。因此運用細胞工程方法(cellular engineering methodologies), 結合區域多點傳輸系統特殊條件，例如雙極化高指向性天線的使用，我們提出細胞群交錯的概念來建置此一固定式網路，並考慮其於分時多工進接與分碼多工進接系統之應用；模擬結果顯示所提出新的細胞架構能有效的降低同頻干擾的影響，且具有極大之基地台配置彈性，同時，由於僅需做好頻道規劃，無須增加硬體設備之額外投資，因此比典型的細胞建置擁有更佳之成效。 The rapid growth of the Internet has increased the demand for broadband services. Users expect high quality, reliability and high-speed communication systems that allow easy access to this resource. Solutions, such as the x-digital subscriber line (x-DSL), which aims at high-bit-rate transmissions over twisted pairs as well as an extension of the existing cable television networks, known as hybrid fiber coaxial (HFC), appears very promising. An alternative technique, the wireless approach to the last mile access is attractive to network operators and service providers because of its reduced deployment time and cost compared to the construction of a wired infrastructure. LMDS, which typically operates at millimeter-wave frequencies above 20 G Hz, is a two-way digital cellular system that offers a wireless access method to broadband interactive services for point-to-multipoint distribution. In this fixed broadband wireless access network, each cell is divided into multiple sectors, with each served by a sector antenna collocated within the hub. Users supported in these systems employ highly directional antennas and signal polarization to communicate with the hub station. The major advantage of using directional terminal antennas is that a large portion of the interference is attenuated by the antenna front-to-back ratio. The system performance is limited by the signal strength over individual radio links and the amount of co-channel interference. Besides the severe attenuation of precipitation in millimetre wave band should be considered in link budget calculation, the effect of antenna and depolarisation also should be integrated into the co-channel interference calculation, since dual-polarized LMDS system is proposed to increase spectrum efficiency and high gain antenna is used to reduce interference. Using cellular engineering methodology, an interleaved cellular pattern, that exploits the high-directivity of the subscriber antenna, is proposed. A cluster-interleaved cellular pattern is proposed by interleaving the cell clusters. The proposed cellular system can be easily developed and without making any additional equipment investment. The coverage performance was investigated by considering high signal propagation impairment for the Taiwan area. A new cellular architecture, termed extended interleaved twisted cell (EITC),reduces the co-channel interference experienced by users located in strong interference regions. The proposed structure delivers the advantages of the interleaved cell, which manipulates the spectral efficiency of fixed broadband wireless networks. This work presents a new cell structure that combines extended cluster planning to eliminate co-channel interference by exploiting the terminal directional antenna for system deployment. The main benefit of this novel design lies in the directivity of the terminal directional antennas. This allows flexible hub deployment. The extended interleaved twisted cell is simple to implement and requires no extra equipment cost. A dual-polarized multimode CDMA based LMDS with hexagonal cell architecture is investigated. Various cellular architectures for improving the system performance are discussed. A novel cellular pattern that exploits the interleaving scheme is proposed to reduce the impact of inter-cell interference to enhance the power of the multimode CDMA scheme in this fixed wireless network. By managing the channel polarization used in the service area, no extra equipment investment is required and better system performance is obtained. Rain attenuation and foliage depolarization play important roles in system deployment. Developing better cellular architecture is an economical approach to enhance spectrum efficiency for LMDS. Results show that the proposed novel cellular system achieves greater improvement for system coverage performance. |
