正交分頻多工存取多細胞蜂巢式系統採用各種干擾處理技術之性能評估

Abstract

本論文主要目標在研究以正交分頻多工 (Orthogonal Frequency Division Multiple Access) 蜂巢式網路系統為基礎，配合不同的干擾處理技術，包括：干擾平均 (Interference Averaging)、干擾迴避 (Interference Avoidance) 及干擾消除 (Interference Mitigation) 等技術，討論這些技術對於系統性能的影響。在蜂巢式網路系統中，細胞之間的干擾會嚴重影響系統在的效能。能有效抑制干擾的影響是蜂巢式網路系統的重要問題，因為干擾擴展函數只能適用在分碼多工系統 (Code Division Multiple Access) 而無法適用於正交分頻多工系統。 首先，排列 (permutation) 是一種干擾平均的機制。在WiMAX中提出了數種分配子載波 (sub-carrier) 到的子通道 (sub-channel)的排列方式，而排列方式又可分成分散式及連續式兩種。分散式將子通道內之子載波打散至整個頻帶中，連續式則將子載波以連續方式在頻帶中排列。本論文分散式採用部分使用次通道配置 (Partial Usage of the Sub-Channels)，連續式則採用頻帶自適應編碼 (Band AMC – Band Adaptive Modulation Coding)。其次，對於干擾迴避技術，針對不同的頻率重用系數 (Frequency Reuse Factor) 作分析，可以將細胞間訊號干擾作空間上的隔離。在WiMAX中，細胞內可以分隔成內外圈兩個區域，而引入了非整數型的頻率重用係數的想法。最後，在干擾消除技術中，不同的指向性天線將能改變系統的干擾，我們將探討三種不同的指向性天線的影響，包括了鑽石扇區式的寬廣波束型 (Wide Beam Tri-Sector with Diamond-Shaped Sector)、狹窄波束型 (Narrow Beam Tri-Sector)以及五邊形扇區式的寬廣波束型 (Wide Beam Tri-Sector with Pentagon-Shaped Sector)方向性天線。本論文也採用全向性天線，搭配全部使用通道配置 (Full Usage Sub-Channel)，以比較之間的差異。 在本論文中，模擬的顯示出一些正交分頻多工存取多細胞蜂巢式系統的重要特性： 1.在時變的無線通道中，採取分散式的排列方式會較有利於系統容量。 2.頻率重用雖然對於連線品質和系統公平性有些益處，但對於系統的總容量都是沒有幫助的。 3.而針對於外圈的使用者而言，在重用係數在2左右時可以得到容量上的改善。 4.針對三種指向性天線而言，本論文並不推薦使用WiMAX建議的模型 (五邊形式的寬廣波束型)，改採用狹窄波束型方向性天線能得到較佳的性能。

The main goal of this thesis is to investigate the performance of orthogonal frequency division multiple access (OFDMA) multi-cellular systems with interference averaging, avoidance, and mitigation techniques. In multi-cellular systems, the inter-cell interference degrades the system performance significantly. Suppressing the interference is an important issue in OFDMA multi-cellular systems because the interference spreading function is only suitable for code division multiple access (CDMA) systems but not for OFDMA systems.

First, permutation belongs to the interference averaging technique. Several permutation methods are proposed to assign sub-carriers in a sub-channel in WiMAX. In the distributed type, the sub-carries in a sub-channel is randomly distributed in the entire usable frequency band. In the contiguous type, the sub-carries in a sub-channel is arranged continuously. In this thesis, we call Partial Usage of the Sub-Channels (PUSC)/ Band Adaptive Modulation and Coding (Band AMC) permutation as randomly distributed/contiguous types. Secondly, in the interference avoidance technique, we focus on frequency reuse. WiMAX introduce the structure of inner circle and outer ring in a cell. Thus, the reuse factor can be an integer or a fraction number. Thirdly, for the interference mitigation technique, directional antenna can change the distribution of the interference in the space. We study three different types of antenna patterns: 1) Wide Beam Tri-Sector (WBTC) with Diamond-Shaped Sector; 2) NBTC (Narrow Beam Tri-Sector); 3) WBTC with Pentagon-Shaped Sector. We will also use the Full Usage of Sub-Channels (FUSC) permutation in omnidirectional antenna pattern for comparison.

The simulation results show some important points about the interference in OFDMA multi-cellular systems: 1) In a time-varying wireless channel, the randomly distributed permutation approach is better than the contiguous sub-carrier method. 2) In this thesis, the frequency reuse with a higher reuse factor can improve the link quality and system fairness, but decrease capacity. 3) The throughput of the outer ring users may be improved when reuse factor is about 2. 4) The antenna pattern suggested by the WiMAX standard, i.e. the WBTC with Pentagon-Shaped Sector, performs worse than NBTC.