蜂巢式OFDMA系統之效能改進技術

Abstract

OFDMA (Orthogonal Frequency Divisional Multiple Access)被視為是下一代行動通訊系統之核心技術，其重要性在可於高速資料傳輸時有效解決符際干擾(Inter Symbol Interference, ISI)的問題。以OFDMA為核心之蜂巢式無線接取網路將會是下一代無線寬頻通訊系統之核心。為了提升及探討蜂巢式OFDMA系統之效能，本論文提出了多種技術以探討或提升系統之效能。首先，本文提出了一套新型之數學分析方法以探討蜂巢式OFDMA系統之下鏈系統容量，且該分析方法可廣泛納入細胞涵蓋範圍、頻率重用係數、頻率分段重用(reuse partitioning)、天線組態等效應，藉由分析數值結果顯示透過頻率分段重用技術搭配細胞內之頻率重複使用可顯著的提升系統容量。本論文並進一步提出一套改良型之快速基地台切換(Fast Base Station Switching, FBSS)技術，其設計在於結合頻率分段重用技術以預留部分干擾較輕微之通道以提升換手中的使用者之訊號品質。模擬結果顯示所提出之技術可可大幅減低(93.6%)使用者在換手過程中所遭受之封包遺失率，且僅些微的損耗(5.82%)部份系統容量。此外，本論文介紹並探討了一種可應用於蜂巢式OFDMA系統之Multi-hop Relay技術標準：IEEE 802.16j，並透過電腦模擬探討採用Multi-hop Relay技術對於系統整體效能之影響。模擬結果顯示於系統加入中繼站(Relay Station)可有效提升使用者傳輸速率及系統涵蓋範圍，但結果亦顯示系統容量有可能因為資料轉傳所額外消耗的資源而惡化。為了有效提升Multi-hop蜂巢式OFDMA系統之容量，本論文進一步提出了新型之頻率規劃及重複使用技術以及以SINR預測為基礎之新型路徑選擇技術。相較於現有之頻率規劃方法，本文所提出之方法可大幅(60.87%~137.75%)提升系統容量且不影響系統有效之涵蓋範圍，而模擬結果亦顯示所提出之路徑選擇方法可進一步提升16.87%~26.31%之系統容量。

OFDMA (Orthogonal Frequency Division Multiple Access) is one of the key multiple access techniques to enable broadband wireless transmission for next generation mobile cellular systems. In order to improve and investigate the performance of cellular OFDMA systems, this dissertation proposes several advanced techniques and part of the designs have been adopt by the international standard organization. The downlink capacity of cellular OFDMA systems is first investigated through the proposed mathematical analysis. The proposed analysis is general and incorporates the effects of propagation environment, cell size, frequency reuse factors, reuse partitioning (fractional frequency reuse), antenna setups and etc. The numerical results show that the downlink capacity of cellular OFDMA system can be significantly improved by reuse partitioning and intra-cell frequency reuse. Then, an improved FBSS (Fast Base Station Switching) handover technique is proposed to improve the packet loss rate for the handover users. Simulation results show that the proposed technique can significantly reduce the packet loss rate for the users during FBSS at a slight expense on system capacity. In addition, a recent advance in multi-hop relay technology is introduced and investigated, and the benefit on user throughput enhancement and coverage extension by relay deployment are justified by simulation results. But it also shows that the capacity may not be improved by deploying relay stations with respect to the cellular OFDMA system with no relay. In order to improve the capacity of multi-hop cellular OFDMA systems, advanced frequency planning (reuse) techniques and path selection techniques are investigated and proposed. Compare to existing techniques, the proposed frequency planning (reuse) technique can lead to 60.87% ~ 137.75% capacity improvements under the guaranteed coverage. In addition, simulation results also show that further 16.87% ~ 26.31% capacity improvement can be achieved by advanced path selection based on proposed SINR prediction technique.