以IP為基礎之OFDMA系統中適用於即時性服務之前置式基地台切換技術

Abstract

以網際網路協定(Internet Protocol, IP)為基礎的正交分頻多重接取(Orthogonal Frequency Division Multiple Access, OFDMA)系統憑藉著與網際網路良好的互連(internetworking)能力以及在多路徑(multipath)傳播環境下可提供更高的資料傳輸率而被採用於IEEE 802.16無線寬頻接取(Broadband Wireless Access, BWA)標準之中。有別於IEEE 802.16標準制定初期以提供固定式無線接取服務為目標，近期該標準正積極發展可支援使用者移動性之版本以期能提供行動通訊服務，而換手技術(handoff)的開發正是該系統能否支援使用者移動性的關鍵。當使用者在系統中移動時，換手技術負責在不同基地台間傳遞使用者的連線以確保服務的連續性，然而現有標準中的硬式換手(hard handoff)技術會因過長的處理時間而導致即時性(real-time)通訊服務中斷過久，並進而造成使用者對服務品質的不滿意。本篇論文提出一套適用於以IP為基礎之OFDMA系統之前置式基地台切換(Proactive Base Station Switching, PBSS)技術, 利用預先註冊(pre-registration)、資料轉遞(data forwarding)與快速調距(fast ranging)等技巧以確保因基地台切換而造成的服務中斷時間(service disruption time)能滿足即時性服務對於品質的要求。文中同時提出符合於IEEE 802.16e標準之網路控制訊號交換流程，及以無線通道品質為依據之基地台切換啟動機制。模擬結果顯示本機制可確保即時性通訊服務的品質，並降低因資料傳送延遲而造成的封包遺漏率。

IP (Internet Protocol)-based OFDMA (Orthogonal Frequency Division Multiple Access) system is adopted in IEEE 802.16 standard by its great internetworking capability with Internet and its capability to achieve high transmission rate in multipath environment. Instead of providing fixed wireless access services, the standard committee also aims to develop a newer version to support user mobility for mobile communications services. Therefore, development on handoff technology, which is the process to transfer the connectivity of mobile station from one base station to another, is the key to achieve this objective. However, the hard handoff process in current standard will lead to high service disruption time and make user feel uncomfortable on service quality. This paper proposes a proactive base station switching (PBSS) mechanism to reduce service disruption time to satisfy the quality of service (QoS) requirement of real-time services, which is achieved by network pre-registration, data forwarding and fast ranging. This paper also proposes an IEEE 802.16e compatible signaling procedure and a link quality-based PBSS initiation algorithm to realize PBSS mechanism in real system. The simulation results show that the service disruption time can be reduced by PBSS mechanism to satisfy the requirement by real-time services; moreover, the packet loss rate can also be reduced by preventing packets timeout.