在 Mobile WiMAX 中基於即時通訊改善掃描與聯繫機制

Abstract

寬頻無線系統是近幾年來廣泛使用的技術,透過這項技術,行動裝置 (mobile device) 也能夠享有寬頻上網的服務。為了因應行動寬頻上網的需求,新世代的寬 頻無線系統也把行動性 (mobility) 視為最重要的特性之一。一個支援行動性的行動 裝置必須在有限的電力之下來維護它的通訊服務,也因此服務品質 (QoS) 和能源 效益 (energy efficiency) 都非常重要。同時,一個行動裝置會因為移動或環境的變 化而影響到其頻道訊號的品質。因此,寬頻無線系統提供了換手 (handover) 機制 給行動裝置,使其可以在基地台之間切換以保持連線服務。 這篇論文首先研究了 Mobile WiMAX 中的換手機制。Mobile WiMAX 中的換 手機制可以讓行動台 (mobile station, MS) 把自己的通訊服務,從正在連線的基地 台 (base station, BS) 轉移到其他基地台。然而,在換手的過程中,可能會造成通 訊服務暫時地中斷,也因此使得即時通訊的 QoS 受到影響。特別是換手機制中 的掃描與聯繫 (scan and association) 這一個步驟總是造成較長的延遲時間,這個 情況在使用了預設的競爭式初始化測距 (contention-based initial ranging) 時更為嚴 重。雖然 QoS 能夠透過調整換手機制的參數來得到改善,但是更長的延遲時間也 可能因應而生。因此,這篇論文分析了 Mobile WiMAX 中的掃描與聯繫機制,並 發展了一個模型來提供合適的系統參數,使得在掃描與聯繫的過程中,能在保證 QoS 的情況下降低延遲時間。同時,這篇論文提出了一個新的方法以改善 Mobile WiMAX 中的聯繫機制。該方法使用了一個不感知競爭機會的後退 (backoff) 演算 法,也因此能減少為了追蹤競爭機會所付出的代價。結果顯示所提出的方法能夠 在維護 QoS 的情況下,顯著地降低延遲時間。除此之外,這篇論文中所提出的新 方法不只能用於改善 Mobile WiMAX 中的換手機制,同時或許也能用於各種寬頻 無線系統中的競爭式存取 (contention-based accessing) 機制。

Broadband wireless system, which could provide mobile broadband access, is widely deployed in recent years. The mobility is regarded as one of the most important feature to meet the requirement of mobile broadband access in a next generation broadband wireless system. A mobile device that supports mobility must maintain its communication services under a limited battery; therefore, quality of services (QoS) and energy efficiency both be- come essential. Moreover, a mobile device may suffer from channel quality degradation due to movement or environment variation. Therefore, broadband wireless systems pro- vide handover mechanism, which allows mobile devices switching between base stations while communication services are still maintained. In this dissertation, we first investigate the handover process in Mobile WiMAX. The handover process is that a mobile station (MS) migrates its communication services from the serving base station (BS) to another BS. However, service disruption could be introduced during the handover process and the QoS for real-time communication would be influenced. Scan and association, which is a part of the handover process, may often introduce a long latency, especially when the mandated contention-based initial ranging is applied. Although the QoS could be guaranteed in the scan and association process, additional latency would be introduced. Therefore, the scan and association process in Mobile WiMAX is analyzed in this dissertation. A model is developed so that suitable parameters for the scan and association process can be obtained for guaranteeing QoS and reducing association latency. Moreover, a novel mechanism is thus proposed to im- prove the association mechanism in Mobile WiMAX. The proposed mechanism is based on an opportunities-unaware backoff algorithm, and the overhead of tracking contention opportunities could be reduced. The results show that the proposed mechanism could sub- stantially reduce the association latency while also maintaining QoS. The proposed mech- anism may be also applied to improve the performance of contention-based accessing in broadband wireless systems.