IEEE 802.16網路之無線資源管理

Abstract

無線城域網路之國際標準IEEE 802.16已被定義出來滿足低成本的大範圍寬頻無線存取(broadband wireless access)，在本篇論文中，我們將充分開發頻譜再利用(spectral reuse)和競爭碰撞解決(contention resolution)之可能性來進一步提昇無線頻寬的使用效率。本篇論文內容分為傳輸排程(scheduling)、封包繞徑(routing)、以及頻寬要求(requesting)三大議題。

在第一個研究主題中，我們深入地研究如何在IEEE 802.16網狀網路(mesh network)的資源配置(resource allocation)中充分開發出頻譜再利用，其中包括繞徑樹建構(routing tree construction)、頻寬配置(bandwidth allocation)、時槽分派(time-slot assignment)、以及即時資料流(real-time flow)的頻寬保證(bandwidth guarantee)。我們提出的頻譜再利用framework 涵蓋了應用層(application layer)的頻寬配置、媒體存取層(MAC layer)的繞徑樹建構與資源分享、以及實體層(physical layer)的頻道再利用(channel reuse)。就目前所知，這是第一個研究成果以數學方式分析出IEEE 802.16網狀網路的頻譜再利用程度，並且設計出完整的framework來提昇頻譜再利用的效率。模擬結果顯示我們所提出之framework大幅度地增加了IEEE 802.16網狀網路的網路產出量(network throughput)。

在第二個研究主題中，當IEEE 802.16網狀網路的網狀傳輸站(mesh station)具備移動能力之後，便形成一個由行動中繼傳輸站(mobile relay station)所構成的行動隨意網路(mobile ad hoc networks)，行動隨意網路由於其極具彈性的網路架構，已經廣受各方的注目，雖然有許多根據不同準則而為行動隨意網路所設計的繞徑協定(routing protocol)，但是其中只有極少數考量到已被許多無線網路設備所支援的多重速率傳輸(multi-rate)之影響。在給定一條傳輸路徑(routing path)的情況下，我們提供了一套數學分析的工具，假設行動中繼傳輸站以離散時間隨機方式(discrete-time, random-walk)來移動，進而計算出此傳輸路徑的期望產出量(expected throughput)，並且將頻譜再利用的因素一併考量進來。模擬結果顯示我們所提出之數學分析方法可準確地計算出傳輸路徑的期望產出量，其推導結果可以作為更佳的傳輸路徑選擇(route selection)準則。

在第三個研究主題中，為了更有效率地使用無線資源，我們進一步地研究了兩個在IEEE 802.16網路中針對best-effort traffics解決頻寬要求(bandwidth request)碰撞的機制。其中一個是定義在標準中的exponential backoff機制，另外一個是我們所提出single-frame backoff的piggyback機制。我們分析並比較了這兩個機制在Poisson traffic下頻寬要求成功機率(request success probability)和封包傳輸延遲(packet delivery delay)方面的效能。分析和模擬結果顯示piggyback機制的效能表現比exponential backoff要來得出色許多，可以大幅度地減低頻寬要求的碰撞。

The IEEE 802.16 standard for wireless metropolitan area networks (WMAN) is defined to meet the need of wide-range broadband wireless access at low cost. In this dissertation, we exploit spectral reuse and contention resolution of IEEE 802.16 networks. This dissertation is composed of three works. In the first work, we exploits spectral reuse in an IEEE 802.16 mesh network through bandwidth allocation, time-slot assignment, and routing tree construction. In the second work, we provides an analytic tool to evaluate the expected throughput of the route with spectral reuse in an IEEE 802.16 relay network. To further improve wireless resource utilization, the last work analyzes and compares two collision-resolution requesting schemes for best-effort (BE) traffics in IEEE 802.16 networks.

In this dissertation, we first study how to exploit spectral reuse in resource allocation in an IEEE 802.16 mesh network, which includes routing tree construction, bandwidth allocation, time-slot assignment, and bandwidth guarantee of real-time flows. The proposed spectral reuse framework covers bandwidth allocation at the application layer, routing tree construction and resource sharing at the MAC layer, and channel reuse at the physical layer. To the best of our knowledge, this is the first effort which formally quantifies spectral reuse in IEEE 802.16 mesh networks and which exploits spectral efficiency under an integrated framework. Simulation results show that the proposed schemes significantly improve the throughput of IEEE 802.16 mesh networks.

On the other hand, when mesh stations have mobility, they form a mobile ad hoc network (MANET) consisted of mobile relay stations. While many routing protocols have been proposed for MANETs based on different criteria, few have considered the impact of multi-rate communication capability that is supported by many current wireless products. Given a routing path, the second work provides an analytic tool to evaluate the expected throughput of the route with spectral reuse in a mobile relay network, assuming that hosts move following the discrete-time, random-walk model. The derived result can be added as another metric for route selection. Simulation results show that the proposed formulation can be used to evaluate path throughput accurately.

To utilize the channel bandwidth more efficiently, the third work studies two collision-resolution requesting schemes for best-effort (BE) traffics in IEEE 802.16 networks. One is the exponential backoff scheme defined in the standard and the other is a piggyback mechanism enhanced by single-frame backoff, called the Request Piggyback (RPB) scheme. We analyze and compare their performance in terms of the request success probability and the packet delivery delay under Poisson traffic. The results show that the RPB scheme outperforms the exponential backoff scheme and can reduce request collision. Based on the designed scheduling, routing, and requesting schemes, we can further improve the efficiency of wireless resource management in IEEE 802.16 Networks.