具壅塞控制能力之低延遲分散式分時多工無線隨意網路存取協定

Abstract

在無線通訊技術中，減少訊息傳輸延遲是一項重要的議題。特別是對於傳輸延遲敏感度高的應用更是格外重要。因此，近來許多智慧型運輸系統 ( Intelligent Transportation System, ITS ) 的安全應用，對於傳輸延遲也有著嚴格的要求。 現有的無線網路媒體存取控制( Medium Access Control, MAC ) 協定，大致上可分成：競爭模式（Contention-based）及排程模式（Schedule-based）兩大類。然而競爭模式的MAC隨機存取機制，在網路密集度很高的時候，常會產生嚴重的爭用情形。 另一方面，排程模式採取的是有限的延遲（bounded-delay）存取機制，其透過將時間分割以及排程藉以達到無競爭傳輸。在密集的網絡中，node 數量可能會超過每一frame 原先所規劃的 slot 數量，導致有 node 無法取得屬於自己的傳輸 slot，因而無法進行訊息傳送。 雖然使用較大的frame size 將允許更多 nodes 無競爭傳輸，但每一個 node 須花費較長的時間來等待下一次傳送週期，而產生較大的傳輸延遲。 在本文中，我們致力於結合排程模式MAC與功率控制技術，以避免通道擁塞，並同時保持較低的傳輸延遲。

Reducing transmission delay is an important issue in wireless communications. It is particular critical to delay-sensitive applications. Many recent safety applications in Intelligent Transportation System (ITS) also have strict requirements on the delay. Existing Media Access Control (MAC) protocols for wireless networks typically fall into two categories: Contention-based and Schedule-based MACs. However, due to the random access nature, contention-based MAC may incur severe contention, especially in high density networks. Otherwise, schedule-based MAC achieves bounded-delay access by dividing time into frames and let each frame contain several slots for collision-free transmissions. However, in a dense network, the number of nodes could exceed the frame size such that some node may not be able to reserve a free slot for its transmission. A larger frame size will allow more nodes to reserve a free slot for their transmissions, but it may also incur a larger delay since each node has to wait for a longer period of time before the next frame coming. In this paper, we aim to combine schedule-based MAC with an adaptive power control technique to avoid channel congestion and at same time to retain a lower end-to-end delivery delay.