長細拓樸下CDMA無線感測網路之效能評估

Abstract

無線感測網路(Wireless Sensor Network)是由眾多的無線感測節點與數個閘道器所組成的無線網路系統。感測節點收集環境中的資料後將其送到閘道器，再由閘道器轉送到伺服器做進一步的處理分析。無線感測網路最初是由美軍發展用於戰場上，後來此項技術繼續發展及應用在不同的用途上，如科學調查、溫濕度監控、火災預報等。 我們觀察到有許多跟安全相關的無線感測網路應用，例如偵測橋樑上的異常振動或張力、礦坑內的易燃氣體濃度等等，其網路拓樸由一些長細狀的分支所組成，呈現”長細拓樸”(long-thin topology)狀。在一般的運作下，感測網路只需定期取得感測節點收集的資料以偵測異常狀況。然而，若發生事故，需要傳送大量資料時，感測網路則需要進入不同的運作模式，例如在礦坑內發生爆炸，感測節點使用更快的速率傳輸則能更快速決定受困礦工之位置，且無線感測網路能被用於傳送受困礦工的聲音和影像以幫助救災行動。 在緊急狀況運作下，無線感測網路需要較大的吞吐量，但目前常見應用在無線感測網路之媒體存取控制協定(MAC protocol)通常為分時多重存取(Time Division Multiple Access)或載波感測多重存取(Carrier Sense Multiple Access)協定，其受到在接收節點的接收範圍內只能有單一傳輸節點做傳輸的限制，整體的吞吐量因此大幅減少。在一般運作下的協定設計目標通常是優化能源消耗的情況下，前面的限制並不構成問題。然而，原有的協定很明顯並不適用於緊急狀況的需求。 分碼多重存取(Code Division Multiple Access)的運作是將原始訊息以偽雜訊碼(Pseudo-noise code)展開成寬頻訊號。此系統除去了前述的限制，並允許多個感測節點在接收節點的接受範圍內同時傳送訊號，因此在使用分碼多重存取之下整體的吞吐量應能得到相當程度的改善。在這篇論文中，我們研究如何設計應用在長細拓樸下的雙模式無線感測網路系統之分碼多重存取協定。此系統在一般運作模式下使用分時多重存取或載波感測多重存取協定，而在轉換到緊急模式時則使用分碼多重存取協定。 我們研究了單鍊(Single-Chain)拓樸及多鍊(Multi-Chain)拓樸。對於單鍊拓樸，我們提出一個實作簡單的啟發式功率分配方法，其所需的開銷少且在吞吐量的表現上較最大功率配置方法好。對於多鍊(Multi-Chain)拓樸，我們提出一個啟發式的排程原則，能增加同個時槽內同時傳輸到閘道器的傳輸數量並得到顯著提升的吞吐量。模擬結果顯示在長細拓樸下使用分碼多重存取協定較分時多重存取得到了約略兩倍的吞吐量。 關鍵詞：分碼多重存取、分時多重存取、無線感測網路、長細拓樸、功率調控、排程

Wireless Sensor Networks (WSNs) are a type of wireless network systems which consist of a large number of wireless sensor nodes and a few gateways. Sensor nodes gather information about the environments and then forward it to the gateways, which in turn relay the information to a server for further processing and analysis. WSNs are initially developed for battlefield purposes by the U.S. military. In the past ten years, the technology continues to develop and begins to serve different purposes, such as scientific investigation, temperature and humidity control, fire forecasting, etc. We observed that for a number of safety-related WSN applications, e.g., abnormal vibration or tension detection on the bridge, flammable gas density monitoring within the mine pit, the network topology consists of a few long and thin branches, or exhibits a “long-thin topology.” During the regular operation, the network only needs to obtain the sensor reading periodically, so that anomaly can be detected. However, if an accident happens, the network instead needs to enter a different operation mode where a much larger amount of data needs to be transferred. For example, when a gas explosion occurs in the mine pit, the sensor nodes transmit at a much faster rate, so that the positions of the trapped miners can be quickly determined, and the WSN can be used to relay voice and video transmissions from the miners to assist the rescue operation. During the emergency operation, the WSNs require a large amount of throughput, but the common MAC protocols used in WSNs, usually a variant of the Time Division Multiple Access (TDMA) protocol or the Carrier Sense Multiple Access (CSMA) protocol, have the constraint that only one node can transmit within the receiving range of a node. The throughput is therefore greatly reduced. As the design objective for protocol during the regular operation is often optimized to reduce energy consumption, this does not pose a problem. However, the original protocol is obviously not feasible for the emergency purposes. The operation of Code Division Multiple Access (CDMA) spreads the original message into a wideband signal by modulating it with a pseudo-noise (PN) code. The scheme removes the aforementioned constraint, and allows multiple nodes to transmit simultaneously within the receiving range of a node. As a result, with a CDMA-based protocol, the throughput could be significantly improved. In this thesis, we investigate how to design a CDMA-based protocol for a dual-mode WSN system with long-thin topologies. The system would use a TDMA- or CSMA-based protocol during its regular operation and the CDMA-based protocol when switching to the emergency mode. Two most commonly used long-thing topologies, single-chain and multi-chain topologies, are studied in this thesis. For single-chain topologies, we propose an easy-to-implement heuristic power allocation scheme, which has low overhead and outperforms the full power allocation scheme in terms of throughput. For multi-chain topologies, we propose a heuristic scheduling principle, which can increase the number of simultaneous transmissions to the gateway in the same time slot and produces significantly higher throughput. Evaluation results suggest that the use of the CDMA-based protocol in WSNs with long-thin topologies approximately doubles the throughput compared to that of TDMA-based protocol. Keywords: CDMA, TDMA, wireless sensor networks, long-thin topologies, power control, scheduling