適用於室內定位系統之跳頻機制

Frequency Hopping for Robust Indoor Localization in Daily Environments

吳意曦(I Hei Ng)

指導教授 ： 黃寶儀

國立臺灣大學/電機資訊學院/電機工程學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.01197

摘要

定位系統的產值在2016年預計將達到兩億七千一百萬美元，並會廣泛地使用在一般的居家環境及商業建築物內，因此近年來定位系統成為了熱門的研究題目。然而要在日常生活環境中使用無線射頻技術來達到穩定的定位是一個難題。透過由二十四個IEEE 802.15.4無線感測節點組成的感測網路平台來實現一以比對信號強度特徵值的定位系統，我們可以有效的分析IEEE 802.11網路封包對室位系統準確度的影響。量測的結果顯示當IEEE 802.11的資料量變高時百分之八十的定位誤差會提高141%。定位準確度的降低是由於信標封包與WiFi封包碰撞而遺失。一個簡單的解決方法是延長信標封包的收集時間，但這樣會增加定位的延遲。為了建構一個強健的實時定位系統，我們提出一跳頻的機制使系統能因應當時無線電波受干擾的程度而跳頻。當系統受到嚴重的干擾時，它能跳至一個新的無線頻帶而避開信標封包被撞掉的情形。實驗結果顯示使用跳頻機制百分之八十的定位誤差在無線網路繁忙時由1.82米降低至1.32米(27%)，最繁忙的20分鐘內由2.74米降低至1.24米(55%)。

關鍵字

室內定位系統 ； 信號強度 ； 跳頻 ； 干擾 ； 共存 ； 隱馬爾可夫模型

並列摘要

With an expected market value of 2.71 billion in 2016, supporting daily use of real-time location systems in households and commercial buildings is an increasingly important subject of study. A growing problem in providing robust location estimations in real time is the use of wireless transmissions in RF frequencies in the daily environments. Having implemented a simple RSSI-signature-based location system on a 24-node IEEE 802.15.4-based sensor network testbed, we are able to analyze the effect of background IEEE 802.11 traffic to the localization error. The measurement results demonstrate that the 80th-percentile of the localization error may increase by 141% when the background 802.11 traffic is high. Such performance degradation is a result of RSSI reading loss as the beacon messages collide with background traffic. A common solution to this problem is to extend the time for beacon message collection. This approach, although effective, adds extra delay before robust estimations can be obtained. Aiming at achieving robust real-time localization in daily environments, we propose a frequency hopping mechanism that enables the system to adapt to the current interference level. When the interference level is high, the system hops to a new channel to avoid the foreseen high loss period. Our experimental results show that the proposed frequency hopping mechanism can reduce the 80th-percentile localization error from 1.82 to 1.32 meters (27%) in a busy hour and from 2.74 meters to 1.24 meters (55%) in a 20-minute period where the 802.11 traffic rate is at its peak.

並列關鍵字

Indoor Localization System ； Received Signal Strength Indicator (RSSI) ； Frequency Hopping ； Interference ； Coexistence ； Hidden Markov Model

參考文獻

[5] P. Bahl and V. Padmanabhan. An in building RF-based user location and tracking system. IEEE INFOCOM 2001, April 2001.

[6] L. E. Baum and J. A. Eagon. An inequality with applications to statistical estimation for probalistic functions of Markov processes and to a model for ecology. Bull. Am. Math. Soc., 73:360–363, 1967.

[7] Y.-C. Cheng, J. Bellardo, P. Benk‥o, A. C. Snoeren, G. M. Voelker, and S. Savage. Jigsaw: solving the puzzle of enterprise 802.11 analysis. In Proceedings of SIGCOMM ’06, pages 39–50, New York, NY, USA, 2006. ACM.

[8] L. Cong andW. Zhuang. Hybrid TDOA/AOA mobile user location for wideband CDMA cellular systems. IEEE Transactions on Wireless Communications, 1(3):439–447, July 2002.

[9] J. Degesys, I. Rose, A. Patel, and R. Nagpal. Desync: self-organizing desynchronization and TDMA on wireless sensor networks. In Proceedings of IPSN ’07, pages 11–20, New York, NY, USA, 2007. ACM.