Proceedings of the 8th World Congress on Intelligent Control and Automation July 6-9 2010, Jinan, China 978-1-4244-6712-9/10/$26.00 ©2010 IEEE An Overview of Range Detection Techniques for Wireless Sensor Networks * Yingfei Diao Minyue Fu Huanshui Zhang School of Control Science and Engineering School of Control Science and Engineering School of Control Science and Engineering Shandong University Shandong University Shandong University Jinan, Shandong Province, China Jinan, Shandong Province, China Jinan, Shandong Province, China yfdiao@mail.sdu.edu.cn minyue.fu@newcastle.edu.au hszhang@sdu.edu.cn * This work was supported by the Taishan Scholar Construction Engineering by Shandong Government, the National Natural Science Foundation for Distinguished Young Scholars of China (No. 60825304), and the Major State Basic Research Development Program of China (973 Program) (No. 2009cb320600). Abstract-Range detection is a key problem in wireless sensor networks (WSN). In this paper, a brief overview is provided for basic techniques for this problem. The fundamental principles for range detection are inherited from RADAR systems, but careful designs need to be made to balance the expected accuracy and the device complexity, considering the limited resources of a WSN. We will introduce some basic measurement principles of range detection for WSN and some available range detection systems. We will also discuss several signal processing algorithms used for range detection. Index Terms – Range detection; Localization, Wireless sensor network. I. INTRODUCTION Target localization is a major application for wireless sensor networks (WSN). Range detection is a key component for localization. In traditional location techniques for RADAR systems, range measurements are complemented with angle measurements for target localization. Large frequency band and expensive equipment, including an antenna array and sophisticated electronic and computing devices, is needed for these systems. In a WSN, however, range measurements are the primary information available for localization. It is not possible to use standard radar techniques because of narrow frequency bands (typically ISM 2.4GHz and 5.8GHz) and limited electronic and computing capability. The fundamental principles for range detection are not much different from those used in radar systems. But new range detection techniques are needed for WSN to balance the resource requirement and measurement accuracy. In this paper, we introduce some of basic range detection principles that are inherited from earlier RADAR systems (Section II). We then discuss several existing range detection platforms, some experimental and some commercial. We will focus on their setups and approaches, implementations and accuracy results (Section III). Afterwards, we will discuss signal processing algorithms that are commonly used for enhancing measurement accuracy in the presence of multi-paths (Section IV). Some concluding remarks will be reached in Section V. II. RANGE DETECTION PRINCIPLES Commonly used range detection techniques employ radio frequency (RF) or acoustic (ultrasonic) signals. Some devices use infrared or global positioning systems (GPS), for example, which we will not discuss in this paper; see [6] for a summary of these techniques. The physical quantities used for range detection are mostly signal travel time and signal power strength. We can divide the detection techniques into several classes according to different measuring principles of these quantities. These techniques include time of arrival (TOA), time difference of arrival (TDOA), round-trip time-of-flight (RTOF) and radio signal strength (RSS). The RSS technique is based on the propagation decay of a radio signal. The other techniques are based on fact the propagation speed of the signal, either radio or acoustic, is almost constant in the measurement environment, so the range detection problem becomes the propagation time detection problem. The sending time and arrival time of a signal in WSN are usually marked using time-stamping at the MAC layer of the network. For acoustic signals, the travel time is relatively large, so its direct measurement is relatively easy. For radio signals, the travel time is extremely small, so the travel time needs to be measured indirectly using various modulation techniques. We will introduce linear frequency modulation (LFM) and a variant of it called chirp spread spectrum (CSS) for WSN which is used as an alternative PHY standard in IEEE 802.15.4a. A. The TOA and RTOF Techniques The TOA technique is used when the transmitter (target) and the receiver (range sensor) are time synchronized. The setup is shown in Fig.1 (a). The sending time is stamped on the same signal or an auxiliary signal transmitted as the same time. Since the nodes are synchronized, the propagation time of the signal is directly calculated by subtracting the sending time of the signal from the arrival time. Commonly used time synchronization algorithms for WSN can give an accuracy of microseconds. This accuracy is enough for range detection using acoustic signals. However, for radio signal based range detection, the TOA technique is inadequate because one microsecond error in time 1150