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Nov 18, 2007 - the battery is needless and its life-span is infinite. The sink node gathers .... network, many wireless sensor nodes monitor the machine, the ship, the ..... and secret keys are commonly used network security technology, and ...
Key Technologies of Passive Wireless Sensor Networks Based on Surface Acoustic Wave Resonators Xiangwen Zhang, Fei-Yue Wang Abstract-In this paper, we present the passive wireless sensor network based on the surface acoustic wave(SAW) resonators. The sensor node consists of the SAW sensor that is small, light, reliable, stable, sensitive, wireless and passive, so the battery is needless and its life-span is infinite. The sink node gathers data from the sensor nodes, processes the data with intelligent algorithms and transmits the needed data to the exterior network timely. The basic structure and the realization of the passive wireless sensor network are elaborated. The five main characteristics of the passive wireless sensor network, that is passive sensor nodes, simple and small sensor nodes, organized sensor nodes, intelligent sink nodes, high security, good extendibility, are explained concretely. Specially, the key techniques in our research, such as coding and decoding techniques of the sensor node, signal frequency measurement techniques of the sensor node, intelligent signal processing techniques, measurement error compensation techniques and network security techniques, are discussed exhaustively. In the end, we point out the problems at present and forecast the application prospect and research direction in the future.

Index Terms-surface acoustic wave(SAW) resonator,

SAW sensor, wireless sensor network, passive, intelligent

wireless and passive. It can be used in bad environments with high temperature, high pollution, high voltage, strong electromagnetic disturbance, closed chambers, moving and rotating parts of the engine to measure parameters which can not be gotten with conventional sensors. In addition, it can be processed with the semiconductor plane manufacture craft and combined with integrated circuit to realize the integrated, intelligent and multifunctional network device. So it has aroused the interest of researchers all over the world, and many productions have been madeli'-li"1. The SAW sensor can be made from the SAW delay line or SAW resonator shown in Figs.l(a)-(b). They consist of the piezoelectric substrate, the interdigital transducer(IDT) and reflectors on the surface of the substrate. The IDT is the electrodes arrayed along the substrate as crossed fingers, and it can translate the electromagnetic wave into SAW or reversely. The reflector is the electrodes arrayed parallelly along the substrate and it can reflect the SAW and change its propagating direction. The sensor based on SAW delay lines can measure the parameters according to the relation between the delay time or phase and the factors acting on the surface of the substrate. It is simple and studied fully at present[1]-[8]

I. INTRODUCTION

S URFACE acoustic wave(SAW) is an elastical wave that propagating along the surface of an elastic substrate and its amplitude decays exponentially with substrate depth. Its velocity is 105 smaller than that of electromagnetic waves and its transmission loss is very little[']~5]. When the pressure, temperature or other outside factors act on the surface of the elastic substrate, the characteristics of the SAW propagating along the surface will change. According to the relation between them, we can make the SAW sensor. The SAW sensor is small, light, reliable, stable, sensitive, Manuscript received September 28th, 2007 and accepted November 18th, 2007. This work was supported in part by Nature Science Fund of GuangXi,China:GuiKeZiO640 172, Doctoral Initial Fund of Guilin University of Electronic Technology and Open Project of the Key Laboratory of Complex Systems and Intelligence Science in the Institute of Automation, Chinese Academy of Sciences. X. Zhang is with the College of Computer and Control, Guilin University of Electronic Technology, Guilin, China and the Key Laboratory of Complex Systems and Intelligence Science, Chinese Academy of Sciences, Beijing, China (corresponding author, phone: 086-0773-560375 1; e-mail: 63.com) F.-Y. Wang is with the Key Laboratory of Complex Systems and Intelligence Science, Chinese Academy of Sciences, Beijing, China (e-mail: eigangLy]zmaiLia.ano

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Reflector (b) Sensors based on SAW resonators Fig. 1 Two types of SAW sensors

The sensor based on SAW resonators can measure the parameters according to the relation between the resonance frequency and the factors acting on the substrate. Compared with the sensor based on SAW delay lines, its bandwidth is narrower, its energy utilization factor, quality factor, frequency stability and measurement accuracy are better, and it can measure parameters wirelessly with farther distance. Owning to these merits, the sensor based on SAW resonators is very promising and there are more and more researches on this sensor in recent years[3][9]-[11] With the development of the embedded system, wireless communication system, network system, semiconductor and MEMS system, the wireless sensor network arises. It is the advanced sensor system and the integration of the many information techniques. It can be used for testing, sensing, collecting and processing information of monitored objects and transferring the processed information to users['2]['3]. Its basic structure is shown in Fig.2. The wireless sensor network is comprised of the data acquisition network and the data distribution network. In the data acquisition network, many wireless sensor nodes monitor the machine, the ship, the animal, the vehicle and the patient, and collect and transmit the signal to the management center where the data are analyzed and transmitted to the base station controller(BSC). From the BSC, the data distribution network can get the needed data. According to the data, we can print the necessary message, monitor the objects online and visit them from the PDA, the notebook, the PC and the cellular phone anywhere and anytime with the wireless network or internet. The wireless sensor networks have been used in the military reconnaissance, the environmental information examination, the agricultural production, the medical healthy guardianship, the building and the family security, the industrial production control, the municipal transportation management, the emergency disaster relief, the dangerous region long-distance control as well as the trade on many domains['4]['5].

The USA, the England, the Japanese and China have attached great importance on this research. Berkeley Branch School of University of California designed the open-source operating system TinyOS for wireless sensor networks. Los Angeles Branch School of University of California gave the model of wireless sensor networks and the simulation condition, designed the wireless integrated network sensors (WINS). The Tsinghua University, the Institute of Software, Chinese Academy of Sciences, and the Haerbin Industry University have done the basic theory and application research actively and built a few experiment platforms at present[ 4].

For the conventional wireless sensor networks, the sensor nodes get the energy from the tiny battery. But the capacity of the battery is limited and it is inconvenient to replace the battery in the special applications, so the life-span of the battery and the sensor node is finite. In addition, the sensor node is very small and cheap, so its computation ability, storage space and traffic capacity are limited. It is difficult to gather, process and send the signals simultaneously. The number of the sensor nodes is large and it is not easy to organize the nodes well. In this paper, we present a passive wireless sensor network based on the SAW resonators. The sensor node is comprised of the sensor based on the SAW resonators. Its life-span and capacity is not problem for the sensor is passive and the battery is needless. The sensor nodes only test and transmit the signals. The sink node gathers data from the sensor nodes, processes the data with intelligent algorithms and transmits the needed data to the exterior network timely. The basic structure and the realization of the passive wireless sensor network are elaborated in section II. The main characteristics and the key techniques in our research of the passive wireless sensor network are discussed in section III and IV. In the end, we point out the problems at present and forecast the application prospect and research direction in the future. II. BASIC STRUCTURE AND REALIZATION The basic structure of the passive wireless sensor network is shown as Fig.3. It comprises of the sensor nodes, the sink node, the transmission network and the application network.

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Fig.3 Basic structure ofthe passive wireless sensor network based on SAW resonators

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The sensor nodes consist of sensors based on the SAW resonators. They only measure and transmit the parameters of the monitoring area. The sink node is the important part of the network. It gathers and processes the sensing signal from the sensor nodes. In signal processing process, the sink node can use the intelligent algorithm to regulate the receiving and sending period and control the sending data size. When there is no obvious change over the monitoring area, the sink node can go into sleep and be awakened after a long fixed time. Otherwise, the sink node decreases the receiving periods, processes the collected data intelligently and sends out the necessary data. The sink nodes communicate with other networks by the Internet or the satellite network. The user can visit and analyze the data with the PC or the PDA by the Internet network. The sensor node based on the SAW resonators is shown in Fig.4. It is composed of the substrate, the IDT, the reflectors, the antenna and the alterable impedance. The antenna receives the interrogation pulse electromagnetic signal from the sink node. The IDT translates the received electromagnetic signal into SAW. The SAW propagates among the cavity that is composed of the IDT and the two reflectors. When the interrogation signal frequency is equal to the resonance frequency of the SAW resonator, the standing wave is aroused and there is a resonance among the cavity. The output signal in resonance situation is the maximum of the different input signals. We can detect the output signal peak to measure the resonance frequency that contains the sensing information and the sensor node code. In order to distinguish different sensor node, we code the sensor by adjusting the alterable impedance matched with the antenna. For the resonance frequency will change with the system impedance, and the alterable impedance matched with the antenna influences the system impedance, we can distinguish the different sensor with the different resonance frequency.

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The sink node is shown in Fig.5. It is composed of the transceiver, the embedded processing unit and the network interface. The embedded processing unit is made up of the micro controller and the memory. The transceiver sends RF interrogation signals with different frequency by the antenna. The sensor nodes receive the signal and send back the sensing signal. The transceiver receives the sensing signals from sensor nodes and sends to the micro controller. The micro controller processes the signal with data fusion to get the useful information and stores in the memory. When the change of the data is bigger than the limitation, it is necessary to send the data to the network interface, otherwise, the controller can go into sleep until receives the obvious changed data. According to the protocol stack of the computer network, we can design the passive wireless sensor network with five layers protocol. In detail, they are the physical layer, the data link layer, the network layer, the transport layer and the application layer. In every layer, the function and property will different from the conventional wireless sensor network in some aspects. The physical layer is related to the sensor based on the SAW resonators. Its main task is to choose the parameters of the sensor, such as the resonance frequency, code method and matched circuit with the antenna etc., according to the measurement requirements. The ISM frequency band is usually selected as resonance frequency for it does not need to register, and it has the wide range frequency band to choose. The data link layer achieves the reliable data communication between the sensor nodes and the sink nodes by coding the data with different frequencies. The network layer transmits the data from the sink node to the outer networks, such as the wireless local-area network(WLAN) and the Internet. The network layer protocol is the key problem to realize the correct data communication. The transport layer is the core of the data communication. It can realize the transport controlling and data scream maintenance to ensure the quality of service(QoS). The application layer realizes functions for special application. Its protocol is related to the detailed application and environment, so the special design is necessary for the given application.

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III. MAIN CHARACTERISTICS

A. Passive Sensor Nodes The sensor node is the sensor based on SAW resonators. The SAW sensor is passive and it can measure the parameters with the energy from the interrogation RF signal of the sink node. The battery is needless for sensor nodes, so the sensor node death question, the energy-efficient and power management are out of consideration. The sink node is powered by the battery, so the number of the sink nodes is small, and the location should be convenient for replacing the battery. B. Simple and Small Sensor Nodes The sensor node is made up of the SAW resonator, the antenna and the matched circuit. They can be processed with the semiconductor plane manufacture craft and integrated together easily, so they are very small. In addition, the sensor node only test and transmit the parameters from the monitoring area, its function is very simple and easy to realize. So the sensor node is very easy to be placed in any parts of the monitoring area and there is no influence on the area. C. Organized Sensor Nodes For the sensor node gets it energy from the sink node, the sensor node should be located around the sink node and the layout should be designed beforehand according to the special requirements. We can organize the sensor nodes to optimize the layout, save the sensor nodes and improve the sensing quality. D. Intelligent SinkNode The sink node has its own micro controller and memory. The memory can store a few groups data collected from the sensor nodes. The micro controller can process the data with intelligent algorithm and data fusion method, so the sink node is intelligent. If the data variation is among the allowable range, the micro controller can go into sleep and the memory stores one group data. If there is an abnormal data received, the micro controller is awoken and the transceiver increases the times of gathering data. At the same time, the abnormal data is sent to the outer network. E. High Security The sink node is rich in the computation and memory source, so it can do the complex data encryption technology to increase the data security in the network transportation. F. Goog Extendibility The sink node can collect signals from a few sensor nodes. Sometimes, the monitoring area needs to expand range, and the new sensor nodes are added. If the new sensor nodes among the range of the sink node, the sink node can detect them soon and collect their sensing signal in time. If the new monitoring area is so large that the old sink nodes cannot detect the new sensor nodes, the new sink node can be added. So the sensor network is extensible.

IV. KEY TECHNIQUES The passive wireless sensor network is the multidisciplinary overlapping research area of the SAW sensor technology, the wireless communication technology, the embedded computation technology and the MEMS technology, and there are many problems and key techniques will appear among the research. In the follows, we list some key techniques in our research.

A. Coding and Decoding Techniques of the Sensor Node In the monitoring area, every sensor node measures the parameters of one special point. In order to distinguish the different measurement from different point, the code of the sensor node is necessary. For the sensors based on the SAW resonators, D. Puccio, D. C. Malocha, D. Gallagher from University of Central Florida studied the coding technique using orthogonal frequency in 2004[16]. Wen,Y.M. and Li,P. from ChongQing University presented a new SAW sensor structure based on the resonator and the delay line '7]. The SAW delay line and the SAW resonator are connected together, and the SAW sensor is coded with different SAW delay line structure and measures the parameters with the SAW resonator. But at present, all the coding methods are very complex and need special design and processing, and they do not fit for the large-scale volume production. So it is the key question to realize the sensor network that seeks the simple practical, realized coding and decoding methods. In our research, we present a new SAW sensor structure with the SAW resonator, the conventional sensor and the matched circuit between them. When the impedance of the conventional sensor and the matched circuit varies, the matched impedance of the resonator changes, and the resonance frequency of the SAW resonator changes, so we can code the sensor with different resonance frequency by adjusting the matched circuit. As the new sensor structure, the SAW resonator, the conventional sensor and the matched circuit can be designed and sealed separately, so there is no problem in fixing wire connector and the special design according to the SAW sensor requirements. In addition, this method decreases the difficulty of the design and seal process, and improves the practicality of the SAW sensor. It will be the key technique of the passive wireless sensor networks at present and in the future. B. Signal Frequency Measurement Techniques of the Sensor

Node The sensing and code information of the sensor nodes is sent to the sink node by the frequency. The sink node detects the frequency for the information. There are many methods of detecting the frequency. In 1998, Pohl, A. and Ostermayer, G. at the University of Technology in Vienna presented a method to detect the frequency by gated phase locked loop circuit['8]. The phase locked loop circuit locks the frequency in a few pulses to keep the sensor and the phase locked loop in-phase, so it is easy to get the sensor frequency by detecting the phase locked loop frequency. But there is restriction for this method because the phase locked

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loop itself has the threshold level. When the input signal receives the environment and the system interior noise, and the noise level surpasses the threshold level, and the output signal signal-to-noise ratio drops suddenly that causes the phase locked loop cannot work well, therefore, this method is unable to be used in big environmental noise conditions. In 2004,Matthias Hamsch and Rene Hoffmann from the Technical University of Ilmenau in Germany detected the resonance frequency by Fourier transform of the output signal"I9]. This method is simple in hardware and the precision is high. In 2003,Wen,Y.M. and Li,P. from ChongQing University obtained the resonance frequency from the received signal envelop and the inherent frequency by modeling the received signal 201. This method is easy to realization and may be the main direction of detecting the resonance frequency in the future. C. Intelligent Signal Processing Techniques The measurement signals from the sensor node are sent in the air and affected by the noise and other multipath propagation signals, so the signal processing by the sink node is not easy. In order to improve the measurement precision, the signal processing method is the key technique. At present, nerve network and fuzzy logic algorithm are new intelligent data processing methods, and they have been used in many signal-processing aspects successfully, so they can be used to study the sensor signals in the wireless sensor network. In addition, data fusion method is a method to carry on the data redundancy processing, and it can be used to improve the signal processing effect. D. Measurement Error Compensation Techniques There is always error in the SAW sensor measurement for the hardware circuit, the noise disturbance and the limitation of the measure method. In order to decrease the error and improve the measurement precision, many error compensation techniques are presented. Chen,M. and Li,S.L. from the Northwestern Polytechnical University presented the method of floating zero to revise the pressure measurement error from the temperature disturbance 21]. This method is limited to a finite application. In recent years, the error theory was researched with the statistic theory and stochastic process theory, and the error model was established with the statistic method 22]. With the error model, we can analyze the reasons of the error and find the effective error compensation techniques. E. Network Security Techniques In order to keep the data safety in the data gathering, transporting and processing process, many encryption and network security techniques are presented. The key technique of the wireless sensor network security is the method to realize the reliability, integrality, secrecy and undeniablity of the data. At present, data integrity distinction, message authentication, watermark technology and secret keys are commonly used network security technology, and they all can be used in the wireless sensor networks.

V. CONCLUSION AND PROSPECT FORECAST In this paper, we present the basic structure of the passive wireless sensor network based on SAW resonators. The realization and its characteristics, key techniques are described in details. It is passive, so the lifetime and the power management of the sensor node are no problem. The

key techniques of the node localization and time synchronization for conventional wireless sensor networks are simple and easy to realization for the passive wireless sensor networks. The node localization can be realized by coding the sensor nodes and laying them in special points according to the design requirements. The time synchronization can be realized by recording the time of the data collected by the sink node according to the sink node clock. The passive wireless sensor network is the main direction of the wireless sensor network and it can be used in monitoring the parameters of the closed chamber, the moving and rotating parts of machines, the bad environment of high temperature, high pollution, high voltage and strong electromagnetic disturbance. In the follows, we list some potential applications. In the food transportation field, the passive wireless sensor network can monitor the temperature, humidity state for all the lifetime in order to give the food deterioration warning timely. In the automobile industry, the passive wireless sensor network can measure the tire pressure and temperature, the motor torque and temperature in real time, and give an alarm in the abnormal situation. In the medical care field, the passive wireless sensor network can realize the uninterrupted nursing and prompt treatment by monitoring patient's bodily condition characteristic, such as the blood pressure, the pulse, the breath, the sleep posture, the body temperature and so on. In addition, the data received by monitoring the patient can be useful to the condition diagnosis in the future. In the environmental monitoring aspect, the passive wireless sensor network can monitor the air pollution degree by measuring and analyzing the atmospheric constituent change. In addition, it also can monitor the soil ingredient variety to provide the basis for the crops cultivation. In the disaster forecast aspect, the passive wireless sensor network can forecast the flood by monitoring the rainfall and river water level, and realize the forest-fire warning by monitoring the air temperature and humidity variety. In the industrial automation production line aspect, we can use the passive wireless sensor network to monitor and analyze the machine working condition. In this way we can decrease the cost of checking the equipments, enhance the efficiency and lengthen the service life of the equipments, find the malfunction in advance and improve the factory operation condition greatly. In the home application aspect, we can connect the refrigerator, the TV set, the PC, the washer, the microwave oven and other domestic electric appliances to the Internet. In this way we can monitor, track and control them whenever we are and where ever we are.

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In conclusion, the application area of the passive wireless sensor network will be more and more widespread, even in all aspects of our work and living. But its research is not mature in some aspects because the passive wireless sensor network is interdisciplinary studies of the SAW sensor, the wireless communication, the embedded computation, the network and the MEMS technique. At present, the multifunctional, miniature, intelligentized and precise SAW sensor, Coding and decoding of the sensor nodes, small, effective and functional micro controller, memory and wireless communication chip, data encryption and safe transmission in the network, and the embedded operation system for the passive wireless sensor networks are all the important research directions. REFERENCES [1]

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