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Wireless sensor networks TOF Ranging Method

In Electronic Infomation Category: W | on April 19,2011

Wireless sensor network is integrated by the large number of random distribution of the sensor unit, data processing unit, communication unit and LT1372HVIS8 datasheet and power unit of the tiny nodes and LT1372HVIS8 price and through self-organization constitutes a distributed network, and LT1372HVIS8 suppliers and its purpose is built by means of small nodes remote monitoring of various sensors to the target or object of interest to carry out the task of sensing, data acquisition and processing.

Wireless sensor network technology enables large-scale sensor networks as possible. But the attendant reduced the reliability of the network, in particular, poor network node location information to determine, for most applications, do not know the location-aware sensor node data is meaningless, since the positioning of nodes function is considered to be one of the basic functions of the system. Therefore, the wireless sensor network positioning technology research is very important, and has become the basis for supporting the wireless sensor network technology.

In general, wireless sensor network is divided into distance-based location technology to locate and position are not based on distance. Distance-based location technology of wireless sensor nodes are generally divided into two stages: first, the wireless sensor measuring the distance between network nodes; then the distance between nodes and existing sensor node localization algorithm, such as trilateration to calculate the A wireless sensor network node. Therefore, the nodes ranging technology is in wireless sensor networks nodes based on distance based location technology.

This wireless sensor networks by studying the domestic and international positioning in the present situation of using T OF node distance ranging technology to improve the node based on distance from the positioning accuracy of positioning technology.

1 wireless sensor networks ranging method

In wireless sensor networks, the commonly used method of measuring the distance between the nodes are TOA (Time of Arrival), TDOA (Time Dif ference of Arrival), ultrasound, RSSI (Received Sig nalStrength Indicator) and TOF (Time of Light ) and so on.

TOA and TDOA Ranging signal propagation time through the speed and signal to calculate the distance of two parameters, the wireless signal transmission rate is large, the time measurement error on the very small distance can lead to large errors , and TOA requires expensive equipment to maintain time synchronization, large consumption of energy. TOA ranging signal transmission involves the measurement of time in order to estimate the distance between two nodes. It can run in high multipath environments, and provide decimeter-level range accuracy.

Ultrasonic ranging method is fired when the ultrasonic transmitter node occurs when an obstacle reflection, the reflected wave received by the receiver, so long as the measured ultrasonic wave reflected back from the sending point to the time interval t, to to distance. Therefore, the ultrasonic wave from the transmitter to the obstacle at the distance between the c t / 2 (c is the propagation of ultrasound in the medium speed). Very precise distance measurement using ultrasound, the measurement error is only 10 cm, but ultrasound is a sound, and sound speed c by environmental temperature, humidity and other factors. In addition, the distance required additional hardware support, an increase of hardware cost and size of the node.

RSSI ranging method is the most basic, basic no additional hardware is simple to achieve. In the received signal strength indicator RSSI-based ranging, the known emission intensity of the node transmission signal, the receiving node received signal strength based on the signal propagation loss is calculated, using the theoretical model of the transmission loss and experience into the distance between nodes . The theoretical model is:


Where: p (d) indicates the signal strength at distance d; n that path length and the ratio between the path loss factor, the range of 2 ~ 4; p (d0), said at a distance d0 signal strength; d nodes that need to calculate the distance between the base station; d0 said reference node and the distance between the base station. Because of sensor nodes with wireless communication capability in itself, so it is a low-power, low-cost ranging technology, RADAR and other projects using the technology.

Although the RSSI in a lab environment to show good properties, but in the actual environment, it sensitive to temperature, the wireless signal reflections and obstructions (such as land, buildings), communication mode, and many other factors, so the technology in practical applications there are still difficulties, often seen as a rough its ranging technology, may produce 50% range error.

TOF distance measuring technology can be understood as jet lag distance (Time ofFlight Measurement) method, is divided into the traditional two-way ranging techniques ranging technology (Two Way Rang ing) and one-way Ranging (OneWay Ranging). T OF ranging ranging method is two-way technology, which mainly use the signal in two asynchronous transceiver (Transceiver) round-trip flight between the time to measure the distance between the nodes. The signal level is better prepared or in the non-line-sight environment, RSSI ranging method based on the results of estimating the ideal; in the sight line of sight environment, ranging method based on T OF Estimation of the ideal result is presented with the distance linear relationship. Therefore, based on TOF-based distance estimation methods can compensate for lack of RSSI distance estimation methods. In addition, specific applications can combine the two methods to improve the positioning accuracy of the system.

TOF distance is D. McCradys, but the technology focuses only on direct sequence spread spectrum (DSSS) communication system. Next, M. Ciur ana T OF Ranging Technology also has research, his first in the IEEE 802. 11b wireless LAN technology using the T OF distance, but require additional hardware assistance. In wireless sensor networks, there are many scholars studied TOF ranging technology. However, they focus on a particular wireless sensor network, a typical scene, such as programmable gate array (FPGA) implementation, and requires a dedicated infrastructure, and this infrastructure is not widely used in IEEE 8021. 11 (Wireless ) network.

In this article, consider the use of narrow-band RF T OF Ranging method and the standard IEEE 802. 11b wireless sensor network chip to complete an accurate estimate of the distance between point to point.

2 TOF distance measurement principle of the method Ranging in the T OF

, the local node A to the remote node sends a data packet B, when node B receives the packet, it will automatically send a confirmation in response to the packet. Implementation shown in Figure 1.


Figure 1 TOF ranging process

A measure node send packets to the receiver from the timing of the recognition, this consumption of the total time recorded for the TT OT time; B records the receipt of data packets from B to B response to the confirmation message of this time period, denoted for the TT AT. By subtracting the total time T TOT TT AT turnaround time is two packets in flight through the round trip time, denoted by TRTT time. Assume that in each direction of flight time equal to 50% of TTO F round-trip time, such as the type (2) below:


TT OF

When calculated, according to D = T c (T on behalf of T TOF; c representative of the speed of light, is 3 108 ms-1) can calculate the distance between nodes.

TOF ranging approach has two key constraints: First, the sending and receiving devices must always be synchronized; Second, the receiving device to provide signal transmission length of time. In order to achieve clock synchronization, TOF ranging method used to solve the clock offset of clock synchronization problems. However, due to the time T OF ranging method relies on local and remote nodes, range accuracy in the end nodes are vulnerable to the impact of the clock offset. To reduce the impact of such errors, the reverse measurement method used here, that remote node to send data packets, the local node receives the packet, and automatically respond to, forward and reverse through the average income, on average, to reduce any clock bias shift effects, thereby reducing the distance error.

3 TOF Ranging Experiment

3. 1 test preparation

Experiment, using the Jennic JN5148 EK010 development platform as an experimental platform to perform TOF ranging trial. JN5148 communication module with ultra-low power, high performance, fully compatible with IEEE 802. 15. 4 and so on. It integrates 32 b of the RSIC MCU core, high-performance 2. 4 GH z IEEE 802. 15. 4 transceivers are used in ZigBeePRO wireless sensor networks [12].

Jennics JN5148 wireless microcontroller includes a hardware time of flight (TOF) engine, capable of measuring 2. 4 GHz radio signal flight time between two nodes. The flight time is proportional with the transmission distance, it can be used to estimate the distance between the nodes.

3. 2 Experiment Description JN 5148

select two nodes, Node A and B are nodes. A node as a Coordinator node, B node as EndDevice node. Where, B node is connected through the serial port and PC, in PC, serial debugging software to view relevant information; A node as a mobile node, used to change the actual distance between nodes. Actual measurement results by comparing the results of TOF to verify the distance ranging method.

Under different experimental environments and test distance, the design of the 4 types of experiments were made for Line of Sight (LOS), No Line of Sight (NLOS), Indoo r, corridors ranging experiments. Which, LOS line of sight up to the region, said in a barrier test Medina; NLOS that sight is not up to the area tested in the orchard, in the straight-line distance between two nodes are buildings, trees and other obstacles; Indoor Ranging experiment was arranged in the laboratory for testing; corridor experiment to test the corridor of a building, barrier wall thickness of 30 cm, the corridor width 3 m, length 30 m.

3. 3 results

Order to reduce the distance error, averaging of multiple distance methods to estimate the distance. At 10. 5 m indoor ranging experiments, EndDev ice nodes in a room, and Coor dinator node on the corridor, separated by a wall, the measurement results shown in Figure 2, measuring 60 times (one point 3, a total of 20 measuring points) the sample data, the average error in 1. 94 m, the maximum error distance is 3. 55 m. 26 m distance in the corridors of the experiment, measuring 60 times the average error of the sample data 2. 01 m, the maximum error distance of 5. 1 m, the sample data shown in Figure 3.

Ranging in the 100 m of the LOS test, measuring 60 times the average error of the sample data in the 3. 62 m, the maximum error distance is 11. 2 m, the sample data shown in Figure 4. 100 m in the NLOS ranging trial, measuring 60 times the average error of the sample data in the 4. 47 m, the maximum error distance is 19. 6 m, the sample data shown in Figure 5. Experimental results show that, TOFs high precision positioning technology to meet the requirements of WSN.


Figure 2 Indoor Range sample plans


corridor ranging sample of Figure 3 Figure


Figure 4 LOS ranging sample plans


Figure 5 NLOS ranging sample plans

4 Conclusions T OF

introduced the node-based ranging techniques. Tests show that the clock offset with time synchronization method can be used both positive and negative direction of the T OF seeking ways to effectively reduce the average error.

TOF ranging experiment in the indoor and outdoor for a variety of tests. The results show that in the 10. 5 m of the room, measure the average error of the sample data 1. 94 m; in 26 m of the corridor, the average measurement error of the sample data in the 2. 01 m; in 100 m of the LOS, measured samples The average error in the data 3. 62 m; in the 100 m in the NLOS, measured data, the average error of the sample 4. 47 m. The above experimental data show that different experimental environment, the error size is different, mainly affected by the signal transmission path. Because the signal propagation process, as reflected by the ground or water and atmospheric refraction effects, the received signals may not be a single path, but by the many paths to the synthesis of many reflected waves, so the transmission of measurement signals time there will be errors, thus affecting the accuracy of estimating distance.

In short, the results show that, T OF is a high ranging accuracy of methods for wireless sensor networks can be widely applied.

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