2MABE Department, University of Tennessee, Knoxville, TN 37996, USA. Abstract -. A high resolution ultra wideband (UWB) positioning radar system based on ...
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Development of an UWB Indoor 3D Positioning Radar with Millimeter Accuracy Cemin Zhang', Michael Kuhn2, Brandon Merkl2, Mohamed Mahfouz2, Aly E. Fathy1 'ECE Department, University of Tennessee, Knoxville, TN 37996, USA 2MABE Department, University of Tennessee, Knoxville, TN 37996, USA Abstract - A high resolution ultra wideband (UWB) positioning radar system based on time difference of arrival (TDOA) has been developed. The UWB radar system provides millimeter accuracy in dense multipath indoor environments for 1D, 2D, and 3D localization. The system is fully compliant with the FCC UWB regulations and utilizes time domain measurements to suppress both multipath signals and Non-Line of Sight (NLOS) errors and has a potential for even sub-mm accuracy.
Index Terms - Localization, indoor positioning, UWB, millimeter resolution, NLOS, time difference of arrival (TDOA)
I. INTRODUCTION
Currently there is a great demand for wireless localization systems to cover numerous indoor applications such as asset tracking, surgical navigation, indoor GPS, etc. The most important feature of these systems is their range resolution, which is highly susceptible to dense multipath effects and NLOS errors within an indoor environment. UWB technology that utilizes very narrow pulses with sub-nano second pulse duration has provided a realistic way for sensing extremely small time differences, which in turn can be used for high-resolution ranging applications. Many different localization techniques have been researched and some commercial systems are currently available. Accuracy of close to 20cm has been achieved using a FMCW radar system while covering a 15x25m2 2-D area [1]. Absolute positioning accuracy of better than 1 foot has been reported utilizing an UWB tracking system in a LOS indoor environment [2]. Recently there has been work done on designing UWB positioning systems with accuracy in the cm range. Low et al. achieved centimeter-range accuracy in a ID short range indoor LOS environment utilizing UWB pulse signals [3]. Zetik et al. reported sub-mm ID accuracy but with only extremely short displacements while accuracy decreased to 1.5cm for 2D localization over a 2x2m2 area [4]. One issue when designing localization systems for sub-cm and sub-mm accuracy is accuracy of the reference data. This has not been strongly emphasized in recent work and in some instances has been completely neglected. Obtaining accurate 3D reference data of transmitters and receivers is critical for testing sub-cm and sub-mm localizations systems, whether ID, 2D, or 3D. Also, there exists a need for quantitative results of UWB 2D and 3D localization systems with sub-cm or mm-range accuracy. Although current UWB positioning systems have not quantitatively achieved sub-cm accuracy for 2D and 3D localization, our previous work [5] shows that UWB
0-7803-9542-5/06/$20.00 ©2006 IEEE
technology offers the potential to achieve mm or even sub-mm accuracy. In this paper, we report an accurate ultra wideband positioning radar system based on a time difference of arrival (TDOA) approach. We have extended our previous work to include ID, 2D, and 3D localization systems. Details on experimental setup and results for each of these systems will be discussed. This will include hardware development, signal processing techniques, and data analysis. Results showed mm-range accuracy for ID, 2D, and 3D systems in a dense multipath indoor environment. II. HARDWARE DEVELOPMENT
We have designed and fabricated basic components that comprise our UWB positioning system including an omni-directional transmitting antenna, 4-element sub-array receiving antenna, electronically tunable pulse generator, 8 GHz oscillator, mixers, and low noise amplifiers. Performance of the transmitting and receiving antennas and pulse generator will be discussed. Fig. la shows the broadband planar monopole transmitting antenna which serves as our positioning tag. It has omni-directional radiation pattern in the horizontal plane with approximately 2dB gain over 5-1OGHz. Fig. lb shows a lx4 Vivaldi sub-array as our receiving antenna. The utilized single element is a flared-antipodal design and has demonstrated almost a constant gain and beamwidth over a wide band beyond 5GHz. A lx4 wideband Wilkinson power divider was also designed, fabricated, and utilized to feed this sub-array [6].
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Fig. 1 - (a)Transmitting tag omni-directional antenna (b) Vivaldi sub-array receiving antenna
A pulse generator with an adjustable width from 300ps to Ins has been developed as the source of our UWB positioning system, which is shown in Fig. 2. This pulse generator can produce either Gaussian pulse or monocycle shape signals with good symmetry and very low ringing level. A Gaussian pulse generated at 300ps has been used in our experiments and has a -10dB bandwidth of more than
2.5 GHz.
107 IV. ALGORITHM A. SIGNAL PROCESSING
Fig. 2 - Electronically tunable pulse generator
III. EXPERIMENAL SETllP
The generated 300ps Gaussian pulse modulates a carrier signal centered at 8 GHz, which is then transmitted through our omni-directional UWB antenna. Multiple directional Vivaldi sub-array receiving antennas located at different positions in our indoor environment receive the modulated pulse signal. Each of the received signals is processed with a multi-channel Tektronix TDS8200 sampling oscilloscope. A block diagram of our system, with an arbitrary number of receivers, is shown in Fig. 3.
We are using a combination of signal strength (SS) and first peak-finding algorithms for accurate detection of pulse peak position. Fig. 6a shows the Gaussian pulse output from our pulse generator. Fig. 6b is the normalized received signal including white noise and multipath pulses. The white noise was removed after applying an averaging filter, as shown in Fig. 6c. Also, a matching filter algorithm was used to detect pulse peaks in signals where pulses from multipath interference overlap significantly with the desired pulse.
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Fig. 3 - Block diagram of indoor localization system
The Optotrak 3020 (Northern Digital, Inc.) is used to obtain accurate reference data. It is a 3D optical tracking system and provides 3D positioning data with accuracy of 0.1mm, which is needed in testing for the mm-range accuracy of our system. Fig. 4 shows the Optotrak system and optical probe.
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Fig. 4 - Optotrak system and optical probe
The indoor environment for our ID, 2D, and 3D experiments is shown in Fig. 5. This room was intentionally selected for its dense multipath effects including reflection from side walls, floor, furniture, ceiling, test equipment, and human bodies.
c) Fig. 6 - Pulse peak detection and filtering a) Transmitted pulse b) Received pulse c) Received signal after averaging filter
B. TDOA APPROACH A GPS-like scheme is utilized along with TDOA to locate the 2D and 3D transmitting tag positions. The four receiving antennas are equivalent to four satellites while the transmitting antenna corresponds to a GPS receiver, as indicated in Fig. 7. In our system the tag transmits an omni-directional signal while a GPS receiver accepts signals
from the satellites. The TDOA approach looks at the difference in arrival time of the pulse signal between each pair of receivers to localize the tag position. The TDOA signals are processed using a least squares iterative triangulation method similar to that used in the GPS technology [7]. Satellite I
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Fig. 5 - Indoor testing environment
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108 V. MEASUREMENTS AND DATA ANALYSIS A. JD EXPERIMENT
Fig. 8 shows the ID experimental setup. Both transmitting and receiving antennas are positioned on a Newport precision optical rail for ID reference positions with sub-mm accuracy. The transmitting antenna is initially placed 10cm away from the receiver and precisely moved along the rail in 5cm increments for a total of nine measurements. Fig. 9 shows the error distribution. Besides 3mm-error at 30cm displacement, error of less than 2mm was consistently achieved.
signal attenuation. First peak-finding algorithm is applied since Signal Strength yields incorrect results due to multipath/NLOS effects. An error of only 2mm was achieved at 50cm displacement even after blocking LOS. B. 2D MEASUREMENT
This experiment demonstrates the capability of our UWB system to accurately perform 2D local positioning within a dense multipath indoor environment. The TDOA technique is used for localization. Three receivers, Rxl, Rx2, and Rx3, are placed in the 2D X-Y plane at well-defined positions: (Omm, Omm), (1546.63mm, -122.87mm) and (895.93mm, 1600.51mm). The 3D reference positions of the three receivers are obtained through the Optotrak system with 0.1mm accuracy. The transmitter is placed at an unknown position (Xu,Yu) on the x-y plane. The experimental scenario is shown in Fig. 11 including side-walls, tables, and measurement instruments. The x-y plane is defined at 1.5m above the ground and 1.5m below the ceiling.
Fig. 8 - ID experiment setup 21
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We also undertook an experiment to test the robustness of our positioning radar to handle severe multipath and NLOS effects. An object with high attenuation (e.g. human hand) was used to fully block the LOS signal path between the transmitting tag and receiver. Fig. 10 shows the received
P1 (i = 1, 2, 3) is the LOS distance between the transmitter and each receiver. The differential distances between the transmitter and three receivers (i.e. dp12 =P - P2,dpI3 =p - p3 ) are used with our TDOA algorithm to triangulate the 2D transmitter location. The transmitter is moved in discrete 5cm steps for a total of nine measurements. An optical probe attached to the transmitting tag provides sub-mm accurate 2D reference data. Fig. 12 shows the 2D error distribution. Maximum error near 3.5mm was achieved for both (x,y) and overall distance.
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Two pulses are detected. The first pulse is the LOS signal traveling through the human hand. The second pulse is the multipath/NLOS signal reflected from nearby instruments. The NLOS signal is stronger than LOS signal due to LOS
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(b) Fig. 14 - 3D error distribution. a) Error in x, y, z b) Overall distance error
(b) Fig. 12- 2D error distribution. a) Error in x, y directions. b) Overall distance error
C. 3D MEASUREMENTS Starting with the 2D experimental setup, an extra receiver out of the x-y plane was added to create the 3D scenario, as shown in Fig. 13.
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TABLE I - OVERALL DISTANCE ERROR FOR ID, 2D AND 3D
ID 2D 3D
Mean Error
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1.49 2.61 3.32
0.69 0.69 1.82
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Table I summarizes the error for the ID, 2D, and 3D experiments. It should be noted that the mean error here represents the mean after taking the absolute value of the error.
VI. CONCLUSION
Fig. 13 - 3D experiment scenario
The same 2D TDOA algorithm, as described in the previous section, was extended for 3D localization. Fig.14a shows the (x,y,z) error for the 3D experiment. The z-axis has larger error because only one receiver is located out of the x-y plane to gather vertical information. A 3mm maximum error was achieved for the x, y dimensions with a 7mm maximum error in the z dimension. 86-
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UWB signals utilizing time domain measurements provide an accurate method for localization. TDOA technique used in conjunction with signal strength (SS) and first peak-finding algorithms provides a robust system that suppresses dense multipath/NLOS effects. ID, 2D, and 3D experiments utilizing our UWB radar system have shown mm-range accuracy with reference to an Optotrak system. With the incorporation of advanced signal processing techniques and continued refinement of our hardware, we are confident that sub-mm accuracy is possible.
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VII. REFERENCES L. Wiebking, et al., "Remote local positioning radar," in IEEE Radio and Wireless Conf, 19-22 Sept. 2004, pp. 191 - 194. R.J. Fontana, "Recent system applications of short-pulse ultra-wideband (UWB) technology," in IEEE Trans. Microwave Theory and Techniques, Vol. 52, Issue 9, Sept. 2004, pp. 2087 -2104. Z.N. Low et al., "Pulse detection algorithm for line-of-sight (LOS) UWB ranging applications," in Antennas and Wireless Propagation Letters, Vol. 4, 2005, pp. 63 - 67. R. Zetik et al., "UWB localization - active and passive approach," in Proceedings of the 21st IEEE IMTC, Vol. 2, May, 2004, pp. 1005-1009. C. Zhang et al., "Accurate UWB Indoor Localization System Utilizing Time Difference of Arrival Approach", in IEEE Radio and Wireless Symposium, Jan. 2006, San Diego, Ca. Y. Yang et al., "Development of an Ultra Wideband Vivaldi Antenna Array", in IEEE AP-S International Symposium on Antennas and Propagation, Washington DC., July 2005. E. D. Kaplan, Understanding GPS: Principles and Applications, Boston, MA: Artech House, 1996.
