| With the rapid development of wireless communication,the demand for Location Based Service(LBS)has also grown.In an open space such as outdoor,the Global Positioning System(GPS)is widely used for achieving high-precision,high-reliability LBS.However,in indoor places where human activities are more frequent,it is difficult for GPS to provide accurate location information due to building obstruction and other reasons.These years,indoor positioning has received extensive attention,and various indoor positioning methods have emerged endlessly.Existing indoor positioning methods,according to different technical categories,can be classified into acoustic,optical and radio frequency(RF)methods.Because RF signals can penetrate obstacles for positioning and communication,RF-based indoor positioning methods has currently become the most promising option.The main metrics for estimating the target location using RF include: Received Signal Strength(RSS),Angle of Arrival(AOA),Time of Arrival(TOA)and Time Difference of Arrival(TDOA).Time-based positioning method(TOA or TDOA)is often studied and applied because of its high accuracy and low system complexity.In particular,the TDOA method is more attractive as it does not require information about the transmitted signal.However,due to multipath effect in indoor environment,TDOA-based indoor positioning accuracy can be severely degraded,especially in narrow spread spectrum systems such as IEEE 802.11 b WLAN.In recent years,researchers have proposed several methods to mitigate the effect of multipath propagation on the accuracy of TDOA-based indoor localization.However,these methods still have significant deficiencies in terms of universality and computational complexity.As is known to all,multipath effect is not only related to the indoor environment,but also to the antenna,especially to the radiation beamwidth and polarization characteristics of an antenna.However,in addition to antimultipath antennas that are widely used for GPS positioning,anti-multipath antennas for indoor positioning are rarely mentioned and studied.Aiming at the above problem,this paper studies the application of a new type of beam-scanning circularly polarized antenna to mitigate multipath effect in TDOA indoor positioning system from three aspects: antenna,hardware and software system and experiment.The main work and innovations of the paper are as follows:(1)A method to mitigate indoor multipath effect by using directional beam and circular polarization characteristics of antenna in TDOA indoor positioning system is proposed,and a new beam steering circular polarization antenna is designed for the first time to solve the problem of small coverage of the directional antenna.Firstly,the propagation characteristics of indoor multipath and its influence on the accuracy of TDOA-based indoor positioning are analyzed theoretically,and a method to mitigate indoor multipath propagation using steering beam and circular polarization characteristics is proposed.By combining steering beam and circular polarization,the beam steering broadband circular polarization microstrip antenna is simulated and designed.The antenna consists of a beam switch network,four identical hexagon patch elements and their respective feeding network.By controlling the state of the RF switch in the beam switch network,the proposed antenna can separately excite four different steering circular polarization beams.After physically fabricating the antenna with standard printed circuit board process,we performed a comprehensive measurement of the antenna,including the function of the feeding network,radiating element and beam switching.When the antenna applied frequency band is set to 2~2.8GHz,the axial ratio of the four radiating elements is basically less than 3dB in the beamwidth of 90°.The measurement results show that the antenna can achieve azimuth omnidirectional circular polarization coverage by switching four steering beams.The proposed antenna can be used as an anti-multipath antenna in a TDOAbased indoor localization system.Compared with the various antennas used for multipath mitigation in indoor positioning at present,the designed antenna has advantages in terms of bandwidth,gain,polarization mode,beam scanning mode and scanning range.To the best of our knowledge,this is the first time that a beam steering circular polarization antenna has been used in TDOA indoor positioning to mitigate multipath.(2)The TDOA indoor positioning system using beam steering circular polarization antenna was designed and completed,and the automatic beam selection algorithm and data correction algorithm suitable for the positioning system are proposed.The system includes both hardware and software aspects.In terms of hardware,it includes the self-designed receiver,the clock synchronization module and the transmitter.The receiver adopts a low-IF architecture,which is significantly improved compared with the universal software radio peripheral of mainstream positioning applications.The RF front-end and intermediate-frequency key modules are explained,including low noise amplifier,demodulators and variable gain amplifier.The clock synchronization module is mainly used to provide sampling time synchronization and frequency synchronization.The existing vector signal source SMBV100 A is used as the transmitter,cooperating with the simulation software to generate digital modulated signals.In terms of software,the initial TDOA estimation value is obtained by the cross-correlation algorithm to realize the distance estimation,and a parabolic interpolation fitting algorithm is added to achieve the estimation accuracy between sub-samples.In order to maximize the use of reasonable measurement data,the trigger signal correction algorithm and outlier removal algorithm are also integrated in the software.Chan algorithm is used in the location calculation algorithm.With the beam steering circular polarization antenna,the designed TDOA indoor positioning system can automatically complete the beam scanning and selection,and finally realize the calculation and display of the position coordinates.(3)A large number of TDOA indoor positioning comparison experiments in the actual environment were designed.The application of beam steering circular polarization antennas in the multi-path effect of TDOA indoor positioning system was systematically and completely studied.The anti-multipath antenna is introduced and the TDOA-based indoor positioning experiment is completed.Specific positioning experiments include wired channel experiment,wireless channel experiment and anti-multipath antenna experiment.In the wired channel,the ranging experiment is carried out,and the calibration method is proposed to eliminate the inherent delay difference between the receivers,and the ranging result with the maximum error of only 2cm is preliminarily obtained.Then the experiments are conducted in wireless channel,including microwave anechoic room and indoor environment,and the antennas used are both directional and omnidirectional.Comparing the ranging and positioning experiments in these two different indoor environments,it can be seen that indoor multipath will significantly deteriorate the positioning accuracy,and different antennas also have an important influence on the positioning accuracy.Finally,we use the proposed anti-multipath antenna as the receiving antenna,and carried out a large number of two-dimensional positioning experiments in the conference room and the laboratory,and obtained an average error of 0.70 m.In contrast to the omnidirectional antenna used together,the proposed anti-multipath antenna can improve positioning accuracy by at least about 51%.Comparing the results of indoor positioning at home and abroad,the advanced positioning of the designed anti-multipath antenna and TDOA indoor positioning system is proved.This work includes both beam steering circular polarized antennas,hardware positioning system test equipment,automatic beam selection,TDOA estimation,data correction and position calculation algorithms,and experimental tests in a large number of practical environments.The application of beam scanning circularly polarized antenna to mitigate multipath effect in TDOA indoor positioning system is studied.The anti-multipath antenna proposed in this paper can not only be applied to TDOA indoor positioning,but also to other positioning methods.In the future,it can be combined with the multipath mitigation method based on signal processing to further improve positioning accuracy,so as to promote the development of indoor positioning in the future. |
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