The DSP Algorithms for Locally Deployable RF Tracking System

Transcription

1 The DSP Algorithms for Locally Deployable RF Tracking System Josef Sypniewski SypTech, Co., San Antonio, TX, < Abstract The recent development in Radio Frequency (RF) positioning technology opens new applications like GPS surveying, accurate navigation, vehicle tracking or guidance systems. The author has developed a novel technique that can further launch new applications and in some cases work in conjunction with the existing GPS system. The proposed Local Positioning System (LPS) is based on local reference stations which form triangulation patterns at every measured point, when they are spread arbitrarily around the desired area. The developed tracking device uses a carrier to estimate the position vector. This approach allows to employ much lower signal frequency than most existing methods, consequently the equipment can exhibit much higher level of penetration then many conventional methods. The local deployment of the system allows its users to measure the position of objects inside the buildings and other non conductive structures where GPS is prohibitive. Background The commercial field of position trackers is a well established growing market and there is a variety of products currently available. The applications that these devices pertain are very diverse. As the enabling technologies become refined, applications will multiply. Please refer to [1] for comparison of most popular technologies currently available. Recent increases in performance and cost effectiveness of Digital Signal Processing (DSP) and data converter products have driven the feasibility of Radio Frequency (RF) techniques in the field of position tracking. Fig.1 Basic system configuration. This technique of a position tracking does not suffer from the obvious tradeoffs and built in limitations of other approaches. This robust method can be employed in a wide variety of applications and it is proven by growing popularity of GPS equipment. The author proposes to apply the techniques developed for the Global Positioning System (GPS) on a local scale. The system consists of a small radio frequency transmitter or Active Radio Tag (ART) located on the target to be tracked. The signal from ART is received by the array digital receiver with multiple antenna elements A m. The position vector Pr n of each antenna element is determined during the calibration process before the measurement. A signal received from ART is acquired and the maximum of cross correlation function

2 between two neighboring antenna elements constitutes the measure of the difference of time of fly to corresponding channels. This locally deployable tracking system is intended for specific applications. The local stations provide a stronger signal at the receiver, therefore, the latency and lock-on time can be reduced in order of magnitude compared to the GPS method. Also the system can be deployed inside the buildings or in other areas where deployment of GPS devices is difficult or impossible. Fig.2 r m d m d n r n Geometry of the local tracking system. Basic System Configuration To determine the position of the transmitter moving in the cube, it is sufficient to measure the differences between arrival time of the carrier at each stationary antenna element A x of an array receiver. These differences can be estimated from the crosscorrelation taken from each antenna element. The set of these correlations will form a correlation matrix S: S mn * { m n } = E s s (1) where: s m - complex value of the received signal from A m antenna element. s n * - conjugate value of If the number of antenna elements N is larger then the number of dimensions by at least one, that is, four for 3D measurement, then the differences in arrival time at each antenna element determine two concurrent position ART1 and ART2 of which only one is correct (Fig.2). s n d't(c (2) Where: d - Euclidian distance between ART and antenna element A x. c t - speed of light. - time of flight. Assuming for now that in an ideal condition the relation (2) will hold, position vector P can be calculated using well known [2] Navigational Equation: *P &Pr 1 *&*P *'c(t 14 *P &Pr 2 *&*P *'c(t 24 *P &Pr 3 *&*P *'c(t 34 (3) Where: P - measured position vector. r n - position vector of n-th antenna element A n. t mn - position of maximum of cross- correlation function from A m and A n element. In this ideal condition the calculation of a position vector P can be summarized by

3 following flow chart: (4) RF Position Tracking in Multipath Environment. In multipath environment the condition (2) usually is not fulfilled and the relation between time of flight t mn and euclidean distance d mn is much more complicated. Also, the estimation of d mn from maximum of crosscorrelation of s mn poses the significant problem. The multiple reflections of the signal can cause s mn to have several local maxima. It is believed that estimation of d mn directly from s mn is not very efficient and some information about the wave distribution is lost. The author proposes to modify flow chart (4) and compute the weighted spatial correlation S(Pr) for every position vector Pr in the area of interest PrhPU even before the final estimation of P is taking place: S(Pr) as a probability density of occurrence Pr'P for all area of interest PrhPU. Several methods can be used to estimate the value of P. The simplest one is to select P at the maximum value of S(Pr) max 'S(P ). A direct path is always shorter than propagation with reflection. The estimation of P can be improved by putting weight on S(Pr), so the reciprocal sum of distances between ART and antenna elements A x will adjust its value. This method is useful where strong multipath propagation is present. In most complicated cases the spatial equalization can be used to remove multipath propagation effect. Example of the Measurement SypTech, Co. has designed the LPS007 tracking device ([3] and [4]), which encompasses eight element phase array receiver and one or more transmitters (ART). At the transmitter the RISC microcontroller generates orthogonal pseudo-random m- sequence [5] which modulates RF carrier. Fig.3 shows a typical spectrum of the transmitted signal. S( τ) (5) S ( τ) S( ) r r This algorithm treats weighted value of Fig. 3. Signals used for ART tracking.

4 The receiver (Fig.4) has three distinct stages. At the first stage signal is down-converted and amplified using analog Radio Frequency Integrated Circuit (RFIC) front end circuit. Also, the initial band formation is achieved at this stage. Following the analog RF circuit signal is digitized and processed by the standard Complex Programmable Logic Device (CPLD). At this stage signal is despreaded and the final band formation is accomplished. Finally the embedded DSP computer calculates s mn matrix and executes time domain equilisation to enhance the spatial resolution of P measurement. In case of multipath propagation the estimation of P is more complicated. Fig. 6 shows (Pr) obtained using three to five antenna elements in presence of strong reflection from the wall. Fig. 5 Demonstration of 2D tracking using spatial estimation: a) geometry, b) measurement of S 01 (Pr), c) measurement of S 02 (Pr), d) final estimation of P. Fig.4 Four channel module of phase array receiver. The phase receiver is connected to the host computer (PC) which resolves values of P for every point of interest. Fig. 5 shows (Pr) measured from two antenna elements A 0 - A 1, A 0 - A 2, and A 0 - A 2, (with no multipath propagation). Dark bands show the region of highest probability for Pr'P from single measurement. The intersection between these bands from two or more measurements resolves the most probable value of P (no multipath propagation). As mentioned, the weighted value of (Pr) can be treated as a probability density for Pr'P. For example to resolve P in 2D only two measurements are needed. The presence of interfering object causes that direct as well as reflected signals will correlate with each others. In this case (t) and (Pr) will exhibit multiple local maxima that make estimation of P very difficult. If one will increase the number of measurements so the system became over-determined than the jamming effect of multipath propagation can be reduced. Generally multipath signals will have different solution for each measurement of (Pr) and by combining the results from each measurement the interference from

5 reflections can be filtered out. Fig. 6 RF tracking in multipath environment. a) geometry of the experiment, b) three antenna, c) four antenna and d) five antenna tracking. Conclusion As experiments showed, there are two main advantages of locally deployable RF positioning system: - The low frequency signal can penetrate most non-conductive objects like walls or the ground. - The local stations can provide a stronger signal at the receiver, therefore, the lock-on time can be reduced in order of magnitude compared to the GPS method. Proposed technique can further open new applications or in some cases, work in conjunction with the existing GPS system. The author has presented a new tracking algorithm. This algorithm can find applications not only in RF tracking, but this approach can be used in wireless communication where multipath effect is the problem. The system is similar to conventional phase array receiver, however proposed solution has much better spatial characteristics. References [1] SypTech, Co.: [2] R.J. Milliken and C.J. Zoller, Principle of Operation of NAVSTAR and System Characteristic, Global Positioning System, Papers, vol. 1, pp. 3-14, 1980 [3] J. Sypniewski, Tracking Sensor Specially for Computer Applications, U.S. Patent 5,790,076. [4] J. Sypniewski, Multidimentional Tracking Sensor, U.S. Patent 6,054,951. [5] R.C. Dixon, Spread Spectrum System with Commercial Applications, John Wiley&Sons, Contact Josef Sypniewski, - SypTech Co., P.O.Box , San Antonio, TX <sypniew@syptech.com>