A High-Precision Wideband Local Positioning System at 24 GHz Christian Meier, Axel Heim, Joachim Brose, Stefan Lindenmeier Abstract – A wideband radio location system for 3D applications with high precision is introduced. For different test scenarios, the system is evaluated with respect to precision, resolution, and reproducibility. Based on ultra wideband direct sequence spread spectrum transmission at 24GHz, the radio location system shows high robustness against interference as well as suppression of the effects of reflections. Distance measurements are processed in a two-stage Kalman filter allowing object tracking with millimeter accuracy even in highly reflective scenarios. Index Terms – local positioning, radio location, DSSS, UWB positioning 1. Introduction Indoor positioning systems – especially radio-based systems – find increasing interest for applications as for example object tracking or automatic gauging in industrial environment, as well as for safety purposes. During the past years, significant effort in research has been reported, achieving accuracies in the range of a few centimeters [1, 2]. These systems show, for example, good performance in applications like tracking of objects in store houses over long distances. The accuracy of such systems, however, is limited by the small bandwidth available at the operating frequencies. New augmented reality systems need real time acquisition of 3D object coordinates for processing and visualization tasks with millimeter-accu- racy. A further critical issue besides the accuracy of a local indoor positioning system is its robustness and guaranteed accuracy even in highly reflective environments. This is of major concern for industrial applications, as it affects the ef- ficiency of construction and production. The new robust highly accurate positioning system was in- troduced in a first version in [3, 4] . In this paper the complete system is shown with special focus on the generation of the ultra wideband (UWB) code with a bandwidth of 1.6GHz, the signal processing and the antennas. Experimental studies show the feasibility of this robust radio location system with high precision in mm-range in an arbitrary reflective scenario. 2. Architecture of the 3D Radio Location System In Fig. 1 the system architecture of the new radio location system is shown. For the accurate 3D positioning of a small transmitting antenna in a highly reflective scenario several receivers are installed with an adequate geometric dilution of precision (GDOP). The transmitting antenna itself is moun- ted on a marker which is wire-bound. The 3D position is calculated via trilateration of distances between the transmitter (Tx) and the receivers (Rx). The di- stance between the Tx and each Rx is calculated using the correlation between the continuous transmitted pseudo-noise (PN)-code c t t ðÞ and the identical, time-shifted PN-code c r t þ k c T C ð Þ in the receiver, where k c T C is the discrete time delay, also called the code phase, with the discrete time index k c . The code phase can be shifted in units of a chip. A chip with duration T C is the smallest part of a code (cf. [5]). The discrete code shift is the variable element of the correlation function (CF) between the delayed code c r t þ k c T C ð Þand the received code c t t  t d ð Þ. t d represents the time-of-arrival (TOA) be- tween the considered Rx and Tx which can be found at the peak of the correlation function where k c T C ¼ t d and hence the distance is d ¼ k c T C c 0 , if the Tx-code generator and the Rx-code generator are synchronous. Especially for indoor applications efficient suppression of multipath effects is required. Otherwise the measurement accuracy might be deteriorated significantly. If the difference between the length of the reflection paths d MP and the length of the line-of-sight (LOS)-path d LOS is more than d LOS  d MP > T C c 0 , the waves propagating along LOS are clearly distinguishable from the reflected waves and hence the latter can be suppressed. Accordingly, in a strongly reflective environment a high distance resolution is necessary which is inversely proportional to the bandwidth. A major requirement was now to process the large band- width B of 1.6GHz in the baseband signal for achieving a distance resolution of Dd ¼ c 0 =B ¼ 187:5mm. There are two practical possibilities to correlate signals with such a high bandwidth: One is to subsample the received signal with an analog-to-digital (AD) converter as described in [6], the another is to correlate the received signal in the RF-path. In [7] the former concept is described with analog correlation in the intermediate frequency-path. The concept as pictured in Fig. 1 divides the analog corre- lation into two sections, the multiplicative and the additive part. First, the received coded signal is multiplied with a del- ayed coded signal at nearly the same radio frequency (RF) frequency. Then the signal is demodulated and low pass (LP) filtered, which represents the integration of the correlation. In order to realize this analogue correlation of the PN- coded radio location system, the codes in the receiver and the transmitter must be identical and the receiver has to be able to shift the code in sections of a chip. This shift of the code phase and the continuous generation in the receiver need to be done in real time. An FPGA with a special parallel to serial con- Fig. 1: Basic principle of the 3-D radio location system Frequenz 62 (2008) 7–8 199 Brought to you by | Monash University Library Authenticated Download Date | 6/16/15 4:44 AM