Abstract
Delay tracking of spread-spectrum signals is widely used for ranging in radio frequency based navigation. Its use in non-coherent optical ranging, however, has not been extensively studied since optical channels are less subject to narrowband interference situations where these techniques become more useful. In this work, an early-late delay-locked loop adapted to indoor optical ranging is presented and analyzed. The specific constraints of free-space infrared channels in this context substantially differ from those typically considered in radio frequency applications. The tracking stage is part of an infrared differential range measuring system with application to mobile target indoor localization. Spread-spectrum signals are used in this context to provide accurate ranging while reducing the effect of multipath interferences. The performance of the stage regarding noise and dynamic errors is analyzed and validated, providing expressions that allow an adequate selection of the design parameters depending on the expected input signal characteristics. The behavior of the stage in a general multipath scenario is also addressed to estimate the multipath error bounds. The results, evaluated under realistic conditions corresponding to an 870 nm link with 25 MHz chip-rate, built with low-cost up-to-date devices, show that an overall error below 6% of a chip time can be achieved.
Highlights
Indoor positioning systems have evolved in the last two decades inspired by global navigation satellite systems (GNSS), and at the same time suffering from their own problems, due to the specific indoor characteristics
In order to validate the theoretical analysis defined so far, the expected results have been compared with measurements carried out in a digital implementation (Simulink) of the defined early-late delay-locked loop (ELDLL) tracking architecture
The results shown correspond to the main error sources analyzed in this work, i.e., tracking error caused by additive white Gaussian noise (AWGN) and dynamic error, together with some preliminary results of the expected multipath error in the defined scenario
Summary
Indoor positioning systems have evolved in the last two decades inspired by global navigation satellite systems (GNSS), and at the same time suffering from their own problems, due to the specific indoor characteristics. In contrast with GNSS, which is the accepted technology for global positioning outdoors, there is not one single solution, nor a unique technology, universally accepted as best solution for indoor localization. Regardless of the technology used, the most common problems that indoor systems must deal with are related to signal quality, positioning-anchors deployment strategy (antennas, receivers, emitters, etc.), non-line-of-sight situations, dynamic localization, interferences from other devices, and, like with. Non-coherent optical ranging is usually carried out by time-of-flight measurements on pulsed signals [4] or phase-shift measurements on continuous wave modulated signals [5]. Phase-based techniques do not inherently provide any multipath mitigation and pulsed systems, usually implemented with laser, require very high bandwidth to discriminate multipath components close to the direct path
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