Response adaptive modelling for reducing the storage and computation of RSS-based VLP

The precise (location) tracking of automated guided vehicles will be key in enlarging the productivity, efficiency and safety in the connected warehouse or production infrastructure. Combining the modest price tag, the adequate coverage and the potential centimetre accuracy makes Visible Light Positioning (VLP) systems appealing as replacements for the current, high-cost, tracking systems. Model-fingerprinting-based received signal strength (RSS) VLP enables the required accuracy. It requires an elaborate optical channel model fingerprinted in a fine-grained, and predefined positioning grid. Depending on the grid's granularity, constructing the fingerprint database demands a significant computation and storage effort. This paper employs response adaptive or sequential experimental design to form sparse channel models, vastly reducing the storage and computation. It is shown that model-fingerprinting-based RSS only requires modelling less than 1 percent of the grid points, in an elementary positioning cell. The sparse model can be re-evaluated as a way to cope with environment changeover. Model recomputation as a way of compensating for LED ageing is also studied

An analysis of the impact of LED tilt on visible light positioning accuracy

Whereas the impact of photodiode noise and reflections is heavily studied in Visible Light Positioning (VLP), an often underestimated deterioration of VLP accuracy is caused by tilt of the Light Emitting Diodes (LEDs). Small LED tilts may be hard to avoid and can have a significant impact on the claimed centimeter-accuracy of VLP systems. This paper presents a Monte-Carlo-based simulation study of the impact of LED tilt on the accuracy of Received Signal Strength (RSS)-based VLP for different localization approaches. Results show that trilateration performs worse than (normalized) Least Squares algorithms, but mainly outside the LED square. Moreover, depending on inter-LED distance and LED height, median tilt-induced errors are in the range between 1 and 6 cm for small LED tilts, with errors scaling linearly with the LED tilt severity. Two methods are proposed to estimate and correct for LED tilts and their performance is compared

Assessment of the influence of photodiode size on RSS-based visible light positioning precision

This work discusses the influence of a photodiodes effective area size on the precision of received signal strength-based visible light positioning. It analyzes how two silicon-based photodiodes with different effective area perform as a receiver under varying illumination conditions. The two main findings are that it is not particularly needed to select a photodiode with a large surface area, despite the higher received signal strength, due to a higher noise contribution. On the other hand, the spread on the distance estimation is much smaller than 1 mm under standard illumination levels for the two photodiodes with a significantly different surface area, so that both photodiodes deliver enough precision

On the impact of LED power uncertainty on the accuracy of 2D and 3D visible light positioning

This paper presents a simulation study of the impact of Light Emitting Diode (LED) output power uncertainty on the accuracy of Received Signal Strength (RSS)-based two-dimensional (2D) and three-dimensional (3D) Visible Light Positioning (VLP). The actual emitted power of a LED is never exactly equal to the value that is tabulated in the datasheet, with possible variations (or tolerances) up to 20%. Since RSS-based VLP builds on converting estimated channel attenuations to distances and locations, this uncertainty will impact VLP accuracy in real-life setups. For 2D, a typical configuration with four LEDs is assumed here, and a Monte-Carlo simulation is executed to investigate the distribution of the resulting positioning errors for four tolerance values at seven locations. It is shown that median errors are the highest just below the LEDs, when using a traditional Least-Squares minimization metric. When tolerance values on the LED power increase from 5% to 20%, median errors vary from at most 2 cm to at most 10 cm. Maximal errors can be as high as 17 cm just below the LED, already for tolerance values of only 5%, and increase up to 40 cm for tolerance values of 20%. An alternative cost metric using normalized Least-Squares minimization makes the errors spatially more homogeneously distributed and reduces them by 35%. For a 3D case, median errors of around 5 cm for a tolerance value of 5% increase to as much as 22 cm for a tolerance value of 20%. As the receiver heights increase, positioning errors decrease significantly

Monte-Carlo simulation of the impact of LED power uncertainty on visible light positioning accuracy

This paper presents a simulation study of the impact of Light Emitting Diode (LED) output power uncertainty on the accuracy of Received Signal Strength (RSS)-based Visible Light Positioning (VLP). The actual emitted power of a LED is never exactly equal to the value that is tabulated in the datasheet, with possible variations (or tolerances) up to 20%. Since RSS-based VLP builds on converting estimated channel attenuations to distances and locations, this uncertainty will impact VLP accuracy in real-life setups. A typical configuration with four LEDs is assumed here, and a Monte-Carlo simulation is executed to investigate the distribution of the resulting positioning errors for four tolerance values at seven locations. It is shown that median errors are the highest just below the LEDs. When tolerance values on the LED power increase from 5% to 20%, median errors vary from at most 2 cm to at most 10 cm. Maximal errors can be as high as 17 cm just below the LED, already for tolerance values of only 5%, and increase up to 40 cm for tolerance values of 20%

New photodiode responsivity model for RSS-based VLP

Visible Light Positioning (VLP) might enable auspicious tracking systems, well-suited for low-cost and route-configurable autonomous guided vehicles. Yielding the high accuracy required, necessitates a detailed modelling of a photodiode (PD) receiver's angular characteristics. Still lacking, current RSS-based VLP systems implicitly cope by measuring and (arbitrarily) fitting the received power - distance relation. Upon PD changeover, a recalibration is needed. In this paper, it is shown that adequately modelling the receiver's angular dependencies (i.e. the responsivity) obsoletes the calibrating fit. Hereto, a new responsivity model is proposed, which is a function of the square of the incidence angle rather than its cosine. An extensive measurement set highlights that this model better matches the measured angular characteristics. In terms of the coefficient of determination R2, the new model outscores the baseline Lambertian and generalised Lambertian responsivity models by 1.64% and 0.17% for a Lambertian-like receiver, and by 133% and 1.24% for a non-Lambertian-resembling receiver

A comprehensive study on light signals of opportunity for subdecimetre unmodulated visible light positioning

Currently, visible light positioning (VLP) enabling an illumination infrastructure requires a costly retrofit. Intensity modulation systems not only necessitate changes to the internal LED driving module, but decrease the LEDs' radiant flux as well. This hinders the infrastructure's ability to meet the maintained illuminance standards. Ideally, the LEDs could be left unmodulated, i.e., unmodulated VLP (uVLP). uVLP systems, inherently low-cost, exploit the characteristics of the light signals of opportunity (LSOOP) to infer a position. In this paper, it is shown that proper signal processing allows using the LED's characteristic frequency (CF) as a discriminative feature in photodiode (PD)-based received signal strength (RSS) uVLP. This manuscript investigates and compares the aptitude of (future) RSS-based uVLP and VLP systems in terms of their feasibility, cost and accuracy. It demonstrates that CF-based uVLP exhibits an acceptable loss of accuracy compared to (regular) VLP. For point source-like LEDs, uVLP only worsens the trilateration-based median p50 and 90th percentile root-mean-square error p90 from 5.3cm to 7.9cm (+50%) and from 9.6cm to 15.6cm (+62%), in the 4m x 4m room under consideration. A large experimental validation shows that employing a robust model-based fingerprinting localisation procedure, instead of trilateration, further boosts uVLP's p50 and p90 accuracy to 5.0cm and 10.6cm. When collating with VLP's p50=3.5cm and p90=6.8cm, uVLP exhibits a comparable positioning performance at a significantly lower cost and at a higher maintained illuminance, all of which underline uVLP's high adoption potential. With this work, a significant step is taken towards the development of an accurate and low-cost tracking system

Passive visible light detection of humans

This paper experimentally investigates passive human visible light sensing (VLS). A passive VLS system is tested consisting of one light emitting diode (LED) and one photodiode-based receiver, both ceiling-mounted. There is no line of sight between the LED and the receiver, so only reflected light can be considered. The influence of a human is investigated based on the received signal strength (RSS) values of the reflections of ambient light at the photodiode. Depending on the situation, this influence can reach up to +/- 50%. The experimental results show the influence of three various clothing colors, four different walking directions and four different layouts. Based on the obtained results, a human pass-by detection system is proposed and tested. The system achieves a detection rate of 100% in a controlled environment for 21 experiments. For a realistic corridor experiment, the system keeps its detection rate of 100% for 19 experiments

An experimental analysis of the effect of reflections on the performance of visible light positioning systems in warehouses

In this paper, an experimental evaluation of the effect of multipath reflections on two 3D positioning algorithms is experimentally compared. The VLP algorithms use the received signal strength (RSS) for estimating the receiver`s 3D position without prior knowledge of its height. An experimental overall median accuracy of 10.5 cm was achieved in a 4 x 4 x 4.1 m3 test area. The effect of multipath reflections was recreated using a storage shelf rack that is similar to the ones used in warehouses and was placed 26 cm away from the path. The results demonstrate the degrading effect of reflections on two and three-dimensional positioning systems. The reflection`s effect was especially severe due to reflections from a metal beam. The achieved median errors using the two different algorithms were 7.5 and 6.6 cm before the addition of a shelf rack, and they increased to 11.7 and 12 cm after the shelf rack was added. Multipath reflections increased the median positioning errors by 112% in 2D systems and by 69% for 3D systems. The paper demonstrates the degrading effect of multipath reflections on VLP systems and highlights the need to take it into consideration when evaluating VLP systems