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Increasing interest in indoor navigation has recently been generated by devices with wireless communication capabilities that enabled a wide range of applications and services. The rise of the Internet of Things (IoT) and the inherent end-to-end connectivity of billions of devices is very attractive for indoor localization and proximity detection. Other fields, such as, marketing and customer assistance, health services, asset management and tracking, can also benefit from indoor localization. Different techniques and wireless technologies have been proposed for indoor location, as the traditional Global Positioning System (GPS) has a very poor, unreliable performance in a closed space. The work presented in this research proposes the use of an indoor localization system based on Visible Light Communication (VLC) to support the navigation and operational tasks of Autonomous Guided Vehicles (AVG) in an automated warehouse. The research is mainly focused on the development of the navigation VLC system, transmission of control data information and decoding techniques.
As part of the communication system, trichromatic white LEDs are used as emitters and a-SiC:H/a-Si:H based photodiodes with selective spectral sensitivity, are used as receivers. Through the modulation of the RGB LEDs, the downlink channel establishes an infrastructure-to-vehicle link (I2V) and provides position information to the vehicle. The decoding strategy is based on accurate calibration of the output signal. Characterization of the transmitters and receivers, description of the coding schemes and decoding algorithms will be the focus of discussion in this paper.
Geolocalization and navigation by visible light communication to address automated logistics control
In this work it is proposed a bidirectional communication system between a static infrastructure and a mobile robot (I2V). The LED lamps of the warehouse illumination system are used to lighten the space, and to transmit information about position and about racks content. The mobile robots communicate with the infrastructure (V2I) to transmit information on the items that are being removed and carried to the packaging station. The communication among the autonomous robots (V2V) provides information on the number of items intended to be collected when the vehicles are in the same lane, possibly with the purpose of collecting the same items. Different codification schemes are proposed to establish the V2I, I2V and V2V links. Tri-chromatic white LEDs with the red and blue chips modulated at different frequencies and a photodetector based on a-SiC:H/a-Si:H with selective spectral sensitivity are used at the emitter and receiver. Position information is provided by each LED lamp to the autonomous vehicle by adequate modulation of the RGB emitters. The decoding strategy is based on accurate calibration of the output signal. Different scenarios were designed and tested. Requirements related to synchronous transmission and flickering were addressed to enhance the system performance.
A generic model of cooperative transmissions for vehicular communications services is established, which share the common features among diverse vehicular communications scenarios. Three specific vehicular communications are analyzed. One is for Infrastructure-to-Vehicle (I2V) communications from the street lamps, located on roadside, to the vehicles; the other is for in line Vehicle-to-Vehicle (V2V) communications and the last for Vehicle-to-Infrastructure (V2I) communications from cars to the traffic lights, at the crossroad. For the V2V and V2I communication study, the emitter was developed based on the vehicle headlights, whereas for the study of I2V communication system, the emitter was built based on streetlights, whose primary purpose is to provide illumination, and are also used for data communication if modulated at fast rates. The VLC receivers extract the data from the modulated light beam coming from the LEDs emitters. The receivers consist in a double SiC pi’npin photodetector, with light filtering characteristics, located at the rooftop of the vehicle, for I2V communications; at the traffic lights, for V2I; and at the tails, for V2V reception. Clusters of emitters, in a square topology, are used in the I2V transmission. The encoded message contains ID code of each emitter concomitantly with a traffic message that is received, decoded and resent to another vehicle (V2V) or to traffic light, in the crossroad. An algorithm to decode the information at the receivers is established. A phasing traffic flow is presented as a proof of concept. The experimental results, confirmed that the proposed cooperative VLC architecture is suitable for the intended applications.
Different indoor layouts, using as position technique a four-code assignment for the LEDs, are proposed. Square and hexagon mesh are tested, and a 2D localization design, demonstrated by a prototype implementation, is presented. The key differences between both topologies are discussed. The location and motion information is calculated by position mapping and estimating the location areas along the time. In both topologies, the transmitted data information, indoor position and motion direction of the mobile device are determined.
The results showed that the LED-aided VLC navigation system make possible not only to determine the position of a mobile target inside the unit cell but also in the network and concomitantly to infer the travel direction along the time.
The selector filter is realized by using a two terminal double pi’n/pin a-SiC:H photodetector. Five visible communication channels are transmitted together, each one with a specific bit sequence. The combined optical signal is analyzed by reading out the photocurrent, under near-UV front steady state background. Data shows that 25 current levels are detected and corresponds to the thirty-two on/off possible states. The proximity of the magnitude of consecutive levels causes occasional errors in the decoded information. To minimize the errors, four parity bit are generated and stored along with the data word. The parity of the word is checked after reading the word to detect and correct the transmitted data. Results show that the background works as a selector in the visible range, shifting the sensor sensitivity and together with the parity check bits allows the identification and decoding of the different input channels. A transmission capability of 60 kbps using the generated codeword was achieved. An optoeletronic model gives insight on the system physics.
The system topology for positioning is a self-positioning system in which the measuring unit is mobile. This unit receives the signals of several transmitters in known locations, and has the capability to compute its location based on the measured signals. LED bulbs work as transmitters, sending information together with different IDs related to their physical locations. A triangular topology for the unit cell is analysed. A 2D localization design, demonstrated by a prototype implementation is presented. Fine-grained indoor localization is tested. The received signal is used in coded multiplexing techniques for supporting communications and navigation concomitantly on the same channel. The position is estimated through the visible multilateration metodh using several non-collinear transmitters. The location and motion information is found by mapping position and estimates the location areas.
Data analysis showed that by using a pinpin double photodiode based on a a-SiC:H heterostucture as receiver, and RBGLEDs as transmitters it is possible not only to determine the mobile target’s position but also to infer the motion direction over time, along with the received information in each position.
An optoelectronic characterization of the devices used in the integrated system is presented to support the main results, namely the decoding strategy. The photodetector is a pin-pin heterostructure that works as an optical filter, presenting a selective spectral sensitivity dependent on the external optical bias. The red and blue light emitted from the white RGB LEDs were modulated at different frequencies. With this configuration each cardinal direction becomes assigned to a specific set of optical excitation (wavelength and frequency). The decoding of the output photocurrent allows the identification of the input optical signals and the determination of the correspondent spatial direction. The localization algorithm makes use of the Fourier transform to identify the frequencies present in the photocurrent signal and the wavelength filtering properties of the sensor under front and back optical bias to detect the existing red and blue signals. The viability of the system is demonstrated through the implementation of an automatic algorithm to infer the photodetector cardinal direction. Additional research on the light intensity is presented to investigate the accuracy of the spatial position along a cardinal direction.
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