Mobility is one of important issues for line-of-sight visible light communication (VLC). For a high-speed VLC system, the field of view (FOV) of the transceivers is very narrow for high optical gain at the receiver so that precise alignment between the transceivers is required. To enhance the mobility performance, in this paper, a simple and effective scheme of tracking light source is designed and experimentally demonstrated for practical VLC systems. To detect the target light source, a beacon light encoded with a low-frequency label signal is used along with the information light at the transmitter, whilst a high-speed camera with wide FOV is used for location detection by differentiating received images at the receiver. The detected signal of the light source location is then digitally processed to control a two-axis gimbal for auto-alignment between two transceivers. To mitigate the effects of ambient and interference lights, adaptive threshold and dynamic search window are used. A field-programmable gate array (FPGA)-based on-off keying (OOK) transceiver with the adaptive tracking function is developed for evaluation of the tracking performance. Experimental results show that with the proposed scheme 25 Mbps OOK transmission is successfully demonstrated at a bit error rate of < 10-3 over a 3 m VLC link with the transmitter horizontally moving at a speed of 1m/s, which corresponds a tracking speed of >18°/s. The angular accuracy and latency of tracking the LED transmitter are < 0.02° and < 21 ms, respectively.
In recent years, extensive investigations have been made on high-speed visible light communication (VLC) for indoor applications including data centres. For high-speed VLC systems, precise alignment between the VLC transceivers is usually required to establish an optical link with less optical loss. In this paper, an effective scheme of auto-alignment and tracking of transceivers is proposed for practical VLC systems with a high-speed camera for location of the target light source and a two-axis gimbal for initial auto-alignment between transceivers. To explore the feasibility of the proposed scheme, real-time dynamic 200 Mb/s carrierless amplitude-phase (CAP)-VLC transmission is experimentally demonstrated in ≥ 3 m VLC links with red light emitting diodes (LEDs) at an angular accuracy of ≤ 0.02º (0.35 mrad), a tracking speed of ≤ 27º/s and a latency of ≤ 21 ms. The proposed scheme can also be applied for coarse alignment in highspeed laser-based free space optical communication (FSO) systems with visible beacons. 10 Gb/s on-off keying (OOK) transmission is successfully demonstrated over a 1.8 m FSO link with the coarse alignment and the spatial light modulator (SLM)-based fine alignment. Experimental results indicate good flexibility and effectiveness of the proposed scheme for an FSO system with high spatial efficiency and low cabling complexity in data centres.
KEYWORDS: Photography, 3D imaging standards, 3D metrology, Dynamical systems, 3D acquisition, Standards development, Visualization, Commercial off the shelf technology, Image quality, Opto mechatronics
The aircraft flight attitude can be obtained by the dynamic visual measurement system for aircraft (hereinafter short for MSA). It is crucial for MSA to evaluate its flight attitude measurement accuracy. There are several indoor evaluation methods for the MSA’s attitude measurement accuracy which is not suitable outdoors. Therefore, we present a method for evaluating its flight attitude measurement accuracy at outdoor working site. A three-dimensional standard verification field can be established by reasonable distribution of mark targets on the surface of outdoor building group. We construct a verification system for flight attitude measurement accuracy at outdoor working site. The building group whose threedimension scale is similar to the aircraft’s three-dimension scale is selected to construct the standard verification field. Paste mark points on the surface of the building group and their coordinates in 3D space are measured by the threedimensional coordinate measuring station consisting of two electronic theodolites. Mark points with known coordinates construct the standard verification field. Still photographs of the standard verification field are taken by the MSA. the attitude solved from the still photographs is used as reference attitude. Manipulate the MSA to shoot and record dynamically to simulate the real working condition, and photographs are taken to solve the dynamic measurement attitude at the same time. Accuracy analysis and evaluation can be performed using the dynamic measurement attitude and the reference attitude to provide scientific basis for debugging, checking outdoor parameters and acceptance of equipment.
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