The limited bandwidth and nonlinearity of light-emitting diodes (LEDs) causes inter-symbol interference (ISI) and nonlinear distortions, which restrict capacity and/or spectral efficiency of LED-based visible light communication (VLC) systems. In this paper, a light-weight polynomial activation neural network (PANN)-based equalizer is investigated for mitigation of the ISI and nonlinear distortion effects. As a variant of classic deep neural networks (DNNs), PANN has polynomial activation functions instead of classical activation functions such as sigmoid and rectified linear unit (ReLU). Therefore, small parameter volume and good interpretability are key features of PANN. The relation between mathematical expressions of the PANN and the traditional DNN using continuous derivable non-polynomial activation functions (such as sigmoid) can be obtained by the Taylor series expansion. The polynomial function can thereby be regarded as a partial summation of the expanded Taylor series. To evaluate the effectiveness of the PANN solution, we experimentally investigate the transmission performance of the PANN-based equalizers compared with traditional linear/nonlinear equalizers, and DNN-based equalizers. Experimental results show that a well-designed PANN equalizer with relatively small parameter volume improves the transmission performance, compared with the Volterra and Chebyshev equalizers. 500Mb/s CAP256-based VLC transmission over 1m is demonstrated with phosphorescent white LEDs and a light-weight PANN equalizer with only 94 parameters. The number of parameters is 6.9% and 48% less than the DNN equalizer using sigmoid and parametric rectified linear unit (PReLU) activation functions, respectively. The error vector magnitude (EVM) performance with the PANN equalizer is 0.6dB better than the third-order Chebyshev equalizer.
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.
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