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Wireless visible light communications employing feed-forward pre-equalization and PAM-4 modulation

Li, X. and Bamiedakis, N. and Guo, X. and McKendry, J. J. D. and Xie, E. and Ferreira, R. and Gu, E. and Dawson, M. D. and Penty, R. V. and White, I. H. (2016) Wireless visible light communications employing feed-forward pre-equalization and PAM-4 modulation. Journal of Lightwave Technology, 34 (8). pp. 2049-2055. ISSN 0733-8724

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Abstract

In this paper, feed-forward pre-equalization in conjunction with a PAM modulation scheme are proposed for use in wireless visible light communication (VLC) systems in order to enable the transmission of data rates > 1 Gb/s. Simulation results demonstrate that simple few-tap feed-forward pre-equalization is able to remove the inter-symbol-interference (ISI) caused by the limited link bandwidth of a line of sight (LOS) VLC link, providing up to 5 dB better receiver sensitivity compared with post-equalization. The pre-equalization scheme is implemented for a free-space VLC link using a PAM modulation scheme, which provides an enhanced spectral efficiency compared to NRZ modulation. Micro-pixelated LEDs (μLEDs) are used as the transmitter in this work, as they exhibit higher modulation bandwidth than conventional large-diameter LEDs. An avalanche photodiode (APD) is used at the receiver to provide an enhanced link power budget. Error-free (BER<10-12) 2 Gb/s freespace VLC transmission over 0.6 m is demonstrated experimentally using a simple 3-tap feed-forward pre-equalizer and a PAM-4 modulation scheme. The results show that feedforward pre-equalization with only a few taps can improve the μLED-based link performance greatly, providing a simple and cost-effective solution for high speed VLC links.