Wireless Visible Light Communications Employing Feed-Forward Pre-Equalization and PAM-4 Modulation

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 intersymbol interference caused by the limited link bandwidth of a line of sight VLC link, providing up to 5 dB better receiver sensitivity compared with postequalization. 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. Micropixelated LEDs (μLEDs) are used as the transmitter in this study, as they exhibit higher modulation bandwidth than conventional large-diameter LEDs. An avalanche photodiode is used at the receiver to provide an enhanced link power budget. Error-free (BER <10−12) 2 Gb/s free-space 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 feed-forward 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.