Proposal of a Pre-equalizer with Nyquist Filtering to Mitigate LED Nonlinearity in PAM-4 VLC Systems

Visible light communication (VLC) systems are widely considered for adoption in the upcoming sixth-generation (6G) systems. This is because they provide a power-efficient, noncomplex, unlicensed frequency band, safe, and secure solution for data transmission. High data rates up to gigabit per second have been achieved in research over the past years using VLC. However, in order to achieve high data rates in such systems utilizing light emitting diodes (LEDs) already in the infrastructure, it is crucial to mitigate its impairments such as limited frequency response (tens of MHz bandwidth) and nonlinearity. In this paper, a novel transmitter system is proposed to compensate for the two main limitations in LED-based VLC systems. Different pulse shaping Nyquist filtering techniques are applied to minimize inter-symbol interference (ISI) caused by the LED's small modulation bandwidth. Additionally, a preequalizer is presented to alleviate LED nonlinearity. Results show an overall linear transmission and a significant enhancement in symbol-error rates (SER). Moreover, out-of-band side lobes are successfully suppressed, thus requiring smaller bandwidth for signal transmission without loss of information.