The widespread deployment of LEDs for illumination purposes has open the door to the use of these devices for visible light communications (VLC). Most lighting fixtures are mounted with phosphor-based white LEDs, and a driver connected to the LED is also required for VLC. This paper shows that the parasitic effects introduced by this setup change the frequency response of the intrinsic LED. A linear model to characterize the whole setup is proposed, as well as a methodology to extract its parameters. This methodology allows the designer to characterize the frequency response of LEDs without the additional difficulty of knowing the specific parasitic components introduced by the setup. The proposed model offers an accurate estimation of the slope of the LED frequency response in order to broaden the frequency range in which the model is useful to characterize and simulate VLC links. This was corroborated with the characterization of three commercial white LEDs whose measured and modeled frequency responses matched perfectly.

LED lamps have become the predominant technology for lighting thanks to their low power consumption. Recently, the joint use of these devices for lighting and data transmission has aroused interest in the research community as a future green communication technology. During this time, many of the advances developed in this area have been demonstrated with experimental setups using LED drivers based on bias-T, thanks to their ease to combine signal modulation and LED bias. This article discusses the impact of the output impedance of the bias-T driver and the driver-LED connection parasitics on the performance and energy efficiency of the VLC system. It is concluded that the output impedance of the driver not only affects energy efficiency but also modifies the frequency response and, therefore, the LED's capacity to be modulated. A low output impedance driver is shown to offer a substantial improvement in the energy efficiency of the VLC driver.

LED lighting has become the standard solution for illumination purposes thanks to its energy efficiency. Nowadays, there is growing interest in the use of LEDs for data transmission to develop future-generation communication systems. The low cost and widespread deployment of phosphor-based white LEDs make them the best candidate for visible light communications (VLC), although they have a limited modulation bandwidth. This paper presents a simulation model of a VLC link based on phosphor-based white LEDs and a method to characterize the VLC setup used to perform the data transmission experiments. Specifically, the simulation model incorporates the frequency response of the LED, the noise levels coming from the lighting source and the acquisition electronics, and the attenuation due to both the propagation channel and the angular misalignment between the lighting source and the photoreceiver. In order to validate the suitability of the model for VLC, carrierless amplitude phase (CAP) and orthogonal frequency division multiplexing (OFDM) modulation signals were employed for data transmission, and simulations with the proposed model and measurements over the equivalent scenario show high agreement.

In recent years, there has been extensive research in carrier-free phase and amplitude (CAP) modulation for optical communications, especially in the field of visible light communications (VLC). While the majority of the published works have been focused on characterizing its performance in optical channels, both theoretically and experimentally, only a limited number of publications have presented practical solutions to solve signal synchronization and demodulation, probably because experiments are usually conducted with externally synchronized equipment. The interest in CAP modulation comes from its lower complexity compared to quadrature amplitude modulation (QAM), but this advantage comes at the expense of a worse performance with time jitter. This paper presents a low-complexity, low-jitter, fully-digital CAP receiver with synchronization that improves CAP demodulation performance. Its enhanced behavior with time jitter is achieved by preprocessing the input of the timing error detector (TED) without changing the rest of the stages of standard CAP processing. That is, the receiver preserves the main advantage of CAP: it does not require frequency synchronization as neither the transmitter nor the receiver uses oscillators for passband up and down conversion. The proposed receiver operates with 4 samples per symbol and comprises only three stages: a matched filter, a time synchronizer, and a blind equalizer.