Wavelength Division Multiplexing in Visible Light Communications Using Fluorescent Fiber Antennas

The use of fluorescent antennas in visible light communications (VLC) can significantly enhance the transmission performance by providing both a high optical concentration gain and a wide field of view (FOV) without being constrained by étendue. Moreover, as these antennas have the function of optical filtering, they can be used for supporting wavelength-division multiplexing (WDM) to boost the transmission data rate. However, there are several notable challenges when fluorescent antennas are used in WDM transmission. One challenge is that, due to the relatively wide absorption spectra of the fluorophores, the signal transmitted from different light sources can often only be partially separated in the optical domain. This causes crosstalk between different channels and consequently, the transmitted signals need to be further separated in the electrical domain. Another challenge arises from the performance imbalance among different decoded data streams, mainly caused by the differences in the frequency response between different transmitters. To address the first challenge, this paper introduces a new channel estimation method which enables the effective acquisition of the channel matrix for each orthogonal frequency-division multiplexing (OFDM) subcarrier without encountering interference. Thus, the signal separation step can be implemented more successfully in the frequency domain. Moreover, to overcome the imbalance between the data streams transmitted by different transmitters, we incorporate pairwise coding (PWC) into the system. Its performance was demonstrated via a WDM VLC testbed in which different colors of light-emitting diodes (LEDs) are configured to transmit independent data and commercially available fluorescent fibers are used to construct the optical antennas. Taking into account practical indoor illumination needs, we measured the system's performance across different illuminance levels. The results show that the use of PWC can well balance the performance between different channels, always leading to significant transmission data rate improvement.