Closed form approximation of the actual spectral power emission of commercial color LEDs for VLC
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Multi-color Light-Emitting Diode (LED) technology enables a simple approach to increase the throughput of a Visible Light Communications (VLC) system, by using Wavelength-Division Multiplexing (WDM) to transmit independent data streams on different colors. However, to compute the data rate that is achievable in such WDM VLC link, the optical power that leaks between the different colors needs to be estimated accurately, especially when low-cost optical filters are used in reception. So far, the approximations that have been reported in the literature to model the spectral power emission of different color LEDs are not good enough to perform these calculations. Starting from the theoretical spectral emission of a color LED, a closed form expression is derived based on an asymmetric Pearson type VII function, which is shown to approximate accurately the measured spectra of the color LEDs at different working regimes. In addition, the effect that the DC-bias current has on the key parameters of the approximated spectral power emissions, namely the peak and half-maximum wavelengths, as well as the peak spectral emission, are studied. Finally, a new approach is proposed to assess the level of fitness of the derived closed form approximations, using for this purpose the step-size of the MacAdam ellipses that corresponds to the different color LED spectral emissions in the CIE 1931 chromaticity diagram.