Predistortion and pre-equalization for nonlinearities and low-pass effect mitigation in OFDM-VLC systems.

The orthogonal frequency division multiplexing (OFDM) transmission is a promising candidate for visible light communication (VLC) applications. However, the nonlinearity in the optical power emitted by the high-power light-emitting diode (HPLED) as a function of current and temperature implies drastic OFDM-VLC performance degradation. The first part of this work experimentally confirms and models this degradation due to temperature in a white HPLED. The higher attenuation at high frequencies, which is inherent to the HPLED and which is accentuated by the effect of the intrinsic capacitance of the photodiode, is another factor of degradation due to the reduction in the signal-to-noise ratio (SNR) at the receiver for such frequencies. For the mitigation of these combined effects, we propose a predistortion and a digital pre-equalization scheme using a luminous feedback signal at the transmitter side. The VLC system is modeled so that the operating points are mathematically deduced and evaluated by simulations and by an experimental setup. By allowing the linearization of the transmitted light signal and the maintenance of a uniform SNR among all OFDM subcarriers, the performance improvement is confirmed in comparison with other schemes, such as a VLC system without predistortion, predistortion with fixed parameters, as well as a simple post-equalization.