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

Abstract

The orthogonal frequency division multiplexing (OFDM) transmission has shown promise in applications of visible light communication (VLC). However, the variation of the nonlinearity of the optical power emitted by the high power light emitting diode (HPLED) as a function of current and temperature implies in drastic OFDM-VLC performance degradation. The first part of this work, experimentally confirms and models this degradation due to temperature in a high power 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 of the signal-to-noise ratio (SNR) at the receiver for such frequencies. For the mitigation of these effects, we propose a pre-distortion and digital preequalization scheme using a luminous feedback signal in the transmitter module. The 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 an average SNR in all OFDM subcarriers, the performance improvement is confirmed in comparison with other schemes, such as with non-predistortion, pre-distortion with fixed parameters, and simple post-equalization.

Wireless and Optical Convergent Access Technologies Toward 6G

“…For instance, Wei et al reported a 6.9 Gbit/s VLC system with functional transmission distance based on white-light phosphor laser [187]. In addition, as reported in some of the previously highlighted works, techniques to linearize and compensate the LED response over the frequency can be used, applying pre-distorter or equalizer, and also MIMO techniques, to enhance the total throughput [172], [173], [175], [183].…”

“…Mejia and Georghiades [174] presented an overview of color-shiftkeying (CSK), based on red, green and blue (RGB) LEDs, by considering different coding methods, with emphasis on CSK trellis-coded modulation (TCM) and CSK finite-state-machines (FSM) codings, which provided higher SNR. In [175], an experimental work discussed DPD and pre-equalization techniques to mitigate the degrading nonlinear effects of VLC systems.…”

Low complexity OFDM VLC system enabled by spatial summing modulation

“…To modulate the LED transmitter, a DC bias current and Power Amplification (PA) are further needed so as to obtain a sufficient unipolar driving current to modulate an illuminating LED. To mitigate the distortion caused by LED non-linearity to the modulated signal, complex pre-distortion circuitry is involved in many existing proposals [20,21]. However, the novel spatial summing enabled transmitter has much simpler control logic compared to the traditional one.…”

Modelling and Design of Pre-Equalizers for a Fully Operational Visible Light Communication System

“…Moreover, these methods can be applied in two different ways: digital or analog. On one hand, digital pre-equalizer methods have been developed through the different types of techniques, such as adding weights to subcarriers, or the use of finite impulse response (FIR) filters [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. Besides, analog pre-equalizer methods have been developed using techniques, such as the cascaded constant-resonance amplitude equalizer and the multiple-resonant-driving equalization technique [ 23 , 24 ].…”

“…In the literature, several digital pre-equalizer methods have been proposed for VLC systems [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. In [ 13 ], in order to improve the modulation speed of 1 MHz bandwidth white LED communication, digital FIR and matched filters were used.…”