Impact of LED nonlinearities on optical wireless OFDM systems

Nonlinearities can drastically degrade the performance of optical communication systems based on intensity modulation (IM) of the optical carrier using the time domain orthogonal frequency division multiplexing (OFDM) electrical signal variations. The light emitting diode (LED) transfer function distorts the signal amplitude and forces the lower signal peaks to be clipped at the LED turn-on voltage (TOV). Additionally, the upper signal peaks are purposely clipped before modulating the LED to avoid chip overheating. The induced distortion can be controlled by optimizing the bias point and/or backing-off the signal power. In this paper, a model that incorporates amplitude distortion and that provides a parameterized upper clipping to control nonlinearity induced distortion is proposed. Through Monte Carlo simulations, the model can be used to determine the optimum bias point and to optimize the signal power to obtain best performance. In this context, an analytical approach to evaluate symbol error probability is proposed. A comparison with Monte Carlo simulation results is carried out to verify the accuracy of this approach.