Hany Elgala

• Recently, indoor OW communication systems based on LEDs as light sources and OFDM as a modulation technique have gained significant attention. For instance, in VLC, in addition to lightening capabilities of white LEDs, they can be modulated to achieve high-speed wireless data transmission. However, only little research has been done in this area, and many open questions exist such as the performance of O-OFDM systems in the presence of LED nonlinearities. The time domain OFDM signal exhibits a random-like variations. Therefore, special attention must be paid to its sensitivity to nonlinearities. This thesis analyzes the performance of indoor OW systems based on OFDM in the presence LED nonlinearities, i.e. amplitude and clipping distortions. The distortion analysis shows that LED nonlinearities can drastically degrade system performance. As a consequence, a linearization technique is proposed. A digital pre-distorter is applied to compensate for LED nonlinearities. It is found that the degradation can greatly be mitigated by using the proposed predistortion technique. In a practical system, the OFDM signal is forcibly clipped at the LED TOV and purposely clipped prior to the LED modulation for voltage levels above the saturation voltage. The OFDM pre-conditioning prior to the LED modulation is to ensure that the LED chip does not overheat. Therefore, there is a need to describe the nonlinear behavior of an LED with a model that fits the LED measured transfer function and to level off at both extremes to represent the TOV at one end and the saturation voltage at the other end. A model offering the desired "S-shaped" curve is proposed. Analytical methodologies to evaluate error probability of uncompensated O-OFDM system and compensated O-OFDM system are proposed. A comparison with Monte Carlo simulation results are carried out. The simulation curves and the analytical curves show a close match in terms of SER.