Predistortion in Optical Wireless Transmission Using OFDM

The nonlinear characteristic of an LED (light emitting diode) imposes limitations on the performance of indoor optical wireless (OW) systems when using intensity modulation in combination with OFDM (orthogonal frequency division multiplexing). First, the impact of the nonlinear characteristic on bit-error performance is analyzed using a commercially available LED (OSRAM, SFH 4230). Second, the paper proposes a predistorter to overcome the nonlinearities. Key features of the predistorter reside in the use of the LED inverse characteristics as nonlinear distortion compensator. A DC biased optical OFDM (DCO-OFDM) system is considered and the performance without compensation and after compensation is analyzed via simulations in an AWGN (additive white Gaussian noise) environment. In this context, the bit-error performance is determined for different bias points and power back-off values applied to the OFDM signal modulating the LED intensity. It is shown that LED nonlinearity can significantly degrade the performance. However, it is demonstrated that this degradation can greatly be mitigated by using the proposed predistortion technique.