LED Nonlinearity Mitigation Techniques in Optical Wireless OFDM Communication Systems

Light emitting diode (LED) nonlinearities have a significant impact on the performance of optical wireless (OW) communication systems based on orthogonal frequency division multiplexing (OFDM) modulation. In this paper, different techniques to compensate for the induced nonlinearity distortions are presented and analyzed. First, single-carrier frequency division multiple access (SC-FDMA) is proposed instead of OFDM for OW systems. In SC-FDMA, some of the major OFDM problems such as high peak-to-average power ratio are inherently resolved. It is shown that the performance of OW OFDM systems and their counterpart SC-FDMA systems are traded off for certain input signal powers and modulation orders. Second, OW OFDM systems with hard, soft, and turbo channel coding techniques are considered. It is reported that significant performance gains can be achieved through channel coding. Finally, a novel method to reduce signal clipping distortion through the use of iterative signal clipping (ISC) and multiple transmit LEDs is proposed. The ISC technique is based on iterative clipping of the time domain OFDM signal and transmission from a multiple LED transmitter. Transmit LEDs are synchronized, located close to each other, and placed to emit light in the same direction. Hence, the channel path gains from each LED to the receiver photodiode are similar. The received signals from the different LEDs add coherently at the receiver. Reported results demonstrate that the effect of distortion is eliminated or significantly reduced based on the considered number of transmit LEDs.