D. Tsonev, S. Sinanovic, and H. Haas, "Novel Unipolar Orthogonal Frequency Division Multiplexing (U-OFDM) for Optical Wireless Communication", in Proc. of Vehicular Technology Conference (VTC Spring 2012), to appear.
In this paper, we propose a novel modulation technique for optical wireless communications using incoherent LED (light emitting diodes). The basic principle to convey information in this case is to use intensity modulation (IM). This means that the information resides in the way how the intensity (power) of the light is varied. As a consequence, the signal that modulates the LED strictly has to be positive and real valued (power can never be negative or complex). In contrast, higher order, capacity achieving modulation techniques used in radio frequency (RF) systems require bi-polar and complex signals. The use of higher order modulation techniques such as M-level quadrature amplitude modulation (M-QAM) is essential to achieve data rates that are close to the Shannon capacity limit. Therefore, the key question in optical wireless communications is how to map bi-polar and complex valued signals into uni-polar (positive) and real valued signals without offsetting the bit-error performance. The proposed technique achieves the first goal, i.e., the mapping of bi-polar to uni-polar signals, in a way that the it outperforms existing methods such as DCO-OFDM (direct current optical - orthogonal frequency division multiplexing) and ACO-OFDM (asymmetrically clipped optical – orthogonal frequency division multiplexing).