Offset and Power Optimization for DCO-OFDM in Visible Light Communication Systems

Direct-current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) is used in visible light communications (VLC) for high rate transmission. Due to the unipolarity of optical signals, a direct current (DC) offset along with proper clipping is adopted in DCO-OFDM. A high DC-offset causes a waste of power and a low DC-offset causes a severe clipping distortion. Thus, the DC-offset and information-carrying power shall be jointly optimized to balance clipping distortion and power consumption. We consider the offset and power optimization problem under the optical and/or electrical power constraints over flat and dispersive channels. In the flat case, we analytically characterize the optimal solution, based on which several important insights are provided. We also investigate the relation and impact of the two constraints, and propose efficient algorithms to obtain the optimal solution. In the dispersive case, we identify the scope of the optimal solution, and provide efficient algorithms to jointly optimize the offset and power. The optimality and superiority of the proposed methods are validated by numerical results.