Diversity Combining for Asymmetrically Clipped Optical OFDM in IM/DD Channels

This paper looks at the problem of performance enhancement for orthogonal frequency-division multiplexing (OFDM) transmission in intensity modulated, direct detected (IM/DD) optical systems. Due to the unipolarity of the IM/DD channel, asymmetric clipping and associated biasing is inevitable, which results in a significant signal degradation. ACO-OFDM has been previously proposed to avoid this detrimental distortion with the cost of a lower spectral efficiency. In this paper, with the help of a detailed analysis of such distortion, a simple combining algorithm is proposed. This algorithm extracts the instructive information inside the clipping distortion, and requires no changes to the existing optical plant and no extra transmitting power. By exploiting this spectral diversity gain, the receiver SNR can be significantly improved.