An optical power efficient asymmetrically companded DCO-OFDM for IM/DD systems

Unipolar forms of orthogonal frequency division multiplexing (OFDM) such as DC biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are widely used in intensity-modulated and direct detection (IM/DD) systems. At low data rate, ACO-OFDM is more efficient in terms of optical power but it suffers from spectral inefficiency. For large constellations, DCO-OFDM with large DC bias has more optical power efficiency, since it requires lower constellation size than ACO-OFDM. However, the required DC bias to obtain an acceptable clipping noise degrades the optical power efficiency. To overcome these drawbacks, we propose an optimized DCO-OFDM technique capable to transmit large constellations with a moderate DC bias. To reduce the clipping impact, a linear companding function is used in order to compress the negative part of the bipolar signal and therefore to reduce the amount of the clipped peaks and the loss of information. We demonstrate that the proposed technique has better bit error rate and optical power performances than conventional DCO-OFDM and ACO-OFDM. For a bit rate/normalized bandwidth of 6, a gain of 4dB in optical power is reached.