  Asymmetrically clipped optical orthogonal frequency-division multiplexing
(ACO-OFDM) is a technique that sacrifices spectral efficiency in order to
transmit an orthogonally frequency-division multiplexed signal over a unipolar
channel, such as a directly modulated direct-detection fiber or free-space
channel. Several methods have been proposed to regain this spectral efficiency,
including: asymmetrically clipped DC-biased optical OFDM (ADO-OFDM), enhanced
U-OFDM (EU-OFDM), and spectral and energy efficient OFDM (SEE-OFDM). This paper
presents a new method that offers the highest receiver sensitivity for a given
optical power at spectral efficiencies above 3 bit/s/Hz, having a 7-dB
sensitivity advantage over DC-biased OFDM for 1024-QAM at 87.5% of its spectral
efficiency, at the same bit rate and optical power.
