  We consider performance enhancement of asymmetrically-clipped optical
orthogonal frequency division multiplexing (ACO-OFDM) and related optical OFDM
schemes, which are variations of OFDM in intensity-modulated optical wireless
communications. Unlike most existing studies on specific designs of improved
receivers, this paper investigates information theoretic limits of all possible
receivers. For independent and identically distributed complex Gaussian inputs,
we obtain an exact characterization of information rate of ACO-OFDM with
improved receivers for all SNRs. It is proved that the high-SNR gain of
improved receivers asymptotically achieve 1/4 bits per channel use, which is
equivalent to 3 dB in electrical SNR or 1.5 dB in optical SNR; as the SNR
decreases, the maximum achievable SNR gain of improved receivers decreases
monotonically to a non-zero low-SNR limit, corresponding to an information rate
gain of 36.3%. For practically used constellations, we derive an upper bound on
the gain of improved receivers. Numerical results demonstrate that the upper
bound can be approached to within 1 dB in optical SNR by combining existing
improved receivers and coded modulation. We also show that our information
theoretic analyses can be extended to Flip-OFDM and PAM-DMT. Our results imply
that, for the considered schemes, improved receivers may reduce the gap to
channel capacity significantly at low-to-moderate SNR.
