  We study the information rates of unipolar orthogonal frequency division
multiplexing (OFDM) in discrete-time optical intensity channels (OIC) with
Gaussian noise under average optical power constraint. Several single-,
double-, and multicomponent unipolar OFDM schemes are considered under the
assumption that independent and identically distributed (i.i.d.) Gaussian or
complex Gaussian codebook ensemble and nearest neighbor decoding (minimum
Euclidean distance decoding) are used. We obtain an array of information rate
result. These results validate existing signal-to-noise-and-distortion-ratio
(SNDR) based rate analysis, establish the equivalence of information rates of
certain schemes, and demonstrate the evident benefits of using
component-multiplexing at high signal-to-noise-ratio (SNR). For double- and
multi-component schemes, the component power allocation strategies that
maximize the information rates are investigated. In particular, by utilizing a
power allocation strategy, we prove that several multi-component schemes
approach the high SNR capacity of the discrete-time Gaussian OIC under average
power constraint to within 0.07 bits.
