Information Theoretic Limits of Improved ACO-OFDM Receivers in Optical Intensity Channels With Time Dispersion

From an information-theoretic perspective, we investigate performance of asymmetrically clipped optical OFDM (ACO-OFDM) in intensity modulated optical wireless systems. Our focus is how much the information rate of ACO-OFDM can be boosted by receivers which utilize information contained in even-indexed subcarriers rather than only observing the modulated (odd-indexed) subcarriers as in the conventional receiver. In the presence of time-dispersion, the maximum achievable gain of such improved receivers in information rate is characterized by a multi-letter (vector) conditional mutual information, which is difficult to evaluate. Under independent complex Gaussian input symbols, we derive computationally tractable lower and upper bounds on the maximum achievable gain, which contain only single-letter expressions of conditional mutual information or conditional differential entropy. At low SNR, the gap between our lower and upper bounds vanishes asymptotically. By numerical studies, it is shown that time dispersion significantly reduces the maximum achievable gain at low SNR. At high SNR, our lower bounds and asymptotic analysis reveal that the achievable gain is still evident, measured in terms of the proportion to that in the case of no time dispersion.