Information Theoretic Limits of Improved ACO-OFDM Receivers

Xiaozhen Liu, Jing Zhou, Nuo Huang, Wenyi Zhang

Abstract: We consider performance enhancement of asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM), which is a variation of OFDM in intensity modulated optical wireless communications. To improve the conventional ACO-OFDM receiver that utilizes only odd-indexed subcarriers, several specific designs have been proposed in the literature. Unlike these studies, this paper investigates information theoretic limits of all possible improved receivers. More specifically, we prove that the maximum achievable information rate of ACO-OFDM is the sum of (i) the information rate of the conventional receiver, and (ii) a single-letter conditional mutual information expression corresponding to the maximum achievable gain of improved receivers. Asymptotic analysis shows that the high-signal-to-noise-ratio (high-SNR) limit of the maximum achievable gain of arbitrary improved receivers is 3 dB in (electrical) SNR, corresponding to an information rate gain of exactly 1/4 bits per channel use. We also show that the low-SNR limit of the maximum achievable gain of improved receivers is approximately 1.35 dB in SNR. Numerical evaluation of finite-SNR gains demonstrate that improved ACO-OFDM receivers may reduce the gap to capacity significantly, especially at low-to-moderate SNR.