Improved Receivers for Optical Wireless OFDM: An Information Theoretic Perspective

TL;DR

This paper analyzes the theoretical limits of improved receivers for optical OFDM schemes, showing significant potential for approaching channel capacity across various SNR levels, with practical implications for optical wireless communications.

Contribution

It provides the first information theoretic characterization of all possible receivers for ACO-OFDM, revealing maximum achievable SNR gains and bounds for practical constellations.

Findings

High-SNR gain asymptotically reaches 1/4 bits per channel use.

Maximum SNR gain decreases to 36.3% at low SNR.

Numerical results show practical bounds are within 1 dB of theoretical limits.

Abstract

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.