Virtual Polarization Modulation: Enabling CSI-Free DCO-OFDM over Dynamic OWC Channels

TL;DR

This paper introduces a virtual polarization modulation scheme for optical OFDM that eliminates the need for channel estimation, improving performance and robustness in dynamic optical wireless channels.

Contribution

It proposes a novel VPM-based DCO-OFDM scheme that maps data onto the Stokes space and derives analytical SER expressions, demonstrating superior performance over traditional QAM methods.

Findings

VPM outperforms QAM with LS and MMSE equalization in practical scenarios.

Achieves approximately 7.5 dB SNR gain over 16-QAM in frequency-selective channels.

Provides a low-latency, robust communication method without channel state information.

Abstract

In dynamically varying optical wireless communication (OWC) links, conventional quadrature amplitude modulation (QAM) in optical orthogonal frequency-division multiplexing (OFDM) requires frequent channel estimation and equalization, incurring pilot overhead and processing latency. This paper proposes a virtual polarization modulation (VPM)-based direct-current-biased optical OFDM (DCO-OFDM) scheme that maps each data symbol onto the three-dimensional Stokes space and places its corresponding Jones vector across two adjacent OFDM subcarriers. Using a rotation-based analytical framework, closed-form symbol error rate (SER) expressions are derived for arbitrary spherical constellations, along with upper and lower bounds and high signal-to-noise ratio (SNR) approximations. The framework is further extended to practical OWC scenarios with frequency-selective channels and atmospheric turbulence. Monte Carlo (MC) simulations validate the theoretical results. The results show that under practical OWC impairments, VPM outperforms QAM with least-squares (LS) channel estimation and minimum mean square error (MMSE) equalization. At a target SER of $10^{-5}$, 16-VPM achieves SNR gains of approximately 7.5 dB and 4 dB over equalized 16-QAM and 8-QAM, respectively, in frequency-selective channels, and a 6 dB advantage over equalized 16-QAM under atmospheric turbulence. By eliminating the need for channel state information, the proposed VPM-based DCO-OFDM provides a robust and low-latency solution for dynamic OWC links.