OAM Mode Selection and Space–Time Coding for Atmospheric Turbulence Mitigation in FSO Communication

El Mehdi Amhoud,Abderrahmen Trichili,Boon S. Ooi,Mohamed-Slim Alouini

doi:10.1109/access.2019.2925680

Abstract

Orbital angular momentum (OAM) multiplexing has recently received considerable interest in free space optical (FSO) communications. Propagating OAM modes through free space may be subject to atmospheric turbulence (AT) distortions that cause intermodal crosstalk and power disparities between OAM modes. In this paper, we are interested in multiple-input-multiple-output (MIMO) coherent FSO communication systems using the OAM. We propose a selection criterion for the OAM modes to minimize the impact of the AT. To further improve the obtained performance, we propose a space-time (ST) coding scheme at the transmitter. Through numerical simulations of the error probability, we show that the penalty from AT is completely absorbed for the weak AT regime, and considerable coding gains are obtained in the strong AT regime.

Highlights

In analogy to mode division multiplexing (MDM) in optical fibers where several spatial modes are used for multiplexing, orbital angular momentum (OAM) multiplexing is proposed as a versatile technique to transmit multiple signals over free space channels [1], [2]
In summary, we have shown that an optimal selection of Orbital angular momentum (OAM) modes is relevant to improve the performance of OAM free space optical (FSO) systems over the turbulent atmosphere
The selection criterion is based on the minimization of the MDL without any algorithm required to update the set of optimal OAM modes

Summary

INTRODUCTION

In analogy to mode division multiplexing (MDM) in optical fibers where several spatial modes are used for multiplexing, orbital angular momentum (OAM) multiplexing is proposed as a versatile technique to transmit multiple signals over free space channels [1], [2]. The mitigation of the effects of atmospheric turbulence can be done either at the beam level using adaptive optics (AO) compensation, or by using digital signal processing (DSP) techniques such as channel coding or equalization. For DSP approaches, MIMO equalization associated with heterodyne detection was shown to mitigate turbulence-induced crosstalk for 4-OAM beams carrying 20 Gbit/s QPSK signals in [8]. Pre-channel combining phase patterns allowed to reduce the crosstalk by 18-dB for a 2-OAM modes transmission in [9] Coding techniques such as channel coding [7] and coded modulation [10] were proposed to mitigate atmospheric turbulence in OAM FSO transmissions. Amhoud et al.: OAM Mode Selection and Space-Time Coding for Atmospheric Turbulence Mitigation in FSO Communication. Where u∗q(r, z) refers to the normalized field distribution of OAM mode of order q at distance z and r refers to the radial position vector

OAM PROPAGATION IN TURBULENCE

CONCLUSION