title: Recent advances in high-capacity free-space optical and radio-frequency communications using orbital angular momentum multiplexing.
abstract: There is a continuing growth in the demand for data bandwidth, and the multiplexing of multiple independent data streams has the potential to provide the needed data capacity. One technique uses the spatial domain of an electromagnetic (EM) wave, and space division multiplexing (SDM) has become increasingly important for increased transmission capacity and spectral efficiency of a communication system. A subset of SDM is mode division multiplexing (MDM), in which multiple orthogonal beams each on a different mode can be multiplexed. A potential modal basis set to achieve MDM is to use orbital angular momentum (OAM) of EM waves. In such a system, multiple OAM beams each carrying an independent data stream are multiplexed at the transmitter, propagate through a common medium and are demultiplexed at the receiver. As a result, the total capacity and spectral efficiency of the communication system can be multiplied by a factor equal to the number of transmitted OAM modes. Over the past few years, progress has been made in understanding the advantages and limitations of using multiplexed OAM beams for communication systems. In this review paper, we highlight recent advances in the use of OAM multiplexing for high-capacity free-space optical and millimetre-wave communications. We discuss different technical challenges (e.g. atmospheric turbulence and crosstalk) as well as potential techniques to mitigate such degrading effects.This article is part of the themed issue 'Optical orbital angular momentum'.
authors.affiliation: Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA willner@usc.edu.
authors.forename: Alan E
authors.identifier: http://orcid.org/0000-0002-7339-4376
authors.initials: AE
authors.lastname: Willner
authors.affiliation: Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA.
authors.forename: Yongxiong
authors.initials: Y
authors.lastname: Ren
authors.affiliation: Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA.
authors.forename: Guodong
authors.initials: G
authors.lastname: Xie
authors.affiliation: Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA.
authors.forename: Yan
authors.initials: Y
authors.lastname: Yan
authors.affiliation: Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA.
authors.forename: Long
authors.initials: L
authors.lastname: Li
authors.affiliation: Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA.
authors.forename: Zhe
authors.initials: Z
authors.lastname: Zhao
authors.affiliation: Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People's Republic of China.
authors.forename: Jian
authors.initials: J
authors.lastname: Wang
authors.affiliation: School of Electrical Engineering, Tel Aviv University, Ramat Aviv 69978, Israel.
authors.forename: Moshe
authors.initials: M
authors.lastname: Tur
authors.affiliation: Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA.
authors.forename: Andreas F
authors.initials: AF
authors.lastname: Molisch
authors.affiliation: NxGen Partners, Dallas, TX 75219, USA.
authors.forename: Solyman
authors.initials: S
authors.lastname: Ashrafi
keywords: free-space optical communications; millimetre-wave communications; orbital angular momentum; space division multiplexing