Communicating Using Spatial Mode Multiplexing: Potentials, Challenges, and Perspectives

Abstract

Time, polarization, and wavelength multiplexing schemes have been used to satisfy the growing need of transmission capacity. Using space as a new dimension for communication systems has been recently suggested as a versatile technique to address future bandwidth issues. We review the potentials of harnessing the space as an additional degree of freedom for communication applications including free space optics, optical fiber installation, underwater wireless optical links, on-chip interconnects, data center indoor connections, radio frequency and acoustic communications. We focus on the orbital angular momentum (OAM) modes and equally identify the challenges related to each of the applications of spatial modes and the particular OAM modes in communication. We further discuss the perspectives of this emerging technology. Finally, we provide the open research directions and we discuss the practical deployment of OAM communication links for different applications. ). 4 in the future. We also summarize the open problems and the future research directions. A discussion of the practicability and cost of multi-OAM communication is also included. II. OAM POTENTIALS A. Free Space Optical CommunicationFSO is a license free wireless communication configuration that has recently received much interest for a variety of applications. FSO is an attractive solution for last mile connectivity problems, particularly for communication networks, when the installation of fiber optics is costly or not possible [66]. FSO can be also used to establish inter-building secure communication, and can be deployed as a backup to optical fibers. Wireless optical communication can guarantee a line-of-sight (LoS) high bit rate wireless transmission over long distances, up to several kilometers. Furthermore, FSO communication is considered as a promising technique to scale down bandwidth challenges in future 5G networks [67].Multiple wavelength FSO provides better transmission, as demonstrated in [68], [69]. Data can be mapped on advanced modulation formats to achieve high bit rates and high spectral efficiency levels [70]- [72]. Another option is multiple-input and multiple-output (MIMO) FSO communication in which multiple lasers are positioned to transmit Gaussian beams to multiple receiving apertures [73]. Over the last few years, it has been proven that it is possible to transmit information over spatially structured light beams [21], [74], [75] including OAM beams and plane waves. Our focus is on the OAM modes of light.The first PoC communication experiment incorporating OAMs in free space was carried out in 2004 by Gibson and co-workers [21]. Over a 15-meter long link, 8 -spaced values were chosen along the Gaussian beam as an alphabet for the communication. OAM beams were generated and detected using two light modulators. Since then, much progress has been made and free space transmission capacity of over 1 Tbit/s is, today, possible [76]- [78]. By performing three dimensional multiplexing, Huang and co-workers were able to attain a 100 Tbi...

A Unified Statistical Model for Atmospheric Turbulence-Induced Fading in Orbital Angular Momentum Multiplexed FSO Systems

“…In optical communications, SDM can be realized by sending different spatial modes in a multi-mode fiber or using many cores in a multi-core fiber [1]. In analogy to optical fiber transmission systems, orbital angular momentum (OAM) is proposed to transmit multiple signals over free space channels [2], [3]. This simultaneous transmission of information on OAM modes is possible thanks to the orthogonality property of OAM beams that allows propagation without interference between signals.…”

Identifying structured light modes in a desert environment using machine learning algorithms

“…To correctly identify spatial modes in turbulent channels, machine learning techniques can also be exploited without the need for AO or DSP equalization algorithms [14]. Machine learning is a powerful tool that can be used as a "regressor" or a "classifier", and has been applied thoroughly for modulation formats classification and impairments monitoring in optical networks [15,16].…”

A CNN-Based Structured Light Communication Scheme for Internet of Underwater Things Applications

“…The concept consists of using the spatial structure of light as an extra degree of freedom for communication systems. A particular mode of choice for spatial multiplexing is the orbital angular momentum (OAM) [12]. OAMs form a subset of the Laguerre Gaussian (LG) mode family, which is detailed in section III-A.…”

“…Also, similarly to FSO, using digital signal processing (DSP) techniques, it is possible to undo the effect of turbulence [21]. Although AO and DSP methods have shown to be of high performance, implementing such techniques can be costly or may lead to an increase in system complexity [12]. However, using the transverse patterns of spatially structured light beams, such as the LG, as code-words can be a way to deliver information in turbulent underwater channels without the need to compensate for propagation effects through AO or DSP techniques.…”