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Communicating Using Spatial Mode Multiplexing: Potentials, Challenges and Perspectives

Abderrahmen Trichili, Ki-Hong Park, Mourad Zghal, Boon S. Ooi, Mohamed-Slim Alouini

arXiv:1808.02462v3cs.ITeess.SPphysics.optics

TL;DR

The paper addresses how spatial modes, particularly OAM modes, can expand communication capacity across diverse propagation media and applications. It reviews reported demonstrations and challenges, then evaluates deployment prospects and open research directions. The review finds substantial progress in free-space and fiber links, while underwater and long-distance RF deployment remain bounded by unresolved practical conditions.

  • Problem

    Growing transmission-capacity demand motivates using space as an additional communication dimension across optical, RF, acoustic, and other links.

  • Method

    The paper comprehensively reviews OAM communication applications, generation and detection approaches, deployment challenges, practical design considerations, and future research directions.

  • Results

    The review identifies the greatest advances in multiple-mode transmission in free-space and optical-fiber communications, with OAM also demonstrated or considered across underwater, chip, indoor, RF, and acoustic systems.

  • Takeaways & Limitations

    OAM links offer application-specific routes to higher-capacity, lower-cabling, lower-energy, switching, routing, and independent-stream communication, subject to propagation and implementation constraints.

  • Takeaways & Limitations

    Long-distance OAM radio has limited practical interest relative to state-of-the-art MIMO, while underwater turbulence studies and real-life multi-link deployments remain limited.

Abstract

from arXiv · show

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.

I. INTRODUCTION

Spatial mode multiplexing uses spatial modes as independent information-bearing carriers to address transmission-capacity demands, with OAM as a prominent mode set. The review surveys OAM generation, detection, communication applications, deployment challenges, and future research directions.

  • Spatial modes can act as independent information-bearing carriers, scaling total transmission capacity by several orders of magnitude.
  • OAM modes use an azimuthally varying phase and provide an unbounded set of states indexed by the topological charge ℓ.
  • OAM communication has been implemented or investigated across free-space optics, optical fibers, underwater links, indoor links, optical interconnects, RF, and acoustic systems.
  • Information can be transmitted either by using OAM states as symbols or by encoding independent data streams on multiple OAM carriers.
  • OAM generation and detection employ techniques including spiral phase plates, holograms, spatial light modulators, resonators, metamaterials, metasurfaces, and antenna or sensor arrays.
  • Practical deployment remains constrained by propagation effects, turbulence, crosstalk, mode preservation, and application-specific generation and detection requirements.
  • The review synthesizes reported OAM communication results, analyzes application-specific challenges, and discusses open problems, future directions, and deployment practicality.

II. OAM POTENTIALS

OAM and other spatial modes extend communication capacity by adding independent spatial carriers across free-space and fiber systems. Demonstrations span high-capacity free-space links, OAM multicasting, vortex-fiber transmission, and combined few-mode multicore fibers.

  • Free-space optical communication: Free-space OAM multiplexing reached 1.036 Pbit/s at 112.6 bit/s/Hz using 26 OAM states.A later link reported 435 bit/s/Hz using dual-polarization 52 OAM modes and Nyquist 32-QAM signals.
  • Free-space optical communication: 100 Gbit/s QPSK data carried by one OAM mode was duplicated onto 5 and 7 equally spaced OAM transmissions.This demonstrates OAM multicasting from one mode to multiple output modes.
  • Optical fiber communication: A 6×6 MIMO DSP compensated inter-modal energy exchange when transmitting 28 Gbaud-QPSK signals over six spatial modes.Combining space-division approaches also enabled up to 200 Tbit/s over 1 km of few-mode multicore fiber.
  • Optical fiber communication: LP modes can couple linearly because they are not exact fiber eigenmodes, increasing demultiplexing complexity and requiring MIMO DSP.OAM fiber modes were proposed as an option for reducing inter-modal coupling and MIMO complexity.
  • Optical fiber communication: A 1.6 Tbit/s transmission used two OAM modes over 10 wavelength channels in a 1.1 km vortex fiber.The reported orthogonality of OAM modes reduced the need for MIMO DSP during demultiplexing.

C. Underwater Communication

Underwater optical communication offers high-rate, short- to moderate-range links, and spatial multiplexing with OAM modes is used to increase capacity. Demonstrations include multi-gigabit transmission through water, including turbulent conditions.

  • Underwater communication: Offline underwater sensors cannot provide real-time information needed for applications such as surveillance and disaster prevention.Robotic underwater missions also require communication systems that avoid large antennas and high transmission powers.
  • Underwater optical communication: UWOC can exceed 1 Gbit/s over short and moderate ranges while offering greater bandwidth than acoustic communication.The blue-green spectrum around 400–500 nm attenuates less in pure water than other visible and mid-infrared regions.
  • Underwater optical communication: 40 Gbit/s was transmitted using four green OAM modes over 1.2 m of turbulent tap water.Each of the four beams carried a 10 Gbit/s signal.

D. Wireless Indoor Connections

Data centers face rising traffic and energy demands, motivating indoor wireless optical links that can provide high-capacity, low-latency, and dynamically reconfigurable connections. OAM multiplexing is proposed for indoor inter-rack communication, with design guidelines and encoding approaches reported.

  • Motivation: Data centers must accommodate growing traffic while addressing fiber-density and energy-consumption costs.Global data-center electricity use is estimated at 1.3% worldwide and almost 2% in the United States.
  • Wireless optical links: Indoor wireless optical links can provide high transmission capacity over short distances with low latency and reduced energy consumption.They can also support flexible and dynamic network reconfiguration.
  • OAM multiplexing: OAM multiplexing has been proposed as a basis for indoor wireless connections in data centers.
  • Design approaches: Engineering guidelines and an OAM-pattern encoding and decoding algorithm have been reported for data-center applications.
  • Optical configuration: A potential inter-rack configuration must account for the reversal of OAM handedness when beams reflect from a mirror.

E. On-chip Photonic Circuits

On-chip photonic circuits are presented as an energy-efficient, high-bandwidth alternative to electrical interconnects, with OAM enabling multimode wireless chip communication and switching. Reported work covers generation, encoding, multicasting, and reconfigurable manipulation of OAM states.

  • Motivation: Integrated photonic circuits can maintain high bandwidth and reduce energy consumption in on-chip communication.CMOS advances have enabled energy-efficient, high-bandwidth photonic circuits.
  • OAM interconnects: A proposed on-chip OAM link encoded and decoded data using topological charges from ℓ = −4 to 3.
  • Power efficiency: OAM-based optical wireless interconnects have the highest power efficiency among the compared electrical, microbump, and dense-WDM interconnects.
  • Generation and multicasting: Experimental and proposed techniques include four-mode generation, digital-holography-based multiplexing, and V-shaped-array multicasting.The V-shaped array was designed for low-crosstalk one-to-many distribution.
  • Switching functions: OAM switching supports common index shifts, charge-order exchange, and selective manipulation of one or more modes.A shift changes every charge by k, exchange reverses charge order, and selective manipulation leaves unaffected modes unchanged.
  • Reconfigurable switching: A 2 × 2 reconfigurable OAM switch was demonstrated using spatial light modulators, while scaling to N × N may require up to 2N + 1 modulators.Liquid-crystal spatial light modulators can limit switching rates to about 1 kHz.

G. Radio Frequency Communication

OAM modes are reviewed as a radio-frequency multiplexing and modulation dimension, with proposed benefits for spectrum use, energy efficiency, security, and resilience. The section also identifies propagation, turbulence, modeling, and implementation challenges across RF and acoustic links.

  • RF opportunities: Multiple RF OAM beams can share a frequency, offering a potential response to spectrum scarcity.
  • RF applications: RF OAM research has proposed multidimensional modulation beyond 1 Tb/s, massive-MIMO combinations, mode hopping for jamming resistance, and radar applications.These claims include numerical and proposed results across different studies.
  • RF modulation: An OAM spatial-modulation scheme reported more than 200% higher maximum energy efficiency than OAM-MIMO and greater resilience to path-loss attenuation.The study did not investigate antenna misalignment.
  • Acoustic communication: For underwater acoustic communication, 8 OAM states achieved 8.0 ± 0.4 bit/s/Hz at 20 dB SNR with BER below 10^-6.Measured crosstalk was lower than −8.54 dB.
  • Propagation challenges: Atmospheric propagation introduces absorption, scattering, turbulence-induced distortion, beam spread, and wandering for optical OAM links.OAM-beam divergence increases with |ℓ|, which can cause power loss at limited receiving apertures.
  • Scattering effects: Ballistic scattering causes power loss, whereas diffuse scattering induces inter-OAM-channel crosstalk.These effects were investigated for four co-propagating OAM beams carrying 20 Gbit/s QPSK signals.
  • Turbulence modeling: The Kolmogorov turbulence model has limitations for lower atmospheric layers and stable atmospheric conditions.The Strehl ratio is described as decreasing with atmospheric distortions, with 1 representing a turbulence-free channel.
  • Open challenges: A theoretical framework for BER, outage probability, and capacity that determines whether OAM performance depends on ℓ remains missing.Indoor turbulence from server-cooling ventilation is also identified as relevant to wireless interconnections.

B. Fiber Propagation Effects

OAM propagation in optical fibers is challenged by linear and nonlinear effects, including attenuation, chromatic and modal dispersion, polarization effects, and mode coupling. Fiber studies examine mode properties, differential delays, and coupling between opposite-charge OAM states.

  • Fiber challenges: Linear and nonlinear effects are the main propagation challenges for OAM in optical fibers.
  • Attenuation: Material-absorption attenuation in OAM-capable fibers is usually small and comparable with standard single-mode fibers.
  • Chromatic dispersion: Chromatic dispersion arises because phase and group velocities depend on optical frequency.
  • Dispersion: Intermodal dispersion and polarization-mode dispersion introduce additional mode- and polarization-dependent delays in fibers.Differential group delay quantifies polarization-mode-dispersion strength, while modal dispersion is quantified by differential mode delay.
  • Mode coupling: Studies have analyzed OAM walk-off, tolerance to fiber-core ellipticity, differential mode delays, and coupling between +ℓ and −ℓ states.The latter coupling has been described as OAM polarization-mode dispersion by analogy with single-mode fibers.

2) Fiber Nonlinear Effects:

Fiber nonlinearities alter optical phase, intensity, and frequency, creating crosstalk and transmission penalties; their behavior in OAM modes remains largely unexplored. Underwater optical links additionally face attenuation, turbulence, and environmental fluctuations that require characterization and compensation.

  • Fiber Nonlinear Effects: Kerr nonlinearities include self-phase modulation, cross-phase modulation, and four-wave mixing, while Raman and Brillouin scattering involve phonons.Self-phase modulation changes refractive index with optical intensity; cross-phase modulation can couple co-propagating beams with different modes, wavelengths, directions, or polarizations.
  • Fiber Nonlinear Effects: Cross-phase modulation can cause significant crosstalk between communication channels in optical fibers.
  • Fiber Nonlinear Effects: Nonlinear effects are detrimental for long-distance transmission in both single-mode and multimode fibers, although four-wave mixing can support mode amplification and wavelength conversion.
  • Fiber Nonlinear Effects: Nonlinearity for OAM modes remains a largely unexplored field.
  • Underwater Propagation Effects: Underwater optical communication offers larger bandwidth than acoustic techniques but requires further research before large-scale deployment.Key requirements include quantifying and compensating propagation effects, including absorption, scattering, turbulence, and environmental fluctuations.

D. Inter-modal Crosstalk

Inter-modal crosstalk transfers power between OAM modes and can prevent signal recovery. RF OAM links are especially constrained by divergence, multipath, distance-dependent power loss, sensitivity to alignment, and long-distance propagation limits.

  • D. Inter-modal Crosstalk: Disturbances can transfer power between OAM modes or generate neighboring modes, producing inter-modal crosstalk that limits communication implementation.The resulting power redistribution may leave insufficient power to recover the signal, especially in chip-to-chip interconnects.
  • D. Inter-modal Crosstalk: RF OAM performance is reported to offer no significant gain over conventional single-mode MIMO, while short-distance operation requires DFT beamforming for high performance and low complexity.
  • D. Inter-modal Crosstalk: RF OAM beams diverge with increasing transmitter–receiver distance, and high-ℓ beams are more sensitive to power loss.
  • D. Inter-modal Crosstalk: Multipath causes receivers to collect both the transmitted OAM mode and a reflected mode with opposite charge.
  • D. Inter-modal Crosstalk: RF OAM arrays require near-perfect alignment for spectral efficiency comparable to traditional spatial multiplexing, while aperture theory disfavors long-distance transmission.

A. Towards a Complete Mode Set

The paper argues that OAM is only a subspace of the complete Laguerre–Gaussian basis, so broader modal sets can provide better choices for scaling free-space capacity. Experiments demonstrate multiplexing across radial and azimuthal mode dimensions, while adaptive optics mitigates turbulence-induced distortions.

  • A. Towards a Complete Mode Set: OAM can be outperformed by a complete modal basis for increasing free-space communication capacity.The conclusion follows from singular-value-decomposition analysis of the coupling operator between transmitting and receiving sources.
  • A. Towards a Complete Mode Set: Laguerre–Gaussian modes form a complete orthonormal basis characterized by azimuthal index ℓ and radial index p, whereas OAM occupies only a subspace.
  • A. Towards a Complete Mode Set: Encoding both radial and azimuthal degrees of freedom enabled transmission of 105 modes over three wavelengths.
  • A. Towards a Complete Mode Set: LG mode multiplexing has comparable complexity to OAM multiplexing because beam size, phase shift, and divergence follow the beam quality factor M².
  • A. Towards a Complete Mode Set: A 200 Gbit/s transmission using two LG modes with distinct radial components achieved BER below the forward error correction limit.
  • A. Towards a Complete Mode Set: Adaptive optics and signal-processing methods are used to mitigate atmospheric turbulence, while reciprocity supports channel-state estimation and pre-compensation.
  • A. Towards a Complete Mode Set: Adaptive feedback correction suppressed inter-state crosstalk in 4-fold and 8-fold OAM multicasting channels and improved BER performance.

2) Coding and Equalization Techniques:

The review covers digital and optical approaches for mitigating turbulence, crosstalk, and alignment impairments in OAM links. These include coding, equalization, modal diversity, machine learning, and beam tracking across free-space and underwater settings.

  • Coding and equalization: Digital coding and equalization mitigate turbulence-induced crosstalk and errors in OAM links.Reported approaches include LDPC coding, MIMO equalization, pilot-assisted LS processing, and space-time coding.
  • Modal diversity: Modal diversity encodes the same information on distinct co-propagating modes that experience different distortions.Simulations using OAM modes ℓ=1, ℓ=8, and ℓ=15 reduced outage probability in turbulent FSO links.
  • Alternative turbulence mitigation: A Gaussian local oscillator combined with an OAM signal improved simulated BER without adaptive optics, coding, or modal diversity.The approach may be costly for free-space applications because coherent detectors are expensive.
  • Machine learning: Machine learning can recognize OAM intensity patterns before information retrieval, avoiding explicit turbulence compensation.The review identifies potential extension of this approach to optical-fiber and underwater transmissions.
  • Beam tracking: Beam tracking compensates alignment errors caused by beam displacement in outdoor FSO and underwater links.Underwater demonstrations tracked displacement caused by water-level changes, tides, and surface waves.
  • Underwater links: Underwater turbulence mitigation remains less developed than free-space mitigation, and adaptive optics has not yet been tested on OAM-based UWOC.Bessel Gaussian modes are proposed as self-healing alternatives to OAM Laguerre-Gaussian modes for bubble obstruction.

F. OAM Maintaining Fibers

The section reviews fiber technologies and system techniques for maintaining OAM modes, emphasizing mode separation, amplification, and information encoding. It also surveys RF OAM solutions and ongoing debates over complexity and link practicality.

  • OAM-maintaining fibers: OAM propagation in fibers requires separation between relevant mode groups, supported by conventional, few-mode, multimode, and specially designed fiber technologies.Reviewed designs include photonic crystal, graded-index, air-core, multi-ring, and multicore supermode fibers.
  • Fiber amplification: Long-distance OAM fiber transmission requires inline amplifiers, but demonstrated designs remain theoretical or limited to few modes.A proposed erbium-doped fiber amplifier supports 12 OAM modes with quasiuniform gain.
  • RF OAM techniques: RF OAM generation and detection use devices including antennas, phase plates, reflectors, metasurfaces, lenses, and FFT-based processing.These designs address beam purity, divergence, chromatic aberration, Doppler shift, multipath, and link-budget estimation.
  • RF information encoding: RF OAM states can encode information directly, including mapping bits 1 and 0 to ℓ=1 and ℓ=−1.Index modulation has also been proposed as an alternative way to convey information through OAM states.
  • RF system assessment: Experimental and mathematical analyses reported that multiple OAM modes can reduce receiver complexity relative to single-mode MIMO without affecting system capacity.Other work derives link-budget descriptions that account for the spatial structure of beams and uniform circular antenna arrays.
  • RF perspectives: Near-field OAM links and modal-diversity approaches are presented as alternatives in continuing debates about RF OAM practicality.A Cassegrain reflector demonstrated multiplication and cancellation of OAM modes in the near field, while modal diversity was proposed to increase capacity.

V. OPEN PROBLEMS AND FUTURE RESEARCH DIRECTIONS

The paper identifies deployment barriers and research priorities across FSO, optical-fiber, underwater, on-chip, and RF OAM communication. Priorities include alignment, relaying, amplification, realistic turbulence studies, integrated devices, and practical validation.

  • FSO: FSO OAM deployment requires compact generation devices, reliable alignment, and relay nodes for shadowed non-line-of-sight terminals.Relaying suitable for spatial modes remains largely unexplored beyond mirror-based connections.
  • Optical fibers: Fiber deployment needs experimentally demonstrated amplifiers supporting more modes and better understanding of fiber nonlinearity.The review also identifies nonlinear propagation as a possible basis for new lasers and optical amplifiers.
  • Underwater communication: Underwater OAM research must experimentally and theoretically characterize multiple-beam propagation under realistic turbulence scenarios.The work also calls for underwater alignment devices and mitigation strategies based on adaptive optics or digital signal processing.
  • On-chip communication: On-chip OAM communication requires small-footprint generation devices and chip-size detectors for links between photonic integrated circuits.Both components are identified as necessary for implementing spatial-mode communication at chip scale.
  • RF communication: RF OAM remains challenging for far-field applications, while near-field systems require further antenna-design investigation.The review characterizes RF OAM as a growing field with unresolved deployment challenges.

VI. DISCUSSION

The discussion presents OAM multiplexing as a cross-domain capacity and connectivity technology, with strongest progress in free space and optical fibers. It also emphasizes application-specific trade-offs, unresolved underwater and RF deployment issues, and broader mode-basis choices.

  • Free-space communication: Free-space OAM links can provide reliable outdoor connectivity over short and moderate distances, while AO and DSP reduce turbulence-induced crosstalk.Modal diversity is described as a potentially cost-effective alternative to complex equalization and adaptive optics.
  • Optical fibers: OAM fiber transmission is promising for exploiting existing multimode-fiber installations and supporting data-center or fiber-to-the-home links.Customized integrated OAM laser sources could contribute to deployment and reduce cabling.
  • On-chip communication: OAM switching and routing may support all-optical photonic circuits, while OAM-based chips could accelerate computer and server processing.The review characterizes on-chip progress as gradual but continuing.
  • Underwater communication: Underwater OAM studies remain limited, especially for simultaneous multiple-beam transmission and realistic turbulence conditions.The review calls for crosstalk quantification and real-life deployment tests of multiple underwater links.
  • RF communication: Long-distance RF OAM deployment remains debated because divergence and power-penalty concerns limit its contribution relative to established MIMO systems.The spatial profile may nevertheless support radar and satellite-localization operations.
  • Deployment considerations: OAM system cost remains a deployment challenge, particularly because commercially available OAM fibers are expensive and largely laboratory-oriented.OAM-shaped laser sources could reduce mode-generation costs and equipment density.
  • Overall perspective: The review identifies OAM multiplexing as a spatial-capacity approach demonstrated across FSO, fibers, underwater links, optical networks, chips, RF, and acoustic communication.It also notes that the full Laguerre-Gaussian basis, Hermite-Gaussian basis, and vortex-vector basis deserve consideration.
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