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A Contemporary Survey on Free Space Optical Communication: Potential, Technical Challenges, Recent Advances and Research Direction

Abu Jahid, Mohammed H. Alsharif, Trevor J. Hall

arXiv:2012.00155v1cs.ITeess.SY

TL;DR

Growing 5G and IoT traffic intensifies RF spectrum scarcity, while long-range FSO reliability is challenged by turbulence, weather, and pointing effects. The paper surveys FSO principles, technologies, architectures, applications, reliability techniques, and hybrid RF-optical systems. It identifies optical spectrum and hybrid deployment as promising directions while outlining unresolved deployment challenges.

  • Problem

    Growing 5G and IoT traffic motivates alternatives to scarce RF spectrum, while long-range FSO links remain vulnerable to atmospheric, weather, pointing, and scintillation effects.

  • Method

    The paper surveys FSO principles, classifications, architectures, applications, reliability methods, MIMO, relaying, and hybrid RF-optical networking.

  • Results

    Larger receiver apertures reduce turbulence-induced scintillation, while spatial diversity and relay transmission improve FSO reliability and performance under fading and poor-link conditions.

  • Takeaways & Limitations

    Unlicensed optical bandwidth and hybrid FSO/RF or RoFSO deployment are presented as promising ways to complement RF networks and address growing traffic demand.

Abstract

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Optical wireless communication (OWC) covering an ultra-wide range of unlicensed spectrum has emerged as an extent efficient solution to mitigate conventional RF spectrum scarcity ranging from communication distances from nm to several kilometers. Free space optical (FSO) systems operating near IR (NIR) band in OWC links has received substantial attention for enormous data transmission between fixed transceivers covering few kilometers path distance due to high optical bandwidth and higher bit rate as well. Despite the potential benefits of FSO technology, its widespread link reliability suffers especially in the long-range deployment due to atmospheric turbulence, cloud induced fading, some other environmental factors such as fog, aerosol, temperature variations, storms, heavy rain, cloud, pointing error, and scintillation. FSO has the potential to offloading massive traffic demands from RF networks, consequently the combined application of FSO/RF and radio over FSO (RoFSO) systems is regarded as an excellent solution to support 5G and beyond for improving the limitations of an individual system. This survey presents the overview of several key technologies and implications of state-of-the-art criteria in terms of spectrum reuse, classification, architecture and applications are described for understanding FSO. This paper provides principle, significance, demonstration, and recent technological development of FSO technology among different appealing optical wireless technologies. The opportunities in the near future, the potential challenges that need to be addressed to realize the successful deployment of FSO schemes are outlined.

I. Introduction

The paper introduces OWC and FSO as responses to RF spectrum scarcity and growing 5G/IoT traffic demands. It surveys optical bands, architectures, technologies, and deployment trade-offs, emphasizing both OWC advantages and line-of-sight and environmental constraints.

  • I. Introduction: Growing 5G and IoT traffic is increasing pressure on RF spectrum, whose limited range, regulation, and interference constrain wireless networking.
  • I. Introduction: OWC offers broad optical bandwidth, high data rates across distances from nanometers to several kilometers, electromagnetic-interference-free transmission, and improved security.
  • I. Introduction: OWC deployment is constrained by limited coverage, line-of-sight dependence, multiple-light-source interference, outdoor atmosphere, low transmit power, and reduced SINR.
  • I. Introduction: Hybrid RF-optical networks combine complementary frequency bands because RF performs better under non-line-of-sight conditions while OWC addresses RF spectrum congestion.
  • I. Introduction: OWC technologies differ in communication protocol, architecture, modulation, channel medium, transmission, reception, and application scenario.The survey compares VLC, LiFi, OCC, LiDAR, and FSO, commonly using LEDs or laser diodes with photodetectors or cameras.
  • I. Introduction: FSO commonly uses the near-infrared band for long-distance point-to-point links, while optical bands are classified into infrared, visible light, and ultraviolet categories.

1) Light Fidelity (LiFi):

This section contrasts optical and RF communication technologies across their operating principles, capabilities, and limitations. LiDAR uses reflected optical signals for sensing, while FSO supports long-distance optical links and competing RF systems provide mobility or non-line-of-sight coverage.

  • 1) Light Fidelity (LiFi):: VLC can support 100 Gbps and offers substantially greater bandwidth than RF, but its application scenarios are constrained by optical-link conditions.
  • 1) Light Fidelity (LiFi):: OCC uses LEDs and cameras with infrared or visible-light spectra, achieving 45 Mbps at zero BER and 54 Mbps at a BER of 10^-5.
  • 4) Light Detection And Ranging (LiDAR):: LiDAR uses near-infrared or visible-light laser beams and analyzes reflected signals to determine target distance, size, and other characteristics.
  • 5) Free Space Optical (FSO) Communication:: FSO uses narrow laser beams and photodetectors, typically in the near-infrared band, to establish high-speed long-distance point-to-point links without illumination.
  • 1) Light Fidelity (LiFi):: WiFi supports up to 8 Gbps with mobility under LOS and NLOS conditions, whereas Bluetooth reaches up to 2 Mbps over short-range 2.4 GHz links.
  • 1) Light Fidelity (LiFi):: Microwave links can exceed 100 km and demonstrate 12.6 Gbps, but electromagnetic interference, fog, rain, and atmospheric turbulence degrade performance.

D. Related Works

The paper situates FSO within optical wireless research and surveys its principles, architectures, applications, hybrid schemes, recent trends, and deployment challenges. It also compares FSO with other optical wireless technologies and synthesizes related surveys.

  • Related OWC research: Earlier OWC research examined technologies including VLC, LiFi, LiDAR, and FSO across several communication applications.
  • Related FSO research: Related FSO studies evaluate coding, multiplexing, BER, link capacity, multi-user interference, crosstalk, and refractive-index variation.
  • Survey scope: This survey presents FSO principles, transmitters, receivers, channel characterization, modulation, architectures, applications, and hybrid FSO systems.
  • Comparative analysis: The survey compares optical wireless technologies using communication distance, data rate, and modulation techniques, while discussing RF/FSO, mmWave/FSO, acoustic/FSO, and OCC/FSO.
  • Technical coverage: It further addresses FSO link budgets, reliability, QoS, RoFSO, MIMO FSO, multi-user communication, physical and TCP layers, backhaul networking, and next-generation systems.
  • Challenges and directions: The paper identifies FSO challenges, mitigation techniques, research directions, and lessons learned for successful deployment.

A. Advantages and Disadvantages of FSO

FSO offers narrow-beam, high-bandwidth optical communication across multiple distances and settings, but its reliability is constrained by atmospheric conditions, alignment, and propagation limitations. The section classifies applications from chip-scale and indoor links to terrestrial, satellite, and deep-space scenarios.

  • Advantages: Optical wavelengths and beam properties enable extremely high bandwidth, high data rates, parallel links, strong received intensity, and smaller antenna sizes than RF systems.
  • Advantages: FSO uses narrow laser beams between fixed nodes over distances up to a few kilometers, providing inherent security, spectrum reuse, and electromagnetic-interference immunity.
  • Disadvantages: FSO link reliability deteriorates under atmospheric turbulence, heavy rain, fog, snow, clouds, wind-related pointing instability, background light, and shadowing.
  • Disadvantages: Pointing error arises from transceiver misalignment and mechanical vibration, making precise pointing, acquisition, and tracking necessary for directional optical links.
  • Application scenarios: Geographical scenarios include atmospheric terrestrial, indoor home networking, space, and underwater communications, with indoor links typically spanning a few to tens of meters.
  • Classification by distance: FSO applications span ultra-short chip-to-chip, short personal-area and underwater, medium indoor and vehicle, long inter-building, and ultra-long satellite links.

C. Applications of FSO

FSO applications span high-capacity networking, inter-building and last-mile connectivity, monitoring, security, IoT/5G, and satellite communications. Its main appeal is high-speed optical connectivity, but backhaul capacity remains challenging as traffic grows.

  • Last-mile and inter-building connectivity: FSO can bridge fiber infrastructure and destination users for last-mile connectivity, including remote users requiring Gbps-range links.It can also provide temporary service when existing fiber connectivity is unavailable.
  • Last-mile and inter-building connectivity: FSO can connect buildings across campuses, enterprises, and residences without installing dedicated fiber links.These links address heterogeneous traffic demands such as voice, internet data, fax, and multimedia services.
  • Monitoring, broadcasting, and security: FSO applications include ultra-quality video transmission, cellular backhaul backup, live broadcasting, disaster monitoring, and secure communication.The cited uses cover military, commercial, public-safety, metropolitan, remote-zone, and disaster scenarios.
  • IoT, 5G, and satellite communications: FSO is applied to IoT/5G connectivity and satellite communications, including power-efficient inter-satellite links and high-capacity OAM-based transmission.These applications target high bandwidth, bit rate, security, low latency, interference-free transmission, and spectral efficiency.
  • Backhaul networking: FSO supports high-capacity backhaul between access and core networks for 5G/B5G and IoT traffic.The paper associates this use with high traffic handling, low power consumption, cost effectiveness, and spectral efficiency.
  • Device and machine connectivity: FSO supports D2D, M2M, V2X, multipoint-to-multipoint, and end-to-end mechanisms in healthcare, transportation, commercial, and industrial settings.The paper states that high-speed FSO mesh connections can overcome RF scalability limitations involving fairness and throughput over many hops.

D. FSO in IoT, 5G and B5G

FSO is positioned as a high-speed complement to dense 5G/B5G and IoT networks, including terrestrial backhaul and satellite links. Its deployment is constrained by atmospheric attenuation, beam divergence, cloud blockage, turbulence, and space-link acquisition challenges.

  • FSO in IoT, 5G and B5G: 5G/B5G uses heterogeneous networks with nested small cells to increase capacity and support extensive device connectivity and QoE.FSO is presented as supporting these high-throughput and high-connectivity requirements.
  • FSO in IoT, 5G and B5G: FSO supports high-speed 5G communication, high-capacity backhauling, and massive IoT connectivity while potentially reducing energy consumption relative to traditional RF systems.The paper also identifies low implementation cost, security, energy efficiency, and support for many smart devices as IoT requirements.
  • Space communications: FSO satellite communications include ground-to-satellite, satellite-to-ground, inter-satellite, and deep-space links.Atmospheric losses are higher for uplinks because optical beams spread and distort near the ground-based transmitter.
  • Atmospheric constraints: Dense fog with visibility below 50 m can produce signal losses higher than 350 dB/km.High-power lasers operating in the third transmission window are described as a means to improve reliability under this condition.
  • Atmospheric constraints: Heavy rain at 25 mm/hr causes ten times the attenuation of light rain at 2.5 mm/hr, with attenuation ranging from 1 dB/km to 10 dB/km at 1500 nm.Hybrid RF/FSO systems are identified as a suitable choice for improving availability under heavy rain.
  • Space-link design: Beam divergence causes geometrical loss that increases with transmission distance regardless of receiver-aperture diameter.The paper therefore describes narrow-beam lasers and receive diversity as preferable design choices for space uplinks.
  • Atmospheric constraints: Thick clouds can disrupt or completely block FSO links for minutes to hours, while turbulence causes intensity fluctuations and arrival-angle variation.Tempo-spatial diversity is described as a way to combat optical beam loss from cloud attenuation.
  • Space communications: Inter-satellite links avoid weather and atmospheric limitations but remain challenged by acquisition and tracking, satellite stability, Doppler shift, and background radiation.Coherent detection is described as more appropriate than direct detection for longer inter-satellite distances.

F. FSO Transceiver

An FSO transceiver comprises atmospheric transmission, a turbulent and noisy channel, and electronic receiver processing. The transmitter shapes and modulates optical signals, while the receiver converts photons into electrical data using photodetection and filtering.

  • Transceiver architecture: FSO communication has three stages: atmospheric optical transmission governed by Beer-Lambert’s law, channel accumulation of noise and turbulence, and receiver-side electronic processing.This division describes the end-to-end transceiver pipeline.
  • FSO transmitter: The optical transmitter encodes incoming bits optionally, modulates them, amplifies the optical intensity when needed, and focuses the laser beam before transmission.Its principal components are a light source, modulator, optional optical amplifier, and beam-forming mechanism.
  • FSO transmitter: LEDs offer energy efficiency, long lifecycle, low heat production, and fast switching for high-speed OWC transmission.The passage presents these properties as advantages of solid-state lighting used in optical transmitters.
  • FSO transmitter: LEDs can suffer interference from natural and artificial light, whereas LDs are limited by narrow apertures, thermal effects, cost, and color-mixing complications.The paper notes that narrow LD apertures restrict communication to point-to-point operation in the cited context.
  • FSO transmitter: Semiconductor laser diodes are typically used in FSO systems because they provide intense optical power and wide modulation bandwidth across temperature ranges.Eye safety is identified as an important laser-transmitter consideration, while LEDs are preferred indoors at low data rates.
  • FSO receiver: A receiver collects photons, converts them to electrical current through a photodetector and transimpedance circuit, then filters noise before recovering data.The low-pass filter restricts background and thermal noise in the described receiver chain.
  • FSO receiver: Commercial FSO receivers commonly use 850 nm and 1550 nm wavelengths, with Si photodetectors suited to 850 nm and InGaAs detectors sensitive around 1550 nm.The passage also identifies graphene, plasmonic nanomaterials, two-dimensional materials, and quantum dots as emerging detector-related materials.
  • Optical amplification: Optical amplifiers can improve long-distance multihop FSO performance, but amplified spontaneous emission noise can reduce achievable receiver SNR.EDFA is described as preferred over SOA at 1550 nm in the cited deployment.

V. Reliability of FSO communication systems

FSO reliability depends on maintaining sufficient link margin and received power despite atmospheric variability, distance, and turbulence. The section frames wavelength, visibility, attenuation, and threshold intensity as central reliability considerations.

  • Reliability factors: FSO reliability is challenged by atmospheric fluctuations that cause signal loss, phase or intensity variations, increased BER, and reduced link availability.Fog, humidity, scintillation, and molecular absorption are identified as important environmental effects.
  • Link-budget design: Long-term reliable link design requires surveying average visibility and the statistical behavior of atmospheric conditions at a specific geographic location.These analyses support subsequent link-budget formulation.
  • Link-budget design: Link margin expresses optical-link reliability using transmit power, receiver sensitivity, and propagation loss, while required loss margin compensates turbulence over the covered distance.The required received power is tied to a specified BER and transmission rate.
  • Reliability target: 99.99% link reliability corresponds to approximately five minutes of downtime per year for telecommunications applications.An example with PT X = 30nW, SRX = 25nW, La = 3dB, and Lo = 4dB estimates RLM =54 dB.
  • Reliability modeling: Link reliability can be represented through cumulative irradiance probability against a threshold intensity and varies between 0 and 1.The formulation includes transmission distance, average turbulence-free intensity, and irradiance variance.

VI. Mitigation Techniques

The survey presents aperture averaging, adaptive optics, relay transmission, and diversity as approaches for reducing FSO degradation from turbulence, misalignment, and channel variability. Aperture sizing improves fading reduction but requires an efficiency-aware trade-off because larger apertures increase background noise.

  • Mitigation approaches: PAT, diversity, and hybrid RF/FSO systems are identified as approaches for improving FSO link reliability under atmospheric fluctuations and temporary line-of-sight blockage.Temporary obstructions include birds, drones, smoke, vehicles, building sway, and tree limbs.
  • Mitigation approaches: Temporal diversity addresses channel variations associated with temperature, location, wind, altitude, pressure, and humidity, while PAT and wavefront correction address received-beam distortion.PAT helps maintain beam-centroid stability in long-haul terrestrial and inter-satellite links.
  • Aperture averaging: Increasing receiver aperture reduces channel fading and turbulence-induced fast fading by collecting more of the received beam.Aperture averaging reduces physical-layer fading and turbulence-induced scintillation for larger apertures.
  • Aperture averaging: The receiver aperture radius should be chosen optimally because increasing aperture size reduces fading but also increases background noise and can reduce power efficiency.The aperture area depends on transmission length and the strength of limiting factors.
  • Aperture averaging: Aperture averaging is quantified by the receiver-side factor Fa, based on the variance of received signal intensity for different aperture sizes.The cited formulation uses the variance at aperture D relative to the variance at reference aperture D0.

B. Adaptive optics (AO)

The section surveys adaptive optics, relay-assisted transmission, and modulation strategies for improving FSO alignment, coverage, efficiency, and resilience. It also notes implementation trade-offs involving steering hardware, synchronization, bias power, and nonlinear distortion.

  • Adaptive optics (AO): Adaptive optics uses closed-loop control and steering mirrors to transmit a distortion-free beam and achieve precise beam pointing in position and angle.Piezoelectric electromechanical steering devices are noted as occupying substantial space and requiring longer processing time.
  • Relay-assisted transmission: Relay-assisted transmission extends FSO distance through distributed cooperative nodes when the source-destination link has poor quality.Relay nodes combat turbulence and can improve system performance without sacrificing QoS.
  • Modulation schemes: OOK is widely used for simplicity, whereas PPM or variable PPM is preferred for energy-efficiency applications such as deep-space communication.OOK is less efficient for ultrafast and complex systems because of poor spectral and energy efficiency.
  • Modulation limitations: OFDM-based MCSIM suffers from PAPR because increasing subcarriers requires more DC bias to prevent nonlinear distortion and clipping.The added bias reduces power efficiency.
  • Modulation schemes: PWM offers improved spectral efficiency, lower peak transmit power, and better ISI performance than PPM, but requires an extra guard slot and synchronization.Both PPM and PWM are described as synchronous modulation schemes.
  • Modulation schemes: M-ary PAM, PSK, and QAM provide higher spectral and energy efficiency than binary modulation by using multiple emission-intensity levels.DPSK and CAP are also considered among multilevel intensity-modulation and direct-detection schemes.

E. FSO channel modeling

FSO channel modeling must account for random atmospheric fading using capacity, outage, BER, and coding analyses. The survey links these models to adaptive transmission, coding, power control, spatial diversity, and background-noise mitigation.

  • Channel-capacity modeling: Atmospheric turbulence makes FSO fading random, motivating the use of ergodic capacity and outage probability to characterize channel performance.Ergodic capacity averages instantaneous capacity, while outage probability concerns failure to reach the information rate.
  • Channel models: FSO capacity studies consider Gamma-Gamma, Rayleigh, Nakagami-m, Malaga, Rician, and log-normal fading with AWGN receiver noise.Related analyses also address outage probability and bit error rate.
  • Fading mitigation: Channel coding, temporal-spatial diversity, and sequence detection are presented as fading-mitigation techniques for weak and strong turbulence conditions.Atmospheric beam fluctuations otherwise degrade performance through increased receiving bit errors.
  • Adaptive transmission: Because FSO fading is slow varying, estimated CSI can be fed back to adapt transmit power, modulation level, and code rate.Adaptive transmission is identified as suitable for full-duplex feedback operation.
  • Noise modeling: Background radiation noise increases at lower operating wavelengths, so spatial filtering and adaptive modulation must account for signal-arrival angle, Doppler shift, and laser spectral width.High-PAPR modulation is included in the proposed noise-mitigation context.
  • Diversity: Time, frequency, and spatial diversity generate multiple correlated signal copies to reduce atmospheric disturbances and can reduce the need for active tracking during misalignment.The approach is associated with improved BER performance and desired QoS.

H. Coherent FSO

Coherent FSO mixes the received optical signal with a local oscillator before photodetection, improving sensitivity and noise rejection while increasing implementation complexity. The section also situates coherent detection within hybrid and multiplexed FSO architectures.

  • Coherent detection: Coherent FSO mixes the received signal with a local-oscillator beam before photodetection, then amplifies and filters it to reject noise and interference.The receiver uses the local oscillator to process the optical field before detection.
  • Coherent detection: Coherent FSO provides greater receiver sensitivity than IM/DD systems, while direct detection remains prevalent because it is simpler and cheaper.Coherent schemes are increasingly implemented as ultra-speed digital processing advances.
  • Hybrid architectures: FSO-WiMAX uses OFDM to divide the channel into N narrowband subcarriers, improving spectral efficiency and dispersion performance.The scheme combines a coherent optical beam with a spatial light modulator for broadband transmission.
  • Multiplexing: Multiple-wavelength SCM can achieve 1 Tb/s over a 3 Tb/s-km bandwidth-length product, supporting cost reduction for last-mile FSO applications.N multiplexed subcarriers are transmitted using Mach-Zehnder modulation.
  • Hybrid architectures: Hybrid RF/FSO systems improve link reliability and load balancing by combining FSO capacity with RF backup during optical outages.RF is less affected by pointing errors, fog, and atmospheric turbulence, although it offers lower data rates than FSO.
  • Applications and evaluation: Hybrid FSO research includes WiFo indoor femtocells, underwater RF/acoustic/optical systems, FSO/OCC vehicular links, and FSO/VLC coordination.The surveyed performance analyses use metrics including throughput, BER, spectral efficiency, outage probability, secrecy rate, SINR, and power efficiency.

VII. Radio over FSO (RoFSO) System

RoFSO transports broadband RF signals bidirectionally over an FSO link by modulating the optical carrier with RF signals. The section surveys multiuser, relaying, spectrum-sensing, and generalized MIMO directions for this architecture.

  • RoFSO architecture: RoFSO modulates RF signals onto an optical carrier before transmission, enabling simultaneous bidirectional broadband RF delivery over an FSO link.The architecture combines radio-over-fiber concepts with free-space optical transmission.
  • RoFSO architecture: RoFSO addresses last-mile broadband deployment where fiber installation is restricted, while offering high transmission rates, low energy consumption, and deployment flexibility.The section frames RoFSO as relevant to growing 5G and B5G user demand.
  • Multiuser communication: Multiuser RF/FSO studies examine multihop relaying, point-to-multipoint MISO, two-way relaying, outage probability, BER, and energy-efficient power allocation.Amplify-and-forward relaying uses fixed gain without CSI and adaptive gain when CSI is available.
  • Multiuser communication: Multiuser MIMO transmission is presented as a capacity-oriented alternative to spatial multiplexing for FSO communications.The generalized multiuser MIMO configuration is illustrated in Fig. 12.
  • Spectrum sensing: Optical spectrum sensing monitors channel occupancy and helps avoid interference among multiple users, including under strong atmospheric turbulence.The surveyed policy uses SNR estimation for unknown optical signals across optical wavelengths.

IX. MIMO FSO Systems

MIMO and diversity techniques improve FSO reliability and capacity under turbulence, fading, and misalignment, while higher-order schemes introduce implementation and processing trade-offs. The section also covers transport-layer resilience and machine-learning-assisted network functions.

  • Diversity techniques: Spatial diversity uses multiple transmit beams or receive apertures to improve FSO transmission over fading channels and introduce signal redundancy.It addresses turbulence-induced fading, beam wandering, waveform distortion, and transceiver misalignment.
  • Diversity techniques: Aperture averaging reduces fading, with several small receiver apertures being especially advantageous under strong turbulence but more complex to implement.SIMO systems may use equal-gain combining for lower complexity, while combining benefits depend on channel estimation.
  • MIMO transmission: MIMO FSO supports simultaneous multiuser transmission and spatial multiplexing, increasing bit rate and signal quality while requiring precoding to control inter-user interference.Multiple co-located apertures provide multiplexing gain under a shared frequency band.
  • MIMO transmission: OSTBC-based MIMO addresses timing misalignment and improves robustness to inter-symbol interference while preserving orthogonality conditions.The performance gap between RC and OSTBC becomes more significant for higher-order MIMO FSO systems.
  • MIMO evaluation: MIMO-FSO studies evaluate ergodic capacity, BER, and outage probability using Alamouti transmit diversity and switch-and-examine combining over Malaga turbulence channels.These results represent experimental or analytical investigations of specific diversity configurations.
  • Reliability and QoS: Reliability-oriented FSO networking includes retransmission, rerouting, QoS control, and cross-layer physical/TCP designs, with rerouting improving reliability but increasing delay and cost.QoS requirements include throughput, jitter, latency, signal loss, spectral efficiency, and energy efficiency.
  • Intelligent FSO networks: Machine learning and deep learning are surveyed for low-cost channel estimation, optical performance monitoring, modulation selection, detection, error correction, and demodulator design.Perfect channel estimation is challenging because of its cost and power consumption.

XII. Summary

The survey presents FSO as a high-capacity complement to RF for 5G, B5G, IoT, and other wireless services, while emphasizing unresolved deployment challenges. It concludes that hybrid architectures and intelligent networking are important research directions.

  • Summary: FSO offers enormous bandwidth, narrow directional beams, electromagnetic-interference-free transmission, security, and energy efficiency for long-haul wireless connectivity.The survey contrasts these properties with RF bandwidth shortage, interference, and lower throughput.
  • Summary: FSO is positioned as an attractive license-free-spectrum solution because existing RF systems cannot satisfy intended data rates for 5G and B5G services.The survey describes narrow directional LOS propagation over long-haul links.
  • Summary: The paper surveys FSO applications from residences to space and identifies unresolved challenges and open problems for efficient deployment.The authors state that fully harvesting FSO’s inherent potential remains immature.
  • 5G/B5G and IoT/IoE solutions: Dense deployment of optical nodes can create inter-cell interference and flickering, motivating coordinated multipoint techniques and further network coordination.The passage identifies CoMP as a way to reduce inter-cell interference and maximize throughput.
  • 5G/B5G and IoT/IoE solutions: Machine-learning-based networking is identified as a future direction for intelligent network reassignment, traffic management, automatic error correction, and higher data rates.The passage links these functions to reinforcement learning and machine learning in 6G networks.
  • 5G/B5G and IoT/IoE solutions: Hybrid FSO connectivity is proposed for interoperating heterogeneous IoT technologies as conventional RF spectrum becomes more congested.The cited examples include RFID, WSN, cloud networks, Bluetooth, WiFi, and ZigBee.

B. THz and Quantum Communications

The survey connects FSO with THz and quantum communications while examining capacity expansion, channel impairments, pointing mechanisms, and mobility challenges. It emphasizes that long-range and mobile optical links require mitigation of turbulence, misalignment, and tracking constraints.

  • B. THz and Quantum Communications: FSO provides high-rate LOS links over long outdoor distances, while THz communication supports high-rate LOS and NLOS applications.The passage contrasts FSO’s long-distance LOS capability with THz links that can operate through reflections in NLOS environments.
  • B. THz and Quantum Communications: Relay-assisted FSO can mitigate atmospheric turbulence by forwarding data through an intermediate node instead of a direct link.Reported relay configurations include serial multi-hop transmission for extending coverage and parallel cooperative diversity.
  • B. THz and Quantum Communications: Dense wavelength division multiplexing increases FSO capacity, with reported demonstrations of 320 Gbps and 200 Gb/s links.The cited demonstrations use LOS/long WDM FSO links, including OOK modulation for the 320 Gbps realization.
  • B. THz and Quantum Communications: Atmospheric turbulence, scattering, scintillation, absorption, refraction, storms, fog, rain, and dust can severely degrade FSO links.These effects motivate channel models including log-normal, lognormal-Rician, and Gamma-Gamma distributions.
  • B. THz and Quantum Communications: Narrow-beam FSO links offer high speed, long reach, low interference, and energy efficiency but require pointing, acquisition, and tracking coordination.The beam divergence is described as smaller than a few µrad, making synchronization between coordinating nodes an inherent design issue.
  • B. THz and Quantum Communications: Mobile FSO for V2X must preserve LOS connectivity while tracking fast-moving vehicles under dense urban conditions.The survey identifies agile, compact, low-complexity, multidirectional, and vibration-mitigating PAT mechanisms as open design needs.

G. Underwater communication (UWC)

The survey presents underwater optical communication as a high-speed, long-coverage technology while identifying hybrid-network adaptation, mobility, handover, security, uplink, and cross-layer control as deployment challenges. It frames hybrid optical-RF architectures and broader OWC integration as approaches for addressing growing communication demand.

  • G. Underwater communication (UWC): Adaptive modulation and coding is an important issue for underwater optical links used in monitoring, investigation, and object detection.The passage highlights oil-pipe investigation, offshore monitoring, and object detection as relevant UWC applications.
  • G. Underwater communication (UWC): Hybrid underwater optical/acoustic networks require adaptive switching between acoustic and optical transmission modes.Such switching is identified as necessary for various underwater applications.
  • G. Underwater communication (UWC): Dense 5G/B5G heterogeneous networks can create frequent and unnecessary handovers between optical and RF networks.The survey identifies suitable horizontal and vertical handover algorithms as necessary for seamless mobility support.
  • G. Underwater communication (UWC): User association and load balancing remain research concerns when users select among heterogeneous optical and RF access networks.The cited discussion links association policies with joint resource allocation and transferring user equipment to improve performance under load balancing.
  • G. Underwater communication (UWC): Physical-layer security for FSO remains immature despite analyses of secrecy outage under turbulence, misalignment, and eavesdropping conditions.The survey covers MISO, MIMO, and cooperative FSO security schemes alongside external-eavesdropper scenarios.
  • G. Underwater communication (UWC): SDN can manage hybrid optical networks through a controller, while application and control layers address rerouting, security, flow control, and network selection.The passage assigns network-demand functions to the application layer and flow-control functions to the control layer.
  • G. Underwater communication (UWC): Optical wireless systems are commonly designed for downlink communication, making uplink transmission an open research issue because of portable-device energy and narrow-beam control constraints.The cited discussion also notes limited attention to data-link-level metrics in RF/optical hybrid research and calls for cross-layer QoS analysis.
  • G. Underwater communication (UWC): The review identifies hybrid wireless deployment as a way to address the limitations of standalone RF or optical networks while supporting rising IoT and cellular traffic.Its scope includes OWC technologies and FSO deployment issues across spectrum, architecture, applications, and research challenges.
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