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Optical Communication in Space: Challenges and Mitigation Techniques

Hemani Kaushal, Georges Kaddoum

arXiv:1705.10630v1cs.IT

TL;DR

FSO communication offers high-capacity optical links but is limited by atmospheric impairments and link-specific propagation challenges. This paper surveys those challenges and mitigation techniques across multiple protocol layers, while reviewing OAM and space-based backhaul applications. Reported demonstrations include 1.25 Gbps FSO backhaul downlinks and OAM transmissions up to 100.8 Tbps, although the cited high-capacity OAM transmissions were limited to short distances.

  • Problem

    Atmospheric absorption, scattering, turbulence, cloud blockage, and related impairments limit the performance and reliability of ground-to-satellite, satellite-to-ground, and inter-satellite FSO links.

  • Method

    The paper provides a comprehensive survey of impairments and mitigation techniques spanning physical, link, network, and transport layers, including adaptive optics, coding, routing, and OAM approaches.

  • Results

    1.25 Gbps FSO backhaul downlink has been demonstrated experimentally, while OAM demonstrations reported 2.56 Tbps and 100.8 Tbps transmissions.

  • Takeaways & Limitations

    OAM offers substantial capacity potential and FSO can support relatively low-cost gigabit backhaul, but atmospheric mitigation remains necessary for space and near-Earth links.

  • Takeaways & Limitations

    High-capacity OAM transmissions reported in the survey were limited to short transmission distances.

Abstract

from arXiv · show

In recent years, free space optical communication has gained significant importance owing to its unique features: large bandwidth, license-free spectrum, high data rate, easy and quick deployability, less power and low mass requirements. FSO communication uses the optical carrier in the near infrared band to establish either terrestrial links within the Earth's atmosphere or inter-satellite or deep space links or ground-to-satellite or satellite-to-ground links. However, despite the great potential of FSO communication, its performance is limited by the adverse effects viz., absorption, scattering, and turbulence of the atmospheric channel. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of the paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. The latter part of the paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers i.e., link, network or transport layer to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of the optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high-capacity and low-cost backhaul solutions.

A. FSO Communication - An Overview

Optical wireless communication uses unguided optical carriers for indoor and outdoor links, including terrestrial, ground-to-satellite, satellite-to-ground, inter-satellite, and deep-space connections. The surveyed space links support high-capacity communication but face distinct uplink and downlink propagation conditions.

  • A. FSO Communication - An Overview: Optical wireless communication transfers modulated information through an unguided atmospheric or free-space channel using optical carriers.It offers high bandwidth, unlicensed spectrum, reduced power consumption, smaller equipment, and narrow-beam line-of-sight communication.
  • A. FSO Communication - An Overview: Outdoor optical wireless communication, termed free-space optical communication, includes terrestrial and space optical links.Space links include ground-to-satellite, satellite-to-ground, inter-satellite, and deep-space connections.
  • A. FSO Communication - An Overview: Space-based optical communication has advanced through theoretical studies, experiments, and missions demonstrating ground-to-satellite, satellite-to-satellite, and satellite-to-ground links.Examples include AFTS, LCLS, and OCD demonstrations, alongside earlier uplink and relay-mirror experiments.
  • A. FSO Communication - An Overview: Ground-to-satellite uplinks encounter atmospheric distortion and pointing instability immediately, whereas satellite-to-ground downlinks are dominated by geometric beam-divergence loss.The different propagation paths make uplink and downlink impairments distinct.

B. Advantages of FSO Communication over RF Communication

FSO communication offers optical bandwidth, narrow beam divergence, lower power and mass requirements, unlicensed spectrum, and improved security compared with RF systems. These benefits are offset by tight acquisition, tracking, and pointing requirements and dependence on atmospheric conditions.

  • B. Advantages of FSO Communication over RF Communication: 100 THz of allowable bandwidth at optical frequencies is nearly 10^5 times that of a typical RF carrier.The comparison follows from the much higher optical carrier frequency.
  • B. Advantages of FSO Communication over RF Communication: Optical beam divergence is narrower than RF divergence, increasing received signal intensity for a given transmitted power and enabling smaller antennas.A typical optical spacecraft system is 0.3 m versus 1.5 m for an RF spacecraft antenna.
  • B. Advantages of FSO Communication over RF Communication: FSO systems use an unlicensed optical spectrum, reducing initial setup cost and development time compared with spectrum-licensed RF systems.Optical beams also provide narrow directivity and make interception difficult.
  • B. Advantages of FSO Communication over RF Communication: FSO communication requires tight acquisition, tracking, and pointing and is vulnerable to unpredictable atmospheric conditions that degrade performance.These limitations accompany its narrow beam divergence and atmospheric propagation.

C. Choice of wavelength in FSO communication

Wavelength selection in FSO communication balances antenna gain, link quality, pointing-induced fading, receiver sensitivity, solar background, and safety constraints. Space applications generally favor longer wavelengths when these trade-offs improve link conditions.

  • C. Choice of wavelength in FSO communication: Lower wavelengths improve antenna gain, while higher wavelengths provide better link quality and lower pointing-induced signal fades.The operating wavelength therefore requires design optimization.
  • C. Choice of wavelength in FSO communication: The NIR range is 750 nm to 1450 nm, while 1530 nm to 1560 nm is a dominant SIR range for long-distance communication.These ranges are identified within the broader IR-A, IR-B, and IR-C classifications.
  • C. Choice of wavelength in FSO communication: Space-based optical wavelength selection trades receiver sensitivity against pointing bias caused by thermal variations across Earth’s surface.Longer wavelengths are generally preferred because they reduce solar background and surface solar scattering.
  • C. Choice of wavelength in FSO communication: FSO wavelengths must satisfy eye- and skin-safety constraints, with most systems using Class 1 and 1M lasers.For the same safety class, operation at 1500 nm permits higher transmitted power than shorter wavelengths.

D. Related Surveys

Earlier FSO surveys mainly addressed terrestrial links and physical-layer mitigation, leaving space-based links and higher-layer techniques less comprehensively covered. This paper surveys space-link challenges and mitigation across physical, link, network, and transport layers, including OAM and backhaul applications.

  • D. Related Surveys: Most earlier surveys focused on terrestrial FSO links, while relatively few addressed space-based optical links.The cited prior work emphasized terrestrial losses, transceivers, and communication-theory issues.
  • D. Related Surveys: This survey covers challenges in ground-to-satellite, satellite-to-ground, and inter-satellite optical communication links.It addresses both uplink and downlink space-link contexts.
  • D. Related Surveys: The paper reviews atmospheric mitigation techniques at the physical layer and at link, network, and transport layers.It identifies higher-layer mitigation as a distinctive aspect relative to surveys that mainly covered physical-layer methods.
  • D. Related Surveys: The survey also covers high-capacity backhaul links using HAPs or satellite-based networks and OAM-based techniques for deep-space and near-Earth optical communication.These topics extend the survey beyond atmospheric impairments alone.

E. Paper Organization

The paper organizes its discussion around major space-link challenges, acquisition and pointing, and mitigation techniques across physical and higher protocol layers.

  • Narrow optical beam divergence makes accurate acquisition and pointing challenging in space-based optical links.
  • The paper discusses major challenges for laser uplinks, downlinks, and space-based optical links.
  • It provides an exhaustive discussion of acquisition, tracking, and pointing for space-based optical links.
  • Mitigation techniques are covered at physical, link, network, and transport layers.

II. CHALLENGES IN SPACE-BASED OPTICAL COMMUNICATION

Space-based FSO links using the atmosphere face random, location- and weather-dependent channel conditions that can strongly attenuate signals and limit link distance.

  • Atmospheric properties vary randomly across space and time, making FSO communication dependent on weather and geographical location.
  • Clouds, snow, fog, rain, and haze can strongly attenuate optical signals and limit link distance.
  • Ground-to-satellite and satellite-to-ground FSO communications are subject to atmospheric effects during signal passage through the atmosphere.

A. FSO Uplink/Downlink

Space-based FSO uplinks and downlinks encounter wavelength-dependent absorption and scattering, turbulence, cloud blockage, pointing-related effects, and atmospheric seeing. Their impact varies with propagation direction, wavelength, weather, and receiver conditions.

  • FSO Uplink/Downlink: Uplink losses are much larger than downlink losses because distortion and beam spreading begin immediately after ground transmission.
  • Absorption and scattering: Atmospheric absorption depends on wavelength and involves interactions with water, carbon dioxide, ozone, gas molecules, and aerosols.
  • Absorption and scattering: Rayleigh scattering is prominent below 1 µm but can be neglected near-IR wavelengths, while Mie scattering occurs when particle size is comparable to wavelength.
  • Weather effects: Dense fog can produce more than 350 dB/km attenuation at visibility below 50 m, limiting FSO link availability.
  • Weather effects: Rain attenuation ranges from 1 dB/km to 10 dB/km for 2.5–25 mm/hr around 850 nm and 1500 nm.
  • Atmospheric turbulence: Turbulence-induced scintillation redistributes signal energy and produces temporal and spatial fluctuations in received irradiance.
  • Atmospheric turbulence: Higher-wavelength FSO links experience less turbulence impact because r0 varies as λ6/5.
  • Cloud blockage: Clouds can intermittently disrupt or completely block ground-to-satellite and satellite-to-ground line-of-sight communication for seconds to hours.

B. Inter-satellite Links

Inter-satellite optical links avoid atmospheric weather effects but face acquisition, tracking, pointing, Doppler, vibration, and noise challenges. Their long distances also favor power-efficient, highly sensitive coherent transmission schemes.

  • Inter-satellite links avoid weather and cloud outages, making acquisition and tracking the principal challenge as satellites move relative to one another.
  • Long inter-satellite distances favor homodyne or heterodyne transmission because these phase-coherent techniques provide high receiver sensitivity and capacity.
  • Point-ahead angle offsets the transmitted beam from the apparent beacon location to compensate for signal travel time across long cross-link distances.
  • Relative motion produces Doppler shift in inter-orbit links, requiring compensation for reliable data reception and frequency synchronization.
  • Optical phase-lock loops cooperatively tune the transmitter and local oscillator to acquire and track the heterodyne or homodyne frequency.
  • Satellite vibrations and onboard noise sources deviate the transmitted beam and cause misalignment between satellites.

III. ACQUISITION, TRACKING AND POINTING

Acquisition, tracking, and pointing establish and maintain alignment between space optical terminals despite narrow beam divergence, platform jitter, and tracking disturbances. The process progresses from beacon-based scanning and coarse detection to fine closed-loop steering.

  • Narrow beam divergence and platform jitter make pointing and focusing critical for ground-to-satellite, satellite-to-ground, and inter-satellite links.
  • Acquisition scans a narrow beacon across an uncertainty region, using sufficient peak power and low pulse rate for detection amid background radiation.
  • After beacon detection, beam-steering elements direct a steady beacon toward the initiating terminal, often with a fixed point-ahead-angle offset.
  • The satellite narrows its FOV, performs coarse tracking with a control loop, and then uses fine steering to keep the received signal bore-sighted.The transition from acquisition to tracking typically takes less than one second; the control-loop bandwidth should be ≥1KHz.
  • The steering error signal is the position difference between the onboard laser and ground beacon, and fine steering continues until this error is minimized.
  • Hybrid pointing architectures combine inertial sensors, celestial references, and uplink beacons, while beaconless designs can save power by removing the extra beacon.

IV. MITIGATION TECHNIQUES

Atmospheric-channel degradation reduces FSO reliability, so mitigation is applied at the physical layer and at network or transport layers. Reviewed approaches include optical, architectural, and protocol-level techniques.

  • Atmospheric-channel impairments degrade received-signal quality and BER, motivating mitigation techniques for reliable operation under varying weather conditions.
  • Physical-layer mitigation approaches include multiple-beam transmission, increased receiver FOV, adaptive optics, relay transmission, and hybrid RF/FSO links.
  • Mitigation can also be implemented at network or transport layers, including techniques reviewed beyond the physical link.

A. Physical Layer Methods

Physical-layer mitigation methods address atmospheric turbulence, fading, scintillation, jitter, background noise, and bandwidth–power trade-offs in space-based FSO links. The survey covers aperture averaging, diversity, adaptive optics, modulation, coding, and coherent detection approaches.

  • Aperture Averaging: Larger receiver apertures average fast fluctuations from small turbulent eddies, reducing channel fading and atmospheric scintillation.Aperture averaging is quantified by the aperture averaging factor A; increasing aperture diameter can improve BER performance.
  • Diversity and Relaying: Spatial and time diversity mitigate turbulence-induced fading by combining independent paths or transmitting symbols across different coherence periods.Convolutional codes suit weak turbulence, while Turbo-codes provide significant coding gain under strong turbulence.
  • Adaptive Optics: Conventional adaptive optics becomes difficult in very strong turbulence, motivating performance-optimized control using high-bandwidth MEMS deformable mirrors and efficient algorithms.Tip/tilt correction tracks the focused spot and compensates turbulence-induced angle-of-arrival fluctuations.
  • Modulation and Coding: Bandwidth-efficient PAM and QAM reduce power level, making them unsuitable for high turbulence or power-limited systems, whereas higher-order M-PPM improves robustness against background radiation.Multi-pulse PPM increases constellation size by placing multiple pulses among symbol slots.
  • Modulation and Coding: DHPIM uses predefined headers to initiate symbols, while 4-DPIM increases transmission capacity by eliminating unused time chips within each symbol.The 4-PPM and 4-DPIM mapping is presented in Table VI.
  • Coherent Detection: Phase-coherent homodyne and heterodyne techniques provide high receiver sensitivity and capacity for long-distance inter-satellite links, but require accurate phase alignment.Coherent receivers can also suppress background radiation, while phase alignment increases receiver complexity.

B. TCP Upper Layer Methods

Upper-layer mitigation methods improve FSO reliability by retransmitting lost packets, dynamically rerouting traffic, and enforcing quality-of-service requirements under adverse atmospheric or device conditions. These methods complement physical-layer techniques.

  • Cross-Layer Mitigation: Research on link, network, and transport-layer modeling and evaluation complements physical-layer methods for mitigating atmospheric effects in FSO communication.The survey identifies retransmission, rerouting, and cross-layer techniques as higher-layer mitigation approaches.
  • Retransmission: ARQ retransmits unacknowledged packets until acknowledgment or a preset counter limit is reached, but stop-and-wait and go-back-N schemes incur delay, energy, and bandwidth penalties.Selective repeat ARQ continuously transmits packets and retransmits only frames presumed lost.
  • Retransmission: Rateless Round Robin provides error control during very strong turbulence, including conditions where channel availability is less than 45%.The protocol is described as effective for practical FSO applications under significant outages.
  • Reconfiguration and Rerouting: Path reconfiguration and rerouting improve link availability and reliability during line-of-sight loss, adverse atmospheric conditions, or device failure.Traffic may be redirected through alternate optical links or lower-data-rate RF links using physical and logical control mechanisms.
  • Quality of Service Control: FSO quality of service is evaluated using data rate, latency, delay jitter, data loss, energy consumption, reliability, and throughput efficiency.Traffic must satisfy the requirements of its assigned QoS class for end users to receive acceptable service.

V. ORBITAL ANGULAR MOMENTUM FOR THE FSO SYSTEM

OAM multiplexing can substantially increase FSO capacity and spectral efficiency, but atmospheric turbulence redistributes energy among modes, causing time-varying crosstalk and fading. Coding, wavefront correction, and optical demultiplexing are reviewed as mitigation approaches.

  • OAM fundamentals: OAM encodes information through helical phase fronts, providing infinitely many possible states indexed by the topological charge l.Unlike spin angular momentum, OAM is associated with spatial wavefront structure and can support different integer values of l.
  • Capacity enhancement: Multiplexing beams with different OAM values, WDM, polarization multiplexing, and concentric rings increase FSO capacity and spectral efficiency.Reported demonstrations include 2.56 Tbps at 95.7 bps/Hz and 100.8 Tbps, although these transmissions used short distances without turbulence.
  • Atmospheric impairment: Atmospheric turbulence redistributes energy among OAM states, producing time-varying crosstalk, signal fading, and bursty errors.Adjacent multiplexed OAM channels can interfere, while the beam’s doughnut profile and phase fluctuations complicate practical implementation.
  • Mitigation techniques: Suitable error-correcting codes and wavefront-correction techniques reduce turbulence effects and can achieve satisfactory BER performance during strong turbulence.The reviewed approaches include Shack–Hartmann wavefront correction, holographic ghost imaging, and adaptive optics.
  • Receiver processing: At the receiver, OAM beams are demultiplexed and an inverse spiral phase mask removes the azimuthal phase before detection, demodulation, and decoding.The inverse mask uses charge (-l) to recover a plane phase front.

VI. FSO BACKHAUL COMMUNICATION

FSO backhaul links using HAPs, satellites, and ground stations are presented as rapidly deployable, high-capacity alternatives for locations lacking terrestrial connectivity. Demonstrations and analyses indicate feasibility across stratospheric and satellite-related links, while weather, turbulence, positioning, and laser nonlinearities remain operational concerns.

  • Backhaul motivation: FSO backhaul is pursued for gigabit-capacity connectivity because it offers lower cost and faster deployment than RF or optical-fiber alternatives.The survey considers links among HAPs, satellites, and ground receivers.
  • HAP architecture: HAPs at 17–25 km can provide broadband links with large coverage, rapid deployment, flexible capacity, and lower atmospheric impact than ground-level links.Their cloud-free stratospheric placement supports links among HAPs, aircraft, satellites, and ground systems.
  • Experimental demonstrations: The CAPANINA STROPEX experiment demonstrated a high-bit-rate optical downlink from a FELT mounted at approximately 22 km to a transportable optical ground station.The terminal transmitted at multiple rates, including 270 Mbps and 622 Mbps.
  • Link feasibility: A theoretical HAP-to-HAP link at 20 km was feasible up to almost 600 km, with BER 10^-6 at 384 Mbps using 800 mW transmit power.The analysis used intensity modulation at 1550 nm.
  • Operational constraints: Backhaul reliability is affected by atmospheric turbulence, weather, stratospheric winds, HAP positioning, and laser nonlinear distortion in OFDM systems.Large-dynamic-range amplifiers or signal clipping can reduce OFDM inter-modulation distortion.

VII. FUTURE SCOPE

Future applications emphasize FSO’s role in expanding high-capacity connectivity across terrestrial, aerial, satellite, and deep-space environments. Demonstrated missions and proposed platforms show opportunities for remote access, mobile operations, and interplanetary communication.

  • Deployment prospects: FSO is positioned for continued growth because it combines rapid deployment, low capital expenditure, and license-free operation.The survey notes that commercial FSO products are already available.
  • Remote connectivity: Aerial FSO platforms can connect remote or sensitive areas where physical access to conventional 3G or 4G infrastructure is difficult.Proposed uses include solar-powered drones for suburban regions and satellites for sparsely populated areas.
  • Mobile and defense applications: FSO mobile platforms can support battlefield communications and real-time dissemination of large image and video datasets for ISR applications.Near-Earth spacecraft and drones are also identified for high-resolution Earth observation.
  • Deep-space communication: NASA’s LLCD demonstrated 20 Mbps uplink and 622 Mbps downlink communication between the Moon and Earth.The uplink supported high-density video and the mission renewed interest in deep-space optical communication.
  • Deep-space requirements: Future deep-space optical links require specialized systems including kilowatt-class lasers, photon-counting detectors, larger apertures, point-ahead angles, and stable beam pointing.The LCRD project is described as a long-duration space-based optical communication mission.

VIII. CONCLUSIONS

The survey frames FSO as a high-capacity alternative to RF whose practical performance is constrained by atmospheric impairments. It reviews mitigation across protocol layers and identifies OAM and optical backhaul as promising directions for deep-space, near-Earth, and broadband connectivity.

  • Motivation: Growing multimedia and internet traffic increases pressure on low-data-rate RF systems and motivates a shift toward optical communication.FSO offers line-of-sight wireless links with very high bandwidth and capacity.
  • Challenges: Absorption, scattering, turbulence, and adverse weather limit FSO performance across atmospheric channels.The paper identifies these effects as challenges to fully exploiting the system’s terabit capacity.
  • Mitigation scope: Mitigation techniques are reviewed at the physical, link, network, and transport layers to combat atmospheric degradation.The survey specifically emphasizes mitigation beyond the physical layer.
  • OAM and capacity: OAM-based FSO is identified as a candidate for deep-space and near-Earth links when combined with coding, modulation, or adaptive optics.These techniques address atmospheric turbulence affecting OAM transmission.
  • Backhaul conclusions: FSO backhaul has demonstrated downlinks up to 1.25 Gbps, while theoretical studies indicate feasibility up to 10 Gbps for HAP-HAP or HAP-satellite links.The survey presents these results as evidence of high-capacity backhaul potential.
  • Outlook: The paper concludes that FSO has high near-term growth prospects, with commercial link products already available.This conclusion is stated alongside the survey’s review of advances in FSO communication.
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