PPT mainly talks about Free Space Optics (FSO) Impairments. Highlighting the understanding factors affecting FSO communication performance.

1. Free Space Optics (FSO)
Impairments
• Understanding Factors Affecting FSO
Communication Performance
• Presented by: Tarun Dhingra
• SID : 22105070

2. Introduction to Free Space
Optics (FSO)
• FSO transmits data using light through free space
instead of optical fiber.
• Operates in infrared or visible light spectrum (780–1550
nm).
• Offers high bandwidth, license-free spectrum, and fast
deployment.
• Performance depends heavily on atmospheric conditions.

3. What are FSO Impairments?
Impairments degrade signal strength or quality
during transmission.
Classified into:
1)Atmospheric impairments
2)Geometric & alignment impairments
3)System-related impairments
It is the key for designing reliable FSO links.

4. Classification of Impairments
• Atmospheric: Fog, Rain, Snow, Scintillation,
Turbulence
• Geometric: Pointing error, Beam divergence,
Misalignment
• System-related: Optical power loss, Detector noise,
Background light

5. Atmospheric Attenuation
Loss of optical signal power as light passes through the
atmosphere due to absorption and scattering by gases,
fog, dust, and other particles.
Causes:
• Absorption: Energy loss by gas molecules (H O, CO ,
₂ ₂
O ).
₃
• Scattering: Deflection of light by particles
1)Rayleigh: by small molecules (<0.1 µm)
2)Mie: by fog and aerosols (≈ wavelength)
3)Non-selective: by large particles (rain, snow)
• Weather Conditions: Fog, rain, and snow cause major
attenuation.
Equation (Beer–Lambert Law): = _0 ^(− )
𝐼 𝐼 𝑒 𝛾𝐿

6. Atmospheric Attenuation
• Effects on FSO Links:
1) Reduced received power
2) Lower Signal-to-Noise Ratio (SNR)
3) Higher Bit Error Rate (BER)
4) Link outage during severe fog or rain
• Mitigation Techniques:
1) Use 1550 nm wavelength
2) Keep shorter link distances
3) Implement adaptive power control
4) Use predictive weather monitoring

7. Fog, Rain and Snow
Attenuation
• Major limiting factor for FSO systems.
• Causes high signal loss (up to 300 dB/km).
• Fog: Tiny droplets (1–20 µm) cause Mie scattering
→ highest attenuation.
• Rain: Large drops (0.5–5 mm) cause moderate
scattering (5–20 dB/km).
• Snow: Depends on type — wet snow > dry snow in
attenuation.

8. Fog, Rain and Snow
Attenuation
• Fog dominates in urban areas, causes frequent
outages.
• Rain & snow degrade visibility and reduce
received signal.
• Mitigation:
• Maintain shorter link range.
• Use diversity paths or hybrid FSO-RF.
• Prefer 1550 nm band for better penetration.

9. Atmospheric Turbulence
• Caused by temperature and pressure variations →
random refractive index changes.
• Effects:
• Beam wander (lateral movement)
• Beam spreading (widening)
• Scintillation (intensity flickering)
• Measured using Scintillation Index (σ²).

10. Atmospheric Turbulence
• Leads to fluctuating received signal, reduced link
stability.
• Severe at long distances and daytime heat.
• Mitigation:
• Aperture averaging (large receiver lens)
• Adaptive optics
• Spatial diversity (multiple beams)

11. Pointing Errors
• Occur when transmitter and receiver are
misaligned due to:
• Building sway
• Vibration
• Wind or thermal expansion
• Even small angular errors → major signal loss due
to narrow beam width.

12. Pointing Errors
• Effects: Power loss, intermittent link, or complete
outage.
• Mitigation:
• Auto-tracking systems for real-time correction.
• Beam expanders to increase tolerance.
• Mechanical stabilization on rooftops or towers.

13. Beam Divergence & Geometric
Loss
• Optical beams diverge naturally with distance (diffraction).
• Larger divergence → lower received power.
• Geometric loss (receiver diameter / total beam
∝
diameter)².
• Excess divergence causes power spread → weak received
signal.
• Too narrow beam → harder to align.
• Mitigation:
• Optimize beam divergence for link distance.
• Use large receiver aperture.
• Maintain stable mounting and alignment.

14. Background Light and Optical
Noise
• Caused by sunlight, city lights, vehicle lights, etc.
• Adds unwanted optical noise to detector → reduces
SNR.
• Strongest during daytime or near bright sources.
• Effects: increased Bit Error Rate (BER), signal
distortion.
• Mitigation:
• Use narrow-band optical filters.
• Operate at 1550 nm (infrared) — minimal interference.
• Use synchronous detection or modulation coding.

15. Combined Effects on Link
Performance
• Multiple impairments in real-world FSO systems act
simultaneously — fog, turbulence, and misalignment often
occur together, amplifying each other’s effects. The result
is much greater overall degradation than any single factor
alone.
• Scattering and turbulence combine to cause deep signal
fading, as fog reduces light intensity while turbulence
makes it fluctuate randomly, lowering the Signal-to-Noise
Ratio (SNR) and increasing the Bit Error Rate (BER).
• Link availability drops significantly during combined
adverse conditions.
• Under heavy fog or rain, attenuation may exceed 200–300
dB/km, reducing system uptime from 99.9% to below 80%,
especially on longer-distance links.

16. Mitigation Techniques
Summary
• Fog/Rain: Shorter links, multiple paths
• Turbulence: Aperture averaging, adaptive optics
• Pointing Error: Auto-alignment, tracking
• Background Noise: Filtering, wavelength selection
• Geometric Loss: Optimal divergence angle

17. Conclusion
• Free Space Optics (FSO) offers a high-speed,
license-free, and secure communication alternative
to fiber optics, but its performance is strongly
affected by environmental and geometric
impairments.
• Atmospheric conditions such as fog, rain, and
turbulence are the most critical challenges, as they
cause severe attenuation, scintillation, and signal
fading that limit link distance and reliability.
• Adaptive system design is essential for achieving
reliable performance.
• Techniques like adaptive power control, error
correction coding, and real-time link monitoring
help maintain stable communication even under
changing atmospheric conditions.