1. On The Study of the FSO Link Performance under Controlled Turbulence and Fog Atmospheric Conditions
S. Rajbhandari, Z. Ghassemlooy, J. Perez, H. Le Minh, M. Ijaz,
E. Leitgeb, G. Kandus and V. Kvicera
Optical Communications Research Group,
School of Computing, Engineering and Information Sciences, Northumbria University, UK

2. Outline Design of Chamber Experiment Set up for Controlled FSO Channel
Results and Discussions
Conclusion and Future Work
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3. Motivations: Laboratory Chamber
Dedicated laboratory prototype chamber to simulate and demonstrate the atmospheric effects on the FSO link in a control environment.
Chamber enables the study of a wide range of atmospheric effects, for a range of wavelengths and modulation schemes.
The channel condition can be changes to accurately mimic the atmospheric episodes in real environment.
Reduces the long term observation time of outdoor links.
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4. Indoor Chamber Set Up Transmitter
Laser diode + Arbitrary waveform generator (AWG)
On‑off‑keying (OOK), pulse amplitude modulation (PAM) and subcarrier intensity modulation (SIM) based on binary phase shift keying (BPSK)
Channel
FOG, turbulence episodes replicated
Fog machine, hot plates, ventilation controlled
Receiver
Optical telescope + photodetector + transimpedance amplifier (TIA)
Post‑processing of the raw data acquired from TIA using Matlab
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5. Indoor Chamber Set Up Transmitter front end (Laser + AWG)
Receiver front end (photodiode + lens)
Operating wavelength
830 nm
Lens focal length
10 cm
Average optical output power
10 mW
Lens diameter
30 mm
Laser modulation depth, m
20%
Photodetector responsivity nm
Laser 3-dB bandwidth
50 MHz
PD active area
1 mm2
PRBS length
210‑1 bits
PD half angle view
± 75º
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6. FSO - Challenges λ= 0.01 - 0.05 mm Effects Options Remarks
Using Mie scattering to predict fog attenuations
m and r are the refractive index and radius of the fog droplets, respectively.
Qext is the extinction efficiency
n(r) is the modified gamma size distribution of the fog droplets.
[1/km]
Effects
Options
Remarks
Mie scattering
Photon absorption
Increase transmit optical power
Hybrid FSO/RF
Thick fog limits link range to ~500 m
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7. Experimental Evaluation: Fog
The transmittance and Q-factor under different fog densities are measured.
Fog is characterized by the transmittance T and hence the visibility V is calculated from fog the attenuation using Kim model.
βλ = attenuation or the scattering coefficient due to fog , z is the propagation length and I(f) and I(0) are average received optical intensities in the presence and absence of fog, respectively.
Link Margin
M = Ptx + Spd ‑ Lfog ‑ Lgeo ‑ Ldev
Spd: photodetector sensitivity
Ptx: Total optical power transmitted
Lgeo : Geometrical loss
Lfog : Attenuation due to the fog
Ldev : Transceiver losses
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8. Experimental Evaluation: Fog
Normalized same Ptx , 5 Mbit/s
For 4‑PAM and BPSK the Q‑factor is normalized indicates that at T = 1 are equal to OOK‑NRZ Q-factor
BPSK and OOK‑NRZ more robust to fog impairments than 4‑PAM.
BPSK has an optimum behaviour under fog conditions
Dense fog conditions:
4‑PAM not achieve BER values lower than 10‑6
OOK‑NRZ and BPSK schemes overcome that values for T > 0.2
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9. Atmospheric Turbulence
Air temperature and density variations thermal gradients
 randomly distributed cells with different reflective index.
Turbulence cell size: 10 cm - 1 km.
Cause scattering, multipath variation of the arriving signal.
Refractive index is highly dependent on the small scale temperature fluctuations in air defined by
where no is mean index of refraction (no = 1), n1(R,t) is the random deviation of index from its mean value, R is the vector position in three dimension and t is the time.
Transmitter
Receiver
Transmitter
Receiver
Refraction and diffraction of beam
n(R,t) = no + n1(R,t)
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10. FSO Experimental: Turbulence
Scintillation index is calculated using
@ 5 Mbit/s
Absolute Q-factor
normalized to
OOK-NRZ Q-factor
The Q‑factor decreases linearly with the logarithmic scale of Rytov variance
BPSK Q‑factor
decrease less sharply
offer much improved performance compared to OOK and PAM
~6.5 times higher Q-factor than OOK and ~ 16 times higher than that of 4‑PAM
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11. FSO Experimental: Turbulence
without turbulence
20 Mbit/s
The expected received signal level for ‘1’ and ‘0’ for OOK are well separated
The Euclidean distance 4‑PAM is reduced by 67% and hence the small turbulence variance can cause signal overlapping (d)
4‑PAM
OOK
Rytov variance of 0.005
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12. Conclusion
A test-bed within a controlled laboratory environment is develop to mimic atmospheric condition.
Comparative study of different modulation schemes under the effect of fog and turbulence were carried out.
BPSK and OOK‑NRZ modulation signalling format are more robust to fog and turbulence impairments in comparison with 4-PAM.
BPSK show significantly higher resilience to turbulence and offers up to 16 times higher Q‑factor than 4-PAM at scintillation index of ~0.2 .
BPSK signalling increase resilience under different environmental conditions, nevertheless implies a higher receiver complexity.
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13. Thank You Prof. Z. Ghassemlooy OCRG, CEIS Northumbria University, UK
@NorthumbriaCEIS
/NorthumbriaCEIS
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