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 http://soe.northumbria.ac.uk/ocr/ z.ghassemlooy@northumbria.ac.uk

Outline Design of Chamber Experiment Set up for Controlled FSO Channel Results and Discussions Conclusion and Future Work Contel 2011

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. Contel 2011

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 Contel 2011

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 0.59 A/W @ 830 nm Laser 3-dB bandwidth 50 MHz PD active area 1 mm2 PRBS length 210‑1 bits PD half angle view ± 75º Contel 2011

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 Contel 2011 6

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 Contel 2011

Experimental Evaluation: Fog Normalized Q‑factor @ 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 Contel 2011

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) Contel 2011

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 Contel 2011

FSO Experimental: Turbulence without turbulence Histograms @ 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 Contel 2011

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. Contel 2011

Thank You Prof. Z. Ghassemlooy OCRG, CEIS Northumbria University, UK @NorthumbriaCEIS /NorthumbriaCEIS Contel 2011