Experimental Characterisation and Modelling of Atmospheric Fog and Turbulence in FSO

Free space optical (FSO) communication uses visible or infrared (IR) wavelengths to broadcast high-speed data wirelessly through the atmospheric channel. The performance of FSO communications is mainly dependent on the unpredictable atmospheric channel such as fog, smoke and temperature dependent turbulence. However, as the real outdoor atmosphere (ROA) is time varying and heterogeneous in nature as well as depending on the magnitude and intensity of different weather conditions, carrying out a proper link assessment under specific weather conditions becomes a challenging task. Investigation and modelling the ROA under diverse atmospheric conditions is still a great challenge in FSO communications. Hence a dedicated indoor atmospheric chamber is designed and built to produce controlled atmosphere as necessary to mimic the ROA as closely as possible. The experimental results indicate that the fog attenuation is wavelength dependent for all visibility V ranges, which contradicts the Kim model for V < 0.5 km. The obtained result validates that Kim model needs to be revised for V < 0.5 km in order to correctly predict the wavelength dependent fog attenuation. Also, there are no experimental data and empirical model available for FSO links in diverse smoke conditions, which are common in urban areas. Therefore, a new empirical model is proposed to evaluate the wavelength dependent fog and smoke attenuation by reconsidering the q value as a function of wavelength rather than visibility. The BER performance of an FSO system is theoretically and experimentally evaluated for OOK- NRZ, OOK-RZ and 4-PPM formats for Ethernet line data-rates from light to dense fog conditions. A BER of 10-6 (Q-factor ≈ 4.7) is achieved at dense fog (transmittance, T = 0.33) condition using 4-PPM than OOK-NRZ and OOK-RZ modulation schemes due to its high peak-to-average power ratio albeit at the expense of doubling the bandwidth. The effects of fog on OOK-NRZ, 4-PAM and BPSK are also experimentally investigated. In comparison to 4-PAM and OOK-NRZ signals, the BPSK modulation signalling format is more robust against the effects of fog. Moreover, the effects of using different average transmitted optical communication powers Popton the T and the received Q-factor using the OOK-NRZ modulation scheme are also investigated for light and dense fog conditions. The results show that for an FSO system operating at a Q-factor of 4.7 (for BER = 10-6), the required Q-factor is achieved at T of 48% under the thick fog condition by increasing Popt to 1.07 dBm, whereas the values of T are 55% and ~70% for the transmit power of 0.56 dBm and -0.7 dBm, respectively. The experimental characterisation and investigation of the atmospheric turbulence effect on the Ethernet and Fast-Ethernet FSO link is reported using different modulation schemes. The experiment is carried out in a controlled laboratory environment where turbulence is generated in a dedicated indoor atmospheric chamber. The atmospheric chamber is calibrated to mimic an outdoor turbulence conditions and the measured data are verified against the theoretical predictions. The experiment also demonstrates methods to control the turbulence levels and determine the equivalence between the indoor and outdoor FSO links. The results show that the connectivity of Ethernet and Fast-Ethernet links are highly sensitive to atmospheric turbulence. The results also show that the BPSK and OOK-NRZ modulation signalling formats are more robust against the weak atmospheric turbulence conditions than PAM signal

Experimental study of bit error rate of free space optics communications in laboratory controlled turbulence

This paper reports experimental results for the performance of an free space optical (FSO) communication link employing different modulation schemes under the influence of the atmospheric scintillation. A dedicated experimental atmospheric simulation chamber has been developed where weak and medium turbulence can be generated and its effect on the FSO link is investigated. The experimental data obtained is compared to the theoretical prediction. The paper also shows that the effect on the data transmission performance depends on the position of turbulence source positioned within the chamber

Route diversity analyses for free-space optical wireless links within turbulent scenarios

Free-Space Optical (FSO) communications link performance is highly affected when propagating through the time-spatially variable turbulent environment. In order to improve signal reception, several mitigation techniques have been proposed and analytically investigated. This paper presents experimental results for the route diversity technique evaluations for a specific case when several diversity links intersects a common turbulent area and concurrently each passing regions with different turbulence flows

Implementation and Evaluation of a Gigabit Ethernet FSO Link for 'The Last Metre and Last Mile Access Network'

In this paper, we propose a simple and a low cost 1 Gbps Ethernet free-space optical (FSO) communications link, which can be used both for the last meter and last mile access networks. In the emerging fifth generation wireless systems, which require at least an order of magnitude increase in the peak data rates and three orders of magnitude increase in network capacity with reduced latency, deploying multiple technologies will play a crucial role to meet these requirements. One possible complementary wireless technology to the radio frequency is the unlicensed FSO, which can bridge the gap between the existing RF wireless and optical fibre communication networks by providing high data rates, low installation costs and high energy efficiency. In this work, we propose a high-speed FSO system, which can be readily implemented using off the shelf components, and assess its performance experimentally under turbulence and fog conditions using the dedicated indoor atmospheric chamber. We show that, the proposed system under the turbulence condition with a scintillation index of 0.43 offers almost the same data rate (i.e., ∼99%) as the link under a clear channel, while the packeterror-rate reduces from 10−3 to 2 × 10−2

Combined effect of turbulence and aerosol on free-space optical links

[EN] Despite the benefits of free-space optical (FSO) communications, their full utilization is limited by the influence of atmospheric weather conditions, such as fog, turbulence, smoke, snow, etc. In urban environments, additional environmental factors such as smog and dust particles due to air pollution caused by industry and motor vehicles may affect FSO link performance, which has not been investigated in detail yet. Both smog and dust particles cause absorption and scattering of the propagating optical signal, thus resulting in high attenuation. This work investigates the joint impact of atmospheric turbulence and dust particle-imposed scattering on FSO link performance as part of the last-mile access network in urban areas. Propagation of an optical wave is at first analyzed based on the microphysic approach, and the extinction caused by small particles is determined. An experimental measurement campaign using a dedicated test chamber is carried out to assess FSO link performance operating wavelengths of 670 nm and 830 nm and under dust and turbulent conditions. The measured attenuation and the &#119876;Q factor in terms of the velocity of particle flow and turbulence strength are analyzed. We show that for an airflow of 2 m/s, the &#119876;Q factor is almost 3.5 higher at the wavelength of 830 nm than at 670 nm. However, for a wavelength of 670 nm, the FSO link is less affected by the increase in airflow compared to 830 nm. The &#119876; factor reduces with turbulence. Under similar turbulence conditions, for ash particles, the &#119876;Q factor is higher than that of sand particles.European Social Fund (ESF) (CZ.1.07/2.3.00/30.0034); Ministerio de Economia y Competitividad (MINECO) (JCI-2012-14805); European Cooperation in Science and Technology (COST) (IC 1101); Ceske Vysoke Uceni Technicke v Praze (CVUT) (SGS14/190/OHK3/3T/13).Libich, J.; Perez, J.; Zvanovec, S.; Ghassemlooy, Z.; Nebuloni, R.; Capsoni, C. (2017). Combined effect of turbulence and aerosol on free-space optical links. Applied Optics. 56(2):336-341. https://doi.org/10.1364/AO.56.000336S336341562Khalighi, M. A., & Uysal, M. (2014). Survey on Free Space Optical Communication: A Communication Theory Perspective. IEEE Communications Surveys & Tutorials, 16(4), 2231-2258. doi:10.1109/comst.2014.2329501Wang, C.-X., Haider, F., Gao, X., You, X.-H., Yang, Y., Yuan, D., … Hepsaydir, E. (2014). Cellular architecture and key technologies for 5G wireless communication networks. IEEE Communications Magazine, 52(2), 122-130. doi:10.1109/mcom.2014.6736752Parca, G. (2013). Optical wireless transmission at 1.6-Tbit/s (16×100  Gbit/s) for next-generation convergent urban infrastructures. Optical Engineering, 52(11), 116102. doi:10.1117/1.oe.52.11.116102Kedar, D., & Arnon, S. (2004). Urban optical wireless communication networks: the main challenges and possible solutions. IEEE Communications Magazine, 42(5), S2-S7. doi:10.1109/mcom.2004.1299334Awan, M. S., Horwath, L. C., Muhammad, S. S., Leitgeb, E., Nadeem, F., & Khan, M. S. (2009). Characterization of Fog and Snow Attenuations for Free-Space Optical Propagation. Journal of Communications, 4(8). doi:10.4304/jcm.4.8.533-545Nauerth, S., Moll, F., Rau, M., Fuchs, C., Horwath, J., Frick, S., & Weinfurter, H. (2013). Air-to-ground quantum communication. Nature Photonics, 7(5), 382-386. doi:10.1038/nphoton.2013.46Perez, J., Zvanovec, S., Ghassemlooy, Z., & Popoola, W. O. (2014). Experimental characterization and mitigation of turbulence induced signal fades within an ad hoc FSO network. Optics Express, 22(3), 3208. doi:10.1364/oe.22.003208Kim, I. I., McArthur, B., & Korevaar, E. J. (2001). Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications. Optical Wireless Communications III. doi:10.1117/12.417512Rekab-Eslami, M., Esmaeili, M., & Aaron Gulliver, T. (2017). Generic Linear Network Code Construction Using Transversal Matroids. IEEE Communications Letters, 21(3), 448-451. doi:10.1109/lcomm.2016.2619706Corrsin, S. (1951). On the Spectrum of Isotropic Temperature Fluctuations in an Isotropic Turbulence. Journal of Applied Physics, 22(4), 469-473. doi:10.1063/1.1699986Ghassemlooy, Z., Le Minh, H., Rajbhandari, S., Perez, J., & Ijaz, M. (2012). Performance Analysis of Ethernet/Fast-Ethernet Free Space Optical Communications in a Controlled Weak Turbulence Condition. Journal of Lightwave Technology, 30(13), 2188-2194. doi:10.1109/jlt.2012.2194271Clifford, S. F., Ochs, G. R., & Lawrence, R. S. (1974). Saturation of optical scintillation by strong turbulence*. Journal of the Optical Society of America, 64(2), 148. doi:10.1364/josa.64.00014

Experimental Error Performance of Modulation Schemes Under a Controlled Laboratory Turbulence FSO Channel

This paper experimentally investigates the performance of different modulation schemes under the atmospheric turbulence conditions for free space optical communication links. The experiments were carried out in a dedicated and controlled indoor atmospheric chamber. The turbulence environment was created by introducing hot air, while the temperature profile was monitored throughout the chamber to maintain a constant environment. By evaluating the error performance of different modulation schemes under identical conditions, it was observed that pulse position modulation offers the best performance, followed by subcarrier intensity modulation under weak turbulence environments

Experimental characterization and mitigation of turbulence induced signal fades within an ad hoc FSO network

Optical beams propagating through the turbulent atmospheric channel suffer from both the attenuation and phase distortion. Since future wireless networks are envisaged to be deployed in the ad hoc mesh topology, this paper presents the experimental laboratory characterization of mitigation of turbulence induced signal fades for two ad hoc scenarios. Results from measurements of the thermal structure constant along the propagation channels, changes of the coherence lengths for different turbulence regimes and the eye diagrams for partially correlated turbulences in free space optical channels are discussed. Based on these results future deployment of optical ad hoc networks can be more straightforwardly planned

Telecommunication Systems

This book is based on both industrial and academic research efforts in which a number of recent advancements and rare insights into telecommunication systems are well presented. The volume is organized into four parts: "Telecommunication Protocol, Optimization, and Security Frameworks", "Next-Generation Optical Access Technologies", "Convergence of Wireless-Optical Networks" and "Advanced Relay and Antenna Systems for Smart Networks." Chapters within these parts are self-contained and cross-referenced to facilitate further study