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Optical Wireless Communications

The proliferation of wireless communication systems can be considered as one of the most remarkable changes in human life over the past thirty years. Wireless devices and technologies have become pervasive very quickly and are among the most integral elements of modern society. The RF band of the electromagnetic spectrum is fundamentally limited in capacity due to the fact that most of the sub-bands are exclusively licensed and strictly regulated by the local and international authorities today. While very high data-rate wireless systems are receiving an ever-growing popularity, the demand for RF spectrum is outstripping the available resources. This has appealed for new technologies in order to use the upper parts of the electromagnetic spectrum. One promising approach is that of optical wireless communications, which can enable wireless connectivity using infrared, visible, or ultraviolet bands. It has powerful features such as high bandwidth, low cost, robustness to electromagnetic interference, operation in an unregulated spectrum, high degree of spatial confinement, and inherent physical security.

FSO links for point-to-point Inter-building connections

Our main research activities have been focusing on three related technologies : outdoor free-space optical communications (FSO), indoor visible-light communications (VLC), and underwater wireless optical communications (UWOC).

A. Free-Space Optical Systems

FSO is a relatively mature technology, operating in near IR band and is typically used for high rate communication between two fixed points over distances up to several kilometers. FSO systems are today used for a wide range of applications such as metropolitan area network extension, LAN-to-LAN connectivity, fiber backup, backhaul for wireless cellular networks, disaster recovery, high definition TV transmission, etc. [A1].

A.1. Our Research and Main Contributions :

We have been working since about 2005 on different aspects of FSO systems mainly for mainly concerned fixed point-to-point links. The main related topics are :
• Atmospheric Turbulence : Fading Statistical Modeling and Mitigating :
o Comprehensive study of modeling and characterization of the atmospheric channel, in particular for the case of space-diversity FSO systems [A2],
o Study of the link performance under realistic propagation conditions including correlated fading [A3].
• Channel coding for FSO links :
o Efficacy of time diversity [A4],
o Coding for non-binary modulations [A5].
• Differential signaling to mitigate background radiations and turbulence effect [A6].
• Cooperative (relay assisted) FSO links using all-optical relays [A7].
• Optimizing transmitter beam parameters to reduce pointing error effect.

Schematic of a (1x3) FSO system and phase screen simulations illustrating turbulence-induced fading, moderate turbulence regime.

A.2. Collaborations

• Northumbria University, Newcastle, UK
• Czech Technical University in Prague, Czech Republic
• Özyegin University, Istanbul, Turkey
• Beheshti University, Tehran, Iran
• Indian Institute of Technology, Delhi, India

A.3. Selected Publications :

[A1] M.A. Khalighi, M. Uysal, “Survey on Free Space Optical Communication : A Communication Theory Perspective,” IEEE Communications Surveys & Tutorials, Vol. 16, No.8, pp. 2231–2258, Nov. 2014.
[A2] M.A. Khalighi, N. Schwartz, N. Aitamer, S. Bourennane, "Fading Reduction by Aperture Averaging and Spatial Diversity in Optical Wireless Systems," IEEE/OSA Journal of Optical Communications and Networking (JOCN), vol.1, no.6, Nov. 2009, pp. 580-593
[A3] G. Yang, M. A. Khalighi, S. Bourennane, Z. Ghassemlooy, “Fading correlation and analytical performance evaluation of the space-diversity free-space optical communications system,” IOP Journal of Optics, vol. 16, no. 3, pp. 1–10, Feb. 2014
[A4] F. Xu, M.A. Khalighi, P. Caussé, S. Bourennane, “Channel coding and time-diversity for optical wireless links,” Optics Express, vol. 17, no. 2, pp. 872–887, Jan. 2009
[A5] F. Xu, M.A. Khalighi, S. Bourennane, "Coded PPM and Multipulse PPM and Iterative Detection for Free-Space Optical Links," IEEE/OSA Journal of Optical Communications and Networking (JOCN), vol. 1, no. 5, Oct. 2009, pp. 404-415
[A6] M. R. Bhatnagar, Z. Ghassemlooy, S. Zvanovec, M. A. Khalighi, M. Mansour Abadi, "Quantized Feedback Based Differential Signaling for Free-Space Optical Communication System," IEEE Transactions on Communications, Vol. 64, No. 12, Dec. 2016, pp. 5176-5188
[A7] N. A. M. Nor, Z. Ghassemlooy, S. Zvánovec, M.A. Khalighi, M. R. Bhatnagar, J. Bohata, M. Komanec, "Experimental Analysis of a Triple-Hop Relay-Assisted FSO System with Turbulence," Elsevier Journal of Optical Switching and Networking, accepted, article in Press

B. Visible-Light Communication Systems

Another interesting application area of OWC is in indoor environments where it is estimated that more than 70% of the wireless traffic takes place. In its most classical form, the VLC technology aims at using the incoherent solid-state lighting elements such as LEDs for data transmission and promises very high data rates up to several Gbps. By exploiting the illumination facilities for data transmission, it provides a unique opportunity to avoid RF radiations in restrictive areas (such as hospitals or within aircrafts) and at the same time allows a significant reduction in power consumption of the users ; it is hence regarded as an example of “green communication.”

Illustration of VLC (Li-Fi) for high-speed indoor wireless access.

B.1. Our Research and Main Contributions :

One of the challenges in the development of commercial VLC systems is the slow modulation response of commercial white LEDs, which is due to the slow response time of the phosphorous layer in these components. This limits the modulation bandwidth of the device to a few MHz (or around 20 MHz in the case of using a blue filter at the receiver) [B1]. We have been working on spectrally efficient modulation schemes, namely pulse amplitude modulation (PAM) and carrier-less amplitude and phase (CAP) modulation, allowing high data rate with simplified hardware at the transmitter side, while keeping the peak-to-average power ratio (PAPR) quite low. Another issue has been to comprehend the limitations arising from the optical channel, and in particular, its possible frequency selectivity. This latter can be due to multipath reflections in the case of LOS blockage, or (as we have demonstrated) due to the asymmetry between the multiple LOS paths corresponding to the multiple LED emitters.

Block diagram of VLC link employing CAP signaling with frequency-domain equalization.

Our main contributions are summarized in the following.

• Channel modeling for multiple transmitter scenarios [B2] :
o Main factors affecting channel delay dispersion,
o Influence of the receiver filter.
• PAM and CAP modulation with frequency-domain equalization at the receiver [B3].
• Optimization of transmitter-receiver optics [B4].
• VLC-based Indoor positioning.

B.2. Collaborations

• Northumbria University, Newcastle, UK
• Czech Technical University in Prague, Czech Republic
• Ilmenau University, Ilmenau, Germany
• Instituto de Telecomunicações, Aveiro, Portugal
• OLEDCOMM Co., France

We are currently coordinating an exciting H2020 ITN project on the VLC technology : VisIoN (Visible-light communications for Interoperability and Networking), launched in September 2017. Different applications of VLC are under investigation, including VLC for smart-home, smart-city, smart-transportation, and for industrial and medical applications. VisIoN brings together an exceptional consortium, composed of the most prominent academic and industrial institutions involved in R&D in the VLC domain, i.e., Ecole Centrale Marseille (Institut Fresnel), Northumbria University, Özyegin University, Czech Technical University, University of Las Palmas Gran Canaria, Institute of Telecommunications in Aveiro, Fraunhofer Heinrich Hertz Institute, OSRAM, FORD Otosan Co., and OLEDCOMM Co.

More information : http://www.fresnel.fr/spip/spip.php?article2037&lang=en

B.3. Selected Publications :

[B1] Z. Ghassemlooy, L. N. Alves, S. Zvánovec, M. A. Khalighi, Editors, VISIBLE LIGHT COMMUNICATIONS, THEORY AND APPLICATIONS, CRC Press, 2017.
[B2] S. Long, M.A. Khalighi, M. Wolf, Z. Ghassemlooy, S. Bourennane, “Investigating Channel Frequency Selectivity in Indoor Visible Light Communication Systems,” IET Optoelectronics, vol. 10, no.3, May 2016, pp.80-88
[B3] M. A. Khalighi, S. Long, S. Bourennane, Z. Ghassemlooy, “PAM and CAP-based Transmission Schemes for Visible-Light Communications”, IEEE Access Journal, Special issue on Optical Wireless Technologies for 5G Communications and Beyond, vol.5, Dec. 2017, pp. 27002-27013.
[B4] D. Wu, Z. Ghassemlooy, W.-D. Zhong, M.A. Khalighi, H. Le Minh, C. Chen, S. Zvanovec, A.C. Boucouvalas, "Effect of Optimal Lambertian Order for Cellular Indoor Optical Wireless Communication and Positioning Systems," Optical Engineering, vol. 55, no.6, June 2016, pp. 066114-1-8.

C. Underwater Wireless Optical Communications

The UWOC technology can provide very high data transmission rates ( Mbps to Gbps) over short to moderate ranges (typically up to one hundred meters or so). Its use has been especially motivated by the need to high-speed and reliable transmission links due to the increasing use of robotics in underwater missions and the need to communicate with autonomous underwater vehicles, for instance. In addition to its ability to provide unprecedentedly high data rates, this technology is highly energy efficient, as compared to the traditional technique of acoustic communication, and also has much less impact on the marine animal life [C1].

C.1. Our Research and Main Contributions :

We have been firstly investigating the aquatic channel characterization and modeling mainly based on numerical methods, including Monte Carlo simulations [C2]. We evaluated the underwater optical channel impulse response taking into account the different transmitter and receiver parameters as well as the water type. We have been also investigating suitable modulation techniques for UWOC systems. Also, we studied the impact of the background solar noise on the link performance in relatively shallow waters for different types of the receiver photo-detector, as well as the interest of channel coding in improving the link reliability and range. More recently, we have focused on the use of silicon photomultipliers (SiPMs) at the receiver in order to increase the transmission range while keeping the implementation complexity of the receiver relatively low.

Simplified schematic of an UWOC link.
Illustration of solar noise affecting a relatively shallow UWOC link.

Our main contributions are summarized in the following.

• Channel characterization via Monte Carlo simulations based on the two-term Henyey-Greenstein model for the volume scattering function [C2].
• Energy efficient modulation and robust channel coding solutions.
• Study of the impact of link misalignments.
• Investigating the effect of solar noise on the link performance [C3].
• Use of SiPMs for UWOC links and the related practical limitations [C4].
• Practical interest of channel coding in improving link performance [C5].

C.2. Collaborations

• IFREMER, la Seyne sur Mer, France
• INSEC-TEC, Porto, Portugal
• McMaster University, Hamilton, Canada

C.3. Selected Publications :

[C1] M. A. Khalighi, C. J. Gabriel, L. M. Pessoa, B. Silva, Underwater Visible Light Communications, Channel Modeling and System Design, in VISIBLE LIGHT COMMUNICATIONS, THEORY AND APPLICATIONS, CRC Press, 2017.

[C2] C. Gabriel, M.A. Khalighi, S. Bourennane, P. Léon, V. Rigaud, "Monte-Carlo-Based Channel Characterization for Underwater Optical Communication Systems," IEEE/OSA Journal of Optical Communications and Networking (JOCN), vol.5, no.1, Jan. 2013, pp. 1-12

[C3] T. Hamza, M. A. Khalighi, S. Bourennane, P. Léon, J. Opderbecke, "Investigation of Solar Noise Impact on the Performance of Underwater Wireless Optical Communication Links," Optics Express, Vol. 24, No. 22, 31 Oct. 2016, pp. 25832-25845

[C4] M. A. Khalighi, T. Hamza, S. Bourennane, P. Léon, J. Opderbecke, "Underwater Wireless Optical Communications Using Silicon Photo-multipliers," IEEE Photonics Journal, Vol. 9, No. 4, Aug. 2017, DOI 10.1109/JPHOT.2017.2726565

[C5] F. Mattoussi, M. A. Khalighi, S. Bourennane, “Improving the Performance of Underwater Wireless Optical Communication Links by Channel Coding,” Applied Optics, accepted.

Contact : Ali Khalighi