Student Blog: Examining optical wireless communications

During WEP 2015, the wireless communications theory research group, supervised by Dr. Mohamed-Slim Alouini, hosted a two-day optical wireless communications workshop -- where guests working in this field came form all over the world. Each speaker was given a two-session lecture allowing general and technical discussions. Invited speakers were Dr. Julian Cheng from the university of British Columbia, Canada,Dr. Maïté Brandt-Pearce from the University of Virginia, USA,Dr. Mitsuji Matsumoto from Waseda University, Japan, as well as Dr. Harald Haas, Chair for Mobile Communications at the University of Edinburgh in Scotland.

An Overview of the Technology

In the mid-1990s, I have witnessed the Internet boom, starting with dial-up and its noisy sound -- which kids today probably cannot recognize. Internet access was limited and no simultaneous usage was possible until the advent of DSL. Then, Wi-Fi came to dominate and it became necessary to buy Wi-Fi cards to outfit our outdated laptops in order to connect. Nowadays, you can find Wi-Fi everywhere; you can also carry your "mobile" Wi-Fi router in your pocket. So, if we observe those trends, what is next chapter for wireless communications?

Conventional wireless communications, involving both cellular and data, depend on radio frequencies to carry the signal -- whose range varies between 30 kHz and 300 GHz in the electromagnetic spectrum. Governments usually regulate and license the spectrum for phone operators and TV broadcasters. Due to the exponential growth both in users and in the demand of data applications and services, the radio frequency (RF) spectrum has become exhausted. This has motivated researchers to look for alternatives and consider the upper part of the spectrum that is the optical carriers.

Optical wireless communications (OWC) is defined as the use of optical frequencies to carry the electrical signals. OWC can replace or complement RF operating systems because of some advantages such as high data rate, immunity to electromagnetic interference and inherent physical security. In addition to that, it is operating on unregulated bandwidths, which makes it a cost-effective technology. The applications of OWC depend on the operating frequency (or wavelength).

Detailing the Optical Band

The optical band is divided into three segments. The first one is the infrared band (750–1600 nm) used in most terrestrial point-to-point OWC systems that is called free-space optical (FSO) communications. These systems are applicable for inter-building connections, which make it attractive for services such as enterprise connectivity, cellular back-haul and broadcasting. For instance, during the 2010 FIFA World Cup, BBC installed FSO links for the Ethernet-based transmission of high definition video between temporary studio locations set up in Cape Town, South Africa. Finally, FSO links can be deployed in case of disaster recovery.

The second segment is the visible band (390-750 nm) that is used to operate visible light communications taking advantage of light-emitting diodes (LEDs) to transmit and receive data -- ensuring human eye safety. These systems have become interesting for various applications such as indoor local area networks.

The third segment is the ultra violet band (200 – 280 nm) -- used by the ultraviolet communication (UVC) systems for applications that require non-line-of-sight configurations.

The History of Optical Wireless Communications

The science behind Optical wireless communications (OWC) cannot be considered as a newly emerging technology. The earliest usage was in the form of beacon fires and smoke. In 1880, Bell invented the photophone that can be considered as the first wireless telephone system. After that, the photophone was developed for military usage. Following the invention of lasers in 1960, there were many attempts for optical transmission, but most of them failed due to two reasons: laser beam divergence and atmospheric turbulence.

Over the years, OWC remained limited to military or space applications. Recently, due to the scarcity of RF spectrum, the interest in OWC systems to be deployed commercially and develop novel and efficient systems for various applications returned to meet the demand of high data rates.

For example, CNN money in March 2014 reported on Anova technologies' plans to install a 50-km FSO link between two stock exchange data centers in addition to the original RF link to ensure availability -- an essential component of stock market. Furthermore, in Scotland, Dr. Herald Hass initiated the Li-Fi that uses LEDs for bidirectional and high speed wireless communications for indoor applications, it is believed that Li-Fi data rate can reach up to 1.67 Gbps on a single color/LED. PureLiFi demonstrated the first commercially available Li-Fi system. Although the OWC sounds promising, it still suffers from some issues that encourage further research around sensitivity to weather conditions and the advancement of point-to-point systems that require perfect alignment between the transmitter and the receiver.

Highlights from the WEP 2015 Workshop

During the WEP workshop at KAUST, Dr. Julian Cheng explained his research area on FSO communications and went further on the channel modeling. Dr. Maïté Brandt-Pearce stated the need of wireless communications technologies convergence through hybrid systems for better link connectivity by exploiting each technology's advantages. Dr. Mitsuji Matsumoto presented the results of some experiments conducted in his lab and showed the effect of turbulence conditions on the performance of FSO systems. Dr. Haas has explained more about some commercial products that employ Li-Fi and the challenges faced.

In our group, one of the research areas is in FSO communication systems. We mainly characterize and evaluate the performance of such systems theoretically, propose some solutions to main issues of FSO systems and validate them mathematically, and finally study the integration of such systems into the conventional RF systems to observe and analyze the advantages of hybrid systems. Moreover, some of our group members work closely with the Photonics Lab to develop an underwater FSO system to be used by the Red Sea Research Center.

The Way Forward

The hosting of such a workshop during WEP has had a great impact on the progress of our research. During the workshop, we exchanged ideas and insights with pioneers in the field -- fostering potential collaborative partnerships in the long run. All gathered to discuss the new research trends in OWC and agreed that the mission of current OWC research is to offer Internet connectivity to rural areas -- especially in developing countries -- and to provide high data rate connections to current users who demand more services. Furthermore, the workshop can enable theorists and experimentalists to integrate research and technology development in Saudi Arabia. This certainly demonstrates KAUST's continuous commitment and mission aimed at conducting cutting-edge research.

- By Hessa Al-Quwaiee, CEMSE Student