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Introduction to Visible Light Communications

1.1 History

Visible light communication (VLC) is an age-old technique which uses visible light to transmit messages from one place to another. In ancient China, communication by flames was an effective way to relay signals from border sentry stations to distant command offices on the Great Wall. Similarly, lighthouses were distributed along seashore or on islands to navigate the cargo ships on oceans. Nowadays, visible lights are also mounted on modern skyscrapers to not only indicate its presence at particular locations, but also provide reference signals to pilots flying a plane.

Along with the evolution of telecommunication science and technology, using visible lights instead of other electromagnetic waves to transmit information started to attract attentions from scientists, tracing back to the famous photophone experiment by Alexander Graham Bell in 1880 [1]. In his experiment, the voice signal was modulated onto the sunlight and the information was transmitted over a distance of about 200 m. Efforts to explore natural lights and artificial lights for communication continued for decades. In 1979, F. R. Gfeller and G. Bapst demonstrated the technical feasibility of indoor optical wireless communication using infrared light emitting diodes (LEDs) [2]. Built upon fluorescent lamps, VLC at low data rates was investigated in [3]. As LED illumination industry advanced, the fast switching characteristic of visible light LEDs prompted active researches on high-speed VLC. A concept was first proposed by Pang et al. in 1999 [4], using the traffic light LED as the optical signal transmitter. Later on, a series of fundamental studies were carried out by S. Haruyama and M. Nakagawa at Keio University in Japan. They investigated the possibility of providing concurrent illumination and communication using white LEDs for VLC systems [5, 6]. Meanwhile, they not only discussed and analyzed effects of light reflection and shadowing on the system performance, but also explored VLC applications at relatively low rates [7, 8]. Using LED traffic lights to transmit traffic information was experimented based on avalanche photodiode (APD) and two-dimensional image sensor receiver, respectively [9, 10]. VLC and powerline communication (PLC) were coherently integrated to provide a network capability [11], where the performance of an advanced orthogonal frequency division multiplexing (OFDM) modulation format was evaluated [12]. Applications were extended to brightness control [13] and high-accuracy positioning [14] in addition to communications.

As mobile broadband grows rapidly, the demand for high-speed data services also increases dramatically. VLC emerges as an alternative to alleviate radio spectrum crunch. Higher rate VLC has attracted global research attentions, in particular, from European researchers at the beginning, by maximally exploring the LED capabilities and increasing the spectral efficiency. Using a simple first-order analogue equalizer, a data rate of 100 Mbps was realized with on-off keying non-return-to-zero (OOK-NRZ) modulation in 2009 [15]. Meanwhile, 125 Mbps over 5 m using OOK and 200 Mbps over 0.7 m using OFDM were reported by Vucic et al. [16, 17], where photodiodes (PDs) were used in those VLC systems to detect optical signals. By adopting a 2 × 1 array of white LEDs and an imaging receiver consisting of a 3 × 3 photodetector array, a multiple-input multiple-output OFDM (MIMO-OFDM) system could deliver a total transmission rate of 220 Mbps over a range of 1 m [18]. The data rate can be further increased if APD is adopted. In 2010, the data rate of the OOK-based system reached 230 Mbps [19] and the data rate of the OFDM-based system approached 513 Mbps with bit- and power-loading [20]. In 2012, the highest data rate of a single LED-based VLC system achieved 1 Gbps with OFDM [21]. Additionally, carrierless amplitude and phase modulation (CAP) was introduced into VLC systems, and a data rate of 1.1 Gbps was achieved [22]. Using an MIMO structure, a 4 × 9 VLC system achieving 1.1 Gbps was presented, where the parallel streams were transmitted by 4 individual LEDs and detected by a 3 × 3 receiver array [23].

In the previous studies, a phosphor-converted LED (pc-LED) was adopted as optical signal transmitter. The bandwidth of a pc-LED is however limited by slow response of the phosphorescent component. In 2014, a post-equalization circuit consisting of two passive equalizers and one active equalizer was proposed to extend the bandwidth from tens of MHz to around 150 MHz [24]. If other types of LEDs having higher bandwidth are employed, it has potential to increase the throughput significantly. For example, using micro LEDs as transmitters in VLC systems could be firstly attributed to McKendry et al. and a data rate of 1 Gbps was reported at a price of low luminous efficiency [25]. Multicolor LEDs, radiating particularly red, green, and blue lights, can provide high-rate transmission by wavelength division multiplexing (WDM). Data were simultaneously conveyed in parallel by different colors such as red, green, and blue lights. In principle, the data rate could be tripled in the absence of color crosstalk. An OFDM-based VLC system using a multicolor LED was realized supporting a data rate of 803 Mbps over 0.12 m [26]. Using multicolor LED as the transmitter and APD as the receiver, the data rate of OFDM-based VLC systems was increased from 780 Mbps over 2.5 m to 3.4 Gbps over 0.3 m, where WDM and bit- and power-loading techniques were jointly applied [27-29]. In another study [30], the bandwidths of multicolor LED chips were extended to 125 MHz and modulated by 512 quadrature amplitude modulation (QAM) and 256WDM, respectively, and the frequency domain equalization based VLC system finally reached a data rate of 3.25 Gbps. The data rate of CAP-based VLC systems using multicolor LEDs was increased up to 3.22 Gbps, also benefiting from WDM technology [31].

It is well known that lighting LEDs typically serve as transmitters for downlink information transmission to mobile devices. In 2013, an asynchronous bidirectional VLC system was demonstrated in [32] where a 575 Mbps downlink transmission was realized by red and green LEDs, and a 225 Mbps uplink transmission by a single blue LED. From a network perspective, a spectrum reuse scheme based on different colors was proposed for different cells in an indoor optical femtocell, where multiple users can share the spectrum and access the network simultaneously [33]. User-centric cluster formation methods were proposed for interference-mitigation in [34]. A VLC system can also be combined with a wireless fidelity (WiFi) system to provide seamless coverage after a judicious handover scheme was designed and applied [35].

In multicolor LED-based VLC systems, signals from three color light sources were transmitted independently in most experiments, leaving room for capacity increase. In 2015, Manousiadis et al. used a polymer-based color converter to generate red, green, and blue lights emitted by blue micro LEDs [36]. Three color lights were modulated and mixed for white light illumination. The aggregate data rate from three colors was 2.3 Gbps. Techniques to explore spatial and temporal capabilities of devices were also investigated. A MIMO VLC system employing different field of view (FOV) detectors in order to improve signal-to-noise ratio (SNR) was analyzed in [37]. An optical diversity scheme was proposed, where the original data and its delayed versions were simultaneously transmitted over orthogonal frequencies [38]. Data rate can be significantly enhanced by employing different degrees of freedom. Combining with WDM, high-order CAP, and post-equalization techniques, Chi et al. showed that a multicolor LED-based VLC system could provide a data rate of 8 Gbps [39]. A novel layered asymmetrically clipped optical OFDM scheme was proposed to make a tradeoff between complexity and performance of an intensity-modulated direct-detection (IM/DD) VLC system [40]. Under lighting constraints, DC-informative modulation and system optimization techniques were proposed [41-43]. Some receiver design issues were particularly addressed in weak illuminance environments and several bidirectional real-time VLC systems with low complexity were reported [44, 45].

Besides individual research groups, there are also many large scale organizations and research teams worldwide that have contributed to the development and standardization of VLC technology. In Europe, the HOME Gigabit Access (OMEGA) project was launched in 2008 to develop a novel indoor wireless access network, providing gigabit data rates for home users [46]. The project members included France Telecom, Siemens, University of Oxford, University of Cambridge, and many other companies and universities. This project finally demonstrated a real-time VLC system using 16 white LEDs on the ceiling to transfer HD video streams at 100 Mbps. Another organization called OPTICWISE was funded by the European Science Foundation under an action of the COST European Cooperation in Science and Technology (COST), which allowed coordination of nationally funded VLC researches across European countries. Significant research results and professional activities were reported from its various groups [47].

In Japan, Visible Light Communication Consortium (VLCC) consisting of many Hi-tech enterprises and manufacturers in...