A review of gallium nitride LEDs for multi-gigabit-per-second visible light data communications

Sujan Rajbhandari, Jonathan J D McKendry, Johannes Herrnsdorf, Hyunchae Chun, Grahame Faulkner, Harald Haas, Ian M Watson, Dominic O'Brien, Martin D Dawson
2017 Semiconductor Science and Technology  
A review of gallium nitride LEDs for multi-gigabit-per-second visible light data communications. Semiconductor Science and Technology, 32. ISSN 0268-1242 , http://dx. Abstract The field of visible light communications (VLC) has gained significant interest over the last decade, in both fibre and free-space embodiments. In fibre systems, the availability of low cost plastic optical fibre (POF) that is compatible with visible data communications has been a key enabler. In free-space applications,
more » ... he availability of hundreds of THz of the unregulated spectrum makes VLC attractive for wireless communications. This paper provides an overview of the recent developments in VLC systems based on gallium nitride (GaN) light-emitting diodes (LEDs), covering aspects from sources to systems. The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored. Furthermore, advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are also outlined. The evolution of computing, consumer electronics and mobile communications technologies is leading to an exponential increase in end-user data requirements. Reports by Cisco, for example, predict that there will be a nine-fold increase in mobile communications traffic from 2014 to the end of 2020, almost doubling every two years [1] . If the global traffic volume increases at the current rate, there will soon be 'a spectrum crisis' as radio frequency (RF) technology cannot keep pace with the demand [2] . It is also very unlikely that a single communication technology can support these growing data requirements in all places at all times. Hence, future generations of networks must support the co-existence and cooperation of different wireless technologies (RF, millimetre wave, and optical wave) [3] . With the evolution of the new wireless communication standards like the fourth generation (4G) and fifth generation (5G) networks, the available RF spectra are efficiently utilized by using advanced signal processing concepts such as massive multiple-input multiple-output (MIMO) systems and shrinking cell size leading to femtocells. In parallel, untapped frequencies at millimetre and nanometre wavelengths are also being considered. It is widely expected that visible light communications (VLC) systems and their extension to fully networked, bi-directional multiuser wireless systems referred to as LiFi ( see [4] for detail) will play a key part in 5G-and-beyond connectivity, especially for indoor environments [2], [5], [6] . Indoor lighting has also undergone a revolution with the recent advances in solid-state lighting (SSL) devices based on visible light emitting diodes (LEDs). Incandescent bulbs with an extremely low efficiency of 13-18 lumens per watt are being phased out. The compact fluorescent light (CFL) which was introduced in the 1990s offers a better energy conversion (55-70 lumens per watt). However, these efficiencies have recently been superseded by LED and laser diode (LD) based lighting. The average efficiency of Gallium-Nitride LEDs is higher than 100 lumens per Watt and is expected to reach 200 lumens per Watt by 2020 [7], [8]. Besides higher efficiencies, the LEDs also offer other advantages including a long operational lifetime (up to 50,000 hours), compact form factor, no emission of harmful ultraviolet or infrared radiation, mercury-free operation and a low maintenance cost. Additionally, the manufacturing cost of LEDs is dropping as the technology continues to mature for mass production, making LED based illumination economical in comparison to CFLs over its lifetime. Hence, the adoption of LED-based illumination is rising steadily [9], [10]. The popularity of SSL-based illumination has created a unique opportunity as each light bulb can potentially serve as a communication hotspot. Unlike traditional lighting devices, SSL devices can be modulated at a rate imperceptible to the human eye. This has paved the way for the dual function of illumination and communication using LEDs, in principle enabling communication at minimal extra cost and energy consumption [11]. The wide availability of lighting infrastructure and the feasibility of achieving communication rates beyond 100's of Mb/s makes VLC a cost effective and attractive complementary technique to RF technology. The first reported VLC system based on a visible LED was in 1999 [12] when Pang et al. used modulated LED traffic lights for broadcasting audio and other information. Tanaka et al. adopted LEDs for illumination and communication in the early 2000s [13]. Interest in VLC now is growing rapidly with a number of practical demonstrations. In Japan, the visible light communication consortium (VLCC) [14] was formed in 2003 which is now superseded by the visible light communication association (VLCA) [15]. A number of key technologies were demonstrated in the European Union funded 'hOME Gigabit Access' project (OMEGA) project [16]. Recently, the UK government funded Ultra-parallel visible light communications (UP-VLC) [17] project has demonstrated a 10 Gbits/s VLC system [18]. In parallel to these research activities, there have been efforts to establish a VLC standard and IEEE802.15.7 was proposed in 2011 [19], [20]. A task group on shortrange optical wireless communication (OWC) was formed in 2015 to revise the IEEE 802.15.7 standard [21]. Early VLC demonstrations were based on conventional chip, broad-area (0.1-1mm 2 ) Gallium Nitride (GaN) LEDs originally developed for lighting, which enabled data rates typically up to 100 Mbit/s [22] . Recently, there has been work on increasing the bandwidth of these devices by developing LEDs with dimensions of less than 100 µm ('µLEDs') [23] . These µLEDs can be driven at a significantly higher current density than the broad-area LEDs and, hence, they offer an optical bandwidth in excess of several hundred MHz [24]. This is an order of magnitude higher than the traditional broad-area LEDs and makes µLEDs attractive sources for multi-Gbit/s VLC systems [25] , [26] . The illumination levels required for lighting can be achieved by operating multiple micro-LEDs in a ganged fashion or by a hybrid approach where micro-LEDs are used in conjunction with high-power LEDs.
doi:10.1088/1361-6641/32/2/023001 fatcat:iyl2iqfnzjbrfjblw4e3q3oeq4