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Is Light Communications a Solution in Search of a Problem?

James Crawshaw
7/30/2019
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When Bell Labs' Rudi Kompfner visited Oxford University in the 1960s to recruit physicists to work on laser and optical communications research, his key selling point to skeptical PhD students was "Think of all this bandwidth!"

A similar enthusiasm was felt in the optical networking industry in the late 1990s and was the inspiration for this website's name. Today, the amazing science behind optical communications carried along miles of glass fiber are taken for granted: Far more attention is lavished upon the radio communications technology behind 5G. Yet from a physicist's perspective, radio and light are just words to describe different parts of the electromagnetic spectrum and there's plenty of R&D activity in wireless optical connectivity, with the cunningly-named LiFi the most high-profile of recent developments (more on that later).

But there have been plenty of efforts to bring wireless optical to market this century.

Acronym soup: OWC, VLC, FSO, LED
The past decade has seen innovations that could allow light to break free from its glass waveguide and bring photons back into fashion in the telecom industry. The umbrella term for light-based communication is optical wireless communication (OWC). OWC uses unguided visible, infrared or ultraviolet light to carry a signal. OWC can be subdivided into Visible Light Communication (VLC) and Free-space Optical (FSO). VLC sends communication signals in the human-visible band (380–700nm), typically using LEDs (Light Emitting Diodes). VLC works, a bit like Morse code, by switching the current to the LEDs on and off. This happens at such high frequency that it is imperceptible to the human eye.

FSO systems use laser diodes in the infrared range (750-1600nm) which is less susceptible to diffraction than shorter wavelength visible light. FSO employs the same concepts as VLC but is focused on long distance communication. In 2008, MRV Communications (acquired by ADVA Optical Networking in 2017) introduced an FSO system that promised 10 Gbit/s over 2km: The product was discontinued three years later.

The nice thing about standards is there are so many to choose from
The IEEE 802.15.7 VLC Task Group was inaugurated in 2009 with the goal of using LEDs to send communication signals in the visible band. However, the group is described as inactive on its wiki; its last document was published in 2015. IEEE 802.15 formed a task group to write a revision to IEEE 802.15.7 (TG7r1) that accommodates infrared and near ultraviolet wavelengths, in addition to visible light. The revision also covers Optical Camera Communications (OCC), which enables positioning/localization, and message broadcasting using devices such as the flash, display and image sensor on a smartphone. The first meeting of the task group took place in 2015 and the most recent meeting was this month.

The original VLC concept remains alive and well in the form of LiFi, a term that alludes to its potential as an alternative to WiFi. The University of Edinburgh's LiFi R&D center describes it as "a subset of VLC." We can't speak to the technology, but in terms of marketing, LiFi is certainly a catchier name than 802.15.7…

The IEEE 802.11bb Task Group on Light Communications was formed in May 2018 and is focused on light communications in the 380-5,000nm band. All PHY modes must achieve minimum single-link throughput of 10 Mbit/s. The most recent meeting of the task group was May 2019. The group is chaired by Nikola Serafimovski (of pureLiFi), vice-chaired by Tuncer Baykas (of Vestel), and the PHY layer technical editor is Volker Jungnickel (of Fraunhofer HHI).

Yet another IEEE task group -- 802.15.13 -- is working on multi-Gigabit/s OWC using wavelengths from 190-10,000nm. The group says it is continuing with work on "high-rate photodiode communications (also denoted as LiFi) done previously in TG7r1, which is now focusing on optical camera communications." Work on 802.15.13 started in March 2017. The most recent meeting took place in May 2019. The chairman, vice chairman and secretary are also members of the 802.11bb task group. Heavy Reading spoke with the task group's vice chairman, Nikola Serafimovski, who described 802.15.13 as a "conceptual standard."

Outside of the IEEE the only other standards body active in light communications appears to be the ITU. The ITU-T work program G.9991 (formerly G.vlc) is focused on high speed indoor communication using visible light. According to an ITU press release issued in January 2019, "ITU G.9991 has achieved first-stage approval ('consent') and is now in the final phase of its development cycle."

Consortia and alliances
In 2011 the Fraunhofer Institute for Photonic Microsystems, illumination module supplier TriLumina and two now defunct companies created the LiFi Consortium. The initiative appears to have been short-lived and is no longer active. Filling this void, in June the Light Communications Alliance was formally launched with the aim of raising awareness and adoption of both LiFi and OCC . Founding members include networking heavyweight Nokia, telecom operators du (UAE Telecom), Orange and Liberty Global, as well as technology suppliers such as pureLiFi -- which claims to have developed the world's first LiFi ASIC -- and Zero.1, which says it has developed a plug-in that enables smartphone cameras to receive an OCC signal.

Prof. Harald Haas, pureLiFi's co-founder, first introduced the term LiFi in a TED talk in 2011. The company launched the first commercially-available LiFi systems at MWC Barcelona 2014. Zero.1's solution has been deployed as an alternative to Bluetooth beacons for retail applications and QR codes for museum tours. (See Eurobites: WiFi's Lightbulb Moment.)

What's so great about light versus radio?
The spectrum of human visible light ranges from 380 to 700 nanometers, which equates to 430 to 790 tera-Hertz in the frequency domain. The potential bandwidth of 360 tera-Hertz (360,000 GHz) is more than 10,000 times greater than the radio portion of the electromagnetic spectrum. Mobile communications typically operate in slivers of spectrum between 800 and 3000 MHz, though with 5G, millimeter wavelengths are now being used (20-30 GHz). As such, LiFi has the potential to offer very high data rates in unlicensed spectrum.

Direct line of sight is not necessary for LiFi as light, can be reflected off walls and surfaces. However, light waves cannot penetrate walls, which makes LiFi more secure against eavesdroppers than radio waves. While LiFi is designed for high data rate, bi-directional communication, OCC is a low data rate, unidirectional technology used for broadcasting information to the camera on a smartphone and for indoor positioning in shops or convention centers.

Why should communications service providers care?
CSPs do not generally generate much revenue from providing WiFi services today, while Bluetooth beacons for retail applications are similarly not big money-spinners for them.

Nevertheless, LiFi and OCC present potentially interesting complements to existing unlicensed wireless technologies. Simon Clement, Advanced Technology Consultant with Liberty Global, told Heavy Reading, "Most smartphone traffic is offloaded onto WiFi. LiFi could be a useful way to offload WiFi traffic in increasingly congested wireless LANs. This could be in the home for residential broadband or in transport applications such as aviation where everyone already has a light source situated directly above their seat."

My two cents
A Google Trends search for LiFi and LiFi show these terms peaked in interest in late November 2015 when an article in International Business Times was widely referenced by numerous other publications. IBT reported that an Estonian start-up called Velmenni, a founding member of the Light Communications Alliance, used a Li-Fi-enabled light bulb to transmit data at 1Gbit/s.

While the IBT story went 'viral' in 2015, the same cannot be said of Li-Fi: Four years on it remains a niche technology. The fact that major equipment vendors, such as Nokia, and large CSPs, such as Orange, and Liberty Global, are supporting the Light Communications Alliance is encouraging for Li-Fi and OCC. However, I'm not convinced there is an immediate need for the additional bandwidth they promise, beyond what WiFi and Bluetooth already offer. As such, mainstream adoption is likely to be light years away.

— James Crawshaw, Senior Analyst, Heavy Reading

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