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25 July 2019

Potential for high bandwidth local LC but forecasts overblown

Momentum is building behind light communications technology both as an alternative to, and complement for, radio frequency technologies (RF) such as WiFi and cellular. This has culminated in formation of the Light Communications Alliance (LCA) focusing on promoting the technology and developing standards for a variety of use cases, including various Internet of Things domains but also video distribution in the home. This is reflected in the identity of the founders, including Nokia, Orange and Liberty Global alongside a few specialist bodies such as the Lifi Research & Development Centre. Lifi is one of the options for light communications and one that has got some analysts overexcited.

The whole field of light communications (LC), sometimes referred to as optical wireless communications (OWC) or incorrectly as visible light communications (because that wrongly excludes infrared or ultraviolet) has been a subject of research for years but only really started to come of age with the advance of LED technology around 8 years ago. This in turn was driven by concerns over anthropogenic global warming and a drive to reduce drastically energy consumed by light bulbs. The principle is to modulate binary data as on and off states of LED sources which switch so rapidly that they are imperceptible to the human eye. Because visible light waves have between 1,000 and 100,000 times the frequency of radio waves, they have a correspondingly broader spectrum and therefore potential data capacity. Typically, LC is considered to be capable of around 10,000 times the capacity of say WiFi and already speeds up to 250 Gbps have been demonstrated in the lab.

The pros and cons of LC intersect, so that the requirement for line of sight increases security because beams are contained within the building or even room where they were generated, but also restricts use on a campus wide basis. Then LC avoids the issues of RF interference but at the same time requires filtering out of ambient light sources, including sunlight for outdoor use especially. Electronics are relatively simple, associated with the big drive for LED lighting, but at the same time beams suffer from dispersion and also atmospheric absorption so the technology is confined to ultra-short distances.

There are at least two variants, of which Lifi is the one that has generated the most excitement so far despite not yet being formally standardized. Indeed, the reason LCA was formed was to address the fragmented and immature state of LC standards.

The Lifi specifications so far illustrates how LC is not just aimed at high bandwidth applications but also very low ones under the IoT banner, being suited for a number of outdoor use cases such as obtaining data from environmental sensors where RF communications are not convenient or readily available.

LiFi, which embraces visible, infrared and ultraviolet light, has been defined by IEEE 802.11 protocols like WiFi. There is the IEEE 802.15.7, but this does not yet support the higher speeds because it fails to incorporate technologies already standard in the RF world, such as orthogonal frequency-division multiplexing (O-OFDM).

IEEE 802.15.7 merely defines the physical (PHY) layer and media access control (MAC) layer and some specific issues for LC such as compatibility with artificial lighting present in infrastructures and interference from ambient lighting. There are three PHY layers, one for outdoor use at very low bit rates from 11.67 Kbps to 267.6 Kbps and then two others extending up to 96 Mbps, still very modest compared with the potential. One task for the LCA then is to galvanize work within the IEEE and elsewhere to develop a state-of-the-art version of Lifi.

The second variant of LC called Optical Camera Communication (OCC) is different as the name suggests because the primary function is to capture images rather than raw data. OCC does transmit underlying data from light sources, but to a camera. It allows a typical camera comprising a lens and 2D spatial sensor to be used without modifying hardware to demodulate spatially separated multiple light sources simultaneously, with potential for rapid image transfer that way. However, the potential of OCC has been only appreciated quite recently and adopted for standardization by the IEEE 802.15.7m Task Group (TG7m).

The LCA is working on both Lifi and OCC, identifying the latter’s potential for both broadcast communications and indoor positioning in environments such as office buildings, convention centers, and parking lots.

So far, Lifi has attracted the most bullish forecasts, with the LCA itself parroting absurd analyst predictions that the field will at least double in value each year for the next six years to reach $75 billion by 2025. It is worth about $1.8 billion at present, largely on the back of outdoor and smart city applications.

The absurdity of such forecasts can be seen by considering the projected total is around four times what the whole global WiFi market is expected to be worth by 2025, around $20 billion. Yet today the WiFi market is at about $7 billion annually, or four times what Lifi is now.

So those projections are for Lifi to leap from being four times smaller than WiFi today to four times bigger by 2025, scarcely credible. But by dowsing the hype we are not denying that LC has great potential and will play a major part in short range communications both at low and ultra-high bit rates.