The high frequency of light/data transmission is undetectable
to the human eye. The LED lights used to transmit Li-Fi signals
are modulated at such a fast rate that the eye cannot
perceive the modulation or flicker.
Li-Fi's OPERATIONAL LIGHT SPECTRUM
For Li-Fi technology, communication data information
is provided through the following spectrum ranges:
• Visible light wavelengths from 380 nm to 740 nm
(430 to 770 THz light frequency)
• Infrared light wavelengths from 700 nm to 1 mm
(430 THz to 300 GHz light frequency)
• Ultraviolet light wavelengths from 10 nm to
400 nm (8 × 1014Hz to 3 × 1016Hz light frequency)
Because UV light has damaging effects on human
health, only visible and infrared light spectrums are used
in Li-Fi technology. This opens up 10,000 times greater
bandwidth than existing Wi-Fi radio frequencies (RFs)
on the visible spectrum only. Therefore, visible light
spectrum is predominantly used. Options are also being
explored using the invisible infrared light spectrum, since
it is already being used to send information (uplink) back
to the LED light bulb.
Because light bounces off surfaces and light reflected
from surfaces also carries data, Li-Fi is not strictly a lineof
sight technology. Rather, Li-Fi is a cellular communication
system, and the data rate is not dependent on the
line of sight but on the signal quality at the device. Signal
quality can be defined by the ratio of the desired data
versus any interfering data and noise.
It is also important to understand that Li-Fi
is a bidirectional wireless communications technology
that allows high speed transmission in both uplink and
downlink simultaneously.
LIGHT LEVELS FOR Li-Fi
Light fidelity can operate in daylight and even in direct
sunlight conditions, as the modulated light can still be
detected. It relies on detecting the fast changes in light
intensity and not on the absolute or slowly varying levels
18 I ICT TODAY
caused by natural disruptions in daylight or sunlight.
Since sunlight is constant light and Li-Fi technology
modulates the light at very high rates, the sunlight can
be filtered out at the receiver. Outdoor receivers have
been tested under 77,000 lux of sunlight (or 7,700
foot-candles fc).4 As a reference, a brightly lit office
is normally at 500 lux (50 fc).
If all power to a light is turned off, then there is no
Li-Fi. However, Li-Fi technology can be enabled to low
enough light levels in a room that make it appear dark
and still transmit data. There is consistent performance
between 10 and 90 percent illumination.5
Communications with Li-Fi’s technology has been
achieved at light levels down to 60 lux (6 fc). As a comparison,
the IESNA standard light level for reading
is between 200 and 500 lux (20-50 fc). Lighting along
a perimeter pathway around a building at nighttime
is around 5 to 6 fc.
Li-Fi SPEED
As a dense and fast wireless communication technology,
Li-Fi offers greater speed and density than RF solutions.
The following notable achievements exemplify
its evolution:
• August 2013: Data rates of over 1.6 Gb/s were
demonstrated over a single-color LED with
an upstream communication rate of about
600 Mb/s.6
• April 2014: A company from Russia developed
a Li-Fi wireless local network module transferring
data at 1.25 Gb/s, while striving to achieve
up to 5 Gb/s.7
• In 2014, a new record was established by
a company in Mexico at transfer data speeds
of up to 10 Gb/s across the light spectrum
emitted LED lamps.8