© http://visiblelightcomm.com/top-10-li-fi-myths - RIPRODUZIONE RISERVATA visiblelightcomm.com
One only needs to read some of the daft comments posted at the foot of online visible light comms articles to understand that there are a large number of misconceptions out there. Here is my Li-Fi myth buster top 10.
We subtly modulate the current supply to the LED devices at relatively high speeds. We are not harshly switching the LEDs on and off, and we are not modulating at speeds anywhere near those perceptible to the human eye. Your TV and computer displays do flicker at just higher than perceptible rates, the same is true of some LED dimming technologies. VLC does not flicker the lights like this, it will not give you a headache!
There are VLC patents pending on methods to dim the LED while maintaining high data rates until the current is dimmed to about 50%. After that the data rates will begin to diminish in a very graceful manner. So yes, you can dim the lights and maintain communications reliably.
VLC can be used for transmission in either direction. The uplink and downlink can be isolated in a number of ways – wavelength, time, code and also by spatial or optical isolation. For practical and cost reasons VLC might be implemented for downlink only since this is where bottlenecks exist with existing technologies, e.g. Wi-Fi may already provide a reliable uplink where congestion is less likely and Li-Fi provides a high capacity uncongested downlink.
It is relatively simple to eliminate the vast majority of interference from natural and artificial sources using optical filters (which avoids receiver saturation). After the photo-detector further analogue and digital filtering ensure remaining interference is negligible.
To use VLC the lights do need to be on. However in the vast majority of industrial, commercial and retail environments the lights are on when the area is occupied. Given that the lights are usually on, VLC transmission power comes free as it is already used for illumination so this is highly efficient.
In domestic environments we do tend to switch off lights during daylight. Where the lights would have been off the power required for VLC is not free but the lights only need to be dimmed up to transmit data. The illumination need not be above ambient levels so will not be noticed. The power consumed is comparable with the watts/bit for radio transmission and so on aggregate even in domestic environments there is a significant net saving in power.
Line of sight is a definite advantage because the signal will be stronger. However, if you look under the table you can still see despite there being no line of sight from the sun or from artificial sources. If a VLC receiver can collect photons, it can receive data, albeit at a lower data rate if light levels are low. Radio technology is similar in that indirect signals have a lower power and hence the data rate reduces. Visible light can be reflected but generally does not penetrate materials which can be a security advantage and perhaps a coverage disadvantage. Radio can suffer multipath interference from non-line of sight reflected signal cancelling each other by being in anti-phase – leads to signal fading. VLC signal always add and cannot cancel each other and cause fading which is a significant advantage.
VLC is often regarded as a disruptive technology relative to radio technology. I do not believe this should be considered to be the case. I believe VLC is totally complementary to radio. In the same way as Wi-Fi is seen as complimentary to cellular data, VLC or Li-Fi is complementary to Wi-Fi. Cellular data is automatically off-loaded to Wi-Fi when in-doors, in the office or home. Cellular operators insist that smart phones used on their networks are Wi-Fi enabled for this reason, and for quality of service reasons we tend to turn this feature on automatically to the relief of the cellular operators. Unfortunately and consequently Wi-Fi is now becoming heavily congested. Li-Fi can provide a high speed, high density bearer onto which the congested Wi-Fi downlink traffic can be off-loaded. VLC is radio’s friend we should not be considered enemies.
Specialist LEDs with ideal characteristics for VLC would be great. However, solid state LED lighting is currently being sold based on its performance for illumination purposes (colour temperature, efficacy, CRI, lifetime, etc). Communications performance is not even a secondary consideration, so it is wholly unrealistic to expect the lighting industry to factor this into designs at this stage.
In a practical sense we can achieve excellent results with COTS LED devices, if better devices are available great, but to implement VLC we can use existing LED devices. When VLC becomes a significant part of the LED industry then we can start to influence the specification of these devices.
VLC is a very simple technology since it uses direct modulation and direct demodulation . Infra-red remote controls are very low-cost for exactly the same reason. On the other hand radio technology is complex since it requires radio frequency circuits to modulate the data onto the radio bearer and then it requires an antenna system to transmit the signal. The radio receiver is often even more complex requiring an antenna system, radio receiver and carrier synchronisation circuits. Therefore VLC is much simpler than the equivalent radio system.
VLC technology has been proven to work by a number of companies and research establishments. The reliability of lighting systems has rarely been questioned but the reliability of wireless communications is increasingly in question. To my mind the question we need to be asking is; “Could we ever make wireless communications as reliable as lighting technology?”
© https://arxiv.org/ftp/arxiv/papers/1606/1606.02831.pdf - RIPRODUZIONE RISERVATA arxiv.org
Indoor wireless communication is an essential part of next generation wireless communication system.For an indoor communication number of users and their device are increasing very rapidly so as a result capacity of frequency spectrum to accommodate further users in future is limited and also it would be
difficult for service providers to provide more user reliable and high speed communication so this short come can be solve in future by using Li-Fi based indoor communication system. Li-Fi which is an emerging
branch of optical wireless communication can be useful in future as a replacement and backup of Wireless Fidelity (Wi-Fi)for indoor communication because it can provide high data rate of transmission along with
high capacity to utilize more users as its spectrum bandwidth is much broader than the radio spectrum. In this paper we will look at the different aspects of the Li-Fi based indoor communication system,summarizes
some of the research conducted so far andwe will also proposed a Li-Fi based communication model keeping in mind coverage area for multiple user and evaluate its performance under different scenarios .
Light Fidelity (Li-Fi) is a new paradigm of revolution which is a continuation of the trend to
move toward higher frequency spectrum in the field of indoor wireless communication. If we
talk about high speed indoor wireless communication, Li-Fi can bring new dimension in term of
data communication speed by utilizing visible light spectrum. The concept behind this technology
is that the data can be transmitted with the help of light emitting diode (LED) bulbs and
transmission rate can be control by using intensity of LED bulb which can be varies even faster
than light intensity human eye can observe . Keeping in mind number of users increase day by
day and heavy traffic of data, Li-Fi technology can be used as a solution to provide users an
environment of high speed data transmission.
Li-Fi system is a bi-directional multiuser communication system and it could be classified as
nanometer-wave communication system . Li-Fi based communication system is different from
Visible light communication (VLC) system because VLC is only a point to point
communication system while Li-Fi is a proper wireless based networking system which supports
point to multipoint communication.In Li-Fi system, data rate can be related with LEDs so LEDs
selection plays a vital role. Parameters like LEDs size, ON-OFF speed and number of LEDs can
affect data rate of the communication model. Data rate is inversely proportional to the Size of
LEDs which means smaller the size of LED more will be the data rate. If ON-OFF speed of LED
is faster than we can transmit data at higher rates in the form of 1’s and 0’s. Higher the number
of LED’s in a system results in more transmission of data. In Li-Fi based system LED Panel (LP) is
a Light source which can perform the function of illumination and data communication at a same
time. Depending upon the situation and requirement of indoor communication model a Single
LED bulb can also act as a LP and it is also possible that multiple LEDs bulb are combined
together to form a single LP . Number of LEDs in a single LP depends upon the size of room and
number of users to accommodate at a specific time. LED Placement approach (LPA) plays an
important role in Li-Fi based indoor communication system because it can limit data rate and
affect the communication. The placement of LP in indoor environment must be adjusted in such a
way that every Li-Fi user can achieve high intensity of light.
Significant research effort is being directed toward the development of Li-Fi indoor system. For
an indoor system the LED is declared as most power efficient illuminating device for future
indoor lighting system . In  researchers investigate how the distance between LEDs can
change the behavior of indoor communication system. The impact of multipath reflections on a
two-dimensional in door positioning is investigated in  .In current indoor visible light
positioning systems, several algorithms are proposed to calculate the receiver position as in 
researchers propose a novel architecture system that can be used for both indoor positioning and
communications. In  light positioning architecture for a typical room has been investigated by
considering different performance related parameters. In  researcher designed an optimal
constellation for Indoor 2×2 MIMO based Communication system under arbitrary channel
correlation. Keeping in mind all the research conducted by different researchers until now this
Paper is organized as follow In Section II, we have discussed theoverview of Li-Fi based indoor
communication system which is consists of detail regarding key elements that Indoor
communication system must possess in order to satisfy the demand of implementation of
Wireless communication system. In Section IIIwe have described Scenario based indoor Li-Fi
architecture design in which we have proposeddifferent types of LP in term of coverage area and
Section IV is consists of performance evaluation of proposed designed system followed by
The transmitter in an indoor Li-Fi system is an LED bulb. The most likely candidate for front-end
transmitter devices is incoherent solid-state lighting LEDs due to their low cost. Due to the
physical properties of these components, information can only be encoded by using the intensity
of the emitted light. Different LED’s of different color like red, blue, orange, yellow can be used
in Li-Fi Communication System. But if we talk about high data rates, 1 Giga bits per second has
been reported using phosphor-coated white LEDs  and 3.4 Giga bits per second has been redgreen-blue
(RGB) LEDs , the highest speed that has ever been reported from a single color
incoherent LEDs is 3.5 Giga bits per second.
LED luminaire commonly use white light to perform both the function of illumination and
communication. One way of producing white light is to use blue LED with yellow phosphor
coating. When a beam of blue light passes through yellow phosphor coating layer it becomes
white light. Another way is to use a combination of red, green and blue (RGB) LEDs .when red,
green and blue light properly mixed together it becomes white light.As the light emitted by LEDs
are incoherent in nature so therefore there is a need of Intensity Modulation (IM). In IM signal is
modulated in to optical signal of instantaneous power. This signal is received at a receiver by
using Direct Detection (DD) method. In Direct Detection (DD) a photodiode is used to convert
the optical signal power into a proportional current.
As Li-Fi system is based on IM/DD therefore Avalanche Photo Detector (APD) is more effective
as compare to PIN based PD.
In Li-Fi based system, Dimming based modulation schemes are most commonly used modulation
schemes which are single carrier based schemes. In dimming based modulation schemes desire
data rate is achieve by controlling the On-Off level of LED. On-off keying (OOK),Pulse Width
Modulation (PWM),Pulse position modulation (PPM), Variable pulse position modulation (VPPM),Overlapping PPM (OPPM) and optical spatial modulation (OSM) are the main dimming
based modulation schemes which can be implemented in Li-Fi based indoor system .Dimming
based modulation schemes are explained in table 2.
To achieve higher data rate and to decrease the effect of distortion and interference, multicarrier
modulation can also be useful in Li-Fi based communication system but multicarrier modulation
schemes are less energy efficient. One of the most common schemes is OFDM  but OFDM
based signal is complex and bipolar in nature so to implement OFDM for Li-Fi system some
modifications are required in conventional technique for better performance. In  researchers
proposed a Asymmetrically-Clipped Optical OFDM (ACO-OFDM) in which odd subcarriers are
modulated DC-biased Optical OFDM (DCO-OFDM)  is a scheme in which all subcarriers are
modulated and unipolar signal is generated by adding positive direct current. ACO-OFDM is
more energy-efficient as compare to DCO-OFDM. The relationship between light emitted by
LED and current is nonlinear so this nonlinearity based nature of LED affects the performance of
OFDM based modulation schemes.
There are some modulation schemes which are designed to support both purpose of
communication and illumination by using multicolored LEDs. Color shift keying (CSK) is a
scheme in which signals are encoded into color intensities emitted by red, green and blue
(RGB) LEDs .The constant color is maintained by mapping the transmitting bits in to
instantaneous chromatics of LEDs to ensure constant luminous flux. CSK has Reliability on LED
performance due to constant luminous flux and has no flicker effect over all frequencies. In 
researchers proposed a Metameric modulation (MM) which modulate data in the visible spectrum
while maintaining a constant lighting state.MM has a better Color quality control and higher
energy efficiency. Color intensity modulation (CIM) proposed in  provides dimming in color
space. CIM also satisfy the need of color matching and increases the data rate in signal space for
multicolored LED based system.
One possible mode of communication in a Li-Fi system can be a Bi-directional based Indoor
Communication System .If we considered a case where two users want to exchange their data
using an indoor Li-fi based system then there is a need of duplexer on both sides. Duplexer is
used here to support bi-directional communication by separating the transmitted and receive
signal. When user1 wants to send data it will pass through the LED driving circuit toward the
LED Light source panel.LED light source panel is consists of LED bulbs while LED driving
circuit is used to power LED bulb. This circuit must provide sufficient current to light the LED at
the required brightness, but must limit the current to prevent damaging the LED. Receiver can
receive an optical signal with the help of photo detector and then data will be passing through the
trans-impedance amplifier and duplexer. Then eventually receiver will receive its data as shown
in figure 1.
If we consider multiple input and multiple output (MIMO) based Indoor Communication system
then signal processing play a vital role to ensure desire data rates in the presence of high Signal to
noise ratio (SNR).
In an indoor system, MIMO technique can be useful to have parallel data transmission to increase
the data rate  But the MIMO channel need to be highly correlated, which increases the
complexity of decoding parallel channels in the receiver  We consider here as shown in
At transmitter end we can avoid the complications of multiple user interface (MUI) by processing
data signal at each terminal using Data Processing nodes. Data Processing Unit as shown in figure
3 is consists of Modulation and pre-coding Technique. This can allow us easy demodulation of
signal at receiver end without any complication of high processing and will reduce burden on user
terminals in result we can also achieve low energy consumption.
For an indoor communication system coverage area is important therefore we have proposed a
scenario based Li-Fi system design in which coverage area is taken in term of LP. We can control
the coverage area by considering fixed, moveable and hybrid types of LP.
To consider the importance of coverage area in a fixed LP case we have consider here a scenario
in which we have 2 users one is using laptop while other is using tablet personal computer .If we
suppose in our Li-Fi system 1 LED is acting as a LP then covering area of LP can be adjusted in
two ways dedicated LP approach and single LP with wide coverage area as shown in figure 4.
In section A we have placed a fixed LP of wider coverage area to accommodate two users. U1
and U2 are both connected to T1 because they both lie inside the coverage area of single LED. In
section B we used a dedicated approach in which for each user individual LP is assigned.
The moveable LP is an approach in which LED bulb can rotate at a certain angle to adjust the
coverage area according to the requirement of receiver placement. As shown in figure 5 A
coverage area of a transmitter is controlled by rotation of LED at certain angel. As angle of
irradianceis the angle with respect to the transmitter perpendicular axis and angle of incidence is
the angle with respect to the receiver axis. By using moveable LP approach the light intensity on
PD can be controlled by using certain angle of irradiance as a result we can also control the
distance between the receiver and the transmitter to achieve desire normalized received power.
Hybrid approach can also be used by deploying both moveable and fixed LP at a same time as
shown in figure 5 B and 6.
In our selected scenario we can achieve desire BER value when angle of irradiance is smaller
then 70 while keeping angle of incidence constant for each case .when angle of irradiance is
greater then 70 we cannot achieve satisfactory BER which can affect the data rate and
performance of system.
LP of wider coverage area which covers multiple users is better approach if we keep in mind cost
efficiency but the small distance separation between the LED and PD can affect signal to noise
ratio (SNR). With dedicated LP Approach we can achieve more secure, high data rate and fast
communication because coverage area formed by adjacent LED is not overlapped so in this case
there is a tradeoff between cost and high speed.
Moveable LP approach is also useful for practical implementation in indoor communication
system because in moveable LP approach we can adjust the transmission angles of the LEDs. By
transmitting a beam of light at certain angle of irradiance we can achieve desire angle of
incidence as a result we can get efficient communication without overlapping coverage area
between adjacent LEDs. But due to wide Field of view (FOV) of receiver the reflection
components can cause the signal spread and channel bandwidth will also be decrease as a result it
will limit overall data rate of system. So in moveable LP approach there is a need of tunable or
fixed FOV to overcome signal spread to achieve high data rates.
Hybrid approach can also be utilized if we need to accommodate more number of users at a time
and it is an efficient option of implementation without any adjacent overlapping coverage area.
We have proposed a scenario based Li-Fi system design in which coverage area is taken in term
of LP. We can control the coverage area by considering fixed, moveable and hybrid types of LP
We have explained which type of LP is better in which scenario and also evaluated the effect of
proposed design in term of SNR. We have also presented an overview of Li-Fi based indoor
communication system. Li-Fi based indoor communication network can provide us more efficient
and genuine substitute of RF based indoor wireless network and this technology has the ability to
turn every light Bulb in to a Wireless Hotspot.Li-Fi based Indoor communication system has high
Initial Installation cost but when it is implemented at large scale area it can accommodate us by
its less operating cost like electricity bills, less operational staff and limited maintenance charge
sas compare to RF system. Li-Fi communication user always need line of sight connectivity with
its light source therefore some advance research work is required to overcome this limitation to
implement this technology in practical use. Service Providers while providing Li-Fi Indoor
services has to consider major issues like reliability and availability of system and companies also
need to consider how to maintain network for better performance.
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received his BS Telecommunication Engineering degree in 2013 from
Foundation University, Islamabad, Pakistan. Currently he is pursuing master degree in
Information and Communication Engineering from University of Science and
Technology Beijing, China. His research area is Mobile adhoc network (Manets), Light
fidelity (Li-Fi) and Network coding.
Muhammad Nafees Ulfat khan
received his degree ofBachelor of Science in
Electronics and Communication in 2013 from University of Lahore, Pakistan. Currently
he is pursuing master degree in Information and Communication Engineering from
University of Science and Technology Beijing, China. His research area is Light fidelity
(Li-Fi) and visible light communication (VLC).
Received his degree of M.Sc computer Science in 2013 from Abdul wali
khan Mardan, Pakistan. Currently he is pursuing master degree in Information and
Communication Engineering from University of Science and Technology Beijing,
China. His research area is Wireless Communication.