20-10-2016, 09:37 AM
1459999087-LIGHTFIDELITYTECHNOLOGY.docx (Size: 124.59 KB / Downloads: 4)
Abstract
Li-Fi light fidelity which implies visible light communication or optical wireless communication uses light emitting diodes (LED’s) for data transmission. It is designed to use LED light bulbs similar to those currently in use in offices and homes. It can serve as an option to access the 5G technology. The VLC has various strengths such as wide regulated spectrum, high spatial reuse, security and high energy efficient. This technology can be an alternative to RF communication and can overcome most of the limitations which are been faced during RF communication. VLC offers large communication bandwidth which can be used for high data rate services such as large data files and HD video transfers. But on the other hand there are certain challenges of VLC. The path used for data transmission, it cannot penetrate through walls and obstacles, therefore the information is not received until a transmission unit is found by the receiver. Hence it has low mobility. Secondly for indoor environments the IF signals can be hazardous to the human eyes. The current research is going in to use this technology for Vehicle to vehicle communication which can be the most effective solution that has been used in order to reduce vehicles’ accidents.
INTRODUCTION
Demand for mutli media devises have increased due to the extensive use of data demanding mobile application which mainly rely on maximum capacity due to limited availability of radio frequency.Li-Fi has availability of 30THz licience free and unused optical spectrum. It uses different part of electromagnetic spectrum which does coincide with the existing radio frequency. Li-Fi is confined to small cells and mostly used for indoor communication. Li-Fi uses full duplex communication system which add on for high speed and network wireless communication system technology has similar to Wi-Fi functioning. Li-Fi technology is termed in the form of visible light communication using light emitting diodes has medium for transmission and receiving of data. It workings by switching current to on/off of LED’s to quick to be noticed by human eye. Li-Fi LED’s have to be kept on to transmit data that could be dimmed below human visibility while still emitting enough light to carry data. These light waves cannot be penetrated through walls so it is more secured from hacking. Li-Fi also uses electromagnectic spectrum to transmit data. It is measured that it is 100 times faster than Wi-Fi speed which is 224 gigabites/s. VLC is communication medium which uses visible light between 400 and 800 THz (375-780nm). For data transmission LED use 500Mb/s. light emitting diodes can br pulsed at very high speeds without noticeable by human eye. Normally data rate of single LED is more than 3Gb/s. Li-Fi add on for hybrid communication with Wi-Fi by increasing the spectrum efficiency. This type of application can be used for vehicle to vehicle communication, which is one of the most effective mechanisms that is implemented in automobiles to provide safety and protocol of communication.
FUNCTIONING OF Li-Fi
In visible light communication we use a course of illumination but also send information using the same light so we say that VLC is illumination along with communication. It uses light LED bulbs at the downlink transmitter. Normally constant current is applied across the LED’s to use them but by varying the current very fast the optical output can be made to vary at very high speed. If the LED is on we transmit a digital one and if it is off we transmit a digital zero, we can easily transmit data by switching the LED’s on and off very rapidly. Thus we need some Led’s and controller which can code data into those LED’s in order to setup the system. Now by varying the rate at which the LED’s flicker, we can encode the desired data and thus transmit the data very easily. A certain improvements to the system can be made by using an array of Led’s for parallel data transmissions or by using a mixture of red, green and blue LED’s to alter the light frequency, with each frequency encoding a different data channel. A speed up of 10Gb/s can be achieved by using such a system.
MODULATION TECHNNIQUES
While sending a data using Li-Fi it is necessary to modulate the data into a signal which can be transmitted. Signals consists of light pulses:
1. OFDM- Orthogonal Frequency Division multiplexing is used for encoding digital data on multiple carrier frequencies. Here a large number of closely spaced orthogonal sub carrier signals are used to carry data on several parallel data streams or channels. Each sub carrier is modulated wit quadrature amplitude modulation or phase shift keying at a low symbol rate, maintaining total data rates similar to conventional single carrier modulation in the same bandwidth.
2. OOK (On/Off Keying) It is amplitude-shift keying (ASK) modulation that represents digital data in the presence or absence of a carrier wave The presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a binary zero. It is analogous to unipolar encoding line code. It is very easy to generate and decode but is not very optimal in terms of illumination control and data throughput.
3. PWMPulse-width modulation is used to encode the signal into pulsating signal. The main use is to allow the control of the power supplied to electrical devices, especially to inertial loads such as motors. PWM transmits the data by encoding the data into the duration of the pulses. More than one bit of data can be conveyed within each pulse.
LI-FI CHANNEL MODEL
The optical channel gain in indoor scenarios consists of the line of sight (LoS) component and the reflection component. The LoS channel gain is expressed as
Hμ,α = [ (m + 1)Ap /2π(z2 μ,α + h2 )]* g(θ)Ts (θ)cosm (φ)cos(θ), θ< ΘF 0, θ< ΘF
where m is the Lambertian index which is a function of the half-intensity radiation angle θ1/2 , expressed as m = −1/ log2 (cos(θ1/2 )); Ap is the physical area of the receiver photo-diode; zμ,α is the horizontal distance between Li-Fi α to user μ; h is the height of the room; φ is the angle of irradiation; θ is the angle of incidence; ΘF is the half angle of the receiver’s FoV; Ts (θ) is the gain of the optical filter; and g(θ) is the concentrator gain, which can be written as
g(θ) = ( χ2 /sin2 ΘF , 0 ≤ θ ≤ ΘF
0 , θ> ΘF )
where χ is the refractive index. Here it is assumed that B = 20 MHz and the reflection component is not considered in the Li-Fi channel model. In Li-Fi systems, the baseband communication with intensity modulation and direct detection is used. The Li-Fi signals are transmitted in the form of optical power, which should be positive and real. working region of the LED.
The conversion between the average electric power of signals and the average optical power obeys the following relationship
L = Popt/ sqrt(Pelec)
Where Pelec is the electric power of the signals. An increase of ι results in a decrease of the probability of the Li-Fi signals being outside the LED linear working region.
The signal-to-interference-plus-noise ratio (SINR) for user μ and AP α can be written as:
SINRμ,α = [ (κPoptHμ,α )^2/ι2N0B + summation(κPoptHμ,else)^2]
where κ is the optical to electric conversion efficiency at the receivers; N0 [A2 /Hz] is the noise power spectral density; Hμ,α is the channel gain between user μ and Li-Fi AP; and Hμ,else is the channel gain between user μ and the interfering Li-Fi APs, accordingly.
Since only half of the bandwidth can be used for data transmission in the system, the achievable data rate is expressed as:
R(n) μ,α = B log2 (1 + SINR(n) μ,α )
FUTURE
In future, we will consider the application of Li-Fi technology in vehicles. It mainly deals on how the communication takes place between two vehicles using LED’s by sending data through light spectrum as an optical wireless medium for signal propagation.