Introduction
Wireless communication has been dominated by RF (Radio Frequency) communication, since James Maxwell and Michael Faraday demonstrated that Electromagnetic wave travels in a straight line through space.
Radio frequency spectrum for wireless communication is in between 3kHz ‘ 300 GHz, and every portion of this frequency spectrum is already in use one way or the other. Number of mobile data usage per month in 2014 is 3.8 Exabyte per month, assumed to be 6.3 in 2015.
Radio Spectrum
RF band limitations
To meet the required data rate we have to use the spectrum in most efficient method.
According to Shannon theory
C=log2 (1 +PTx Gpath /N)
Where C is the capacity in bps
B is the bandwidth
Ptx is the transmitted power
Gpath is the gain of the channel (<<1)
As the number of user increases, Capacity decreases exponentially by same factor. As each user will take some band( if there are two users we have to split band in two part ) .
Cells and microcells are the concept of using frequency reuse so we don’t have to split the band (reuse at cellular level)
MIMO is the other reuse factor (at link level) to efficiently use the band.
If capacity of single-input-single-output (SISO) is given by
Csiso =B log2(1 +Prx/N0B)
Then For CMIMO =min(M,N)Csiso
number of tx antenna N number of rx antenna
As the above graph shows Using MIMO we are increasing the data rate (bps) for same SNR.
MIMO is very sensitive to interference; to avoid the concept of spatial modulation comes.
Spectral efficiency Gain graph we can see the bits/sec/Hz from 3GPP to NGMN is getting constant means we can use radio frequency up to a certain extent its capacity is drying up and the requirement is way greater then that.
For the limitation of RF spectrum Dr. Harald Haas comes up with an idea of using some other band from EM spectrum and introduced the VLC using visible light spectrum
Rapid development in the field of lightning and illumination, to take out the energy consuming incandescent bulbs and replace them with energy efficient solid-state sources. These solid-state sources commonly known as LED’s.
These solid-state sources offer the high probability of high data communication in addition to illumination. Idea is simple to modulate the sources at high speed using sources for illumination and data communication as well. Visible light band is 10,000 times bigger then that of RF band. VLC uses visible light between 400 THz(780nm) and 800 THz(375nm) as carrier for data transmission .
Li-FI (Light Fidelity)
Li-FI is the transmission of data through illumination by taking out the fiber from fiber optics, by sending data through light LED bulb that varies in intensity faster then the human eyes.
Working Principle
Li-fi is optical wireless network, if light is on we transmit a logic ‘1’ if light is off logic ‘0’.These transaction between on and off of light is too fast that human eye can’t detect the change.
There are two main components of this system 1) High brightness LED act as a source 2) high sensitive photo-detector can be a silicon photodiode and one last thing a controller that codes data into LED’s.
Basic block diagram is shown in figure above, the data from servers and internet, Lamp driver or controller process the signal to feed to LED lamp now the light beam from LED has data embedded into it which falls on the Photo-detector (Photo-diode). Photodiode captures the light and convert it into electrical signals then electrical signal is amplified and processed and we get the desired output on our devices
VLC Link
VLC link consist of a transmitter, receiver and a propagation channel.
Transmitter or Source is a White-light LED which use red, blue and green LED to provide the desired color or a single LED that excites the phosphor. Different color approach is helps to send data to multiple devices.
Figure shows the emission spectrum of high- brightness LED with peak intensity at 550nm of wavelength.
Receiver, consist of a optical element which is lens or non imaging concentrator with maximum gain it collect and concentrate that radiation on photodetector. This photodetetor converts the optical radiation into photocurrent.
The VLC channel ‘ There are different channel exit between source and terminal, channel of Line of sight and channel created by reflection of light from different surfaces both these channel are modeled differently and combined to obtain overall power at the receiver.
Figure 3.3 SNR at different points of room
Potential applications of Li-Fi
Green technology ‘ As radio and other communication waves are harmful to human body, this technology has no effect on human health .LED are very power efficient makes this technology energy efficient and cheaper.
Aircrafts ‘ RF wireless communication is not acceptable in aircraft because these RF signal can interfere with the aircraft system. Li-FI can help to provide multimedia access in aircraft cabins and as no wired infrastructure is required it helps to reduce the weight .
Underwater communication ‘ RF does not work underwater, ultrasound is too slow but through VLC underwater communication is possible.
Military and hospitals (Security)’ RF communication is avoided in these are due to security purposes, VLC is more secure because it does not penetrates the solid object.
Information display communication ‘ All the shopping malls use LED to display, through these display owners can also transmit there new offers through these LED , in Museum light shedding on object can have information stored in it
Visual signaling ‘ In traffic lights and car lights to avoid accident and to update the traffic information.
Figure 4.1 Communication between cars through head and tail light
Challenges and possible solutions –
Increasing data rate
Simple on ‘off modulation known as pulse modulation
We only have two signals ‘1’ for on and ‘0’ for off.
figure 5.1 On-off modulation scheme
Other modulation scheme, which can overcome this low data rate, is orthogonal frequency division multiplexing.
We are converting sets of symbols in real signals, by varying intensity heavily, which was a problem in RF where we don’t want to vary the amplitude. But in this case we are varying the intensity at high speed that is not recognizable to human eyes.
Orthogonal frequency division multiplexing in Li-fi
We are breaking the frequency in smaller bands or pipe each pipe carrying one symbol, as in the above figure 1 symbol carrying 4 bits. In this 16 symbols on real and imaginary axis, which was not the case in on ‘off modulation where we had only two symbols.
We are increasing the capacity by factor 4′?, so if we have 10 pipes we can increase the capacity by factor 40.
Optical MIMO
Illumination applications many LEDs are used to provide the necessary lighting intensity. This offers the opportunity of transmitting different data on each device or on different groups of emitters. For this to be successful a detector array is required at the receiver, and this creates a Multi-Input Multi-Output (MIMO) system. Radio-frequency MIMO techniques can be applied to such optical transmission systems to relax the necessary alignment between the array of detector and array of sources.
figure 5.3 MIMO in optical communication
Conclusion
Li-Fi is the future technology that will one day replace all the RF based wireless communication system .It is faster, cheaper, cleaner, power efficient. This technology don’t need infrastructure because it is there, every place has a light bulb just need to replace with LED with smart chips. According to DR Harlod Hass every street light bulb could be wireless spot of Internet access.
REFERENCES ‘
Visible Light Communications Consortium,www.vlcc.net, 2008
IEEE: IEEE 802.15 WPAN Visual Light Communication Study Group (IGvlc) http://www.ieee802.org/15/pub/IGvlc.html, 2008
the-gadgeteer.com/2011/08/29/li-fi-internet-at-the-speed-of-light
Visible-light communication: Tripping the light fantastic: A fast and cheap optical version of Wi-Fi is coming’, Economist, dated 28Jan 2012.
Visible Light Communications: challenges and possibilities Dominic C. O’Brien, Lubin Zeng1, Hoa Le-Minh1, Grahame Faulkner1, Joachim W. Walewski, Randel, University of Oxford (UK); Siemens AG, Corporate Technology, Information and Communications, Munich (Germany).
www.lificonsortium.org