What is LIFI ?

 

You will find on this page:

- Top 10 Li-Fi myths (a text wrote in the 2014 but always topical)

- Light Fidelity (LI-FI) based indoor communication system

 

© http://visiblelightcomm.com/top-10-li-fi-myths - RIPRODUZIONE RISERVATA visiblelightcomm.com

Top 10 Li-Fi myths

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.


1. The lights flicker.

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!


2. You cannot dim the lights.

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.


3. VLC is uni-directional (downlink or broadcast only).

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.


4. There will be interference from sunlight.

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.


5. Lights need to be on so this is inefficient.

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.


6. You must have line-of-sight.

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.


7. This is a disruptive technology.

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.


8. You need special LEDs

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.


9. VLC is a complex technology

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.


10. It will never work!

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

LIGHT FIDELITY (LI-FI) BASED INDOOR COMMUNICATION SYSTEM

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 .

KEYWORDS

 

Light Fidelity, Indoor communication, Visible light communications, Wi-Fi.

1. INTRODUCTION

Light Fidelity (Li-Fi) is a new paradigm of revolution which is a continuation of the trend to move toward higher frequency spectrum[2] 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 [1]. 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 [2]. Li-Fi based communication system is different from Visible light communication (VLC) system[2] 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 [3]. In [4] 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 [5] .In current indoor visible light positioning systems, several algorithms are proposed to calculate the receiver position as in [6] researchers propose a novel architecture system that can be used for both indoor positioning and communications. In [7] light positioning architecture for a typical room has been investigated by considering different performance related parameters. In [8] 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 conclusion. 

2. LI-FI BASED INDOOR COMMUNICATION SYSTEM OVERVIEW

2.1.LI-FI TRANSMITTER AND RECEIVER

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 [9] and 3.4 Giga bits per second has been redgreen-blue (RGB) LEDs [10], 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.

 

2.2. MODULATION

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 [11] 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 [12] researchers proposed a Asymmetrically-Clipped Optical OFDM (ACO-OFDM) in which odd subcarriers are modulated DC-biased Optical OFDM (DCO-OFDM) [13] 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 [14]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 [15] 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 [16] 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.

 

2.3. COMMUNICATION MODES IN LI-FI 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 [17] But the MIMO channel need to be highly correlated, which increases the complexity of decoding parallel channels in the receiver [18] We consider here as shown in figure 3.

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.

3. SCENARIO BASED INDOOR LI-FI ARCHITECTURE DESIGN

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.

3.1. FIXED 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.

3.2. MOVEABLE LP

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.

3.3.HYBRID LP

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.

4. PERFORMANCE EVALUATION OF PROPOSED DESIGN

 

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.

5.CONCLUSION

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.

REFERENCES

[1] Taner Cevik and Serdar Yilmaz “An overview of visible light communication system” International Journal of Computer Networks & Communications (IJCNC) Vol.7, No.6, November 2015. [2] Harald Haas, Member, IEEE, Liang Yin, Student Member, IEEE, Yunlu Wang, Student Member,and Cheng Chen, Student Member, IEEE, ”what is LiFi?”Journal of Light wave, vol. 34, no. 6, MARCH 15, 2016. [3] T.Komine and M. Nakagawa,“Fundamental analysis for visible light communication system using LED lights,” IEEE Trans.consumerElectron., vol. 50, no. 1, pp. 100–107, Feb. 2004. [4] J.Ding, K. Wang, and Z. Xu, “Impact of different LED-spacing in arrayed LED transmitter on VLC channel modeling,” in Proc. 6th Int. Conf. Wireless Commun. Signal Process. (WCSP) , Oct. 2014, pp. 1–6. [5] Indoor Visible Light Positioning System with Multipath Reflection Analysis Wenjun Gu, Student Member, IEEE, Mohammadreza Aminikashani, Student Member, IEEE, and Mohsen Kavehrad, Fellow, IEEE, 2016 IEEE International Conference on Consumer Electronics (ICCE). [6] Indoor Positioning with OFDM Visible Light Communications, Mohammadreza Aminikashani, Wenjun Gu, and Mohsen Kavehrad, 2016 13th IEEE Annual Consumer Communications& Networking Conference (CCNC). [7] Impact of Multipath Reflections on the Performance of Indoor Visible Light Positioning Systems Wenjun Gu, Student Member, IEEE , Mohammadreza Aminikashani , Student Member, IEEE, Peng Deng, and Mohsen Kavehrad , Fellow, IEEEJOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 34, NO. 10, MAY 15, 2016. [8] Optimal Constellation Design for Indoor 2×2 MIMO Visible Light Communications ,Heng-Bin Cai, Jian Zhang, Yi-Jun Zhu,Member, IEEE, Jian-Kang Zhang,Senior Member, IEEE,and XinYang, IEEE COMMUNICATIONS LETTERS, VOL. 20, NO. 2, FEBRUARY 2016. [9] Khalid, A. M., Cossu, G., Corsini, R., Choudhury, P and Ciaramella, E “1-Gb/s Transmission Over a Phosphorescent White LED by Using Rate-Adaptive Discrete Multitone Modulation,” IEEE Photonics Journal 4, 1465–1473 (Oct. 2012). [10] Cossu, G., Khalid, A. M., Choudhury, P., Corsini, R., and Ciaramella, E., “3.4 Gbit/s Visible Optical Wireless Transmission Based on RGB LED,” Optics Express 20, B501–B506 2012. [11] M.Z.Afgani, H. Haas, H. Elgala, and D. Knipp, “Visible light communication using OFDM,” in Proc. 2nd Int. Conf. Testbeds Res.Infrastruct. Develop. Netw. Commun., 2006, p. 134, doi: 10.1109/ TRIDNT.2006.1649137. [12] J.Armstrong and A. Lowery, “Power efficient optical OFDM,” Electron.Lett., vol. 42, no. 6, pp. 370– 372, Mar. 2006. [13] H.Elgala, R. Mesleh, H. Haas, and B. Pricope, “OFDM visible light wireless communication based on whiteLEDs,” in Proc. IEEE 65th VTC-Spring, Apr. 2007, pp. 2185–2189. [14] IEEE Std. 802.15.7-2011, IEEE Standard for Local and Metropolitan Area Networks, Part 15.7: Short-Range Wireless Optical Communication Using Visible Light, IEEE Std., 2011. [15] P.M.Butala, J. C. Chau, and T. D. C. Little, “Metameric modulation for diffuse visible light communications with constant ambient lighting,” in Proc. Int. Workshop Opt. Wireless Communication., Pisa, Italy, Oct. 2012,pp. 1–3. [16] K.-I.Ahn and J. K. Kwon,“Color intensity modulation for multicolored visible light communication”, IEEE Photon. Technol. Lett., vol. 24, no. 24, pp. 2254–2257, Dec. 2012. [17] D.Tsonev, S. Sinanovic, and H. Haas, “Practical MIMO capacity for indoor optical wireless communication with white LEDs,” in Proc. IEEE VTC—Spring, 2013, pp. 1–5. [18] F.Bohagen, P. Orten, and G. Oien, “Design of optimal high-rank line of-sight MIMO channels,” IEEE Trans. Wireless Commun., vol. 6, no. 4,pp. 1420–1425, Apr. 2007. [19] Seok-Ju Lee and Sung-Yoon Jung “A SNR Analysis of the Visible Light channel Environment for Visible Light Communication” IEEE APCC 2012.

AUTHORS

Farooq Aftab

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).

Shahzad Ali

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.