· Web viewABSTRACT- This paper introduces the concept of Visible Light Communication (VLC). VLC is used to overcome the problem faced in RF wireless communication. The visible light

A Survey on Visible Light Communication

Anil Semwal M. Tech ECE PITK

[email protected],

Associate Prof. A. S. ButtarH.O.D- E.C.E (Wireless-Communication)

PITK, Jalandhar Punjab (India),[email protected]

ABSTRACT- This paper introduces the concept of Visible Light Communication (VLC). VLC is used to overcome the problem faced in RF wireless communication. The visible light portion of quanta particle of the electromagnetic spectrum is used in VLC to transmit information via Li-Fi which is similar to established form of wireless communication such as Wi-Fi which uses radio frequency (RF) signal to transmit information. While in VLC, communication takes place by modulating the intensity of the LEDs light in such a way that it is undetectable to the human eyes, LED is used as a Transmitter but the forward voltage and light emission of the LED vary with temperature and driving current. A photodiode is used as a receiver which can detect and modulates the incident light signal into electrical signal, this work provides a huge overview of applications, design challenges and the impact of channel equalization, modulation techniques for VLC that uses white LEDs is described.

KEYWORDS - Line Of Sight (LOS), Light Fidelity (Li-Fi), Physical Layer (PHY), Visible Light communication (VLC).

I. INTRODUCTION

At present scenario, wireless communication has become fundamental to our lives and we transmit a lot of data every day by using electromagnetic waves, in particular radio waves. Due to restricted spectrum availability and interference, radio waves can support only limited bandwidth. Furthermore, radio spectrum is full to bursting and it is difficult to find radio capacity to support media applications. Visible Light Communication (VLC) is an emerging wireless communication with a promising future and which can be a complement of radio waves. VLC is a data

communication technology that uses a visible light spectrum in the range of wavelength between 380 –780nm. One of the first visible light wireless communication systems was reported by Alexander Graham Bell. In 1880, he transmitted the wireless telephone message on his newly invented “Photophone” [1], [2]. In VLC system the visible light source as a signal transmitter, the air as a transmission medium or channel and a signal receiving device. Generally, the transmitters are Light Emitting Diodes (LEDs) while the principal device of the receiver is a photo-detector, usually a photodiode. By using VLC in short distance applications, we can supplement radio waves achieving high data rates and a larger bandwidth. A LED is a semiconductor device that has the advantages of fast switching, power efficiency and emits visible light that is safe for the human because it is not harmful to vision. Therefore, we can both illuminate and transmit data everywhere. A basic model of a VLC system using LEDs was firstly introduces by Toshihiko komine and Masao Nakagawa in [3]. LED light bulbs have high intensities and can achieve very large data rates [4]. The Government of US and UK had decided to replace all inefficient high power consuming incandescent fluorescent light bulbs/tubes with highly efficient and low power consuming LEDs [5].

II. A Visible Light Indoor Communication System Model

Fig.1. Visible light indoor optical wireless system using LED lights

A. Transmitter

It is assumed that an LED has a Lambertian radiant intensity,

R0 (∅ )=[m+12π ]cosm (∅ ) [6]

where m is the order of Lambertian emission, m=ln2/ln(cos¿¿)) and ∅ 1/2 is the transmitter semi-

angle (at half power). The transmitted power is

Ptx=PLED∗R0 (∅ )

B. Channel Light beam propagates from the LED to the receiver (Photodiode) via two main channels: line of sight (LOS) and diffuse channels.

1) LOS Channel: The transfer function for LOS channel

(2)

The total power of i LEDs in the directed path, see Fig.1 is calculated in Equ.3:

(3)

where H LOSi (0) is the ith LED channel DC gain.

2) Diffuse Channel: A integrating sphere model for the optical wireless diffuse signal was introduced in [7]. The receiver is assumed as a small part of a the room surface, so the received diffused power Pdiff with the receiving area Arx is

Pdiff=Arx Iwhere I is the total intensity of light emitted by source. The diffused channel loss is

ƞdiff=¿ Pdiff /P total LED

C. Receiver

The received power is :

Prx = (P¿¿ LOS+Pdiff )¿ * T f (ψ )* g(ψ ¿

where T f ¿) is the transmission coefficient and g(ψ ¿ is the concentrator gain. The photodiode will convert the received optical power into electrical current, and the current is:

i = Prx* Rwhere R is the photodiode responsivity (A/W).The SNR is given by:

SNR=Prx

2

σ total2

where σ total2 is total noise variance.

Fig.2. SNR distribution in a room¿) dimension [30]

The BER is calculated as:

BER=Q√(SNR)

where Q(x) = 1√2 π∫x

∞

e− y2

2 dy

The SNR (dB) is a function of the receiver’s position is shown in Fig.2

III. APPLICATIONS VLC applications are classified into six categories: Homeland Security Defense, Transport, Hospitals, Industrials, Public sector, and. Domestic.

Homeland Security Defense: The VLC technology can be activated in the visible spectrum and by gaining natural advantages of Wireless Optical Communication (OWC) like secure and covert communication. Unique VLC positioning and range technology can replace GPS based systems in many

places [8]. In commercial market some applications may be similar to some of the Homeland Security applications. The case in which two or more vehicles are required to establish communication and to accurately detect their relative position and distance specially in RF jammed environment. This can be translated to any manned or unmanned moving platforms. Due to quick deployment and fast communication supplied, VLC system can be used as a system of choice in a disaster recovery situation. VLC based FSO communication solutions are already in service with several military organizations [9], [25].Transport: At present street lamps, signage and traffic signals are also using LEDs. This can be used for vehicle to vehicle and vehicle to roadside communications [10]. Road safety and traffic management will become effective with VLC via street lighting and traffic lights. VLC can be implemented for aircraft traveler lighting to listen music and watch video [11], [12]. Fig. 3 shows the communication between vehicle to vehicle and vehicle to traffic control infrastructure [13].

Fig. 3 Application of VLC in Transport [14]

Hospitals: VLC prevent medical equipment from Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI). VLC does not emit EMI or RFI. So it does not interfere with medical instruments and also it does not interfere with MRI scanners. Hence VLC provides equipment and staff communications with no EMI and RFI problems [11].

Industrial: In environments such as petro-chemical plants, mines, etc. EMI from RF is potentially dangerous because there are explosion risks, so communication becomes difficult. VLC can be interesting in this area as it is a safe technology and provides illumination and communication at the same

time. Excess capacity demands of Wi-Fi networks can be offloaded to VLC networks [4]. VLC is especially effective on the downlink in the RF congestion area.

Public sector: Transmission of a unique ID is all that is required for basic positioning for providing local information in museums, communications for civil contingencies. Multiple LED light bulbs can be used with relative location for more accurate indoor positioning and navigation [10]. Also in the malls we can provide VLC tags for positioning and localization purpose [15], [16].

Domestic: RF technology is relatively expensive and difficult to implement. VLC is working on unlicensed band not produces E-smog and easy to deploy and implement. Any lighting lamps can be used to provide VLC hotspots and the same communications and sensor infrastructure can be used to monitor and control lighting and data. In fig.4 Laptops, computers, printers, mobile phones, tablets and other mobile devices are interconnected using VLC. Over short range links it provides security via the visible pairing method and also gives very high data rates.

Fig. 4 Boston Smart Lighting office [26]

Underwater Communications: VLC can support high data rates beneath the water, where other wireless technologies like RF do not work. Thus, communications between divers or remote operated vehicles are possible which is given in Fig.5.

Fig.5 VLC in underwater communications Naka Lab

IV. VLC CHALLENGES

VLC is still in the early stage that there are many severe problems or limitations needed to be solved.

Line Of Sight (LOS): LOS has an advantage of the signal strength. Visible light signals can be reflected but does not penetrate most of the objects which can be a security advantage and perhaps a coverage disadvantage [17], [18]. However, if the link from LOS is block there is no signal is detected at receiver from LOS light sources. Only NLOS path is detected from multipath after reflection, diffraction and scattering, reflection can absorb much energy so that the rate of communication without LOS between the transceivers is greatly limited. Not any optical spread signal under power regulation can be strong enough to let reflected signals still preserve enough power for communication. If light levels are low and VLC receiver can collect photons, it can receive data at a lower data rate. Like radio technology that indirect signals have a lower power and hence the data rate reduces [13], [19].

Transmitter Sources: There are three types of white LED: RGB, Blue chip + Phosphor, and Organic LED (OLED). The use of RGB LED is limited, and problem with balancing each R, G, B component to create white light. The popular Blue chip + Phosphor is an efficient and cheap technology for today general lighting, and the third type of LED OLED is an expensive and short life time. It is, however, very potential.

Equalization: By Equalization the modulation bandwidth of the white LED increases from (3MHz to 50MHz) and the data rate increases from 16Mb/s to 32Mb/s with NRZ-OOK modulation [6].

Modulation: VLC channel is ‘low-bandwidth, high signal to noise ratio. High-order modulation schemes increases data rates. High PAR due to non-linearity of amplifier is utilized by introducing OFDM will

also increase the communication range [20]. Complex modulation Discrete Multi-Tone Modulation is used to increase data rates [21]. VPPM modulation supports illumination with dimming control and communication simultaneously [22]. Color shift key modulation with non-uniform signaling support the RGB weights combinations group according to the actual parameters of the RGB light source, the maximum color coordinate distance cannot directly lead to the best BER performance [31].

Multipath Distortion: When the transceivers are equipped with wide beam, the copies of the same signal from different paths arrive the destination via different multipath with different amount of delay, because each path has different length from source to destination [23]. This creates a problem called multipath distortion which can cause Inter Symbol Interference that severely degrades the performance.

Interference from sunlight: When the ambient light is present where the VLC system is deployed this becomes more critical since the ambient light is could be very strong that the resulting SNR is low [23]. The cost will effective by equipping a good receiver to distinguish such low signal with encountering high signal attenuation. It is relatively simple to eliminate the huge amount of interference from natural and artificial sources using optical filters [19] and digital filtering the interference is negligible.

Links: VLC can transmit data in either uplink or downlink [10]. The uplink and downlink can be established via number of ways like wavelength, time, code and also by spatial multiplexing. Due to cost reasons and high bandwidth VLC might be implemented for downlink. Wi-Fi or IR may provide a reliable uplink where congestion is less likely and VLC provides a high capacity uncongested downlink. V. Need of VLC

The bottleneck problem in RF communication due to demand of spectrum for communication is increasing at the rate of 108% per year. RF is medically proved to be unfit for humans also it gives rise several neurological diseases and other medical problems too.

Fig. 6 Haitz graph [24].

VLC system works on the principle of intensity of light. The new LEDs coming in the market known as Resonant Cavity (RC) LEDs are very highly efficient. They have very bright light and low power consumption [24]. In table 1 the typical efficiency and lifetime values of common white light sources, are shown, as along with the predicted values of white light LED’s around the year 2000.

Table1. Comparison of different Light sources Efficiency and Lifetime [24].

VI. COMMUNICATION ARCHITECTURE

The VLC system transmits signals by controlling the ON/OFF repetition of LED or the color of transmitting light. VLC system is a different from RF system. In VLC system LED plays the role of a

transmitter and the photo diode plays the role of a receiver. There are two parts of VLC architecture one is a transmitter part and the other one is a receiving part. The sending part can use any kind of LED illumination. The sending part of VLC must have Physical Layer (PHY) & Data Link Layer (DLL) functions for illumination and transmission performance. The receiving Part of VLC can support any kind of Photo Diode with prevention from interference of any other light source. There are a PHY and a Media Access Control (MAC) as a common part of the transmitter and receiver of VLC. PHY has a modulation and a line coding for a wireless communication and VLC MAC has to support different Applications [25]. The OSI reference model of VLC system is illustrated in Figure 7. In this PHY and DLL plays main important role in communication. OSI LAYER VLC Communication Protocol

SAP–Service Access Point

Fig.7 OSI reference model of VLC Architecture [27]. The physical layerThe PHY defines the electrical and hardware specifications for devices. Exactly it defines the relationship between a device and a physical channel. One device transmits information to the channel, and another device receives data from channel based on the PHY. IEEE 802.15.7 specification defines 7 colors channel for physical layer in VLC. In PHY to achieve full duplex communication it requires a separate uplink and downlink. VLC link mainly used for downlink and for uplink IR or RF is preferred.

The communication takes place in terms of packets in the PHY. The Synchronization Header (SHR), PHY Header (PHR), and PHY Service Data Unit (PSDU) together form the PHY packet which is known as a Physical Protocol Data Unit (PPDU) [28]. Several modulation methods are used in high data rate PHY such as Color Code Modulation (CCM), High Hamming Weight code (HHW), On-Off Keying (OOK) and VPPM (Variable PPM). A modulation of VLC might be decreasing the brightness of LED illumination [25], [29]. Three physical layer communication options are currently specified as per system requirement. PHY I, PHY II and PHY III [25].

VII. FUTURE WORK VLC system have major research challenge of LOS, NLOS and filtering of ambient light from the useful portion of signal. It can be mitigated by the application of “Quantum theory of Bosons”, the interaction of light (photon) with mater.

VIII. CONCLUSION

We focus on the concept of Visible Light Communication, the several modulation techniques used in VLC, the designed challenges and the impact of equalization, on VLC system furthermore the future work which may be mitigate the present challenges on VLC by means of Quantum theory of Bosons.

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