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Prof. Brandt-Pearce

Lecture 4Visible Light Communications

Optical Wireless Communications

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Visible Light Communications (VLC)

Introduction

Applications

White LED

Illuminance Distribution

Channel Model

Challenges and Solutions

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(Ref: Wikipedia)

Visible light is only a small portion of the electromagnetic spectrum.

Visible Light Communications (VLC)

Dates back to 1880, when Alexander Graham Bell invented the photophone

VLC is used for Vehicle to vehicle communication Networking in indoor environments

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Indoor VLC

Can provide network access at Home Office Shopping Center Plane Hospital Convention Centers

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Advantages Safe for health Secure No interference on RF signals High speed Confined to small geographical area

Challenging Problems Connectivity while moving Multiuser support Dimming Shadowing Confined to small geographical area

Indoor VLC

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Indoor VLCLight-emitting-diodes (LED) are preferred sources for dual purpose of lighting and data communications

Eye-safety regulations (compared to Laser) Longevity Lower cost Are mercury free Less consumption High speed Have smaller and compact size Minimum heat generation higher tolerance to humidity A much higher energy conversion efficiency (white LEDs with

luminous efficacy greater that 200 lm/W are now available)

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WiFi Has limited capacity, and cannot increase it easily, because

it covers a wide area, services potentially many users, and limited bandwidth. Higher order modulation of limited use since SNR limited.

E.g., office buildings, conference centers, stadiums

Bluetooth Single user system for personal area communications.

Very small range and low data rate. Less shadowing so good around moving people.

E.g., wireless microphones

Millimeter Wave? UWB?

Rival Technologies

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VLC vs. Infrared (IR) and Radio-frequency(RF)

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White LEDs

Two technologies in white LEDs Phosphor-based LEDs

This technique involves the use of blue LED coated with a phosphor layer that emits yellow light

The phosphor layer absorbs a portion of a short wavelength light emitted by the blue LED and then the emitted light from the absorber experiences wavelength shift to a longer wavelength of yellow light

Are cheap and are less complex

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White LEDs

Trichromatic Generates white light by combining red (~625 nm), green

(525 nm), and blue (470 nm) (RGB) in a correct proportion Are high-speed Enables color control Typically, these triplet devices consist of a single package

with three emitters and combining optics Are attractive for VLC as they offer the possibility of

wavelength division multiplexing (WDM)

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White LEDs

The most important factor in VLC is the switching properties of the visible LEDs

They have the ability to be switched on and off very rapidly

thereby making it possible to impress data on their radiated optical power/intensity

Modulation speed of white LEDs is limited due to the relaxation time of the LEDs BW of trichromatic LEDs < 20 MHz BW of phosphor-based LEDs < 5 MHz

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An illustration of the VLC concept

This is for the downlink only, and a parallel similar system is needed for the uplink.

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Signal Distribution

Three main options: Electrical network – extension of Internet Passive optical network (PON) Wireless-over-fiber Power-line communication system

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A block diagram of a VLC system

Precise dimming appears to be challenging for incandescent and gas-discharge lamps

With LEDs it is quite convenient to accurately control the dimming level

The illumination requirement is that the illuminance must be 200–1000 lx for a typical office environment

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Generally there are 4 configurations for indoor optical links 1

(a) Directed – line-of-sight (LOS) link(b) Non-directed LOS link(c) Diffuse link(d) Quasi diffuse link

Indoor VLC Configurations

1 H. Elgala, R. Mesleh, and H. Haas, “Indoor optical wireless communication: Potential and state-of-the-art,” IEEE Commun. Mag., vol. 49, no. 9, pp. 56 – 62, Sep 2011.

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Since LEDs are used for the dual propose of illumination and communication, it is necessary to define the luminous intensity and transmitted optical power Transmitted optical power indicates the total energy radiated from an LED Luminous intensity is used for expressing the brightness of an LED Luminous intensity is the luminous flux per solid angle and is given as

where Φ is the luminous flux and Ω is the spatial angle

Φ can be calculated from the energy flux Φe as

where V(λ) is the standard luminosity curve, and Km is the maximum visibility,

which is ~683 lm/W at 555 nm wavelength

Illuminance Distribution in VLC

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In fulfilling the lighting requirements, a single high luminous efficiency LED can only provide limited luminous flux and over a limited area To illuminate a much larger environment, spatially distributed LED clusters would be needed LED array, and illuminance distribution for (b) one transmitter and (c) four transmitters

Illuminance Distribution in VLC

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An example of a VLC system

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Optical power distribution in received optical plane for using four sources and a FWHM of

(a) 70°(b) 12.5°(c) with reflection from walls for 70°

Illuminance Distribution in VLC

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Channel ModelThe output PSD of a white phosphor-based LED (solid line, which corresponds to the left axis) is compared to the measured spectral reflectance (which corresponds to the right axis) of plaster and plastic wall (dash-dot line), floor (dash line) and ceiling (dot line) 1

1 L Kwonhyung, P Hyuncheol and J R Barry, IEEE Communications Letters, 15, 217–219, 2011.

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Channel Model Multipath effect limits the channel bandwidth The impulse response of the channel is modeled as a short

impulse (caused by LOS path) followed by a broad pulse (multipath effect/NLOS path)

The delay between the two parts is determined by room geometry and size

The NLOS part is usually modeled as a Gaussian pulse

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Performance Limits

System is typically either

bandwidth limited

background-light limited (if daylight falls on PD)

If channel-bandwidth limited, use higher-order modulation

or equalizer

If background-noise limited:

Shot noise due to receive intensity – nothing can be done

Decrease symbol rate

Channel state information (CSI) is needed at the transmitter

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Signal Processing Optical and electrical filtering:

Block out-of-band background light Remove electrical harmonics

Equalization required: Bandwidth limited by LED response and by multipath Types of equalizers:

FIR filters, adapted using an LMS algorithm Decision-feedback equalizer MLSE – very complex

Remove multiuser interference

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Challenges and Solutions

As discussed before, main challenges for indoor VLC systems are

Connectivity while moving: users need to be connected

when they move inside the indoor environment

Multiuser support: in large areas is vital, many users need

to have access to the network at the same time

Dimming: is an important feature in VLC when

communications is integrated with lighting

Shadowing: happens when the direct paths from user to all

sources are blocked

Some solution has been proposed for each one

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Challenges and SolutionsSolution for connectivity

This problem is similar to the connectivity problem in cellular network when you move from one area of the city to another area while speaking with cell-phone

The solution is called “handover”, using which the user is transferred from one BS to another

Handover is done in the area that two BS’s have common coverage Similar solution can be used in signal processing domain for VLC The user can be transferred from one light source to another in the

area that is under the coverage of both

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Challenges and Solutions

Solution for multiuser support One solution is time division

multiplexing (TDM) Each frame is divided into equal

time slots Each user transmit data in one time

slot in a predefined order

The other solution is code division multiple access (CDMA) Codes are assigned to users Each user transmit its data using the assigned signature pattern It is used in 3G and 4G cellular networks CDMA has been adopted and developed for optical systems Optical orthogonal codes (OOC) are used as signature pattern for

users

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Challenges and Solutions

Solution for multiuser support Last solution is spatial multiplexing Can use to increase data rate or to add users Rely on LED arrays and multiple receivers Or can use an imaging receiver (camera)

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Challenges and Solutions

Solution for dimming Two main solutions are proposed for solving dimming

problem in VLC systems Pulse width modulation (PWM) is combined with other

modulation schemes in order to control the duty cycle of the transmitter signal

By controlling the width of the PWM signaling, the dimming level can be controlled

The other solution is using modified forms of PPM In these schemes multiple pulses are transmitted instead of

one pulse By controlling and changing the ratio between the number of

pulses and the length, the dimming level can be altered

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Challenges and Solutions

Solution for shadowing As shown before, the impulse

response in VLC systems has two parts

When the line-of-sight (LOS) part (which is received via direct path) is blocked, the impulse response is only the second part

Then the data can be recovered using the second part which is indeed the received data from the indirect paths (multipath signal)