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1 Prof. Brandt-Pearce Lecture 4 Visible Light Communications Optical Wireless Communications.

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Presentation on theme: "1 Prof. Brandt-Pearce Lecture 4 Visible Light Communications Optical Wireless Communications."— Presentation transcript:

1 1 Prof. Brandt-Pearce Lecture 4 Visible Light Communications Optical Wireless Communications

2 Visible Light Communications (VLC)  Introduction  Applications  White LED  Illuminance Distribution  Channel Model  Challenges and Solutions 2

3 3 (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

4 4 Indoor VLC Can provide network access at  Home  Office  Shopping Center  Plane  Hospital  Convention Centers

5 5 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

6 6 Light-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)

7 77 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

8 8 VLC vs. Infrared (IR) and Radio-frequency(RF)

9 White LEDs 9 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

10 White LEDs 10  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)

11 White LEDs 11  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

12 12 An illustration of the VLC concept This is for the downlink only, and a parallel similar system is needed for the uplink.

13 Signal Distribution 13 Three main options:  Electrical network – extension of Internet  Passive optical network (PON)  Wireless-over-fiber  Power-line communication system

14 14 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

15 15 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.

16 16  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 Φ e as where V(λ) is the standard luminosity curve, and K m is the maximum visibility, which is ~683 lm/W at 555 nm wavelength Illuminance Distribution in VLC

17 17  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

18 18 An example of a VLC system

19 19 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

20 20 Channel Model The 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.

21 Channel Model 21  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

22 Performance Limits 22  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

23 Signal Processing 23  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

24 Challenges and Solutions 24 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

25 Challenges and Solutions 25 Solution 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

26 Challenges and Solutions 26 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

27 Challenges and Solutions 27 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)

28 Challenges and Solutions 28 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

29 Challenges and Solutions 29 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)


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