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

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

Visible Light Communications (VLC) Visible light is only a small portion of the electromagnetic spectrum. (Ref: Wikipedia) Dates back to 1880, when Alexander Graham Bell invented the photophone VLC is used for Vehicle to vehicle communication Networking in indoor environments

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

Indoor VLC Advantages Challenging Problems 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

Indoor VLC 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)

Rival Technologies WiFi Bluetooth 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? 7

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

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

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)

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

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

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

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

Indoor VLC Configurations 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 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.

Illuminance Distribution in VLC 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 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

An example of a VLC system

Illuminance Distribution in VLC 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°

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.

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

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

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

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

Challenges and Solutions 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

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

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)

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

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)