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Visible Light CommunicationsSnigdha SharmaShivani Sheth

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Presentation Outline

1. Introduction2. How it Works3. Modulation Techniques4. Implementation5. Application6. Further Research

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INTRODUCTION1

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Visible light communication (VLC) is a data communications variant which uses visible light between 400 and 800 THz (780–375 nm). VLC is a subset of optical wireless communications technologies.

What is Visible Light Communication?

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● Relies on optical radiations to convey information in free space, with wavelengths in IR to UV range

● Transmitter○ converts electrical signal to an optical signal○ Can use LEDs or laser diodes (LDs)

● Receiver○ converts the optical power into electrical

current○ Can use photodetectors

Optical Wireless (OW) Systems

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Indoor Optical Wireless Potential

● OW is essential for short-range communication with high throughputs

● Beneficial because○ Unregulated○ Low cost○ No license needed

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● Pros:○ WLEDs for data transmission and lighting using

existing electrical wiring○ Distributed ceiling installations → dominant LOS

component and small path loss ○ estimated bandwidths > 88 MHz

● Challenges:○ Link can be lost due to movement or rotation of

the receiver units. ○ link recovery and handover mechanisms

needed○ Room illumination → high SNR ratio (>60 dB)

Indoor Optical Wireless Potential

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● The free space loss (FSL)● Ambient light noise and/or interference● Multipath dispersion causing intersymbol

interference (ISI).

Key Design Challenges

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Earlier Systems

● Concept dates back to 1880s with Alexander Graham Bell’s photophone○ transmitted speech on modulated sunlight over

several hundred meters● 1979:

○ First IR system○ Diffuse link operating at ~950 nm and 1 Mb/s ○ Gfeller and Bapst

● 1996:○ Data rate of 50 Mb/s. ○ March and Khan

● 2000: ○ Quasi-diffuse systems○ Data rate of 70 Mb/s ○ Carruther and Kahn

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HOW IT WORKS2

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Tradeoffs between LOS and FOV as well as transmission power and complexity:a) directed-LOSb) undirected-losc) diffuse linkd) quasi-diffuse

Indoor Link Configurations

Optimal Operating Range

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● 780 nm to 950 nm wavelengths● low-cost optical sources readily available● coincides with the peak sensitivity of

inexpensive photodiodes

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● driven by progress in white LED (WLED) technology for solid state lighting and the potential of simultaneously using such LEDs for wireless data transmission

● two types○ Trichromatic

■ data rates up to 400 Mb/s○ blue-chip LEDs

■ modified version of OFDM achieved data rates higher than 500 Mb/s

White LEDs in VLC

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Typical Indoor OW Systems

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● Consists of○ light source (LEDs)○ propagation medium (free

space)○ light detector (PIN PDs)

● Signals modulated to light source and then into an optical system and then processed

● Only intensity of optical wave is detected → no phase or frequency information (different from regular wifi processing)

Application: Indoor OW Systems● Wired, wireless, and satellite technologies.● Wired base stations + WLED-based illumination

equipment (reading lamps) provide passengers with Internet access and a range of multimedia services.

● Power over Ethernet (PoE) technology used to transport data traffic and supply the base stations and reading lamps with the required power

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MODULATION TECHNIQUES3

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Single-Carrier Pulsed Modulation

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● Time-dependent characteristics of the optical pulse is used to convey information

● Two methods:○ On-off keying (OOK) ○ Pulse-position modulation (PPM)

Multiple-Subcarrier Modulation

● OFDM is a practical realization of multiple subcarrier modulation (MSM). ○ Do not require complex channel equalizers○ Possibility to combine OFDM with any multiple

access scheme makes it an excellent preference for indoor OW applications

○ In general, the output of the OFDM modulator is complex. In IM optical systems, quadrature modulation is not possible (i.e., phase information detection is not possible for IM/DD systems). Therefore, the OFDM commonly used in RF communications must be modified.

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IMPLEMENTATION4

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Option 1: Using Cameras and LCDs

● LCD can encode data into visual frames● Highly directional due to short wavelengths in

visible light spectrum● Interference free wireless communication

(multiple LCD camera links can operate in the same area)

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Challenges1. Perspective distortion:

a. need LOS to work, so user has to be really careful about where LCD/camera are placed

b. when we try to increase the view angles, distortion happens

2. Blur: motion can cause blurring3. Ambient light: error due to noise in environment

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Solution: PixNet

● Encodes information in frequency domain in 2D● Perspective Correction Algorithm: address

irregularities in sampling domain bc it’s in frequency (by generalized OFDM)

● Blur-Adaptive Coding: freq domain is more resilient to blurring (lower frequencies remain intact while higher frequencies attenuate)

● Ambient light filter: ambient light only affects 0 Hz (since only affects luminance = DC)

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PixNet’s Transmitter

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● Translates bits into luminance values on LCD● 2D LCD screens x time (so 3D information)● Bleeding from nearby pixels is inevitable - causes ISI and

light interference (like multipath)○ → add cyclic prefix

● Blur adaptive coding: attenuates high frequencies in an image → only read lower frequencies. Doesn’t include information from objects that are too close or far either

● Frame Synchronization○ Frame shadowing: image captured by

camera is actually two LCD frames (shutter is open during two frames)

○ → Syncs cameras frame rate to 2x LCD’s● Frame splitting: same image is rendered into

two separate frames but overlap

Cross Tx-Rx Techniques

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Evaluation Environment

● 30” display + fancy camera● Compared with QR code recognition● 60 fps● QR code results: balance between enough

information while minimizing distortion

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Results

● Increasing distance highly decreases the throughput capacity of the camera (but still much better than QR code

● PixNet is able to provide up to half throughput at angle of 70 degrees

● PixNet responds well to blue at different nightings

● Overall responded very well to different factors

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APPLICATIONS5

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Applications

● Camera/LCD● High user density where RF based

communication often fails● Cellphones

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Special Application: Vehicle to Vehicle Communication● Use camera from front sensors and LED lights

from brake and signal lights● Specific needs:

○ Needs higher data rate○ Accurate and quick LED detection

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Special Application: Vehicle to Vehicle Communication● Requires LOS● Communication range limited in areas

overlapping the light radiation angle of the LED transmitter and the view angle of the camera receiver○ No multipath○ Simpler link design

● 32-bit preamble, 32-bit unique word, 2392-bit payload, and 8-bit postamble

● 10 Mb/s data rate for reliability

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Experiment Setup

◦ All experiments are conducted while driving outside.

◦ experimental hours are from daytime to nighttime.

◦ The maximum vehicle speed is 25 km/h.

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Measured for:1. LED light detection2. Bit error rate and

packet arrival3. Vehicle Internal Data

and Image Data Transmission

Results: Detecting LED

→ The method using the flag image is very effective and achieves the correct and real-time LED detection even in challenging outdoor lighting environments.

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Results: Bit Error Rate & Packet Loss

PAR measured every 5 seconds

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◦ The bumps are caused by uneven roads◦ By an improvement of the LED detection rate, i.e.,

the output rate of the flag image, packet losses will be decreased.

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Results: Image Data Transmission

Daytime: 13 fps, Nighttime 8.9 fps

FURTHER RESEARCH6

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● Light reflection by a corner cube● Time division duplexing● Wavelength division duplexing● Need to compare alternatives and determine best

performance

Further Research: Uplink Channel

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● VLC performance limited by modulation bandwidth in blue clip LEDS

● Can set frequency response of each LED which allowed a high net bandwidth

Further Research: LED Modulation Bandwidth

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● LEDS are nonlinear and cause problems with analog OFDM techniques

● Degreades the LED● can clip upper peaks so LED doesn't overheat● Can optimize type of LED too

Further Research: LED Nonlinearity

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Thanks!ANY QUESTIONS?

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Works Cited

◦ Takai, Isamu, et al. “Optical Vehicle-to-Vehicle Communication System Using LED Transmitter and Camera Receiver.” IEEE Photonics Journal, vol. 6, no. 5, 28 Aug. 2014, doi:10.1109/JPHOT.2014.2352620.

◦ Perli, Samuel David, et al. “PixNet: Interference-Free Wireless Links Using LCD-Camera Pairs.” ACM Mobicom, 2010, doi:10.1145/1859995.1860012.

◦ Elgala, Hany, et al. “Indoor Optical Wireless Communication: Potential and State-of-the-Art.”IEEE Communications Magazine, vol. 49, no. 9, 2011, pp. 56–62., doi:10.1109/mcom.2011.6011734.

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