4/16/18 1 Visible Light Communications Snigdha Sharma Shivani Sheth 1 Presentation Outline 1. Introduction 2. How it Works 3. Modulation Techniques 4. Implementation 5. Application 6. Further Research 2 INTRODUCTION 1 3 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? 4
10
Embed
Visible Light Communications - Carnegie Mellon School of ...prs/wirelessS18/handouts/1-visible-light.pdf · Visible Light Communications Snigdha Sharma Shivani Sheth 1 Presentation
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
1. Introduction2. How it Works3. Modulation Techniques4. Implementation5. Application6. Further Research
2
INTRODUCTION1
3
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?
4
4/16/18
2
● 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
5
Indoor Optical Wireless Potential
● OW is essential for short-range communication with high throughputs
● Beneficial because○ Unregulated○ Low cost○ No license needed
6
● 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
7
● The free space loss (FSL)● Ambient light noise and/or interference● Multipath dispersion causing intersymbol
interference (ISI).
Key Design Challenges
8
4/16/18
3
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
9
HOW IT WORKS2
10
11
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
12
● 780 nm to 950 nm wavelengths● low-cost optical sources readily available● coincides with the peak sensitivity of
● 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
13
Typical Indoor OW Systems
14
● 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
15
MODULATION TECHNIQUES3
16
4/16/18
5
Single-Carrier Pulsed Modulation
17
● 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.
18
IMPLEMENTATION4
19
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)
20
4/16/18
6
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
21
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)
22
PixNet’s Transmitter
23
● Translates bits into luminance values on LCD● 2D LCD screens x time (so 3D information)● Bleeding from nearby pixels is inevitable - causes ISI and
● 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
24
4/16/18
7
Evaluation Environment
● 30” display + fancy camera● Compared with QR code recognition● 60 fps● QR code results: balance between enough
information while minimizing distortion
25
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
26
APPLICATIONS5
27
Applications
● Camera/LCD● High user density where RF based
communication often fails● Cellphones
28
4/16/18
8
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
29
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
30
Experiment Setup
◦ All experiments are conducted while driving outside.
◦ experimental hours are from daytime to nighttime.
◦ The maximum vehicle speed is 25 km/h.
31
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.
32
4/16/18
9
Results: Bit Error Rate & Packet Loss
PAR measured every 5 seconds
33
◦ 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.
34
Results: Image Data Transmission
Daytime: 13 fps, Nighttime 8.9 fps
FURTHER RESEARCH6
35
● Light reflection by a corner cube● Time division duplexing● Wavelength division duplexing● Need to compare alternatives and determine best
performance
Further Research: Uplink Channel
36
4/16/18
10
● 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
37
● 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
38
Thanks!ANY QUESTIONS?
39
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.