Low-Complexity Visible Light Networking with LED-to …people.networks.imdea.org/~domenico_giustiniano/img/LED2LED... · Low-Complexity Visible Light Networking with LED-to-LED Communication

Low-Complexity Visible Light Networking withLED-to-LED Communication

Domenico Giustiniano, Nils Ole Tippenhauer, Stefan Mangold

Wireless Days IFIP Conference, Dublin, November 2012.

Introduction

Visible Light Communication (VLC)

VLC is an emerging technology,I it uses the visible light spectrumI light emitters (LEDs) transport information wirelessly

VLC is a low-power, low-cost alternative to traditional short-rangewireless RF communications

Image: Bytelight

D. Giustiniano, N. Tippenhauer, S. Mangold (Wireless Days IFIP Conference, Dublin, November 2012.)Visible Light Networking with LED-to-LED 2 / 23

Motivation

LED-to-LED communication

Most of the research on VLC has focused on wideband white LEDsused in ambient illumination.

I Wideband: 400 nm

Narrowband and visible LEDs have received little attention.I Narrowband: 15− 150 nm


Motivation

Application: Toy-to-Toy Communication

(a) Police car approach-ing....

(b) Car detecting themessage and reacting!

1) Light is safe. 2) We can control the communication through thedirectivity and the visual field of light propagation → this makes itinteractive (in contrast to infrared or RF).


Summary

Summary of the Rest of the Talk

We investigate the limit of communication using a single LED andmicrocontroller (Arduino) for applications that require low PHY rate.

We introduce an efficient MAC protocol to access the channelI CSMA/CA is here inefficient.

We study and resolve the problem of light flicker.I Previous work does not consider that the light flicker when

1 LEDs operates in ad-hoc network and2 the LED is also a receiver

We implement and evaluate the solution in a network of up to fourtransceivers communicating on the same visible color.


Transceiver

Principles

The brightness of LEDs can be adapted rapidly,I at speeds orders of magnitude higher than for conventional light

emitting devices.

By varying the intensity of the LED light source,I data messages can be communicated through visible light to a receiver

sensitive to light.F In our approach, the receiver is the same LED used for transmission.F Only one microcontroller and one LED.


Transceiver

Transmission

Station transmits the bits using the binary OOK modulation:

bit = 0→ symbol = ZERO

bit = 1→ symbol = ONE


Transceiver

Reception

ZERO ONE

Vth


Transceiver

Reception

Vca(T ) > Vth → symbol = ZERO

Vca(T ) < Vth → symbol = ONE.

ZERO ONE

Vth


Flicker Elimination

Flicker transmitting data

11 00001 1 0 11 100 110

No Data sent → Inter-data �i ker

Data sent → Intra-data �i ker

Single light channel to transmit and receive data → flicker effects.

Intra- and inter-data flickers


Flicker Elimination

Frequency of Intensity Modulation

Visualemotion/feedback

Flickeringregion VLC Data

FrequencyLig

ht I

nten

sity


Flicker Elimination

Data and Energy symbols

Data symbolsI Transmitter:

F carry information (bit) from the transmitter to the intended receiver.

I Receiver:F either receive any data from stations in rangeF or listen the channel before transmitting any data.

I Only one station is allowed to output energy in each Data Symbol.

Energy symbolsI No data is transmitted.I Any station is free to output energy to the optical medium.


Flicker Elimination

Flicker Elimination Scheme

D EE D

Time

symbol time T

(c) Data and Energy symbols.

Time

ZERO ONEZEROONE

(d) Reception.

Time

ONE ONEZERO ZERO

(e) Transmitter, bit ’10’.


Flicker Elimination

Frequency analysis

0 200 400 600−60

−50

−40

−30

−20

−10

0

Frequency [Hz]

Out

putV

olta

ge[V

(dB

)]

0 200 400 600−60

−50

−40

−30

−20

−10

0

Frequency [Hz]

Out

putV

olta

ge[V

(dB

)]

Flickerregion

Observed average

Flickerregion

Observed average

CFF=Critical flicker frequency=166 Hz

T < 14CFF = 1.5 ms


Flicker Elimination

LED noise

B5 Y5 G5 R5 B3 Y3 G3 R30

10

20

T ZERO[m

s]

B5 Y5 G5 R5 B3 Y3 G3 R30

2.5

5

T ZERO[m

s] With Ambient Noise

DarknessFlicker Region

Flicker Region

No Flicker Region

No Flicker Region1/(4CFF)

1/(4CFF)

Using blue and red LEDs, we can transmit with symbol durationbelow T = 1.5 ms, without being affected by flicker and LED noise.

We use T = 512µs unless otherwise stated.


Collision Detection Protocol

Transmission and Reception with One LED

Charging: Reception:

Symbol ZEROSymbol ONE

Long time wasted in collisions due to low PHY rate →Key idea: sense the channel transmitting a ZERO symbol



CSMA/CD MAC access protocol

RBT new RBT

Sta

tion

AS

tatio

n B

new RBTRBT

Message a1 Message a1

Message b1 Message b1

RBTcont'd

ACK

t

Detection is possible at the transmitter using Data Symbols ZERO, wherelight can be sensed.



Data packet

12 bit 8 bit 8 bit 24 bit 0−1280 bit 16 bit

Sync SFD Length MHR Data CRC

Software-based synchronization protocol.

State machine for MAC protocol.

All implemented in Arduino.I hex file of about 11 kB over the 30 kB available.



Two Contending LEDs

ACK Data

Data

Data

ACK ACK



Three Contending LEDs



Full flickering elimination

Data

Data

DataACK

ACK ACK

Data

ACK

Rx Rx

RxRx



MAC Throughput

1 2 3 4

600

700

800

900

Throughput[b/s]

Number of Stations

CSMA/CA,40 byteCSMA/CD,40 byteCSMA/CA,160 byteCSMA/CD,160 byte



CSMA/CD MAC Throughput

From renewal theory, the maximum system throughput Smax is:

Smax =E [P]

E [Tm]=

160

352 · T = 0.45/T (1)

For T = 512µs, Smax = 878 b/s, very close to our experiments.

We obtain a Jain’s fairness index of 0.98− 1, that indicates that bothour protocol and prototype implementation are fair.


Conclusion

Conclusion

Short-range free-space LED-to-LED communication enables a widevariety of entertainment applications.

These applications benefits from:I the explicit visual feedback to the user when pointing a device,I the system simplicity and the low-cost nature of the LED-based

systems.

We proposed and evaluated an LED-to-LED communication system,I including physical layer transmission and networking protocols.

Our evaluation addressed technical challenges,I such as the elimination of flicker and detection of collisions.


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Low-Complexity Visible Light Networking with LED-to …people.networks.imdea.org/~domenico_giustiniano/img/LED2LED... · Low-Complexity Visible Light Networking with LED-to-LED Communication