Doc.: IEEE 802.15-09-0635-00-0007 Submission September 2009 Roberts [Intel], Xu [UCR]Slide 1...

September 2009

Roberts [Intel], Xu [UCR]Slide 1

doc.: IEEE 802.15-09-0635-00-0007

Submission

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title: Update on VLC Link Budget WorkDate Submitted: September 2009Source: Rick Roberts [Intel], Zhengyuan Xu [University of California, Riverside]Address Voice: 503-712-5012, E-Mail: [email protected], [email protected]

Re:

Abstract: Update on the VLC link budget work. The one remaining issue is the calculation of the noise density.

Purpose:

Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

VLC Link Budget

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Outline

• Relation between radiometric and photometric parameters

• Ascertaining the LED parameters of interest• Path loss due to light propagation• Received optical and electrical power• Receiver noise density• An example based on vendor data

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Radiometric (Physical) vs. Photometric (Visual)

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

LED DetectorDiode

Eyeball

A white LED spews optical power across a range of wavelengths (mW/Hz)

The human eye and the detector diode have different frequency responses and hence perceive the same LED source differently.

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

For data link budgets we want to use Radiometric units

For illumination applications we want to use Photometric units (which include the frequency response of the human eye)

LED vendors generally only provide Photometric data since illumination is the market today and the use of LEDs for data is an obscure usage.

Radiometric (Physical)

Photometric (Visual)

Total Flux Watts (W) lumens (lm)

Flux Density W/cm2 lm/cm2

Source Intensity W/sr candela = lm/sr

Illuminance Lux (lx) = lm/m2

Irradiance W/m2

Units

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Ascertaining the LED parameters of interest

On the following pages, equation (2.2.1) and Figure 2.1.1 are from the book Introduction to Solid-State Lighting by A. Zukauskas, et.al. The equation relates the power spectral distribution S() (W/nm) to luminous flux v (lm).

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

V() is the relative luminous efficiency function defined by CIE and given in the table (from internet) and curve (from the book Fig 2.1.1)

780

380

683 ( ) ( )nm

t t

nm

F S V d The LED total luminous flux Ft (lumens) is given as (2.2.1)

Sometimes it is convenient to use a Gaussian curve fitting for V() (from internet)

2285.4( 0.559)( ) 1.019 : inV e m ,

Find transmitted power and spectral density

Fig 2.1.1

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

'( ) ( )t t tS c S

Typically we only know a normalized spectral curve St’() instead of St() in (2.2.1). Denote their relation as St()=ct St’() with an unknown scaling factor ct that can be found from

780'

380

683 ( ) ( )

tt nm

t

nm

Fc

S V d

Warm WhiteNeutral WhiteCool White

780'

380

683 ( ) ( )nm

t t t

nm

F c S V d

Remark: The above step to find St() can be skipped if St() can be either measured using a spectrometer or supplied by the LED vendor.

780

380

( )nm

t t

nm

P S d H

L

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

A normalized spatial luminous intensity distribution gt() is provided by a vendor. We need to find the axial intensity I0 that is defined as the luminous intensity (candelas) on the axis of the source (zero solid angle). Since the luminous flux Ft is also a spatial integral of spatial luminous intensity in addition to spectral integral we used before, we have the following relation

max max

0 0

0 0

* ( ) 2 ( )sint t tF I g d I g d

max0

0

2 ( )sin

t

t

FI

g d

where max and max are the source beam solid angle and maximum half angle respectively and max=2(1-cosmax).

normalized spatial luminous intensity distribution

Find transmitter luminous spatial intensity distribution I0 gt()

Note: if the axial intensity is provided by the vendor then one only need convert the intensity from candelas to watts/sr.

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Path loss due to line-of-sight (LOS) light propagation

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

α

β

θmax

Transmitter

LOS Link Model

The receiver distance to the source is D The receiver aperture radius is r and receiver area is Ar

The angle between receiver normal and source-receiver line is The angle between source beam axis and source-receiver line (viewing angle) is The solid angle of the receiver seen by the source is r

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

The luminous angular intensity of the source at the receiver direction is I0gt(), and therefore the receiver ingested luminous flux Fr=I0gt()r.

The luminous path loss can be represented as

where r is the receiver solid angle which satisfies Arcos()D2r .

max max max

0

20

0 0 0

( ) ( ) ( ) cos

2 ( )sin 2 ( )sin 2 ( )sin

t r t r t rrL

tt t t

I g g g AFL

FI g d g d D g d

Power path loss Lp can be proven equal to luminous path loss LL as follows:

Optical power can be written as

In LOS free space propagation, path loss is assumed independent of wavelength. Power path loss can be represented as Lp=S2()/S1()=P2/P1.

Luminous flux is related to S() as , which is linear with S()

Therefore, LL=F2/F1=Lp.

( )H

L

P S d

683 / ( ) ( )H

L

F lm W S V d

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Now that we have known the optical power loss due to LOS propagation, we can obtain the received optical spectral density from transmitter optical spectral density as

Suppose we use a photodiode detector to receive the signal light. We can obtain the electrical power of the signal as

( ) ( ) ( )r p t L tS L S L S

The detector diode vendors are giving us the info we need

Received optical power , ( ) ( ) rH

rL

r o r fP S R d

Find received optical and electrical power

( )D

qR

hc

21150

,

250

( ) ( ) ( )nm

r e r r D L

nm

P S R R d R

f

where Sr() is the received light power spectral density (W/nm)Rr() is the receiver filter spectral responseRD() is the detector responsivity (A/W at )f

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

ideal

typical

Exemplary Optical Filter Response

http://www.newport.com/images/webclickthru-EN/images/2226.gif

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Summary of key steps to obtain received optical power and electrical power

• Calculate transmitter (source) optical power from given luminous flux Ft (lumens) and normalized spectral curve St’()

• Find the transmitter axial intensity I0 from given luminous flux Ft and luminous spatial intensity distribution gt()

• Find receiver ingested luminous flux Fr from receiver solid angle and transmitter luminous spatial intensity distribution I0 gt()

• Find the luminous path loss LL from Ft and Fr

• Prove power path loss Lp is equal to luminous path loss LL from which to find received optical power spectral curve Sr()

• Calculate received optical power and electrical power

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Comment on RX aperture vs. magnification factor

It is the author’s opinion that the RX aperture determines the “brightness” of an observed object and that the field of view determines the magnification factor of an observed object. That is, for a given aperture the magnification factor determines how big an object appears but not how bright an object appears. The observed brightness is solely a function of the aperture size. It should be noted that the field of view, and hence the magnification factor, is a function of the focal distance.

Same aperture lens

Long focal distance

Short focal distance

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Receiver Noise Density

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Determining the noise density NoWhat are the sources that contribute to the noise density?

Photodetector Noise• Transimpedance Amplifier Noise

• Ambient “in-band” noise• Out-of-band cross modulation noise due to photodetector non-linearity

• Others?

Ndiode

Ntransamp

Nambient

NOOB

Signal Of Interest (SOI)

Ambient Noise Floor

Modulation Domain Spectrum

SOI

Out-of-Band (OOB)Interference Signal

Non-linear noisebleed over

ModulationDomainSpectrumAnalyzer

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Ambient “In-Band” Noise Floor

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Cross-modulation of Out-of-Band noise from other VLC signals into the SOI

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

CL

R2

2 2 2 ( ) 4shotnpd thermal D S B SHi i i q I I I kT R

q is the electron charge (1.6e-19 coulombs)ID is the dark currentIS is the signal currentIB is the background light induced currentB is the bandwidth (B=1 Hz for N0)k is Boltzmann’s constant (1.38e-23 J/K)T is the Kelvin temperature (~290 K)RSH is the shunt resistance

The detector itself contributes a noise density Ndiode (W/ Hz)

(A/ Hz)

R2C2

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

TI OPA111R2=10e6 ohms

Transimpedance Amplifier Noise Analysis

Ref. TI/Burr-Brown Application Bulletin SBOA060“Noise Anaysis of FET Transimpedance Amplifiers”

Assume R3=0

http://focus.ti.com/lit/an/sboa060/sboa060.pdf

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

The resistors and capacitors form critical corner frequencies as shown below:

22 2

1

2f

R C 2121 ||||2

1

CCRRfa

TI OPA111

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Typically op-amps have three noise regions … the above noise regions are for the TI OPA111 op-amp. It is anticipated most outdoor VLC implementations will be bandpass systems operating in noise region 3.

Noise Regions for the TI OPA228

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

The approximation output noise is given by

niRn NNNN 003

00

2

2

123

30 1

C

CKN n

20 4kTRN R

where

22

222

220 4)(2 RRkTIIIqiRiiN SHBSDnopnpdnopin

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Then electrical SNR is

( ) ( ) ( )rH

rL

sig r f DI S R R d

The signal current is given as

used in numerical example later on

22

22

2

2

123

2

2

04)(241

/)()()(

RRkTIIIqikTRCC

K

RatedLRSR

N

E

SHBSDnop

b

rH

rL

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

22

22

2

2

123

2780

380

2

04)(241

/)()()(

RRkTIIIqikTRCC

K

RatedLRSR

N

E

SHBSDnop

nm

nmb

Hz

V

Hz

V 22

Hz

VsV

sA

J

s

sV

A

J

A

VK

K

J 2

Hz

V

A

V

Hz

A 222

Hz

VsVV

sC

CV

A

C

A

VAC

2222

2

2

/

Hz

V

sW

AW

A

V 22

1

1

Eb/No Dimensional Analysis

AVs

JW

sA

J

A

WV

s

CA

sA

J

A

V2

sHz

1

KT

KJk

coulombsCq

/

)(

Hz

V

AV

AV

KK

J 22

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

An example based upon vendor data

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Example OSRAM white LED: LUW W5SM-JYKY-4P7R-Z

Used parameters in red box

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Normalized radiant power spectrum density

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Normalized spatial luminous intensity distribution

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Thorlabs High Speed Detector: FDS010 PIN Silicon Diode

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

An example responsivity curve (A/W)

FDS-010 http://www.thorlabs.de/Thorcat/0600/0636-S01.pdf

optical filter band430nm ~ 470nm

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Thorlabs filter response

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Slide 36

ParametersTransmitter luminous flux Fs 66lm

Distance between transmitter and receiver D

2m

Angle 0

Angle 0

Receiver diameter 2.54cm

Receiver area 5.07cm2

Optical filter band 430nm~470nm

Optical filter transmittance 0.7

Focusing lens transmittance 0.9

Electrical signal bandwidth 250kHz or 100MHz

Photodiode dark current 2.5nA

Photodiode capacitance 3.2pF

Photodiode spectral responsivity

approximately 0.17 A/W within the band

Photodiode shunt resistance RSH

108Ω

Measured average background light optical power

60μW

Temperature T 300K

Electron charge q 1.6×10-19C

Boltzmann constant k 1.38×10-23

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Low data rate100 kbps

High data rate10 Mbps

Load resistance

Electrical signal power

Signal induced shot noise

Background light induced shot noise

Dark current induced shot noise

Thermal noise

SNR

Path Loss 43.8dB

Calculated received optical power 2.1μW

Measured received optical power 1.2μW

Results

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Companion Xcel Spreadsheet Analysis

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Link BudgetSpreadsheetResults

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Backup Slides

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doc.: IEEE 802.15-09-0635-00-0007

Submission

Flu

x /

ba

nd

wid

th (

W/n

m)

LED Spectrum(white LED)

2

21150

250

)()()( RdLRSPnm

nm

RX

Where T() is the transmitter power spectral density (W/nm) R() is the detector responsivity (A/W at ) L() is the propagation loss (loss at )

The detector diode vendors are giving us the info we need

This is the information we need from the LED vendor

For best performance we want the detector spectral responsivity to be “matched” to the LED spectral density. In general this is hard to due, especially for white LEDs.

September 2009


doc.: IEEE 802.15-09-0635-00-0007

Submission

Doc.: IEEE 802.15-09-0635-00-0007 Submission September 2009 Roberts [Intel], Xu [UCR]Slide 1...

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