Optical transmitter characteristics for GEPOF technical...

IEEE 802.3 GEPOF Study Group - May 2014 Interim

POF

Knowledge Development

Rubén Pé[email protected]

Optical transmitter characteristics for GEPOF technical feasibility

mailto:[email protected]

mailto:[email protected]


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Supporters• Frank Aldinger (Mitsubishi International)

• Yutaka Tanida (Mitsubishi Corporation)

• Y.Tsukamoto (Mitsubishi Rayon)

• Eric Chan (Boeing)

• Philippe Bolle (Skylaneoptics)

• 曹�文 / Mike Cao (Dongguan ipt Industrial Co,.LTD.)

• John Lambkin (Firecomms)

• Hugh Hennessy (Firecomms)

• Josef Faller (Homefibre)

• Manabu Kagami (Toyota R&D Labs)

• Bas Huiszoon (Genexis)

• Oscar Rechou (Casacom)

• Naoshi Serizawa (Yazaki)

• Thomas Lichtenegger (Avago Tech)

2


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Agenda• Objectives

• The optical transmitter ➤ main characteristics

• LED non-linear response and capacity penalties

• Conclusions

3


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Disclaimer• Technical characteristics provided in this presentation are limited to those

directly affecting the optical link budget and, therefore, the Shannon’s capacity analysis.

• Other characteristics, like the ones related to the physical semiconductor parameters, integration, manufacturing process, etc. are intentionally left outside of the sope of this presentation

4


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Objectives• This presentation provides technical characteristics of the optical transmitter

used today for automotive applications as well as for consumer applications• This optical transmitter is a red LED, and it is the light emitter most widely used by the

industry for POF communications• The red LED has been qualified for automotive applications, being demonstrated its

reliability during the last +10 years

• The main objective of this presentation is to analyze the red LED from the perspective of the aspects that directly relates to the Shannon’s capacity based technical feasibility assessment

• The results presented here will be used for Shannon’s capacity analysis in [perezaranda_01_0514_shannoncap]

5


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The optical transmitter ➤ main characteristics


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The optical transmitter - architecture• The optical transmitter is composed by the current driver IC and the LED IC

• The red LED converts the electrical current into optical power• In general, the I-P characteristic of LED is not linear; this topic is covered later on• Electrical-to-electrical response is well approximated by a 1st order low pass system• Achievable -3dB bandwidth of LED itself is between 75 and 95 MHz, depending on the

internal structure of LED• Wavelength center ~650 nm; wavelength width ~30 nm

• Typically, the driver is a trans-conductance amplifier in charge to convert the voltage communication signal from the PHY into the adequate current to drive the LED, providing:• Bias current control to ensure reliability of the LED• Extinction Ratio (ER) control, to avoid switching off the LED (optical power clipping) and

ensure the quantum noise from PD is low• Typical target ER = 10 dBo• Typical process and temperature variation of ER < ±2 dBo• Frequency pre-emphasis, to enhance the bandwidth of the LED• Frequency pre-emphasis gain is limited based on reliability criteria ➤ max peak current

7


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The optical transmitter - architecture

8

Pre-emphasis

+

-DAC

PHY

+

-

CurrentBiasing

VDDExtinction Ratio

Control +

+

+

+

Zin

Peak

ing

gain

Peak

ing

zero

cut-o

ff fre

q

Bias

Cur

rent

ER

TCAmp

DRIVER LED

Pre-emphasis


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The optical transmitter - pre-emphasis

9

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

ILEDavg

ILEDmin

ILEDmaxILEDpk+

ILEDpk−

Arbitrary time unit

LED

cur

rent

(A) a

nd v

olta

ge o

ut (a

rbitr

ary

units

)

LED response for ILEDavg: 20 mA; ER: 10.0 dB; LED Fc−3dB: 100 MHz; Preemphasis: Fz is 60 MHz, GHF is 0 dBSaturated samples ratio: 0.0e+000

LED currentRx Vout

No pre-emphasis, MOST line-coding


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0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000−0.03

−0.02

−0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

ILEDavg

ILEDmin

ILEDmax

ILEDpk+

ILEDpk−

Arbitrary time unit

LED

cur

rent

(A) a

nd v

olta

ge o

ut (a

rbitr

ary

units

)


LED currentRx Vout


10

Pre-emphasis, MOST line-coding


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0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

ILEDavg

ILEDmin

ILEDmaxILEDpk+

ILEDpk−

Arbitrary time unit

LED

cur

rent

(A) a

nd v

olta

ge o

ut (a

rbitr

ary

units

)


LED currentRx Vout


11

No pre-emphasis, high M PAM


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0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000−0.03

−0.02

−0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

ILEDavg

ILEDmin

ILEDmax

ILEDpk+

ILEDpk−

Arbitrary time unit

LED

cur

rent

(A) a

nd v

olta

ge o

ut (a

rbitr

ary

units

)


LED currentRx Vout


12

Pre-emphasis, high M PAM


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The optical transmitter - response

13

10−1 100 101 102 103−40

−35

−30

−25

−20

−15

−10

−5

0

5

X: 150.6Y: −3.011

Frequency (MHz)

Elec

trica

l−to−e

lect

rical

mag

nitu

de re

spon

se (d

B)

Driver + MOST red LED E−to−E response

Lab measurement of real product qualified forautomotive


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Performance with temperature

14

−40 −20 0 20 40 60 80 100−8

−7

−6

−5

−4

−3

−2

−1

Temp (oC)

AOP

(dBm

)

ATX16125: real AOP vs. temperature

#15#41#27Avg

AOP coupled into POF (lab measurements)


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−40 −20 0 20 40 60 80 100−7

−6

−5

−4

−3

−2

−1

0

Temp (oC)

OM

A (d

Bm)

ATX16125: real OMA vs. temperature

#15#41#27Avg

Performance with temperature

15

OMA coupled into POF (lab measurements)


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Non-linear response and capacity penalties


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Non-linear distortion (-40 ºC, 15.6 MHz)

17

0 2 4 6 8 10 12 14 16x 107

−120

−100

−80

−60

−40

−20

0

Frequency (Hz) − RBW 9999.94 Hz, equiv NFFT 31250.2

Ampl

itude

(dBF

S)

fs 312.5 MHz, fc 15.625 MHz

0 MHz−80.6957 dBFS

HD(1) 15.6217 MHz0 dBc

HD(2) 31.2507 MHz−29.9664 dBc

HD(3) 46.8725 MHz−38.0823 dBc

156.246 MHz−92.0123 dBFS

Lab. measurements of a real product


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Non-linear distortion (+25 ºC, 15.6 MHz)

18

0 2 4 6 8 10 12 14 16x 107

−120

−100

−80

−60

−40

−20

0


Ampl

itude

(dBF

S)

fs 312.5 MHz, fc 15.625 MHz

HD(1) 15.6217 MHz0 dBc

HD(2) 31.2507 MHz−26.1667 dBc

HD(3) 46.8725 MHz−35.9205 dBc

109.258 MHz−79.562 dBFS

156.246 MHz−90.6931 dBFS



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19

0 2 4 6 8 10 12 14 16x 107

−120

−100

−80

−60

−40

−20

0


Ampl

itude

(dBF

S)

fs 312.5 MHz, fc 15.625 MHz

0 MHz−78.672 dBFS

HD(1) 15.6217 MHz0 dBc

HD(2) 31.2507 MHz−25.1816 dBc

HD(3) 46.8725 MHz−35.5948 dBc

156.246 MHz−93.1443 dBFS



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Non-linear distortion (-40 ºC, 44.6 MHz)

20

0 2 4 6 8 10 12 14 16x 107

−120

−100

−80

−60

−40

−20

0


Ampl

itude

(dBF

S)

fs 312.5 MHz, fc 44.6429 MHz

7.36723 MHz−80.9863 dBFS

15.4472 MHz−81.8577 dBFS

39.5707 MHz−81.3665 dBFS

HD(1) 44.6397 MHz0 dBc

HD(2) 89.2868 MHz−24.9942 dBc

HD(3) 133.926 MHz−37.3326 dBc

156.246 MHz−94.6294 dBFS



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21

0 2 4 6 8 10 12 14 16x 107

−120

−100

−80

−60

−40

−20

0


Ampl

itude

(dBF

S)

fs 312.5 MHz, fc 44.6429 MHz

HD(1) 44.6397 MHz0 dBc

HD(2) 89.2868 MHz−23.1844 dBc

HD(3) 133.926 MHz−35.3223 dBc

156.246 MHz−88.4621 dBFS



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22

0 2 4 6 8 10 12 14 16x 107

−120

−100

−80

−60

−40

−20

0


Ampl

itude

(dBF

S)

fs 312.5 MHz, fc 44.6429 MHz

HD(1) 44.6397 MHz0 dBc

HD(2) 89.2868 MHz−22.7772 dBc

HD(3) 133.926 MHz−32.7958 dBc

156.246 MHz−88.6409 dBFS



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Non-linear distortion - preliminary conclusions• Based on previous measurements we can do some conclusions:

• The non-linear response of the LED depends on the temperature• The harmonic distortion measurement with input single tone depends on the frequency

of the tone

• Based on this very basic measurements we could conclude that only low spectral efficiency modulation schemes would be feasible with the LED

• However, we are going to demonstrate that this conclusion is false, by analyzing the non-linear response in deeper detail

• The idea behind the following analysis is that the non-linear response of the LED can be adaptively compensated by the PHY in the same way the ISI is equalized in modern Ethernet PHYs to approach the channel capacity

23


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Non-linear response - the Volterra model• In order to analyze the effect of LED HD in the communication system we need to

develop a correct model for the non-linear response

• Truncated Volterra series expansion is selected to model the optical TX non-linear response• Volterra series expansion is a well known technique and it have been used by the industry in a

wide range of engineering fields to model non-linear systems• It is attractive from the mathematical point of view ➤ linear combination of non-linear functions

of the input signal• It fits a large class of non-linear systems• Well known adaptive filtering algorithms are suitable for Volterra series estimation

24

y(k) = wo0 + wo1(l1)x(k − l1)l1 =0

L

∑ +…

+ wo2 (l1,l2 )x(k − l1)x(k − l2 )l2 =0

L

∑l1 =0

L

∑ +…

+ … woP (l1,l2 ,…lP )x(k − l1)x(k − l2 )…x(k − lP )lp =0

L

∑l2 =0

L

∑l1 =0

L

∑

DC offset + linear filter

2nd order convolution

Higher-order convolutions


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Non-linear response - the Volterra model

25

z-Δ01

x(k) z-Δ1 z-1 z-1 z-1...

...z-Δ2

z-1

z-1 z-1 z-1...

...

z-1 z-1 z-1...

...

z-2

z-1 z-1 z-1...

...

y(k)

ω0(k) ω1(k) ω2(k) ω11(k)

ω0,0(k) ω1,1(k) ω2,2(k) ω8,8(k)

ω0,1(k) ω1,2(k) ω2,3(k) ω7,8(k)

ω0,2(k) ω1,3(k) ω2,4(k) ω6,8(k)

ωoO(k)

gap 0

gap 1

gap 2

gap n

DC offset

1st order response

2nd order response


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Non-linear response - the Volterra model

26

3rd order response

z-Δ3

z-1

z-1 z-1 z-1...

...

z-1 z-1 z-1...

...

z-1 z-1 z-1 z-1...

...

y(k)

ω0,0,0(k) ω1,1,1(k) ω2,2,2(k) ω6,6,6 (k)

ω0,0.1(k) ω1,1,2(k) ω2,2,3(k)ω5,5,6(k)

ω0,1,1(k) ω1,2,2(k) ω2,3,3(k) ω5,6,6(k)

• The optical transmitter is well modeled by a 3rd order Volterra system.

• Higher order kernels are negligible

gap 0 0

gap 0 1

gap 1 0


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Non-linear response: Volterra DC and 1st order

27

0 2 4 6 8 10 12 14 16−0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.80th and 1st order Volterra kernels

Vol0Vol1

-40 ºC

FS = 312.5 MHz

+105 ºC

0 2 4 6 8 10 12 14 16−0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.90th and 1st order Volterra kernels

Vol0Vol1


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Non-linear response: Volterra 2nd order

28

0 2 4 6 8 10 12 14 16−0.06

−0.05

−0.04

−0.03

−0.02

−0.01

0

0.01

0.02

0.032nd order Volterra per gap

Vol2 gap 0Vol2 gap 1Vol2 gap 2Vol2 gap 3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

−60

−50

−40

−30

−20

−10

Normalized Frequency (×π rad/sample)

Mag

nitu

de (d

B)

Magnitude Response (dB)

Vol1Vol2 gap 0Vol2 gap 1Vol2 gap 2Vol2 gap 3

-40 ºC

FS = 312.5 MHz


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Non-linear response: Volterra 2nd order

29

105 ºC

0 2 4 6 8 10 12 14 16−0.1

−0.08

−0.06

−0.04

−0.02

0

0.02

0.042nd order Volterra per gap

Vol2 gap 0Vol2 gap 1Vol2 gap 2Vol2 gap 3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9−55

−50

−45

−40

−35

−30

−25

−20

−15

−10

−5


Mag

nitu

de (d

B)


Vol1Vol2 gap 0Vol2 gap 1Vol2 gap 2Vol2 gap 3

FS = 312.5 MHz


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Non-linear response: Volterra 3rd order

30

-40 ºC

0 2 4 6 8 10 12 14 16−0.04

−0.03

−0.02

−0.01

0

0.01

0.02

0.03

0.043rd order Volterra per gap

Vol3 gap 0 0Vol3 gap 0 1Vol3 gap 0 2Vol3 gap 0 3Vol3 gap 1 0Vol3 gap 1 1Vol3 gap 1 2Vol3 gap 2 0Vol3 gap 2 1Vol3 gap 3 0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

−100

−90

−80

−70

−60

−50

−40

−30

−20

−10


Mag

nitu

de (d

B)


Vol1Vol3 gap 0 0Vol3 gap 0 1Vol3 gap 0 2Vol3 gap 0 3Vol3 gap 1 0Vol3 gap 1 1Vol3 gap 1 2Vol3 gap 2 0Vol3 gap 2 1Vol3 gap 3 0

FS = 312.5 MHz


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Non-linear response: Volterra 3rd order

31

105 ºC

0 2 4 6 8 10 12 14 16−0.05

−0.04

−0.03

−0.02

−0.01

0

0.01

0.02

0.03

0.043rd order Volterra per gap

Vol3 gap 0 0Vol3 gap 0 1Vol3 gap 0 2Vol3 gap 0 3Vol3 gap 1 0Vol3 gap 1 1Vol3 gap 1 2Vol3 gap 2 0Vol3 gap 2 1Vol3 gap 3 0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

−90

−80

−70

−60

−50

−40

−30

−20

−10


Mag

nitu

de (d

B)


Vol1Vol3 gap 0 0Vol3 gap 0 1Vol3 gap 0 2Vol3 gap 0 3Vol3 gap 1 0Vol3 gap 1 1Vol3 gap 1 2Vol3 gap 2 0Vol3 gap 2 1Vol3 gap 3 0

FS = 312.5 MHz


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Non-linear response: Volterra analysis• Bandwidth of the optical TX increases with temperature, although impulse

response could be considered approximately constant

• The magnitude of the 2nd and 3rd order Volterra kernels increases with temperature and frequency ➤ it confirms the basic single tone HD measurements

• It is important to note that most part of energy of 2nd and 3rd order responses is delayed respect to 1st order• We can conclude that optical TX cannot be modeled as a Wiener or a Hammerstein non-

linear system

• The morphology of Volterra (2nd and 3rd) kernels basically does not change with temperature ➤ good from the implementation point of view

32


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Capacity penalties - channel linearization

33

Light Source

(Driver + LED)

POF PhotodiodeTrans-

Impedance Amplifier

(TIA)

Antialias Filter

POF non-linear channel

Wo1

Wo2

WoP

Wo0

Do1

Do2

DoP

Do0

n(k)

x(k) v(k)

y(k) z(k)

HC(k)

n’(k)

v(k)x(k)

POF non-linear channel Linearizer

Wo1

Wo2

WoP

Wo0

Do1

Do2

DoP

Do0

n(k)

x(k) v(k)

y(k) z(k)

HC(k)

n’(k)

v(k)x(k)

Wo1(k)

n’(k)

v(k)x(k)

Linear Channel


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Capacity penalties - Linearizer is not implemented

34

0 2 4 6 8 10 12 14 16x 107

−110

−100

−90

−80

−70

−60

−50

−40

Frequency (Hz) − MBW 200040.74 Hz

PSD

(dBm

/Hz)

− H

anni

ng W

elch

, Z =

1.0

ohm

s, A

vg 1

RX signalNoiseNoise + NL

0 2 4 6 8 10 12 14 16x 107

−80

−70

−60

−50

−40

−30

−20


PSD

(dBm

/Hz)

− H

anni

ng W

elch

, Z =

1.0

ohm

s, A

vg 1

DFE: Detector signalDFE: Detector NoiseDFE: Noise bound

Ideal MMSE DFE

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

−6

−4

−2

0

2

4

6

8

10

12

14


Mag

nitu

de (d

B)


DFE: FFE+FBFDFE: FFEDFE: FBF

25.4 dB 39.8 dB

14.4 dB

SNRe = 39.8 dB

PHY input DFE output


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0 2 4 6 8 10 12 14 16x 107

−80

−75

−70

−65

−60

−55

−50

−45

−40

−35

−30


PSD

(dBm

/Hz)

− H

anni

ng W

elch

, Z =

1.0

ohm

s, A

vg 1

Linearizer + DFE: Detector signalLinearizer + DFE: Detector NoiseLinearizer + DFE: Noise Bound

Capacity penalties - Linearizer is implemented

35

0 2 4 6 8 10 12 14 16x 107

−110

−100

−90

−80

−70

−60

−50

−40


PSD

(dBm

/Hz)

− H

anni

ng W

elch

, Z =

1.0

ohm

s, A

vg 1

RX signalNoiseNoise + NL

Linearizer+

Ideal MMSE DFE

36.7 dB 39.8 dB

3.1 dB

SNRe = 39.8 dB

PHY input DFE output

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

−4

−2

0

2

4

6

8

10


Mag

nitu

de (d

B)


DFE after linearizer: FFE+FBFDFE after linearizer: FFEDFE after linearizer: FBF


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Capacity penalties

36

15 17.5 20 22.5 25 27.5 30 32.5 35 37.5 400

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Cap

acity

pen

alty

in d

etec

tor (

dB)

Channel SNRe (dB)

Capacity penalty caused by the LED non−linear response

Linearizer + DFEDFE

Capacity loss < 1dB for SNRe < 30 dB

High spectral efficiency schemes are feasible


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Conclusions• Technical characteristics of the optical transmitter used today for automotive

applications as well as for consumer applications have been presented

• The non-linear response of I-P characteristic of LED has been analyzed in detail, concluding that high spectral efficiency modulation schemes are also feasible with low capacity penalties, opening the use of LED beyond OOK schemes

• The results presented here will be used for Shannon’s capacity analysis in [perezaranda_01_0514_shannoncap]

37


POF


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