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Flexible Optische Übertragungssysteme mit

Direktdetektion für Verbindungen zwischen

Datenzentren Simon Ohlendorf, Riya Joy, Werner Rosenkranz

Workshop der ITG-Fachgruppe 5.3.1 Modellierung Photonischer Komponenten und Systeme

Hamburg, 23.02.2018

Chair of Communications 23.02.2018 | Simon Ohlendorf | Workshop der ITG-Fachgruppe 5.3.12

Outline

1. Introduction

2. System Setup

3. Multidimensional Modulation

1. Principle

2. Optimization of Gray Penalty

3. Signal Processing

4. Experimental Setup & Results

1. Single Channel

2. DWDM

5. Conclusions


Data center clusters built from geographically separated sites

optical links with up to ~100 km reach

Motivation

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ToR

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Agg.Agg.

Core

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<100 km

Intra-DC

< 2 km MMF, CWDM

Intensity Modulation with Direct Detection

On-Off Keying- low reach + low complexity

- low capacity + low cost

+ small footprint

+ low power consumption

Long-Haul

< 10.000 km SMF, DWDM

Polarization Multiplexing, IQ-Modulation with Coherent Receiver

e.g. PM-16-QAM

+ high reach - high complexity

+ high capacity - high power consumption

- high cost

- large footprint

Intra-Cluster Interconnects

< 100 km:

find cost-efficient and flexible solution with high capacity


IM/DD Transmission Setup

Tx-DSP

ADC

SSMF

direct detection

50 GHz DWDM grid

intensity modulation

DAC

ECL,𝑓𝑐 ≈ 193.4 THz

MUX

30 GHz electrical

bandwidth

DEMUX

Rx-DSP

Accumulated dispersion causes destructive interference between lower and upper sidebands with direct detection: Power fading

SSB transmission

Use optical (De-)Mux as SSB-filters

2≜𝐸 𝑡 𝐸 𝑡

𝐸 𝑡 2 = 1 + 2𝛼𝑠 𝑡 + 𝛼2[𝑠2 𝑡 + Ƹ𝑠2(𝑡)]

DCuseful signal signal-signal beating

interference (SSBI)

Nonlinear mixing terms after detection (SSBI)

Optimization of optical signal to carrier ratio (OSCR)

0 < 𝛼 ≤ 0.5

Lower OSCR: less SSBI – but: high carrier power

detuned from DWDM grid


Motivation – Conventional Modulation Formats

0 0.5 1 1.5 2

2

1

0

-1

-2

0 0.5 1 1.5 2

2

1

0

-1

-2

0 0.5 1 1.5 2

3

2

1

0

-1

-2

-3

15 20 25 30 35 40 4510

-4

10-3

10-2

10-1

OOK

PAM-8

OSNR [dB]

BE

R

PAM-4

> 10 dB OSNR

Only integer values for

spectral efficiency

(bits/symbol) possible!

Limited flexibility

Improved fine tuning,

if we allow for fractional values,

e.g. 1.5 bits/symbol

B2B-Simulation, 56 GBd

DMT

Probabilistic Shaping

Time Domain Hybrid Modulation

Multidimensional Modulation

HD-FEC limit

BER = 3.8e-3


Multidimensional Modulation – Principle

PAM-M 𝑀 = 2𝑁B, with integer 𝑁B: PAM-4, PAM-8,…

Composition of words with length 𝑁Sym

Here: fractional values for 𝑁B

𝑁Sym

allows usage of e.g. PAM-3, PAM-5

condition for M: 𝑀𝑁Sym ≥ 2𝑁B

Example: 𝑁B

𝑁Sym= 1.5 3 bits mapped on 2 symbols (2 dimensions) [𝑑1, 𝑑2]

23 = 8 symbols required

2 Symbols: PAM-3 provides 32 = 9 combinations

𝑑1

𝑑2“constellation point” with most power is not used

0 0.5 1 1.5 2

2

1

0

-1

-2

[1] J. Leibrich and W. Rosenkranz, “Multidimensional constellations for power-efficient andflexible optical networks,” IEEE Photonics Technol. Lett., vol. 26, no. 8, pp. 753–756, 2014.


Multidimensional Modulation – Mapping

Mapping: Gray coding is not possible

Gray Penalty 𝐺P due to non-ideal Gray mapping

Number of bit errors per symbol error

111

100

101

110

000

001

110

010

011

𝑑1

𝑑2

0 1 2

0

1

2

[2] K. Zeger and A. Gersho, “Pseudo-Gray coding,” IEEE Trans. Commun., vol. 38, no. 12, pp. 2147–2158, 1990.

B

N

B

( )2

1 1B BP2

S

N

1

dist ( ), ( ( ))

( )

N

N

N m

m

m q

m

m qN P

GP

N m

b b

Example: Mapping of 3 bits 𝐛 onto 2 symbols [𝑑1, 𝑑2]

0 00 1 0 1 1

1 0 2 0

00 1 1 1 1

0 0 0 2

1 0 1 1 0 0 1

2 0 1 0

1 0 0 1 0

1 2 2 1

1 . . .

. . .

Bits

Symbols

Possible mapping combinations: 2𝑁B !

𝑁B = 3 ≈ 4 ⋅ 104 permutations

𝑁B = 6 ≈ 1 ⋅ 1089 permutations

Optimization of mapping with Binary Switching

Algorithm2

𝑑1 𝑑2 𝑑1 𝑑2 𝑑1 𝑑2


Pseudo Gray Coding – Binary Switching Algorithm

Pre-implementation task, lookup-tables with mappings

Minimized Gray-Penalty: Pseudo-Gray Coding2

1. Initialization with random mapping

2. Compute list with cost 𝐶 𝑚 for all words in decreasing order

3. Take word 𝑚 with highest cost

4. Try switching bit vector )𝐛(𝑚 with all words and compute

variation of 𝐺P for every trial

No reduction of Gray penalty: take next word from list

5. Select switching partner which reduces 𝐺P the most and switch

6. Start again with (2.) until no further reduction is found

local optimum

Multiple executions with different initializations: global optimum

[2] K. Zeger and A. Gersho, “Pseudo-Gray coding,” IEEE Trans. Commun., vol. 38, no. 12, pp. 2147–2158, 1990.

N ( )

1

N

dist ( ), ( ( ))

( )( )

N m

m

q

m q

C mN m

b b

111

100

101

110

000

001

110

010

011

𝑑1

𝑑2


Multidimensional Modulation

Data rate𝑁B𝑁Sym

𝐺PModulation

format

56 Gb/s 1 BpS 1 PAM-2

67.2 Gb/s 6/5 = 1.2 BpS 1.13 PAM-3

74.67 Gb/s 4/3 = 1.33 BpS 1.09 PAM-3

84 Gb/s 3/2 = 1.5 BpS 1.14 PAM-3

98 Gb/s 7/4 = 1.75 BpS 1.55 PAM-4


130.67 Gb/s 7/3 = 2.33 BpS 1.5 PAM-6

140 Gb/s 5/2 = 2.5 BpS 1.06 PAM-6

149.33 Gb/s 8/3 = 2.67 BpS 1.76 PAM-7


Constant symbol rate 𝑓Sym = 56 GBd

5 7.5 10 12.5 15 17.5 20 22.5 25 27.5SNR [dB]

10-4

10-3

10-2

10-1

BE

R

1

6/5

3/2 2

3

4/3

7/4

5/2

8/3

7/3HD-FEC limit

BER = 3.8e-3

Baseband simulation:

AWGN channel, unipolar transmission


Multidimensional Modulation - DSP

Mapping

RRACOS Pulse Shaping

Resampling to DAC Rate

Pre-compensation

DAC

Matched Filter

DC-Block

Resampling to

2 ⋅ 𝑓sym

ADC

Synchronization

Equalization

Resampling to 𝑓sym

Demapping

MUX

DEMUX

Bitstream Bitstream

correlation with training sequence

FFE, number of taps is adjusted to fiber length

channel estimation with training sequence using the

MMSE criterion


𝐺PMod.

format

56 Gb/s 1 BpS PAM-2

67.2 Gb/s 6/5 BpS 1.13 PAM-3

74.67 Gb/s 4/3 BpS 1.09 PAM-3

84 Gb/s 3/2 BpS 1.14 PAM-3

98 Gb/s 7/4 BpS 1.55 PAM-4

112 Gb/s 2 BpS PAM-4

130.67 Gb/s 7/3 BpS 1.5 PAM-6

140 Gb/s 5/2 BpS 1.06 PAM-6

149.33 Gb/s 8/3 BpS 1.76 PAM-7


Constant symbol rate 𝑓Sym = 56 GBd


Experimental Setup

𝑓Sym 56 GBd

𝑁FFE 12 (B2B) … 60 (125 km) taps

RRACOS roll-off factor 0.1

50 GHz

63 GHz

Tx-DSP

160 GS/s Scope

Rx-DSP

SSMF < 125 km

dispersion: 17.6 ps/nm/kmattenuation: 0.22 dB/km

PL = 0…6.5 dBm

84 GS/s AWG

ECL, 193.42 THz 20 GHz detuning

45 GHz193.4 THz

8 bits resolution

(ENOB = 5.5)32 GHz

28 GHz

32.1 GHz

OSNR

44.3 GHz193.4 THz PR = 0…9 dBm

SSB filter SSB filter

f [THz]

-30

-25

-20

-15

-10

193.3 193.4 193.5

S21

[d

B]


System Frequency Response & SSB Filtering

f [GHz]

S2

1 [

dB

]

10 20 30

-15

-10

-5

0

0

PS

D [d

Bm

]

λ [nm]

-75

-50

-25

1549.75 1550.251550

50 GHz

63 GHz160 GS/s Scope

84 GS/s AWG32 GHz

28 GHz

32.1 GHz

Tx-DSP

160 GS/s Scope

Rx-DSP

84 GS/s AWG

Low-pass frequency response: Pre-compensation in Tx-DSP

SSB-filtering

1 2


Experimental Results – Single Channel, B2B

Symbol rate: 56 GBd

B2B transmission with SSB filter (interleaver) + Rx-filter

Optimized OSCR for each measurement

15 20 25 30 35 40OSNR [dB]

10-4

10-3

10-2

10-1

BE

R

1

23/2

7/37/4

6/5

4/3 3

5/2

8/3

HD-FEC limit

BER = 3.8e-3


𝐺PMod.

format

56 Gb/s 1 BpS PAM-2

67.2 Gb/s 6/5 BpS 1.13 PAM-3

74.67 Gb/s 4/3 BpS 1.09 PAM-3

84 Gb/s 3/2 BpS 1.14 PAM-3

98 Gb/s 7/4 BpS 1.55 PAM-4


130.67 Gb/s 7/3 BpS 1.5 PAM-6

140 Gb/s 5/2 BpS 1.06 PAM-6

149.33 Gb/s 8/3 BpS 1.76 PAM-7


PAM-3

PAM-4

PAM-6


Experimental Results – Single Channel

Symbol rate: 56 GBd

Required OSNR @ BER = 3.8e-3

Launch Power L 0 dBmP

0 20 40 60 80 100 120

l [km]

PL = 4 dBm

20

25

30

35

40

req

. O

SN

R [

dB

]

8/3

1

3/2

2

5/2

6/54/3

7/4

7/3 PL = 6.5 dBm


𝐺PMod.

format

56 Gb/s 1 BpS PAM-2

67.2 Gb/s 6/5 BpS 1.13 PAM-3

74.67 Gb/s 4/3 BpS 1.09 PAM-3

84 Gb/s 3/2 BpS 1.14 PAM-3

98 Gb/s 7/4 BpS 1.55 PAM-4


130.67 Gb/s 7/3 BpS 1.5 PAM-6

140 Gb/s 5/2 BpS 1.06 PAM-6

149.33 Gb/s 8/3 BpS 1.76 PAM-7



Experimental Setup - DWDM

Tx-DSP

160 GS/s Scope

Rx-DSP

SSMF < 125 km

dispersion: 17.6 ps/nm/kmattenuation: 0.22 dB/km

84 GS/s AWG 193.4 THz

45 GHz

OSNR

3xECL, 193.322 THz193.422 THz193.522 THz

2xECL, 193.372 THz193.472 THz

PS

D [

dB

m]

λ [nm]1549 1550 1551

-80

-60

-40

-20

PS

D [

dB

m]

λ [nm]

1549 1550 1551

-60

-40

-20

f [THz]

-30

-25

-20

-15

-10

193.3 193.4 193.5

S21

[d

B]

22 GHz detuning from DWDM grid

22 GHz detuning from DWDM grid


Experimental Results – DWDM B2B

Reception of center channel

Symbol rate: 56 GBd

HD-FEC limit

BER = 3.8e-3

15 20 25 30 35 40

OSNR [dB]

10-4

10-3

10-2

10-1

BE

R

1

23/2

7/3

7/4

6/5

4/3

5/2

8/3


𝐺PMod.

format

56 Gb/s 1 BpS PAM-2

67.2 Gb/s 6/5 BpS 1.13 PAM-3

74.67 Gb/s 4/3 BpS 1.09 PAM-3

84 Gb/s 3/2 BpS 1.14 PAM-3

98 Gb/s 7/4 BpS 1.55 PAM-4


130.67 Gb/s 7/3 BpS 1.5 PAM-6

140 Gb/s 5/2 BpS 1.06 PAM-6

149.33 Gb/s 8/3 BpS 1.76 PAM-7



Experimental Results – DWDM

Symbol rate: 56 GBd

Black marker: req. OSNR @ maximum reach for each data rate

Launch power per channel

20

25

30

35

40

req

. O

SN

R [

dB

]

0 20 40 60 80 100 120

l [km]

0 20 40 60 80 100 120

l [km]

1

6/54/3

3/2

7/4

2

LP

L 6 dBmP

L 1dBmP

L 6dBmP


𝐺PMod.

format

56 Gb/s 1 BpS PAM-2

67.2 Gb/s 6/5 BpS 1.13 PAM-3

74.67 Gb/s 4/3 BpS 1.09 PAM-3

84 Gb/s 3/2 BpS 1.14 PAM-3

98 Gb/s 7/4 BpS 1.55 PAM-4


130.67 Gb/s 7/3 BpS 1.5 PAM-6

140 Gb/s 5/2 BpS 1.06 PAM-6

149.33 Gb/s 8/3 BpS 1.76 PAM-7



Conclusions

IM/DD system can cover the requirements for DC interconnects

SSB-transmission to avoid power fading

Optimized OSCR to minimize influence of SSBI

Gradual scaling of reach and data rate at a fixed symbol rate is possible with Multidimensional Modulation

Pseudo Gray Coding: minimized Gray penalty using the Binary Switching Algorithm

Highest data rate below HD-FEC limit in single channel setup:

75 km: 112 Gb/s - 100 km: 98 Gb/s – 125 km: 84 Gb/s

DWDM-transmission with 5 channels

error floor for data rates above 112 Gb/s

75 km: 112 Gb/s - 100 km: 84 Gb/s – 125 km: 67.2 Gb/s

Thank you

[email protected]

Acknowledgements:

mailto:[email protected]

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