Optical Power Budgets Session Chair: Tony Frisch, Xtera

Presenter: Company:

Power Budgets Key to the Supply or Upgrade of any Submarine System?

Tony Frisch, Priyanth Mehta, Tim Stuch Xtera Ciena Microsoft

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Contents

Presenter Profile

Tony started at BT's Research labs and then moved to Alcatel Australia, becoming involved in testing submarine systems. A move to Bell Labs gave him experience in terminal design and troubleshooting, after which he went back to Alcatel France, where he worked in Alcatel Submarine Networks’ Technical Sales before moving to head Product Marketing. He is now SVP, Repeaters and Branching Unit for Xtera Communications.

Tony Frisch SVP Repeaters and Branching Units tony.frisch@xtera.com

TF  

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Presenter Profile

Priyanth Mehta Submarine Optical Systems Designer prmehta@ciena.com

Priyanth Mehta is an optical systems designer for the submarine research and development team at Ciena in Ottawa, Canada. He received his B.Sc. (Hons) (2007) and M.Sc. (Hons) (2009) in Optical Physics from the University of Auckland, New Zealand. He then obtained a PhD in the nonlinear properties of semiconductor optical fibres from the Optoelectronics Research Centre, University of Southampton, in 2013. At Ciena, his primary fields of research are focussed on improving transmission capacity, reach, and user operability through modem and line terminal enhancements. Priyanth also serves as a contributing delegate in standardisation on the International Telecommunication Union (ITU) for Optical Transport and Access.

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Contents

Presenter Profile

Tim Stuch is a Principal Network Engineer for Microsoft’s Azure WAN transport team, where he is the engineering lead for all of Microsoft’s Subsea engagements from contract through to operations. Currently he is working on extending Microsoft’s cloud globally with a focus on making open cable concepts into real world deployments. Working on subsea for the last five years, Tim has been designing and deploying IP and transport networks for 20 years with companies like Bay Networks, Nortel, and Ciena. Tim received his B.S. Physics from Guilford College and his Physics Ph.D. from the University of Denver.

Tim Stuch Principal Network Engineer tstuch@microsoft.com

Other contributors

•  P Murphy AJC •  O Ait Sab Alcatel-Lucent •  L Moskowitz AT&T •  J Gaudette Microsoft •  N Brochier Orange •  D Welt Tata •  C Mott Telstra •  P Booi Verizon •  E West Vodafone

Contents 1.  Introduction and uses 2.  Fundamentals with a few simplifications 3.  How to construct a Power Budget 4.  How it feeds into Acceptance 5.  Refinements removing the simplifications

handling ROPA or DRA 6.  Discussion comparisons / sanity checks

“open” systems

A brief history 1.  On-Off OOK

Amplitude ASK

2.  Phase DPSK Differential encoding

3.  Multi-level 16QAM Coherent transmission Soft Decision FEC Pulse-shaping A lot has changed…

DEC UI

FEC

DEC FEC

~

ADC

ADC

ADC

ADC

DSP

FEC

Framing

Local oscillator

λ/4

Polarizing beam splitter/combiner

Specified in ITU-T Rec. 977

"Characteristics of optically amplified optical fibre submarine cable systems"

•  Annex A provides templates – in the latest version there are two –  Example 1 The original

–  Example 2 “This (2015) edition introduces a new power budget template for the implementation of coherent

systems”

Looks different, but the principles are mostly the same

Presenter: Company:

Where it's used

TS

Bidding

•  Defines key Requirements –  System Length –  Traffic capacity –  Repair and Ageing –  Commissioning limit i.e. start of life performance to guarantee end of

life margin –  Error performance –  Any constraints e.g. keep some/all existing traffic when

upgrading

•  Should make it easy to compare different proposals

System design

•  May not use ITU-T G.977 template

•  Used to explore possible system parameters –  Fibre loss / effective area … –  Number of repeaters, Power, Bandwidth –  Transmission formats, Encoding, FEC –  …

Acceptance

•  Verify budget in particular:

•  Measure average Q-factor and time-varying Q-factor (5 sigma) •  Estimate worst case Q-factor over time (5 sigma calculation) •  Worst performing wavelength … and compare with … •  Commissioning limit – calculation of required start of life margin to obtain

end-of-life performance commitment

Maintenance and long-term operation

•  Track system performance –  Q measured by SLTE, delivered Error Performance

•  Track repairs –  Additional cable –  Repair joints –  Can affect tilt

–  Can measure impact on Q a single repair generally has a small impact, but they accumulate

–  Can determine remaining margin and risk

Presenter: Company:

Fundamentals

TF

OSNR, Q …

•  OSNR Optical Signal to Noise Ratio [ dB/0.1 nm ]

•  Q Quality factor [ dB ]

•  Frequency Speed of light / Wavelength [ GHz ] •  BW Bandwidth

[ GHz ] or [ nm ]

•  Important to use consistent units, ideally All SI Linear or dB

•  Useful approximations –  1.0 nm ~125 GHz (at 1550 nm) –  1550 nm ~193 THz –  50.0 GHz ~0.4 nm –  37.5 GHz ~0.3 nm

Definition – OSNR

•  Power in one channel / Noise Spectral Density (NSD)

•  Power [ mW ] / NSD [ mW per 0.1 nm ]

•  OSNR per 0.1 nm 10 log (Sig/NSD) dB per 0.1 nm

•  Obtained by: –  Calculation –  Measurement using an Optical Spectrum

Analyser (OSA) care needed to get accurate measurements

Constellation diagrams

•  Represents signals in phase space I = In phase, Q = Quadrature

•  Receiver can display in same format •  Gives an idea of the signal quality

00 01

10 11

I

Q

Amplitude Phase

Q ~12 dB Q ~6 dB

Definition – Q

•  The key parameter used in budgets

•  Q original defined a “Quality factor” at the detection point

•  Q = Signal / RMS noise (Linear) Q = 20 log(Sig / Noise)

•  Assuming noise is Gaussian •  Error Rate before FEC = erf(Q) = 1-NORM.DIST(Q,0,1,TRUE) •  Q = -NORM.INV(ERBF,0,1)

Q = 6 dB

Calculating noise – simplified

•  Amplifier behaves as if there were an input noise of NF x hv [ W/Hz ]

–  NF Noise Figure depends on the amplifier

–  h Plank’s constant ~ 6.6E-34 –  v frequency = speed of light / wavelength

~ 3E8/1.55E-6 = 1.93E14 = 193 THz

–  hv ~ 1.28E-19 –  BW Bandwidth ~ 1.25E10 Hz for 0.1

nm

G NF hv BW G NF hv BW

Amplifier characteristic

•  Is self-stabilising: low input produces higher gain •  Amplifier gain tends to be close to fibre loss

12

13

14

15

16

17

18

-­‐10 -­‐5 0 5 10 15

Output  (dBm)

Input  (dBm)

Operating point with 16 dB loss

Partial restoration after 1 amplifier

Noise accumulation – identical sections

•  A good simplification for long systems

After N amplifiers (don’t forget that the SLTE contains an amplifier) Noise = N G NF hv BW

= N L NF hv BW where L is fibre loss ~ G OSNR = Pch / (N L NF hv BW) LINEAR

G NF hv BW G NF hv BW

G 2 NF hv BW 2 G NF hv BW

Noise accumulation – logarithmic units

•  More commonly used

NF = 10 log(linear Noise Figure) dB Pch = 10 log(Power in mW ) dBm/channel L = Section length x fibre attenuation dB/km OSNR = Pch – 10 log(N) – L – NF + 58 dB/0.1 nm

G NF hv BW G NF hv BW

G 2 NF hv BW 2 G NF hv BW

5

9

13

10 14 18 22OSNR (dB/0.1 nm)

Q(OSNR)

Q dependence on OSNR

•  Simple theory suggests Q = Coefficient x OSNR •  Coefficient depends on:

–  Receiver optical bandwidth electrical bandwidth Modulation format

•  Practice is more complex Best to measure

QPSK

Example for illustratiom Depends on implementation

Different Formats

•  Higher line-rates need significantly higher OSNR

•  Theoretical penalties – reality will be a little worse – particularly as the

number of levels increases – Q vs. OSNR may

have a different shape

QPSK 8 QAM 16 QAM

4 dB 7 dB

2 bit/symbol 3 bit/symbol 4 bit/symbol

More power / OSNR than QPSK

BPSK

1 bit/symbol

0 dB -3 dB

Different Formats – Practice

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0

QPSK

8QAM 16QAM

OSNR

Q

4 dB 8 dB

Example for illustratiom Depends on implementation

Presenter: Company:

Constructing a Power Budget

Optical budget (simplified) – 1

Q from OSNR may include effect of SLTE – Propagation effects non-linearity, dispersion etc. – Imperfections pre-emphasis etc. – Supervisory small effect – Time variations 5 sigma i.e. worst case – Effects of terminal if not already included = Q Line at Beginning Of Life (BOL / SOL)

Optical budget (simplified) – 2

•  Need to calculate for Beginning of Life (BOL) AND End of Life

(EOL) •  Budget must consider

–  Cable repairs –  Ageing effects –  Pump failures

•  Must be some Operating margin, typically 1.0 dB (Q), at EOL

•  Consider reducing EOL margin –  Fewer amplifiers in a new build OR –  More traffic capacity in an upgrade

Margins for ageing and repairs

•  Extra loss due to repairs, typically 0.4 dB every 40 km (shallow)

3 dB every 1000 km (deep) 0.4 dB per land cable repair

•  Extra loss due to fibre ageing, typically 0.005 dB/km •  Pump failures, typically 5% of

amplifiers; 1.5-3.0 dB depends on amplifier

•  Calculate impact of ALL on OSNR, then repeat budget calculation •  NOTE that nonlinearity generally gets better with repairs/ageing

repairs

Pump failure

repair repair

Gain Tilt

•  Impact of extra attenuation e.g. due to repair or ageing is Gain Tilt

•  4400 km transmission example

32 nm

1.64 dB

Nominal conditions With all repairs and EOL ageing

3 dB repair

OAS

Non-linearity?

Low OSNR

Possible solutions

1 Include Active Tilt Equalizer (ATEQ) in initial system design

–  Includes sufficient adjustment to maintain gain flatness –  Drawback: Adds loss, so more repeaters

2 Add a fixed Tilt Equalizer (TEQ) or repeater during repair

–  Reduces losses and simplifies the system –  Drawback: More complex repair

ATEQ ATEQ

repair TEQ

OAS

ITU-T G.977 Annex A – Example 1 Line Parameter BOL EOL 1 Mean Q value (from a simple OSNR calculation) 1.1 Propagation impairments: chromatic dispersion, non-linear effects etc. 1.2 Gain flatness impairments 1.3 Non-optimal optical pre-emphasis impairment 1.4 Wavelength tolerance impairment 1.5 Mean PDL penalty 1.6 Mean PDG penalty 1.7 Mean PMD penalty …

5 Segment Q value (computed from 3 and 4) 5.1 BER corresponding to segment Q without FEC 5.2 BER corresponding to segment Q with FEC 5.3 Effective segment Q value with FEC 6 Q limit [ Q at which post-FEC objective is met ] 7 Repair margins 8 Segment margins 9 Unallocated supplier margin 10 Commissioning limits

PM

ITU-T G.977 Annex A – Example 2 Line Parameter BOL A BOL OSNR at full loading1 (XX dBm channel power) B EOL OSNR at full loading1 (XX dBm channel power) 1 Back-to-back Q at BOL OSNR 2 Propagation impairments 3 Other impairments 3.1 Non-optimal optical pre-emphasis impairment 3.2 Wavelength tolerance impairment 3.3 Mean penalty due to polarization-dependent effects …

5 BOL segment Q 6 Repair and Ageing Impairments 6.1 Cable repair and ageing 6.2 TTE ageing 7 EOL segment Q 8 FEC limit [ Q at which post-FEC objective is met ] 9.1 Customer Segment EOL margin 9.2 Extra margin 10 Commissioning limit

EOL

Example 1 compared with Example 2

Example 1

Determine B-B OSNR 1 Q (from simple OSNR) 1.2 Gain Flatness Impairment 4 Specified TTE Q value (back-to-back) 5 Segment Q value 5.1 BER without FEC 5.2 BER with FEC 5.3 Effective segment Q value 7 Repair margins

Component and fibre-ageing penalty Pump(s) failure penalty Non-optimal decision threshold

Example 2 A OSNR (at full loading) 1 Back-back Q at BOL OSNR

includes modem (RX) effects 3.6 Unspecified Impairment

5 BOL Segment Q 6 Repair and ageing impairments 6.1 Cable repair and ageing 6.2 TTE Ageing

Power Budget Tables (Non-Coherent TTE)

Wet-­‐Plant  SLTE   SLTE  

OSNR [dB/0.1nm]

Q2 [

dB]

Propagation Penalties

Q2 [

dB] TTE Penalty

•  TTE/Modem  implementa6on  penalty  very  low  

•  Mean   𝑄↑2 = 𝐵↓𝑜 𝑂𝑆𝑁𝑅↓ /𝐵↓𝑒    

•  Linear  rela6onship  

•  Propaga6on  penal6es  due  to  the  wet-­‐plant  are  subtracted  

•  Modem  implementa6on  penalty  subtracted  

•  1/𝑄↓𝑠𝑒𝑔↑2  = 1/𝑄↓𝑝𝑟𝑜𝑝↑2  + 1/𝑄↓𝑇𝑇𝐸↑2    

•  Repairs  and  aging  done  on  OSNR  

OSNR [dB/0.1nm]

Q2 [

dB]

OSNR [dB/0.1nm]

Propagation Penalties

Repairs/Aging/Extra Margin

FEC Limit FEC Limit FEC Limit

Power Budget Tables (Coherent TTE)

•  TTE/Modem  implementa6on  penalty  maEers  

•  Actual  Q:  

𝑄↑2 = 𝐸𝐶/ 𝐵↓𝑒 / 𝐵↓𝑜 𝑂𝑆𝑁𝑅↓  + 1/𝑆𝑁𝑅↓𝑇𝑇𝐸     

•  Propaga6on  penal6es  due  to  the  wet-­‐plant  are  subtracted  

•  Repairs  and  aging  margin  calcula6ons  performed  in  OSNR  

Wet-­‐Plant  SLTE   SLTE  

OSNR [dB/0.1nm]

Q2 [

dB]

OSNR [dB/0.1nm]

Q2 [

dB] Propagation

Penalties

FEC Limit FEC Limit

Del

iver

ed

OS

NR

Q2 [

dB]

OSNR [dB/0.1nm]

Propagation Penalties

FEC Limit

Repairs/Aging/Extra Margin

Power Budget Tables (Coherent) – higher modulation

OSNR [dB/0.1nm]

Q2 [

dB]

FEC Limit

EOL Margin (Q budgeting)

OSNR [dB/0.1nm]

Q2 [

dB]

FEC Limit

EOL Margin (OSNR budgeting) Example  2  

Example  2  Example  2  

Example  2  

Presenter: Company:

How it feeds into Acceptance

Acceptance – Budget Verification

1.  Q measured by FEC correction rate –  Compare with commissioning limit (Line 10)

–  Should also consider effect of Q variations –  BOL Margin = Q Line (average) – Time variations (5 sigma) –

Q Limit OR = Q Line (worst measured) – Q Limit

–  The period over which average / worst case is measured is important Long periods remove the effect of short-term variations

2.  Error performance after FEC

System Acceptance : G.828 and G.8201

•  G.8201 for OTN (up to 100G) •  G.828 for SDH, where performance is assessed by measurement of:

–  BBER : Background Block Error Ratio –  SESR : Severely Errored Second Ratio (>15% errored blocks in 1

sec.)

–  Depends on link distance: e.g. 5,000 km link at 100G –  BBER <3.50E-4 BER < 3.75E-12 –  SESR <4.38E-7

•  Limit Q usually specified at 1E-13 to 1E-15 •  In practice require much better than G.828 – often 0 errors in 7 days

OAS

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Refinements

Refinements

1.  Extra losses e.g. Equalizers, Branching Units

2.  Correct Assumptions that are not quite true – typical correction 0.5-1.0 dB –  Amplifier gain = Cable loss –  All sections the same –  All wavelengths the same –  System is symmetrical

3.  Special cases –  ROPA –  Distributed Raman gain –  Terminal upgrades

Extra Losses

•  Equalizer Units •  Branching Units •  OADM filters

•  Example: 1 Equaliser every 10 sections Equaliser loss is 5 dB Section loss was 20 dB not including equalizer units Increase normal section length Decrease length of section with equaliser Section loss is now 20.5 dB including equalizer units Adds a penalty of 0.5 dB to OSNR

Non-identical sections – Examples

•  Repeater positions are not quite regular •  Branching unit is not in the optimum position

•  Mix of submarine and terrestrial – different NF and spacing

Sections are not identical – Calculation

•  1/OSNR = 1/OSNR(1) + 1/OSNR(2) + … •  1/Q2= 1/Q2

Line + 1/Q2SLTE

•  Adds complexity, but can be important –  Example:

10 sections at 20.5 dB 9 sections at 20.0 dB + 1 section at 25.0 ~0.4 dB difference

1 2 OSNR(2) OSNR(1)

SLTE

Wavelengths are not identical

•  Noise figure, loss, non-linearity ... generally vary with wavelength •  Channel power needs to be adjusted to suit

•  Need to ensure worst case wavelength has adequate margin •  Must consider non-linear effects

Noise Figure

Average

Worst case

Average

Amplifier Gain not equal to Section Loss

•  G = Po / (Pi + Ni) not Po / Pi •  H = Pi / (Pi + Ni) actual gain / ideal gain close to 1

•  After N amplifiers •  Signal = Po HN •  Noise = Ni G (1 + H + H2 + ... + HN)

•  Penalty = HN / (1-HN) / (1-H) A small effect (<0.5 dB in general)

G Ni G Ni

Pi G Pi Po = G (Pi + Ni)

System symmetry

•  Not usually an issue, but worth considering

•  Example: one section is long Solution : increase TX power

•  Works in one direction •  Not in the other direction cannot increase amplifier

power

•  Did the budget consider the worst case?

T R

R T

Systems with ROPA or DRA

•  Most often used in unrepeatered systems •  May use Remote Optically Pumped Amplifiers (ROPA)

•  Distributed Raman Amplification (DRA)

ROPA

TX RX

P

TX RX

P

Budgets involving ROPAs and DRA

•  Raman / ROPA can be treated as an optical amplifier with a defined gain and noise figure

•  Note, however, that a loss in the pump section reduces both the Raman / ROPA gain and adds loss to the signal path –  Affects repair margins more than with other amplifier types

•  Impact is position-dependent –  Loss at (1) less significant than at (2)

RX

P

2 1

Terminal Upgrades

•  Often necessary to retain some old wavelengths –  Important not to take too much power from old WLs

–  Budget must consider both old and new WLs, Idlers/Loaders

•  Getting budget parameters not always easy –  Sometimes possible to obtain OSNR measurements –  Often includes a trial/demonstration – allows the budget to be tested –  May need to demonstrate inter-working of ASK and PSK

Old WLs New WLs

Presenter: Company:

Other topics / discussions

Other topics

•  Typical characteristics of different systems –  Can help to spot anomalies

•  Open systems –  What is different? –  How is acceptance handled?

Characteristics of different systems

•  Short systems •  Repeater separation is large •  Gain flatness easy •  Larger repair allowances •  Usually have some extra margin

Length (km) Repair margin Extra margin 1,000 1.8 1.1 2,000 1.4 0.4 4,000 1.1 0.3 8,000 0.8 0.0

•  Long systems •  Repeater separation is smaller •  Gain flatness hard •  Smaller repair allowances •  Usually have little extra margin

Open systems – Why

•  Separation of SLTE from Cable System offers:

•  Transparency of Cable System performance –  Separates cable specification and performance-monitoring from

SLTE •  Integration of terrestrial and subsea network infrastructure under one

management system –  Allows purchasers to integrate preferred terrestrial solutions where

possible – for most who run networks, subsea is a small percentage of total capacity

•  Flexibility in the operations model, protecting Purchasers from supply chain uncertainty and/or disparate technology cycles

TS

Open systems – how they could look

Open  Cable  

Interface

Open  Cable  System

North  BoundSupervisory  &  Management

Terminal  Station  

Equipment

Open  Cable  

Interface

TxRx

TxRx

Clear  D

emark

Supervisory  &  Management

Terminal  Station  

Equipment

………….

………….

TxRx

TxRx

Clear  D

emark

SLTESLTE

North  Bound

Provides  broadband  access  for  2  (or  more)  SLTE

Incorporates  Cable  Supervisory  and  PFE  functionality

Choice  of  SLTE

Share  Alarms  and  Supervisory  Data  with  SLTE

Open systems

•  Specification and Acceptance

•  ‘Performance’ and Acceptance defined on line system characteristics, Most notably OSNR, Power, and Power Tilt

•  Difficult set of measurements

•  Potential for standardization

Name

Segment

Landing  Sites CLS1 CLS2

Length xxx km

Quantity  of  Channels  at  Full  Loading xxx carriers

Nominal Worst  Case

1.1    Slope  of  T ilt  [dB/THz]

1.2    Gain  Deviation  from  tilt  [dB]

1.3    Power  per  carrier  [dB]  

1.4    Span  Loss  [dB]  at  1550nm

1.5    Span  Length  [km]

1.6    Equalized  OSNR  [dB/0.1nm]  across  the  Passband  at  full  loading

1.7    Passband  Start/Stop  [THz]

1.8    Average  DGD  across  the  Passband  [ps]

1.9    Mean  PDL  [dB]

1.10    Total  accumulated  Chromatic  Dispersion  [ps/nm]  at  1550nm

 

2.1    Repeater  Total  Output  Power  [dBm]

2.2    Average  Repeater  Noise  Figure  across  Passband  [dB]

2.3    Number  of  Repeaters

 

3.1    Fiber  Effective  Area  [um^ 2]

3.2    Fiber  Chromatic  Dispersion  [ps/nm/km]

3.3    Fiber  Attenuation  [dB/km]

 

4.1    Deep  Water  [km  between  repairs]

4.2    Shallow  Water  [km  between  repairs]

4.3  Estimated  Loss  per  Deep/Shallow  Repair  [dB]

3.  Fiber  Specification  at  1550nm

2.  Repeater  Specification

1.  System  Specification  (Open  C able  System)

4.  Repair  Guidance

Open  Cable  Performance  SpecificationExample

Example

S tart-­‐of-­‐Life  [SOL]

Thank  you  for  listening    Any  Ques6ons?