The document discusses the comparison between single-carrier and multi-carrier transmission methods for achieving over 800G coherent data rates in advanced roadm networks. It highlights the benefits of both approaches, including cost efficiency and spectral efficiency, while emphasizing the importance of optimizing symbol rates and managing network configurations to enhance performance and enable higher capacity. The document also indicates future trends in optical technology and network management that will continue to influence transmission methods.

1. Single vs. multi-carrier in ROADM networks
OFC 2022 workshop – Single carrier vs. multi-carrier for >800G coherent
Sorin Tibuleac

2. 2
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
10G through 800G
Will the trend continue?
• Multi-carrier transmission = first option for higher data rates across longer distances
• 10G as 4x2.5G
• 100G as 2x50G coherent or 10x10G
• Higher symbol rates have replaced multi-carrier options
• 400G & 800G available as SC or MC channels
• Key enabler of single-carrier option:
• Advances in electronics
• Driver for SC option
• Fewer optics = lower cost
• Simplify network management
Historical perspective

3. 3
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
High capacity ROADM network Sparse add/drop
Core Metro/Regional Long-haul
Data center
Different optimum solution using single or multi-carrier channels
High capacity/SE required in all cases with specific constraints
Network applications
• Standard compliant
• Pluggable
• Low power/size
• Uniform channel rate
• Simplicity of operation
• Standard & Proprietary
• Mixed data rate & flex grid
• ROADM impairments
• Wavelength contention
• Routing flexibility
• Proprietary
• High spectral fill factor
• Fewer routing constraints
• Fine-granularity channel
spacing possible
• Maximum capacity per
fiber – no compromise
Compromise on capacity and spectral efficiency

4. 4
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
These solutions will coexist, may have different optimum applications
Commercial 800G for Core Networks
Digital subcarriers (4-8) 95Gbd
• Sharp roll-off, tight SC spacing
• Flexibility in assigning information rate per digital subcarrier
• Low symbol rate aids nonlinear mitigation for QPSK & long distance
High spectral efficiency
Optical subcarriers 70Gbd x 2
• Simple and scalable, less demanding O/E BW
• Tolerant to ROADM filtering
• High flexibility in symbol rate & SE -> highly adaptive to ROADM networks constraints
Long reach transmission per channel
Single carrier - Next generation ~140Gbd
• Follows historical trend, benefits from higher BW electronics
• High symbol rate aids mitigation of polarization effects and nonlinearities in DSP
• Flexible symbol rate
Improve SE over wide range of applications
Pittala et al. IEEE PTL Feb. 2022
Porto et al. JLT Feb. 2022
Searcy et al. JOCN, under review

5. 5
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
What is a channel?
This trend will continue
• Between end points in a network, channels are multiplexed to higher data rates and/or
multiple carriers
• Super-channels are formed at ROADM nodes
• Maximize spectral efficiency
• Reduce filter penalties
• Reduce crosstalk (WSS shaping)
• Super-channels are jointly routed through the network
• Reduce filter and adjacent channel crosstalk
• Simplify route selection
• Assumes contiguous BW available
• Multiple optical carriers – option for separate routing
Wavelength aggregation and routing
AWG
ROADM
WSS
Tx

6. 6
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
Available BW for channel routing
Increase S.E. by reducing guard-band spectrum
• ROADM granularity limited in SW for network management
• Spectral gaps in meshed networks
• Wavelength contention
• Mix of symbol rates, modulation formats, legacy traffic
• Disaggregation and spectral services
• Guard-bands used to avoid filtering and crosstalk
• Optimum symbol rate for transceiver will depend on
• Transceiver properties (bandwidth of OE components)
• Spectral fill factors (guard-bands)
SE limitations in ROADM Networks
Guard-band
Spectral
gap
Guard-band
Guard-band
Dual-carrier
SE (capacity)
Symbol rate
Transceiver limitations
Guard-bands
Higher
transceiver BW
Optimum
symbol rate
Lorences-Riesgo et al. PTL Feb. 2022

7. 7
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
Need for fine tuning of symbol rate to maximize spectral efficiency
Flexible transceiver on bandwidth constrained links
• Increase SE by avoiding large guard-bands
• Optimization of symbol rate & bits/symbol per optical carrier for a specific ROADM link
• Use optimum setting to increase SE, distance, or margin
Maximize margin
Increase
ROADMs
per link
Searcy et al. ECOC 2021
Bandwidth narrowing specific
to ROADM link
Number of ROADMs
Net
Bandwidth
Transceiver with high-resolution
symbol rate and bits/symbol
Wavelength

8. 8
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
Higher margin/longer distance with dual-carrier routing
Dual-carrier 800G
• Increased reach/margin in dual-carrier transmission
• Use optimum symbol rate per carrier considering
• Net filter bandwidth
• Transceiver performance
• Minimize spacing between carriers
• Sharp roll-off & stable spectrum
• Benefits from single-side filtering
• Assumes a minimum bandwidth is available
Symbol rate
Q
Dual-sided filtering
Searcy et al. JOCN, under review
Single-sided
filtering
Use higher S.R. per carrier
enables lower bits/symbol

9. 9
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
800G transmission over LH distances
Optical path 1,376km:
• 800G DP-16QAM setting, 4.0 bits/sym/pol, 2x69GBd
• 18 WSS modules, set to 150GHz channel
• 26 EDFA amplifier modules + 11x Raman
• G.652 standard fiber
Optical path 2,266km:
• 800G DP-16-32QAM setting, 4.14 bits/sym/pol, 2x67GBd
• 28 WSS modules, set to 150GHz channel
• 44 EDFA amplifier modules + 16x Raman
• G.652 standard fiber
2,000km live ROADM network
800G long haul network trial

10. 10
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
400G terrestrial ultra long haul transmission
QPSK setting, 2 bits/sym/pol, 27% FEC, 2x69GBd
Optical path 7,168km:
• 101 WSS modules, set to 150GHz channel
• 168 EDFA amplifier modules + Raman
• G.652 standard fiber
Optical path 10.215km:
• 89 WSS modules, set to 200GHz channel
• 206 EDFA amplifier modules + Raman
• G.652 standard fiber
10,000km trial on a live ROADM network
400G Multi-domain transmission

11. 11
OFC 2022 Workshop – Single carrier vs. Multi-carrier for >800G coherent
Beyond 800G
• Improvements in opto-electronics & DSP enable high SE per optical carrier
• Multi-carrier channels increasingly used for longer distances
• Benefit from continued progress in optics integration
• For any path and data rate, there is an optimum channel bandwidth
• Granularity in occupied bandwidth (symbol rate, nr. of sub-carriers)
• Granularity in ROADM bandwidth
• Network-wide spectral management
• Increased filter tolerance, highly stable transmitter and WSS spectrum
• Wider bandwidth channels from transceivers
• Fewer ports/connections, lower-loss ROADM aggregation
• Simpler network management
Maximizing throughput in ROADM networks

12. Thank you
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