Ch 02 — Modulation, Coherent Detection, and DWDM Architecture

Modulation, Coherent Detection, and DWDM Architecture

This chapter covers how bits get encoded onto a wavelength and how those wavelengths are combined into a working DWDM line system. It builds directly on the OSNR and Shannon-capacity framework from Optical Physics and Link Engineering: every modulation format trades capacity for reach against the OSNR budget, and the DWDM components — transponders, MUX/DEMUX, ROADMs, OSC — are the physical realisation that makes those tradeoffs concrete.

Modulation Formats and Coherent Detection

Key Insight

Every modulation format trades capacity for reach [21, 28]. More bits per symbol means higher throughput per channel but demands a cleaner signal (higher OSNR), which limits transmission distance. Modern transponders resolve this tradeoff dynamically through probabilistic constellation shaping [29].

The fundamental relationship is: bits per symbol = log2(constellation size). Dual polarisation (DP) doubles the effective bits per symbol by transmitting two independent signals on orthogonal polarisations (X and Y) through the same fibre simultaneously.

Modulation	Constellation Points	Bits/Symbol	With DP (×2)¹	Typical Rate	Terrestrial Reach²	Required OSNR³
OOK/NRZ	2	1	N/A (direct-detect)	10G	Long	~10 dB electrical SNR (not OSNR-comparable)
QPSK	4	2	DP-QPSK = 4 b/sym	100–200G	2000–4000 km terrestrial; 12 000+ km subsea	~12 dB (SD-FEC) [15]
8QAM	8	3	DP-8QAM = 6 b/sym	300–400G	800–1500 km	~16 dB (SD-FEC)
16QAM	16	4	DP-16QAM = 8 b/sym	400–800G	80–500 km (32 GBd) / 200–600 km (64 GBd)	~19 dB (SD-FEC) / ~16 dB with PCS [29]

¹ DP-QPSK is two independent QPSK streams on orthogonal polarisations — 4 bits per dual-pol symbol slot, not 4 bits per single-pol constellation point. ² Reach is heavily implementation- and FEC-dependent; numbers are planning ranges. ³ All OSNR values are at 0.1 nm reference bandwidth, with modern SD-FEC; legacy HD-FEC values run ~5 dB higher.

Coherent detection is the enabling technology behind all modern modulation formats [21, 28]. Unlike legacy direct-detect systems that measured only light intensity (on/off), a coherent receiver mixes the incoming signal with a local oscillator laser and recovers amplitude, phase, and polarisation state simultaneously — accessing all four dimensions of the optical field. This allows the receiver’s DSP to compensate chromatic dispersion, polarisation mode dispersion, and nonlinear impairments digitally, eliminating the need for external dispersion-compensating fibre.

Current-generation transponders (Ciena WaveLogic 6, Nokia PSE-6s, Infinera ICE7) auto-select the optimal modulation based on real-time channel conditions. The technique — probabilistic constellation shaping — places constellation points non-uniformly to approach the Shannon limit more closely, squeezing maximum capacity from each link without manual configuration [29].

graph TD
    subgraph "Modulation — reach vs capacity"
        QPSK["DP-QPSK<br/>4 bits/sym<br/>100-200G<br/>Reach: 6000 km"]
        QAM8["DP-8QAM<br/>6 bits/sym<br/>300-400G<br/>Reach: 1500 km"]
        QAM16["DP-16QAM<br/>8 bits/sym<br/>400-800G<br/>Reach: 500 km"]
    end
    QPSK -->|"Higher capacity<br/>shorter reach"| QAM8
    QAM8 -->|"Higher capacity<br/>shorter reach"| QAM16
    style QPSK fill:#1D9E75,stroke:#0F6E56,color:#fff
    style QAM8 fill:#BA7517,stroke:#854F0B,color:#fff
    style QAM16 fill:#D85A30,stroke:#993C1D,color:#fff

The same tradeoff plotted as bar charts makes the non-linear scaling obvious — required OSNR roughly doubles between adjacent formats while reach drops by 3–4×:

xychart-beta
    title "Maximum Reach by Modulation Format (400G, standard fibre)"
    x-axis "Modulation" ["DP-QPSK", "DP-8QAM", "DP-16QAM", "DP-64QAM"]
    y-axis "Approximate Reach (km)" 0 --> 7000
    bar [6000, 1500, 500, 80]

xychart-beta
    title "Required OSNR by Modulation Format"
    x-axis "Modulation" ["DP-QPSK", "DP-8QAM", "DP-16QAM", "DP-64QAM"]
    y-axis "Required OSNR (dB)" 0 --> 30
    bar [12, 16, 19, 25]

Key Insight

As required OSNR roughly doubles between adjacent formats, reach drops by 3-4×. Always select the highest modulation the link’s OSNR can sustain with at least 3 dB of margin — that maximises capacity per wavelength.

Constellation Layouts

Each format places symbols on the complex I/Q plane. More symbols means more bits per symbol, but tighter symbol spacing demands cleaner OSNR to avoid decision errors:

OOK / NRZ                         QPSK
                                       Q
  I axis →                             │
  ●─────────────────●             01 ● │ ● 00
  0                 1                  │
                                  ─────┼─────── I
  2 points = 1 bit/symbol              │
                                  11 ● │ ● 10
                                       │
                                  4 points = 2 bits/symbol
                                  DP-QPSK = 4 bits/symbol

8QAM                              16QAM
       Q                                 Q
       │                                 │
    ●  │  ●                       ●  ● │ ●  ●
  ●    │    ●                     ●  ● │ ●  ●
  ─────┼─────── I                 ─────┼─────── I
  ●    │    ●                     ●  ● │ ●  ●
    ●  │  ●                       ●  ● │ ●  ●
       │                                 │
  8 points = 3 bits/symbol         16 points = 4 bits/symbol
  DP-8QAM = 6 bits/symbol          DP-16QAM = 8 bits/symbol

Dual-Polarisation Multiplexing

The “DP” prefix doubles spectral efficiency by transmitting two independent symbol streams on orthogonal polarisation states of the same wavelength. The coherent receiver separates them using a polarisation beam splitter and a local oscillator:

flowchart LR
    LASER["Laser<br/>source"] --> SPLIT["Polarisation<br/>beam splitter"]
    SPLIT -->|"X polarisation"| MODX["Modulator X<br/>e.g. QPSK = 2 bits"]
    SPLIT -->|"Y polarisation"| MODY["Modulator Y<br/>e.g. QPSK = 2 bits"]
    MODX --> COMBINE["Polarisation<br/>beam combiner"]
    MODY --> COMBINE
    COMBINE --> FIBER["Single λ on fibre<br/>carries 2 + 2 = 4 bits/symbol"]
    style LASER fill:#378ADD,stroke:#185FA5,color:#fff
    style MODX fill:#1D9E75,stroke:#0F6E56,color:#fff
    style MODY fill:#1D9E75,stroke:#0F6E56,color:#fff
    style COMBINE fill:#7F77DD,stroke:#534AB7,color:#fff
    style FIBER fill:#BA7517,stroke:#854F0B,color:#fff

DWDM System Architecture

A DWDM line system consists of a small number of component types chained together in a repeating pattern [16, 26, 27]. Understanding each component’s role and its contribution to the link budget is the foundation of optical network design.

flowchart LR
    TX["Transponders<br/>lambda1...lambda96"] --> MUX["MUX"]
    MUX --> BA["EDFA<br/>Booster"]
    BA --> F1["Fiber<br/>Span 1<br/>~80 km"]
    F1 --> ILA["EDFA<br/>ILA"]
    ILA --> F2["Fiber<br/>Span 2<br/>~80 km"]
    F2 --> ROADM["ROADM<br/>Add/Drop"]
    ROADM --> F3["Fiber<br/>Span 3"]
    F3 --> PA["EDFA<br/>Pre-amp"]
    PA --> DMX["DEMUX"]
    DMX --> RX["Receivers<br/>lambda1...lambda96"]
    ROADM -.->|"Drop lambda5"| LOCAL["Local<br/>client"]
    style TX fill:#378ADD,stroke:#185FA5,color:#fff
    style RX fill:#378ADD,stroke:#185FA5,color:#fff
    style MUX fill:#7F77DD,stroke:#534AB7,color:#fff
    style DMX fill:#7F77DD,stroke:#534AB7,color:#fff
    style BA fill:#1D9E75,stroke:#0F6E56,color:#fff
    style ILA fill:#1D9E75,stroke:#0F6E56,color:#fff
    style PA fill:#1D9E75,stroke:#0F6E56,color:#fff
    style ROADM fill:#BA7517,stroke:#854F0B,color:#fff
    style LOCAL fill:#D85A30,stroke:#993C1D,color:#fff

End-to-end DWDM system: transponders feed the MUX, amplified fibre spans carry the composite signal, ROADM nodes add/drop individual wavelengths, and the far-end DEMUX delivers each channel to its receiver.

Component	Function	Key Detail
Transponder	Converts client signals (100GbE, 400GbE) into DWDM wavelengths [17]	Contains coherent DSP, DAC/ADC, modulator, and tuneable laser. Modern “universal” transponders handle all modulation formats in software.
MUX / DEMUX	Combines (or separates) multiple wavelengths onto a single fibre [27]	Technologies: AWG (Arrayed Waveguide Grating) and thin-film filters. Typical insertion loss 4-7 dB.
EDFA	Amplifies all wavelengths simultaneously in the optical domain [12, 13, 24]	Three roles: booster (post-transmitter), inline/ILA (mid-span every 80-120 km), pre-amplifier (pre-receiver). Adds ASE noise.
ROADM	Remotely selects wavelengths to add, drop, or pass through [27]	Uses WSS (Wavelength Selective Switch) technology [23]. Modern configurations: 1x9 or 1x20 degree for mesh topologies. Enables software-defined optical networking.
OSC	Carries management data on a dedicated out-of-band channel	Operates at ~1510 nm (outside C-band). Transports amplifier status, alarms, firmware updates, remote configuration.
DCM	Compensates chromatic dispersion	Legacy systems used DCF (dispersion-compensating fibre). Modern coherent DSP handles dispersion digitally — no external DCM required (DCMs disappeared from coherent line systems around 2012).

The ITU-T G.694.1 Channel Grid — How 100G, 400G, and 800G Pack Into Spectrum

The DWDM channel plan is set by ITU-T G.694.1 [1]. The C-band is anchored at 193.100 THz, with channels offset by integer multiples of a fixed step. Two grids coexist:

• Fixed grid — channel spacings of 12.5 / 25 / 50 / 100 GHz, centre frequencies on the same step as the spacing [1].
• Flex-grid (G.694.1 §7) — slot widths quantised to integer multiples of 12.5 GHz, centre frequencies on a finer 6.25 GHz raster, so each channel claims exactly the spectral width its symbol rate needs [1].

Key Insight — 100G uses a 50 GHz slot

The standard channel bandwidth for 100G coherent (DP-QPSK at ~32 Gbaud) is 50 GHz on the fixed grid [1]. This is the dominant deployed format on long-haul DWDM today and is what most operators mean by “a wavelength”. Higher rates spill onto the flex-grid because their broader spectra no longer fit a 50 GHz slot. Slot width is configurable per-design — these are typical baud-rate-driven minima, not fixed mandates:

Line rate	Typical modulation	Symbol rate	Typical slot width
100G	DP-QPSK	~32 Gbaud	50 GHz (fixed grid) [1]
200G	DP-QPSK / DP-16QAM	~32–64 Gbaud	50–75 GHz (flex)
400G	DP-16QAM (e.g. 400ZR)	~60 Gbaud	75–100 GHz (flex) [18]
600G	PCS-64QAM	~64–69 Gbaud	~100 GHz (flex) [19]
800G	DP-16QAM / PCS-shaped	~118–130 Gbaud	100–150 GHz (flex)
1.2T	PCS super-channel	~140 Gbaud	~200 GHz (flex)

C-band arithmetic: ~4.4 THz of usable spectrum at 50 GHz spacing yields ~88 channels; the commonly quoted “96 channels” figure assumes the slightly extended 1528–1568 nm window used by gain-extended EDFAs. L-band roughly doubles the available channel count at the cost of higher noise.

The end-to-end client-to-client path makes the layering concrete. Both client routers see nothing but a 100GbE Ethernet link; everything between them — OTN framing, DWDM multiplexing, coherent modulation, optical amplification — is completely transparent:

flowchart LR
    CLIENT1["Client router<br/>100GbE port"] -->|"100GBASE-LR4"| TXP1["Transponder<br/>Maps to ODU4<br/>Wraps in OTU4<br/>Modulates λ42"]
    TXP1 --> MUX["MUX<br/>Combines<br/>96 λ"]
    MUX --> BOOST["EDFA<br/>Booster"]
    BOOST --> SPAN1["80 km<br/>fibre"]
    SPAN1 --> ILA1["EDFA<br/>ILA"]
    ILA1 --> SPAN2["80 km"]
    SPAN2 --> ILA2["EDFA<br/>ILA"]
    ILA2 --> SPAN3["80 km"]
    SPAN3 --> PREAMP["EDFA<br/>Pre-amp"]
    PREAMP --> DEMUX["DEMUX<br/>Splits<br/>96 λ"]
    DEMUX --> TXP2["Transponder<br/>Demodulates λ42<br/>Unwraps OTU4<br/>Extracts ODU4"]
    TXP2 -->|"100GBASE-LR4"| CLIENT2["Client router<br/>100GbE port"]
    style CLIENT1 fill:#378ADD,stroke:#185FA5,color:#fff
    style CLIENT2 fill:#378ADD,stroke:#185FA5,color:#fff
    style TXP1 fill:#7F77DD,stroke:#534AB7,color:#fff
    style TXP2 fill:#7F77DD,stroke:#534AB7,color:#fff
    style MUX fill:#534AB7,stroke:#3D2F99,color:#fff
    style DEMUX fill:#534AB7,stroke:#3D2F99,color:#fff
    style BOOST fill:#1D9E75,stroke:#0F6E56,color:#fff
    style ILA1 fill:#1D9E75,stroke:#0F6E56,color:#fff
    style ILA2 fill:#1D9E75,stroke:#0F6E56,color:#fff
    style PREAMP fill:#1D9E75,stroke:#0F6E56,color:#fff

Key Insight

The client routers at both ends see nothing but a 100GbE Ethernet link. Everything between them — OTN framing, DWDM multiplexing, coherent modulation, optical amplification — is completely transparent.

Walking the transmit side: the client router sends 100GbE frames over a short-reach optic [17]; the transponder maps the payload into an ODU4 (adding PM and TCM overhead) [3], wraps it in OTU4 (adding FEC, typically oFEC or SD-FEC) [14, 19], and modulates onto a specific ITU lambda (e.g. λ42 = 193.200 THz) using the format appropriate for the link distance; the MUX combines this lambda with up to 95 others; a booster EDFA amplifies the combined signal for the first span; ILAs restore power after each subsequent span; the pre-amp, DEMUX, and remote transponder reverse the entire process at the far end.

See Also

• Optical Physics and Link Engineering
• Optical Amplifiers — EDFA, Raman, and Wideband C+L
• OTN, SDH, and Network Design
• MPLS Traffic Engineering — RSVP-TE, SR-TE, and Fast Reroute

References

Standards (ITU-T)

1. ITU-T G.694.1 — Spectral grids for WDM applications: DWDM frequency grid (10/2020). https://www.itu.int/rec/T-REC-G.694.1
2. ITU-T G.694.2 — Spectral grids for WDM applications: CWDM wavelength grid (12/2003). https://www.itu.int/rec/T-REC-G.694.2
3. ITU-T G.709/Y.1331 — Interfaces for the Optical Transport Network (06/2020). https://www.itu.int/rec/T-REC-G.709
4. ITU-T G.798 — Characteristics of optical transport network hierarchy equipment functional blocks (12/2017). https://www.itu.int/rec/T-REC-G.798
5. ITU-T G.872 — Architecture of optical transport networks (10/2017). https://www.itu.int/rec/T-REC-G.872
6. ITU-T G.873.1 — Optical Transport Network — Linear protection (03/2022). https://www.itu.int/rec/T-REC-G.873.1
7. ITU-T G.652 — Characteristics of a single-mode optical fibre and cable (11/2016). https://www.itu.int/rec/T-REC-G.652
8. ITU-T G.654 — Characteristics of cut-off shifted single-mode optical fibre and cable (03/2020). https://www.itu.int/rec/T-REC-G.654
9. ITU-T G.655 — Characteristics of non-zero dispersion-shifted single-mode optical fibre and cable (11/2009). https://www.itu.int/rec/T-REC-G.655
10. ITU-T G.657 — Characteristics of a bending-loss insensitive single-mode optical fibre and cable (11/2016). https://www.itu.int/rec/T-REC-G.657
11. ITU-T G.707/Y.1322 — Network node interface for the synchronous digital hierarchy (SDH) (01/2007). https://www.itu.int/rec/T-REC-G.707
12. ITU-T G.661 — Definitions and test methods for the relevant generic parameters of optical amplifier devices and subsystems (07/2018). https://www.itu.int/rec/T-REC-G.661
13. ITU-T G.662 — Generic characteristics of optical amplifier devices and subsystems. https://www.itu.int/rec/T-REC-G.662
14. ITU-T G.975 — Forward error correction for submarine systems (10/2000). https://www.itu.int/rec/T-REC-G.975
15. ITU-T G.975.1 — Forward error correction for high bit-rate DWDM submarine systems (02/2004). https://www.itu.int/rec/T-REC-G.975.1
16. ITU-T G.698.2 — Amplified multichannel dense wavelength division multiplexing applications with single channel optical interfaces. https://www.itu.int/rec/T-REC-G.698.2

Standards (IEEE / OIF / MSA)

17. IEEE 802.3 — Ethernet; specifically 802.3bs (400 GbE), 802.3ck, 802.3df (800 GbE). https://standards.ieee.org/standard/802_3-2022.html
18. OIF 400ZR Implementation Agreement (OIF-400ZR-01.0). https://www.oiforum.com/documents/
19. OpenZR+ MSA specification. https://www.openzrplus.org/
20. OIF Common Management Interface Specification (CMIS). https://www.oiforum.com/documents/

Books

21. G. P. Agrawal, Fiber-Optic Communication Systems, 5th ed., Wiley, 2021.
22. G. P. Agrawal, Nonlinear Fiber Optics, 6th ed., Academic Press, 2019.
23. I. P. Kaminow, T. Li, A. E. Willner (Eds.), Optical Fiber Telecommunications VI-A & VI-B, Academic Press, 2013.
24. P. C. Becker, N. A. Olsson, J. R. Simpson, Erbium-Doped Fiber Amplifiers — Fundamentals and Technology, Academic Press, 1999.
25. C. Headley & G. P. Agrawal (Eds.), Raman Amplification in Fiber Optical Communication Systems, Academic Press, 2005.
26. V. Alwayn, Optical Network Design and Implementation, Cisco Press, 2004.
27. R. Ramaswami, K. N. Sivarajan, G. H. Sasaki, Optical Networks: A Practical Perspective, 3rd ed., Morgan Kaufmann, 2009.

Papers

28. K. Roberts et al., “Beyond 100 Gb/s: Capacity, Flexibility, and Network Optimization,” J. Lightwave Technol. 35 (2017).
29. J. Cho et al., “Probabilistic Constellation Shaping for Optical Fiber Communications,” J. Lightwave Technol. 37, 1590 (2019).
30. A. D. Ellis et al., “Performance limits in optical communications due to fiber nonlinearity,” Adv. Opt. Photon. 9, 429 (2017).