Cascaded ROADM Tolerance of mQAM Optical Signals Employing Nyquist Shaping

IPC 2014, La Jolla CA (TuF1.2)

Mark Filer and Sorin Tibuleac

ADVA Optical Networking / Atlanta, GA

[email protected]

Cascaded ROADM Tolerance of mQAM Optical Signals Employing Nyquist Shaping

mailto:[email protected]

© 2014 ADVA Optical Networking. All rights reserved.22 IPC 2014, La Jolla CA (TuF1.2)

• Currently deployed 100G DP-QPSK at 50 GHz spacing transmission through many ROADMs with minimal penalty

• Paths to increased capacity:

• Higher-order QAM at current baud rates

• Tighter channel spacing (Nyquist)

• Issues:

• Higher-order QAM more sensitive to ISI

• Narrower ROADM passbands for tighter channel spacing

• This study: assess cascaded ROADM tolerance considering

• Optical filter (WSS) characteristics

• Transmitter pulse shaping + channel spacing

• DSP implementation

All of the above utilizing technology available today

Introduction


16QAM, r=1.0, Δf = 50GHz

• Transmitter variables:

• mQAM, m = {4,8,16}

• 32GBaud, Nyquist signaling

• 40-tap RRC w/ rolloff, r = {0.1,1.0}

• Channel spacing, Δf = {50, 37.5} GHz

• Colorless multiplexing

Simulation setup: transmitter

16QAM, r=0.1, Δf = 37.5GHz


50GHz WSS

• Optical noise (ASE) added at Tx output before ROADM cascade consistent OSNR regardless of filtering applied

• ROADM passband profiles from commercially-available WSSs

• Multiple devices + ports averaged for typical shape

• Center ±6.25 GHz section of ‘flat’ spectrum in center removed to emulate 37.5 GHz shape

Simulation setup: noise-loading + ROADM

cascading

37.5GHz WSS


• Receiver/DSP configuration:

• Polarization-diverse balanced coherent receiver

• Matched RRC FIR filter

• Timing recovery via NDA feed-forward digital square and filter

• 2x2 TDE based on ICA with 13 T/2-spaced taps

• Frequency offset and carrier phase recovery with decision-directed algorithm

Simulation setup: receiver + DSP


r=1.0 r=0.1

• Rolloff r = 1.0

• 0.2-0.3dB colorless add crosstalk

• Moderate impact − for ΔOSNR 1dB:

• QPSK @ >48 WSSs

• 8QAM @ 46 WSSs (BW=27.5GHz)

• 16QAM @ 42 WSSs (BW=28GHz)

Result: Δf = 50GHz channel spacing

• Rolloff r = 0.1

• no colorless add penalty

• Larger impact − for ΔOSNR 1dB:

• QPSK slightly better for ≤46 WSSs

• 8QAM @ 32 WSSs (BW=29GHz)

• 16QAM @ 26 WSSs (BW=29.5GHz)

ΔOSNR1dB


• Rolloff r = 0.1 only

• Huge impact – notice x-axis range!

• For ΔOSNR 1dB:

• QPSK @ ≤4 WSSs (BW=24.6GHz)

• 8QAM @ ≤3 WSSs (BW=26.0GHz)

• 16QAM @ ≤2 WSSs (BW=28.1GHz)

Result: Δf = 37.5GHz channel spacing

• Alleviate impact by:

1. Optimized WSS with higher order filter shape

2. More taps in DSP time-domain equalizer (TDE)

ΔOSNR1dB


• Previous WSSs assumed are representative of flex-grid ROADMs currently widely deployed (“standard” below)

• Next-gen WSS with higher-order filter shapes have been developed (“higher-order” below) – enhanced cascadability

Optimized WSS

3dB BW = 33.0GHz2.4-order Gaussian

3dB BW = 35.5GHz3.3-order Gaussian


• Begin with previous result (solid lines)

• Overlay with result using optimized WSS shape (dotted lines):• QPSK increased from ≤4 to ≤7 WSSs

• 8QAM increased from ≤3 to ≤4 WSSs

• 16QAM increased from ≤2 to ≤3 WSSs

Impact of optimized WSS shape

ΔOSNR1dB


• Swept number of taps in DSP TDE over range of practical values for the “standard” WSS shapes:

• Too few taps has large impact

• Diminishing returns for increasing taps beyond 15, at expense of increased complexity, power consumption, convergence

Dependence on TDE taps

16QAM, r=1.0, Δf = 50GHz 16QAM, r=0.1, Δf = 37.5GHz


• Transmission of QPSK, 8QAM, and 16QAM at 32 GBaud through cascaded ROADMs was studied

• 50 GHz channel-spaced systems robust to cascading (>40 WSSs)

• 37.5 GHz channel-spaced systems incur high penalties (<10 WSSs)

• To alleviate high penalties for 37.5 GHz systems, we explored:

• Enhanced WSS filter shape for approx. twofold increase

• Increased DSP TDE tap count for slight additional benefit

• Additionally, the following may be applied (for further study):

• Broadcast-and-select architecture

• Rolloff factor optimization

• Timing recovery algorithm optimization

• Spectral compensation in DSP and/or optically

Conclusions

IPC 2014, La Jolla CA (TuF1.2)

[email protected]

Thank you

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Cascaded ROADM Tolerance of mQAM Optical Signals Employing Nyquist Shaping

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