This document summarizes a study on the cascaded ROADM tolerance of mQAM optical signals employing Nyquist shaping. It found that 50GHz channel spaced systems can pass through over 40 ROADMs with minimal penalty, while 37.5GHz spaced systems incur high penalties after less than 10 ROADMs. To improve the tolerance of narrower spaced systems, the study explored using ROADMs with enhanced filter shapes, which provided around a twofold increase in tolerance, and increasing the DSP tap count for a slight additional benefit. Optimizing additional factors like rolloff, timing recovery, and spectral compensation may further improve the cascaded tolerance of closely spaced mQAM signals.

1. IPC 2014, La Jolla CA (TuF1.2)
Mark Filer and Sorin Tibuleac
ADVA Optical Networking / Atlanta, GA
stibuleac@advaoptical.com
Cascaded ROADM Tolerance of
mQAM Optical Signals Employing
Nyquist Shaping

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

3. © 2014 ADVA Optical Networking. 3 IPC 2014, La Jolla CA (TuF1.2) All rights reserved.
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

4. © 2014 ADVA Optical Networking. 4 IPC 2014, La Jolla CA (TuF1.2) All rights reserved.
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

5. © 2014 ADVA Optical Networking. 5 IPC 2014, La Jolla CA (TuF1.2) All rights reserved.
• 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

6. © 2014 ADVA Optical Networking. 6 IPC 2014, La Jolla CA (TuF1.2) All rights reserved.
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

7. © 2014 ADVA Optical Networking. 7 IPC 2014, La Jolla CA (TuF1.2) All rights reserved.
• 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

8. © 2014 ADVA Optical Networking. 8 IPC 2014, La Jolla CA (TuF1.2) All rights reserved.
• 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 = 33.0GHz
2.4-order Gaussian
3dB BW = 35.5GHz
3.3-order Gaussian

9. © 2014 ADVA Optical Networking. 9 IPC 2014, La Jolla CA (TuF1.2) All rights reserved.
• 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

10. © 2014 ADVA Optical Networking. 10 IPC 2014, La Jolla CA (TuF1.2) All rights reserved.
• 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

11. © 2014 ADVA Optical Networking. 11 IPC 2014, La Jolla CA (TuF1.2) All rights reserved.
• 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

12. IPC 2014, La Jolla CA (TuF1.2)
stibuleac@advaoptical.com
Thank you