Check out Mark Filer and Sorin Tibuleac's latest research

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