cascaded roadm tolerance of mqam optical signals employing nyquist shaping
DESCRIPTION
Check out Mark Filer and Sorin Tibuleac's latest researchTRANSCRIPT
IPC 2014, La Jolla CA (TuF1.2)
Mark Filer and Sorin Tibuleac
ADVA Optical Networking / Atlanta, GA
Cascaded ROADM Tolerance of mQAM Optical Signals Employing Nyquist Shaping
© 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
© 2014 ADVA Optical Networking. All rights reserved.33 IPC 2014, La Jolla CA (TuF1.2)
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
© 2014 ADVA Optical Networking. All rights reserved.44 IPC 2014, La Jolla CA (TuF1.2)
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
© 2014 ADVA Optical Networking. All rights reserved.55 IPC 2014, La Jolla CA (TuF1.2)
• 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
© 2014 ADVA Optical Networking. All rights reserved.66 IPC 2014, La Jolla CA (TuF1.2)
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
© 2014 ADVA Optical Networking. All rights reserved.77 IPC 2014, La Jolla CA (TuF1.2)
• 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
© 2014 ADVA Optical Networking. All rights reserved.88 IPC 2014, La Jolla CA (TuF1.2)
• 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
© 2014 ADVA Optical Networking. All rights reserved.99 IPC 2014, La Jolla CA (TuF1.2)
• 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
© 2014 ADVA Optical Networking. All rights reserved.1010 IPC 2014, La Jolla CA (TuF1.2)
• 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
© 2014 ADVA Optical Networking. All rights reserved.1111 IPC 2014, La Jolla CA (TuF1.2)
• 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