Maximizing Subsea Cable System Capacity: Recent Lab Demos and Field Trials (4/4)

Subsea Cable Systems, Technical Papers

Fourth and Last Post of a Series of 4

This post is the fourth and last of a series of 4 about the information rate that can be transported by submarine cable systems.  The first post of this series presented in a simple way the Shannon limit that drives the ultimate subsea cable system capacity.  The second post examined the various multiplexing options that can be put at work for increasing subsea cable system capacity and lowering the cost per transported bit as well.  The third post looked at some technologies that can be used to get closer to the Shannon limit with the objective of maximizing the capacity transported by a single communication channel (in our case, a single fiber core today).  This post reviews the most recent and meaningful results from laboratory demonstrations and field trials for increasing further subsea cable system capacity so one can get a good view of what will be feasible, capacity wise, in the short- and mid-terms.

 

Laboratory Demonstrations for Higher Subsea Cable System Capacity

This section presents a selection of lab experiments about long-haul, ultra-high-capacity transmission along subsea cable systems reported in technical conferences or published in peer-reviewed technical journals since the beginning of year 2017.  Out of the five lab demonstrations summarized below, two of them make use of C band EDFA amplification scheme while the other three ones rely on C+L band EDFA repeaters.  For each of the works, the main technologies involved in reaching the Capacity / Reach results are summarized, while the results themselves are reported in terms of total capacity, spectral efficiency and transmission distance.  The five lab demonstrations are listed in chronological order.

  • Capacity-Approaching Transmission Over 6,375 km Using Hybrid Quasi-Single-Mode Fiber Spans (NEC – JLT – February 2017)
    • Fiber spans: 56.1-km / 8.9-dB span (26.1 km at 176 μm2 / 0.163 dB/km and 30 km at 150 μm2 / 0.153 dB/km)
    • Repeaters: 4.2 THz bandwidth of C-band EDFA (1529.75 – 1563.05 nm), with +20 dBm total output power launched into the line fiber
    • Channels: 168 x 208 Gbit/s channels spaced at 25 GHz, modulated at 24.8 Gbaud
    • Modulation format: 64APSK (Amplitude-Phase Shifted Keying)
    • Average received Optical Signal-to-Noise Ratio (OSNR) is about 21 dB / 0.1 nm with <1 dB tilt across the C band.
    • 34.9 Tbit/s at 8.3 bit/s/Hz over 6,375 km
  • 70.4 Tb/s Capacity over 7,600 km in C+L Band Using Coded Modulation with Hybrid Constellation Shaping and Nonlinearity Compensation (TE SubCom – OFC 2017 – March 2017)
    • Fiber spans: 52.8-km / 7.92-dB fiber spans with 0.150 dB/km loss and ~150 μm2 effective area
    • Repeaters: 9.74 THz bandwidth of C+L EDFA, with +22.5 dBm total output power launched into the line fiber
    • Channels: 295 x 239 Gbit/s channels spaced at 33 GHz, modulated at 32.6 Gbaud
    • Modulation format: 56APSK (Amplitude-Phase Shifted Keying)
    • Average received OSNR is about 20.2 dB / 0.1 nm with 1.8 dB tilt across C and L bands.
    • 70.4 Tbit/s at 7.23 bit/s/Hz over 7,600 km
    • Comments:
      • Multi-dimensional coded modulation (CM) based on 56APSK
      • Hybrid (geometrical and probabilistic) constellation shaping
      • Adaptive-rate FEC decoding
      • Multi-stage nonlinearity compensation
  • Advanced C+L-Band Transoceanic Transmission Systems Based on Probabilistically Shaped PDM-64QAM (Nokia Bell Labs – JLT – April 2017)
    • Fiber spans: 55-km / 8.6-dB span (150 μm2 / 0.157 dB/km). The total span loss, including fiber, C/L-demultiplexer, connectors and splicing loss was 10.2 dB.
    • Repeaters: 8.9 THz bandwidth of C+L EDFA, with +22 dBm total output power launched into the line fiber
    • Channels: 179 x 363.1 Gbit/s channels spaced at 50 GHz, modulated at 49 Gbaud
    • Modulation format: PS64QAM (Probabilistically-Shaped 64QAM)
    • Average received OSNR is about 11 dB / 0.1 nm with 2 dB tilt across C and L bands.
    • 65 Tbit/s at 7.3 bit/s/Hz over 6,600 km
    • Comments:
      • Digital nonlinearity compensation
      • Adaptive-rate spatially-coupled low density parity check (SC-LDPC)
      • Note: Average net data rate of 363.1 Gbit/s per 50 GHz channel slot for a full-band transoceanic lab demonstration!
  • Near Capacity 24.6 Tb/s Transmission over 10,285km Straight Line Testbed at 5.9 b/s/Hz Spectral Efficiency Using TPCS-64QAM and C-Band EDFA-Only (ASN – ECOC 2017 – September 2017)
    • Fiber spans: 54.4-km span at 110 μm2 (no loss information)
    • Repeaters: 4.2 THz bandwidth of C-band EDFA (1529.75 – 1563.05 nm), with +16.6 dBm total output power launched into the line fiber
    • Channels: 84 x 293 Gbit/s channels spaced at 50 GHz, modulated at 49 Gbaud
    • Modulation format: Truncated Probabilistic Constellation Shaping 64QAM (TPCS-64QAM).
    • Average received OSNR is about 9.3 dB / 0.1 nm with 1 dB tilt across the C-band.
    • 24.6 Tbit/s at 5.9 bit/s/Hz over 10,285 km
  • 51.5 Tb/s Capacity over 17,107 km in C+L Bandwidth Using Single Mode Fibers and Nonlinearity Compensation (TE SubCom – ECOC 2017 – September 2017)
    • Fiber spans: 52.8-km / 7.92-dB fiber spans with 0.150 dB/km loss and ~150 μm2 effective area
    • Repeaters: 9.74 THz bandwidth of C+L EDFA, with +22.5 dBm total output power launched into the line fiber
    • Channels: 295 x 175 Gbit/s channels spaced at 33 GHz, modulated at 32.6 Gbaud
    • Modulation format: 40APSK (Amplitude-Phase Shifted Keying)
    • Average received OSNR is about 16.3 dB /0.1 nm with 2.1 dB tilt across C and L bands.
    • 51.5 Tbit/s at 5.29 bit/s/Hz over 17,107 km
    • Comments:
      • Multi-dimensional coded modulation (CM) based on 56APSK
      • Hybrid (geometrical and probabilistic) constellation shaping
      • Adaptive-rate FEC decoding
      • Multi-stage nonlinearity compensation

 

Improvements in Spectral Efficiency in Technical Demonstrations

We have reviewed a selection of technical achievements within the past 9 months.  Taking a broader view encompassing the past 6 years, we can get a good picture of the continuous improvement in spectral efficiency over time in technical demonstrations of transoceanic transmission through the below selected works:

  1. 2011: 4 bit/s/Hz spectral efficiency, using 8 quadrature-amplitude modulation (8QAM) and single-mode fiber with 0.16 dB/km loss and 148 μm2 effective area (NEC)
  2. 2015: 7.1 bit/s/Hz spectral efficiency, using 64QAM, multi-dimensional 9/12 single-parity-check (SPC) coded modulation (TE SubCom)
  3. 2017: 8.3 bit/s/Hz spectral efficiency, using 64APSK, low-density parity check (23 090, 16 163, 0.7) codes, and a mix of 0.163 dB/km / 176 μm2 and 0.153 dB/km / 150 μm2 fibers (NEC)

References for the above spectral efficiency achievements are reported below:

  1. D. Qian et al., “Transmission of 115×100 G PDM-8QAM-OFDM Channels with 4 bits/s/Hz Spectral Efficiency over 10,181 km,” presented at the Eur. Conf. Exhib. Optical Communication, Geneva, Switzerland, 2011
  2. J. Cai et al., “64QAM based coded modulation transmission over transoceanic distance with >60 Tb/s capacity,” presented at the Optical Fiber Communication Conf., Los Angeles, CA, USA, 2015, Paper Th5C.8.
  3. S. Zhang et al., “Capacity-Approaching Transmission Over 6375 km Using Hybrid Quasi-Single-Mode Fiber Spans,” Journal of Lightwave Technology, Vol. 35, No. 3, pp. 481-497, 2017

 

Field Trial Demonstrations

In parallel to lab demonstrations, we have seen several field trials during the first three quarters of 2017 aiming at assessing the capacity performance of long-haul subsea cable systems that were designed 2 to 4 years ago.  These field trials are based on commercially-available products, offering stable, commercial-ready performance margin.  Not all the field trials reported below were presented in technical conferences or journals so fewer technical details are available than for lab demonstrations.  The four field trials below are listed chronologically.

  • Trans-Atlantic Field Trial Using Probabilistically Shaped 64-QAM at High Spectral Efficiencies and Single-Carrier Real-Time 250-Gb/s 16-QAM (Nokia Bell Labs / Facebook – OFC 2017 conference – March 2017)
    • Field trial over transatlantic AEC-1 subsea cable system (wet plant supplied by TE SubCom)
    • Fiber spans: 89-km / 14.4-dB span at 130 μm2 and 0.156 dB/km
    • Repeaters: 4.3 THz bandwidth of C-band EDFA, with +19 dBm total output power launched into the line fiber
    • Channels: 69 x 250 Gbit/s channels spaced at 62.5 GHz, modulated at 62.5 Gbaud
    • Modulation format: 16QAM.
    • Average received OSNR is about 9.3 dB with 1 dB tilt across the C band.
    • 17.2 Tbit/s at 4 bit/s/Hz over 5,523 km
  • Infinera XTS-3300 Meshponder Delivers Industry-leading 19 Terabits of Capacity on Trans-Atlantic Route (Infinera 11 July 2017 press release)
    • Field trial over an unnamed “modern transatlantic route”
    • Channels: 600 Gbit/s super-channel in 140 GHz of spectrum
    • Modulation format: 8QAM.
    • 19 Tbit/s at 4.3 bit/s/Hz over transatlantic distance
  • FASTER Open Submarine Cable (Google / NEC / ASN – ECOC 2017 conference – September 2017)
    • Field trial over FASTER subsea cable system (wet plant supplied by NEC)
    • Channels: spaced at 50 GHz
    • Modulation format: 8QAM.
    • 4.0 bit/s/Hz over 10,940 km
  • Infinera and Seaborn Set Subsea Industry Benchmark for Capacity-Reach with XTS-3300 on Seabras-1 (Infinera 20 September 2017 press release)
    • Field trial over Seabras-1 subsea cable system (wet plant supplied by ASN)
    • Channels: 100 Gbit/s spaced at 22 GHz, modulated at 22 Gbaud
    • Modulation format: 8QAM.
    • 18.2 Tbit/s at 4.5 bit/s/Hz over 10,500 km

 

To Conclude with a Few Thoughts

For the current subsea cable system design in commercial service, it looks like that the objective is to operate these submarine cable systems at the limit of the optical power regime where the nonlinear effects become noticeable, with an optical signal-to-noise ratio of about 13 dB/0.1 nm.

Nyquist pulse-shaping and ultra-high wavelength stability enable to decrease the channel spacing down to the baud rate (for instance, if the optical carriers are modulated at the speed of 49 Gbaud, the carriers can be spaced 50 GHz apart).  Starting from this remark, the game for increasing the fiber capacity becomes very simple.  Assuming that C+L EDFA optical amplification offers a maximal spectrum of 10 THz, and that today’s commercially-available electronics speed is 50 Gbaud, the game is then to split the 10 THz optical bandwidth into 50 GHz slots and to find a way to maximize the capacity transmitted within in each of these 50 GHz slots.  As an illustration, the work reported by Nokia Bell Labs in the April 2017 issue of the Journal of Lightwave Technologies achieved an average net data rate of 363.1 Gbit/s per 50 GHz slot!  This must be compared with the customary practice of placing one 10 Gbit/s carrier on a 50 GHz grid 10 years ago…

As technology is getting closer to the Shannon limit, the options identified for the short- to mid-term for further increasing the subsea cable system capacity are wider repeater bandwidth and/or higher fiber count.

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