The subsea cable market is booming with approximately $9.8 billion in new cables expected to enter service between 2016 and 2018 driven largely by the cloud. The deployment of a subsea cable of over 10,000 kilometers (km) in length can cost more than $500 million, with multiple investors typically financing the venture. Further, a subsea cable has relatively few fibers compared to a terrestrial network because each fiber requires its own repeaters spaced about every 80 km. Given that a single subsea cable usually houses only up to eight fiber pairs, you can probably guess what an investor is thinking: how can I maximize my return on investment with only a few fibers? The answer is simply to get as much revenue as possible out of the cable during its roughly 25-year lifespan. This means maximizing the capacity on the cable is critical. Thus, subsea cable operators are eager to deploy the fastest technology available for maximum capacity-reach performance, or more saliently, revenue-reach performance. In this blog let us look at the latest technology subsea cable operators can use to get the most potential from their cable.
In the late 1990s, 10 gigabits per second (10 Gb/s) and 16-channel dense wavelength-division multiplexing (DWDM) technology were used. These optical channels were intensity modulated, and their capacity-reach performance was directly impacted by chromatic and polarization dispersion. To achieve the capacity-reach necessary, the cable had to be meticulously compensated at periodic intervals with counter-dispersion characteristics that would produce a net zero dispersion effect for the channels over the length of the cable. The good thing was that such dispersion is linear and thus can be calculated and managed. Since the early 2000s, compensated fiber pairs with 10 Gb/s DWDM technology have been able to provide up to 960 Gb/s (96 channels of 10 Gb/s) of capacity. Given that subsea capacity demand growth is more than 45 percent year-over-year and 100 Gigabit Ethernet connectivity is dominant, these cable systems quickly run out of capacity. However, the good news is that the limitation is not the cable itself but the optical transport technology currently implemented. Upgrading these cables with the latest technology will add significant capacity and value to the subsea cable operator’s greatest asset.
From 10 Gb/s to 100 Gb/s Yields More than 10 Times the Capacity
The shift from 10 Gb/s to 100 Gb/s required a significant technology change, from intensity modulation of light to coherent modulation, which varies the light’s amplitude, phase and polarization in much the same way as in radio and wireline modems. One might think that going from 10 Gb/s to 100 Gb/s results in a tenfold improvement in a DWDM subsea cable system. However, coherent modulation combined with various digital signal processing (DSP) techniques yields even greater capacity. This is because some modulation formats allow for more than one bit per symbol, or state of information, and thus provide better spectral efficiency. For example, in Figure 1, 8 quadrature amplitude modulation (8QAM) has three bits per symbol, at a symbol rate of 22 gigabaud (Gbaud) on 2 polarizations with a Forward Error Correction (FEC) overhead of 20 percent to carry 100 Gb/s in a 25 gigahertz (GHz) channel. The spectral efficiency can further be increased by spacing channels closer together, as opposed to the International Telecommunications Union’s Telecommunication Standardization Sector’s (ITU-T) fixed grid spacing of 100 GHz for 10 Gb/s systems. Even if a fixed channel spacing of 50 GHz is used, the capacity increases 20-fold over an older 10 Gb/s system. However, as seen in Figure 1, coherent modulation allows for a variety of spectral widths and thus a flexible spectral grid, or gridless spectrum, enabling even tighter channel spacing on a cable.
As in radio and wireline transmission, coherent optical phase modulation is more robust than amplitude modulation. As distances increase, the modulation format capacity must decrease, with a corresponding decrease in spectral efficiency. This is the crux of the subsea cable business. As seen from Figure 2, trans-Atlantic systems can support more capacity than trans-Pacific ones because of the modulation that can be supported over the distance.
The latest subsea capacity-reach performance is 100 Gb/s using 8QAM over 10,500 km in a 22 GHz channel for a spectral density of 4.5 b/s/Hz as showcased recently on Seaborn Networks’ Seabras-1 cable. To achieve this level of performance, innovative advanced coherent techniques are required to overcome the physical limitations of the fiber for optimal capacity-reach performance.
Dispersion Can Be a Good Thing
Coherent optical transmission is truly a disruptive technology for subsea. Not only does it give significantly more capacity on the cable but the cable doesn’t need to be compensated. In fact, contrary to what one might intuitively think, transmission performance is even better if the cable is uncompensated. Since linear dispersion can be calculated for the length of the cable, it can be compensated not by the fiber as used to be the case, but by the DSP of the optical engine. However, the real benefit of an uncompensated cable is that linear chromatic and polarization dispersion mitigates the non-linear effects of cross-phase modulation (XPM) and self-phase modulation (SPM). How is that possible? Consider multiple wavelengths propagating at their various speeds down the length of a fiber. Instead of each modulated wavelength briefly occupying the same point in time on the fiber, allowing for XPM and SPM effects, linear dispersion makes them move past each other quicker and thereby independently, mitigating XPM and SPM.
Sharing Is Always a Good Thing
To achieve the highest capacity over the length of a cable means that one sets the system up, or commissions it, to operate as close as possible to its performance limit. However, the performance margin across the entire spectrum may not be flat, and thus the limit must be set to either accommodate the weakest portion of the spectrum or for a compromised level that optimizes capacity. Using a technique called soft-decision forward error correction (SD-FEC) gain sharing, each individual optical channel’s performance can be monitored and the available energy of the system can be balanced across the spectrum to compensate for channels whose margin of performance is below a desired commissioning level. As seen in Figure 3, two channels out of 20 were compensated to operate above the commissioning level for a 10 percent gain in capacity.
Harry Would Be Pleased
Harry Nyquist’s theorems are the foundation of digital signal processing. Infinera utilizes Nyquist in a unique way to further mitigate XPM and SPM for even better capacity-reach performance in much the same way as digital subscriber line (DSL) technology uses discrete multitone to mitigate line impairments and noise effects. Instead of a single carrier bearing 100 percent of the capacity, multiple carriers are spaced at just the right distance and modulated at just the right baud rate to bear a portion of the overall capacity so that the aggregate of each Nyquist subcarrier equals the total capacity of just the single carrier. The effect is an overall increase of 0.75 decibels (dB) of performance margin, which is almost 20 percent.
Since the Nyquist subcarriers can be spaced six times closer than a single carrier, wavelength stability and locking are essential. Infinera’s fourth-generation Infinite Capacity Engine (ICE4) uniquely assures that the subcarriers will not drift, and because of the super-channel precision from a single ICE4, tighter channel spacing can be achieved for increased spectral density.
As new uncompensated subsea cables connect the continents, advanced coherent technologies like Nyquist subcarriers and SD-FEC gain sharing, now available with Infinera’s ICE4 Advanced Coherent Toolkit (ACT), can increase the capacity of a cable up to 40 percent by allowing for an 8QAM system (22 GHz spacing) where only a quadrature phase-shift keying (QPSK) system (37.5 GHz spacing) would have been used previously. Also, cables that with can operate with 100 Gb/s 8QAM can realize an improvement in spectral efficiency of up to 12 percent (from 25 to 22 GHz). Such capacity improvements are not just marginal improvements, but real game changers for subsea operators.
About the Author:
Peter Zwinkels, Sr. Sales Account Director, Infenera.