Bell Labs is claiming an industry first in demonstrating the recovery in real time of multiple signals sent over spatial-division multiplexed fibre. Gazettabyte spoke to two members of the research team to understand more.
Part 2: The capacity crunch and the role of SDM
The argument for spatial-division multiplexing (SDM) - the sending of optical signals down parallel fibre paths, whether multiple modes, cores or fibres - is the coming ‘capacity crunch’. The information-carrying capacity limit of fibre, for so long described as limitless, is being approached due to the continual yearly high growth in IP traffic. But if there is a looming capacity crunch, why are we not hearing about it from the world’s leading telcos?
“It depends on who you talk to,” says Peter Winzer, head of the optical transmission systems and networks research department at Bell Labs. The incumbent telcos have relatively low traffic growth - 20 to 30 percent annually. “I believe fully that it is not a problem for them - they have plenty of fibre and very low growth rates,” he says.
Twenty to 30 percent growth rates can only be described as ‘very low’ when you consider that cable operators are experiencing 60 percent year-on-year traffic growth while it is 80 to 100 percent for the web-scale players. “The whole industry is going through a tremendous shift right now,” says Winzer.
In a recent paper, Winzer and colleague Roland Ryf extrapolate wavelength-division multiplexing (WDM) trends, starting with 100-gigabit interfaces that were adopted in 2010. Assuming an annual traffic growth rate of 40 to 60 percent, 400-gigabit interfaces become required in 2013 to 2014, and the authors point out that 400-gigabit transponder deployments started in 2013. Terabit transponders are forecast in 2016 to 2017 while 10 terabit commercial interfaces are expected from 2020 to 2024.
In turn, while WDM system capacities have scaled a hundredfold since the late 1990s, this will not continue. That is because systems are approaching the Non-linear Shannon Limit which estimates the upper limit capacity of fibre at 75 terabit-per-second.
Starting with 10-terabit-capacity systems in 2010 and a 30 to 40 percent core network traffic annual growth rate, the authors forecast that 40 terabit systems will be required shortly. By 2021, 200 terabit systems will be needed - already exceeding one fibre’s capacity - while petabit-capacity systems will be required by 2028.
Even if I’m off by an order or magnitude, and it is 1000, 100-gigabit lines leaving the data centre; there is no way you can do that with a single WDM system
Parallel spatial paths are the only physical multiplexing dimension remaining to expand capacity, argue the authors, explaining Bell Labs’ interest in spatial-division multiplexing for optical networks.
If the telcos do not require SDM-based systems anytime soon, that is not the case for the web-scale data centre operators. They could deploy SDM as soon as 2018 to 2020, says Winzer.
The web-scale players are talking about 400,000-server data centres in the coming three to five years. “Each server will have a 25-gigabit network interface card and if you assume 10 percent of the traffic leaves the data centre, that is 10,000, 100-gigabit lines,” says Winzer. “Even if I’m off by an order or magnitude, and it is 1000, 100-gigabit lines leaving the data centre; there is no way you can do that with a single WDM system.”
SDM and MIMO
SDM can be implemented in several ways. The simplest way to create parallel transmission paths is to bundle several single-mode fibres in a cable. But speciality fibre can also be used, either multi-core or multi-mode.
For the demo, Bell Labs used such a fibre, a coupled 3-core one, but Sebastian Randel, a member of technical staff, said its SDM receiver could also be used with a fibre supporting a few spatial modes. By increasing slightly the diameter of a single-mode fibre, not only is a single mode supported but two second-order modes. “Our signal processing would cope with that fibre as well,” says Winzer.
The signal processing referred to, that restores the multiple transmissions at the receiver, implements multiple input, multiple output or MIMO. MIMO is a well-known signal processing technique used for wireless and digital subscriber line (DSL).
They are garbled up, that is what the rotation is; undoing the rotation is called MIMO
Multi-mode fibre can support as many as 100 spatial modes. “But then you have a really big challenge to excite all 100 spatial modes individually and detect them individually,” says Randel. In turn, the digital signal processing computation required for the 100 modes is tremendous. “We can’t imagine we can get there anytime soon,” says Randel.
Instead, Bell Labs used 60 km of the 3-core coupled fibre for its real-time SDM demo. The transmission distance could have been much longer except the fibre sample was 60 km long. Bell Labs chose the coupled-core fibre for the real-time MIMO demonstration as it is the most demanding case, says Winzer.
The demonstration can be viewed as an extension of coherent detection used for long-distance 100 gigabit optical transmission. In a polarisation-multiplexed, quadrature phase-shift keying (PM-QPSK) system, coupling occurs between the two light polarisations. This is a 2x2 MIMO system, says Winzer, comprising two inputs and two outputs.
For PM-QPSK, one signal is sent on the x-polarisation and the other on the y-polarisation. The signals travel at different speeds while hugely coupling along the fibre, says Winzer: “The coherent receiver with the 2x2 MIMO processing is able to undo that coupling and undo the different speeds because you selectively excite them with unique signals.” This allows both polarisations to be recovered.
With the 3-core coupled fibre, strong coupling arises between the three signals and their individual two polarisations, resulting in a 6x6 MIMO system (six inputs and six outputs). The transmission rotates the six signals arbitrarily while the receiver, using 6x6 MIMO, rotates them back. “They are garbled up, that is what the rotation is; undoing the rotation is called MIMO.”
Demo details
For the demo, Bell Labs generated 12, 2.5-gigabit signals. These signals are modulated onto an optical carrier at 1550nm using three nested lithium niobate modulators. A ‘photonic lantern’ - an SDM multiplexer - couples the three signals orthogonally into the fibre’s three cores.
The photonic lantern comprises three single-mode fibre inputs fed by the three single-mode PM-QPSK transmitters while its output places the fibres closer and closer until the signals overlap. “The lantern combines the fibres to create three tiny spots that couple into a single fibre, either single mode or multi-mode,” says Winzer.
At the receiver, another photonic lantern demultiplexes the three signals which are detected using three integrated coherent receivers.
Don’t do MIMO for MIMO’s sake, do MIMO when it helps to bring the overall integrated system cost down
To implement the MIMO, Bell Labs built a 28-layer printed circuit board which connects the three integrated coherent receiver outputs to 12, 5-gigabit-per-second 10-bit analogue-to-digital converters. The result is an 600 gigabit-per-second aggregate output digital data stream. This huge data stream is fed to a Xilinx Virtex-7 XC7V2000T FPGA using 480 parallel lanes, each at 1.25 gigabit-per-second. It is the FPGA that implements the 6x6 MIMO algorithm in real time.
“Computational complexity is certainly one big limitation and that is why we have chosen a relatively low symbol rate - 2.5 Gbaud, ten times less than commercial systems,” says Randel. “But this helps us fit the [MIMO] equaliser into a single FPGA.”
Future work
With the growth in IP traffic, optical engineers are going to have to use space and wavelengths. “But how are you going to slice the pie?” says Winzer.
With the example of 10,000, 100-gigabit wavelengths, will 100 WDM channels be sent over 100 spatial paths or 10 WDM channels over 1,000 spatial paths? “That is a techno-economic design optimisation,” says Winzer. “In those systems, to get the cost-per-bit down, you need integration.”
That is what the Bell Lab’s engineers are working on: optical integration to reduce the overall spatial-division multiplexing system cost. “Integration will happen first across the transponders and amplifiers; fibre will come last,” says Winzer.
Winzer stresses that MIMO-SDM is not primarily about fibre, a point frequently misunderstood. The point is to enable systems with crosstalk, he says.
“So if some modulator manufacturer can build arrays with crosstalk and sell the modulator at half the price they were able to before, then we have done our job,” says Winzer. “Don’t do MIMO for MIMO’s sake, do MIMO when it helps to bring the overall integrated system cost down.”
Further Information:
Space-division Multiplexing: The Future of Fibre-Optics Communications, click here
For Part 1, click here