Part 1 of 2
Bell Labs, the research arm of Alcatel-Lucent, has used a real-time receiver to recover a dozen 2.5-gigabit signals sent over a coupled three-core fibre. Until now the signal processing for such spatial-division multiplexed transmissions have been done offline due to the computational complexity involved.
“The era of real-time experiments in spatial-division multiplexing is starting and this is the very first example,” said Peter Winzer, head of the Optical Transmission Systems and Networks Research Department at Bell Labs. “Such real-time experiments are the next stepping stone towards a true product implementation.”
Spatial-division multiplexing promises to increase the capacity of optical fibre by a factor of between ten and one hundredfold. Multiple input, multiple output [MIMO], a signal processing technique employed for wireless and for DSL broadband access, is used to recover the signals at the receiver.
MIMO also promises optical designers a way to tackle crosstalk between components, enabling cheaper integrated optics to be used at the expense of more complex digital signal processing, said Winzer.
For the demo, Bell-Labs used MIMO to recover twelve 2.5-gigabit transmitted signals down a three-core fibre, in effect three polarisation-multiplexed, quadrature phase-shift keying (PM-QPSK) signals. The result is a 6x6 MIMO system [six inputs, six outputs] due to the coupling between the three signals, each with two polarisations. The signal couplings cause an arbitrary rotation in a 6-dimensional space, says Winzer: “They are garbled up, that is what the rotation is. Undoing the rotation is called MIMO.”
The signals were transmitted at 1,550nm over a 60 km spool of coupled-core fibre. The three 10 gigabit PM-QPSK signals are a tenth the speed of commercial systems but this was necessary for an FPGA to execute MIMO in real time.
According to Bell Labs, the coupled-core fibre was chosen for the real-time receiver demonstration as it is the most taxing example. The Bell Labs team is now working on optical integration to reduce the overall spatial-division multiplexing system’s cost-per-bit. “Making those transponders cheaper, we are trying to figure out what are the right knobs to turn,” said Winzer.
Bell Labs does not expect telcos to require spatial-division systems soon. But traffic requirements of the web-scale data centre operators could lead to select deployments in three to five years, said Winzer.
For Part 2, a more detailed discussion with Bell Labs about spatial-division multiplexing and the 60km 6x6 MIMO demonstration, click here