NEC announced that it has demonstrated transmission capacity of 50.9 Tbit/s on a single fibre over a distance of more than 11,000 km, which the company claims represents the first time 50 Tbit/s have been achieved over 10,000 km using C+L band erbium-doped fibre amplifiers (EDFA).
NEC stated that the high capacity, long haul transmission demonstration corresponds to a record capacity-distance ratio of 570 Pb-km (Petabit/s per kilometre). NEC noted that its efforts to extend the benchmark 50 Tbit/s transmission capacity to trans-Pacific distances supports the current trend of increasing the reach of ultra-high capacity submarine cable segments.
The company stated that achieving the high capacity transmission, even employing extremely wide bandwidth EDFAs, requires the efficient use of bandwidth at a level approaching the Shannon limit, the fundamental spectral efficiency limit of optical communications. NEC added that while there is more than one way to design modulation formats that allow transmission close to the Shannon limit in the linear regime, these generally do not perform well in the non-linear regime, where the performance gap increases approaching the non-linear Shannon limit.
To address this issue, NEC researchers developed a multilevel, linear and non-linear constellation optimisation algorithm. Leveraging this algorithm, it created an optimised 32QAM (opt32) constellation that allows performance close to Shannon capacity and, more importantly, enables a higher non-linear capacity limit that is more appropriate to submarine transmission.
Additionally, the new modulation format developed by NEC is simpler to implement as it does not require iterative decoding or non-uniform coding. As a result, opt32 modulation enabled NEC researchers to achieve spectral efficiency of 6.14 b/s/Hz over a trans-Pacific route.
The demonstration also featured C+L amplification to maximise the capacity per fibre pair, and as part of the solution NEC researchers also developed a patent-pending bi-directional amplifier that is designed to reduce the effective noise figure and overall device complexity.
