400G solutions optimised for ultra-long distance and ultra-large capacity
FlexOTU for 100G and beyond
Feng Yonghua: Two of FiberHome’s 400G solutions are
optimised for ultra-long distance and ultra-large capacity
Huge demands on bandwidth, driven by internet video, cloud computing, the internet of things and other services is making our out-of-date transport network a bottleneck. Since the service access data rate provided by both wireless and wireline is approaching 1000 megabits a second, networks able to carry 100 gigabits and and beyond are the most promising solution to meet the ever increasing bandwidth requirement.
Capacity, distance and cost are the main evaluation indexes of optical transmission system. The history of optical communication is the process of maximising transmission capacity and distance using technology while lowering the cost per bit.
It is efficient and cost effective to improve the capacity of existing networks by upgrading optical line cards to a high-speed channel data rate.
The channel data rate of DWDM transport system has been upgraded from 2.5 to 100 gigabits a second in the past 20 years, helped by the introduction of advanced modulation format PM-QPSK (polarisation multiplexing-quadrature phase shift keying) and digital coherent detection technology.
FiberHome’s 100G DWDM system has delivered a great improvement in transmission performance and network maintenance. The system can provide transmission capacity up to 9.6 terabits a second in C band, which can greatly enhance utilisation efficiency of fibre, reducing construction cost.
The non-electricity relay transmission distance is more than 3,000 kilometres, which means more flexible network deployment. Since there is no need to worry about CD and PMD of optical path, the cost of dispersion compensation fibres (DCF) and extra power compensation amplifiers can be saved.
A reduction in the nonlinear noise from DCF and amplifier stimulated emission (ASE) noise originally generated from extra amplifiers means better transmission performance or longer transmission distance. What’s more, the simplified line path means decreased probability of faults, and without DCFs there is lower propagation delay.
The goal of the proposal of high-speed data rate beyond 100 gigabits is to further improve the capacity of the fibre. The key is to improve the spectrum efficiency with advanced modulation formats and to improve utilisation efficiency with advanced frequency multiplexing techniques.
It is generally recognised that a data rate of 400 gigabits is more practicable than 1 terabit with current technology.
The 400G DWDM system will carry forward the key idea of 100G to enhance spectrum efficiency using an advanced modulation format, increasing receiver sensitivity and equalisation capability with digital coherent detection. What’s more, multicarrier technology would be adopted to lower the bandwidth requirement of electro-optic devices.
There is always a trade-off between transmission capacity and distance for a noise-limited channel. When the channel data rate is increased to 100 gigabits, the nonlinearity of the fibre becomes the main cause of distortion — instead of dispersion — which limits the transmission performance.
The nonlinearity of the fibre, self-phase modulation and cross-phase modulation in particular rotate the phase of the optical carrier along the propagation path, making the precise recovery of the data difficult and leading to error.
Since the generation of fibre nonlinearity is complex and is influenced by many random factors, the effective equalisation mechanism is still under development.
The conflict between the transport capacity and distance is more noticeable at 400 gigabits due to the nonlinearity of the fibre. With higher order QAM modulation, such as 16QAM and 64QAM, the transmission capacity and spectrum efficiency can be improved, but the transmission distance would decrease exponentially, which is not feasible in long-distance optical transmission.
Considering the capacity-distance trade-off, FiberHome has developed several 400G solutions, two of which are optimised for ultra-long distance and ultra-large capacity application scenarios.
For ultra-long distance applications, the company provides optical transport units with PM-QPSK modulation format. The transmission capacity is about 15 terabits a second in C band and the transmission distance is up to 3,000 kilometres. For ultra-large capacity application, the company provides OTUs with PM-16QAM modulation format. The transmission capacity is up to 25 terabits in C band and the transmission distance is about 800 kilometres.
In addition to these 400G solutions, FiberHome is developing a next generation 400G flexible optical transport unit, FlexOTU, which is flexible and can be adapted to different application scenarios.
FlexOTU is in essence a software-definable optical transponder. With a fixed transceiver and universal hardware, the transmission data rate, modulation format, encapsulation procedure, FEC coding, and other features of FlexOTU can be reconfigured adaptively according the available network resource and transport requirement to optimise the transmission distance, performance and cost.
FlexOTU supports broad data rate ranging from 10 to 400 gigabits a second. FlexOTU is compatible with flexible grid recommendation G.694.1 and with the OpenFlow protocol.
Dr Feng Yonghua is the optical transport networks technology director at FiberHome