Optical transport network (OTN) featuring high bandwidth, low latency, and long-distance transmission provides quality bearers for global operator networks. Using revolutionary technologies such as coherent communications, polarization multiplexing, and soft-decision error correction, 100G transport has quickly replaced 40G transmission and become a new-generation of long-life technology. 100G has been deployed on a large scale for nearly five years.
With the rapid development of 5G, big video, and high-performance private line services, as well as the growing demands for IDC interconnection brought by cloud networks, the demands for higher optical network bandwidth are witnessing an exponential growth. Accordingly, beyond-100G coherent transport represented by single-carrier 200G/400G has become a hot topic. Beyond-100G uses high-order QAM modulation, constellation shaping (including probabilistic shaping), and nonlinear compensation for signal processing, uses low-loss large-effective-area fiber, low noise amplifiers, and distributed Raman amplifiers to improve transmission performance, and uses optical integration including Silicon Photonics (SiPh) integration, Indium Phosphide (InP) integration, optical hybrid integration, hybrid optoelectronic integration, and digital-to-analog conversion with high sampling rate and high resolution to improve energy efficiency. The objective is to settle the conflicts between spectral efficiency and transmission distances and between network performance and energy efficiency for large-scale commercial use of beyond-100G.
In 2016, China Unicom carried out a laboratory test on the transmission performance of beyond-100G using new optical fibers, and deployed trial sites on the existing network for verification. In 2017, China Mobile conducted laboratory tests to verify transmission functions and performance of single-carrier 400G, preparing for subsequent standards formulation and commercial deployment. In January 2018, American telecom operator Verizon completed a 400G field trial by establishing connections between core routers for 400GE services over its OTN.
The year 2018 is crucial for launching beyond-100G and will be followed by sustained high growth. Cignal Al, an analyst firm, forecasts that beyond-100G will take nearly a quarter share of OTN bandwidth market by 2020. Another well-known consulting firm, OVUM, predicts that beyond-100G market will account for over one-third of the whole OTN market in 2022.
Progress of Standardization
The commercial availability of OTN is inseparable from healthy development of the industrial chain. The beyond-100G standards are being perfected. Both the ITU-T G.709 (OTUCn) and IEEE 802.3bs (200GE/400GE) standards have been released. The Optical Internetworking Forum (OIF) has launched the flex coherent DWDM transmission framework and is developing the 400G ZR standards for short-haul DCI as well as TROSA and HB-CDM standards for high-speed optical devices and interfaces.
Beyond-100G OTN is a total solution for multiple application scenarios involving long-haul, MAN, and short-haul interconnection. ZTE has taken a lead in drafting the Flex Coherent DWDM Transmission Framework that provides a comprehensive analysis of various application specifications. The 400G application modes are listed in Table 1.
It can be seen from the table that transmission distance and spectral efficiency (channel spacing/number of carriers) conflict with each other. Major concerns for beyond-100G in different application scenarios are balancing transmission bandwidth and distance or achieving the optimal transmission distance and the optimal spectral efficiency respectively by limiting the bandwidth and shortening transmission distance.
Beyond-100G also faces technical challenges such as channel modulation, line transmission and product integration and packaging.
Baud rate is the basic means to increase single-channel transport rates from 32G baud in the 100G era to 64G baud in the 400G era and 96G/128G baud in the future 800G era. The increase in baud rate can reduce the number of optical components. However, the baud rate increase is limited by the bandwidth of modulators/drivers and receivers and the manufacturing level of components. In addition, simply increasing the baud rate cannot increase spectral efficiency or total transmission capacity.
High-order QAM can increase spectral efficiency and transmission capacity, but requires narrower linewidth lasers and better linear optoelectronic components. Moreover, closer arrangement of constellation diagrams in high-order modulation results in shorter transmission distance.
As for probabilistic shaping, constellation probability in an additive white Gaussian noise (AWGN) system gets close to Maxwell-Boltzmann distribution (a shaping gain of 1.53 dB per dimension can be achieved in ideal conditions). High redundancy ratio, optimized soft-decision error correction, probabilistic shaping, and optimized constellation arrangement that increases the Euclidean distance, are used to increase the transmission distance of high-order QAM and expand the coverage of beyond-100G applications.
Compared with the ever-changing development of channel modulation, line transmission has developed at a relatively slow pace. New fibers (such as G.654E) with low loss and large effective area can reduce line loss and increase the incident optical power, thus reducing the number of electrical repeaters. On the equipment side, low-noise optical amplifiers are used to improve transmission performance, and the commercial deployment of distributed Raman amplifiers are accelerated. These technical means are adopted to ensure engineering safety and reduce maintenance difficulities of Raman amplifiers and will play a more significant role in extending transmission distance.
Product Integration and Packaging
Optical components, which are key to the beyond-100G technology, are developed from traditional discrete devices (such as optical sources, modulators, and integrated receivers) to integrated optical devices based on InP and SiPh technologies. With the group III-V elements, InP can be used for lasers and other active gain devices. SiPh compatible with the CMOS technology has a great potential for large-scale and low-cost production, but faces the challenge of implementing an active gain structure. As InP and SiPh based Die and optical components are small in size, highly integrated and pluggable optical modules can be made.
The OIF is developing IC-TROSA standards for packaging. Type-A standards are specified for the SiPh technology that integrate modulators, drivers and receivers. Type-B standards are defined for the InP technology, integrating lasers, modulators, drivers and receivers. Leveraging the advantage of non-hermetic packaging, SiPh enables BGA packaging, while the InP technology can integrate optical components into a Goldbox.
The explosive growth of bandwidth is beyond imagination. The industry was discussing 100G yesterday, but today it has started to study beyond-100G. Beyond-100G transport brings both challenges and opportunities. In general, standards, applications, trials and technical implementations all help to accelerate the deployment of beyond-100G, which will open a new chapter in optical communications.
Beyond-100G, OTN, Channel Modulation, Line Transmission, Product Integration and Packaging