With the advancement of the social informationization, the demand of bandwidth is increasing rapidly. With the introduction of video services such as video on demand, P2P and IPTV as well as the emerging applications like 3G, LTE, cloud computing and the internet of things, it is predicted that the gross annual growth rate of trunk (backbone) traffic will exceed 80% in the next five years. Since the network bandwidth demand will be increased 10 to 15 times in five years, the current 10G and 40G WDM system cannot meet the ever increasing demand of transmission capacity.
Considering the cost efficiency and compatibility, it is optimal and feasible to improve the system capacity by upgrading optical transport unit (OTU) to increase the data rate of single wavelength channel while making full use of existing optical fiber. Being the two main parts of physical layer of optical transmission system, the transmission characteristics of optical channel determines the system’s requirements of OTU. On one hand, fiber loss window and the narrowband filtering effect arising from cascaded optical devices make the available spectrum resources limited. The limitation of available bandwidth demands the maximization of the efficiency of optical spectrum. This requires high-capacity WDM systems to take full advantage of the optical signal modulation dimensions (such as amplitude, frequency, phase and polarization state) to carry data. On the other hand, the nonlinear effects of the optical transmission channel demands the maximization of energy efficiency of the optical transmission power. This requires coherent detection to improve the receiver sensitivity, and phase modulation to reduce optical launch power.
Currently, 100G WDM system based on coherent detection and polarization multiplexed quadrature phase shift keying (PM-QPSK) has been widely recognized by the industry. The Institute of Electrical and Electronics Engineers (IEEE) has defined the interface medium, data rate, PCS and MAC layer of 100 Gigabit Ethernet in IEEE802.3ba. The International Telecommunication Union (ITU-T) has made amendments on OTU4 in G.709. Optical internetworking Forum (OIF) has completed 100G modulation format, high-speed physical layer specifications, forward error correction coding as well as mechanical and electrical interface recommendation.
Different from traditional binary modulation, PM-QPSK is operated in multiple dimensions including the state of polarization, phase and amplitude with greater degree of freedom but lower complexity of each dimension. Constant amplitude quad-phase modulation combined with orthogonal polarization multiplexing is applied to achieve identical effect of hexadecimal modulation.
The modulation symbol rate is reduced to a quarter of data rate. With the reduction of symbol rate, the signal baseband bandwidth is decreased to less than 32GHz, the bandwidth requirements on electro-optical device is reduced, which make the high speed digital signal processing more feasible.
In digital coherent receiver, the optical properties of the received signal are mapped into electrical domain through polarization diversity and phase diversity. In electrical domain, cheap and mature digital signal processing technology is applied to realize polarization de-multiplexing, the channel distortion (CD、PMD and fiber nonlinear effects) compensation, timing recovery, carrier phase estimation, symbol decision and linear decoding. With the digital coherent detection, the complexity of transmission channel is transferred to the receiver. The design of optical dispersion compensation and polarization de-multiplexing in line is simplified, and the transmission dependence on low CD and PMD fiber is eliminated.
In addition, Forward Error Correction coding techniques (FEC) can further improve the transmission performance. FEC can be used to optimize OSNR requirements, improve the tolerance to channel distortion, ensure the reliability, stability and robustness of the optical transmission. In recent years, iterative FEC such as Turbo, LDPC coding is getting wide attention for its higher coding gain (to about 12dB). In addition to adopting new encoding algorithm, soft decision can also increase the FEC coding gain. When FEC coding overhead is 7% and 20%, the soft-decision coding gain is higher than the hard decision by 1.1dB and 1.3dB respectively. Soft-decision based LDPC has been successfully applied to 100G optical transmission systems.
As an excellent equipment and network solutions provider, FiberHome has worked on high-speed optical transmission technology for years. FiberHome has achieved 1.92Tb/s optical transmission with single optical source and 30.7Tb/s optical transmission in C-band in the laboratory firstly in the world in 2011. In response to the greedy demand of backbone network capacity, FiberHome can provide systematic and comprehensive 100G WDM solutions. With technical advantage, FiberHome has taken the lead in passing the industry’s first large scale 100G WDM equipment and system test organized by China Telecom in December 2011. Currently, FiberHome is carrying on field validation with China Telecom.
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