In order to achieve 400G long-haul (LH) transmission, three 400G OTN (Optical Transport Network) technologies come into being to meet the needs: single-carrier 400G, dual-carrier 400G, and quad-carrier 400G. They differ from each other mainly in the number of wavelengths used for transmission. This post will reveal what are they and their respective pros and cons.
Single-carrier 400G, or single-wavelength 400G, means there is 400G capacity on a single wavelength. The single-carrier 400G adopts high-order modulation formats such as PM-16QAM (Polarization-Multiplexed-16 Quadrature Amplitude Modulation), PM-32QAM and PM-64QAM.
Take PM-16QAM as an example. PM refers to a process where the 400G (448Gbit/s) optical signal is separated into two signals and modulated to transmit in two polarization directions -X and Y, which can cut the original signal rate in half (224Gbit/s). QAM is a process of separating the signals in X and Y to further reduce the rate. 16 stands for 4 bits, which means the signal in X and Y is respectively divided into 4 signals and the rate will accordingly decrease to 1/4 on the basis of the previous 224Gbit/s. By using PM-16QAM, the signal rate at this moment becomes 448÷2 (PM)÷4 (16QAM)=56G Baud. Why do we need to cut down Baud (the rate of electrical processing)? Because in current circuit technology, 100Gbit/s has approached the limit of the electronic bottleneck. If the rate continues to increase, problems like signal loss, power dissipation, and electromagnetic interference will remain a hassle, which will, even if solved, require tremendous costs.
Advantages: compared with the multi-carriers scheme, single-carrier 400G is an easier wavelength allocation solution, with simpler structure and small size. Also it provides easy network management, low power consumption. With higher-order QAM, single-carrier can increase signal rates and spectrum efficiency, which will significantly expand network capacity and increase the number of users to support. Also, with high system integration, it can connect the separate subsystems into a complete one and make them work in coordination with each other and achieve the best overall performance. single-carrier can give you an cost-effective solution.
Disadvantages: since single-carrier adopts more advanced QAM, it requires a higher OSNR (Optical Signal Noise Ratio) and greatly reduces transmission distance (less than 200km). Also, single-carrier is more susceptible to laser phase noise and fiber nonlinear effects. It is the best solution only for some specific applications that don't require ultra long-haul transmission distance, but need large bandwidth capacity. Normally, single-carrier 400G is used only in network access, metro, or DCI (Data Center Interconnection) transmission.
Dual-carrier 400G, also named dual-wavelength 400G, offers 400G capacity via two 200G wavelengths. Compared with single-carrier 400G solution, the dual-carrier 400G system based on the 2×200G super-channel scheme adopts lower-order modulation formats like PM-QPSK (Quadrature Phase Shift Keying, a symbol represents two bits, which means the rate is reduced to 1/2), PM-8QAM or PM-16QAM. Dual-carrier is applied in more complex metro networks to achieve 400G long-haul transmission.
Advantages: the spectrum efficiency of dual-carrier 400G has increased by more than 165%, with relatively high system integration, small size, low power consumption. Dual-carrier 400G is regarded as the most commonly-used technology for 400G OTN. The span of dual-carrier 400G is longer than single-carrier 400G, which can reach up to 500km for commercial use. When deployed with low-attenuation fiber optic cable and EDFA (Erbium Doped Fiber Amplifiers), dual-carrier 400G can cover more than 1000 km, which can basically satisfy the 400G long-haul transmission application.
Disadvantages: even with EDFA and lower-attenuation fiber cable, dual-carrier 400G still fails to reach as long as quad-carrier 400G does, not suitable for ultra long-haul (ULH) transmission over 2,000km.
Quad-carrier 400G refers to a solution that offers 400G capacity through four 100G wavelengths. It is achieved by constructing a 400G super-channel based on 100G PM-QPSK with four carriers.
Advantages: quad-carrier 400G adopts the mature 100G transmission technology that has been widely-used for commercial purpose. It can achieve ultra long-haul transmission of more than 2000km at relatively low cost.
Disadvantages: quad-carrier 400G system makes sense only when spectrum compression technology is introduced to improve spectrum efficiency, and the 100G chip is upgraded to solve the problems of integration and power consumption. Otherwise, a 400G system built on the current 100G chip is essentially a 100G system.
At present, 400G long-haul transmission is mainly realized by single-carrier, dual-carrier and quad-carrier. Single-carrier 400G with PM-16QAM/32QAM/64QAM is limited in transmission, which can only cover a distance of less than 200km. By using EDFA and ordinary G.652 fiber, dual-carrier 400G with PM-16QAM is the ideal solution for MAN transmission, and dual-carrier 400G with PM-QPSK is fit for medium long-haul transmission. The quad-carrier 4x100G scheme is essentially 100G technology, which has the same transmission distance as 100G and is appropriate for ULH transmission. As the global data traffic keeps climbing, there is no end for bandwidth demands. While it may take time to transit to 400G, you can learn about 400G Ethernet Market Current and the Future to make preparations first.