With the commercialization of 5G and the emergence of new services such as cloud computing and big data, the pressure on network bandwidth has increased dramatically. Compared to earlier technologies like 25G/100G, 400G offers the advantages of larger bandwidth, lower latency and lower power consumption. As a result, the deployment of 400G optical transport network (OTN) has become the trend. At present, there are three transmission technologies, namely single-carrier, dual-carrier and four-carrier, to realize a 400G optical transport network (OTN), what are the differences between these three transmission technologies besides the different number of carriers? What are the advantages and disadvantages of each? You will find the answer after reading this article.
Single Carrier 400G Technology Overview
Single-carrier 400G technology uses a high-order modulation format to build 400G channels by single-carrier modulation based on 400G PM-16QAM, PM-32QAM and PM-64QAM signals. It is suitable for short-range applications such as metro networks, data center interconnect (DCI) that need large bandwidth capacity but do not require long-distance transmission ).
Here is an example of 400G PM-16QAM technology. PM” refers to the separation of a 400G (448Gbit/s) optical signal into two polarization directions (X and Y directions), and then modulating the signal to these two polarization directions for transmission, as shown in the figure below. This is equivalent to “splitting” the data, reducing the rate by half. “QAM” refers to the process of separating the X and Y signals, at which point the rate is reduced by half, that is, 224Gbit/s. “16” means that the X and Y signals are divided into four signals, reducing the rate from the previous 224Gbit/s to 56Gbit/s. Some people will surely ask, why do we need to reduce the baud rate? Because from the current stage of circuit technology, 100Gbit / s is close to the limit of the “electronic bottleneck”, if we continue to increase the rate, issues such as signal loss, power dissipation, and electromagnetic interference become difficult to solve, even if they are solved, it requires a huge cost.
Benefits: Compared to multi-carrier light source technology, single-carrier 400G technology is a simpler wavelength modulation solution with a simpler architecture, smaller size and relatively lower power consumption. Not only that, it also provides network management. Because single carrier 400G technology uses a higher-order modulation format, it can increase the signal rate as well as improve the spectral efficiency by more than 300%, thus greatly expanding network capacity to support a larger number of users. Moreover, it has a high degree of system integration that allows individual subsystems to be connected into a complete system, enabling them to work in concert with each other for optimal performance. This means that single carrier is an economical and efficient solution.
Disadvantages: precisely because single carrier uses a higher-order modulation format, it requires a higher optical signal-to-noise ratio, which greatly reduces the transmission distance (less than 200 km), and if the technology does not break through, the application in long-distance transmission is not optimistic. At the same time, single carrier is susceptible to laser phase noise and fiber nonlinear effects.
Dual Carrier 400G Technology Overview
For single-carrier 400G technology, dual-carrier 400G adopts 2*200G super channel technology scheme, which mainly constructs a 400G super channel using modulation formats such as 8QAM, 16QAM and QPSK, and is suitable for long-distance and complex metro networks. Dual-carrier 400G mainly uses DSP for signal processing to divide one 400G optical signal into two 200G signals, and one 200G occupies a spectrum of 37.5GHz. This allows 400G to only needs a spectrum of 75GHz, achieving a spectrum efficiency of 5.33bit/s/Hz. The actual data processing baud rate for the 400G(448 Gbit/s) signal is calculated as 448 ÷ 2 (dual-carrier) ÷ 2 (PM) ÷ 4 (16QAM) = 28G Baud.
Benefits: Dual-carrier 400G offers a spectral efficiency improvement of over 165%, along with higher system integration, smaller size and lower power consumption. Currently, the transmission technology is commercially available and widely used in 400G OTN applications. At the same time, compared to single-carrier 400G, dual-carrier 400G can achieve a transmission distance of 500km, slightly longer. When combined with low-loss fiber and EDFA, the transmission distance can reach more than 1000km, effectively meeting the needs of long-distance transmission applications.
Disadvantages: Although dual-carrier 400G with low-loss optical fiber and EDFA can reach a transmission distance of more than 1000km, it is unable to meet the demand for ultra-long distance transmission exceeding 2000km.
Quad-carrier 400G technology
Four-carrier 400G technology refers to the use of four subcarriers (each carrying a 100G signal) using Nyquist WDM (Nyquist Wavelength Division Multiplexing) PDM-QPSK modulation to create 400G channels, which are suitable for ultra-long distance backbone network transmission.
Advantages: Quad-carrier 400G uses mature technology, which is now commercially available on a large scale, with low costs and transmission distances of up to 2,000 kilometers.
Disadvantages: Quad-carrier 400G relies solely on chip upgrades to solve the system integration and power consumption issues. The introduction of spectrum compression technology is essential to improve spectrum efficiency, otherwise the current 100G chip-based 400G system is still the essence of the 100G system.