With the increasing demand for video traffic, cloud computing and mobile data, the optical network bandwidth is increasing at a rate of about 2 dB per year. High-speed coherent optical communication is developing rapidly. It took only five years for the 100G coherent optical single-carrier communication system to develop from laboratories to actual layout and use in the existing network. As single-carrier 100G signal transmission has been widely used, there is a pressing need for single-carrier 400G signal transmission.
Unlike single-carrier 100G transmission where polarization-multiplexed quadrature phase shift keying (PM-QPSK) is commonly used, single-carrier 400G transmission will have different options due to certain limitations and challenges it may face. If single-carrier 400G transmission follows the 100G system by still using PM-QPSK modulation that is based on digital signal processing, its baud rate will reach 128 Gbaud, but its frequency offset efficiency can only reach 2 bit/s/Hz. The communication capacity cannot be effectively increased; therefore, a product developed this way is unattractive. For the 400G system, a more advanced modulation mode is needed to improve spectrum efficiency and transmission capacity. In this way, the baud rate required by signals will be reduced, and so will the bandwidth requirements of devices. For example, if 64QAM is used, the necessary baud rate is only about 44 Gbaud. However, advanced QAM requires a higher signal-to-noise ratio, which results in limited transmission distance and cannot meet the requirement of long-haul transmission. The advanced modulation optimization technologies that have been researched including probabilistic shaping can greatly extend the transmission distance because they change the probability distribution of signals and therefore provide a transmission capacity that is closer to the Shannon limit.
Using High Baud Rates to Improve Single-Channel Transmission Rates
Based on past research, increasing the baud rate is an effective and popular way to increase the transmission rate of each channel. A high baud rate can achieve high-speed transmission of a single channel, reduce the number of channels and optical components, and thereby reduce costs. ZTE has demonstrated the generation of 128.8 Gbaud PM-QPSK signals and their long-haul terrestrial transmission through optical fibers, where the transmission distance can reach tens of thousands of kilometers. Recently, ZTE has also demonstrated the wavelength division multiplexing (WDM) transmission of 128 Gbaud PM-16QAM optical signals. For these solutions, the two main factors that limit system performance are modulation bandwidth limitation, and non-linear loss that is introduced by electro-optical devices for signal modulation and detection. ZTE can use advanced digital signal processor algorithms to solve these limiting factors at the sending and receiving ends, including the pre-/post-equalization compensation algorithm at the sending and receiving ends.
Using Advanced QAM to Increase Signal Rates and Spectrum Efficiency
Ultra-high-speed signal transmission can be achieved by using a high baud rate and advanced QAM. Advanced QAM can reduce the baud rate and increase spectrum efficiency. However, coherent detection by using advanced QAM requires more sophisticated digital signal processing technologies, and signal detection by using advanced QAM requires optoelectronic devices with a higher signal-to-noise ratio, narrower laser linewidth, and better linearity. The non-linearity caused by optoelectronic devices and optical fibers can also be compensated by using digital signal processing. In 2017, ZTE first achieved the generation and coherent detection of 400G PM-256QAM signals. At present, the most advanced QAM mode in the world is 4096QAM, but its rate is only about 50 Gb/s.
Using New Optical Fiber and Amplification to Extend Transmission Distance
The transmission of optical signals in optical fibers is affected by dispersion, loss and nonlinearity. In the coherent optical communication system, fiber dispersion is no longer a major issue since it can be effectively compensated by using digital signal processing. The loss of C-band signals in standard single-mode optical fibers is about 0.2 dB/km, so the total loss after fiber transmission over 100 km is about 20 dB. To reduce loss and improve the signal-to-noise ratio after the transmission, a new ultra low loss fiber (ULLF) has been introduced. Now, the minimum loss of ULLF can be about 0.14 dB/km, so the total loss after the transmission over 100 km is only about 13 dB, which is 6 dB less than that of standard single-mode optical fibers. Moreover, increasing the aperture of an optical fiber can reduce both the optical power per unit area and the non-linear effect in the fiber. Currently, the aperture of an optical fiber can exceed 150 square micrometers. Different from centralized amplification of EDFA, Raman amplification can reduce the optical power of signals in fibers (at places near the EDFA) and thus reduce the non-linear effect in optical fibers. Now, the records of long-haul terrestrial transmission are basically all achieved through Raman amplification. If the digital signal-processing algorithm is used at the receiving end, the impact of the non-linear effect in optical fibers will be further reduced. However, the current complicated algorithm is basically not applicable and needs to be further researched.
Using Probabilistic Shaping to Achieve Long-Haul Transmission
Probabilistic shaping (PS) is the most popular digital signal processing algorithm used in the recent two years. Since the birth of the information theory, narrowing the gap between the communication system capacity and the Shannon limit has become an eternal topic. In the PS constellation diagram, constellation points are equally spaced, but each has a different probability. The PS technology is a very important method in the additive white Gaussian noise (AWGN) channel. Transmission power can be reduced by making symbols with lower energy appearing more frequently than those with high energy. Though the non-uniform distribution reduces the entropy output by the transmitter and decreases the average bits (or bit rates), the saved energy is sufficient to compensate for the loss of bit rates. In addition, by increasing the Euclidean distance under fixed power, the PS technology also increases noise immunity of the system.
For a given average bit rate or fixed transmission entropy, the optimal distribution of constellation points that can minimize the average transmission energy is the Maxwell Boltzmann distribution, which can achieve the maximum information rate in the AWGN channel. In principle, when constellation points comply with the Maxwell Boltzmann distribution, a 1.53 dB shaping gain (or sensitivity gain) can be achieved in each dimension. The non-uniform signal generation mechanism can be achieved by mapping simple variable-length prefixes.
Fig. 1 shows four probability distributions of PS-64QAM, where the histogram height indicates the probability of modulation symbols. As probabilistic shaping becomes more and more significant from (a) to (d) in Fig. 1, the entropy becomes smaller and smaller, which is 5.73 bits/symbol, 5.23 bits/symbol, 4.60 bits/symbol and 4.13 bits/symbol in the four figures respectively.
ZTE Using New Technologies to Create a 400G Long-Haul Transmission Record
The PS technology can greatly extend transmission distance. Recently, ZTE, together with several leading device and optical fiber companies overseas, successfully transmitted the 66 Gbaud PS-16QAM 400 Gb/s signals over more than 6000 km under the 75 GHz channel spacing on 100 km amplification spacing, refreshing the record of terrestrial transmission distance. The system adopts a series of new technologies and devices, including the high-sensitivity probabilistic shaping and pre-/post-equalization coherent detection technologies developed by ZTE, the low-power and small-size high-bandwidth coherent drive modulation (HB-CDM) module made by NeoPhotonics, and the large-aperture low-loss TeraWave™ optical fiber manufactured by OFS. In this experiment, the transmission distance of single-carrier 506 Gb/s PDM-16QAM signals that are probabilistically shaped exceeds 6000 km under the channel spacing of 75 GHz on 100 km spans, and the spectrum efficiency of the line exceeds 5.3 b/s/Hz. Compared with the case where no PS technology is used, the transmission distance is extended by more than 40%.
The PS technology is more effective for advanced QAM than that for low-level QAM. ZTE also uses this technology to achieve long-haul transmission of 400G 64QAM signals. In the experiment, ZTE transmitted 528 Gb/s single-carrier PM-64QAM signals in eight channels (under the channel spacing of 50 GHz). Table 1 summarizes the experimental results that ZTE got by using or not using the two different PS technologies respectively. Under the 5×10-2 SD-FEC threshold, signal transmission over 3200 km can be achieved by using both PS technologies. However, if no PS technology is used, the maximum transmission distance is 2000 km. It can be seen that the transmission distance is extended by more than 60% after the PS technology is used.
The 400G high-baud-rate advanced modulation coherent communication technologies based on digital signal processing are developing rapidly. By using these advanced technologies, ZTE has achieved the transmission records of 10,000 km transmission of 400G QPSK signals, 6,000 km transmission of 16QAM signals, and 3,000 km transmission of 64QAM signals. In these transmission systems, the highest baud rate of transmitted signals can reach 128.8 Gbaud, and advanced QAM has achieved 400G 256QAM signal modulation. ZTE used small-loss large-aperture optical fibers, advanced devices such as Raman amplifiers as well as PS technology and coherent detection based on advanced digital signal processing to effectively extend the transmission distance.
Single-carrier 400G transmission, PM-QPSK, High baud rates, advanced QAM, spectrum efficiency, probabilistic shaping, long-haul transmission