Reviewing the past

Huawei's optical networks have gone through three generations, through 2.5G, 10G, and 40G, before finally achieved a good market share in 2008. However, we still have a higher goal. In the 100G and beyond 100G era, Huawei will provide customers with industry-leading solutions.

The PM-QPSK (polarization multiplexing phase modulation) coherent technology used by 100G requires real-time digital signal processing to demodulate optical signals. This is a new field in the industry. At the beginning of 2008, Huawei set up a special project team for 100G optical transmission comprising global experts in opto-electronics and algorithms. This led to Huawei's release of a 100G optical transmission solution in 2011.

Since Huawei's 100G solution became commercially available, it has been widely applied on operator networks around the world. Here are some of the success stories.

• Conquering the Siberian wilderness

In 2012, a Russian operator planned to bring its Eurasian backbone 100G network into commercial use. The entire network reached more than 5000 km east-west, and was located in the extremely cold environment of Siberia. The live-network test at the beginning of the project identified many problems, such as irregular and transient link interruptions.

The project team sent engineers several times to check the links onsite and found that a certain segment of the optical cable was deployed along the railway in aerial mode. The transient service interruptions occurred when a train passed through. It was determined that the root cause of the problem was that aerial optical cables were located on both sides of the railway. When a train passed through, the phase jitter of the phase discrimination signals in the receiver became thousands of times higher than that in the normal environment, which was far beyond the designed value. The project team quickly rectified the problem by modifying the preset configurable logic circuit and optimizing the parameters. Huawei became the only vendor to pass this network test, and became a long-term partner of the operator.

• Mysterious bit errors from lightning

In 2015, the 100G products of Huawei and other vendors were deployed simultaneously in a tropical country. After the products had been running for a period of time, the customer notified the vendors that some transmission bit errors occasionally occurred on the link, hoping that the vendors could locate the fault. However, the errors on the network seemed irregular. Experts from the vendors carried out multiple rounds of analysis of the live network and failed to locate the fault.

Huawei's expert team did not give up. Using its ability to capture transient changes in real time and rounds of analysis of the bit error characteristics and the customer's environment, Huawei's team determined that the likely cause of the fault was lightning. When lightning occurred near the optical cable, the strong ionization effect in the air made the polarization state in the optical fiber change rapidly, which exceeded the preset tracing capability of the digital equalizer, causing errors on the link.

After the root cause was located, Huawei's optical transmission experts began to rectify the fault. A preliminary solution was soon developed, and bit errors were eliminated by adjusting configuration parameters on the live network. It seemed that the fault was rectified and the customer was satisfied.

However, Huawei's R&D team did not stop there. What if there is even worse lightning somewhere else in the world? Is the current solution fit for all environments? To prevent similar issues from occurring again, Huawei invested millions of dollars and built a lab that could simulate the lightning. After multiple technical breakthroughs, Huawei developed an innovative algorithm to further improve the State of Polarization (SOP) tolerance by 10 times. This algorithm has already been implemented in Huawei's ultra-high-speed products, further improving link quality for Huawei's customers.

After the success of the 100G solution, Huawei's R&D team continued to dedicate itself wholeheartedly to innovation. They used real-time phase jump capturing and compensation algorithms to resolve the cycle slip problem, a Trellis phase tracking algorithm to implement accurate phase recovery [1], and a new clock phase discrimination algorithm and non-linear equalization algorithm to compensate for signal distortion caused by overly narrow bandwidth. Between 2013 and 2015, Huawei successfully released the non-differential PM-QPSK 100G solution, providing industry-leading performance and the first dual-carrier PM-16QAM 400G solution. Moreover, it led the industry by providing the first single-carrier PM-QPSK 200G and 400G solutions with optimal transmission distance implemented by the 60G baud rate[2].

Working on the present

In the mobile Internet era, smartphones and 4G networks are developing quickly, increasing backbone traffic at a rate of 50% to 80% each year. Operators must use new technologies to further improve network capacity and reduce the per-bit transmission cost, relieving the increasing pressure from the service traffic and continuously increasing network bandwidth.

Some vendors have promoted 400G systems, which use the dual-carrier PM-16QAM implementation method. PM-16QAM modulation provides higher spectral efficiency than a 100G PM-QPSK modulation system, improving network capacity and reducing the per-bit cost. However, the 16QAM modulation format has a fatal disadvantage in commercial deployment. The transmission distance is significantly lower than that of a 100G system. Theoretically, the back-to-back OSNR tolerance of the PM-16QAM is 6.7 dB less than that of the PM-QPSK. Therefore, the transmission distance of the PM-16QAM is less than a quarter that of the PM-QPSK, and the industry struggles to improve the transmission distance of the 400G system.

In addition, the rapid growth of network traffic increases the challenges brought by heat dissipation and power supply in equipment rooms. The increasing construction cost makes equipment room expansion impractical. Operators need an environmentally friendly and energy-efficient technology to cope with the challenges of power supply and heat dissipation.

To meet the complex requirements of 400G, Huawei has released two series of products in the sixth-generation ultra-high-speed solution, optimal for the high-performance scenario and low-power-consumption scenario.

• High performance

The high-performance series supports flexible adjustment of 100G to 600G rates and 50 GHz to 75 GHz channel spacing. To address the inherent defects of 400G systems – that is, the transmission distance – Huawei carefully analyzed the optical noise, component bandwidth and response characteristics, and non-linear transmission characteristics, and developed the Channel-Matched Shaping (CMS) technology. The basic advances of technology are its provision of advanced encoding and equalization algorithms; accurate network and optical-layer modeling; real-time monitoring of the actual situation of each link for adaptive optimization, shaping, and compensation; and maximizing the spectral efficiency and channel entropy. With the help of this innovative technology, Huawei increases the 400G transmission distance from 1,000 km to 3,000 km or even higher, resolving the largest difficulty faced by operators in 400G systems deployment.

• Low power consumption

Huawei has designed a specific product series to tackle the problems of heat dissipation and power supply in equipment rooms. Thanks to its industry-leading 16 nm FinFet technology and the innovative sub-sampling equalization architecture and algorithm design[3], it reduces high-performance series power consumption by 50%. This optimal energy-efficient solution greatly reduces operators' power consumption.

Additionally, Huawei has integrated its unique AI-powered neural optical networks into these two series of products to further promote the digital evolution of optical networks and smoothly upgrade intelligent network management and control. Optical layer labeling technology is used to quickly mark, detect, and collect data for all wavelengths on the WDM network at the optical layer in real time, requiring no extra modules or optical-to-electrical conversion processes. Together with Huawei's NCE-T centralized management and control platform, operators can achieve self-optimization, self-protection, and advance-warning of performance deterioration in optical networks. In this way, operators can smoothly evolve their legacy networks to the Intent-Driven Network (IDN).

Looking to the future

Five to ten years from now, 5G networks will support the further development of the mobile Internet. In the meantime, new technologies such as AI pose high requirements on network bandwidth. Larger capacity, higher spectral efficiency, lower energy consumption, and high intelligence will be rigid demands for ultra-high-speed transmission on next-generation optical networks.

Huawei has launched multiple key technologies, including next-generation algorithms and AI optical network technologies. With this continuous innovation, Huawei's networks are continuously evolving towards ultimate capacity, performance, energy consumption, and intelligence.

[1] “Trellis-based feed-forward carrier recovery for coherent optical systems References and links”, Optics Express, 2016, 24(20) [2] “Coherent Transceiver operating at 61GBaud/s”, Optics Express, 2015, 23(15)

[3] “Low-Complexity Design of Noninteger Fractionally Spaced Adaptive Equalizers for Coherent Optical Receivers”, IEEE Signal Processing Letters, 2016, 23(9)