In response to the needs of 5G networks, G.metro technology is gradually becoming mature and applied

Compared with LTE, 5G networks that introduce features such as Massive-MIMO and ultra-dense networking and meet business needs such as the Internet of Vehicles have put forward higher requirements for the bandwidth, latency and synchronization of the transport bearer network, including larger mobile fronthaul and backhaul bandwidth, smaller end-to-end latency, denser networking and higher-precision network synchronization. Research on the networking mode and application technology of 5G bearer transport networks is imminent.

With the construction of 3G and LTE mobile networks, the architecture of local bearer transmission networks that meet the needs of mobile fronthaul and mobile backhaul has become stable, and the infrastructure for the construction of 5G bearer transmission networks has been prepared in terms of optical fiber resources. As the main means of 3G and LTE mobile fronthaul, optical fiber direct drive has a huge demand for the number of optical fibers in the 5G era with ultra-dense networking, coverage encryption, and increased sites. The optical fiber capacity of the existing network is difficult to meet the demand; a large number of newly built optical cables require too many pipeline resources, and optical fiber construction is becoming more and more difficult, especially in dense urban areas. How to meet the fronthaul needs of 5G networks has become one of the technical hotspots.

G.metro's technical features and adaptability to 5G networks

China Unicom, together with major domestic and foreign operators (including AT&T, Verizon, China Telecom, BT, DT, TI, Telefonica, SKT, NTT and SoftBank) and equipment manufacturers, launched the G.metro technology standardization work in ITU-T as early as 2014. The G.metro system block diagram is shown in the figure.

Figure 1 G.metro system block diagram

In view of the bandwidth, latency and synchronization requirements of 5G transport bearer networks, G.metro has obvious advantages over other solutions:

• Large bandwidth and adaptive adaptation: Adopting dense wavelength division multiplexing (DWDM) technology, single wavelength 10G rate, and further support 25G/50G and higher rates in the future, the port rate is adaptive and can be upgraded independently on demand; the total capacity of the system is large, and can support 20 channels, 40 channels and 80 channels.
• Low latency: Zero-layer WDM transparent technology is used to transmit service signals completely transparently, reducing the electrical layer encapsulation and processing latency and jitter caused by OTN, and avoiding the latency and jitter problems of the packet solution, which can better meet latency requirements.
• Good synchronization support capability: single-fiber bidirectional transparent transmission is adopted, which naturally supports the symmetrical transmission of time synchronization signals. At the same time, since the 1588 timing information is transmitted along the optical path, the multi-hop accumulation of synchronization timing information caused by electrical layer processing is avoided, thereby ensuring the accuracy of 1588 transmission.

In addition, G.metro technology uses low-cost wide-range wavelength tunable lasers with port-independent and wavelength automatic adaptation functions, which greatly simplifies network construction and operation and maintenance, reduces the types and quantity of spare parts, and reduces network construction costs; provides optical wavelength-level connections, independent channel operation, physical isolation between users, and high security. Adding OTN encapsulation on the basis of G.metro technology can realize low-cost bearing of mobile backhaul signals; G.metro technology also supports multiple topologies such as star, chain and ring, and supports full-scenario access applications for indoor and outdoor use.

The low-cost G.metro technology is currently a more mature fixed-mobile ultra-wideband converged access technology, which is very suitable for 5G mobile fronthaul and backhaul, and creates a unified bearer transmission network (UniHaul) based on WDM technology. It can effectively solve the dependence of multi-service integrated access in the metropolitan area on the number of optical fiber cores of the access trunk optical cable, making network planning and construction more economical and reasonable, reducing the depreciation and ineffective precipitation of optical cable resources and fixed assets; avoiding the huge consumption of fiber cores caused by direct drive of mobile fronthaul optical fiber; the simple and effective OAM mechanism greatly improves the maintainability and manageability of the mobile fronthaul network.

Industrialization and application of G.metro technology

While promoting the standardization of G.metro, China Unicom has actively promoted the development of the industry chain and the process of technology industrialization, and has also received extensive support from mainstream equipment manufacturers and optical device manufacturers at home and abroad. Many optical module suppliers at home and abroad (including the world's top 3 optical module suppliers) are developing low-cost tunable lasers, and some manufacturers have demonstrated wavelength automatic tuning products based on G.metro technology at OFC2017. China Unicom has also invited multiple equipment manufacturers to conduct G.metro equipment testing and verification trials in laboratories and existing networks, fully verifying the performance of G.metro technology and its compatibility with commercial wireless equipment.

Both mobile and fixed access are developing towards ultra-broadband, and building a converged ultra-broadband metropolitan transmission and access system has become inevitable. The metropolitan all-optical layer network based on WDM is of far-reaching significance. G.metro technology uses optical wavelength as a service (λ as a service), extending from the current optical fiber to base stations/users to optical wavelengths to base stations/users/dedicated lines/data centers, etc., to create Gbit/s and above converged ultra-broadband all-optical access, and achieve more efficient and resource-saving metropolitan ultra-high bandwidth service provision capabilities.

The technical standardization of G.metro is about to be completed and is expected to be released at the SG15 Geneva Plenary Meeting in February 2018. At the same time, the industrial chain of G.metro equipment has basically matured, and the research and development and industrialization of related core components have gradually improved. It is expected to be deployed on a small scale in the existing network in 2018 to welcome the upcoming 5G network.

Author: Wang Guangquan and Shen Shikui, Network Technology Research Institute of China Unicom