“With the acceleration of 5G commercial use, operators have begun to plan the construction of 5G bearer networks. Compared with 4G, 5G places higher requirements on the bearer network in terms of bandwidth, delay, synchronization, reliability, and flexibility. As an important part of the bearer network, the 5G fronthaul network not only needs to meet the above requirements, but also faces the difficulty of laying optical fiber resources required for the deployment of the exponentially increased 5G base station. In new 5G or hotspot areas, the C-RAN fronthaul architecture can help operators reduce DU site rental costs, maintenance costs, and power consumption, which is favored by operators.However, the current C-RAN optical fiber direct connection fronthaul solution consumes a lot of optical fiber resources.
With the acceleration of 5G commercial use, operators have begun to plan the construction of 5G bearer networks. Compared with 4G, 5G places higher requirements on the bearer network in terms of bandwidth, delay, synchronization, reliability, and flexibility. As an important part of the bearer network, the 5G fronthaul network not only needs to meet the above requirements, but also faces the difficulty of laying optical fiber resources required for the deployment of the exponentially increased 5G base station. In new 5G or hotspot areas, the C-RAN fronthaul architecture can help operators reduce DU site rental costs, maintenance costs, and power consumption, which is favored by operators. However, the current C-RAN fiber direct connection fronthaul solution consumes a lot of fiber resources and has high deployment costs. Operators hope to find other more cost-effective solutions.
Advantages of 25G WDM-PON for 5G Fronthaul Networking
According to 3GPP specification definitions and industry discussions, the technical requirements under the C-RAN fronthaul architecture include:
Data interface and rate: Standardized 25Gbps eCPRI fronthaul interface is the mainstream technology choice;
Delay: It should have low delay characteristics, and the one-way forward delay should not exceed 100μs;
Synchronization: synchronous signal transmission should be supported to meet the ±1.5μs synchronization requirements of 5G basic services;
Optical power budget: Support the link optical power budget requirements corresponding to the 5G fronthaul technical solution;
Management: support equipment management, business configuration and monitoring, fault diagnosis and other management functions;
Others: The single-fiber bidirectional transmission technology is conducive to saving the fiber consumption of the fronthaul network and reducing the cost.
As a combination of WDM and TDM-PON technologies, 25G WDM-PON has the following technical characteristics:
1) Physical point-to-multipoint topology, using combiner/demultiplexer to achieve wavelength routing, saving fiber;
2) Logical point-to-point topology, OLT and ONU communicate through independent wavelengths, and wavelength channels are relatively isolated;
3) A single wavelength can support a rate of 25Gbps, meeting the eCPRI signal bearing requirements;
4) ONU adopts colorless tunable technology, which can realize flexible wavelength allocation and routing;
5) The AWG loss is about 5.5dBm, and the optical power budget can meet the 10km deployment requirements;
Under the C-RAN fronthaul architecture, compared with fronthaul technologies such as fiber direct connection, passive wavelength division, and active wavelength division, 25G WDM-PON has the following advantages in fronthaul networking:
High technology matching: 25G WDM-PON technology can meet the interface, speed, delay and other requirements of 5G fronthaul networking. 25G WDM-PON technology will be commercialized in 2019-2020, perfectly matching the rhythm of 5G commercial deployment;
Low construction cost: The operator’s fixed network FTTH ODN has been constructed, and the FTTH ODN architecture can match the construction requirements of 5G base stations. Recycling the old FTTH ODN can greatly save the backbone fiber, reduce the difficulty of planning and deployment of the optical cable and the overall cost, and has better economy;
Fast network construction: The FTTH ODN network can flexibly face the future 5G base station network construction method, and quickly provide fronthaul links through the deployment of optical fibers and cables. Through the SFP ONU, it can provide the ability to quickly deploy to AAU equipment without power-on;
Simple operation and maintenance: Central office equipment is centrally deployed in the access room to achieve centralized maintenance and improve operation and maintenance efficiency.
5G fronthaul solution based on TITAN platform
Among the many features of the next-generation optical flagship platform TITAN, 5G fronthaul is a major feature. The TITAN platform has 12 high-density 25G WDM-PON line cards. Each PON port is an exclusive channel with an independent wavelength for 1 AAU eCPRI fronthaul bearer. 20 exclusive channels can be aggregated into one backbone fiber for transmission through the WDM combiner (as shown in Figure 1), which can save 97.5% of the backbone fiber compared to the fiber direct connection solution. The TITAN platform provides the original TDM-Like low-latency switching channel (as shown in Figure 1), and realizes cut-through forwarding of fixed code blocks based on the core self-developed chip, ensuring that the processing delay of fronthaul services in OLT&ONU equipment is less than 7μs. When the OLT and DU are deployed in the same equipment room, the system forwarding delay is less than 57μs (including 50μs delay of 10km fiber), which is 43% lower than the 5G URLLC delay requirement (100μs).
Figure 1 5G fronthaul 25G WDM-PON network architecture
TITAN supports network slicing based on access service types (as shown in Figure 2), meeting the network slicing requirements of 5G fronthaul services. The 5G fronthaul access can have an independent OLT uplink port, and the OLT downlink can be divided according to the 2 levels of PON port/PON board as needed, providing decentralized management and differentiated QoS guarantee for 5G fronthaul access.
Figure 2 OLT network slicing supports 5G fronthaul access
In conclusion, the 5G fronthaul 25G WDM-PON solution based on the TITAN platform has the following highlights:
Save fiber: share FTTH ODN, which can save more than 90% of the backbone fiber;
Low latency: The original TDM-Like low-latency switching channel, OLT&ONU processing latency is less than 7μs, meeting the requirements of 5G uRLLC services;
Easy to manage: Provide independent network slicing for 5G fronthaul access, realize decentralized management and differentiated QoS guarantee;
ZTE is a core member and an important contributor to the WDM-PON technical standard. In June 2017, he served as the white paper editor of the ITU-T G.sup.5GP project, actively promoting PON technology to achieve 5G fronthaul technology standards. In December 2017, it hosted the “FSAN and ITU-T SG15 Q2 Conference” to discuss the WDM-PON 5G fronthaul solution with the industry. In February 2018, he was the editor-in-chief of the ITU “Single-fiber bidirectional point-to-point standards for 5G applications” group.
ZTE is committed to the advancement of the 25G WDM-PON optical device industry, and has long-term in-depth technical cooperation with leading optical module manufacturers such as Finisar, Oclaro, and Hisense.
ZTE has maintained long-term and in-depth cooperation with China Telecom, and actively participated in the formulation of technical specifications and technical verification. In December 2018, ZTE, China Telecom’s optical access research team and Suzhou Telecom took the lead in completing the industry’s first N×25G WDM-PON-based 5G fronthaul live network verification in the Suzhou 5G field. The verification content includes ONU adaptive tuning, fronthaul 25G eCPRI service transparent transmission, system forwarding delay less than 57μs (including 10km fiber delay), system management (configuration, alarm, statistics, etc.) and long-term service operation stability. The verification results show that 25G WDM-PON can stably and transparently carry 5G fronthaul services, and the service rate and forwarding delay are comparable to those of point-to-point fiber direct connection.
In 2019, ZTE will continue to promote industrial cooperation and promote the commercialization of 25G WDM-PON technology in 2020.
It can be seen that the 25G WDM-PON technology fully meets the technical requirements of 5G fronthaul. Under the C-RAN architecture, FTTH ODN resources can be fully utilized, backbone fibers can be saved, and network construction can be achieved quickly. ZTE provides 25G WDM-PON line cards on the TITAN platform, using innovative low-latency switching channels and fronthaul network slicing, which is an excellent fronthaul bearer solution.