The acceleration of 5G rollout prompts operators to plan the construction of 5G transport networks. Compared with 4G, 5G imposes higher requirements for transport networks in terms of bandwidth, latency, synchronization, reliability and flexibility. As an important part of the transport network, the 5G fronthaul network needs to meet these requirements and also address the difficulty of laying optical fibers to accommodate the exponential growth in 5G base stations.
Since the C-RAN fronthaul architecture can reduce the site rental fees, maintenance costs, and power consumption of distributed units (DUs) in 5G greenfield and hotspot areas, it has gained the favor of operators. Based on 3GPP specifications and industry trends, the C-RAN fronthaul architecture has the following technical requirements:
—Data interface and rate: The standard 25G eCPRI is the 5G fronthaul interface of choice.
—Latency: Support for one-way latency requirement of not exceeding 100 μs.
—Synchronization: Support for synchronous signal transmission to meet the ±1.5 μs synchronization precision requirements for basic 5G services.
—Optical power budget: Support for the power budget of fronthauling links.
—Management: Support for device management, service configuration and monitoring, fault diagnosis, and other management functions.
—Others: For example, adopting the single-fiber bi-directional transmission technology to save optical fibers needed by the fronthaul network.
However, the current direct fiber connection solution for the C-RAN fronthaul consumes considerable fiber resources, resulting in high deployment costs. Operators need a more economical and efficient solution to address this problem.
5G Fronthaul Based on 25G WDM-PON
25G WDM-PON is an integration of WDM and TDM-PON with the following technical features:
—The physical topology is point-to-multipoint, and wavelength routing is performed by a multiplexer/demultiplexer to save optical fibers.
—The logical topology is point-to-point. The OLT communicates with ONUs through independent, mutually isolated wavelengths.
—A single wavelength supports data rates up to 25 Gbps, meeting the signal transport requirements of the eCPRI.
—The ONU is colorless, tunable to allow flexible wavelength assignment and routing, thus reducing its deployment cost.
—The AWG incurs an optical power loss of around 5.5 dBm, which is less than that of the conventional optical splitter, and supports an optical power budget for 10 km.
Under the C-RAN architecture, fronthaul via 25G WDM-PON has many advantages over other fronthaul schemes like direct fiber connection, passive wavelength division, and active wavelength division.
—High technical suitability: The 25G WDM-PON technology meets the interface, rate, latency, and other requirements of 5G fronthaul networking. It will be put into trial commercial use in 2019-2020 to match with the progress in 5G rollout.
—Low construction cost: Many operators have built and operationalized their FTTH ODNs. By using the existing FTTH ODN resources including fibers and pipes to also carry 5G fronthaul, operators can vastly save trunk fibers, reduce the difficulty of optical cable planning and deployment as well as the overall cost of the 5G fronthaul network.
—Fast deployment: The FTTH ODN can adapt to a denser grid of 5G base stations in the future, and can rapidly provide fronthaul links through scheduling the distribution cables. The SFP ONU can be quickly deployed in the AAU without being powered on.
—Simple operation and maintenance (O&M): All the central-office devices are deployed in the access office to enable centralized maintenance and improve O&M efficiency.
TITAN-Based 5G Fronthaul Solution
To meet the requirements and challenges of 5G fronthaul deployment, ZTE launched a 5G fronthaul solution based on its next-generation optical access platform—the TITAN OLT. The solution uses premium, abundant fiber broadband network resources to achieve 5G+FTTH integration, which is a variant of fixed-mobile convergence (FMC), thereby enabling 5G+FTTH integrated service access in dense residential communities and transport service for 5G indoor distributed antenna system.
TITAN provides 12-port high-density 25G WDM-PON line cards. Each PON port is a dedicated channel with an independent wavelength, which is used for eCPRI fronthaul transport of one AAU. Up to 20 dedicated channels can be converged by a WDM combiner into a single trunk fiber. Compared with the direct fiber connection scheme, the WDM-PON solution reduces the required trunk fiber by at least 90 percent, as shown in Fig. 1. The TITAN platform provides innovative TDM-like low-latency channels that handle traffic from 5G AAUs without buffering, forwarding, routing and search processes. ZTE's self-developed core chips allow TITAN to forward fixed code blocks using shortcuts, ensuring that the OLT and ONU together introduce a processing latency of less than 7 μs for fronthaul services. When the OLT and DU are installed in the same equipment room, the system forwarding latency (including the 50 μs latency over the 10 km fiber) is less than 57 μs. That is 43% lower than the 100 μs latency required by 5G URLLC applications.
As shown in Fig. 2, TITAN supports network slicing based on service types, including 5G fronthaul service. The OLT can offer independent uplink ports for 5G fronthaul. In the downlink direction, the OLT can realize network slicing at both the PON port and PON card levels. The slicing can also be based on ONUs. All these mechanisms enable rights- and domain-based management as well as differentiated QoS assurance for 5G fronthaul.
Meanwhile, ZTE has introduced an innovative solution that embeds blade servers in the PON OLT platform to support latency-sensitive services. TITAN is the industry's first PON OLT platform that contains built-in blade servers. As a network functions virtualization infrastructure (NFVI), the blade servers can be used by the operator itself or leased to third parties. Meanwhile, they provide edge computing capabilities for both mobile and fixed services, can be leveraged to cache and accelerate 5G real-time video services or deliver more value-added services in the future to improve quality of experience (QoE) while reducing pressure on the data centers. The TITAN-based solution for carrying 5G fronthaul over 25G WDM-PON has the following highlights:
—Fiber savings: The FTTH ODN is reused to save least 90 percent trunk fibers.
—Low latency: Innovative TDM-like low-latency channels ensure a processing latency of less than 7 μs in OLT and ONU, which meets the latency requirements of 5G URLLC services.
—Easy management: Independent network slices are provided for 5G fronthaul to enable rights- and domain-based management as well as differentiated QoS assurance.
—High scalability: The built-in blade servers provide edge computing capabilities for multi-service access. They can serve as an NFVI to enable FMC, thereby helping operators achieve network and operation transformations.
ZTE's Contribution to the Industry
ZTE is a major contributor to WDM-PON standardization. In June 2017, ZTE became an editor of ITU-T G.sup.5GP whitepaper to promote the PON-based 5G fronthaul technology. In December 2017, ZTE held a joint conference for FSAN and ITU-T SG15 Q2 to discuss solutions for 5G fronthaul over WDM-PON. In February 2018, ZTE became an co-editor of ITU-T Recommendation that specifies bidirectional single-fiber point-to-point systems oriented to 5G applications. Meanwhile, ZTE is committed to advancing the development of the 25G WDM-PON optical component industry and has in-depth technical cooperation with leading optical module manufacturers.
ZTE works closely with operators to promote specification formulation and technology validation. In December 2018, ZTE and China Telecom jointly completed the industry’s first validation of 5G fronthaul over N×25G WDM-PON on a live network in Suzhou. The validation covered many metrics including transmission performance, automatic wavelength tuning and delivery, and long-time stability. The N×25G WDM-PON devices could be connected to 5G DUs and AAUs that support standard eCPRI interfaces. The combined processing latency of OLT and ONU was as low as 7 μs, which well met 5G requirements for ultra-low latency. As demonstrated by the validation, 25G WDM-PON could carry 5G fronthaul services stably and transparently, with the data rate and forwarding latency equivalent to those in a point-to-point direct fiber connection.
Moving forward, ZTE will continue to work with industry partners to promote the improvement, maturation and commercialization of the 25G WDM-PON technology.