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An optimal solution for 5G fronthaul based on 25G WDM-PON

With the advent of the 5G era, the bearing networks for 5G fronthaul (5G fronthaul bearer networks) are facing challenges. 5G base station fronthaul requires a bandwidth of up to 25 Gbps, the number of base stations multiplies, and the bearing of base stations brings tremendous challenges to fiber infrastructure networks and deployment costs. Moreover, the uRLLC services require ultra-low and high-reliability networks for end-to-end traffic transmission.

In 5G greenfield or hotspot scenarios, operators face pressure to reduce the number of sites and leased equipment rooms so that the Capital Expenditure (CapEx) on sites can be significantly cut through centralized deployment. Consequently, employing a Centralized Radio Access Network (C-RAN) architecture for 5G fronthaul holds great appeal for operators, but the current dark fiber solution for the C-RAN architecture requires a large amount of mobile backhaul fibers. Therefore, most operators are seeking an optimal solution to meet the needs of 5G fronthaul.

Features and advantages of 25G WDM-PON for 5G fronthaul

In January 2018, the 3rd Generation Partnership Project (3GPP) released the first version of the specification on the Ethernet Common Public Radio Interface (eCPRI) used for 5G fronthaul. The 25G eCPRI has almost certainly been designated as the 5G fronthaul interface. The 25G single-channel interface will be a mainstream interface for 5G fronthaul, and 25G WDM-PON is the perfect enabler of such an interface.

WDM-PON is a high-performance access technology, which combines advantages of both the WDM technology and the TDM-PON (Such as GPON and 10G-GPON) topology, and disallows bandwidth sharing among users, hence a better solution to handle a surge in bandwidth demand. The solution for carrying 5G fronthaul over WDM-PON provides benefits including high bandwidth, low latency, fiber savings, plug-and-play Optical Network Units (ONUs), and simplified Operation and Maintenance (O&M).

5G networks need a lot of fiber resources, and an architecture based on the point-to-multipoint tree topology of a PON can save large amounts of trunk fibers. The existing FTTx fiber networks are big in size with abundant line and port resources, together with racks, ODFs, equipment rooms, and power supply. Making full use of these resources can reduce 5G network deployment costs, avoid overlapping investment and increase existing network utilization, while extending the coverage of 5G networks. It is forecasted that the commercial trials of 25G WDM-PON will be commenced in 2019 to 2020. It is an appropriate choice for 5G fronthaul applications and has attracted widespread attention from the industry in recent years.

Compared with existing mobile fronthaul technologies including dark fiber, passive WDM and active dense wavelength division multiplexing (OTN, DWDM) which require a large amount of trunk fibers, WDM-PON has advantages including high bandwidth, low latency, fiber savings to reduce network costs. These make WDM-PON a better choice to satisfy 5G fronthaul requirements such as dense deployment of sites, burgeoning bandwidth demand and a shorter latency.

The solution for carrying 5G fronthaul over 25G WDM-PON has the following technical advantages:

  • Support for CPRI and eCPRI standards as well as 4G/5G hybrid networking.

  • High bandwidth with single-channel 25G rates, which can smoothly evolve to single-wavelength 50G in the future.

  • High density with 20 pairs of wavelengths on a single trunk fiber.

  • Bandwidth delivered by an exclusive single wavelength to ensure high transmission efficiency. Abundant bandwidth resources.

  • Colorless ONU technology allows flexible wavelength allocation and wavelength routing.

  • In the future, a colorless Small Form-Factor Pluggable (SFP) ONU can be directly inserted into the Active Antenna Unit (AAU) to increase the equipment integration density.

  • The Arrayed Waveguide Grating (AWG) incurs a power loss of about 5.5 dBm, which is lower than that of the optical splitter.

ZTE’s 25G WDM-PON solution for 5G fronthaul

To address the challenges and meet the requirements of 5G fronthaul, ZTE proposed a 5G fronthaul solution – a flagship optical access platform TITAN based on the 25G WDM-PON technology, which makes use of the premium and enriched fiber broadband network resources to implement the convergence of 5G and FTTH (FMC), and has a number of new features to meet the high bandwidth, ultra-low and high-reliability technical requirements of 5G services.

TITAN is deployable in the Access Office (AO), providing a unified solution for 5G fronthaul bearing. The solution takes advantage of the existing resources to enable the deployment of a centralized Distributed Unit (DU) pool. When conditions permit, the wireline and wireless AOs can be co-located. For example, the PON OLTs and the DUs can be deployed in the same Access Office.

Figure 1. TITAN-Based WDM-PON Architecture for 5G Fronthaul

The TITAN access platform in the Access Office (AO) was launched specifically with an eye towards carrying 5G fronthaul. The TITAN platform contains high-density 25G WDM-PON cards, and multiple channels with each channel having different wavelength and 25G bandwidth capability. The channels are combined into one trunk fiber by a WDM combiner (AWG as shown in Figure 2). Thus the logical topology of 25G WDM-PON is point-to-point, with each 5G AAU (Active Antenna Unit) and the DU (Distributed Unit) link using a single channel wavelength with high security and reliability. Each 25G channel delivers CPRI/eCPRI fronthaul while reducing the trunk fibers by over 90% comparing to the direct fiber connection solution.

Figure 2. 25G WDM-PON Network for 5G Fronthaul

The TITAN platform provides innovative TDM-like channels to ensure a latency of less than 10us in PON OLT. In the OLT and DU co-sited scenarios with the OLT coverage of 5 km, we achieve an end-to-end latency of less than 41μs, which is 59% lower than the 5G uRLLC requirements and extends the coverage of DUs/OLTs. The mechanism of the TDM-like channel processes data in real-time, which is different from the forwarding of traditional Ethernet packets. The dataflow sent from 5G AAUs does not need to be processed, no queue is required, and the processes including Ethernet packet buffer, forwarding, routing and searching are omitted, which allows it to meet the stringent technical requirements of 5G fronthaul for low-latency forwarding.

TITAN supports service-based network slicing. With service isolation and domain-based management, it fulfills the network slicing requirements of 5G services. TITAN supports diverse network slices based on uplink port, PON port, and ONU to support leasing different AAUs from the same operator to different operators, or connecting the AAUs from multiple operators to different service network through one operator’s OLT. TITAN can provide household slices to home users in residential communities and provide mobile slices to 5G services.

Figure 3. Serial Slices Guarantee Differentiated 5G and Fixed Network Services

TITAN is the industry’s first platform to provide built-in blade for MEC within PON OLT, which provides edge computing resources simultaneously to mobile and fixed-line services. The built-in blade for MEC is able to accelerate the real-time 5G video services. The blade makes it easier to provide services near end users, enhances QoE of video services, and reduces the pressure of Edge Data Centres.

Figure 4. Built-in Blade for MEC Sharing the Edge Computing Resources

In a word, besides the WDM-PON advantages, ZTE’s 25G WDM-PON for 5G fronthaul based on the new-generation platform TITAN has the following advantages:

  • The point to multi-point topology delivers CPRI/eCPRI fronthaul while saving resources, and reduces the trunk fibers by over 90% comparing to the direct fiber connection solution.

  • The 25G WDM-PON solution provides separate slices to differentiate the access of 5G, home users, and government/enterprise services.

  • Innovative TDM-like channels ensure a latency of less than 10us in OLT, and an end-to-end latency of less than 41μs, which is 59% lower than the 5G uRLLC requirements and extends the coverage of DUs/OLTs.

  • ZTE is first in the industry to support built-in MEC over PON OLT, which provides edge computing resources to accelerate real-time 5G video services near end users for higher QoE.

ZTE’s achievements in WDM-PON and 5G fronthaul

ZTE is an important contributor to the WDM-PON technical standards, and has made some contributions to push the industry chain ahead. ZTE is an editor of the ITU-T G.sup.5GP whitepaper to promote the 5G fronthaul technology standards based on PON technologies, a chief editor of the Single-Fiber Bi-Directional Point-to-Point Standard for 5G Applications, and had held joint conference for the FSAN and ITU-T SG15 Q2 in December 2017. In recent years, ZTE has been dedicated to promoting the 25G WDM-PON optical component industry and has deep technical cooperation with our partners to lead in the industry chain.

In January 2018, ZTE was among the first vendors to complete solution feasibility verification of WDM-PON for the fronthaul interface of China Telecom’s 5G pilot in Shanghai.

In December 2018, ZTE was first in the industry to verify the delivery of wireless services through 5G base stations at China Telecom Suzhou Branch with 25G WDM-PON, The results of the verification show that WDM-PON can carry 5G fronthaul traffic stably and transparently, and the service rate and forwarding delay meet the 5G requirements. ZTE will continue to promote the industrialization and commercial process of the WDM-PON technology.

Summary

The 5G fronthaul over WDM-PON solution can make the most of the advantages of integrated network construction and shared investment. ZTE has been committed to the mobile bearing solution for years, and 25G WDM-PON for 5G fronthaul solution over the TITAN access platform is an optimal one that can greatly reduce the construction costs and energy consumption of the auxiliary equipment and sites.