Research on 25G WDM-PON Bearer for 5G Fronthaul

 

 

Development Progress  

Wavelength division multiplexing passive optical network (WDM-PON) combines the WDM technology and PON topology structure to provide high bandwidth, low latency, fiber savings, plug-and-play optical network units (ONUs), simple OAM, and low costs. Thanks to these strengths, WDM-PON is uniquely suited to 5G fronthaul applications and has attracted widespread industry attention in recent years.
The ITU-T G.9802 standard defines the universal requirements of wavelength routed/wavelength selective (WR/WS) PON application scenarios as well as those of wavelength allocation, tuning and management. In 2015, ITU-T Q2 defined the indices of 8-wavelength point-to-point (PTP) WDM in 989.2 Am1—a specification on the physical media dependent (PMD) layer of 40-Gigabit-capable passive optical networks 2 (NG-PON2). WDM-PON requires mature and reliable OAM mechanisms. Annex B of 989.2 Am1 defines the auxiliary management and control channel (AMCC) to transport wavelength allocation information and OAM data, thereby achieving wavelength control and transparent service transmission for WDM-PON. Annex C of 989.2 Am2 defines the network architecture and optical layer indices of WR WDM-PON, with the modulation rate specified as 10 Gbps.
International standards for 25G WDM-PON have not been released, but discussions are already under way. To meet the need of higher bandwidth for 5G fronthaul, ITU-T Q2 has begun to work on a technical white paper about using single-wavelength 25G WDM-PON for 5G fronthaul. The research on efficient fronthaul network architecture that produces breakthroughs in high-speed colorless ONU and AMCC technologies is key to speeding up the industrialization of WDM-PON standards. In 2014, ITU-T SG15 initiated the G.metro standardization work, which was led by China Unicom and involved active participation of mainstream operators and vendors. The standard that was finalized in February 2018 as G.698.4 V1.0 defines the 20-/40-wavelength 10G interface and focuses on a 20 km transmission distance. G.698.4 will undergo more revisions in the future to upgrade its interface support to 25G and higher rates.
In China, the China Communications Standards Association (CCSA) has initiated a WDM-PON project and is expected to release the relevant standard in Q3 2018. As for enterprises, China Telecom started related standards formulation in Q2 2018 through its Shanghai Research Institute, while China Unicom plans to publish its 25G WDM-PON standard at the end of 2018.
As an important supplementary solution for 5G fronthaul, 25G WDM-PON is forecast to commence large-scale commercial trials in 2019 to 2020.

Technological Analysis 

In 5G greenfield or hotspot scenarios, operators face pressure to reduce the number of sites and leased equipment rooms. The capital expenditure (Capex) on sites and equipment rooms can be significantly cut through centralized deployment. Consequently, employing centralized radio access network (C-RAN) architecture for 5G fronthaul has found great favor with operators. WDM-PON OLT can take advantage of the access office (AO) to enable the deployment of a centralized distributed unit (DU) pool. When conditions permit, wireline and wireless AOs can be co-located. The WDM-PON architecture is based on a point-to-multipoint tree topology of passive optical networks and therefore can vastly cut the fiber resources required. It can also use the idle fiber resources of the existing ODN to reduce network construction and maintenance costs.
WDM-PON is a high-performance access scheme that leverages advantages of the WDM technology and PON topology structure. WDM-PON does not permit bandwidth sharing among users and hence is the best solution to handle a surge in bandwidth demand. Compared with existing mobile fronthaul technologies such as dark fiber, passive WDM and active dense wavelength division multiplexing (OTN, DWDM), WDM-PON has numerous advantages including high bandwidth, low latency, fiber savings, plug-and-play ONUs, simple OAM, and low costs. These advantages make it better for WDM-PON to satisfy 5G fronthaul requirements such as dense site deployment, growing bandwidth needs, and shorter latency.
In January 2018, 3GPP released the first version of Ethernet common public radio interface (eCPRI) specification for 5G fronthaul. 25G eCPRI has been basically designated as the 5G fronthaul interface, and 25G single channel will be a mainstream interface for 5G fronthaul. 25G WDM-PON is the perfect enabler of such an interface.
ZTE has been researching WDM-PON technology since 2010. Collaborating with China Telecom and the Chinese broadcast and TV sector, ZTE conducted research on 64-wavelength 10G WDM-PON equipment and applications, which was supported by China’s 863 Program. In 2014, ZTE launched demonstration networks in Wuhan and Shanghai to offer converged services such as internet access and high-definition video for thousands of users. In January 2018, ZTE was among the first vendors to complete development and testing of 10G WDM-PON equipment that met the fronthaul device interface requirements of China Telecom’s 5G trial in Shanghai. According to the plan, ZTE will also verify the delivery of wireless services through 5G base stations on the existing network.
ZTE has launched its TITAN access platform for 5G fronthaul. The platform provides high-density 25G WDM-PON cards and addresses technical challenges of low-latency forwarding and high-precision time synchronization for 5G fronthaul. Because TITAN is deployed in the AO, it can provide a unified 5G bearer solution for fronthaul, middlehaul and backhaul. The network architecture of WDM-PON 5G fronthaul based on TITAN OLT is shown in Fig. 1.

 

ZTE's WDM-PON 5G fronthaul solution has the following technical advantages:
● It supports CPRI and eCPRI standards as well as 4G/5G hybrid networking.
● It has high bandwidth that supports 25G per channel and can smoothly evolve to single-wavelength 50G in the future.
● It has high density, with a single trunk fiber providing an access capacity of 20×25G.
● It provides exclusive bandwidth for a single wavelength, high transmission efficiency, and abundant bandwidth resources.
● It provides colorless ONU technology that can be used for flexible wavelength allocation and wavelength routing.
● Its future colorless small form-factor pluggable (SFP) ONU can be directly inserted into the active antenna unit (AAU) to improve equipment integration.
● Its arrayed waveguide grating (AWG) has smaller loss than power splitter. The loss is about 5.5 dBm.

The WDM-PON 5G fronthaul solution allows for sharing of the existing fiber infrastructure. While a 5G network needs a lot of fiber resources, the architecture based on the point-to-multipoint tree topology of a PON can save large amounts of trunk fibers. The existing FTTx networks are big in size with rich line and port resources. A full use of these resources can reduce 5G network deployment costs, avoid overlapping investment and increase existing network utilization while improving network coverage.
The WDM-PON 5G fronthaul solution also allows for sharing of the AO resources by wireline and wireless access. The re-architected AO, in particular, can make the most of the solution in integrated network construction and sharing investment. After DUs are pooled, wireless and OLT resources can be shared and built as needed. OLT can serve as the shared equipment or platform for both wireline and wireless services. Both the OLT platform and the DU pool can be deployed in the same AO to remove the operator’s dependence on sites and rooms and shrink their auxiliary equipment and site costs as well as energy consumption. 
Besides, the solution also allows one OLT to deliver multiple services, providing unified access to home users, government/enterprise users, and 5G base stations. This further enhances equipment utilization, saves network deployment costs and reduces the number of sites and rooms being used.

Prospects 

Photoelectric devices in a WDM-PON system are cutting-edge and highly sophisticated and require the high-end integrated photoelectric chip R&D and manufacturing capabilities. Mastering the core technologies of optical sources is essential to bringing WDM-PON to fruition. 
25G optical receivers such as positive-intrinsic-negative (PIN) photodiodes and avalanche photodiodes (APDs) have matured, with some of them already in large-scale commercial use. 25G tunable laser is an important 25G WDM-PON component. The wavelength tunable ONU technology is based on the tunable laser and can easily implement high-speed transmission when integrated with the modulator. Currently, there are many types of tunable lasers supplied by a variety of vendors, but their rates have not yet met the requirements for 5G fronthaul. Although the digital supermode-distributed Bragg reflector (DS-DBR) has obvious advantages among the tunable lasers in terms of tuning range, integration, modulation rate and technology maturity, its cost is relatively high. Of many technical schemes proposed for tunable lasers, the combination of DS-DBR and Mach-Zehnder modulator (MZM) is the most mature and expected to achieve a transmission rate of 25 Gbps in 2018. Compared with DS-DBR, other schemes are more cost-effective and have a brighter future after addressing the issues like transmission rate, tunable range and optical power.

In general, the 25G WDM-PON industry chain is basically mature but still needs sustained investment in chips, modules, equipment and systems to develop key technologies, reduce the cost of core devices, establish unified standards, and accelerate the productization process.