1. WDM-PON bears 5G application scenarios

1.1 Pre-transmission network architecture definition

With the rapid development of mobile Internet and the emergence of a variety of new applications such as the Internet of Things, in order to cope with the future explosive mobile data traffic growth and massive device connectivity, the fifth generation of mobile communication (5G) technology came into being.

In 2015, ITU-R officially defined three typical application scenarios for 5G: eMBB, mMTC and URLLC. eMBB corresponds to large-capacity mobile broadband services such as 3D/Ultra HD video. mMTC corresponds to large-scale Internet of Things services, while URLLC corresponds to services such as driverless, industrial automation, etc. that require low latency and high reliability.

On November 28, 2016, China Mobile released a white paper on the 5G C-RAN requirements, architecture and challenges.

WDM-PON bearer 5G application scenarios and technology research
Figure 1 BBU research architecture: from 4G single node to 5G CU/DU two-level architecture

The 5G BBU function will be reconstructed into two functional entities, CU and DU (see Figure 1). The CU mainly includes non-real-time wireless high-layer protocol stack functions, and supports partial core network function sinking and edge application service deployment. The Layer 2 function, which mainly deals with physical layer functions and real-time requirements, is divided into two levels, one-level pre-transmission and two-level pre-transmission.

The existence of the CU implements some of the functions of the original BBU, that is, it is compatible with the complete centralized deployment, and also supports distributed DU deployment.

The two-stage preamble C-RAN architecture provides network support for DU pooling or CU pooling. One of the prequels is Fronthaul, which can support pre-pass agreements such as eCPRI; the second-level prequel is also known as Midhaul (Zhongchuan). Fronthaul and Midhaul have different latency and bandwidth requirements for different 5G services.

The large bandwidth requirements of 5G pre-transmission and backhaul drive the large interface requirements of the transmission network, including high-speed Ethernet interfaces or transmission interfaces such as OTN/DWDM. At the same time, with the definition of eCPRI’s new preamble interface, the network architecture evolves into a new architecture supporting DU and CU pooling, which presents new opportunities and challenges for wireless access and 5G bearers.

1.2 WDM-PON bears 5G application scenarios

WDM-PON bearer 5G application scenarios and technology research
Figure 2 5G new scene or hotspot scene

The typical application scenario of WDM-PON in 5G is shown in Figure 2 above.

When the operator has the site selection pressure, or needs to release the BBU room, or the dense area needs to deploy the DU pool in a centralized manner, the DU location can be moved up and deployed in these scenarios. Especially for the new scenarios of operators with existing wired and wireless services, it is very suitable to carry the bearer of the WDM-PON for the pre-transmission interface. The OLT can use the advantage of accessing the equipment room to centrally deploy the DU, the DU pool and the OLT common site. For the 5G URLLC service, it is recommended that the CU also share the site with the OLT and the DU. The DU pooling co-site will make the user plane data transmission path of the coordination demand (such as CoMP) between the DUs closer; the DU and CU co-station will make the Midhaul disappear, and the 5G RAN user plane transmission delay will be smaller.

2. The realization of WDM-PON technology and its significance of 5G bearer

WDM-PON architecture:

WDM-PON bearer 5G application scenarios and technology research

Figure 3 WDM-PON architecture

The key technologies of WDM-PON are currently focused on: colorless ONU technology, ie ITU-T G. 989.2 and CCSA WR WDM-PON standard explicitly adjustable technology to achieve colorless ONU; the other is the auxiliary management channel (AMCC) technology, namely RF Pilot-tone and Baseband Overmodulation.

In the WDM-PON solution, the development of key technologies such as optical module technology, OAM management, and protection switching plays an important role in the large-scale planning and use of WDM-PON 5G bearers. The relevant agreements are being discussed and developed in the standards organization.

2.1 Pre-transmission scheme

WDM-PON bearer 5G application scenarios and technology research

Figure 4 WDM-PON pre-transmission network solution based on OLT unified optical access platform

The WDM-PON OLT device unified optical access platform is used for 5G mobile pre-transmission and supports wired optical access services. The 5G DU or BBU pool and the RRU are connected through a WDM-PON passive optical network to implement mobile service pre-transmission.

WDM-PON as the 5G pre-transmission scheme reflects the following technical features:

1. The WDM-PON technology has a small delay and can provide separate networking and service performance guarantee for 5G, government and enterprise services;

2. Large bandwidth, supporting 10G and 25G per channel, meeting the bandwidth requirements of 25G eCPRI preamble signals;

3. High transmission efficiency. Reflected in two aspects, one is exclusive bandwidth without DBA scheduling, logical point to point. The other is the management aspect, which uses AMCC signal tuning technology, the management channel is superimposed on each wavelength, there is no OMCC reservation, and there is no waste caused by GEMPORT resource reservation.

WDM-PON as a 5G pre-transmission solution will also reflect value in engineering applications:

1. The WDM-PON solution is suitable for coverage of densely populated urban residential areas. Because of “natural tree-shaped cable topology”, “solid-shift convergence business”, “intensive coverage.”

2. Existing fiber infrastructure settings can be shared. 5G network deployment requires a large amount of fiber resources. The network architecture is based on a point-to-multipoint tree network topology of passive optical networks, which can save a lot of fiber routing resources. At present, the FTTx fiber network covers a wide area, and the line and port resources are abundant. The full utilization of the FTTx fiber network can reduce the deployment cost of the 5G network, reduce the repetitive investment, improve the utilization of the existing network resources, and quickly improve the dense coverage of the 5G network.

3. Multiple wavelengths are distributed by the AWG and then distributed to the branch fiber, saving a lot of backbone fiber resources.

4. AWG has less loss than POWER SPLITER. In the case of the same ODN networking, replacing the SPLITER with the AWG means a longer transmission distance.

5.5G and wired access can share room resources, such as Local PoP access points. In particular, the equipment room based on AO reconstruction can give full play to the advantages of comprehensive network construction and equal investment.

6. The OLT can be shared to achieve access to the home user, the government enterprise user, and the 5G base station. Further improve equipment utilization, save network equipment deployment costs, and reduce resource requirements such as equipment rooms.

7. After the pooling of the DU, it helps to realize the sharing and sharing of resources for wireless and wired access, and build a future-oriented fixed-mobile convergence network. Including solid-state control plane fusion, to achieve authentication, billing and user information unified; to achieve a solid-transfer and transfer surface integration UPF fixed-shift common platform equipment; also can achieve solid storage resource integration, such as CDN, MEC resources.

2.2 Progress of WDM-PON standards and operators’ demands

At present, the WDM-PON standard mainly studies WDM-PON systems with single wavelengths below 10G (1.25G/2.5G/10G). When the single wave rate reaches 10G, the mainstream technology of WDM-PON colorless ONU is adjustable technology. Standards organizations are beginning to focus on the application of WDM-PON in 5G pre-transmission, especially the single-wave 25G-rate WDM-PON system. ITU-T G. Sup. The 5GP discussion group is advancing and discussing 25G WDM-PON.

International operators since 2017 have studied WDM-PON as a key program for 5G prequel. Legal Orange, the current network is mostly D-RAN, C-RAN is not much, 5G C-RAN, consider placing CU in NG-POP point, 5G pre-transmission pay close attention to and study WDM-PON. DT, considering C-RAN, CU/DU separation, CU is concentrated in the aggregation layer. WDM-PON prequel is of interest. Telstra, for the C-RAN prequel, compares active DWDM and WDM-PON solutions. China’s three major operators, China Telecom Research Institute actively promoted WDM-PON testing and trial commercial; China Unicom’s 5G bearer pushes G. Metro, WDM-PON technology is similar, can be used as a simplified version of G. Metro; China Mobile will promote FlexE for 5G bearers, and WDM-PON can be integrated with it.