Research: WDM-PON technology and its application to 5G Bearing

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1. Technical Requirements on the Network Architecture for Carrying 5G Services

The rapid development of the mobile Internet and the emergence of new applications such as the Internet of Things (IoT) are causing an explosive growth in mobile data traffic and connected devices. To address the network challenges posed by the growth, the 5th generation mobile telecommunications technology, or 5G, is being developed.

In 2015, ITU-R defined three classes of typical application scenarios for 5G: eMBB, mMTC and URLLC. eMBB includes services such as 3D and ultra-high definition video, mMTC includes services provided by large IoT providers, and URLLC includes services requiring low-latency, high-reliability connectivity such as self-driving vehicles and industrial automation.

On November 28, 2016, China Mobile released the white paper “Towards 5G C-RAN: Requirements, Architecture and Challenges”. The paper presents the technical architecture and key technologies required to carry 5G applications, which are characterized by high frequency bands, multiple antennae, massive connections, low latency, and so on. As shown in Figure 1, the functionality of the 5G BBU will be reconstructed as two functional entities: the Centralized Unit (CU) and the Distributed Unit (DU). The CU mainly includes the non-real-time part of wireless upper-layer protocol stacks and supports distributing some core-network functions and deploying applications at the network edge. Thanks to the CU, the BBU centralizes some functions. That is, the BBU is compatible with both complete centralization and distributed installation. The DU mainly processes physical-layer functions and real-time L2 functions, so the fronthaul comprises two levels.

Figure 1: BBU Architecture: From a Single Node in 4G to CU/DU Two Levels in 5G

The C-RAN architecture, which includes two levels of fronthaul, provides network support for DU or CU pooling. L1 fronthaul can support fronthaul protocols such as eCPRI, while L2 fronthaul, also called midhaul, can meet the latency and bandwidth requirements of different 5G services.

The big bandwidth required of 5G fronthaul demands that the transport network has big bandwidth interfaces such as high-speed WDM transmission interfaces. With eCPRI defined as a new fronthaul interface, the network architecture evolves towards a new structure which supports DU and CU pooling. These factors present new opportunities and challenges for WDM PON as a solution to carry 5G services.

Research on the Application of WDM-PON Technology to 5G Bearing

Figure 2: Research on 5G Greenfield or Hotspot Scenarios

Figure 2 above shows the requirements of applying WDM-PON technology to 5G bearing. Under certain circumstances, the operator needs to relieve the site selection pressure caused by the increasing number of base stations, release the BBU equipment room, deploy and centralize the DU pool for a dense urban area, and so on. In these cases, the DUs can be moved upwards and deployed in a centralized way. The solution of using WDM-PON as a fronthaul interface to carry 5G services is especially suitable for those operators who have to provide both wireless and wireline services in a greenfield scenario. In such a case, the operator reuses the access office to deploy the centralized DUs and co-locate them with the OLT. If a 5G URLLC service is to be carried, it is suggested that the CU, OLT and DU is co-located. The pooling and co-location of DUs will shorten the user-plane data transmission paths that are required by the mechanism of coordinated transmissions, such as CoMP, between DUs. After the DUs and CUs are co-located, the midhaul will disappear and the transmission latency at the user plane of the 5G RAN will diminish.

2. WDM-PON technology and its advantages

The key technologies of WDM include:

1) Colorless ONU technology. Both the ITU-T G.989.2 and the CCSA WR WDM-PON standards specify how to use tuneable technology to enable colorless ONUs, as shown in Figure 3.

2) Auxiliary Management and Control Channel (AMCC), which consists of two mechanisms: RF pilot tone and baseband over modulation.

3) Other key technologies such as optical modules, OAM, and protection switching. These technologies play an important role in the plan for deploying WDM-PON in 5G bearing, and their protocols are being formulated by the related standards bodies.

Figure 3: WDM-PON Architecture

Figure 4 shows the solution for carrying 5G fronthaul over WDM-PON. The OLT can be used for 5G mobile fronthaul. The 5G DU or BBU pool is connected to the RRU through the WDM-PON to fronthaul mobile services. In the same OLT platform, the WDM-PON can support wireline optical access as well.

Figure 4: WDM-PON as Fronthaul Solution Based on an OLT Unified Optical Access Platform

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

  1. Low latency: The solution can provide separate network and performance assurance to 5G, enterprise, government, and other services.
  2. Big bandwidth: The solution supports 10 Gbps and 25 Gbps per channel, meeting the bandwidth requirements of sending eCPRI fronthaul signals at 25 Gbps speeds.
  3. High transmission efficiency: Thanks to exclusive occupation by the service without dynamic bandwidth allocation and the point-to-point logic, the solution saves time and increases transmission efficiency. AMCC signal modulation technology is employed to stack a management channel onto each wavelength. No OMCC and Gemport resources need to be reserved, thereby saving resources and improving transmission efficiency.

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

  1. With attributes such as a natural tree-like topology, fixed-mobile convergence service and dense coverage, the WDM-PON solution better suits densely populated urban residential areas.
  2. The existing ODN fiber infrastructure can be shared. A 5G network requires a large amount of fiber resources. A network architecture based on the point-to-multipoint tree topology of the WDM-PON can save massive amounts of fiber. The existing FTTx networks have abundant optical fiber and port resources in extensive deployments. These resources can be fully reused to reduce 5G network deployment costs, cut overlapping investments and rapidly achieve dense 5G network coverage.
  3. Multiple wavelengths are first converged by an AWG and then distributed to the branch fibers, which saves substantial backbone fiber resources.
  4. The AWG suffers less attenuation compared with the traditional power splitter. For the same ODN networking, using the AWG means a longer transmission range.
  5. 5G and wireline services can share the equipment room resources such as local POPs. Equipment rooms based on Access Office (AO) re-architecture, in particular, can make the most of the advantages of the WDM-PON solution in integrated network construction and shared investment.
  6. The OLT can provide access for home users, government and enterprise users, and also 5G base stations in a unified manner. This further increases equipment utilization, reduces equipment deployment cost, and decreases the requirements for resources such as equipment rooms.
  7. After the DUs are pooled, wireless and wireline resources can be jointly built and shared to construct a future-proof FMC network. Within the network, fixed-mobile convergence at the control plane unifies authentication, accounting, user information, and so on. Fixed-mobile convergence at the forwarding plane results in the User Plane Function (UPF) serving as the shared platform for both fixed and mobile services. In addition, fixed storage resources, such as CDN and MEC, can be converged.

3. Research and Application Developments of WDM-PON Technology

Since 2017, the 5G fronthaul over WDM-PON technology has attracted the attention of multiple operators, for whom it has become a research priority. DT is intensely interested in the technology and is researching technical details such as C-RAN, CU-DU separation, and concentration of DUs in the convergence layer. In the Chinese market, China Telecom Research Institute is actively driving the testing and trial of WDM-PON. China Unicom treats WDM-PON as a simplified version of G.metro and promotes it heavily. China Mobile gives priority to FlexE as 5G bearing technology and intends to integrate WDM-PON with FlexE transmission.

The 5G fronthaul over WDM-PON technology, especially the WDM PON system with a 25G rate per wavelength, has also attracted the attention of standards organizations. The ITU-T G.sup.5GP study group is currently discussing and advancing the 25G WDM-PON standard.

The rapidly advancement of the WDM-PON technology solution will accelerate the arrival of 5G, which in turn will offer the technology a bright application prospect.

This sponsored article was brought to you by ZTE.

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