Evolution of optical access networks in the 5G era

Challenges and Opportunities Faced by Optical Access Networks in the 5G Era

As the most important information and communication infrastructure for the digital transformation of the entire society, 5G will enable internet of everything (IoE), connect people, machines, and the environment more closely and efficiently. The convenient, fast, intelligent and reliable ubiquitous communication connection will lead the innovation and evolution of the production model, business model and lifestyle of the whole society.

Compared with 4G, 5G can provide stronger telecommunications services and can be divided into three scenarios. One is the enhanced mobile broadband (eMBB) with a peak rate of up to 10Gbps, and the other is the massive machine-type communication (mMTC) with connections up to 1 million per square kilometer. The third is the ultra-reliable low latency communication (uRLLC) with an end-to-end delay of 1ms, and can generate new applications such as Internet of Vehicles (IoV).

With increased bandwidth, reduced latency, we can see that 5G's stronger support for the IoT, but have also created huge challenges for traditional optical access networks. First, 5G increases the bandwidth of mobile users to 1~10Gbps and reduces the delay to 1~10ms, basically equivalent to fixed optical access performance. Therefore, compared with 5G, the bandwidth and delay advantages of traditional optical reception are lost, and the convenience of mobile network will increase the transfer of more traffic to 5G wireless networks. Second, compared with the IoT carried by an optical access network requires gateways to operate, 5G IoT has a wide coverage, convenient service provisioning, and standardized interfaces, which have simpler operation and maintenance (O&M) and lower cost.

On the other hand, the development of 5G brings new opportunities to the optical access network. First of all, 5G use a separate architecture of AAU and DU, and because of the high frequency introduction, the number of 5G AAUs is huge, which is more than twice that of the 4G era. Therefore, the 5G fronthaul network becomes very important, and fiber resources become the key to deployment. The existing high-density coverage ODN network can quickly access 5G AAU at low cost as needed, providing a wide range of application prospects for WDM-PON and other technologies. Secondly, 5G uses high-frequency signals, has weak penetration through walls, and also has problems of declining bandwidth and unstable access quality at the edge of a 5G wireless network. In contrast, the bandwidth and quality of service (QoS) are unrelated to, is a huge advantage for optical access network.

Operators can consider combining the advantages of 5G access and optical access, complement each other, using the existing huge ODN fiber resources and stable high bandwidth access, and provide users with stable and reliable 5G + FTTH dual gigabit access.

Optical Access Network Evolution Trends and Technology Hotspots

In order to achieve 5G+FTTH dual gigabit access, an optical access network needs to consider the evolution of wireline-wireless convergence, which is specifically reflected in planning and construction, network architecture, and technology roadmap.

From the perspective of planning and construction, in planning optical network coverage and constructing an access office (AO), current service coverage and future expansion need to be considered. Establishing independent integrated access areas is an effective method, that is, according to the access requirements of services, like fixed home broadband, base station transport, and enterprise private line, combined the division of administrative and natural, road network architecture and customer distribution. Each integrated service access area includes a dense-coverage ODN and a converged AO where OLTs, BBUs/DUs and wireline transmission devices are deployed in a unified manner for fixed-mobile convergent access.

From the perspective of network architecture, the converged AO serves as a point of presence (POP) for user access, is an important node to where services are identified and steered to the cloud. The large-capacity converged AO reduces the number of AOs, which meets the requirements of simplified network. Through the establishment of converged AO, unified service models, AO specifications, technology choices and networking solutions, facilitates the evolution to SDN, and introduces AI to achieve smart O&M. This will significantly simplify the construction and operation of the entire optical access network and reduce operators cost.

Technically, in order to meet the user's ultimate experience requirements for new services such as 4K/8K/VR/AR, 10G PON wireline access and 5G wireless access are introduced to achieve a bandwidth of more than 1Gbps per user. Through deploying network functions virtualization infrastructure (NFVI) to the AO and adopting multi-service edge computing (MEC) technology, to meet the real-time low-latency service needs, such as VR, IoV and remote control. 

Focusing on the goal of more effective use of ODN resources, PON technology based on point-to-multipoint (P2MP) architecture presents multiple directions, including WDM-PON for 5G fronthaul and 50G PON for higher bandwidth.

WDM-PON is a P2MP architecture (see Figure 1), which uses independent wavelengths to provide rigid pipes for each user at a rate of up to 25Gbps to meet the requirements of 5G fronthaul. At the same time, WDM-PON matches existing ODN, saves trunk fiber resources, and is suitable for 5G coverage in dense urban areas. It is one of the major technological option for 5G fronthaul. At present, WDM-PON still has the problems of high cost and low reliability of working temperature conditions, which need to be solved through advancing the industry chain.

Figure 1. WDM-PON architecture

ITU-T launched the standardization of 50G PON as the next-generation PON technology in 2018. 50G PON employs the single-wavelength technology, compatible with XG(S) PON or GPON. Through low-latency dynamic bandwidth allocation (DBA) technology, the upstream latency performance is greatly improved, which can not only meet the needs of home broadband, but also can be used for new applications including enterprise private line and 5G base station backhaul. This have greatly expanded the application scope of PON, and are the best technological evolution path for operators full use of existing ODN networks.

Reflections on the construction of optical access networks in the 5G era

The key of the optical access network construction in the 5G era is to develop the converged AO into an intelligent fixed-mobile converged AO, which meet the technical requirements of speed, easy maintenance, flexibility, intelligence, and reliability. In the case of retaining the existing power supply system (including power backup equipment), cooling system, monitoring system, and wiring routes, the internal network of the AO is divided into four major functions (see Figure 2).

Figure 2. Logical architecture of AO in the 5G era

  • Connection function: Using a leaf-spine architecture of the data center, AO can establish a high-bandwidth, scalable, and reliable internal communication network to meet the complex communications and QoS assurance between the DU (wireless), OLT (wireline), uplink transmission equipment, and NFVI.
  • Access function: The DU(wireless) is used for wireless access processing, and the OLT(wireline) is used for wireline access processing.
  • NFVI(computing and storage function): As a remote module of the edge data center (EDC), the NFV services running on it are unifiedly managed by the 5G core network, ensuring fast processing of low-latency, real-time services to improving user experience.
  • Transport function: Provide network-side interface to centrally carry wireline and wireless traffic, and the transport equipment can be OTN, IPRAN or SPN.

In reality, there are a large number of AOs, and there are large differences in hardware conditions and environments. The capital investment and workload of the entire equipment renovation are large. In the specific implementation, the following three principles should be fully considered to implement step by step.

  • Openness: The access function, connection function, NFVI and transport function in the AO should be open. The NFVI is shared by all functions and users in the AO.
  • Scalability: The specific conditions of AOs vary greatly, including the area of AO, power supply system and cooling equipment. The access function, connection function, NFVI and transport function in the AO should be based on the actual service needs and supports smooth expansion by function and capacity.
  • Flexibility: The reconstruction of the AO should support smooth evolution based on the existing AO architecture. On the premise of ensuring the normal operation of the existing services, the relevant function deployment should be flexibly selected according to the conditions of the equipment room.

The optical access network in the 5G era still has great value. Based on the ubiquitous ODN resources, through the construction of integrated access areas, the wireline-wireless access services can be matched with the area of the AOs, and the resource sharing between the AOs and the MEC can be achieved. It also combines the evolving of PON technologies and introducing SDN and NFV. Realize the intelligent reconstruction of the converged AOs, simplify service deployment and O&M.

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