China Mobile 5G network construction, introduction of fronthaul bearer solutions

5G is ready to go, China Mobile's 5G mid-backhaul solution has been basically determined, but the 5G fronthaul bearer solution has not been studied in depth. To this end, several solutions are proposed for the fronthaul, and their advantages and disadvantages and application scenarios are analyzed. The analysis will provide a reference for the later 5G fronthaul network construction.

1 background

5G is ready to go, and China Mobile has also started a pilot test of 5G on the existing network. Compared with 4G, the architecture of 5G has a big change, as shown in Figure 1:

China Mobile 5G network construction, introduction of fronthaul bearer solutions

Figure 1 Comparison of 4G and 5G structures

Compared with 4G, the network structure in the 5G era has been adjusted to four parts: AAU-DU-CU-core network. The corresponding bearer is also divided into three segments: AAU-DU is called fronthaul, and DU-CU is called middle. Transmission, CU-core network is called backhaul. For the mid-backhaul paragraph, China Mobile has basically determined its carrying plan, but the fronthaul solution directly affects the planning and construction of future computer rooms, power supplies, pipelines, etc. Therefore, it is necessary to conduct research and discussion on the fronthaul program before 5G is officially commercialized. .

2 Several solutions for the fronthaul

The currently recognized interface types for 5G fronthaul include CPRI interface and eCPRI interface. The CPRI interface rate is 100GE and the eCPRI interface rate is 25GE. There are several solutions:

2.1 Scheme 1: Optical fiber direct drive

In the fronthaul, the transmission distance of the CPRI and eCPRI interfaces is generally controlled within 10 kilometers, so point-to-point optical fiber direct connections can be used between the BBU/DU and each AAU port.

The solution is simple and easy to implement, and can meet the requirements of fronthaul carrying. According to research, at present, each wireless equipment manufacturer needs 2-12 cores for a 5G base station fronthaul. Considering 10-20 AAUs for a BBU/DU, it will consume a lot of fiber resources. The BBU/DU side fiber management requirements are high, and the export pipeline , Optical cable resources have become the bottleneck of this scheme. From the perspective of technical solutions, AAU cascading can also be used to reduce the consumption of optical fiber resources. At the same time, the wireless side equipment can complete the protection, OAM and network management of the optical fiber direct drive line through the fronthaul signal itself. Therefore, the feature of this solution is that the deployment cost is relatively low, but it is limited by the end fiber resources, and is suitable for scenarios with abundant fiber resources and small-scale concentration of BBU/DU.

Combined with the actual network situation of China Mobile, in most application scenarios, the existing network fiber resources cannot meet the requirements of 5G fronthaul fiber resources, and new optical cables need to be built. New optical cables should adopt a large core number ring network architecture (similar to the backbone optical cable network in the integrated service access area. ), as shown in Figure 2, select 144-288 core optical cables according to the planned number of AAUs, 12-24 core optical fibers at each node terminal, and connect to the convergence room through the contact optical cable. Using this deployment method can avoid the concentration of a large number of small-core fiber optic cables in the BBU/DU machine room, which will cause great pressure on the pipeline; this method requires that the deployment nodes be predicted in advance and the large-core fiber optic cable deployment is carried out.

China Mobile 5G network construction, introduction of fronthaul bearer solutions

Figure 2 Schematic diagram of the construction plan of the fronthaul optical cable network

However, in the actual network construction, the construction of AAU is gradually carried out. There is still some uncertainty in the use of large-core number optical cable ring networking. Star-shaped optical cable connection can also be used. In order to avoid the impact on the outlet pipeline of the computer room, the BBU Deploy large-core fiber optic cables at the exit of the DU computer room, and set up optical fiber splitting points near the computer room to divide the large-core fiber optic cable into multiple small-core fiber optic cables, and then connect them to each AAU as needed.

2.2 Scheme 2: WDM/OTN

When the existing optical fiber resources are limited and the new optical cable is difficult, the WDM/OTN solution can also be used.

(1) WDM bearer scheme

WDM technology has been very mature, adopting passive multiplexing and demultiplexing + color optical direct drive solution, the optical modules on BBU/DU and AAU use color light modules with wavelength respectively, and optical multiplexing/demultiplexing OMD and AAU are configured at the front end of BBU/DU. The node is equipped with optical add/drop multiplexer OAD and adopts WDM technology, which can greatly save the consumption of optical fiber resources. WDM equipment uses pure transparent transmission processing for fronthaul services, so the impact on delay characteristics is extremely limited. The disadvantage is that the color light optical module puts forward new requirements for AAU base station management, and cannot use signal multiplexing technology to improve wavelength utilization, and the business OAM management function is limited. In addition, each BBU/DU and AAU wavelength connection is physically a point-to-point connection, so the power budget is a key issue that needs to be considered for the direct drive of the color light. At present, CCSA is discussing and formulating "Metro-Access Wavelength Division Multiplexing (WDM) System Technical Requirements" to make it more suitable for wireless base station fronthaul requirements.

This solution requires a good plan for the color light wavelength between the BBU/DU and AAU, so it is relatively complicated in actual deployment and maintenance management.

(2) OTN bearer solution

To reduce the wavelength planning between AAU and BBU/DU, it can also be carried in OTN mode. Connect to the client side of OTN equipment, map and multiplex into high-speed OTN signals and convert them into color optical interfaces, and transmit them in one or a pair of optical fibers after wavelength division multiplexing, which greatly saves optical fiber resources.

The solution uses OTN technology for signal multiplexing, which can also improve wavelength utilization, and use OTN overhead bytes to provide richer OAM functions and fault diagnosis capabilities, and can support network protection. At the same time, AAU and BBU/DU devices do not require color light. The module avoids the complexity of wavelength allocation and management for wireless devices. The networking is more flexible, and it can support multiple network structures such as ring, tree, and MESH.

However, the use of OTN bearer mode needs to consider its shortcomings: OTN equipment is active. In fronthaul scenarios, most of them are applications with no machine room. Miniaturized OTN equipment needs to use industrial-grade devices to increase temperature control capabilities. , Considering more severe environmental applications and complex installation conditions; on the other hand, the cost of traditional OTN equipment is relatively high. If deployed on a large scale, the amount of OTN equipment is huge, and related functions are realized by ASIC, which can greatly reduce equipment costs.

With business needs such as large-grain customer collection and home broadband connection, China Mobile OTN is gradually sinking to the edge. At this time, sharing OTN can further dilute costs and adapt to the development of large business volumes in the future.

2.3 Scheme 3: WDM-PON

The PON star network structure meets the requirements of 5G fronthaul and is a more suitable solution. However, because the WDM-PON technology is not mature enough, the current use of PON for fronthaul is a relatively small way to consider. When meeting 5G fronthaul, WDM PON Need to have the following characteristics:

(1) The line rate is 0.6~10GBps, a single PON port supports multiple rates, and the user side supports multiple interface types. Since some of China Mobile's LTE base stations also use the remote method, the initial stage of 5G construction is basically the same site as LTE, so there are multiple standard base station co-site methods. At this time, WDM-PON is required to have the capability of multi-rate bearer. Under the mouth, different wavelengths can correspond to different transmission rates to meet the bearer requirements of base stations of different wireless standards.

(2) The transmission distance is 10~40km. The CPRI interface generally requires a transmission distance of not less than 10 kilometers. According to China Mobile's actual network situation and the scale of BBU/DU concentration, the transmission distance of optical cable between BBU/DU-AAU in urban areas is generally within 4 kilometers; the transmission distance of optical cable between BBU/DU-AAU in rural areas is generally within 10 kilometers.

(3) Transmission delay

(4) In addition, the WDM PON system is used for mobile fronthaul, and needs to have forwarding functions, error correction functions, framing functions, protection functions, and optical link diagnostic functions.

In short, WDM PON provides abundant bandwidth, small delay and good security, which can well meet the bandwidth requirements of 5G base station fronthaul, and can be used as one of the main technology options for fronthaul in the future. However, the current industry WDM-PON still lacks mature commercial technology, and the device cost is high. Large-scale applications need to reduce system costs and colorless ONU technology.

2.4 Scheme 4: IP+Optical

The IP+ optical solution can also be used in the fronthaul network. Its large bandwidth and low delay characteristics can well meet the fronthaul requirements of 5G base stations:

In terms of service bandwidth capability: The IP+ optical solution provides a flexible bearer solution for 5G base station fronthaul. On the one hand, it uses the characteristics of IP to adapt to various bandwidth demand scenarios. IP packet technology makes the bandwidth utilization before BBU/DU and AAU higher. The resources of the fronthaul network are used efficiently; on the other hand, the large bandwidth provided by the optical layer rigid pipe is used to meet the large bandwidth requirements of the fronthaul.

In terms of service delay guarantee: In response to low-latency fronthaul requirements, IP+Optics can use exclusive low-latency solutions at the IP layer and the optical layer respectively. The device-level ultra-low-latency forwarding technology is adopted at the IP layer. In the device forwarding chip, the message is divided into two types: normal and accelerated by the preamble of the Ethernet frame, and the accelerated message is used to preempt common message resources and Cut- Through forwarding mode, the electrical layer processing delay of the node can be reduced from tens of milliseconds to several milliseconds. In the optical layer, the optical layer pass-through method is adopted, and only the service wavelengths that need to be accessed or landed are processed by the electrical layer, and other wavelengths are directly passed through at the optical layer to achieve one-hop direct service.

The IP+optical solution can well meet the bandwidth and delay requirements, but the technical aspects of the solution still need to be further promoted and developed. There is still no real commercial product, but it provides a new technology for China Mobile's 5G fronthaul deployment. plan selection.

2.5 Scheme comparison

Various technical solutions such as optical fiber direct drive, WDM/OTN, WDM-PON, and IP+ optical have their own advantages and disadvantages. China Mobile must consider the current actual network conditions and future business development needs, network deployment planning and other aspects. Therefore, comprehensive considerations:

From the perspective of meeting the requirements of fronthaul, optical fiber direct drive, WDM/OTN, and WDM-PON solutions are more suitable for providing the ability to carry rigid pipes, while IP+ optical can be well adapted to some scenarios of 5G fronthaul due to its specific packet and optical capabilities; On the other hand, in terms of the degree of delay satisfaction, the above-mentioned solutions can meet the delay requirements very well, but the slight difference in the delay introduced by the optical device will cause the transmission distance to be not completely the same, but in general Said they are all in a quantity level.

From the perspective of networking flexibility, the optical fiber direct drive and WDM-PON solutions are suitable for point-to-point networking and chain networking, while the WDM/OTN and IP+ optical solutions are suitable for point-to-point networking, chain networking and ring networking. Various networking methods such as networking, support single-fiber bidirectional and dual-fiber bidirectional transmission at the same time, to meet the needs of various networking methods of wireless networks. At the same time, 1+1 protection on the line side can be realized in ring networking, which improves service security.

In terms of fiber resource consumption, direct drive fiber consumes the most resources, and other solutions are comparable. However, how to deploy needs to be combined with the existing network optical fiber and system construction. For example, for areas where PON networks are deployed on a large scale, WDM-PON is also a good choice to make reasonable use of existing network resources for transformation; and for large-scale deployments For the area of ​​LTE base station fronthaul, it is a more sensible choice to carry out 4G/5G fronthaul planning in combination with the current fronthaul solution.

From the perspective of fronthaul network management, the fronthaul management of fiber direct drive and WDM can only rely on the AAU and BBU/DU units themselves, and related fault detection can only rely on the limited monitoring and management fields of AAU and BBU/DU, and other types With the help of corresponding byte overhead, OSC, ESC and other means, the solution has stronger network management capabilities, and is more convenient in terms of fault management, performance management, security management, configuration management, maintenance management, and system management. The comparison is shown in Table 1.

Table 1 Comparison table of fronthaul solutions

China Mobile 5G network construction, introduction of fronthaul bearer solutions

In short, the above solutions have their own advantages and disadvantages, and the applicable scenarios are also different; from the current stage, optical fiber direct drive is more suitable; but with large-scale deployment, WDM and OTN solutions also have certain advantages; with WDM-PON technology Mature, adopting WDM-PON is also a good solution choice. China Mobile's current network has various actual conditions. In actual network deployment, it is advisable to select appropriate technologies and solutions according to actual scenarios to achieve cost-effective, operation and maintenance management and other appropriate solutions to meet various fronthaul bearer requirements.

3 summary

China Mobile's 5G network is about to be built. How to reasonably plan and deploy fronthaul networks is also an important part of 5G network construction. The above solutions are only briefly discussed based on the current technology and network conditions, and are only for your reference. With the advancement of technology in the future And the evolution of the network, the fronthaul scheme may have a better solution.

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