5G has arrived. Today we discuss several solutions for 5G fronthaul, and analyze their advantages and disadvantages, as well as application scenario analysis, to provide a reference for the construction of 5G fronthaul networks in the later period.

1, Background

China has started a pilot test of 5G live networks. The architecture of 5G has changed significantly compared to 4G, as shown in Figure 1:

Comparison diagram of 4G and 5G architecture comnen

Comparison diagram of 4G and 5G architecture comnen

2, Several solutions of fronthaul

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

2.1, Solution 1: Fiber Optic Direct Drive

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

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

Combined with the actual network situation, in most application scenarios, the existing network optical fiber resources cannot meet the requirements of 5G fronthaul optical fiber resources, and new optical cables need to be built. When building new optical cables, a large core number ring network architecture (similar to the backbone optical cable network in the integrated service access area) should be used, and You can select 144-288 core optical cables according to the number of AAUs planned to be connected, 12-24 core optical fibers at each node terminal, and connect to the convergence computer room through contact optical cables. This deployment method can avoid the concentration of a large number of small-core-count optical cables in the BBU/DU equipment room, which causes great pressure on the pipeline; this method requires that the deployment node be predicted in advance and large-core-count optical cable deployment.
However, in the actual network construction, the construction of AAU is carried out gradually. There is still some uncertainty in the ring networking of large-core optical cables. Star-shaped optical cable connections can also be used. To avoid the impact on the outlet pipes of the computer room, you can Deploy the large-core-count optical cable at the exit of the /DU equipment room, and set the fiber distribution point near the equipment room, divide the large-core optical cable into multiple small-core optical cables, and then connect to each AAU as needed.

2.2, Solution 2: WDM/OTN

WDM/OTN can also be used when existing optical fiber resources are limited and it is difficult to build new optical cables.

(1) WDM bearing solution

The WDM technology is very mature, adopting passive combining and demultiplexing + color light direct drive solution, the optical modules on the BBU/DU and AAU respectively use color light modules with wavelengths, and the optical combining and demultiplexing devices OMD and AAU are arranged at the front end of the BBU/DU The node is equipped with an optical add-drop multiplexer OAD and adopts WDM technology, which can greatly save the consumption of optical fiber resources. WDM equipment adopts pure transparent transmission processing for fronthaul services, so its impact on delay characteristics is extremely limited. The disadvantage is that the colored light optical module will put forward new requirements for AAU base station management, signal multiplexing technology cannot be used to improve wavelength utilization, and the OAM management function of the service 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 color light direct drive. CCSA is currently discussing the formulation of “Technical Requirements for Metropolitan Access Wavelength Division Multiplexing (WDM) Systems” to make it more suitable for wireless base station fronthaul requirements. COMNEN can provide different solutions such as DWDM/CWDM to meet your 5G fronthaul requirements. If you have WDM requirements, please kindly contact us (Email: sales@comnen.com)
This solution requires that BBU/DU and AAU plan the colored light wavelength well, so it is relatively complicated in actual deployment and maintenance management.

(2) OTN bearing solution

To reduce the wavelength planning between AAU and BBU/DU, it can also be carried by OTN. Connected to the client side of OTN equipment, mapped and multiplexed into high-speed OTN signals and converted into color light interface, and then transmitted 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 the wavelength utilization rate, 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 need colored light. The module avoids the complexity of wavelength allocation and management of wireless devices. The networking is relatively flexible, and can support a variety of 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 the preamble scenario, the vast majority of applications are in the inorganic room. Miniaturized OTN equipment needs to use industrial-grade devices to increase temperature control capabilities. Consider more severe environmental applications and complex installation conditions; on the other hand, traditional OTN equipment costs are higher. If deployed on a large scale, OTN equipment is used in a large amount, and related functions are implemented by ASIC, which can greatly reduce equipment costs.

2.3 Solution 3: WDM-PON

The PON star network structure meets the 5G fronthaul requirements and is a more suitable solution. COMNEN can provide different solutions such as DWDM/CWDM to meet your 5G fronthaul requirements. When meeting 5G fronthaul, WDM PON needs 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 LTE base stations of China Mobile also use the remote method, the initial 5G construction is basically the same site as LTE, so there are multiple standard base station co-site methods. Under the mouth, different wavelengths can correspond to different transmission rates to meet the requirements of different wireless standard base stations.

(2) The transmission distance is 10~40km. The general transmission distance of CPRI interface is not less than 10 kilometers. According to the actual network situation of China Mobile and the concentration of BBU/DU, the transmission distance of optical cable between urban BBU/DU-AAU 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 <200us, frequency jitter <0.002ppm, frequency synchronization ±0.05ppm, time synchronization <±8.138ns. WDM-PON equipment should have a perfect synchronization interface and support time and frequency synchronization functions.

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

In short, WDM PON provides rich bandwidth, small delay and good security. It can well meet the bandwidth requirements of 5G base station fronthaul and can be used as one of the main technical options for future fronthaul. If you have WDM requirements, please kindly contact us (Email: sales@comnen.com)

2.4 Solution 4: IP+Optical

The IP+optical scheme can also be used in the fronthaul network. Its large bandwidth and low latency characteristics can meet the 5G base station fronthaul requirements well:

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

In terms of service delay guarantee: for the low-latency fronthaul requirements, IP+Optical can adopt exclusive low-latency solutions at the IP layer and the optical layer, respectively. At the IP layer, device-level ultra-low latency forwarding technology is used. Within the device forwarding chip, the preamble of the Ethernet frame divides the message into ordinary and accelerated types. For accelerated messages, preemption of ordinary message resources and Cut- Through the forwarding method, the node’s electrical layer processing delay can be reduced from tens of milliseconds to several milliseconds. In the optical layer, the optical layer penetration method is used, and only the service wavelengths that need to be accessed or landed are processed at the electrical layer, and other wavelengths are directly penetrated in the optical layer to achieve direct service in one hop.

The IP+optical solution can well meet the requirements of bandwidth and delay, but the technical aspects of the solution still need to be further promoted and researched 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 Solutions comparison

Optical fiber direct drive, WDM/OTN, WDM-PON, IP+optical and other technical solutions have their own advantages and disadvantages. China Mobile must combine existing actual network conditions and future business development needs, network deployment planning and other aspects, so comprehensive consideration:

From the perspective of meeting the needs of fronthaul, fiber optic direct drive, WDM/OTN, and WDM-PON solutions are more suitable to provide the ability to carry rigid pipelines, and IP+optical can be well adapted to some 5G fronthaul scenarios due to their specific grouping and optical capabilities; On the other hand, in terms of delay satisfaction, the above-mentioned solutions can meet the requirements of delay well, but the slight difference in the delay introduced by the optical device will cause the transmission distance to be not exactly the same, but overall Say it’s all at a quantitative level.

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

From the perspective of optical fiber resource consumption, direct-drive optical fiber consumes the most resources, and other solutions are equivalent. However, how to deploy it needs to combine the existing network optical fiber and system construction, for example: for the area where the PON network is deployed on a large scale, the WDM-PON is also a good choice for the rational use of the resources of the existing network for transformation; In 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, fiber optic direct drive and WDM fronthaul management 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. With the help of corresponding byte overhead, OSC, ESC and other means, the solution has stronger capabilities in network management, and is more convenient in fault management, performance management, security management, configuration management, maintenance management, and system management. For example, as shown in Table 1.

Table 1 Comparison table of Fronthaul Solutions

Fronthaul Solutions Delay Network flexibility Fiber consumption Management ability Technology maturity Investment
Fiber Optic Direct Drive Satisfy Point-to-point, star many weak mature Lower
WDM-PON Satisfy Point-to-point, star many Strong mature general
WDM Satisfy Point-to-point, star Moderate weak mature Lower
OTN Satisfy Point to point, star, ring Moderate Strong mature high
IP+Optical Satisfy Point to point, star, ring Moderate Strong mature high

In short, the above solutions have their own advantages and disadvantages, and the applicable scenarios are also different; from the current stage, fiber optic direct drive is more suitable; but with the scale of deployment, WDM, OTN solutions also have certain advantages; with WDM-PON technology Mature, adopting WDM-PON is a good choice. The actual situation of each mobile operator’s existing network is diverse. When deploying the actual network, it is appropriate to select the appropriate technology and solution according to the actual scenario to achieve cost-effective, operation and maintenance management and other suitable solutions to meet various fronthaul bearer requirements.