The general principle of 5G fronthaul is based on direct optical fiber connection. When optical fiber resources are insufficient and CU/DU are deployed in a centralized manner, a basic WDM technology equipment bearing solution can be adopted. The specific implementation forms mainly include: passive WDM, active WDM/OTN and There are 3 types of WDM-PON.
The CU/DU and the ports of each AAU are all directly connected by point-to-point optical fibers. This solution is simple and easy to implement, but is limited by the fiber resources at the end. 10G/25G grey light optical modules (click here) are usually adopted in this solution. It is suitable for scenarios with abundant fiber resources and small-scale centralized CU/DU.
In this solution, the color light module is installed on the AAU/RRU and CU/DU equipment on the wireless side, and the WDM function is completed through an external passive combination/demultiplexer device, and the fronthaul signal of multiple wavelengths is transmitted by a single backbone fiber. At present, the coarse wavelength division technology is mainly used to realize the 10G/25G fronthaul bearing, and there are also dense wavelength division technologies such as more than 24 waves. Deploy a combiner to aggregate the multi-channel wavelength signals of a group of color light modules on the CU/DU device and transmit them through a trunk access fiber, and deploy a demultiplexer on the remote side to separate the multi-channel wavelength signals on the trunk fiber. , respectively access the wireless network element (AAU or RHUB). When the coarse wavelength division technology is used to realize the passive color light fronthaul bearing, 6-wave or 12-wave CWDM is usually used.
6-wave CWDM: realizes the fronthaul bearing of 3 AAU/RRU, and the uplink/downlink each occupies 3 wavelengths;
12-wave CWDM: realizes the fronthaul bearing of 6 AAUs/RRUs, and the uplink/downlink each occupies 6 wavelengths.
Active WDM solution: This solution connects AAU/RRU and CU/DU to OTN equipment, realizes fronthaul bearing through wavelength division multiplexing and OTN technology, transmits multi-wavelength fronthaul signals through a single fiber, and uses OTN overhead Implement maintenance management.
When the active WDM solution is adopted, traditional OTN equipment can be used for service bearer, but in order to reduce the construction cost of fronthaul bearer equipment, the industry has proposed the use of turbo frequency technology, that is, the use of 10G rate optical chips through DMT (Discrete Multi-tone) Algorithm to overclock ordinary 10G optical chips to 50G speed. At the same time, with the help of reasonable compression technology, the access rate of 12 channels of 10G can be realized through 1 pair of 100G optical modules. The 100G optical module consists of 2 channels of 10G optical chips overclocked to 50G rate.
This solution follows the FTTx point-to-multipoint networking topology, inserts the modular ONU into the AAU/RRU or the room division RHUB, connects the CU/DU to the central office OLT equipment, and completes the WDM function through passive AAWG devices (multiple The fronthaul signal of the channel wavelength is transmitted by a single fiber). At present, the ONU adopts a colorless dimmable optical module, which supports 12-channel/20-channel (24-wave/40-wave) 10G/25G rate fronthaul bearer.
The 5G room division fronthaul bearer adopts WDM-PON technology and has the following four characteristics.
?Fixed-mobile integration: recently based on 10G-EPONOLT common platform. In the long run, OLT network slicing technology can also be used to realize the full service bearing of fixed-mobile convergence;
?Modular ONU: Plug and play on AAU/RHUB, free of mains introduction and installation; colorless tunable technology automatic wavelength adaptation, engineering-free manual wavelength pairing, to achieve rapid installation and opening;
?Passive AAWG: 20-channel (40-wave) general-purpose passive device, support 12-channel/20-channel, compatible with macro station AAU and room RHUB connection, deployment position similar to FTTH optical splitter, which is convenient for maintenance and management;
?Smooth upgrade: By replacing the ONU module and the WDM-PON board at the central office (same as the OLT frame), the old passive AAWG devices are utilized to achieve a smooth upgrade from 10G to 25G.
In view of the above-mentioned three fronthaul equipment carrying technologies and optical fiber direct connection technologies, an in-depth analysis is made on the demand for optical modules, backbone fibers, passive optical components, and active equipment in the carrying network, as well as the industrial chain level. Comparing the operation and maintenance process, each scheme has its own advantages and disadvantages.
Passive WDM, active WDM and WDM-PON, these three equipment bearing technologies can save a lot of access backbone optical fiber resources in the fronthaul network. Among them, the passive WDM bearing solution technology and industry chain based on coarse wavelength division technology are relatively mature, but The multi-channel fronthaul bearer puts forward higher requirements for procurement, construction and maintenance; the active WDM bearer solution adopts turbo frequency and compression technology, and the equipment accesses multiple wireless devices through 100G, thereby reducing the cost of each channel. However, a small OTN device needs to be installed on the remote side, which requires power supply and generates a large power consumption; while the WDM-PON bearer solution can use the existing FTTH architecture. When the ONU is miniaturized by using the SFP ONU optical module, it can Realize plug and play, rapid deployment and opening. The current industry chain of colorless tunable technology is not yet mature, so the price of tunable light modules is relatively high.
To sum up, under different RAN architectures and in different application scenarios, the construction of 5G fronthaul networks will adopt different deployment methods and bearer technologies. In the 5G indoor scenario, when the CRAN architecture is adopted, equipment carrying technologies such as active WDM and WDM-PON will effectively save fiber resources and reduce the overall cost of the fronthaul network. However, due to the differences in existing infrastructure resources of operators, the construction of 5G fronthaul network will involve massive optical fibers and computer rooms. For this reason, the current problems faced by operators are not only the construction of 5G wireless equipment and 5G core network, but also how to effectively utilize the existing infrastructure resources such as optical cable networks and equipment rooms that have been deployed on a large scale to serve the planning and development of 5G fronthaul networks. Construction and operation are also a key and important topic. Facing the above requirements and challenges, China Telecom is continuing to promote the research, testing and experimentation of 5G fronthaul network bearer technology from multiple dimensions such as 5G fronthaul construction scenarios, technical requirements and network planning.