Research on 5G Fronthaul Bearer Scheme Based on CWDM

By Ankit
December 15, 2022

The existing 5G fronthaul adopts the traditional CWDM passive color light technology, which can only deploy 25G optical modules in the first 6 waves (1271nm~1371nm) of the O-band on a large scale to meet the requirements of the 3-channel fronthaul bearer. In order to meet the needs of 5G multi-frequency networking and the co-construction and sharing of China Unicom, operators have also proposed multi-channel solutions such as MWDM, LWDM, and DWDM. However, the above-mentioned solutions require a new development of optical chips and the industry chain is not mature. , Insufficient localization capacity, high prices, and other deficiencies. To this end, the project team led the proposed CWDM/circulator solution, that is, based on the CWDM technology O-band first 6-wave optical module, and introduced a new type of circulator into the passive multiplexer/demultiplexer to achieve simultaneous Wavelength bidirectional transmission, complete 6-channel fronthaul bearer.

CWDM/Circulator Technology Principle

CWDM/circulator technology mainly uses the optical transceiver of the existing CWDM passive color light system and adds a new type of circulator to the multiplexer and demultiplexer of the passive device. The circulator meets the 1271nm ~ 1371nm band Work requirements. With the help of the new all-band circulator, the uplink and downlink directions of the 5G fronthaul system can use the same 6 working wavelengths (1271-1371nm) optical modules.

Take the fronthaul working channels used in pairs of 1271nm and 1331nm as an example. In the upstream direction of the working channel, the 1271nm wavelength optical module on the AAU device sends an optical signal with a working wavelength of 1271nm through the multiplexer and circulator components in the multiplexer and then passes through the backbone optical fiber to reach the circulator in the demultiplexer on the CU/DU side The components of the demultiplexer and demultiplexer are received by the 1331nm optical module on the CU/DU device. When passing through the multiplexer and demultiplexer, the circulator has the feature of directional isolation to avoid conflicts and interference with the working wavelength of 1271nm in the downlink direction. However, after the uplink 1271nm wavelength optical signal encounters an optical reflection point on the trunk fiber, part of the reflected optical signal and the downlink 1271nm wavelength service optical signal will be received by the AAU side optical module at the same time, which will affect the fronthaul system. The principle of the downlink direction of the working channel is the same as that of the uplink direction.

Working Wavelength Pairing Scheme of 5G Fronthaul Equipment

One of the key factors affecting the large-scale deployment of CWDM/circulator equipment is how to pair and deploy the uplink and downlink working wavelengths. At present, there are mainly two working wavelength pairing schemes, which are described in detail as follows:

Same Wavelength Pairing Scheme

The first 3 channels of the CWDM/circulator use 3 wavelength optical modules of 1271nm, 1291nm, and 1311nm, and the last 3 channels use 3 wavelength optical modules of 1331nm, 1351nm, and 1371nm, and the uplink and downlink directions of each working channel use the same working wavelength.

Different Wavelength Pairing Scheme

The first 3 channels and the last 3 channels of the CWDM/circulator all use 6-wavelength optical modules, of which the first 3 channels of AAU equipment using 1271nm, 1291nm, and 1311nm 3-wavelength optical modules, and CU/DU equipment uses 1331nm, 1351nm, and 1371nm 3-wavelength optical modules module; the working wavelength of the optical module used by the latter 3-channel AAU equipment and CU/DU equipment is just the opposite.

The above two working wavelength pairing schemes have their own advantages and disadvantages. Comparing the above two schemes comprehensively, it is recommended to adopt the different wavelength-matching schemes. On the one hand, the traditional CWDM passive color light deployment method is used, and there is no need to redefine the CWDM passive color light equipment. Training for installation and maintenance personnel; on the other hand, is also conducive to smooth upgrades and continuous business in the future.

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