Application of CWDM Technology in 5G Base Station Construction

Operators can currently build 5G base stations in two modes, D-RAN and C-RAN, for deployment. In order to realize the function of resource pool sharing and flexible scheduling, communication operators usually choose C-RAN mode to deploy 5G base stations. 5G BBU devices are located in several aggregation layer rooms, and a large amount of fiber optic cable resources are required for the Fronthaul system from the base station AAU (Active Antenna Processing Unit) to the aggregation layer BBU (baseband unit) devices. The choice of a 5G fronthaul technology solution will directly affect the network construction investment and the efficiency of the base station launch. The development and application of Coarse Wavelength Division Multiplexing (CWDM) technology can significantly alleviate the problem of tight optical cable resources in 5G fronthaul systems.

Introduction of 5G Transmission Network Construction Solution

Taking the STN solution as an example, the 5G bearer network adopts a converged bearer, 5G regional integrated business services access to the original 4G base station in the area, gradually cut to the new equipment bearing. Old equipment is transferred to non-5G areas for use (preferentially remote areas). The deployment of C-RAN can improve the bearer efficiency. Promote centralization of BBU, deploy the newly built new A equipment in the access point computer room, and fully adopt the new ER/B/A equipment. New A equipment must be 100% looped. In the initial stage, use 10GbE link networking, and in the later stage, the capacity is expanded according to the actual traffic. After the upstream peak traffic is ≥60G, 50GbE and 100GbE links are used.

The 5G bearer network generally follows the existing 4G bearer network architecture, and supports the perfect two- and three-layers of flexible networking architecture. IPRAN still adopts a three-layers networking structure: core layer (ER device layer), convergence layer (B device layer), and access layer. The core layer is further subdivided into the provincial ER layer, metro ER layer, and convergence ER layer. Currently, communication operators mainly use D-RAN (Distributed Radio Access Network) and C-RAN (Centralized Radio Access Network) schemes to deploy 5G construction, as shown in Figure 1. 

Among them, the C-RAN deployment scheme is the main 5G base station deployment scheme adopted by communication operators, where AAU devices are distributed in each base station and BBU devices are centrally deployed in the aggregation office, and BBU devices realize 5G core network traffic backhaul through A2 devices. and the eCPRI interface of passive Coarse Wavelength Division Multiplexing (CWDM) is used between AAU Radio Frequency devices and BBU devices to provide a 25Gbps fronthaul rate.

Passive WDM Equipment Network Application Solution

In the C-RAN deployment scheme, 5G site backhaul access STN network, and BBU equipment need to be accessed through A equipment and then connected to B equipment. Equipment A is generally deployed in the same room as BBU. According to the bandwidth requirements of the base station, the AAU equipment fronthaul of the 5G site can flexibly choose the following two schemes for the construction of the transmission bearer network, so as to ensure the 5G construction requirements. 

The first solution is to build a new incoming optical cable, using single-fiber bi-directional optical modules or dual-fiber bi-directional optical modules. The second solution is to use a passive wavelength division multiplexing solution based on CWDM to save optical cables. In general, 5G BBU is mainly deployed in a centralized manner, and distributed deployment can be considered for cases where there are insufficient resources such as optical cables or other factors that are restricted. The access layer A equipment is deployed in the access point computer room in the C-RAN mode, and the initial BBU and A equipment are mainly connected to each other using 10GbE optical ports. 5G BBU is concentrated in the integrated service access point and BBU concentration point, and the common sharing mode is as follows

(1) At 3.5G NR 100MHz carrier bandwidth, the fronthaul interface between 5G BBU and single AAU requires 1x25GE eCPRI interface and does not support cascading.

(2) For 3.5G NR 200MHz bandwidth, 2x25GE eCPRI interface is required for the fronthaul interface, and cascading is not supported.

(3) 2.1G NR 50MHz bandwidth requires a 1x10GE CPRI interface for the fronthaul interface and supports up to two levels of cascades.

5G AAU fronthaul does not support cascading, and the fronthaul delay requirement is less than 100us. The 5G site backhaul access IP RAN network, BBU equipment needs to be accessed through A equipment and then connected to B equipment. The A equipment is generally deployed with BBU in the same room.  The 5G site fronthaul will select the following two schemes to construct the bearer network according to the bandwidth requirements of the base station to meet the 5G construction requirements

（1） Build a new incoming optical cable, using single-fiber bi-directional optical modules or dual-fiber bi-directional optical modules

  (2）Use a passive wavelength division solution based on CWDM to save optical cables

When operators build 5G fronthaul networks on a large scale, in order to save fiber resources, a large number of coarse wavelength division multiplexing passive color light equipment are deployed to realize 5G fronthaul 3-channel bearing. At present, the mature wavelength division solutions mainly include passive WDM solutions based on CWDM, which use 6-wave CWDM devices in pairs to realize the transmission and access of AAU devices and BBU devices.

The optical module uses a colored optical module. Dual-core optical modules and fiber core requirements, as shown in Table 1. When using a 6-wave passive wavelength division system, a single set of 6-wave passive WDM systems supports 3 AAU/RRU equipment, according to 2.1G DSS + NR 3.5G (200M) S111 BBU common frame consideration, a single BBU requires 3 sets of 6 waves passive wavelength division system.

Optical fiber layout requirements for CWDM equipment at the access central office: attention should be paid to the bending radius of the interface optical fiber, otherwise damage will occur, resulting in excessive fiber loss and affecting transmission. The radius of curvature at the bend of the fiber optic cable shall not be less than 38-40mm.

The optical fiber connection cable should be protected by a sleeve or a wire groove on the cable tray. The part without casing protection should be tied with a flexible buckle, and the tie should not be tied too tightly. The braided fiber optic cable should be straight on the cable tray without obvious twisting. All patch fibers are labeled according to the specifications, which is conducive to the maintenance of the base station and ensures the safe operation of the base station equipment.

Conclusion

With the rapid growth of 5G network traffic, the demand for 6-channel 25 Gbit/s fronthaul bearers is becoming increasingly evident. CWDM technology uses the same fiber to simultaneously allow two or more optical wavelength signals to transmit information through different optical channels each, multiplexing the tight spectral spacing of individual fibers in order to achieve the best transmission performance with minimal dispersion or attenuation, which reduces the total number of fibers required at the specified information transmission capacity.

CWDM system can significantly improve the transmission capacity of optical fiber, increase the utilization of fiber resources, realize high-quality and stable access to the fronthaul system between AAU equipment of 5G base stations and BBU equipment of access office, provide efficient 5G fronthaul transmission solutions for C-RAN deployment of 5G networks, and save a large amount of access fiber and operation costs. QSFPTEK can provide you with professional telecom solutions with our extensive experience. You can also get the optical module and optical fiber cables in QSFPTEK, welcome to consult via sales@qsfptek.com.