Explanation
According to the 5G EN-DC Architecture and Interfaces, there are two methods that can be used to support X2 interconnection between an eNodeB and a gNodeB in NSA networking:
Interconnection through CI interfaces on the UMPT boards in different subracks: The UMPT is a universal main processing and transmission unit that provides main processing and transmission functions for a subrack. The UMPT has two CI interfaces that can be used for X2 interconnection with other base stations. The CI interfaces support CPRI, OBSAI, and eCPRI protocols and can use optical fibers or electrical cables for transmission. The CI interfaces can be configured as master or slave interfaces depending on the network topology.
IPRAN interconnection between subracks: IPRAN is an IP-based radio access network that uses IP transport technologies to carry base station traffic over Ethernet or MPLS networks. IPRAN can support X2 interconnection betweendifferent base stations by using IP routers or switches. IPRAN can provide flexible networking, high bandwidth, low latency, and high reliability for X2 interconnection.
The other two methods are not valid for X2 interconnection between an eNodeB and a gNodeB:
Interconnection via backplane in a subrack: This method is only applicable for X2 interconnection between two eNodeBs or two gNodeBs that are installed in the same subrack. The backplane is the internal bus that connects different boards within a subrack. The backplane supports CPRI, OBSAI, and eCPRI protocols and can provide high-speed data transmission between boards.
Interconnection through HEI interfaces on the UBBP boards in different subracks: This method is not supported by Huawei base stations. The HEI interface is a high-speed Ethernet interface that is used for data transmission between base stations and core networks or transport networks. The HEI interface supports S1, Xn, F1, E1, and N2 protocols and can use optical fibers or electrical cables for transmission. The HEI interface cannot be used for X2 interconnection between base stations.
Therefore, the correct answer is A and D.