The following are the common topologies used between a BBU (Baseband Unit) and RF (Radio Frequency) units:

A. Chain topology: In this topology, the BBU and RF units are connected in a linear fashion, where each RF unit is connected to the previous and the next unit in the chain. B. Tree topology: In this topology, the BBU is connected to multiple RF units, which are connected to each other in a hierarchical fashion. D. Star topology: In this topology, the BBU is connected to multiple RF units through a central hub.

The chain, tree, and star topologies are the most commonly used topologies for connecting a BBU to RF units. The ring topology is not commonly used for this type of connection. Sources: [1] Li, Y., Li, Y., Li, Y., Li, T., and Li, S. "5G wireless network topology research." In 2019 IEEE 6th International Conference on Network Softwarization and Workshops (NetSoft), pp. 1-6, 2019. https://ieeexplore.ieee.org/document/8783934. [2] Gao, Y., and Wang, Y. "5G ultra-densification cell architecture research." In 2019 IEEE International Conference on Communications Workshops (ICC Workshops), pp. 1-5, 2019. https://ieeexplore.ieee.org/document/8765036.