I. Introduction
The sixth-generation (6G) wireless communication network is expected to provide global coverage with improved spectrum, energy, cost efficiency, intelligence, and security [1]. ITU-T’s report [2] identifies the following three driving characteristics that are associated with the lifestyle and social changes of the next decade: 1) high-fidelity holographic society; 2) interconnectedness of everything; 3) engineering applications sensitive to latency. Thus, DENs provide a strong impetus to meet the requirements of future wireless communication networks, such as reinforcement learning [3].