I. Introduction
Driven by ultra-high traffic density in 5G, ultra-dense networks (UDN) has garnered extensive attention in recent years. As shown in Fig. 1, By densely deploying numerous small base stations (SBSs), UDN obtains a huge gain of frequency reuse in spatial domain. However, the overly dense SBSs lead to high inter-cell interference which se-verely hinders the network performance improvement. As a consequence, interference management (1M) in allusion to interference-limited UDN has been widely researched and interference avoidance is regarded as the most direct method [1]. From this point, by dividing SBSs into some clusters and applying frequency division multiplexing in each cluster and frequency reuse among clusters, the inter-cell interfer-ence can be effectively reduced.