Ultra Dense Network (UDN) in which Base Stations (BSs) are deployed at an ultra high density is a promising network model of the future wireless generation. Due to ultra densification, reuse of frequency bands with an ultra high density is compulsory for this network. Conventionally, the research on frequency reuse technique such as Fractional Frequency Reuse (FFR) classifies the active users into only two groups. However, this approach is not suitable for UDNs where the signal experiences a huge amount of power loss over distances. Thus, this paper proposes a generalized model of FFR for UDNs where the active users are classified into more than two groups The paper introduces a simple approach to obtain the coverage probability of a typical user in the case of a general path loss model. In the case of stretched path loss model for UDNs, the closed-form expression of user coverage probability is derived. From the analytical and simulation results, it is stated that the proposed model can improve user performance without increasing BS power consumption. Furthermore, two additional interesting conclusions are found in this paper: (i) the user coverage probability increases to a peak before passing a decline when the density of BSs increases; (ii) an increase in BS transmission power may decrease the user performance.

以超高密度部署基站 (BS) 的超密集网络 (UDN) 是未来无线一代的有前途的网络模型。由于超高密度化，该网络必须重复使用超高密度的频段。传统上，诸如分数频率复用（FFR）之类的频率复用技术的研究仅将活跃用户分为两组。但是，这种方法不适用于信号在远距离内经历大量功率损耗的 UDN。因此，本文提出了一种用于 UDN 的 FFR 广义模型，其中活动用户被分为两组以上。本文介绍了一种简单的方法来获得通用路径损耗模型情况下典型用户的覆盖概率。在 UDN 的延伸路径损耗模型的情况下，推导出用户覆盖概率的闭式表达式。从分析和仿真结果来看，所提出的模型可以在不增加基站功耗的情况下提高用户性能。此外，本文还发现了另外两个有趣的结论：(i)当基站密度增加时，用户覆盖概率在下降之前增加到峰值；(ii) BS 传输功率的增加可能会降低用户性能。