text: The aggressive spatial spectrum reuse (SSR) by network densification using smaller cells has successfully driven the wireless communication industry onward in the past decades. In our future journey toward ultra-dense networks (UDNs), a fundamental question needs to be answered. Is there a limit to SSR? In other words, when we deploy thousands or millions of small cell base stations (BSs) per square kilometer, is activating all BSs on the same time frequency resource the best strategy? In this paper, we present theoretical analyses to answer such question. In particular, we find that both the signal and interference powers become bounded in practical UDNs with a non-zero BS-to-UE antenna height difference and a finite UE density, which leads to a constant capacity scaling law. As a result, there exists an optimal SSR density that can maximize the network capacity. Hence, the limit to SSR should be considered in the operation of future UDNs.
  @cite_12: In this paper, we present a new and significant theoretical discovery. If the absolute height difference between base station (BS) antenna and user equipment (UE) antenna is larger than zero, then the network performance in terms of both the coverage probability and the area spectral efficiency will continuously decrease toward zero as the BS density increases in ultra-dense networks (UDNs). Such findings are completely different from the conclusions in the existing works, both quantitatively and qualitatively. In particular, this performance behavior has a tremendous impact on the deployment of the 5th-generation UDNs. Network operators may invest large amounts of money in deploying more network infrastructure only to obtain even less network capacity. This paper’s results reveal that one way to address this issue is to lower the BS antenna height to the UE antenna height. However, this requires a revolutionized approach of the BS architecture and deployment, which is also explored in this paper.
summary: First, if we consider a practical non-zero BS-to-UE antenna height difference @math , then the coverage probability is shown to crash as the BS density increases in a fully-loaded UDN. This is caused by a severe in UDNs @cite_12 . The intuition of such is that the signal power becomes in UDNs due to the lower-bound on the BS-to-UE distance, as a UE cannot be closer than @math to its serving BS.