The aggressive spatial spectrum reuse (SSR) by network densification usingsmaller cells has successfully driven the wireless communication industryonward 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 basestations (BSs) per square kilometer, is activating all BSs on the sametime/frequency resource the best strategy? In this paper, we presenttheoretical analyses to answer such question. In particular, we find that boththe signal and interference powers become bounded in practical UDNs with anon-zero BS-to-UE antenna height difference and a finite UE density, whichleads to a constant capacity scaling law. As a result, there exists an optimalSSR density that can maximize the network capacity. Hence, the limit to SSRshould be considered in the operation of future UDNs.
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