In this paper, we present a new and significant theoretical discovery. If theabsolute height difference between base station (BS) antenna and user equipment(UE) antenna is larger than zero, then the network performance in terms of boththe coverage probability and the area spectral efficiency (ASE) willcontinuously decrease toward zero as the BS density increases for ultra-dense(UD) small cell networks (SCNs). Such findings are completely different fromthe conclusions in existing works, both quantitatively and qualitatively. Inparticular, this performance behavior has a tremendous impact on the deploymentof UD SCNs in the 5th-generation (5G) era. Network operators may invest largeamounts of money in deploying more network infrastructure to only obtain aneven less network capacity. Our study results reveal that one way to addressthis issue is to lower the SCN BS antenna height to the UE antenna height.However, this requires a revolutionized approach of BS architecture anddeployment, which is explored in this paper too.
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