Optimal Base Station Antenna Downtilt in Downlink Cellular Networks.

From very recent studies, the area spectral efficiency (ASE) performance ofdownlink (DL) cellular networks will continuously decrease and finally to zerowith the network densification in a fully loaded ultra-dense network (UDN) whenthe absolute height difference between a base station (BS) antenna and a userequipment (UE) antenna is larger than zero, which is referred as the ASE Crash.We revisit this issue by considering the impact of the BS antenna downtilt onthe downlink network capacity. In general, there exists a height differencebetween a BS and a UE in practical networks. It is common to utilize antennadowntilt to adjust the direction of the vertical antenna pattern, and thusincrease received signal power or reduce inter-cell interference power toimprove network performance. This paper focuses on investigating therelationship between the base station antenna downtilt and the downlink networkcapacity in terms of the coverage probability and the ASE. The analyticalresults of the coverage probability and the ASE are derived, and we find thatthere exists an optimal antenna downtilt to achieve the maximal coverageprobability for each base station density. Moreover, we derive numericallysolvable expressions for the optimal antenna downtilt, which is a function ofthe base station density. Our theoretical and numerical results show that afterapplying the optimal antenna downtilt, the network performance can be improvedsignificantly. Specifically, with the optimal antenna downtilt, the ASE crashcan be delayed by nearly one order of magnitude in terms of the base stationdensity.