Significant Gains in Coverage and Downlink Capacity from Proper Antenna Downtilt for Closely-spaced Cells in Wireless Networks

This paper focuses on the impact of antenna downtilt for closely placed cells, which is typical of built-out cellular networks in sub-urban and urban areas. The results show the need for small to no downtilt in coverage-limited scenarios, otherwise (for the case of close placement of cells), downtilt improves performance significantly. The most important conclusion of this study is that there is a large gain in downlink coverage and capacity due to down-tilting to the correct range of values; in fact, without the correct downtilt, all the gain in coverage due to the close placement of cells is lost. The simulations demonstrate that all in-building penetration advantage (up to 40 dB for the examples simulated in this paper) and up to 60% of the capacity can be lost if the tilts are off by 4° to 5° from the optimal for closely placed cells. In-building coverage is one of the challenges for current cellular systems, and it is shown that the same range of downtilt is optimal whether the users are in-building or on-street; hence the concern that downtilt will reduce in-building coverage is demonstrated as in-accurate for closely placed cells. Another question that this paper addresses is: Are the downtilts which have been optimized for 3G systems which have soft handoff also optimal for the 4G system? It is shown that the tilts that are optimal for 3G and voice systems are very good (if not optimal, at least near optimal) for the 4G LTE system. Finally, we also estimate the capacity gain in an existing deployment due to replacing lower gain (and wider vertical beamwidth) antennas with higher gain (and narrower vertical beamwidth) antennas. The simulations show that optimal downtilts can substantially reduce the number of basestations (and hence capital investment) needed from the point of view of capacity. Given the capacity constraints in 4G LTE systems, the study suggests that antenna design and downtilts must receive much more than nominal focus for urban areas.