access (TDMA)] the three- and six-sector plans make sense because the six important interferers are evenly spaced around the serving cell. In CDMA, however, the important interferers are in the 2If the antenna is used for receiving rather than transmitting, then the term reception pattern might be more appropriate, but we still call it the radiation pattern of the antenna. See Sec. 3.1.2 for a detailed explanation. Downloaded from Digital Engineering Library @ McGraw-Hil (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The Base Station The Base Station same cell, so it is possible to design other sector plans to optimize capacity, as discussed in Sec. 31.1. Due to the reciprocity of antenna transmission and reception patterns, the entire set of transmitting and receiving antennas is generally tweaked as a unit. The vertical ra- diation pattern of the base-station antennas determines the coverage range of the base station. We can extend the cell's radius by using high-gain antennas, which have nar- row vertical range. The user terminals near and under the base-station antenna are out of the high-gain zone, but they are very close to the base station, so they still get more than adequate radio coverage. The vertical radiation pattern can be used to shrink the coverage range as well. If we tilt a sector's antennas downward, then their maximum gain is closer to the base sta- tion. As a user terminal moves past the antenna gain peak, its radio path gets weaker both from increased distance and from reduced antenna gain. In this way we can use a high-gain, vertically thin-beam antenna to attenuate the signal path beyond a certain distance. It is not terribly hard to tilt antennas downward. This means that a large- radius coverage cell can be converted into a smaller-radius cell in a growth environ- ment by reaiming the antennas already there, which is a lot easier and cheaper than replacing them. The wirel