ng as we do not make similar changes at neighboring base stations, lowering the antenna path gain will move the equal-radio-path border zones closer to the modified base station and will reduce its service area. Reducing antenna path gain, however, also reduces the ability of the base station to serve hard-to-reach areas. This will aggravate any cover- age problems we have and may create new ones. Keep in mind that coverage problems are not all distance-related. Underground malls and subway trains may have low ra- dio signal path gains even to nearby base stations. Tilting the serving antennas downward can offer the best of both worlds, as shown in Fig. 33.1. Downtilting reduces the radio signal path gain further from the base station by moving the high-gain, narrow-beam vertical lobe of the antenna away from those areas. Downtilting also increases the path gain nearer the base station because that high- gain vertical lobe is now serving user terminals closer in. We think of downtilting as a part of CDMA system evolution. In its early days, a base station is designed to reach as much traffic as possible with the highest path gain possible for maximum large-cell capacity. As neighbors appear, the same base station has a new mission, to serve the 1Recall that high-gain antennas get their high gain by concentrating their transmission and re- ception in a beam with a narrow vertical angle. 2We cannot use downtilting on an omnidirectional antenna, but we are figuring that most CDMA systems are serving enough subscriber demand that they are using sectorized cells with multiple antenna faces, as described in the next section. Downloaded from Digital Engineering Library @ McGraw-Hill (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. Base Station Planning Planning for CDMA Capacity Figure 33.1 Downtilting antennas concentrates signal close to the base station. same amount of traffic in a smaller area and to r