Sec. 28.1 tell us that there is a significant capacity cost to having equal-signal-path traffic. Even so, adding sectors adds some capacity to a CDMA system. A CDMA rule of thumb is that three-sector cells carry 2.5 times as much traffic as omnidirectional cells and that six-sector cells carry four times as much traffic. The re- duced rate of additional benefit is a result of the increase in sector-to-sector interfer- ence when there are more sectors. This is only a rough approximation based on some typical cellular sector antennas. Better antennas and carefully thought out sector ori- entation might do a little better than this. 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. Capacity Issues Specific to CDMA Capacity and Quality Principles There is nothing magic about three or six sectors in CDMA. Cellular engineers tend to think in terms of three or six sectors because of our conventional-reuse tradition. In conventional reuse, we use sectors to manage interference so that we can reuse chan- nels more frequently. Since the conventional-reuse interferers are from other cells in the hexagonal pattern, we use three or six sectors. A CDMA cell could have 5 sectors, 9 sectors, or even 25 sectors if we wanted to put that much equipment at one base station. So why don't we just keep adding sectors? Because each sector we add to a cell adds less capacity than the one before. Using our rule of thumb, doubling three sectors to six only adds 50 percent to the call capacity. This means that doubling the radio equip- ment cost does not double the subscriber revenue. At some point, adding sectors is less cost-effective than adding new base stations. The tradeoff between more sectors and more cells depends on the cost of building construction and of putting up a tower. Cost structures vary dramatically in different parts of the world. We have not seen a 