ility analysis of each com- ponent to form a mean time between failures (MTBF) estimate for the base station. Most likely there will be one or two components that contribute the most to that MTBF, and we have to decide whether we want to spend more money on more reliable equip- ment for those components. More reliable base stations cost more money for redundant components. As discussed in Sec. 4.8, we can make a system more reliable by duplicating some of its functions. When one component breaks, its duplicate takes over its function, and the repair facil- ity is notified. But the overall reliability of the system is determined not only by the re- dundancy but also by how quickly the failed component is repaired. After all, the period between a component failure and its repair is a period of vulnerability during which another failure will result in service outage. A service provider may decide that partial service is an acceptable condition. If the typical antenna face is down 0.1 percent of the time, 9 hours per year, then we can de- cide that it is reasonable to let its subscribers be served by neighboring antenna faces during that time. The decision to live with these failures rather than to design more re- liable base stations depends on whether or not the cell is adequately covered by neigh- boring base stations. Will the other base stations be able to cover the entire cell? 12 There is a joke like that: The bad news is we're lost; the good news is we're making really good time. 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 In making this decision, it is appropriate to consider mean time to repair (MTTR) as well as MTBF. The effect on subscribers of nine 1-hour failures per year is very differ- ent from the effect of one 9-hour failure. Of course, blocking rates will be higher during a 