ng possible across much of the world because GSM and TDMA net- works cover large sections of the global population and together account for 79 percent of all mobile subscribers, as shown in Figure 1-1. Cell Structure In a cellular system, the coverage area of an operator is divided into cells. A cell is the area that one transmitter or a small collection of transmitters can cover. The size of a cell is determined by the transmitter's power. The con- cept of cellular systems is the use of low-power transmitters in order to enable the efficient reuse of the frequencies. The maximum size of a cell is approximately 35 km (radius), providing a round-trip communications path As of May, 2001 Figure 1-1 Market penetrations IS-136 of GSM and TDMA. 64 Mill 51 Mill. 90 Million 550 Million 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. Introduction to GSM Chapter 1 from the mobile to the cell site and back. If the transmitters are very pow- erful, the frequencies cannot be reused for hundreds of kilometers, as they are limited to the coverage area of the transmitter. In the past when a mobile communications system was installed, the coverage blocked the reuse beyond the 25-mile coverage area, and created a corridor of interfer- ence of an additional 75 miles. This is shown in Figure 1-2. The frequency band allocated to a cellular mobile radio system is dis- tributed over a group of cells and this distribution is repeated in all of an operator's coverage area. The entire number of radio channels available can then be used in each group of cells that form the operator's coverage area. Frequencies used in a cell will be reused several cells away. The distance between the cells using the same frequency must be sufficient to avoid interference. The frequency reuse will increase the capacity in the number of users considerably. The patterns can be a four-cel