ns to get the most out of the scarce radio resources as possible. The 21-cell pattern is by far the largest repeating pat- tern in use today. The cells are grouped into clusters. The number of cells in a cluster determines whether the cluster can be repeated continuously within the coverage area. The number of cells in each cluster is very important. The smaller the number of cells per cluster, the greater the number of channels per cell. Therefore, the capacity of each cell will be increased. However, a balance must be found in order to avoid the interference that could occur between neighboring clusters. This interference is produced by the small size of the clusters (the size of the cluster is defined by the number of cells per cluster). The total number of channels per cell depends on the number of available channels and the type of cluster used. Types of Cells The density of population in a country is SO varied that different types of cells are used: Macrocells Microcells Selective or sectorized cells Umbrella cells Nanocells Picocells Macrocells Macrocells are large cells for remote and sparsely populated areas. These cells can be as large as 3 to 35 km from the center to the edge of the cell (radius). The larger cells place more frequencies in the core, but because the area is rural, the macrocell typically has limited frequencies (channels) and higher-power transmitters. This is a limitation that prevents other sites from being closely adjacent to this cell. Figure 1-5 shows the macrocell. 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 Introduction to GSM Figure 1-5 The macrocell. Up to 35 Kilometers Microcells These cells are used for densely populated areas. By splitting the existing areas into smaller cells, the number of channels available and the capacity of the cells are increased. The power level of the tr