ansmitters used in these cells is then decreased, reducing the possibility of interference between neighboring cells. Some of the microcells may be as small as .1 to 1 km depending on the need. Often times the cell splitting will use the reduced power and the greater coverage to satisfy hot spots or dead spots in the network. Another need may well be a below-the-rooftop cell that satisfies a very close-knit group of people or varied users. The picocell will be in a building, and is typically a smaller version of a microcell. The distances covered with a picocell are approximately .01 to 1 km. These are used in office buildings for close in calling, part of a Private Branch Exchange (PBX) or a wireless Local Area Network (LAN) application today. A small group of users will share this cell because of the close proximity to each other and larger cells around. Nanocells also fall into the below-the-rooftop domain where the dis- tances for this type of cell are from .01 to .001 km. These are just smaller and smaller segments that are built within a building, as an example. Fig- ure 1-6 shows a combination of a microcell and picocell. 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 Figure 1-6 Microcell The microcell and picocell. Macrocell Base station Picocell Selective Cells or Sectorized Cells It is not always useful to define a cell with a full coverage of 360 degrees. In some cases, cells with a particular shape and coverage are needed. These cells are called selective cells. Selective cells are typically the cells that may be located at the entrances of tunnels where 360 degrees coverage is not needed. In this case, a selective cell with coverage of 120 degrees is used. This selective cell is shown in Figure 1-7. Tiered Cells A tiered cell is one where an overlay of radio equipment operates in two dif- ferent fr