equencies and uses different sectors. The tiered cell is also a form of a selective cell. Umbrella Cells Alongside of a high-speed freeway, crossing very small cells produces an overabundance of handovers among the different small neighboring cells. 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 Tunnel Figure 1-7 3600 coverage not needed The selective cell. Comes in 60° - 120° - 180° To solve this problem, the concept of umbrella cells was introduced. An umbrella cell covers several microcells, as shown in Figure 1-8. The power level inside an umbrella cell is increased compared to the power levels used in the microcells that form the umbrella cell. How does the cell know when to shift from a microcell to an umbrella cell? When the speed of the mobile is too high, the mobile is handed off to the umbrella cell. The mobile will then stay longer in the same cell (in this case, the umbrella cell). This will reduce the number of handovers and the work of the network. A large number of handover demands and the propagation characteristics of a Figure 1-8 The umbrella cell. 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 mobile can help to detect its high speed. The radio equipment is no longer forced to constantly change hands from cell to cell when using this umbrella. This meets the goal of GSM in that the efficient use of the radio frequency (RF) spectrum is what is being achieved. Analog to Digital Movement In the 1980s, most mobile cellular systems were based on analog systems including AMPS, TACS, and NMT. In fact, 95 percent of the United States has coverage from AMPS services, whereas only 70 percen