itated by more rapid processing. The coverage area of a micro- cell is about 0.6 miles (1 km) in diameter. Compared to macrocells, this architec- ture offers better frequency reuse, lower power, and better battery life, as well as smaller subscriber units. 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. A typical example of selective cells is the cells that may be located at the entrances of tunnels, where 360-degree coverage is not needed. In this case, a selective cell with a coverage of 120 degrees is used. The demand for spectrum is growing beyond what even the microcell design can provide, and further shrinkage of the cell size is mandated, SO we are now employing the picocell-that is, the tiny cell-architecture. This approach is for stationary or very slow-moving users-folks who dash out of a seminar during a conference and stand by a window SO they can conduct a conversation, or coffee Chapter 13 Wireless Communications Basics lovers who check e-mail over lattes. These users are not traveling great distances, maybe 330 to 1,000 feet (100 to 300 m) end to end. The coverage radius of a pico- cell is only about 150 feet (46 m), and because it's such a small coverage area, the base station power is also very small-10 milliwatts or less. Therefore, compared to the microcell design, the picocell design offers even better frequency reuse, even lower power, even smaller subscriber units, and even better, longer battery life. The picocell architecture does create some concerns in the engineering realm; for example, Tokyo needed to plan how to implement more than 40,000 cells for its Personal Handyphone System (PHS) deployment. There are tradeoffs with the var- ious designs: You can serve greater densities with the picocell design than with other designs, but at the cost of a bigger engineering project. The concept of umbrella cells was introduced to address the situation where 