AMPS systems, there were loopback trunks specifically designed to look like PSTN trunks while not going anywhere, and these loopback trunks were used for mobile-to- mobile calls. Telephone network design is often about signaling, the communication among net- work components about calls. Any system that bypasses the PSTN has to make sure that these signaling issues are taken care of, both making the connection and billing the users appropriately for the call. There is a specific reason for avoiding routing through the PSTN on mobile-to-mobile calls whenever possible. Wireless providers pay a lease fee to PSTN service providers for use of the PSTN. Mobile-to-mobile calls that do not go over the PSTN cost less for the wireless provider. Wireless providers can use this either to increase their net rev- enue or to offer special rate plans to attract customers. 5.2 Radio Architecture We have looked at the basic concepts of the design of the cellular system. Now we will examine a cellular system from the vantage point of radio transmission and reception. 5.2.1 Cells and base stations Cells are regions of radio coverage defined by having the same base station; base sta- tions and cells go together in a one-to-one correspondence. Whatever antennas are on top and radio resources are under the roof and however many sectors the cell may be divided into, we think of the cell as a basic entity. There is a solid financial reason for this: Base stations are also the economic units of wireless telephone systems. With the added costs of building, tower, antennas, electrical power, telephone transport, and zon- ing issues for a radio tower, adding a new base station is a very expensive way to in- crease cellular capacity. If reengineering base stations can increase capacity without new construction, the service provider saves a lot of money. In addition, many cus- tomers want cellular service but do not want radio towers in their neighborhoods. A Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibra