The first mobile telephone system in the United States began in St. Louis, Missouri in 1946. By 1947, more than 25 cities in the United States had mobile telephone service available. The systems used a single high-power transmitter for the base station in the center of a metropolitan area. Coverage was provided for 50 miles or more from the transmitter. These initial systems used a human operator at the base station to manually connect the mobile user with the landline network. In most of these systems, service was very poor because too many customers (called subscribers) shared each radio channel (called loading). It was not uncommon to have busy channels over 50% of the time. Despite this poor service, it revolutionized the definition of telephone service and priority was given to police and ambulance service. The waiting list for mobile phones in some cities was more than 7 years. This type of system was improved many times and the last upgrade, called improved mobile telephone service (IMTS), was introduced in the mid 1960’s. While there may still be some original systems in operation throughout the United States, new equipment for these systems is not currently being produced. It has been replaced with cellular systems.
Cellular and Personal Communication Service (PCS)
Cellular and PCS mobile telephone systems allow mobile telephones to communicate with each other or to the public telephone system through an interconnected network of radio towers. In early mobile radio-telephone systems, one high-power transmitter served a large geographic area with a limited number of radio channels. Because each radio channel requires a certain frequency bandwidth (radio spectrum) and there is a very limited amount of radio spectrum available, this dramatically limited the number of radio channels that kept the serving capacity of such systems low. For example, in 1976, New York City had only 12 radio channels to support 545 customers and a two-year long waiting list of typically 3,700.
When linked together to cover an entire metro area, the radio coverage areas (called cells) form a cellular structure resembling that of a honeycomb. The cellular systems are designed to have overlap at each cell boarder to enable a “hand-off” (also called a “handover”) from one cell to the next. As a customer (called a subscriber) moves through a cellular or PCS system, the mobile switching center (MSC) coordinates and transfers calls from one cell to another and maintains call continuity.
Figure 1 shows a mobile telephone system. The wireless network connects mobile radios to each other or the public switched telephone network (PSTN) by using radio towers (base stations) that are connected to a mobile switching center (MSC). The mobile switching center can transfer calls to the PSTN.
When a cellular system is first established, it can effectively serve only a limited number of callers. When that limit is exceeded, callers experience too many system busy signals (known as blocking) and their calls cannot be completed. More callers can be served by adding more cells with smaller coverage areas - that is, by cell splitting. The increased number of smaller cells provides more available radio channels in a given area because it allows radio channels to be reused at closer geographical distances.
There are two basic types of systems: analog and digital. Analog systems typically use FM modulation to transfer voice information and digital systems use some form of phase modulation to transfer digital voice and data information. Although analog systems are capable of providing many of the services that digital systems offer, digital systems offer added flexibility as many of the features can be created by software changes. The trend at the end of the 1990’s was for analog systems to convert to digital systems.
To allow the conversion from analog systems to digital systems, some cellular technologies allow for the use of dual-mode or multi-mode mobile telephones. These telephones are capable of operating on an analog or digital radio channel, depending on availability. Most dual-mode phones prefer to use digital radio channels in the event both are available. This allows them to take advantage of the new features such as short messaging and digital voice quality.
Cellular systems have several key differences that include the radio channel bandwidth, access technology type (FDMA, TDMA, CDMA), data signaling rates of their control channel(s), and power levels. Analog cellular systems have very narrow radio channels that vary from 10 kHz to 30 kHz. Digital systems channel bandwidth ranges from 30 kHz to 1.25 MHz. Access technologies determine how mobile telephones obtain service and how they share each radio channel. The data signaling rates determine how fast messages can be sent on control channels. The RF power level of mobile telephones and how the power level is controlled typically determines how far away the mobile telephone can operate from the base station (radio tower).
