Cellular Technology

The cellular phone is the latest in a long line of mobile, portable, and wireless technologies extending back to the 1930s. Military forces were among the first to use mobile radio communications. Early mobile radio equipment for the military involved large transmitters aboard military vehicles with huge antennas and high transmitter power. The large, robust military vehicles were capable of accommodating the massive, power-hungry equipment.

The first use of radio communications for civilian land vehicles was primarily by police departments. The earliest systems were one-way, where the dispatcher could broadcast to all cars the location and nature of a problem. There was no return communication and thus no verification of a response to the problem, but it was better than nothing.

The first successful installation of a large two-way police radio system was for the Connecticut State Police in 1939. This system used a newly invented type of radio called frequency modulation (FM). This system set the standard for mobile radio for many years.

Two-way radio installed in automobiles inspired the idea for a mobile telephone service. The first police radio was a simple, or one-way, system, meaning that the mobile unit could only receive communications. The two-way police radio was a half duplex system in which both the mobile and base units could transmit and receive but not at the same time. Proper radio procedures were required, such as saying "over" to invite the other station to transmit, and using radio call signs. The frequency was shared by a number of users and conversations were far from private.

Ideally, a mobile telephone is a full duplex system where both stations transmit and receive simultaneously and the channel is not shared. The first full duplex mobile telephone systems were installed in large cities in the 1950s. The systems used base stations connected to the public switched telephone network (PSTN) and had a range of 60 to 80 kilometers (37 to 50 miles). Mobile telephones had a telephone number, rang like a normal telephone, and were full duplex. Because of the large area covered by the base station and the limited number of available channels or radio frequencies, the mobile phone system (MPS) quickly reached full capacity. Priority for new subscribers was given to physicians and others needing emergency communications, and the waiting lists were very long.

A Texas rancher, Tom Carter, played an important role in mobile telephone history. Carter had made a simple device that would allow his private business two-way radio system to be used with his office telephone when he was out on the ranch. The telephone company refused to allow Carter to connect his device to the PSTN and Carter took the case to court. Although it took fifteen years, the Federal Communications Commission (FCC) in 1968 ruled in favor of Carter in the landmark Carterfone decision. The action opened the PSTN to radio connections as well as those for computer data and other devices.

In the 1970s Bell Telephone Laboratories began investigating a replacement system for the MPS. After the Carterfone decision, competitors were gearing up to use new technologies to provide alternative mobile telephone service. The FCC reassigned a number of under-used ultra-high frequency (UHF) television channels for a new, advanced mobile phone system (AMPS). The AMPS had considerably more channels than the older MPS and had two sets—one for the local telephone company, and a second set for a competitor.

The concept of the AMPS was to increase the reuse of the communications channels. Frequency reuse occurs when two stations occupy the same frequency or channel but are separated by such a distance that they do not interfere. The MPS used high antennas to provide a 60 to 80 kilometer (37 to 50 mile) range, but no two base stations could be closer than about 150 kilometers (93 miles) to avoid interference. In the AMPS, the height of the base station antenna and the transmitter power are limited so the range of a cell is only about 11 to 15 kilometers (7 to 9 miles). In addition, the base station controls the transmitter powers of the mobile units. This ensures that the least amount of power is used, which limits the interference and allows the channels to be reused by another cell only 20 to 30 kilometers (12 to 19 miles) away.

The cells are interconnected with wire lines or microwave radio links. When a user leaves the coverage of one cell and enters another, the new cell provides continuing communications, a process called handoff. The cell system must determine which cell is most capable of picking up the user, acquire that user, and connect the user to the correct land line. All of this is invisible to the user.

The handoff process involves a number of algorithms using various data from the mobile telephone. First, every cell phone handset has a digital address that is continuously transmitted. Any cell site, or base station, can positively identify signals being received even though many of the received signals are not communicating with that cell site. Cell sites continually communicate with neighboring cell sites and compare the signal quality of the mobile units being received. If a particular mobile telephone unit has a superior signal in a neighboring cell site, the handoff process begins. This has to be done with care, as certain situations can cause a signal to fade temporarily in one site while improving at another, perhaps only for a few seconds. If a handoff is initiated prematurely, it will be necessary to restore the mobile phone to the original cell site quickly.

In addition to determining which cell site is capable of providing the best communications to the mobile phone, the computer system must also switch the land lines and keep a tally of the airtime for billing purposes.

Early cell telephone systems only allowed customers to use the system to which the user was a subscriber. Later, roaming, or using another company's cell system, was initiated. This came with very high prices and complicated billing procedures. As the cellular mobile phone system became financially successful, more cell sites were constructed, and now most of the continental United States has cell coverage. Proposed regulations would require the cell system to determine the location of a handset in emergency situations. Agreements between cellular telephone companies simplified roaming, and a customer can now travel through much of the country with no loss of service. This is called a seamless system.

AMPS uses frequency modulation (FM), which is the same technology used in the very first mobile two-way radio in 1939. FM has performed well for many years but is inferior to many digital systems. These digital systems opened up the way for more sophisticated applications—especially non-voice communications such as paging, e-mail, and Internet services. Many mobile telephones became not just telephones but personal communications systems or PCSs.

It is important for a global, seamless, wireless cell phone system to have well thought-out standards to which the cell sites adhere. With the advent of the more sophisticated digital cell systems, a large number of new standards have appeared. These include a European standard, global system for mobile (GSM), code division multiple access (CDMA), time division multiple access (TDMA), and others. The complexity of modern handsets has increased because of the need to operate with a number of different standards. The modern cellular telephone is a sophisticated, cost-effective, and worldwide communications device and promises to become more capable in the future.