PDF of this tutorialThe first commercially available radio and telephone system, known as improved mobile telephone service (IMTS), was put into service in 1946. This system was quite unsophisticated—but then there was no solid state electronics available.
With IMTS, a tall transmitter tower was erected near the center of a metropolitan area. Several assigned channels were transmitted and received from the antenna atop this tower. Any vehicle within range could attempt to seize one of those channels and complete a call. Unfortunately, the number of channels made available did not come even close to satisfying the need. To make matters worse, as the metropolitan area grew, more power was applied to the transmitter or receiver, the reach was made greater, and still more erstwhile subscribers were unable to get dial tone.
The solution to this problem was cellular radio. Metropolitan areas were divided into cells of no more than a few miles in diameter, each cell operating on a set of frequencies (send and receive) that differed from the frequencies of the adjacent cells. Because the power of the transmitter in a particular cell was kept at a level just high enough to serve that cell, these same sets of frequencies could be used at several places within the metropolitan area. Beginning in 1983, two companies, one called a wireline company and the other called a nonwireline carrier, were given a franchise to operate in each major territory.
Two characteristics of cellular systems were important to their usefulness. First, the systems controlled handoff. As subscribers drove out of one cell and into another, their automobile radios, in conjunction with sophisticated electronic equipment at the cell sites (also known as base stations) and the telephone switching offices (also known as mobile telephone switching office [MTSO] ), transferred from one frequency set to another with no audible pause. Second, systems were also designed to locate particular subscribers by paging them in each of the cells. When the vehicle in which a paged subscriber was riding was located, the equipment assigned sets of frequencies to it, and conversation could begin (see Figure 6).
Figure 6. In a Cellular System, Vehicle-to-Cell-Site Communications Take Place Using FDMA, TDMA, CDMA, GSM, etc.; Communications between the Cell Site and the MTSO Utilize Conventional Techniques
The initial transmission technology used between the vehicle and the cell site was analog in nature. It is known as advanced mobile phone service (AMPS). The analog scheme used was called frequency division multiple access (FDMA).
But the age of digital transmission was upon us, and many companies operating in this arena concluded that a digital transmission scheme would be preferred. The result was time division multiple access (TDMA). In Europe, the selected scheme was an adaptation of the TDMA used in the United States, and it was called groupe special mobile. Since then, the name has been changed to global system for mobile communications (GSM).
As if that was not enough, a third group of companies determined that a special spread-spectrum or frequency-hopping scheme would be even better, and this also was developed and trialed. This is called code division multiple access (CDMA). Thus, there are at least four schemes that may be used for communications between a vehicle and the cell site. Communications between the cell site and the MTSO utilized more conventional techniques, such as microwave, copper pairs, or fiber optics.
The continuing growth of cellular communications (there are presently about 20,000 new subscribers signing on each day), led government and industry in the United States to search for additional ways to satisfy the obvious need not only for ordinary telephone service but also for special services and features, smaller telephones, and cellular phone use. This search led to the PCS industry. Additional frequency bands were allocated for their use, and rather than assign them to the first comers or by way of a lottery, the FCC auctioned them off through a sophisticated bidding contest that brought the U.S. treasury billions of dollars.
Geosynchronous satellites represent yet another way of providing wireless communications. These satellites, located 22,300 miles above the earth, revolve around the earth once each twenty-four hours—the same as the earth itself. Consequently they appear to be stationary. Communications between two places on earth can take place by using these satellites; one frequency band is used for the uplink, and another for the downlink. Such satellite systems are excellent for the transmission of data, but they leave something to be desired for voice communications. This is a result of the vast distance and the time it takes for an electrical signal to make an earth-satellite-earth round trip. That time amounts to one quarter of a second. A reply from the called subscriber takes another quarter of a second, and the resultant half a second is definitely noticeable. Consequently, voice communications is seldom carried via geosynchronous satellites.
Yet another wireless telecommunications technology is the low earth orbit (LEO) satellite system. LEOs are satellites that communicate directly with handheld telephones on earth. Because these satellites are relatively low—less than 900 miles—they move across the sky quite rapidly.
In a LEO system the communications equipment on a satellite acts much like the cell site of a cellular system. It catches the call from earth and usually passes it to an earth-based switching system. Because of the speed of the satellite, it is frequently necessary to hand off a particular call to a second satellite just rising over the horizon. This is akin to a cellular system, except that in this case it is the cell site that is moving rather than the subscriber.
Several systems are now in the planning stage, and in fact many satellites have already been launched. The most noted is Iridium, created by Motorola, which would utilize sixty-six satellites. A second system, called Globalstar, would employ forty-eight satellites. There are at least two or three others that are advanced in terms of preparations to launch.
