The History of the Radio - Контрольная работа

The 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 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. 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. С помощью УОМТ, высокая башня передатчик была возведена недалеко от центра столичной области.