Н. В. Моина ю. Б. Генина т. В. Шульженко чтение английской научнотехнической литературы
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Cellular PhonesTo start with, one of the most interesting things about a cell phone is that it is actually a radio: an extremely sophisticated radio, but a radio nonetheless. Before cell phones, people who really needed mobile-communications ability installed radio telephones in their cars. In the radio-telephone system, there was one central antenna tower per city, and perhaps 25 channels available on that tower. This central antenna meant that the phone in your car needed a powerful transmitter: big enough to transmit 40 or 50 miles (about 70 km). It also meant that not many people could use radio telephones: there just were not enough channels. The genius of the cellular system is the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously. A good way to understand the sophistication of a cell phone is to compare it to a CB (a citizens band ) radio (or a walkie-talkie): – full-duplex vs half-duplex. Both walkie-talkies and CB radios are half-duplex devices; – channels. A walkie-talkie typically has one channel, and a CB radio has 40 channels. A typical cell phone can communicate on 1,664 channels or more; – range. A walkie-talkie can transmit about 1.6 km using a 0.25W (Watt) transmitter. A CB radio, because it has much higher power, can transmit about 8 km using a 5-W transmitter. Cell phones operate within cells, and they can switch cells as they move around. Cells give cell phones incredible range. Someone using a cell phone can drive hundreds of miles and maintain a conversation the entire time because of the cellular approach. A cell phone handset contains a radio transmitter, for sending radio signals onward from the phone, and a radio receiver, for receiving incoming signals from other phones. Cell phones have such tiny speakers and micro-phones that it is incredible how well most of them reproduce sound. The speaker is about the size of a dime and the microphone is no larger than the watch battery beside it. Speaking of the watch battery, this is used by the cell phone's internal clock chip. Because the radio transmitter and receiver are not very high-powered, cell phones cannot send signals very far. This is one reason why cell phone masts are necessary: with their huge high-powered antennas, they can pick up faint signals from many cell phones and route them onward to their destination. If we didn't have masts, we'd need cell phones with enormous antennas and giant power supplies, and they'd be too cumbersome to be mobile. There is another reason for having masts too. Suppose several people in your area all want to use their cell phones at the same time. If their phones all send and receive calls in the same way, the signals would interfere and scramble together and it would be impossible to tell one call from another. One way to get around this is to use different radio waves for different calls. If each phone call uses a slightly different frequency (the number of up-and-down undulations in a radio wave in one second), the calls are easy to keep separate. They can travel through the air like different radio stations that use different frequency bands. That's fine if there are only a few people calling at once. But suppose you are in the middle of a big city and millions of people are all calling at once. Then you'd need just as many millions of separate frequencies – more than are usually available. The solution is to divide the city up into smaller areas, with each one served by its own cell phone mast. These areas are called cells and they are normally thought of as hexagons on a big hexagonal grid. Each cell has its own mast (also called a base station, or antenna, or the tower) and all the calls made or received inside that cell are routed through its mast. Cells enable the system to handle many more calls at once, because each cell uses the same set of frequencies as its neighboring cells. The more cells, the greater the number of calls that can be made at once. Each cell is usually split into three sectors, which overlap with the sectors of neighbouring cells to create an uninterrupted network. When people travel, the signal is passed from one base station to the next, and typically never has to travel further than the nearest base station. The size and shape of each cell is determined by the features of the surrounding area, such as buildings, trees and hills, which can block signals. Cells are largest in flat open landscapes, where they can cover up to a five kilometre radius. Cells in urban areas cover up to a two kilometre radius. The smallest cells are in built-up areas, where micro-cell base stations are used to provide extra coverage and capacity. Each base station can only handle a limited number of calls at a time. In areas of high demand, additional antennas are sometimes added to a base station to send and receive more calls and other mobile services, or an extra base station is installed. All this means that a large number of base stations are needed to allow more people to use mobile phones, from more locations, and for coverage to be continuous when moving around. Cell phone base stations may be on hills, free standing towers or mounted on existing structures, such as trees, water tanks, or tall buildings. The antennas need to be located high enough so they can adequately cover the area. Base stations usually range in height from 50...200 feet. Without base stations mobile phones will not work and customers' calls cannot be connected. |