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Chapter 2:A Base Station Primer • In the early 1980s,an entirely new form of mobile A. ELECTROMAGNETIC FIELDS AND n What is a radio telecommunications became available to the American RADIO WAVES u wave? people—cellular telephony. This advanced form of telecommunications combines modem radio, telephone, Any electric charge sets up an electric field around and computer technologies. Cellular telecommunica- it. If another electrical charge enters that field,an radio wave is a cons employs fixed relay facilities (base stations)and attractive or repulsive force arises.The force will AA pair of electric mobile and portable phones. Cellular systems are attract the charges to each other if the charges are and magnetic fields designed to provide mobile telephone service to many of the opposite polarity.The force will repel the that can travel through people in a given area even though there is only a limited charges if the charges are of the same polarity.The space at the speed of amount of radio spectrum available for such use. This force's strength depends on the size of the charges light and can convey Chapter provides the basics of cellular technology,and and the distance between them.A moving elec- information. It has a describes trends in cellular system development. tic charge (an electric current) is direct current certain frequency, the Of course, like any new technology, cellular telecom- (DC) if the motion is in one direction only.An way visible light has a electric charge that moves back and forth corn- certain color and con- munirntvns poses a number of questions.Among the prises alternatingcurrent(AC).An electric cur questions about cellular telecommunications that we tains a power. A radio awave ount rent sets up a magnetic field.A magnetic field can, of A wave will answer are: in some cases,bend the path of a moving electric emanates from a trans- "' What is a radio wave? charge.A magnetic field can also exert a physical mining antenna and ■ What is a base station? force on magnetic material such as iron or steel. can be picked up by a ■ Why do base stations have limited communications An electrical charge will be of one polarity only receiving antenna. A range? (positive or negative),but a magnet or magnetic radio receiver of the ■ Why do cellular systems need so many base stations? field always has two poles,North and South. proper design can ■ What is the difference between analog and digital A batteryhas two terminals,onepositive and one extract the information cellular communications? impressedradio So does an electric generator.In opera- mp upon a •negative. tion,a battery or a DC generator's positive terminal wave. always remains positive,and the negative terminal BASIC RADIO CONCEPTS AND RADIO-SYSTEM Transmitters are to some extent like stereo ampli- COMPONENTS fiers, which change 60-Hz AC into AF currents corresponding to music or speech that can drive a ■Flowing electrical charges constitute electricity loudspeaker. or current.Stationary charges constitute static electricity.One can liken charge to water standing III A transmitting antenna converts RF current in a bucket,and current to the water flowing out from the transmitter into radio waves,similar to a of the bucket when the bucket is tipped toward loudspeaker that transforms AF current from a the ground. Current can flow in one direction only, stereo amplifier into sound waves. The radio or it can regularly reverse direction—like the waves travel outward from the antenna like sound tides,or ocean waves,that crash on a beach and waves from the loudspeakers,or ripples from a then retreat back out to sea. Current flowing in disturbance on a pond. one direction only is direct current or DC. Current ■A receiving antenna extracts some of the signal that regularly reverses direction is alternating power from a passing radio wave and converts it current or AC.Power used by batteries is DC;power into an RF current One can liken a receiving used by utilities is AC. The number of times in one antenna to a microphone placed in front of a loud- second that AC reverses direction is its frequency speaker, which captures part of the sound wave in hertz(Hz).n AC with a frequency comparable to and changes it back into AF current a pitch within the range of human hearing(up to ■A radio receiver extracts information encoded 20 kHz)can be called audio-frequency(AF) in an RF current and presents it to the user.A tele- current Higher-frequency AC is radio-frequency vision set;for example,is a specialized radio (RF)current.Radio frequencies range from tens of receiver.it reproduces the images and sounds • kilohertz to thousands of megahertz encoded into the radio wave captured by the ■A radio transmitter converts electricity from a receiving antenna. battery or the power grid into RF current