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• What is a remains negative. But an AC generator's two ter- manned,except during regular maintenance visits. cellular base mink flip polarity at the AC's frequency. Network engineers monitor system performance station? A air of AC electric and from a mobile telephone switching office which P magnetic fields forming communicates with base stations via radio or tele- a right angle make up a radio wave.Each field is also at right angles to the wave's direction of travel. phone links. AA cellular base The fields change polarity ac the frequency of the As mentioned earlier,base stations serve as bridges station is the link between the wired radio wave. to the wired telephone network for portable or telephone network and B. CELLULAR BASE STATION mobile cellular phones,which are specialized a portable or mobile radio sets.A phone's receiver section lets the sub- cellular phone, or FUNDAMENTALS scriber hear the conversation's other half trans- between one portable Whenever anyone uses a cellular telephone,a base miffed by radio from the nearest base station. or mobile phone and station acts as the gateway to the wired telephone When the subscriber talks his portable or mobile another. It is a combi- network or to another cellular phone.A base sta- unit transmits his voice,encoded in a radio wave. nation radio relay tion plays this key role by acting as a radio relay sta- The nearest base station receives that signal, station and telephone non and telephone switching center.Base stations extracts the subscriber's voice,and sends it through switching center. have lower transmission powers than many FM the wired telephone network to the other party.3 broadcast stations(those between 88 and 108 on To make natural conversation possible,the two the dial),and have higher operating frequencies.2 radio links(those to and from the base station) remain active simultaneously4 Base stations vary widely in appearance,but every - base station has a set of transmitting and receiv- C. COMMUNICATIONS RANGE ing antennas which often look like vertical whips Any radio system has a certain limited range • or panels.The antennas are usually 50 to 250 feet within which reliable communications is possible. above the ground.A receiving antenna handles Range limitations are caused by distance, terrain the half of the conversation originating from a obstructions,and interference. subscriber's cellular telephone,while a correspond- ing transmitting antenna relays the other half of 1. Signal Strength vs.Distance.Radio waves the telephone conversation to that cellular phone. weaken over distance.This aspect of their behav- One pair of receiving and transmitting antennas for is much like that of light from a candle;or...� can handle the signals associated with a number heat from a fire.Close up,the light or heat from_. of different telephone conversations.The precise even a modest source can be intense.Five hundred number of antennas and the type of antennas used feet away,however,the same light source is a feeble at a particular base station will depend on the glow, the fire's warmth is imperceptible even amount of radiotelephone traffic handled by that though the total radiant power remains the same. facility and by neighboring base stations. Why is this?What we perceive as brightness or warmth is actually the degree of concentration of To keep their base station antennas up in the air, light or heat.Very close to the source,even mod- cellular systems use various structures.Depending est amounts of power are densely packed. But on the circumstances,the support structure might moving away from the source,the total power be a pole or tower built especially for that purpose, spreads out over an ever-larger surface area.The a preexisting radio or television tower,a short pole power concentrated in a given area—(say,one on the roof of a building,or a short pole or tower square centimeter)—the power flux density installed on a municipal water tower. drops off sharply.Likewise,radio signals near a An equipment shed at the tower's base or an transmitting antenna are also strong,but weaken equipment cabinet in the office budding houses dramatically as they travel outward.This weakening the radio receivers,transmitters,and computerized restricts communications range.Within practical telephone switching gear that make up the rest of limits,one can counter it by increasing: the base station's equipment.Sometimes,an • transmitter output power, • emergency generator or an array of batteries permits ■ transmitting antenna efficiency; a base station to provide uninterrupted service ■ receiving antenna efficiency; during power blackouts. • receiver sensitivity to weak signals;or Base stations operate automatically.They are not is two or more of the above. 6