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<br /> 2. Line Of Sight.We cannot see past the horizon E. BASE STATION INTERFERENCE AREAS Q Why do cellular
<br /> due to the earth's curvature.So unless we are on base stations have
<br /> top of a mountain,our view of short,ground- Undesired signals can be substantially weaker than limited range?
<br /> based objects extends only about four miles.Even desired ones and still cause interference,especially
<br /> then,line of sight rarely exceeds 60 miles.This is if the signals are on the same frequency(are
<br /> because light waves travel in straight lines.Beyond co-channel).So,a base station's cochannel inter AA cellular base
<br /> the horizon,objects are below the line of sight and Terence area always exceeds its service area. station can only
<br /> are not visible.At UHF television and cellular
<br /> frequencies,radio waves also travel in straight lines. F. PRECLUSION AND THE SCARCITY OF cover a limited geo-
<br /> They too have an horizon.When radio wavesgraphic area because
<br /> strike the earth,the earth absorbs at least some of SPECTRUM radio waves weaken
<br /> the power that the radio waves contain.This also A major consequence of interference is preclusion. with distance and
<br /> limits a radio system's range.The horizon for That is,use of a frequency prevents similar uses of because the earth is
<br /> antennas on mountaintops is further.out than for the same and adjacent frequencies in the same and curved and cellular
<br /> antennas that are not as high.5 surrounding areas.There are many keys on a piano, radio waves travel in
<br /> Sometimes,even two close antennas will not share a but never more than 88.Two people can play on straight lines. Rough
<br /> line of sight.Just as we cannot see more than a block the same piano simultaneously, if one uses lower or hilly terrain, limited
<br /> if a skyscraper is in the way,buildings,hills,or dense octaves and the other the upper registers. Both, transmitter power,
<br /> foliage can obstruct a cellular radio signal's path. however,cannot use the same keys at the same limited receiver sensi-
<br /> Depending on how severe the obstruction is,the instant without disrupting each other's melodies. tivity, and the effects of
<br /> obstruction's effect can range from slightly weakening For similar reasons,two transmitters in the same interference also con-
<br /> the radio signal to totally blocking it(a drop out). area cannot use the same channel simultaneously. strict a base station's
<br /> Thisrise to a scarcityof spectrum,because service area. However,
<br /> gives
<br /> 3. Interference.A clear line of sight between as we will see later,
<br /> transm ittin and receivingantennas helpsproduce the number of channels which make up the radio
<br /> g 1p cellular systems turn •
<br /> high-quality communications,but it does not spectrum is finite.Moreover,the radio spectrum the limited range of a
<br /> guarantee them.Just as one can have problems lends itself to many different applications.9 cellular base station
<br /> trying to understand someone speaking nearby in Because of the finite number of radio channels into an advantage.
<br /> available,the FCC allots only certain channels for
<br /> a crowded,noisy room,so,too,a transmitter and
<br /> receiver only a few miles apart and with perfect each use,based on international treaty,relative
<br /> line of sight may not communicate because of demand,and social importance.And in any area,
<br /> strong interference from undesired radio energy. the demand for radio channels outstrips the supply.
<br /> For cellular communications,the FCC has allotted
<br /> There are many different sources of interference, a total of 50 MHz of spectrum,split between two
<br /> and the interfering radio wave can be on the same licensees in each part of the country,the local
<br /> or a nearby frequency.6 A radio transmitter is telephone company(the wireline carrier),and
<br /> often the culprit,but interference can come from another service provider(the nonwireline carrier).
<br /> such generators of radio noise as computers,elec-
<br /> tric motors,car ignition systems,thunderstorms, G. GETTING AROUND THE SCARCITY
<br /> or fluorescent lights. PROBLEM
<br /> D. BASE STATION SERVICE AREAS The first generation of radiotelephone systems
<br /> Modern radio receivers are very sensitive. If inter- (some of which are still in use)employ high trans-
<br /> ference levels are low,a receiver can produce a lis- muter powers and transmitting antennas,because
<br /> tenable output from a very weak signal—one with their primary goal was reliable,wide-area service.
<br /> a field strength measured in microvolts per But even though first-generation systems offer
<br /> meter. The Federal Communications wide-area,drop-out-free coverage, they have a
<br /> Commission(FCC),which licenses base stations, major disadvantage:their preclusion areas are
<br /> deems the 32-dB1V/m contour the outer edge of even larger.Such systems inherently prevent reuse
<br /> a base station's service area.8 The extent of that of their own and adjacent channels over large
<br /> service area depends mainly on the base station's distances.And as more and more people need to •
<br /> antenna height,signal power,and the nature of use mobile telecommunications,the limit on the
<br /> the surrounding terrain.Higher antennas and pow- number of radio channels available for that purpose
<br /> ers,and flatter terrain,yield larger service areas. becomes a major problem—especially in major
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