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• QWhy do cellular cities and their suburbs,and even along heavily systems,which use different radio channels like usystems have so travelled country highways. maps use different colors.To prevent interference, many base stations? a cellular network breaks a region down into sub- Cellular base stations exist to circumvent the natural cap on radio channels.As the public demands units,and builds one base station with a specific more mobile telecommunications service,more set or block of frequencies (channels)for each. A A cellular system Immediately adjacent base stations use ocher and more base stations are needed.In contrast to employs many the traditional handful of wide-coverage base stations, blocks of channels,and ocher base stations a bit base stations with further away can reuse the original block of radio deliberately limited cellular base stations have modest signal powers, range to allow reuse of channels employed at the first base station. antenna heights,and service areas.The primary goals are to suppress preclusion and permit Because base stations have limited range, the strictly limited mobile number of radio than- frequency reuse in nearby areas.Of necessity, this users often reach the edge of a base station's service eels available for requires more base stations-to cover the same area area before the phone call ends.A key aspect of mobile telecommunica- as a first-generation radiotelephone system with cellular technolo'v is that,when a base station tions. The more base just one or two base stations. senses the mobile signal weakening, it"hands off" stations a system uses, In other words,cellular systems invest in items responsibility for servicing that call to the next the more subscribers available in quantity(towers and the other items base station down the line.The transition is the system can handle that comprise base stations) to stretch out a strict- designed to happen so quickly that there is no at any one time. ly limited resource—the radio channels available break in the conversation,and the conversation for mobile telecommunications.Constructing smoothly continues on a different radio channel. This more,but smaller base stations allows a cellularfrees up the first base station to serve another network to serve many more people at the same mobile unit on the original channel. time.Subscribers within the.same general area can H. TYPICAL BASE STATIONS • use and reuse the allotted radio channels,without mutual interference. At the Federal Focus National Symposium on Wireless Transmission Base Station Facilities, A good analogy to spectrum reuse through the cellular engineers explained that a base station's construction of more,lower-power base stations is ERP,antenna height,and complement of channels the map-coloring problem studied in some math depend on the traffic load.At the onset of cellu- courses.Suppose you have a black-and-white lar network operations,when phone traffic is the county map of a state and some colored pencils. lightest,a typical base station might have a 250- You must shade the counties such that no two foot tower, 10 to 15 active channels,and about adjacent ones are the same color.If you have just a 100 watts per channel of ERP.14 Antennas are few colors,you run into problems.The solution is omnidirectional;the service radius of each base to break counties down into smaller units,and to station is about five to eight miles.The base reuse your pencils such that adjacent sub-d stric stations are about 10 to 15 miles apart. do not have the same color.So it is with cellular SIGNAL POWER watts of RF output The missing power becomes Two frequently used,closely related terms are 40 watts of waste heat and the transmitter . energy and power.Energy is the ability to do work; becomes warm to the touch.Of the 100 watts of transmitter RF output power,some will be lost as power is performance of a certain amount of work in a fixed time. heat in the antenna feedline and in the transmit- tingThe physical concepts of energy and power also antenna itself.Let us suppose that loss is apply to radio.A transmitter-generated RF signal 10%.Ninety watts of signal power remain.If the has power.A radio wave leaving a transmitting transmitting antenna is a half-wave dipole(one antenna also has power.It can stimulate an RF whose electrical length is half of the radio signal's current in a receiving antenna. The power is wavelength,or half the distance the radio wave measured in watts,or,if it is weak,in milllwatts or travels during one cycle),the 90 watts escape out microwatts.to into space as a radio wave. The dipole emits the • most signal at right angles to itself,and relatively No radio component is 100%efficient;some power little off its ends,in a doughnut-shaped pattern. is always lost as heat.A transmitter draw The Effective Radiated Power or ERP is 90 140 watts from the power grid to produce just 100 watts."At any point a given distance from the El