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• 9aWhar is the differ- I. SPECIALIZED SIGNAL FORMATS receivers reconstruct the music or speech by ence between converting the acellular system's traffic-handlingfrequency swings(frequency g and digital cello- To increase )of the received signal into AF current. capacity,the licensee must use the limited amount lar communications? Analog FM has good fidelity and immunity to static, of radio spectrum available to it more efficiently. such as that from car ignition systems,fluorescent There are two basic ways to accomplish this.The lights,and electrical appliances.17 However,analog first is to increase channel reuse by suppressing A conventional FM channels take up a good bit of radio spectrum. cellular phone preclusion.This alternative,however,requires Conventional analog FM celh,lar phones use 30-kHz- smaller base station service areas,and more base employs an analog wide channels.18 An FM broadcast station occupies transmission format. stations to serve a given region.And even using a channel about seven times wider. The transmitted radio microcells or picocells, the demand for vacant signal's instantaneous channels can eventually outstrip the supply.The Participants at the Base Station Symposium frequency varies cortin- other course of action,as Symposium participants described various ways cellular systems can use the uously within certain explained,is to change the signal format from analog radio spectrum more efficiently than the conven- limits in step with the to digital.The use of digital signal formats can tio:.al AMPS transmission format does.For example, user's voice. This is mean that a cellular system needs fewer base stations a narrowband version of AMPS called N-AMPS called analog frequency to meet the public's demand for service.In areas of (Narrowband-AMPS)employs channels that are modulation. More extremely high demand for cellular communications, only 10 kHz wide,as opposed to an AMPS chan- for the ultimate in spectral efficiency,a system can nel's 30-kHz bandwidth.N-AMPS permits a advanced systems employ digital trans-mission employ both options simultaneously(use many, threefold increase in traffic density,but at some formats, in which users' closely spaced base stations with very small service sacrifice in coverage and recovered audio at the voices are repeatedly areas,and use a digital signal format). receiver end. sampled and converted Analog Frequency Modulation (FM) is the trans- Also,for greater spectral efficiency than AMPS, filznumerical values• mission format used by FM broadcast stations and cellular networks can also employ digital signal numerical values conventional cellular phones. During moments of formats.Digital Frequency Modulation or its are then transmitted silence,the transmitted signal (the carrier,because cousin Digital Phase Modulation(PM),Time using digital modulation it carries information) is at full power,but the Division Multiple Access (TDMA),and Spread techniques, recovered at carrier rests at the transmission channel's center Spectrum Transmission are the major digital the receiver, and used frequency.As speech or music begins,the carrier technologies currently deployed by cellular systems. to reconstruct the starts swinging up and down within the channel, speaker's voice. The in step with the modulating signal's strength.FM process is similar to the digital techniques corn- BASE STATION PARAMETERS pact discs and CD The table below summarizes the antenna heights,signal powers,number of channels,and distances players use to record between base stations that cellular networks employ. We classify the base stations by phase.Phase is not and play back music. a standard industry usage,just our attempt to convey the changes in base station operating parameters as a cellular network evolves and traffic grows. Cellular systems using digital technology can serve more subscnaers Type of base Antenna height Antenna type ERP per Number Distance than analog cellular station channel of between base systems, all other things channels stations being equal. Rural 250 feet Omnidirectional 500 watts 10 to 15 20 miles or more Phase I 250 feet Omnidirectional 100 watts 10 to 15 10 to 15 miles Phase ll 100 to 150 feet 120'Directional 20 to 50 watts 30 to 45 7 to 10 miles Phase III 100 to 150 feet 120'Directional 10 watts 30 to 57 4 to 5 miles Phase IV 100 feet or less 60'Directional 2 to 5 watts 30 to 90 2 miles or less • Microcell 100 feet or less 60'Directional Less than 1 watt 30 to 90 Less than 1 mile Picocell 100 feet or less 60'Directional 0.2 watt or less 30 to 90 Less than 1 mile 10