1.Modulation Techniques

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Modulation Techniques
   Modulation Techniques __________________________________________________________________________________ _ 1MODULATION TECHNIQUES Introduction Radio communication has been employed as a replacement for copper based cables inthe long distance media for several years. More recent developments in digital radios andadvances in micro-electronic circuits have given rise to wireless in local loop (WILL)systems and cellular mobile systems.Modulators have assumed considerable importance in radio communication systems.heir performance! to a large e tent! determines the #uality of recovered speech andacapacity of the system. In Mobile $ommunications and for fi ed wireless applications! bandwidth is a limited natural resource. %pectrally efficient modulators can accommodatemore information content in the limited available bandwidth. his article describes thevarious parameters and factors governing the choice of a modulator for a mobile or fi edwireless digital cellular radio system. Frequency Band &arious 'Micro and 'Macro cellular digital radio technologies used for cellular mobile systems  fi ed applications i.e. wireless in local loop (WLL) systems available indifferent fre#uency bands and their important parameters are indicated below*+%M +lobal %ystem for Mobile $ommunication,M% ,igital dvance Mobile hone %ervice$,M $ode ,ivision Multiple ccess$-/ $ordless elephony 0 /,1$ ,igital 1nhanced $ordless echnology2% ersonal 2andiphone %ervice34R4RI! 5abalpur   Modulation Techniques __________________________________________________________________________________ _ Fig  ! 1#A$Fig  ! 1 a)mplitude %hift 6eying (%6) b)7re#uency %hift 6eying (7%6)c)hase %hift 6eying (%6)%ometimes a combination of above basic methods is used for a typical application.he signal to be transmitted is a stream of 3s and 8s i.e. 9: or 977 in the shape of  pulses. heoretically an infinite bandwidth is re#uired to transmit such stream of pulses.%ince any communication system has a limited availability of R7 4andwidth! this band/4R4RI! 5abalpur   %;%1M1$2:9L9+;7R1<=1:$;$ell %i>eMultiple ccessR7 $hannel4it RateModulationMacro $ellularMicro $ellular +%M?@8-@3A@BA-@C8Mh>Large,M/D3.?6bEs+M%6 ,M%?/F-?F@?C@-?@FMh>Large,MF?.C6bEs9<%6 $,M?/F-?F@?C@-?@F Mh>Large$,M3//?6bEs<%6E9<%6 $/?CF-?C?Mh>%mall7,MD/ 6bEs/L1&1L7%6 ,1$3??8-3@88Mh>%mall,M33A/6bEs+7%6 2%3?@A-3@3?.Mh>%mall,MB?F 6bEs9<%6    Modulation Techniques __________________________________________________________________________________ _ limiting introduces detection errors at the receiver. hat is why the filter bandwidth must bechosen to optimi>e trade-off between R7 bandwidth and error rates. his constraint forms the basis of design and selection of bandwidth limited digital modulators  demodulators. variety of digital modulation schemes are used in wireless communication systems.We shall discuss the various modulation schemes used particularly in handheldE cellular mobile radio environment. T%e Ce&&u&ar En'iron(ent In a Mobile :etworG it is a challenge to serve thousands of subscribers with a limitedfre#uency resource at an affordable cost. In order to overcome this problem $ellular radiosystems operates on principles of fre#uency reuse where the R7 carrier fre#uencies aresimultaneously reused at geographically separated locations.  typical seven cell pattern isdepicted in fig./.3. 4oth cells numbered as '3 use same set of R7 channels and hence aretermed as co-channel cells. Fig  ! !Frequency )eu*e +attern N,- %uch systems are thus naturally limited by co-channel interference. 2ence cellular radio systems must use modulators that are both bandwidth efficient and are capable of tolerating relatively higher levels of co-channel interference. )equire(ent* o. a good (odu&ation *c%e(e o suit the specific re#uirements of digital cellular radios the modulators should preferably satisfy the following properties.B4R4RI! 5abalpur   Modulation Techniques __________________________________________________________________________________ _ Co(/act +o0er Den*ity S/ectru( o minimi>e the effect of adHacent channel interference it is necessary that the power radiated into adHacent channels is about C8 d4 below as compared to power transmitted indesired channel. In other words the modulation techni#ue which offers narrow main beam inthe desired channel and fast diminishing side lobes is desirable. ood Bit Error )ate +er.or(ance Modulators with a low (good) bit error rate performance (41R) in the presence of atypical cellular environment characteri>ed by appreciable co-channel interference andchannel impairments are desirable. A )e&ati'e&y Con*tant En'e&o/e ortable and Mobile handsets generally use more efficient amplifiers to minimi>e the battery drain. %uch amplifiers are non linear ($lass '$) amplifiers. %uch non linear amplification shall lead to degraded 41R performance of modulation schemes that transmitthe information in the amplitude of the carrier. 2ence it is desirable that a relatively constantenvelope signal (wherein the modulated signal stays within a constant envelope of amplitude)is used alongwith non-linear amplifiers to prevent the regrowth of undesirable spectralsidelobes during non-linear amplification. Modu&ation Met%od* Let us now discuss the following modulation methods and their suitability for digitalcellular radio environment. A(/&itude S%i.t 2eying #AS2$ In mplitude %hift 6eying we modulate the baseband signal into changes inamplitude of the transmitter carrier and the fre#uency of R7 carrier remains the same.=nfortunately the error performance (41R) of %6 signal is inferior to the other forms of digital modulation particularly when non-linear amplifiers are used as it is the case withmobileE WLL systems. 2owever there is a very important use of amplitude modulationcombined with phase modulation e.g! 3C #uadrature amplitude modulation (<M) in fi ed point to point radio applications. Frequency S%i.t 2eying In 7re#uency %hift 6eying! R7 fre#uency is varied in accordance to the amplitude of the modulating signal and the R7 carrier amplitude remains constant. 7%6 allots one fi edfre#uency tone (say) f3 for 8s and another fi ed fre#uency tone (say) f/ for 3s. in other wordsthe input data se#uence is used to switch bacG and forth between these two fre#uenciesaccording to change from 8 to 3 or 3 to 8. (Ref fig /./). In this simplest form as depicted infig././ it has two fre#uencies  is called a two level (binary) 7%6 i.e. (47%6).$/ technology uses 47%6 with ±  /A 6h> deviation from nominal R7 $arrier F4R4RI! 5abalpur


Jul 23, 2017
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