radio modulation

8/8/2019 radio modulation

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IEEE TRANSACTIONS O N COMMUNICATIONS, VOL. COM-27, NO. 1 2, DECEMBER 1979752

A Comparison of Modulation Techniques or Digital RadioJOHN D. OETTING. MEMBER. IEEE

Absmct-This paper describes and summarizes the characteristics ofthe modulation techniques ost applicable to digitaladio. The modulationtechniquesiscussedren-off-keyingOOK) withoherentndnoncoherentetection. quadraturemplitudemodulation (QAM),quadrature partial response QPR), frequency-shift-keying (FsK) withnoncoherent detection, continuoushase FSK (CP-FSK)with coherent andnoncoherentetection,minimum-shift-keying (MSK), binaryndquaternaryhase-shift-keying (BPSK,PSK)ithoherentnddifferentially coherent detection, offset-keyed QpSK (OK-QpSK), M-aryPSK withcoherentdetection (M = 8, 16),and16-aryamplitudeandphase-shift-keying (APK). Functional descriptions of these schemes areprovided ndheirperformance is ompared in a eries f tablessummarizing heresultsof the iterature of thepast20 years. Themodulation schemes are compared with respect to ideal (white Gaussiannoise) performance, spectral properties,signaling speed, complexity, andthe effectson performance of interference,ading and delay istortion.

T I. INTRODUCTIONHE crowded cond itions prevailing in man y regions of theradio spe ctrum co mbined with the increased emphasis ondigital transmission have created a need fo r improved spectr umutilization techn iques. The intelligent application of efficientdigital modulation techniques provides one means of achievingimproved spec tral efficiency at a reasonable cost. This surveyidentifies and describes some of the more impor tant mod ula-tion schemesapplicable to digital radio, includingsome thathave been developed in recent years and are not discussed inthe classic text boo ks. This paper should be of interest to any -one involved in comm unications planning and comm unicationssystem engineering.

Much previously publishedmaterialhasbeendevoted tocomparing the performance of digital modulation methods. Inma ny cases, comparisons were limited to on e performance cri-terion (e.g., performance in a restricted ba nd [2 6 ], spectraloccupancy [76] , effects of phase distortion [1161 , or cross-talk [118, 1151) . In othe r cases [77 ,98 , 165, 1921 ,relativelyfew mo dulatio n schem es were considered. This paper presentsan up-to-date comparison of the modulation methods of par-ticular applicability to digital radio channels, and provides anindexed bibliography for readers interested in greater depth.

Sec tion I1 provides fu nction al descriptions of the mod ula-tion schemes considered to be representative of the techniquesappr opriate for digital radio applications. Sec tion 111 summar-izes the performanceharacteristics of the representativemodulation schemes by means of a series of tables togetherwithsourcereferenceswhere com plete detai i r mn be found.

The mod ulation schemes are compared with respect to idealperformance, spectral prope rties, signaling speed , com plexityand theeffectson performance of interfe rence , fadingan ddelay distortion.

11. DESCRIPTIONS O F THE REPRESENTATIVEMODULATION SCHEMES

There are. three basic mod ulation techniques: amp litudemodulation (AM), frequency modulation (FM), and phasemod ulation (Ph4). Each of these basic techniq ues has a largenumber of variants, the most relevant of which will be dis-cussed briefly in this section. In recent years hybr id schemes(e.g., amplitude-and-phase-shift-keying-APK) ave receivedincreased at tenti on because of th eir inhere nt economical useof bandwidth. Therefore, 16-ary APK is included as being rep-resentative of this large class of mod ulation techniques [1771 .The primary com pone nts of a system for transm itting digitaldata over a radio channel are illustrated in Figure 1 . All of thedigital modulation schemes discussed in this paper can be con-ceptualized in terms of radio frequency sinusoids (carriers)modulatedby low frequency (baseband mod ulation ) signalsthat convey the digital information. These baseband modula-tion signals may be filtered, weighted, or otherwise hapedprior to modulating t he carrier in ord er o achieve desirableresults. At the receiver, the baseband information is recoveredby a detectionprocess. Coherent de tection requires a sinusoidalreference signal perfectlymatched in both frequency ndphase to th e received carrier. This phase reference may be ob-tained either from a transmitted pilot tone or from the modu-lated signal itself. Noncoherent det ectio n, being based onwaveform characteristics in depe nden t of phase (e.g., energy orfrequency) does not require a phase reference:

Usually, detection is followed bya decision process thatconverts the recovered baseband mod ulation signal into a se-quence of digital bits. This process requires bit synchroniza-tion, which is generally extrac ted from the received waveform.With most mod ulation schemes, decisions can be made on abit-by-bit basis with no loss in performance,but withsomeschemes an advantage can be gained by examining the signalover several bit intervals prior to making eachbitdecision.The por tion of the received waveform examined by he de-cision device in making a single bit decision is called t he ob -servation interval.

work was s uppor t ed i n par t by t he Naval Research Labora to ry underManuscript eceivedJanuary 16, 197 9; rev i sed Ju ly1 7 ,1 9 7 9 .Thi s Ampli tude Modulation ( A M ) TechniquesCont rac t N00173-76-C-0253 . The simplest digital AM technique is double sideband (DSB)Division.Booz.Allen & Hami l t on ,nc. ,Be thesda , MD 20014 . AM [79,pp.173 f]mo dula ted by a binary signal. The doub leT h e a u t h o r s wi th t he Communica t i ons and In format ion Technology

0090-6778/79/1200-1752$00.75 0 1979 IEEE


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O E T T I N G : 1753OEMOOULATOR

L- - - _ - _ _ _ JFigure I : Primary Components of a D igital Radio System

sideband waveform is represented as:A2fDsB(t) = - [ l +m(t)]CO S o,t (1 )

where m(t) is the modulating signal and w, is the carrier fre-quency (in radians per second). For the case of 100 percentmod ulation by a non-return -to-zero (NRZ) binary data wave-form (m(t)= kl), we have on-off-keying (OOK) modulation.Such an OOK waveform can be detec ted either coherently ornoncoherently,but he difference in performance is slightcompared to th e required increase in complexity so that co-herent detection of O OK is not employed over radio channels.Th e properties of the DSB waveform can be modified by re-placing the NRZ m(t) by some other type of binary basebandsignall (e.g., the partial response signal [18, 20, 341) .

Since the carrier conveys no information, efficiency can beimproved b y t he use of d oub le sideband suppressed carrier(DSB-SC) AM. The general fo rm of th e D SB-SC signal is:

f sc ( t ) = A m(t)CO S a c t . ( 2 )For the case where m(t) takes on the values 0 and 1, we havethe OOK situation described in the previous paragraph. Whenm(t) akes on the values -1 and 1, we have the case of binaryphase-shift-keying (PSK), which will be discussed under PMtechniques.

Both DSB techniques involve the transmission of a redun-dan t sideband. F or app lications in which spectral efficiency isimp ortan t, the occupied bandwidth can be reduced by a factorof two by the use of single sideband (SSB) mod ulation. TheSSB signal can be w ritten as:

f S S B ( t ) A[m(t)cos w,t + m( t ) sin oc t ] (3 )where i ( t ) s the Hilbert transform [79, p. 311 of m(t). npractice, SSB signals are usually generated b y he use of abandpass filter to suppress the upper or lower sideband. Thesharp cu toff characteristic required for the bandpass filter pre-sents implementation problems. Thus, abandpassfilter withsm ooth roll-off is often used. This procedure results in a vestig-ial sideband (VSB) signal [7 9, p. 1 92 1.

QuadratureAmplitudeModu lation (QAM) is yet anotherAM alternative. Th is technique involves summ ing tw o DSB-SCsignals 90 apart in phase as follows:

fQAM(t) = A [ m I ( t ) os a c t + m&) sin a c t ] . (4)1 Rad ios designed for anal ogAM t r an smiss io n a re so me t imes u sed totransmit d ig ita l s ignals . These radios d is tort the d ig ita l s ignal through in-eff ic ient h igh frequency or low frequency s ignal response. Occasionally ,o th e rdis tort ions a re d e l ib e ra tely n t ro d u ced o mp ro v espectra l p e r -fo rman ce a t th e ex p en se o f co mmu n ica t io n s e f f ic ien cy .

When m ~ ( t )s the Hilbert transform of mI(t),QAM reduces toSSB. When mI(t)an d mQ(t) re independent binary data sig-nals, QAM is as efficient in required power and ban dw idth asideal SSB with ou t t he stringent filtering requirements. If mI(t)and m s ( t ) are three-level duobinary signals (+l , -1 or 0 )coded in such a way as to m inimize intersym bol interferencecaused by filtering, the result is quadrature partial response(QPR),whichha sbeenproposed for use in the Canadian 8GHz frequency band [1781 . All of these techniques requirecoherent detection. Any phase tracking errors that occur resultin interference between th eI an d Q channels, thereby degradingperformance.When m,(t) and mQ(t ) ake on the values * I , QAM is iden-tical to quaternary phase-shift-keying (QPSK) discussed in thesection o n phase modulatio n techniques. These tw o techniqueswill differ, hqwever, w hen mI(t) nd mQ(t) re not rectangularpulses.2Frequency Modulation ( F M ) Techniques

The simplest FM technique is frequency-shift-keying (FSK)involving binary signaling by the use of two frequencies separ-a ted by Af Hz, where Af, the frequency deviation, is smallcompared to the carrier frequency, f , . With FSK sch emes, it iscomm on practice to specify t he frequency spacing in terms ofthe modulation index,d , defined as:

d = A f T (5)where T is the symbol duration (equal to th e inverse of thedata rate for binary schemes).As with other mod ulation schemes, FSK can be detectedeither coherently or nonco herently. Noncoherent detectioncan be effected by two bandpass filters followed by envelopedetectorsand a decision device [79 ,p .297 1. With this ap-proach, the frequency spacing must be at least l / T ( d > 1 ) t oprevent significant overlap of the passbands of the tw o filters.Alternately, adiscriminator can be used to convert the fre-quency variations to amp litude variations, so that AM enve-lope detection can be employed [8 2] . Thisapproach elimi-nates the above constraint ond.Rec ently , considerable interest has arisen in modified ver-sions of FSK , including some coh eren t schemes. These schemesare based on the idea of contin uou s phase FSK (CP-FSK), inwhich the abru pt phase changes at the bit transit ion instantscharacteristic of othe r FSK implem entations are avoided. Thisimplementation of FSK results in rapid spectral roll-off andimproved efficiency. The improvement is attain ed by the useof observation intervals greater than one bit [1 , 4 0 1 .This fea-ture enables narrower filter bandw idths than would otherwisebe feasible. With c ohere nt de tection , values of d in the neigh-borhoo d of 0.7 have been shown to provide optimal perform-ance for a ny observation in terval [1231 .

Ano ther FM tech niqu e hat has received considerable in-terest in recent years is minimum-shift-keying (MSK), alsocalled fast requency-shift-keying. MSK is a special caseof

2 Nonrectangular pulses can be employed t o e l iminate abrupt transi-t io n s in th e m o d u la ted wav e fo rm, th e reb y imp ro v in g sp ec t ra l ch a rac te r-is t ics and easing transmitter implementation .


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1754 IEEER A N S A C T I O N SNO M M U N I C A T I O N S . V O L . COM-27, NO. 12 , DECEMBER 1979CP-FSK for which d = 0.5 and coh eren t d etectio n is used. Thistechnique achieves performance identical to cohe rent PSK andexhib its the superior spectral properties of CP-FSK. MSK hasthe additio nal advantage of the possibility of a relatively sim-ple self-synchronizing implementation [59] , n advantage tha tcoherent C P-FSK with d = 0.7 does not share.Phase Modulation (PM ) Techniques

Almost by defin ition, digital PM schemes require coh eren tdete ctio n. There are hre e basic variations of bina ry phase-shift-keying (BPSK). The most straightforward approach is coherentBPSK, in w hich the carrier phase is shifted by 0 or 180 degrees.Detection requires a precise phase reference, which is normallyobtained by performing a nonlinear operation on the receivedwaveform. Since some phase reference extraction echniquesexhibit 180" phase ambigbities, a modified form of PSK calledDifferentially En cod ed PS k (DE-PSK) is often used. With DE-PSK , inform ation is conveyed via tra nsition s in carrier phase(e.g., no ransition may co rrespo nd oa space anda 180"transition may correspond t o a mark). Since a bit decision er-ror on he current bi t will induc e another error on he sub-seque nt bit, the pe rform ance o f DE-PSK is slightly inferior tothat of coherentPSK.

The third version of bina ry PSK is Differential PSK (DPSK),in w hich, as w ith DE-PSK, the information is differentially en -code d. The difference between DPSK and DE-PSK lies in thedetector. With DPSK, no at tempt s made to extract a coherentphase reference. Rath er, the signal from the previous bit inter-val s used as a phase reference f or hecurrentbit interval.Since the phase reference signal is not smoothed over many bitintervals, the perfo rman ce of DPSK s somew hat worse thanthat of DE-PSK.

Quaternary PSK (QPSK) schemes will also be considered.Cohe rent QPSK involves encoding tw o bits at a time into oneof four possible carrier phases spaced 90" apart. As in the bi-narycase, the data can be differentially encoded and differ-entially detec ted w ith a conc omitan t loss in performance (thisscheme w ill be den oted DQPSIC). In recent years, a modifiedversion ofQPSK, called offset-keyed QPSK (OK-QPSK) orstaggered QPSK (SQPSK), has com e nto use. This schemeoffers advantages over conventional QPSK with regard to spec-tral efficiency, sideband regeneration and synchronization [45 ,50,94, 1441.OK-QPSK can be visualized by considering the QPSK sig-nal to consist of in-phase and quadrature com ponents (as withQAM). With norm al QPSK, during each 2 bit time interval of Tsecon ds, the carrier is binary PSK mod ulated by one bit andth e Q carrier is mod ulated by the othe r bit. The resulting sig-nal can take on any one of four possible phases, and abrup tphase transitionsof 0", 90", or 180" can occur. With OK-QPSK, the Q channel is shifted b y T / 2 seconds with respect t oth e I chan nel. The transitio n rules are designed so that whenth e I and Q channels are adde d togeth er, the resulting signalcan shift abrupt ly by 90" at most (but shifts can occur everyT / 2 seconds, compared t o every T seconds for standard QPSK).

Hyb rid AM/PM TechniquesThe ever increasing need forbandwidth conservationha s

led t o the use of a class of hybrid AM/PM techniques called

0 0

0 0

0 0

0 0

0 0

L O C A T I O N OF I th S I G N A L

0 0

0 0

S i ( d = a i cos Iw t + E i l It= 1, . ., 161Figure 2: Location of 16-ary APK Waveforms in Phase-Amplitude Space

amp litude and phase-shift-keying (APK) [1771 . The resultinginformation signals are best visualized by a representation in aphase-amplitude signal space. Figure 2 shows the signal spacelocation of each of the 16 possible transm itted signals for 16-ary APK . Because this scheme conveys four bits of informa -tion duringeach signaling interval, it hasbeenproposed foruse over digital radio channels [1761 .111. COMPARISON O F REPRESENTA TIVE MODULATION

SCHEMESIn this sectio n, he mod ulation schemes described in Sec-

tion I1 are compared w ith respect to their performance unde ra variety of cond itions characteristic of digital radio channels.A performance measure utilized thro ugho ut this section is thebasebandequivalent E B / N o (defined as the atio of averagesignal energy per b it to noise power spectral de nsity, as meas-ured at the input to the receiver) required t o achieve a bit er-ror rate of lop4. This error rate is adequa te for most generalpurpose digital radio a pplications.Ideal Performance

In ord er to establish a baseline for comp arison, Table 1 pr.e-sents the ideal performance of the representative modula tiontechniques in the presence of additive white Gaussian noise.References are included with each en try o facilitate obtain-ing values of required EB/No or oth er error rates3 Of par-ticular interest is the fact that CP-FSK with coherent detec-tion over a3-bit observation nterval can outp erfor m BPSKand o the r equivalent techniques (which are optim al only whenthe observation interval is confined to one bit). The identicalideal performance of QAM, MSK, and QPSK atte sts to theirunderlyingsimilarities, often discussed in the iteratu re [ 17,601. ndeed,offset-keyed QAM [45], MSK, and OK-QPSKdiffer only in the w eighting functions applied to th e I an d Qchannels.

3 Care must he exerc i sed t o conver t a r ious SN R measures t o E,/NoEB/No s used t h r ougho ut , t he va lues shown for heOOK schemes ar ean d t o conver t symbol e r ro r ra tes to b i t e r ro r ra tes . Also, since averagealways 3 dB helow the peak values given in m any refere nces.


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O E T T I N G : M O D U L A T I O N T E C H N I Q U E S F O R D I G I T A L R A D I O 1755TABLE 1

IDEAL PERFORMANCEO F REPRESENTATIVE M O D U L A T I O NSCHEMES

M i P M I 16-ary AP K 12.4 I 189* FO R B I T E R R O R R A T E O F o 4ASSUM ESTHREE-BITOBSERVATION I N T E R V A L d = F M M O D U L A T I O N N D E X

Spectral C haracteristicsThe spectral characteristics of the mod ulation schemes can

be compared in many ways. Of particular interest is the exte ntto which a signal will interfere withsignals in adjacent chann els.Onemeasureof his quality is the attenuatio n of a signalspower spectrum a specified distance from the center frequency.If , for example, we examhe he at tenuat ion at an arbi t rarydistance of 8/T Hz from the center frequency Tis the symbolduration), we find hatwith AM schemes th e sidelobesaredownbyabout 25 dB,with PM schemes the sidelobesaredown by about 33 dB, a nd w ith continuous hase FM the side-lobes are dow n by 60 dB or more.4 In general, for frequenciesfar from the center frequency (large ( f - ,.)T), the spectrumof AM an d PM signals falls off a s p 2 while that of CP-FM sig-nals falls off as f- 4 ..

While these numbers appea r to indicate a significant advan-tage for FM scheme s, h ey should be put n he proper per-spective. First, the figures quo ted for PM system s assum e thatabrupt phase transitionsoccur. If phase ransitions can bemade to occur more smoothly, improved spectral characteris-tics can beachieved. Second,modifications ca nbe made toAM schemes. Forexample,Shehadehand Chiu [I051 showthat a type of continuous phase AM exhibits a spectrum thatfalls off as f P 4 . Finally, sidelobes can always be reduced bysuitable. post modulation filtering, although a penalty in per-formanc e is incurred. Thus, the spectral merits of the variousschemes can only be judged on the basis of a detaiied study ofthe tradeoffs between cost and performance.Another spectral property of interest is the bandw idth re-quired to transmitata specified information ate.The so -called speed of a modulation technique (equalo R/W,where

4 A modified version of MSK knownas i nuso ida lF S K S F S K )achieves an a t t enua t i on of 95 dB I15 .

TABLE 2RELATIVE S iGNALING SPEEDS OF R E P R E S E N T A T I V EM O D U L A T I O N S C H E M E ST Y P E M O D U L A T I O N S C H E M E SPEEDlb/r PER Hz ) R E F E R E N C E E 8 I N OdBI

OO K ~ C O H E R E N T D E T E C T I O N 262.5.8

A M OO K - E N V E L O P E D E T E C T I O NO AM ,

6 41 . 8 0 . 8 F S K - N O N C O H E R E N T D E T E C T I17 81.7.25PR14 5.5.7

Id = 11CP F S K C O H E R E N T D E T E C T I O N

F M - Id = .71C P-FSK N ON C OH ER EN T D ETEC TIO N 10.7.0

Id = .71 26

MS K I d = 5l 44.41.9 IMS K - D I F F E R E N T I A L E N C O D I N G I d = .51

0.8PSK ~ C O H E R E N T D E T E C T I O N440.4.9

26 t.9 0 .8DE-BPSK269. 4

I DPSK I 0.8 1 10.6 I R7 II I IQPSK 1.9

16 51.8 1.6OPSK57.9PM

I O K - O P S K I I I I8-ary PSK ~ C O H E R E N T D E T E C T I O N

1767. 2.96 -a ry PSK -C OH ER EN T D ETEC TION57 12.8 2.6

AMIPM. FOR B ITR R O RAT EF lo4 d = F MO D U L A T I O NN D E Xt C A L C U L A T E D F R O MR ESU L TS FOR BPSK 1763.4.16 ~ a r y P K

* * D I S C R I M I N A T O RE T E C T I O N

R is the data rate and W is the IF bandwidth) is an importantfigure of mer it. n Table 2 , we list the speeds for each tec hniqu e,togetherwithhe EB/No required for BER when thesignal is filtered at the indicated bandwidth i.e., the degradingeffects of finite band width are included). The results presentedin Table 2 were derived from many different sources, so thatslightly diffe rent ilters5 wereused in obtain ing he results,but these figures are indicative of the results to be expected. Itshould also be noted that, in most cases, no rigorous attemp twas made t o achieve the optim um com binatio n of speed andefficiency.Effects of Interference

Ano ther factor in evaluating potential modu lation schem esfor digital radio is the effects of co-channel and adjacent chan-nel interference. We have already discussed one aspect of adja-cent channel interference-the out-of-band attenu ation of thevarious schemes. It was p ointed o ut tha t MSK and the otherCP-FSK scheines enjoy a large advantage over th e AM and PMschemes,when no post-modulationiltering is employe d.Another aspect of adjacent channel interference is the amo untof performa nce degrada tion caused by a specified level of in-terference. Table 3 illustrates the effe ct of an in-band CW in-terferer (10 dB or 15 dB down in power from the desired sig-nal) on he E B / N o required for a lop4 BER.Thissituationcan be a model either for interfering sidelobes from adjacentchannels or fo r the main lobe from a co-channel interferer. Ofthe schemes for which data are available, noncoherent FSKand BPSK show the least degradation from ideal performance,while the 8-ary and 16-ary schemes exhibit the most degrada-tion compare to Table1). Unfortunately,noanalyticalor

5 In mo st cases, a simple hree-pole fi l ter or a Gaussian fi l ter wasused.


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1756 IEEER A N S A C T I O N SNO M M U N I C A T I O N S , VOL. COM-27, NO. 1 2 , DECEMBER 1979T A B L E 3 ,PERFORMANCE O F REPRESENTATIVE MODULATION SCHEMESIN THE P R E S E N C E OF cw N T E R F E R E N C E

F M I CP-FSK - NONCOHE RE NT E TE CTI ONId = .71 I I . I

PM

MS K Id = .5 lMS K - DI FFE RE NTI AL E NCODI NG Id = .5)BPSK - COHE RE NT DE TE CTI ON

1915.82 0-ary PSK - COHE RE NT DE TE CTI ONOK-OPSK

1474 020OPSK147. 191.82.2PSK

1470.32. 0PSK147.71.0E-BPSK

147, 191.20.5

l 6ary P SK - COHE RE NT DE TE CTI ON 19124AMIPM l 6 a r y APK

F O R B I T E R R O R R A T E OF d = FM MODULATI ONN D E X

simulation results are available for any of the CP-FSK schemesor for OK-QPSK.Effects o f Fading

Fading is another problem oftenencountered on digitalradio inks. If the fading is caused by wo resolvable multi-path com pone nts, then the results of .Table 3 can be utilized(the CW interferer can represent the signal from the secondarypath). If the fading is caused by a large, num ber .of equal ampli-tude com pone nts, he Rayleigh fading model [79, p. 3481 ismore appropriate.6 Table 4 presents th e performance esultsfor a Rayleigh fading channel. Because of the severe effects ofRayleigh fading, a required itrror rate of is assumedin this table (although this erro r rate is rather high fo r digitalradio pplications, rror contr ol coding could be used toachieve the desired ;ate). Because thk values of Table 4 repre-sent simply a weighted average of the ideal performance curves,th erelativ e performance of the schemes does not differmarkedly from hat indicated by Table 1. Itshould be em-phasized tha t for the AM and AM/PM schemes, the fading isassumed t o be slow. enough tha t tde d ecision threshold can becontinuously adjusted to the optimaivalue.Effects ofDelay Distortion

Yet an othe r fac tor that should be considered in selecting amod ulation scheme for digital radio applications is the effectof d e l a y d i s t ~ r t i o n . ~ost of the delay distortion observed on

6 Nei ther o f hese model s akes n to account he fade ra t e of th esignal .Adiscussionof heeffectsof ade a t e on he p roba b i l i t y o ferror i s bey ond th e scop e of this paper, but these effects are discussedin some of t he re fe rences [ 1 4 , 28 , 3 5 ; 3 7 , 4 7 , 6 5 , 7 9 , 1 6 1 1 .7 I f the distortion observed on he received signal can be modeledby passage of he ransm it ted signal through a inear i l ter, he delaydistort ion is t he der i va t i ve o f t he phase- f requency charac t e r i s ti c o f t he

f i l t e r [77 , p . 851.

TABLE 4P E R F O R M A N C E O E REPRESENTATIVE MODULATION SCHEMESON A RAYLEIGH FADING C H A N N E L

TYPE I MODULATI ON S CHE ME I E nI NndBI ' I RE FE RE NCE IV E RAGEI OO K - COHE RE NTETECTION I 17' 1 79 I

AM I OOK ~ ENVELOPEETECTION I 19' I 79 II IOA MOPR

14

I FS K ~ NONCOHE RE NT DE TE CTI ONid =11 1 2 0 1 f f 1 1CP-FSK- COHE RE NT DE TE CTI ON 1743" 'r d = .71FM -C P - F S K - N O N C O H E R E N T D E T E C T I O NId = .71 18". 174

PM

~

MIPM~

-

-A* FOR BIT ERRORRATEOF1AS S UME SOP TI MUMV ARI ABLETHRE S HOLD-

d = F M M O D U L A T I O N I N D E X*' A S S U M E S T H R E EB I T O B S E R V A T I O NN T E R V A L

TABLE 5PERFORMANCE O F REPRESENTATIVE M O D U L A T I O N SCHEMESI N T H E P R E S E N C E O F D E L A Y D I S T O R T I O N (d/T= 1)

1OK-OPSK

16-ary PSK - COHE RE NT DE TE CTI ON17525


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O E T T I N G : M O D U L A T I O N T E C H N I Q U E S FOR DIGITAL RADIO 1 7 5 7

1 1 1 1 1 1LOW COMPLEXITY

t t t t t HIGHDOPSKLL L P S IFEK - N O N C O H E R E N T O E T l C T l O NLCPFSK .lSCRlMlNATOR DETECTI ONOOK - ENVELO PE O L T t C T l O NFisure 3 : Relative Complexity ot'Representative Modulation Schemes

suffers severe degradation fromquadratic delay distortion,while the coherent biorthogonal schemes (QAM, MSK, and thevariations of QPSK) are degraded significantly b y linear delaydistortion. Thus, delay distortion can be an impo rtant criterionin the selection of a mod ulation scheme for digital radio.Cost an d Complexity

Finally, ome mentio n should be made of the costan dcomp lexity of th e mod ulation schemes discussed in this paper.In general, it is difficult to evaluate the cost of a particularscheme withoutconductinga full-scale investigation of thecost and complexity tradeoffs involved with the many alterna-tive implem entation ptions. Nevertheless, themodulationmetho ds can be ranked according to their inher ent com plex ity,and the results of such a ranking are presented in Figure 3.Application of the Results

The results presented in this section in conjun ction with thereferenced source material can be used to solve systems engi-neering problems. In many applications, most of the altern a-tives can be eliminated simply bydetermining and applyingthe physical and economic constraints.

Supp ose, for examp le, hat t is desired to de termin e hebest mod ulation scheme for ransm itting90 Mbits/s over a40 MHz radio channel. A quick glance at Table 2 shows thatonly QP R, 8-ary or 1 6-ary PSK, and 16-ary APK can achievethe necessary signaling speed (2.25 bits/s/H z). Ofth eseschemes,QPR is the most efficient (smallest required E,/N,,) and theleast complex. These considerations have promptedat leastone proposal to employ Q PR in the Canadian 8 GHz frequencyband [1781 .

IV. CONCLUSIONSThis paper has treated some of the more important consid-

eratio ns relevant to the selection of modulation schemes fordigital radio applications. In th e process, we have summarizeda portion of the vast body of digital modulation literature ofthe past 20 years. Hopefully his wo rk will prove useful tocommunication engineers both as a summary of modulationcharacteristics and as a guide to th e available literature .

Clearly, his paper does not cover every aspect of digitalmod ulation. Many factors(e.g., the effects of frequency offset,imprecise carrler phase recovery andimiting) re omittedfrom the discussion. Many of these topics are covered in detail.in th e references at the end 'of this paper. As an aid to usingthese references,Appendix A presents a omple te subjectindex.

ACKNOWLEDGMENTThe author wishes t o acknowledge Mr. D. I. Himes of the

Naval Research Laboratory, Washington, D.C., for suggestingthe study that eventually led to this paper.APPENDIX A: SUBJECT INDEX

Am plitude ModulationAM-FSK:30Binary Coherent

ge ne r a l : 32 , 70 , 77 , 80 , 119 , 192spec trum: 75,105filtering: 26synchronization: 90delay distortion: 1 16fading: 109

Binary Noncoherent: 70,192M-ary: 52, 19 2OK-QASK: 45 ,6 2O O K : 3 5 , 7 0

QAMgeneral: 67, 80spectrum: 98, 145filtering: 98exper imenta l : 98 , 145synchronization: 89delay distortion: 11 6interference: 98

QASK: See Amplitude and Phase Modulatio n,QPR: 188Part ia l Response: 18 ,2 0 ,3 4 ,9 0Radios (digital transmission over): 67

Frequency ModulationChirp modulation: 55CP-FSK

general: 1,4 0, 71 15 7spec trum: 21,76fil ter ing: 71,72, 165experimenta l : 66 ,71 , 1 65demodulation: 123, 157, 164delay d istortion: 165interference: 73, 118, 155, 165frequency stability: 165M-ary: 1, 118, 165

Digital FMspec trum: 21,75f il te ring: 25 ,7 4, 8 2exper imenta l : 63 ,113,122demodula t ion: 126, 133interference: 104, 113


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1758fading: 65biphase FM: 148capture effect: 9 1threshold performance: 43

FSK (coherent)general: 86 , 192spectrum:141experimental:12 4demodulation: 102delay distortion: 1 16phase reference: 110fading: 39M-ary:124

FSK (noncoherent)genera l: 24 ,3 0,8 0, 19 2filtering: 26, 2 7, 30demodulation: 8 2, 121, 164delay distortion: 1 16frequency uncertainty: 178interference: 4 1fading: 28, 3 9, 15 8nonlinearities: 85M-ary: 23, 165, 192

MSK (See also SFSK, TFM)genera l: 1 7 ,4 2, 5 9 , 151, 160s p ec tr um : 1 5 , 7 6 , 8 4 , 1 6 6f il te ring : 44 , 7 1 , 7 2 , 8 4 , 9 7 , 18 0exper imenta l : 68 ,71,96, 144demodulation: 59, 156, 171delay distortion: 96phase reference: 16 9

I E E E T R A N S A C T I O N S O N C O M M U N I C A T I O N S ,VOL. COM-27, NO . 1 2 , D E C E M B E R 1 9 7 9

interference: 118, 153, 155, 171, 182nonlinearit ies: 144, 18 0, 186, 18 7M-ary: 155multiamplitude (MAMSK): 167differential MSK: 17 1

SFSK (Sinusoidal FSK): 153, 1 56, 1 70, 17 1TFM (Tamed FM ) : 166

Phase ModuZationBinary PSK (BPSK)

genera l : 67 ,77,80,86: 119, 192spec trum: 7 6,7 5, 150filtering: 8, 10 , 12 , 26 , 10 7, 1 1 , 136, 143, 154, 162,168experimental: 2 2, 139, 14 4

demodula t ion: 164delay distortion: 1 16phase reference: 46 ,6 9 , 88, 101, 129, 149inte r fe rence : 33 ,4 8, 56 , 11 8, 143 , 147, 14 9, 162fading: 39,47, 161nonl inear it ie s: 2 ,4 ,6 ,9 3 ,9 9 , 144, 159, 168, 172,1 8 3 , 1 8 5

D PE K: 9 8 , 1 6 4 , 1 6 5

DPSKgenera l: 67 , 71 ,77, 1 08, 1 64, 92fil ter ing: 38,71,87, 115, 165e xper im e ntal : 13 , 22 , 66 , 71 , 1 12 , 1 44 , 165frequency uncertainty: 178demodulat ion: 16 ,61, 114 , 163, 164phase reference: 100, 1 12interference: 52, 73, 104, 142, 147, 165fading: 1 4 , 38 , 39nonlinearities: 14 4, 159, 172

DQPSKgeneral: 71spectrum: 9 8fil ter ing: 71,98, 165e xp er im en ta l: 6 6 , 7 1 , 9 8 , 1 1 2 , 1 4 4 , 1 6 5phase reference: 100, 112 , 172interference: 98, 1 47, 1 65fading: 37nonlinearities: 14 4, 172frequency stability: I 6 5

Ail-ary PSK (Co here nt)general: 1 79, 192spectrum:17 5filter ing: 3,49, 57, 58, 175experimental: 175delay distortion: 175interference: 5'2,58, 175, 179, 191fading: 161, 181nonlinearities: 144, 186, 187

OK-QPSKgeneral: 17, 60spectrum:17filtering: 17 ,71 , 18 0, 193experimental: 71, 144demodulation:103synchronization: 45, 184phase reference: 50, 1 73interference: 15 3, 155fading: 1 8nonlinearit ies: 94 , 144, 1 7, 180 , 186, 1 93

QPSK (coh erent)general: 8 6spectrum: 144fil ter ing: 95,96experimental: 144, 145phase reference: 101, 149, 152, 173interference: 47, 161fading : '1 81nonlinearities: 31, 51, 93 , 96 , 99 , 144 , 172 , 187equalization: 1 1

SQPSK (see OK-QPSK)Amplitude an d Phase Modulation (APK): 37 ,52, 167, 176,1 8 9 , 1 9 0


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I. pp. 47-5 .

1969. p. 589-594. *John D. Oetting (S'69-"73) was born inMerchantv i l le,New Jersey on July 3 I . 1947. H ereceived the B.S.E.E. (su mm acum laude). the M.S .and the Ph.D. degrees from the Unive rsity ofPennsylvania. Philadelphia, Pennsylvania. in 1969.1971 and 1973, espectively.For the past f ive years. D r. O ettin ghas been withBooz .Allen and Ham ilton n Bethesda. Marylan d.work ingn the reas o f coding and modulationanalysis. Lo w Probability o f Intercept (LP I) andAnti- jamAJ )omm unication techniques. and

digital ignal rocessing. r ioro that. he worked in the field o fH Fcommunications and over-the-horizon radar at I l T Electro-PhysicsLabo ratories n Colu mbia . Mary land. While n graduate school. he spent twosumm ers as a research assistant at the L os Alamo s S cientific Laborato ry in L n sA lamos .NewMex ico. and one summer working on his dissertation topic(formant extraction b y linear pre diction) t Philco-Ford Corporation inW i l l o wGrove. Pennsylvania.D r . O e t t i n g s a member o fT au Beta P iand Eta Kappa Nu .

radio modulation

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