Angle Modulation And Multiplexing
Chapter4Angle Modulation and MultiplexingContents4.14.24.34.4Angle Modulation . . . . . . . . . . . . . . . . . .4.1.1 Narrowband Angle Modulation . . . . . .4.1.2 Spectrum of an Angle-Modulated Signal .4.1.3 Power in an Angle-Modulated Signal . . .4.1.4 Bandwidth of Angle-Modulated Signals . .4.1.5 Narrowband-to-Wideband Conversion . . .4.1.6 Demodulation of Angle-Modulated SignalsFeedback Demodulators . . . . . . . . . . . . . .4.2.1 Phase-Locked Loops for FM Demodulation4.2.2 PLL Frequency Synthesizers . . . . . . . .4.2.3 Frequency-Compressive Feedback . . . . .4.2.4 Coherent Carrier Recovery for DSB Demodulation . . . . . . . . . . . . . . . . . . .Interference and Preemphasis . . . . . . . . . . . .4.3.1 Interference in Angle Modulation . . . . .4.3.2 The Use of Preemphasis in FM . . . . . . .Multiplexing . . . . . . . . . . . . . . . . . . . 64-604-604-644-65
CONTENTS4.54-24.4.1 Frequency-Division Multiplexing (FDM) . . 4-664.4.2 Quadrature Multiplexing (QM) . . . . . . . . 4-69General Performance of Modulation Systems in Noise 4-70ECE 5625 Communication Systems I
4.1. ANGLE MODULATION Continuing from Chapter 3, we now focus on the .t / term(angle) in the general modulated carrier waveform xc .t / D A.t / cos 2 fc t C .t /4.1Angle Modulation A general angle modulated signal is of the formxc .t / D Ac cosŒ!c t C .t / Definition: Instantaneous phase of xc .t / is i .t / D !c t C .t /where .t / is the phase deviation Define: Instantaneous frequency of xc .t / is!i .t / Dd .t /d i .t /D !c Cdtdtwhere d .t / dt is the frequency deviation There are two basic types of angle modulation1. Phase modulation (PM).t / Dkp„ƒ‚ m.t /phase dev. const.which impliesxc .t / D Ac cosŒ!c t C kp m.t / ECE 5625 Communication Systems I4-3
CONTENTS– Note: the units of kp is radians per unit of m.t /– If m.t / is a voltage, kp has units of radians/volt2. Frequency modulation (FM)d .t /Ddtkf„ƒ‚ m.t /freq. dev. const.orZt.t / D kfm. / d C 0t0– Note: the units of kf is radians/sec per unit of m.t /– If m.t / is a voltage, kf has units of radians/sec/volt– An alternative expression for kf iskf D 2 fdwhere fd is the frequency-deviation constant in Hz/unitof m.t /Example 4.1: Phase and Frequency Step Modulation Consider m.t / D u.t / v We form the PM signal xPM.t / D Ac cos !c t C kp u.t / ; kp D 3 rad/v We form the FM signalZ thixFM.t / D Ac cos !c t C 2 fdm. / d ; fd D 3 Hz/v4-4ECE 5625 Communication Systems I
4.1. ANGLE MODULATIONπ/3 phase step at t 0fc3 Hz frequency step at t 0fc1 1fc1tfc 3 Hz1 11 1 1Phase ModulationFrequency ModulationPhase and frequency step modulation4.1.1Narrowband Angle Modulation Begin by writing an angle modulated signal in complex form xc .t / D Re Ac e j!c t e j .t/ Expand e j .t/ in a power series xc .t / D Re Ac e j!c t 1 C j .t /2.t /2Š The narrowband approximation is j .t /j 1, then xc .t / ' Re Ac e j!c t C jAc .t /e j!c tD Ac cos.!c t /ECE 5625 Communication Systems IAc .t / sin.!c t /4-5t
CONTENTS Under the narrowband approximation we see that the signal issimilar to AM except it is carrier plus modulated quadraturecarrier φ(t)NBFMxc(t) Ac sin(ωct)90oNBFM modulator block diagramExample 4.2: Single tone narrowband FM Consider NBFM with m.t / D Am cos !mtZ t.t / D 2 fdAm cos !m d D Am2 fdfdsin !mt D Am sin !mt2 fmfm Now, fdxc .t / D Ac cos !c t C Am sin !mtfm fd' Ac cos !c t Am sin !mt sin !c tfmAmfdD Ac cos !c t Csin.fc C fm/t2fmAm fdsin.fc fm/t2fm This looks very much like AM4-6ECE 5625 Communication Systems I
4.1. ANGLE MODULATIONfc - fmffc0fc fmSingle tone NBFM spectra4.1.2Spectrum of an Angle-Modulated Signal The development in this obtains the exact spectrum of an anglemodulated carrier for the case of.t / D ˇ sin !mtwhere ˇ is the modulation index for sinusoidal angle modulation The transmitted signal is of the formxc .t / D Ac cos !c t C ˇ sin !mt j!c t jˇ sin !mtD Ac Re e e Note that e jˇ sin !mt is periodic with period T D 2 !m, thuswe can obtain a Fourier series expansion of this signal, i.e.,ejˇ sin !m tD1XYne j n!mtnD 1ECE 5625 Communication Systems I4-7
CONTENTS The coefficients areZ!mYn D2 Z!mD2 !me jˇ sin !mt e !m !mej n!m tdtj.n!m t ˇ sin !m t/dt !m Change variables in the integral by letting x D !mt, then dx D!mdt, t D !m ! x D , and t D !m ! x D With the above substitutions, we haveZ 1Yn De j.nx ˇ sin x/ dx2 Z 1 Dcos.nx ˇ sin x/ dx D Jn.ˇ/ 0which is a Bessel function of the first kind order n with argument ˇJn.ˇ/ Properties Recurrence equation:JnC1.ˇ/ D2nJn.ˇ/ˇJn 1.ˇ/ n – even:J n.ˇ/ D Jn.ˇ/ n – odd:J n.ˇ/ D4-8Jn.ˇ/ECE 5625 Communication Systems I
4.1. ANGLE MODULATION10.80.6J0(β)J1(β)J2(β)J3(β)0.40.2β2 0.246810 0.4Bessel function of order 0–3 plotted The zeros of the Bessel functions are important in spectralanalysisFirst five Bessel function zeros for order 0 – 5J0(β) 02.40483, 5.52008, 8.65373, 11.7915, 14.9309J1(β) 03.83171, 7.01559, 10.1735, 13.3237, 16.4706J2(β) 05.13562, 8.41724, 11.6198, 14.796, 17.9598J3(β) 06.38016, 9.76102, 13.0152, 16.2235, 19.4094J4(β) 07.58834, 11.0647, 14.3725, 17.616, 20.8269J5(β) 08.77148, 12.3386, 15.7002, 18.9801, 22.2178ECE 5625 Communication Systems I4-9
CONTENTSSpectrum cont. We obtain the spectrum of xc .t / by inserting the series representation for e jˇ sin !mt"#1Xxc .t / D Ac Re e j!c tJn.ˇ/e j n!mtnD 11XD Ac Jn.ˇ/ cos .!c C n!m/t nD 1 AcJ1(β) AcJ-1(β) AcJ-2(β) AcJ2(β) AcJ0(β) AcJ3(β) fc 5fmfc 4fmfc 3fmfc 2fmfcfc fmfc - fm AcJ4(β) AcJ5(β) fc - 2fmfc - 3fmfc - 4fm AcJ-3(β) A J (β) AcJ-5(β) c -4fc - 5fmAmplitude Spectrum(one-sided) We see that the amplitude spectrum is symmetrical about fcdue to the symmetry properties of the Bessel functionsf For PMˇ sin !mt D kp .A sin !mt /„ ƒ‚ m.t/) ˇ D kp A For FMZˇ sin !mt D kftA cos !m d DfdA sin !mtfm) ˇ D .fd fm/A4-10ECE 5625 Communication Systems I
4.1. ANGLE MODULATION When ˇ is small we have the narrowband case and as ˇ getslarger the spectrum spreads over wider bandwidthβ 0.2, Ac 1(narrowband ctrum110(f - fc)/fm10(f - fc)/fm10(f - fc)/fm10(f - fc)/fmβ 1, Ac 10.80.60.40.2-501AmplitudeSpectrum55β 2.4048, Ac 1(carrier null)0.80.60.40.2-505AmplitudeSpectrum1β 3.8317, Ac 1(1st sideband null)0.80.60.40.2-505AmplitudeSpectrum1β 8, Ac 1(spectrum becomingwide)0.80.60.40.2-50510(f - fc)/fmThe amplitude spectrum relative to fc as ˇ increasesECE 5625 Communication Systems I4-11
CONTENTSExample 4.3: VCO FM Modulator Consider again single-tone FM, that is m.t / D A cos.2 fmt / We assume that we know fm and the modulator deviation constant fd Find A such that the spectrum of xc .t / contains no carrier component An FM modulator can be implemented using a voltage controlled oscillator (VCO)sensitivity fd MHz/vm(t)VCOCenterFreq fcA VCO used as an FM modulator The carrier term is Ac J0.ˇ/ cos !c t We know that J0.ˇ/ D 0 for ˇ D 2:4048; 5:5201; : : : The smallest ˇ that will make the carrier component zero isˇ D 2:4048 DfdAfmwhich implies that we need to setA D 2:4048 4-12fmfdECE 5625 Communication Systems I
4.1. ANGLE MODULATION Suppose that fm D 1 kHz and fd D 2:5 MHz/v, then wewould need to set1 103D 9:6192 10A D 2:4048 2:5 1064.1.34Power in an Angle-Modulated Signal The average power in an angle modulated signal is 222hxc .t /i D Ac hcos !c t C .t / i 11D A2c C A2chcos 2 !c t C .t / i22 For large fc the second term is approximately zero (why?),thus1Pangle mod D hxc2.t /i D A2c2which makes the power independent of the modulation m.t /(the assumptions must remain valid however)4.1.4Bandwidth of Angle-Modulated Signals With sinusoidal angle modulation we know that the occupiedbandwidth gets larger as ˇ increases There are an infinite number of sidebands, butˇnlim Jn.ˇ/ lim n D 0;n!1n!1 2 nŠso consider the fractional power bandwidthECE 5625 Communication Systems I4-13
CONTENTS Define the power ratio1 2APcarrier C P k sidebandsPr DD 2 cPtotalDJ02.ˇ/C2kXPknD k1 2A2 cJn2.ˇ/Jn2.ˇ/nD1 Given an acceptable Pr implies a fractional bandwidth ofB D 2kfm (Hz) In the text values of Pr 0:7 and Pr 0:98 are single anddouble underlined respectively It turns out that for Pr 0:98 the value of k is IPŒ1 C ˇ , thusB D B98 ' 2.ˇ C 1/fm for sinusoidal mod. only For arbitrary modulation m.t /, define the deviation ratio peak freq. deviationfd DDDmax jm.t /jbandwidth of m.t /W In the sinusoidal modulation bandwidth definition let ˇ ! Dand fm ! W , then we obtain what is known as Carson’s ruleB D 2.D C 1/W– Another view of Carson’s rule is to consider the maximum frequency deviation f D max jm.t /jfd , then B D2.W C f /4-14ECE 5625 Communication Systems I
4.1. ANGLE MODULATION Two extremes in angle modulation are1. Narrowband: D 1 ) B D 2W2. Wideband: D 1 ) B D 2DW D 2 fExample 4.4: Single Tone FM Consider an FM modulator for broadcasting with xc .t / D 100 cos 2 .101:1 106/t C .t /where fd D 75 kHz/v and m.t / D cos 2 .1000/t v The ˇ value for the transmitter isfd75 103ˇDADD 75fm103 Note that the carrier frquency is 101.1 MHz and the peak deviation is f D 75 kHz The bandwidth of the signal is thusAmplitudeSpectrumB ' 2.1 C 75/1000 D 152 kHzB 2(β 1)fm17.51512.5107.552.5-76-50101.1 MHz - 76 kHzECE 5625 Communication Systems I0101.1 MHz5076(f - 101.1 MHz)1 kHz100101.1 MHz 76 kHz4-15
CONTENTS Suppose that this signal is passed through an ideal bandpassfilter of bandwidth 11 kHz centered on fc D 101:1 MHz, i.e., f C fcf fcC H.f / D 1100011000 The carrier term and five sidebands either side of the carrierpass through this filter, resulting an output power of"#52XAPout D c J02.75/ C 2Jn2.75/ D 241:93 W2nD1 Note the input power is A2c 2 D 5000 WExample 4.5: Two Tone FM Finding the exact spectrum of an angle modulated carrier is notalways possible The single-tone sinusoid case can be extended to multiple tonewith increasing complexity Suppose thatm.t / D A cos !1t C B cos !2t The phase deviation is given by 2 fd times the integral of thefrequency modulation, i.e.,.t / D ˇ1 sin !1t C ˇ2 sin !2twhere ˇ1 D Afd f1 and ˇ2 D Afd f24-16ECE 5625 Communication Systems I
4.1. ANGLE MODULATION The transmitted signal is of the form xc .t / D Ac cos !c t C ˇ1 sin !1t C ˇ2 sin !2t D Ac Re e j!c t e jˇ1 sin !1t e jˇ2 sin ˇ2t We have previously seen that via Fourier series expansione jˇ1 sin !1t De jˇ2 sin !1t D1XnD 11XJn.ˇ1/e j n!1tJn.ˇ2/e j n!2tnD 1 Inserting the above Fourier series expansions into xc .t /, wehave()11XXxc .t / D Ac Re e j!c tJn.ˇ1/e j n!1t Jm.ˇ2/e j m!2tnD 1D Ac1X1XmD 1 Jn.ˇ1/Jm.ˇ2/ cos .!c C n!1 C m!2/tnD 1 mD 1 The nonlinear nature of angle modulation is clear, since we seenot only components at !c C n!1 and !c C m!2, but also atall combinations of !c C n!1 C m!2 To find the bandwidth of this signal we can use Carson’s rule(the sinusoidal formula only works for one tone) Recall that B D 2. f C W /, where f is the peak frequencydeviationECE 5625 Communication Systems I4-17
CONTENTS The frequency deviation is 1 d ˇ1 sin !1t C ˇ2 sin !2t2 dtD ˇ1f1 cos.2 f1t / C ˇ2f2 cos.2 f2t / Hzfi .t / D The maximum of fi .t /, in this case, is ˇ1f1 C ˇ2f2AmplitudeSpectrum Suppose ˇ1 D ˇ2 D 2 and f2 D 10f1, then we see that W Df2 D 10f1 and B D 2.W C f / D 2 10f1C2.f1C10f1/ D 2.32f1/ D 64f10.350.30.250.20.150.10.05β1 β2 2, f2 10f 1B 2(W f) 2(10f1 2(11)f1) 64f1(f - fc)-40-2002040f1Example 4.6: Bandlimited Noise PM and FM This example will utilize simulation to obtain the spectrum ofan angle modulated carrier The message signal in this case will be bandlimited noise having lowpass bandwidth of W Hz4-18ECE 5625 Communication Systems I
4.1. ANGLE MODULATION In Python/MATLAB we can generate Gaussian amplitude distributed white noise using randn() and then filter this noiseusing a high-order lowpass filter (implemented as a digital filter in this case) We can then use this signal to phase or frequency modulatea carrier in terms of the peak phase deviation, derived fromknowledge of maxŒj .t /j 4.1.5Narrowband-to-Wideband ConversionNarrowband FMCarrier fc1Peak deviation fd1Deviation ratio D1m(t)NarrowbandFM Modulator(similar to AM)Wideband FMCarrier nfc1Peak deviation nfd1Deviation ratio nD1xnFreq.MultiplierAc1cos[ωct φ(t)]Ac2cos[nωct ideband conversion Narrowband FM can be generated using an AM-type modulator as discussed earlier (without a VCO and very stable) A frequency multiplier, using say a nonlinearity, can be usedto make the signal wideband FM, i.e.,n Ac1 cosŒ!c t C .t / ! Ac2 cosŒn!c t C n .t / so the modulator deviation constant of fd1 becomes nfd1ECE 5625 Communication Systems I4-19
CONTENTS4.1.6Demodulation of Angle-Modulated Signals To demodulate FM we require a discriminator circuit, whichgives an output which is proportional to the input frequencydeviation For an ideal discriminator with inputxr .t / D Ac cosŒ!c t C .t / the output isyD .t / Dxc(t)d .t /1KD2 dtIdealDiscriminatoryD(t)Ideal FM discriminator For FMZ.t / D 2 fdtm. / d soyD .t / D KD fd m.t /OutputSignal (voltage)slope KDfc4-20InputFrequencyECE 5625 Communication Systems I
4.1. ANGLE MODULATIONIdeal discriminator I/O characteristic For PM signals we follow the discriminator with an integratorxr(t)IdealDiscrim.yD(t)Ideal discriminator with integrator for PM demod For PM .t / D kp m.t / soyD .t / D KD kp m.t / We now consider approximating an ideal discriminator with:e(t)xr(t)EnvelopeDetectoryD(t)Ideal discriminator approximation If xr .t / D Ac cosŒ!c t C .t / ddxr .t /D Ac !c Csin !c t C .t /e.t / Ddtdt This looks like AM providedd .t / !cdtwhich is only reasonableECE 5625 Communication Systems I4-21
CONTENTS ThusyD .t / D Acd .t /D 2 Ac fd m.t / (for FM)dt– Relative to an ideal discriminator, the gain constant isKD D 2 Ac To eliminate any amplitude variations on Ac pass xc .t / througha bandpass dpass LimiterFM discriminator with bandpass limiter We can approximate the differentiator with a delay and subtractoperatione.t / D xr .t / xr .t /sincee.t /xr .t /D lim!0!0limthuse.t / 'xr .t/Ddxr .t /;dtdxr .t /dt In a discrete-time implementation (DSP), we can perform asimilar operation, e.g.eŒn D xŒn 4-22xŒn1 ECE 5625 Communication Systems I
4.1. ANGLE MODULATIONExample 4.7: Complex Baseband Discriminator A DSP implementation in MATLAB that works with complexbaseband signals (complex envelope) is the following:function disdata discrim(x)% function disdata discrimf(x)% x is the received signal in complex baseband form%% Mark WickertxI real(x); % xI is the real part of the received signalxQ imag(x); % xQ is the imaginary part of the received signalN length(x); % N is the length of xI and xQb [1 -1];% filter coefficientsa [1 0];% for discrete derivativeder xI filter(b,a,xI); % derivative of xI,der xQ filter(b,a,xQ); % derivative of xQ% normalize by the squared envelope acts as a limiterdisdata (xI.*der xQ-xQ.*der xI)./(xI. 2 xQ. 2); To understand the operation of discrim() start with a generalangle modulated signal and obtain the complex envelopexc .t / D Ac cos.!c t C .t // D Re Ac e j .t/e j!c t D Ac Re Œcos .t / C j sin .t / e j!c t The complex envelope isxQ c .t / D cos .t / C j sin .t / D xI .t / C jxQ .t /where xI and xQ are the in-phase and quadrature signals respectively A frequency discriminator obtains d .t / dtECE 5625 Communication Systems I4-23
CONTENTS In terms of the I and Q signals,.t / D tan1 xQ .t /xI .t / The derivative of .t/ is d xQ .t /d .t /1Ddt1 C .xQ .t / xI .t //2 dt xI .t /0xI .t /xQ.t / xI0 .t /xQ .t /D2xI2 .t / C xQ.t / In the DSP implementation xI Œn D xI .nT / and xQ Œn DxQ .nT /, where T is the sample period0 The derivatives, xI0 .t / and xQ.t / are approximated by the backwards difference xI Œn xI Œn 1 and xQ Œn xQ Œn 1 respectively To put this code into action, consider a single tone message at1 kHz with ˇ D 2:4048.t / D 2:4048 cos.2 .1000/t / The complex baseband (envelope) signal isxQ c .t / D e j .t/ D e j 2:4048 cos.2 .1000/t/ A MATLAB simulation that utilizes the function Discrim() is: 4-24n 0:5000-1;m cos(2*pi*n*1000/50000); % sampling rate 50 kHzxc exp(j*2.4048*m);y Discrim(xc);ECE 5625 Communication Systems I
4.1. ANGLE MODULATION % baseband spectrum plotting tool using psd()bb spec plot(xc,2 11,50);axis([-10 10 -30 30])gridxlabel(’Frequency (kHz)’)ylabel(’Spectral Density (dB)’)t n/50;plot(t(1:200),y(1:200))axis([0 4 -.4 .4])gridxlabel(’Time (ms)’)ylabel(’Amplitude of y(t)’)ECE 5625 Communication Systems I4-25
CONTENTS30Note: no carrierterm since β 2.4048Spectral Density (dB)20100 10 20 30 10 8 6 4 202Frequency (kHz)468100.40.3Amplitude of y(t)0.20.10 0.1 0.2 0.3 0.400.511.52Time (ms)2.533.54Baseband FM spectrum and demodulator output wavefrom4-26ECE 5625 Communication Systems I
4.1. ANGLE MODULATIONAnalog Circuit Implementations A simple analog circuit implementation is an RC highpass filter followed by an envelope detector H(f) 1C0.707Linear operatingregion convertsFM to AMRHighpassfcCRHighpass12πRCfRe CeEnvelope DetectorRC highpass filter envelope detector discriminator (slope detector) For the RC highpass filter to be practical the cutoff frequencymust be reasonable Broadcast FM radio typically uses a 10.7 MHz IF frequency,which means the highpass filter must have cutoff above thisfrequency A more practical discriminator is the balanced discriminator,which offers a wider linear operating rangeECE 5625 Communication Systems I4-27
Filter AmplitudeResponseCONTENTS H2(f) H1(f) ff1Filter AmplitudeResponsef2fLinear region H1(f) - H2(f) Rf1C1L1ReCexc(t)yD(t)L2RC2ReCef2Bandpass Envelope DetectorsBalanced discriminator operation (top) and a passive implementation(bottom)4-28ECE 5625 Communication Systems I
4.1. ANGLE MODULATIONFM Quadrature DetectorsUsually axout(t) lowpassfilter isadded herexc(t)C1xquad(t)LpCpTank circuittuned to fcQuadrature detector schematic In analog integrated circuits used for FM radio receivers andthe like, an FM demodulator known as a quadrature detectoror quadrature discriminator, is quite popular The input FM signal connects to one port of a multiplier (product device) A quadrature signal is formed by passing the input to a capacitor series connected to the other multiplier input and a paralleltank circuit resonant at the input carrier frequency The quadrature circuit receives a phase shift from the capacitorand additional phase shift from the tank circuit The phase shift produced by the tank circuit is time varying inproportion to the input frequency deviation A mathematical model for the circuit begins with the FM inputsignalxc .t / D Ac Œ!c t C .t / ECE 5625 Communication Systems I4-29
CONTENTS The quadrature signal is .t /xquad.t / D K1Ac sin !c t C .t / C K2dt where the constants K1 and K2 are determined by circuit parameters The multiplier output, assuming a lowpass filter removes thesum terms, is 1d .t/xout.t / D K1A2c sin K22dt By proper choice of K2 the argument of the sin function issmall, and a small angle approximation yieldsd .t /11D K1K2A2cKD m.t /xout.t / ' K1K2A2c2dt24.2Feedback Demodulators The discriminator as described earlier first converts and FMsignal to and AM signal and then demodulates the AM The phase-locked loop (PLL) offer a direct way to demodulateFM and is considered a basic building block by communicationsystem engineers4.2.1Phase-Locked Loops for FM Demodulation The PLL has many uses and many different configurations,both analog and DSP based4-30ECE 5625 Communication Systems I
4.2. FEEDBACK DEMODULATORS We will start with a basic configuration for demod
4.1. ANGLE MODULATION 11 1 1 11 1 1 tt /3 phase step at t 0 3 Hz frequency step at 0 Phase Modulation Frequency Modulation f c f c f f 3 Hz Phase and frequency step modulation 4.1.1 Narrowband Angle Modulation Begin by writing an angle modulated signal in complex form x c.t/DRe A ce j!ctej .t/ Expand ej .t/in a power series x c.t/DRe A .
9/18/2016 9Nurul/DEE 3413/Modulation Types of Modulation Pulse Modulation Carrier is a train of pulses Example: Pulse Amplitude Modulation (PAM), Pulse width modulation (PWM) , Pulse Position Modulation (PPM) Digital Modulation Modulating signal is analog Example: Pulse Code Modulation (PCM), Delta Modulation (DM), Adaptive Delta Modulation (ADM), Differential Pulse
modulation and multiplexing techniques in Section 3 to be used to extend the single-bit embedding to multiple-bit embedding. We shall cover amplitude modulo modulation, orthogonal/bi-orthogonal modulation, and TDMA/ CDMA type modulation/multiplexing. These techniques are quantitatively compared in Section 4.
Modulation allows for the designated frequency bands (with the carrier frequency at the center of the band) to be utilized for communication and allows for signal multiplexing. Amplitude modulation (AM) is an analog and linear modulation process as opposed to frequency modulation (FM) and phase modulation (PM).
Orthogonal Frequency Division Multiplexing (OFDM) Modulation - a mapping of the information on changes in the carrier phase, frequency or amplitude or combination. Multiplexing - method of sharing a bandwidth with other independent data channels. OFDM is a combination of modulation and multiplexing. Multiplexing generally refers to
Multiplexing Providing (dynamic) rerouting of channels Electronic multiplexing – signals from different channels are added before optical modulation Optical multiplexing – signals from different channels are coded into light before multiplexing Different schemes – Frequency Division Multiplexing (FDM)
Modulation for multiplexing –This allows for multiple signals to be carried on a single transmission medium (multiplexing is one form of modulation). Modulation to overcome equipment limitations –Modulation is used to place signals in a portion of the spectrum where equipment limitations are minimal or most easily met.
Modulation, Modems and Multiplexing Gail Hopkins Part 2 – Data Communication Basics Introduction Sending signals over long distances Modulation and Modems Leased serial data circuits Optical, radio and dialup modems Multiplexing DSL and Cable modems
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