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Overview of Modulation

Techniquesfor Wireless

EEL 6593 Fall '95 Paul FlikkemaUSF

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Introduction

The purpose of analog modulation is to impressaninformation-bearing analog waveform onto acarrier fortransmission.

The purpose of digital modulation is to convert aninformation-bearing discrete-time symbolsequence into acontinuous-time waveform perhaps impressed ona carrier.

Key concerns | bandwidth e ciency andimplementationcomplexity. These are a ected by:

baseband pulseshapephase transitioncharacteristicsenvelope uctuations channel non-linearities?


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Frequency

ModulationAngle modulation: transmittedsignal isxt = A cos !c t + t = fAej !c t+tg

with instantaneous

phase i t = !ct + t;

and instantaneousfrequency

di t = ! + dt:

!i t = dt c dtt is the instantaneous

phase


instantaneous frequencydeviation.

dtis

thedeviation, and dt

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For phasemodulation,

t = kp mtwhere mt is the message signal, and kp is the phasedeviation

constant rad volt. For FM, we have dt= kfmt or

dt Zt t = kfmsds + t0

to

where kfis the frequency deviation constant radsec volt.

With t0 = ,1 and t0 = 0, we havext = A cos !c t + kp mtPM

Ztxt = A cos !c t + kfmsdsFM

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FM using integrator and phasemodulator

v(t)Integrator Ph. Modulator


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If modulating signal is a sinusoid, mt = Am cos !mt, then

xt = A cos !ct + sin !m twhere = kf!Am , the modulation index de ned forsingle-tonem modulation only. In this

case, it can be shown that xAtisJ0 cos !ct+1X

n=1,1n Jn cos!c ,n!m t+,1n cos!c +n!mt

For small , wehave 2

J0 1, 2 n 1

Jn n! 2 ; n 6= 0

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This implies

xt cos ! t , cos! , ! t + cos! + ! tccmcm A22

or using a trig identity xt cos ! t , sin ! t sin ! tcmc AGeneral rule: if0:3, the bandwidth of the modulatedsignal is

approx. 2!m ; and we have narrow-band FM NBFM.This is themodulation scheme used in AMPS.


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FM DemodulationMethods

Limiter-discriminatorFM feedback FMFB

Phase-locked loopPLL

FM PerformanceCharacterized by signal-to-noise ratio SNR: the

demodulatorinput CNR carrier-to-noise ratio for AMPS is speci edto be 18dB, resulting in an output SNR of 40 dB.


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DigitalModulationCriteria forselection:

BERperformance

Mobile personal channel severefading

Cellular architecture interferenceTypically, req't is 10,2 or betterspeechSpectral e ciency

Adjacent channel

interferencePower e ciency esp. atmobileImplementation complexity cost may requiredual-modemobile


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Digital Modulation | Classi cationConstant-envelope methods: Allow use ofless expensiveampli cation not dependent on signal amplitude at theexpense

of out-of-band emissions. Limited to a spectral e ciencyof about 1bit sec Hz.Examples: MSK, GMSK

Linear methods: Higher spectral e ciency, but mustuse linearampli ers to maintain performance and to limit out-of-bandemissions.

Examples: PSK, QAMEEL 6593 Fall '95 Paul FlikkemaUSF

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Spectral E

ciencySpectral occupancy per channel isroughlySO = B + 2f

where B = bandwidth occupied of RF signal powerspectrum and

fis the maximum one-way carrier frequency oscillatordrift.Remark: Per-channel spectral e ciency for narrowbandsystems

onlyWe can express the bandwidth as

Rd B= nwhere Rd is the channel data rate and n is the spectral

e ciencyin bits sec Hz.


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Rd + 2fSO = n

Thus, to minimize spectral occupancy thus maximizingcapacity innumber of users we can:1. Lower speech encoder rate trade: cost,delity, or2. Improve spectral e ciency of modulation trade:complexity,

or3. Improve transmitter receiver oscillatorstrade: cost.

Combining,


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State of the technology:Bandwidth e ciency: 1 n 2

Speech encoder rate: Rd 4 8 kb sec

Oscillator stability: 1 10,6 year implying f 1 kHz at

900MHz long-termExamples:

NADC TDMA: 48.6 kbps in 30

kHzGSM: 34 kbps in 25kHz


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