chapter6 modulation techniques for mobile radio imp.pdf

7/30/2019 Chapter6 Modulation Techniques for Mobile Radio imp.pdf

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NCCUWireless Comm. Lab

6-1

Modulation Techniques for Mobile Radio

Amplitude Modulation

Angle Modulation

Digital Modulation-an Overview

Pulse Shaping Techniques Linear Modulation Techniques

Constant Envelope Modulation

Combined Linear/Constant Envelope Modulation Techniques

Spread Spectrum Modulation Techniques


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6-3


AM signal can be represented as (see Fig. 1)

)2cos()](1[)( tftmAts ccAM += For a sinusoidal modulating signal

the modulation index is given by

c

m

A

Ak=

)2cos()/()( fmtAAtm cm =

The spectrum of an AM signal can be show to be (see Fig. 2)

)]()()()([2

1)( cccccAM ffMffffMffAfS +++++=


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6-4


1


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6-5



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6-6

Angle Modulation

The two most important classes of angle modulation being

frequency modulation and phase modulation Frequency modulation (FM) can be represented as

+=+=

t

fccccFM dmktfAttfAtS )(22cos)](2cos[)(

Ac : amplitude of the carrier

fc : carrier frequency

kf : frequency deviation constant Phase modulation (PM) can be represented as

)](2cos[)( tmktfAtS ccPM += k: phase deviation

constant


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6-7

FM modulation Methods

Direct method (see Fig. 3)

The carrier frequency is directly varied in accordance with

the input modulating signal

Indirect method (see Fig. 4)

A narrowband FM signal is generated using a balanced

modulator and frequency multiplication is used to increase

both the frequency deviation and the carrier frequency to

the required lever, can be expressed as

tftAtfAtS ccccFM 2sin)(2cos)(


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6-8

FM Modulation Methods

3


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6-10

FM Detection Technique

Slope Detector (see Fig. 5)

Zero-crossing Detector (see Fig.6)

PLL for FM Detection (see Fig. 7)

Quadrature Detection (see Fig. 8)


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6-11


Slop Detector

+=+=

t

fccccin dmktfAttfAtv `)(22cos)](2cos[)(

+=+=

t

fcc dmktfVttfVtv `)(22cos)](2cos[)( 111

))(2sin(2)( 12 tfctdt

dfctVtv +

+=

)(22)(2)( 111 tmkVfVtdt

dfVtv fccout +=

+=

5


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6-12


Zero-crossing Detector

6


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6-13


PLL for FM Detection

7


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6-14


Quadrature detection

8 )()(2)(22

0 tCmtmKAtv fc ==

++= ))](()(22cos[)( tfdmktfAtv ifcc


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6-15


Factors that influence the choice of digital modulation

power efficiency

bandwidth efficiency

p

B

HzbpsBRB /=


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6-16


Bandwidth and power spectral density of digital signals

the power spectral density of a random signal w(t) is define

as [1]

=

T

fWfp T

Tw

2

)(lim)(

the PSD of the bandpass signal is given by

[ ])()(41

)( cgcgs ffPffPfP +=

Where Pg(f) is the PSD ofg(t), and g(t) is the complex

baseband envelop


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6-17

Pulse Shaping Techniques

Nyquist criterion for ISI cancellation

Raised cosine rolloff filter (see Fig. 9-10)

+>

+


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6-18

Raised Cosine Rolloff Filter

9


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6-19

Raised Cosine Rolloff Filter

10


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6-20

Linear Modulation Techniques

z Digital modulation techniques are classified as linear and

nonlinear.

z The amplitude of transmitted signal,s(t), varies linearly

with the modulating digital signal, m(t).

)]2()()2()([

)]2()([)(

tfsintmtfcostmA

tfjexptAmRets

cIcR

c

==

signalmodulatedthe:)()()(

frequencycarierthe:amplitudethe:

tjmtmtm

fA

IR

c

+=


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6-22

Coherent BPSK Receiver with carrier recovery circuits

)2(cos2

)(

)2(cos2

)()( ch

+=

++=

tf

T

Etm

tfT

EtmtS

c

b

b

ccb

bBPSK

The received BPSK signal

=

0,

2

N

EQP bBPSKe

The bit error rate in an AWGN channel

12


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6-23

Differential Phase Shift Keying (DPSK)

z A noncoherent form of PSK

z The input binary sequence is

differentially encoded firstly.

z An example

z The bit error rate in anAWGN channel

{mk} 1 0 0 1 0 1 1 0

{dk-1} 1 1 0 1 1 0 0 0{dk} 1 1 0 1 1 0 0 0 1

1= kkk dmd

=

0, exp

2

1

N

EP bDPSKe

13

14


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6-24

Quadrature Phase Shift Keying (QPSK)

z 2 bits are transmitted an a signal modulation symbol.

z Therefore, QPSK has twice the bandwidth efficiency of BPSK.

.4,3,2,10)2

)1(2(cos2

)( s =+= iTtitfT

EtS c

s

sQPSK

15


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6-25

Spectrum and Bandwidth of QPSK Signals

z The null-to-null RF

bandwidth is equal to the bit

rateRb.

z The BW is half of a BPSK

signal.

z The bit error rate in an

AWGN channel

=

0,

2

N

EQP bBPSKe 16


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6-26

QPSK Signal Transmission Technique

z The bit stream m(t) is split into two bit streams mI(t) and

mQ(t) (in-phase and quadrature stream).

zThe two binary streams are separately modulated by twocarriers, which are in quadrature.

17


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6-27

Coherent QPSK Signal Detection Technique

z The frontend bandpass filter remove the out-of-band noise and

adjacent channel interference.

zThe filtered output is split into two parts, and each part iscoherently demodulated using in-phase and quadrature carriers.

18


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6-28

Offset QPSK

z OQPSK signaling is similar to QPSK, expect for the time

alignment of the even and odd bit streams.

zThus, OQPSK can eliminate 180 phase transition, and

prevent the signal envelope to go to zero.

19


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6-29

/4 QPSK

z /4 QPSK is a quadrature phase shift keying technique.

z It offers a compromises between OQPSK and QPSK.

z The maximum phase change of/4 QPSK is limited to 135

z An extremely attractive feature of/4 QPSK is that it can be

noncoherently detected.

z Further, it has been found that in the presence of multipath

spread and fading, /4 QPSK performs better than OQPSK [2].


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6-30

Constellation diagram of/4 QPSK signal and

Carrier Phase Shifts corresponding to input bit pairs

Information bits Phase shift k1 1 /4

0 1 3/40 0 - 3/41 0 -/4

20


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6-31

Generic /4 QPSK Transmitter

21


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6-32

/4 QPSK Detection Techniques

z Baseband differential detector (see Fig. 22)

z IF differential detection (see Fig. 23)

z FM discriminator (see Fig. 24)


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6-33

/4 QPSK Baseband differential detector [3]

22


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6-34

/4 QPSK IF differential detection

23


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6-35

/4 QPSK FM discriminator

24


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6-36

Constant Envelope Modulation

z The constant envelope family of modulations has following

advantage [4] :

Power efficient Class C amplifiers can be used without

introducing degradation in the spectrum occupancy of the

transmitted signal.

Low out-of-band radiation of the order of -60 dB to -70 dB

can be achieved.

Limiter-discriminator detection can be used, which

simplified receiver design and provides high immunity

against random FM noise and signal fluctuations due to

Rayleigh fading


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6-37

Binary Frequency Shift Keying (BFSK)

z The information of the transmitted signal is modulated into

the frequency of the carrier signal.

z In general, an FSK signal may be represented as

)t22(cos

2

)()(

)t22(cos2

)()(

ffT

E

tvtS

ffT

EtvtS

cb

bLFSK

cb

bHFSK

==

+==


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6-38

Detection of Binary FSK

Coherent

Noncoherent

25

26


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6-39

Minimum Shift Keying (MSK)

z Minimum shift keying (MSK) is a special type of continuous

phase-frequency shift keying (CPFSK).

z MSK is sometimes referred to as fast FSK, as the frequency

spacing used is only half as much as that used in conventional

noncoherent FSK [5].

z An MSK signal also can be thought of as a special form ofOQPSK where the baseband rectangular pulses are replaced

with half-sinusoidal pulses [6].

( ) ( )tfT

ttmtf

T

ttmtS c

bQ

bIMSK 2)sin

2(sin)(2)cos

2(cos)()( c

+=


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6-40

MSK Power Spectrum

z From this figure, the MSK spectrum has lower sidelobes

than QPSK and OQPSK.

27


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6-41

Block diagram of an MSK transmitter

( ) ( )tfT

ttmtf

T

ttmtS c

bQ

bIMSK 2)sin

2(sin)(2)cos

2(cos)()( c

+=

28


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6-42

Block diagram of an MSK receiver

29


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6-43

Gaussian Minimum Shift Keying (GMSK)

z GMSK is a simple binary modulation scheme which may

be viewed as a derivative of MSK.

z In GMSK, the sidelobe levels of the spectrum are further

reduced by passing the modulating NRZ data waveform

through a premodulation Gaussian pulse-shaping filter [7].

z In practice, GMSK is most attractive for its excellent

power efficiency (due to the constant envelope).


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6-45

Block Diagram of a GMSK receiver

31


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6-46

Power Spectrum density of a GMSK signal [7]

32

C bi d Li /C t t E l M d l ti


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6-47

Combined Linear/Constant Envelope Modulation

Techniquesz Digital baseband data may be sent by varying both the

envelope and phase (frequency) of an RF carrier.

z This can offer two degrees of freedom, thus such modulationtechniques are called M-ary modulation.

z In M-ary signaling scheme, two or more bits are grouped

together to form symbols.z Depending on whether the amplitude, phase, or frequency of

the carrier is varied, the modulation scheme is called M-ary

ASK, M-ary PSK, or M-ary FSK.

z M-ary modulation schemes achieve better bandwidth

efficiency at the expense of power efficiency.


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6-48

M-ary Phase Shift Keying (MPSK)

( ) ( )

bsbs

s

s

s

si

TMTEME

tfini

T

Etfi

T

EtS

)(logand)(logwhere

2s

M

21)-(sin

22cos

M

21)-(cos

2)(

22

cc

==

+

=

33


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6-49

M-ary PSK power spectral density

34


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6-50

M-ary Quadrature Amplitude Modulation (QAM)

( ) ( )

integers.tindependenofpairaareandandamolitude,lowestthewithsignaltheofenergytheswhere

2sM

21)-(sin

22cos

M

21)-(cos

2)(

min

cmin

cmin

ii

is

is

i

baiE

tfinibT

Etfia

T

EtS

+

=

35


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6-51

Hertz.21byseparated

aresfrequenciesignaltheand,integerfixedsomefor2/where

,...,2,10)(cos2

)(

s

cscc

scss

si

T/

nTnf

MiTttinTT

EtS

=

=

+=

M-ary Frequency Shift Keying (MFSK)

z

z The orthogonality characteristic of MFSK has led

researches to explore Orthogonal Frequency Division

Multiplexing (OFDM)

S d S M d l i T h i


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6-52

Spread Spectrum Modulation Techniques

z Pseudo-noise (PN) Sequences

36

Di S S d S (DS SS)


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6-53

Direct Sequence Spread Spectrum (DS-SS)

0atanglephasecarriertheis

frequency,carriertheis

sequence,spreadingPNtheis)(

sequence,datatheis)(

)2cos()()(2

)(ss

=

+=

t

f

tp

tm

tftptmT

EtS

c

cs

s

37

I t f S i th S d S t


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6-54

Interference Suppress in the Spread Spectrum

38

F H d S d S t (FH SS)


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Frequency Hopped Spread Spectrum (FH-SS)

39

Performance of Digital Modulation in Slow,


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g ,Flat Fading Channels

40

R f


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References

[1] Couch, L. W.,Digital and Analog Communication Systems, 4th edition, Macmillan, New York,

1993.

[2] Liu, C. L., Feher, K., Noncoherent detection of/4-shifted systems in a CCI-AWGN combinedinterference environment, IEEE VTC89, San Francisco, 1989.

[3] Feher, K., Modems for emerging digital cellular mobile radio systems, IEEE Trans. on Veh.

Techno., vol 40, no 2, pp. 355-365, May 1991.

[4] Young, W. R., Advanced mobile phone service : introduction, background, and objectives, Bell

Systems Technical Journal, vol. 58, pp. 1-14, January 1979.

[5] Xiong, F., Modem techniques in satellite communications, IEEE Communication Magazine, pp.

84-97, August 1994.

[6] Pasupathy, S., Minimum shift keying: a spectrally efficient modulation, Communication

Magazine,pp. 14-22, July, 1979.

[7] Murota, K., and Hirade, K., GMSK modulation for digital mobile radio telephony, IEEE Trans.

on Commun. vol. COM-29, no. 7, pp. 1044-1050, July 1981.

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