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Transcript of Chapter6 Modulation Techniques for Mobile Radio imp.pdf
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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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Amplitude Modulation
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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Amplitude Modulation
1
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Amplitude Modulation
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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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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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FM Modulation Methods
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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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FM Detection Technique
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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FM Detection Technique
Zero-crossing Detector
6
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FM Detection Technique
PLL for FM Detection
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FM Detection Technique
Quadrature detection
8 )()(2)(22
0 tCmtmKAtv fc ==
++= ))](()(22cos[)( tfdmktfAtv ifcc
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Digital Modulation-an Overview
Factors that influence the choice of digital modulation
power efficiency
bandwidth efficiency
p
B
HzbpsBRB /=
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Digital Modulation-an Overview
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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Pulse Shaping Techniques
Nyquist criterion for ISI cancellation
Raised cosine rolloff filter (see Fig. 9-10)
+>
+
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Raised Cosine Rolloff Filter
9
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Raised Cosine Rolloff Filter
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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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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
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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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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
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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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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.
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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.
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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.
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/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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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
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Generic /4 QPSK Transmitter
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/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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/4 QPSK Baseband differential detector [3]
22
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/4 QPSK IF differential detection
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/4 QPSK FM discriminator
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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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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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Detection of Binary FSK
Coherent
Noncoherent
25
26
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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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MSK Power Spectrum
z From this figure, the MSK spectrum has lower sidelobes
than QPSK and OQPSK.
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Block diagram of an MSK transmitter
( ) ( )tfT
ttmtf
T
ttmtS c
bQ
bIMSK 2)sin
2(sin)(2)cos
2(cos)()( c
+=
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Block diagram of an MSK receiver
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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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Block Diagram of a GMSK receiver
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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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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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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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M-ary PSK power spectral density
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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
+
=
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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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Spread Spectrum Modulation Techniques
z Pseudo-noise (PN) Sequences
36
Di S S d S (DS SS)
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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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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.