chapter 6 modulation techniques for mobile radio
TRANSCRIPT
ECE 8708 Wireless Communications : Modulation Techniques
Chapter 6Chapter 6Modulation Techniques for Mobile RadioModulation Techniques for Mobile Radio
Yimin Zhang, Ph.D.Department of Electrical & Computer Engineering
Villanova UniversityVillanova Universityhttp://yiminzhang.com/ECE8708
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ECE 8708 Wireless Communications : Modulation Techniques
OutlinesOutlines
Concept of Modulation
Power and Spectral Efficiency
Pulse Shaping
Digital ModulationsDigital Modulations
Linear – BPSK, QPSK, DPSK, OQPSK, π/4 QPSK
Nonlinear – FSK, MSK, GMSKo ea S , S , G S
M-ary ASK, M-ary PSK, M-ary FSK
Coherent and Noncoherent Detection
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ECE 8708 Wireless Communications : Modulation Techniques
Concept of Modulation and DemodulationConcept of Modulation and Demodulation
Modulation is the process of encoding information from a message source in a manner suitable for transmission.
It generally involves translating a baseband message signal (called the source) to a bandpass signal at frequencies that are very high compared to the baseband frequency.compared to the baseband frequency.
The bandpass signal is called modulated signal and the baseband signal is called modulating signal.
Demodulation is the process of extracting the baseband message from the carrier so that it may be processed and interpreted by the y p p yintended receiver.
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ECE 8708 Wireless Communications : Modulation Techniques
Modulation TechniquesModulation Techniques
ModulationModulation
Analog ModulationAnalog Modulation Digital ModulationDigital Modulationgg(First Generation(First Generation
Mobile Radio)Mobile Radio)
gg(Present & Future(Present & Future
Systems)Systems)
Amplitude modulation
A l d l ti
Amplitude shift keying (ASK)
F hift k i (FSK)Angle modulation (Frequency & phase modulations)
Frequency shift keying (FSK)
Phase shift keying (PSK)
Quadrature amplitude
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pmodulation (QAM)
ECE 8708 Wireless Communications : Modulation Techniques
AM and FM Modulations
AM and FM signalsAM and FM signals produced by a single tone.
(a) Carrier wave(a) Carrier wave.
(b) Sinusoidal modulating signal.signal.
(c) Amplitude-modulated signal.
(d) Frequency-modulated signal.
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g
ECE 8708 Wireless Communications : Modulation Techniques
Frequency Modulation vs. Amplitude Modulation
FM signals have all their information in phase or frequency of th ithe carrier
AM signals have all their information in the amplitude of the carriercarrier
FM has better noise immunity when compared to amplitude modulation
AM signals are able to occupy less bandwidth as compared to FM signals
FM exhibits a so-called capture effect characteristic
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ECE 8708 Wireless Communications : Modulation Techniques
Amplitude Modulation
• AM signal can be represented as
)2cos()](1[)( tftmAts ccAM π+=
• For a sinusoidal modulating signal )2cos()/()( tfAAtm π=• For a sinusoidal modulating signal
the modulation index is given by
A
)2cos()/()( tfAAtm mcm π=
c
m
AAk =
The spectr m of an AM signal can be sho to be• The spectrum of an AM signal can be show to be
)]()()()([21)( cccccAM ffMffffMffAfS ++++−+−= δδ
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2
ECE 8708 Wireless Communications : Modulation Techniques
Amplitude Modulation
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1
ECE 8708 Wireless Communications : Modulation Techniques
Amplitude Modulation
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ECE 8708 Wireless Communications : Modulation Techniques
Angle Modulationg
• The two most important classes of angle modulation are frequencymodulation and phase modulation
• Frequency modulation (FM) can be represented as
⎤⎡ t
⎥⎦
⎤⎢⎣
⎡+=+= ∫
∞−
t
fccccFM dmktfAttfAtS ηηππθπ )(22cos)](2cos[)(
Ac : amplitude of the carrier c pfc : carrier frequencykf : frequency deviation constant
• Phase modulation (PM) can be represented as
)](2cos[)( tmktfAtS ccPM θπ +=
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kθ : phase deviation constant
ECE 8708 Wireless Communications : Modulation Techniques
FM Modulation Methods – Direct Method
• Direct methodThe carrier frequency is directly
varied in accordance with the input modulating signal.
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ECE 8708 Wireless Communications : Modulation Techniques
FM Modulation Methods – Indirect Method
• Indirect method4A 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.
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tftAtfAtS ccccFM πθπ 2sin)(2cos)( −≈
ECE 8708 Wireless Communications : Modulation Techniques
FM Detection Techniqueq
• Slope Detector
• Zero-crossing Detector
• PLL for FM Detection
Q d t D t ti• Quadrature Detection
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ECE 8708 Wireless Communications : Modulation Techniques
Digital Modulation TechniquesDigital Modulation Techniques
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ECE 8708 Wireless Communications : Modulation Techniques
Why Using Digital Modulations?Why Using Digital Modulations?Advantages
Higher transmission efficiencyg y
Noise immunity
Robustness to channel impairments
Easier multiplexing (voice, data, image)
Greater security
C t ffi i & i i i tiCost efficiency & minimization
Software implementation for flexibility
Error control codes (for error detection and correction)
Source coding, encryption
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Equalizations
ECE 8708 Wireless Communications : Modulation Techniques
Basics of Digital Communications
• In digital wireless communication systems, the message (mod lating signal) is represented as a time seq ence of s mbols(modulating signal) is represented as a time sequence of symbols or pulses.
• Each symbol has m finite states
Example m=16Each symbol represents n bits of information
n = log2m =4 bits/symbol
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ECE 8708 Wireless Communications : Modulation Techniques
Power and Bandwidth EfficienciesPower and Bandwidth Efficiencies
Performance of modulation is often measured by
power efficiencyThe ability of a modulation technique to preserve the fidelity ofthe digital message at low power levels.
pη
Expressed in Eb/N0 (ratio of signal energy per bit to noise power spectral density) required at the receiver output for a certain error probability.certain error probability.
bandwidth efficiencyThe ability of modulation scheme to accommodate data within
li it d b d idth
Bη
a limited bandwidth.
Expressed as the ratio of throughput data rate per Hz. R
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bps/HzBR
B =η
ECE 8708 Wireless Communications : Modulation Techniques
Power and Bandwidth EfficienciesPower and Bandwidth Efficiencies
Fundamental upper bound on achievable bandwidth efficiency(Shannon’s channel coding theorem)
For AWGN (additive white Gaussian noise) non-fading channels
⎞⎛ SC
C / W [bits/s/Hz]
Unattainablei
C: channel capacity (bps)
bps/Hz1log2max ⎟⎠⎞
⎜⎝⎛ +==
NS
WC
Bηregion
p y ( p )W: RF signal bandwidth (Hz)S/N: signal-to-noise power ratio (SNR) Practical region
There is a tradeoff between bandwidth efficiency and power efficiency.
C i li l t W d i l ith
SNR [dB]
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C increases linearly to W, and in logarithm to SNR (for high SNR).
ECE 8708 Wireless Communications : Modulation Techniques
Power and Bandwidth EfficienciesPower and Bandwidth Efficiencies
Examples :
For US digital cellular standard, R = 48.6 kbpsRF bandwidth = 30 KHzF SNR 20 dB 100For SNR = 20 dB 100C = 30000 * log2(1 + S/N)
= 30000 * log2(1 + 100) = 199.75 kbpsg2( ) p
For GSM standard, R = 270.833 kbpsC = 1 99 Mbps for S/N = 30 dBC = 1.99 Mbps for S/N = 30 dB
> For most exiting systems, the actual data rate is much lower than th it b d
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the capacity bound.
ECE 8708 Wireless Communications : Modulation Techniques
Shannon’s LimitShannon’s Limit
⎟⎟⎞
⎜⎜⎛+=
CEC b1log⎟⎠⎞
⎜⎝⎛ +=
NSWC 2 1log
⎟⎟⎠
⎜⎜⎝+=
WNW 02 1log
⎩⎨⎧
==
WNNCES b
0
[dB]6169301
:get we,0or As
≈→
→∞→
EWCW
b Shannon limit[dB] 6.1693.0log 20
−≈=→eN
b
– There exists a limiting value of Eb/N0 below which there can be no error-free communication at any information rate.
– By increasing the bandwidth alone, the capacity cannot be increased to any desired value.
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Q: How to increase Eb/N0 in a highly noisy environment?
ECE 8708 Wireless Communications : Modulation Techniques
Shannon’s LimitShannon’s Limit
R>C
R / W [bits/s/Hz]
R=C
M=256Unattainable region
Bandwidth limited M=8
M=16
M=64
M=256
R<CPractical regionM=2
M=4
M 8
M=2M=4M=8
M=16
Power limited
Shannon limit MPSKMQAMMFSK
510−=BP
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[dB] / 0NEb
ECE 8708 Wireless Communications : Modulation Techniques
Power Spectral DensityPower Spectral Density
The power spectral density (PSD) of a random signal w(t) is define asas
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛=
∞→ TfW
fp T
Tw
2)(
lim)(
⎩⎨⎧ <<−
=⇔elsewhere
2/2/0
)()()(
TtTtwtwfW TT
The power of the signal in a given frequency band can be calculated
⎩
by integrating over positive and negative frequencies,
∫∫−
+= 12 )()(ff
dffpdffpP
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∫∫ −+=
21
)()(f wf w dffpdffpP
ECE 8708 Wireless Communications : Modulation Techniques
BandwidthBandwidth
Bandwidth
• Absolute bandwidth
• Null-to-null bandwidth
• Half-power bandwidth (3 dB bandwidth)
• Occupied bandwidth (defined by FCC; contains99% of signal power)99% of signal power)
• Bandwidth with PSDoutside the band is under
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certain stated level
ECE 8708 Wireless Communications : Modulation Techniques
Line CodingLine Coding
• Line codes are used to id ti lprovide particular
spectral characteristics of a pulse train.
• Most commonly used – Return-to-zero (RZ)– Non-return-to-zero
(NRZ)– Manchester Code
• Uni-polar (0,V) or Bipolar (-V, V )
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Bipolar ( V, V )
ECE 8708 Wireless Communications : Modulation Techniques
Pulse Shaping techniques Pulse Shaping techniques
BandlimitedChannelChannel
• ISI – Inter Symbol Interference ⇒ errors in transmission of symbols• Pulse shaping techniques ⇒ reduces the inter-symbol effects
> The objective of pulse shaping is to design causal pulse waveforms that yields no or negligible ISI for propagation in band-limited channels
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channels.
ECE 8708 Wireless Communications : Modulation Techniques
Nyquist Criterion for ISI Cancellation Nyquist Criterion for ISI Cancellation
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ECE 8708 Wireless Communications : Modulation Techniques
Nyquist Criterion for ISI Cancellation Nyquist Criterion for ISI Cancellation
Transmitter Channel Receiverpulse shape of the channel impulse receiver impulse
heff(t)
p psymbol p(t)
presponse hc(t)
receiver impulse response hr(t)
• Mathematical analysis of Nyquist criterion)(*)(*)(*)()(eff ththtptth rcδ=
* denotes convolution operationTwo important considerations• h (t) should have fast decay with a small magnitude near the• heff(t) should have fast decay with a small magnitude near the
sample values for n ≠ 0.• for a ideal channel, hc(t) = δ(t), it should be possible to realize or
closely approximate shaping filters at both receiver and
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closely approximate shaping filters at both receiver and transmitter to produce the desired heff(t).
ECE 8708 Wireless Communications : Modulation Techniques
Pulse Shaping Techniques
• Rectangular “brick-wall” filter0.8
1
s
s
TtTtth
/)/sin()(eff π
π=
0.4
0.6
h eff(t)
⎟⎟⎠
⎞⎜⎜⎝
⎛=
ss ff
ffH rect1)(eff
-0.2
0
0.2h
-20 -10 0 10 20-0.4
t/Ts
)( fH
It satisfies Nyquist’s criterion, but the filter is not causal (not buildable).
f
)(eff fH
Decay at 1/t. An error in the sampling time will cause
Yimin Zhang, Villanova University 28
fs-fs
sampling time will cause significant ISI.
ECE 8708 Wireless Communications : Modulation Techniques
Pulse Shaping Techniques• Alternative
The following filter also
)(/
)/sin()(eff tzTt
Ttth s
ππ
=
satisfy Nyquist’s criterion.
)(rect1)(eff fZff
ffH
ss
⊗⎟⎟⎠
⎞⎜⎜⎝
⎛=
/Tt sπ
ff ss ⎠⎝
)()( fZfZ −=
sTfffZ
21for 0)( 0 =≥=
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ECE 8708 Wireless Communications : Modulation Techniques
Raised Cosine Rolloff FIlter
• Raised cosine rolloff filter
⎪⎪
⎪⎪⎨
⎧
+>+≤<−
−≤≤
⎥⎦
⎤⎢⎣
⎡⎟⎠⎞
⎜⎝⎛ +−
+= ss
ss
RC
TfTfT
TffTfH
2/)1(||2/)1(||2/)1(
2/)1(||0
21||2(cos1
21
1
)(α
ααα
ααπ
⎪⎪⎩
+> sTf 2/)1(||0 α
⎟⎟⎞
⎜⎜⎛⎟⎠⎞
⎜⎝⎛= 2
)/cos()/sin()( ssRC
TtTtth παπ⎟⎟⎠
⎜⎜⎝ −⎟⎠
⎜⎝ 2))2/(4(1
)(s
RC Ttt απ
Most popular pulse shaping filter used in mobile communications
As the value of α (rolloff factor) increases,
- the bandwidth of the filter also increases
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- the time sidelobe levels decrease.
ECE 8708 Wireless Communications : Modulation Techniques
Raised Cosine Rolloff FIlter
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ECE 8708 Wireless Communications : Modulation Techniques
Raised Cosine Rolloff FIlter
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ECE 8708 Wireless Communications : Modulation Techniques
Geometric Representation of Digital Modulation Signalsp g g
• Modulation signal set S = {S (t) S (t) S (t)}S = {S1(t), S2(t),..... SM(t)}
M =2: BinaryM >2 : M-ary
• Number of bits of information possible = log2M bits/symbol
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ECE 8708 Wireless Communications : Modulation Techniques
Types of Digital Modulation
Linear Non-Linear Spread Spectrum
Amplitude of transmitted signal s(t) varies linearly
Amplitude of carrier is constant
Transmission bandwidth >> minimum required
with message signal m(t)
signal bandwidth
Bandwidth efficient- Higher bandwidth Inefficient for single useful for accommodating more users in a li it d t
but high immunity against random FM noise.
user but efficient for multi-users
limited spectrumQPSKOQPSK
FSKGMSK(Gaussian
min. Shift keying)
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π/4 QPSK MFSK
ECE 8708 Wireless Communications : Modulation Techniques
Linear Modulation Techniques
• Digital modulation techniques are classified as linear and nonlinear.
• For linear modulations, the amplitude of transmitted signal, s(t),varies linearly with the modulating digital signal, m(t).
)]2()()2()([ )]2()([)(
tfsintmtfcostmAtfjexptAmRets
cIcR
c
πππ−=
=)]()()()([ tfsintmtfcostm cIcR ππ
amplitudethe:A
signalmodulatedthe:)()()(frequencycarier the:
amplitude the:
tjmtmtmfA
IR
c
+=
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signalmodulated the: )()()( tjmtmtm IR +
ECE 8708 Wireless Communications : Modulation Techniques
Binary Phase Shift Keying (BPSK)
• The information of the transmitted signal is modulated into the phase of the carrier signal.
)2(cos2)( ccb
bBPSK tf
TEtS θπ +=
)1binary (0 Tt b≤≤
2
)2(cos2)( ccb
bBPSK
E
tfTEtS θππ ++=
)y(b
)2(cos2 ccb
b tfTE θπ +−=
)0binary (0 Tt b≤≤
⎥⎤
⎢⎡
⎟⎟⎞
⎜⎜⎛ −−
+⎟⎟⎞
⎜⎜⎛ −
22)(sin)(sin bcbcb TffTffEP ππ )2(cos2)()( cc
bBPSK tfEtmtS θπ +=
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⎥⎥⎦⎢
⎢⎣
⎟⎟⎠
⎜⎜⎝ −−
+⎟⎟⎠
⎜⎜⎝ −
=)()(2 bc
bc
bc
bcbBPSK TffTff
Pππ
)()()( ccb
BPSK fT
ECE 8708 Wireless Communications : Modulation Techniques
Coherent BPSK Receiver with Carrier Recovery Circuits
The received BPSK signal ( ) ( ) 1cos22cos 2 −= xx ( ) ( )[ ]12cos21cos2 += xx
)2(cos2)()( chθθπ ++= tfEtmtS ccb
BPSK
)2(cos2)(
)2(cos)()( ch
θπ
θθπ
+=
++
tfTEtm
tfT
tmtS
cb
b
ccb
BPSK
b
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ECE 8708 Wireless Communications : Modulation Techniques
Coherent BPSK Receiver
The bit error rate in an AWGN channel
)|( 1srP)|( 2srP
⎤⎡
= ∫∞−
0 2
0
11,
)(1
)|()|(
Er
drsrpseP BPSKe
⎥⎤
⎢⎡−=
⎥⎥⎦
⎤
⎢⎢⎣
⎡ −−=
∫
∫−
∞−
/2 2
00
exp1
)(exp1
0
dxx
drN
ErN
NE
b
b
π
⎟⎟⎠
⎞⎜⎜⎝
⎛=⎥
⎦
⎤⎢⎣
⎡−=
⎥⎦
⎢⎣
=
∫
∫∞
∞−
0/2
2 22
exp21
2exp
2
NEQdxx
dx
b
NEπ
π
bEbE−
⎠⎝⎦⎣ 0/2 0NEb
⎟⎞
⎜⎛ 2)|()()|()( EQPPPPP b
Yimin Zhang, Villanova University 38
⎟⎟⎠
⎜⎜⎝
=+=0
2,21,1, )|()()|()(N
QsePsPsePsPP bBPSKeBPSKeBPSKe
ECE 8708 Wireless Communications : Modulation Techniques
Coherent BPSK Receiver
The bit error rate in an AWGN channel ⎟⎟⎠
⎞⎜⎜⎝
⎛=
0,
2NEQP b
BPSKe
10-1
100
10-2
10-4
10-3
BE
R
10-6
10-5
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0 2 4 6 8 10 1210
Eb/N0 (dB)
ECE 8708 Wireless Communications : Modulation Techniques
Quadrature Phase shift keying (QPSK)
• 2 bits are transmitted at a signal modulation symbol.• Therefore QPSK has twice the bandwidth efficiency of BPSK• Therefore, QPSK has twice the bandwidth efficiency of BPSK.
.4,3,2,1 0 )2
)1(2(cos2)( s =≤≤−+= iTtitfTEtS c
sQPSK
ππ2Ts
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ECE 8708 Wireless Communications : Modulation Techniques
Quadrature Phase shift keying (QPSK)
• BPSK BBS TRRBW /222 ===
• QPSK
BBS
BBS TRRBW /12 ===
• Quadrature Phases = 0, π/2, π, 3π/2
BBS
• Average probability of bit error in additive white Gaussian noise
⎟⎟⎞
⎜⎜⎛
=2EQP b
Q S
• The BER of QPSK is identical to BPSK, but twice the bandwidth ffi i
⎟⎟⎠
⎜⎜⎝
=0
, NQP QPSKe
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efficiency.
ECE 8708 Wireless Communications : Modulation Techniques
Spectrum and Bandwidth of QPSK Signalsp g
• The null-to-null RF b d idth i l t thbandwidth is equal to the bit rate Rb.
• The BW is half of a BPSK signal.
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ECE 8708 Wireless Communications : Modulation Techniques
QPSK Signal Transmission Techniqueg q
• The bit stream m(t) is split into two bit streams mI(t) and mQ(t)(in-phase and quadrature stream).
• The two binary streams are separately modulated by two carriers, which are in quadrature.
1
0
1
2
0 0.5 1 1.5 2 2.5 3-1
0 1 0 0
Input binary 0 0.5 1 1.5 2 2.5 3
-1
0 0.5 1 1.5 2 2.5 3-2
0
1
sequence1 1 0 1 0 0
0 0.5 1 1.5 2 2.5 3-1
0
1
1 1 00
1
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0 0.5 1 1.5 2 2.5 3-1
ECE 8708 Wireless Communications : Modulation Techniques
Coherent QPSK Signal Detection Techniqueg q
• The frontend bandpass filter remove the out-of-band noise and adjacent channel interference.
• The filtered output is split into two parts, and each part is coherently demodulated using in-phase and quadrature carriers.
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ECE 8708 Wireless Communications : Modulation Techniques
Offset QPSK
• OQPSK signaling is similar to QPSK, except for the time alignment of the even and odd bit streams.
• Thus, OQPSK can eliminate 180° phase transition, and prevent the signal envelope to go to zero.
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ECE 8708 Wireless Communications : Modulation Techniques
Offset QPSK
T iti f QPSK d l t d i l T iti f OQPSK d l t d i l
Variation of QPSK
Transitions for a QPSK modulated signal Transitions for a OQPSK modulated signal
Q channel is delayed by a ½ bit interval from I channelI and Q channel signals transition at different timesRange of phase transitions is from 0-90 degreesThi li i t th 180 d h hift OQPSK i lThis eliminates the 180 degree phase shift so an OQPSK signal never goes through a zeroIn non-linear amplification, a zero causes regeneration of sidelobes and spectral spreading in the signal
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p p g gThus, OQPSK signals yield a more efficient amplification process
ECE 8708 Wireless Communications : Modulation Techniques
Differential Phase Shift Keying (DPSK)
• Differential PSK is a noncoherent form which avoids the need for a coherent reference signal at the receiver.
• Nocoherent receivers are easy and cheap to build, and hence are widely used in wireless communications.
• Transmitter: input sequence is first differentially encoded and then modulated.
e g (for BPSK) 1⊕= kkk dmde.g., (for BPSK)
dk: k-th transmitted symbol mk: k-th information symbol
1−⊕ kkk dmd
k y
• Receiver: the original sequence is recovered from the demodulated differentially encoded signal through a complementary process
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differentially encoded signal through a complementary process.
ECE 8708 Wireless Communications : Modulation Techniques
Differential Phase Shift Keying (DPSK)
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ECE 8708 Wireless Communications : Modulation Techniques
Differential Phase Shift Keying (DPSK)
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ECE 8708 Wireless Communications : Modulation Techniques
Differential Phase Shift Keying (DPSK)
⎞⎛
The BER of differential BPSK in an AWGN channel
⎟⎟⎠
⎞⎜⎜⎝
⎛−=
0, exp
21
NEP b
DBPSKe
10-1
100
Coherent BPSKDifferential BPSK
The energy efficiency of
10-3
10-2
ER
gy ydifferential BPSK is inferior to coherent BPSK.
10-5
10-4B
E
At BER of 10-3 the difference in SNR is l th 3 dB
0 2 4 6 8 10 12 1410
-7
10-6
SNR (dB)
less than 3 dB.
At BER of 10-5, the difference in SNR is
Yimin Zhang, Villanova University 50
SNR (dB)difference in SNR is less than 1 dB.
ECE 8708 Wireless Communications : Modulation Techniques
Differential Phase Shift Keying (DPSK)
When the M-ary PSK modulations ( ), the 4≥M( ),degradation of the BER due to noncoherent detection is roughly 3 dB.
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ECE 8708 Wireless Communications : Modulation Techniques
π/4 QPSK
• π/4 QPSK is a quadrature phase shift keying technique.
• It offers a compromises between OQPSK and QPSK.
• The maximum phase change of π/4 QPSK is limited to ±135°
• An extremely attractive feature of π/4 QPSK is that it can be
noncoherently detected.
• Further, it has been found that in the presence of multipath spread
and fading, π/4 QPSK performs better than OQPSK.
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ECE 8708 Wireless Communications : Modulation Techniques
Constellation Diagram of π/4 QPSK Signal
Information bits Phase shift φk1 1 π/41 1 π/4 0 1 3π/4 0 0 - 3π/41 0 -π/41 0 π/4
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ECE 8708 Wireless Communications : Modulation Techniques
Generic π/4 QPSK Transmitter
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ECE 8708 Wireless Communications : Modulation Techniques
π/4 QPSK Baseband Differential Detector
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ECE 8708 Wireless Communications : Modulation Techniques
π/4 QPSK IF Differential Detection
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ECE 8708 Wireless Communications : Modulation Techniques
π/4 QPSK FM Discriminator
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ECE 8708 Wireless Communications : Modulation Techniques
Constant Envelope Modulationp
• The constant envelope family of modulations has following advantage:advantage:– Power efficient 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 simplifiedLimiter 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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ECE 8708 Wireless Communications : Modulation Techniques
Binary Frequency Shift Keying (BFSK)
• The information of the transmitted signal is modulated into the frequency of the carrier signalfrequency of the carrier signal.
• In general, an FSK signal may be represented as
2E )tΔ22(cos2)()( ffTEtvtS cb
bHFSK ππ +==
)tΔ22(cos2)()( ffTEtvtS cb
bLFSK ππ −==
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ECE 8708 Wireless Communications : Modulation Techniques
Phase Trajectory Examplej y p
0.5
1
-0.5
0
0 0.2 0.4 0.6 0.8 1-1
t/T
0.5
1
-0.5
0
Yimin Zhang, Villanova University 60
0 0.2 0.4 0.6 0.8 1-1
t/T
ECE 8708 Wireless Communications : Modulation Techniques
Detection of Binary FSKy
• BPSK and BFSK)(tψ )(t
)(
2s1s“0” “1”
)(2 tψ )(1 tψ
1s bE bE221 =− ss“0”BPSK BFSK
)(1 tψbEbE−
bE221 =− ss2s
)(2 tψbE
“1”
2
0, ⎟
⎟⎠
⎞⎜⎜⎝
⎛=
NEQP b
BPSKe 0
, ⎟⎟⎠
⎞⎜⎜⎝
⎛=
NEQP b
BFSKecoherent detection0 ⎠⎝ N 0 ⎠⎝
exp21
0,, ⎟⎟
⎠
⎞⎜⎜⎝
⎛−=
NEP b
NCBPSKe 2
exp21
0,, ⎟⎟
⎠
⎞⎜⎜⎝
⎛−=
NEP b
NCBFSKenoncoherent detection
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2 0 ⎠⎝ N 0 ⎠⎝ N
ECE 8708 Wireless Communications : Modulation Techniques
Detection of Binary FSKy
Coherent
NoncoherentNoncoherent
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ECE 8708 Wireless Communications : Modulation Techniques
Minimum Shift Keying (MSK)y g ( )
• Minimum shift keying (MSK) is a special type of continuous phase-frequency shift keying (CPFSK)frequency shift keying (CPFSK).
• MSK is sometimes referred to as fast FSK, as the frequency spacing used is only half as much as that used in conventional noncoherent FSKnoncoherent FSK.
• An MSK signal also can be thought of as a special form of OQPSK where the baseband rectangular pulses are replaced with half-sinusoidal pulses.
( ) ( )tftttftttS 2)sin(sin)(2)cos(cos)()( πππ+( ) ( )tπf
Ttmtf
TtmtS c
bQ
bIMSK 2)sin
2(sin)(2)cos
2(cos)()( cπ +=
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ECE 8708 Wireless Communications : Modulation Techniques
MSK Waveform
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27
ECE 8708 Wireless Communications : Modulation Techniques
MSK Power Spectrump
• From this figure, the MSK spectrum has lower sidelobes than QPSK and OQPSK.QPSK and OQPSK.
Yimin Zhang, Villanova University 65
27
ECE 8708 Wireless Communications : Modulation Techniques
Gaussian Minimum Shift Keying (GMSK)y g ( )
• GMSK is a simple binary modulation scheme which may be viewed as a derivative of MSK.
• In GMSK, the sidelobe levels of the spectrum are further reduced by passing the modulating NRZ data waveform throughreduced by passing the modulating NRZ data waveform through a premodulation Gaussian pulse-shaping filter.
• In practice, GMSK is most attractive for its excellent power efficiency (due to the constant envelope) and its excellent spectrum efficiency.
• GMSK is used in GSM, the world's most popular standard forGMSK is used in GSM, the world s most popular standard for mobile telephony systems.
Yimin Zhang, Villanova University 66
ECE 8708 Wireless Communications : Modulation Techniques
GMSK Transmitter Using Direct FM Generationg
• The impulse response of the GMSK premodulation filter
BN=BTGMSK premodulation filter
2ln222
⎟⎞
⎜⎛ ππ
BtthG
2ln2 ,exp)( 22 =⎟
⎟⎠
⎞⎜⎜⎝
⎛−= ααπ
απ
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ECE 8708 Wireless Communications : Modulation Techniques
Power Spectrum Density of a GMSK Signaly g
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ECE 8708 Wireless Communications : Modulation Techniques
Combined Linear/Constant Envelope Modulation TechniquesTechniques
• Digital baseband data may be sent by varying both the envelope and phase (frequency) of an RF carrierand phase (frequency) of an RF carrier.
• This can offer two degrees of freedom, thus such modulation techniques are called M-ary modulation.
• In M-ary signaling scheme, two or more bits are grouped together to form symbols.
• Depending on whether the amplitude phase or frequency of the• Depending on whether the amplitude, phase, or frequency of the carrier is varied, the modulation scheme is called M-ary ASK, M-aryPSK, or M-ary FSK.
ff• M-ary modulation schemes achieve better bandwidth efficiency at the expense of power efficiency.
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ECE 8708 Wireless Communications : Modulation Techniques
M-ary Phase Shift Keying (MPSK)y y g ( )
( ) ( )ssi tfini
TEtfi
TEtS 2s
M21)-(sin22cos
M21)-(cos2)( cc ⎥⎦
⎤⎢⎣⎡+⎥⎦
⎤⎢⎣⎡= ππππ ( ) ( )
bsbs
ssi
TMTEME
fT
fT
)(log and )(log whereM
)(M
)()(
22
cc
==
⎥⎦⎢⎣⎥⎦⎢⎣
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ECE 8708 Wireless Communications : Modulation Techniques
M-ary PSK Power Spectral Densityy p y
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ECE 8708 Wireless Communications : Modulation Techniques
M-ary Quadrature Amplitude Modulation (QAM)y p ( )
( ) ( )2sM21)-(sin22cos
M21)-(cos2)( c
minc
miniii tfinib
TEtfia
TEtS ππππ
⎥⎦⎤
⎢⎣⎡+⎥⎦
⎤⎢⎣⎡=
integers.t independen ofpair a are and and amolitude,lowest the withsignal theofenergy thes where
MM
min
ii
ss
baiE
TT ⎦⎣⎦⎣
Yimin Zhang, Villanova University 72
ECE 8708 Wireless Communications : Modulation Techniques
M-ary Frequency Shift Keying (MFSK)
210)(cos2)( s MiTttinEtS ≤≤⎥⎤
⎢⎡
+π
y q y y g ( )
•
bdare sfrequencie signal theand ,integer fixed somefor 2/ where
,...,2,1 0 )(cos)(
cscc
scss
si
/nTnf
MiTttinTT
tS
=
=≤≤⎥⎦
⎢⎣
+=
Hertz.21by separated sT/
• MFSK is not used in traditional way because its spectral efficiency is poor.
• The orthogonality characteristic of MFSK has led researches to g yexplore Orthogonal Frequency Division Multiplexing (OFDM).
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ECE 8708 Wireless Communications : Modulation Techniques
OutlinesOutlines
Concept of Modulation
Analog and Digital Modulations
Power and Spectral Efficiency
Pulse ShapingPulse Shaping
Digital Modulation Schemes
Linear – BPSK, QPSK, DPSK, OQPSK, π/4 QPSKea S , Q S , S , OQ S , π/ Q S
Nonlinear – FSK, MSK, GMSK
M-ary ASK, M-ary PSK, M-ary FSK
Coherent and Noncoherent Detection
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ECE 8708 Wireless Communications : Modulation Techniques
Shannon’s LimitShannon’s Limit
R>C
R / W [bits/s/Hz]
R=C
M=256Unattainable region
Bandwidth limited M=8
M=16
M=64
M=256
R<CPractical regionM=2
M=4
M 8
M=2M=4M=8
M=16
Power limited
Shannon limit MPSKMQAMMFSK
510−=BP
Yimin Zhang, Villanova University 75
[dB] / 0NEb
ECE 8708 Wireless Communications : Modulation Techniques
Homework Assignment 4Homework Assignment 4
1. Using Matlab, generate the QPSK waveform with the followingparameters: Symbol rate: 1000 baud; carrier frequency: 5000 Hz,bit sequence to be transmitted: 1 0 1 1 0 0 1 1 The in-phase andbit sequence to be transmitted: 1 0 1 1 0 0 1 1. The in phase andquadrature waveforms should be separated plotted. Usesufficiently high sampling rate to ensure smooth waveforms.Submit your waveforms with Matlab code.
2. Using Matlab, simulate the BER performance for QPSK signals.Geometric representations should be used. Plot the simulatedBER together with the theoretical result given in (6.79). Theevaluated value of Eb/N0 should be between 0 and 10 dBevaluated value of Eb/N0 should be between 0 and 10 dB.Sufficiently high number of random bits should be used to ensurehigh accuracy of the simulated BER. Submit your waveforms withMatlab code.
3. 6.12 (a) (b) (c) from the textbook.4. 6.13 from the textbook.5 6 15 from the textbook
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5. 6.15 from the textbook.Deadline 2/15/11 Before next class begins