Digital Modulation Techniques
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Transcript of Digital Modulation Techniques
page1Iwao Sasase, Keio University
Digital Modulation Techniques
Iwao Sasase Department of Information and
Computer Science, Keio Universityhttp://www.sasase.ics.keio.ac.jpEmail:[email protected]
page2Iwao Sasase, Keio University
Digital Modulation Techniques
Amplitude Shift Keying (ASK)Frequency Shift Keying (FSK)Phase Shift Keying (QPSK)M (Multi)-ASK ,M-FSK, M-PSKQPSK, π/4shift QPSKQuadrature Amplitude Modulation (QAM)Minimum Shift Keying (MSK) and GMSK
page3Iwao Sasase, Keio University
Waveform and SpectrumBaseband pulse waveform has low frequency components
By modulating the carrier depending on the input data, we can transmit the data in RF frequency
-T/2 T/20
V
T
time
Frequency1/T 2/T 3/T0
Am
plitu
de
Spec
trum
page4Iwao Sasase, Keio University
Principle of Modulation
Depending on the information of the baseband signal, amplitude, frequency or phase of the carrier is changed. tcωcos
Modulation
ctAtS ωcos)()( =amplitude
frequency tttS c )}(cos{)( ωω +=
phase )}({cos)( tttS c φω +=
tcωcos
page5Iwao Sasase, Keio University
Classification of Modulation Techniques
Analog Modulation
Digital Modulation
AM (Amplitude Modulation)
FM (Frequency Modulation)
PM (Phase Modulation)
ASK (Amplitude Shift Keying)
FSK (Frequency Shift Keying)
PSK (Phase Shift Keying)
QAM (Quadrature Amplitude Modulation )
)(tA
)(tφ
)(tω
)(tA
)(tA)(tω
)(tφ
)(tφ
page6Iwao Sasase, Keio University
tf a(t)cos2π(t)S cASK =)(ta amplitude
cf carrier frequency
ASK (Amplitude Shift Keying)Accomplished by changing an amplitude of carrier, depending on the input digital signalASK is also called as “On-Off-Keying”
1 1 10In binary signal case (2ASK),
a(t) is either “1” or “0”.As shown in Fig. 1, carrier becomes “on” and “off”
In binary signal case (2ASK), a(t) is either “1” or “0”.
As shown in Fig. 1, carrier becomes “on” and “off”
Binary OOK modulation waveform
page7Iwao Sasase, Keio University
Non-Coherent DetectionEnvelope Detection of ASK
Bandpass filter Rectifier Lowpass
Filter Decision
Th
Threshold Voltage
page8Iwao Sasase, Keio University
Coherent detectionCoherent Detection of ASK
BandpassFilter
Carrier Recovery
Circuit
Lowpass Filter Decision
Threshold Voltage
tωcos
page9Iwao Sasase, Keio University
FSK (Frequency Shift Keying)Accomplished by changing an instantaneous Frequency, depending on the input digital signal
)2cos()2cos(
{)(22
11
φπφπ
++
=tfAtfA
tS FSK
1 1 10
Input waveform
FSK
Binary FSK modulation waveform
Carrier frequency f1and f2 respond to “1” and “0”, respectively
page10Iwao Sasase, Keio University
PSK (Phase Shift Keying)Accomplished by changing a phase of carrier, Depending on the input digital signal
1 1 10
入力波形
PSK
Binary PSK modulation waveform
)tf cos(2 π(t)S cPSK iA φ+= A amplitudeiφ phase
page11Iwao Sasase, Keio University
SNR (Signal-to-noise ratio in dB)6 8 10 12 14 16 18
10-7
10-6
10-5
10-4
10-3
10-2
BER
(bit
erro
r rat
e)
Pe PSK(coherent)
ASK(coherent)
FSK(coherent)
PSK(differential) FSK(noncoherent)
ASK(noncoherent)
Error Performance
2/10 )4/(
21 NEerfcPe =ASK
2/10 )/(
21 NEerfcPe =PSK
2/1
0
)2
(21
NEerfcPe =FSK
Bit error probability Bit error probability
erfc : complementary error function
E : energy per bit
N0 : power spectrum density (psd) of noiseBER Performance
page12Iwao Sasase, Keio University
PSK has better power and frequency efficiencies compared to ASK and FSK
PSK achieves small bit error rate (BER) for the same C/N(carrier-noise ratio)
PSK has constant envelope (no information in amplitude ), and is robust to time-varying fading channel.
PSK is popularly used in many communication systems such as satellite and mobile communication systems.
Characteristic of PSK
page13Iwao Sasase, Keio University
MASK refers to ASK that transmit more bits by using multilevel amplitude a(t) of baseband waveform. MASK is usually called as Multilevel ASK, or MASK, where k =log 2 M bits of information are transmitted by using M-level amplitude.
Multilevel Amplitude Shift Keying(M-ASK)
In MASK, band, 1/k of occupancy bandwidth can be conserved while transmitting k bit information. That is, by using multilevel amplitude, the bit rate can be increased without the bandwidthexpansion. Larger transmission power is required.
Waveform of MASK signal
page14Iwao Sasase, Keio University
Q
I
Q
I
Q
I
4-ary PSK 8-ary PSK 16-ary PSK
Phase allocation of MPSK signal
In MPSK, signal amplitude is constant. Each signal constellation point is placed on the circumference where the center equals to origin.
Multi-ary Phase Shift Keying (MPSK)
page15Iwao Sasase, Keio University
Quadrature PSK (QPSK)
Serial/Parallel
×
cosωt
×
sinωtQPSK modulator
The transmitter consists of two quadrature modulation components cosωt and sinωt to transmit 2 bits/symbol
×
±1
±1
±1
Input data ++
page16Iwao Sasase, Keio University
QPSK Demodulation(Quadrature Detection)
BPF
LPF×
cosωt
In-phase component
LPF×
sinωt
Quadrate component
QPSK receiver
Signal phase 0° 90° 180° 270°
LPF input 1 0 -1 0
Signal phase 0° 90° 180° 270°
LPF input 0 1 0 -1
page17Iwao Sasase, Keio University
Quadrature DetectionIn quadrature detection, for symbol detection, plus and minus of in-phase and quadrate components can be decided independently.
Quadrate( Q )
In-phase( I )
QPSK
Quadrate( Q )
In-phase( I )
Quadrature detection of cos component
Decision boundary
(1,1)
(1,-1)
(-1,1)
(-1,- 1)
page18Iwao Sasase, Keio University
π/4 shift QPSK
adopted in mobile phone(PDC)2bits/symbol8 signal points are allocated on the circumference
In each symbol transmission, QPSKsignal points are shifted by π/4. Since there are no phase transition crossing origin, the signal envelope does not become zero, and thus,envelope fluctuation is smaller compared to the conventional QPSK.
Q
I
page19Iwao Sasase, Keio University
Quadrature Amplitude Modulation (QAM )QAM changes both amplitude and phase.Quadrature multilevel ASK using two orthogonal carriers(sin and cos )of the same frequency
)2sin(2)2cos(2)( minmin tfbTEtfa
TEts ci
sci
sQAM ππ +=
MiTt ,.....,2,10 =≤≤
16QAM signal points
16QAM
4bits/symbol
Compared to QPSK, frequency efficiency is improved to be double.
page20Iwao Sasase, Keio University
MSK
TffTff cc 4/1,4/1 21 −=+=
MSK is continuous phase FSK with minimum frequency shift
⎥⎦
⎤⎢⎣
⎡+−= k
bQc
b
bMSK T
ttmtmtfTEts φππ
2)()(2cos2)(
Feature of MSK
・continuous phase FSK with constant envelope ・digital FM with modulation index 0.5 ・good spectral efficiency
page21Iwao Sasase, Keio University
Input NRZ pulse sequence is band-limited by Gaussian LPF.
GMSK
GaussianLow pass
filter
FM transmitter with modulation index 0.5
GMSK Output NRZ Data
GMSK transmitter
GMSK has advantages of MSK, and achieves narrow spectrum.
・ GSM(Global System for Mobile Communication)・ DECT (Digital European Cordless Telecommunication)・ CT2