Digital Modulation Techniques

21
page1 Iwao Sasase, Keio University Digital Modulation Techniques Iwao Sasase Department of Information and Computer Science, Keio University http://www.sasase.ics.keio.ac.jp Email:[email protected]

description

digital modulation for telecom copamanies.

Transcript of Digital Modulation Techniques

Page 1: 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]

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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

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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

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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

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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

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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

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Non-Coherent DetectionEnvelope Detection of ASK

Bandpass filter Rectifier Lowpass

Filter Decision

Th

Threshold Voltage

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Coherent detectionCoherent Detection of ASK

BandpassFilter

Carrier Recovery

Circuit

Lowpass Filter Decision

Threshold Voltage

tωcos

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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

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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

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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

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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

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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

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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)

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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 ++

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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

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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)

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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

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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.

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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

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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