# Digital Modulation Techniques

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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, /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. tccos

Modulation

ctAtS cos)()( =amplitudefrequency tttS c )}(cos{)( +=phase )}({cos)( tttS c +=

tccos

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

)(t)(t)(tA

)(tA)(t

)(t

)(t

page6Iwao Sasase, Keio University

tf a(t)cos2(t)S cASK = )(ta amplitudecf 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

In 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

tcos

page9Iwao Sasase, Keio University

FSK (Frequency Shift Keying)Accomplished by changing an instantaneous Frequency, depending on the input digital signal

)2cos()2cos(

{)(22

11

++=

tfAtfA

tS FSK

Input waveform

FSK

Binary FSK modulation waveform

Carrier frequency fand 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

PSK

Binary PSK modulation waveform

)tf cos(2 (t)S cPSK iA += A amplitudei phase

page11Iwao Sasase, Keio University

SNR (Signal-to-noise ratio in dB)

B

E

R

(

b

i

t

e

r

r

o

r

r

a

t

e

)

P

e PSKcoherent

ASKcoherent

FSKcoherent

PSKdifferential FSKnoncoherent

ASKnoncoherent

Error Performance

2/10 )4/(2

1 NEerfcPe =ASK2/1

0 )/(21 NEerfcPe =PSK

2/1

0

)2

(21

NEerfcPe =FSK

Bit error probability Bit error probability

erfc complementary error function

E energy per bit

N 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/Ncarrier-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 KeyingM-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

-ary PSK -ary PSK -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

cost

sintQPSK modulator

The transmitter consists of two quadrature modulation components cost and sint to transmit 2 bits/symbol

Input data +

+

page16Iwao Sasase, Keio University

QPSK DemodulationQuadrature Detection

BPF

LPF

cost

In-phase component

LPF

sint

Quadrate component

QPSK receiver

Signal phase

LPF input

Signal phase LPF input

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

page18Iwao Sasase, Keio University

/4 shift QPSK

adopted in mobile phonePDC2bits/symbol8 signal points are allocated

on the circumferenceIn each symbol transmission, QPSK

signal points are shifted by /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 carrierssin and cos of the same frequency

)2sin(2)2cos(2)( minmin tfbTEtfa

TEts ci

sci

sQAM +=

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

+= kb

Qcb

bMSK T

ttmtmtfTEts

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.

GSMGlobal System for Mobile Communication DECT (Digital European Cordless Telecommunication) CT2