Week 6: Bandpass Modulation MPSK MASK, OOK · PDF fileWeek 6: Bandpass Modulation MPSK ... within the same bandwidth ... FSK (c) ASK (d) ASK/PSK (APK) Amplitude Shift Keying

EE5713 : Advanced Digital Communications

Week 6:

Bandpass Modulation

MPSK

4/14/2014 Advanced Digital Communications, Spring-2014, Week-6 1

MPSK

MASK, OOK

MFSK

� In-phase and Quadrature (I&Q) Representation

– Any bandpass signal can also be represented as

• x(t) is a real-valued signal called In-phase (I)

• y(t) is a real-valued signal called Quadrature (Q)

)sin()()cos()()( 00 ttyttxts ωω −=


– This is often a convenient form which

• Emphasizes the fact that two signals may be transmitted within the same bandwidth

• Closely parallels the physical implementation of the Tx/Rx

Concept of a constellation diagram


Digital Modulation Schemes

� Basic Digital Modulation Schemes:

– Amplitude Shift Keying (ASK)

– Frequency Shift Keying (FSK)

– Phase Shift Keying (PSK)

– Amplitude Phase Keying (APK)


� For Binary signals (M = 2), we have

– Binary Amplitude Shift Keying (BASK)

– Binary Phase Shift Keying (BPSK)

– Binary Frequency Shift Keying (BFSK)

� For M > 2, many variations of the above techniques exit usually classified as M-ary Modulation/detection

Bandpass MOdulation and DEModulation


Overview of Modulation Schemes


digital modulations, (a) PSK (b) FSK (c) ASK (d) ASK/PSK (APK)

Amplitude Shift Keying

� Modulation Process

– In Amplitude Shift Keying (ASK), the amplitude of the carrier is switched between two (or more) levels according to the digital data

– For BASK (also called ON-OFF Keying (OOK)), one and zero are


Keying (OOK)), one and zero are represented by two amplitude levels A1 and A0

� Analytical Expression:

where Ai = peak amplitude

TttAts ci ≤≤= 0),cos()( ω

)cos(2)cos(2)cos()( 02

00 tAtAtAtsrmsrms ωωω ===

2E



Hence,

where

)cos(2

)cos(2 00 tT

EtP ωω ==

MiTttT

tEts i

i ,......2,1,0),cos()(2

)( 0 =≤≤= ω

1,......2,0,)(0

2 −=∫= MidttsET

i

� Where for binary ASK (also known as ON OFF Keying (OOK))

� Mathematical ASK Signal Representation

– The complex envelope of an ASK signal is:

10),cos()()(1 binaryTtttmAts cc ≤≤+= φω00,0)(0 binaryTtts ≤≤=

)()( tmAtg =



– The magnitude and phase of an ASK signal are:

– The in-phase and quadrature components are:

the quadrature component is wasted.

)()( tmAtg c=

0)(),()( == ttmAtA c φ

)()( tmAtx c=

,0)( =ty

ASK, OOK, MASK� The amplitude (or height) of the sine wave varies to

transmit the ones and zeros


� One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation)

Implementation of binary ASK


OOK and MASK� OOK (On-OFF Key)

– 0 silence.

– Sensor networks: battery life, simple implementation

� MASK: multiple amplitude levels


Pro, Con and Applications� Pro

– Simple implementation

� Con

– Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

– Susceptible to sudden gain changes


– Susceptible to sudden gain changes

– Inefficient modulation technique for data

� Applications

– On voice-grade lines, used up to 1200 bps

– Used to transmit digital data over optical fiber

– Morse code

– Laser transmitters

Detectors for ASK: Coherent Receiver

Coherent detection requires the phase information


� Coherent detection requires the phase information� A coherent detector mixes the incoming signal with a locally generated

carrier reference� Multiplying the received signal r(t) by the receiver local oscillator (say

Accos(wct)) yields a signal with a baseband component plus a component at 2fc

� Passing this signal through a low pass filter eliminates the high frequency component� In practice an integrator is used as the LPF

� The output of the LPF is sampled once per bit period

� This sample z(T) is applied to a decision rule

– z(T) is called the decision statistic� Matched filter receiver of OOK signal


� A MF pair such as the root raised cosine filter can thus be used to shape the source and received baseband symbols

� In fact this is a very common approach in signal detection in most bandpass data modems

Noncoherent Receiver– Does not require a phase reference at the receiver

– If we do not know the phase and frequency of the carrier, we can use a noncoherent receiver to recover ASK signal

� Envelope Detector:


– The simplest implementation of an envelope detector comprises a diode rectifier and smoothing filter

Frequency Shift Keying (FSK)� In FSK, the instantaneous carrier frequency is switched between 2 or more levels

according to the baseband digital data

– data bits select a carrier at one of two frequencies

– the data is encoded in the frequency

� In the past, FSK has been the most widely used form of digital modulation;Why?

– Simple both to generate and detect

– Insensitive to amplitude fluctuations in the channel


– Insensitive to amplitude fluctuations in the channel

� FSK conveys the data using distinct carrier frequencies to represent symbol states

� An important property of FSK is that the amplitude of the modulated wave is constant

� Waveform

� Analytical Expression

1,....1,0),cos(2

)( −=+= MitT

Ets i

s

si 321

φω

formAnalog)()(

])([)(

0

0

+==

+= ∫ ∞−

tmfftdt

df

dmtt

dii

t

di

θ

ττωωθ


� General expression is

Where

)()( 0

+== tmfftdt

f dii θ

bsbsi kTTkEEandfiff ==∆+= ,0

1−−=∆ ii fff

1,....1,0),22cos(2

)( 0 −=∆+= MiftitfT

Ets

s

si ππ

Frequency Shift Keying

� One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation)


� Represent each logical value with another frequency (like FM)

( )

=ts( )tfA 12cos π( )tfA 22cos π

1binary 0binary

Multiple Frequency-Shift Keying (MFSK)

� More than two frequencies are used

� More bandwidth efficient but more susceptible to error

• f i = f c + (2i – 1 – M)f d

( ) tfAts ii π2cos= Mi ≤≤1


i c d

• f c = the carrier frequency

• f d = the difference frequency

• M = number of different signal elements = 2 L

• L = number of bits per signal element

Phase Shift Keying

� One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation)


( )

=ts( )tfA cπ2cos( )ππ +tfA c2cos

1binary 0binary

DBPSK, QPSK� Differential BPSK

– 0 = same phase as last signal element

– 1 = 180º shift from last signal element


� Four Level: QPSK

( )

=ts

+4

2cosππ tfA c 11

+4

32cos

ππ tfA c

−4

32cos

ππ tfA c

−4

2cosππ tfA c

01

00

10

QPSK Example


FSK Vs PSK


PSK


BPSK


BPSK Detector


M=4 QPSK


QPSK


QPSK Detection


QPSK Detection: Matched Filter


MPSK


MPSK


MPSK Detection


Matched filter detection


Properties of MPSK Constellation

� In time domain, all M signals have the same amplitudes.

� All M signals have the same energy.

� All M constellation points are the same distance from the origin -- they are equally spaced on a circle


� They differ in phase only (i.e. the data is encoded in the phase of the transmitted carrier).

� Each phase differs by 2p/M radians.

Documents

Week 6: Bandpass Modulation MPSK MASK, OOK · PDF fileWeek 6: Bandpass Modulation MPSK ... within the same bandwidth ... FSK (c) ASK (d) ASK/PSK (APK) Amplitude Shift Keying