DIT

Dar es Salaam institute of Technology (DIT)

ET 7308

Introduction to Communication System

Ally, J

jumannea@gmail.com

DIT

Course OutlinePrinciple of Communication System, Types of signal characteristics and reason for modulation

Analogue Modulation

Angle Modulation

Digital Coding

Digital Modulation

Errors

DIT

Analogue Modulation

DIT

Introduction to ModulationDefinitions

Analog modulation

Both the message signal and the transmitted signal are analog signalsTwo classes: amplitude modulation, angle modulation

Three signals:

Message signal: the information signal to be modulated and transmitted

Carrier signal c(t) : high frequency sinusoidal signal

Modulated signal: the signal to be transmitted, or the signal obtained after modulation

DIT

Modulation

It is the process of facilitating the transfer of information over a medium.

This is done by changing one or more the parameters of a signal including power, frequency, phase and amplitude depending on the requirement of the transmission system.

DIT

Baseband, Passband

Baseband: refers to the signals and systems before modulation, which have frequencies/bandwidth much lower than the carrier frequency

Passband: refers to the signals and systems after (including) modulation, which have frequencies/bandwidth around the carrier frequency

Baseband signal: is usually the message signal

Passband signal: is usually the modulated signal, or transmitted signal

Base band and band pass signals

DIT

Base band and band pass signals

• Base band signal is the original signal having the original frequencies when delivered by transmitters.

• In base band communication, signals are transmitted without modulation.

• Band pass signal is a signal which is modulated by one of the modulation schemes.

• Demodulation is the process of extracting the baseband message from the carrier so that it may be processed and interpreted by the intended receiver

DIT

Message signal m(t) modifies:Amplitude: AM linear modulationPhase: PMFrequency: FMExample Compare signal waveforms

( )A t

( ) ( )f t d t dtφ=Non-linear modulation

0 0.5 1 1.5 2 2.5 3 3.5 4−2

0

2

4

6

8

10

message signal

AM signal

FM signal

carrier

)(tφ

DIT

Concept of Modulation

DIT

DIT

Checkpoints for studying each modulation

Modulated signal (time-domain)

Spectrum (frequency-domain)

Parameters: bandwidth, power, etc

Modulator and demodulator (Principles, block diagrams or circuits)

Major properties (advantages/disadvantages over other modulations)

DIT

List of modulation methods we will learn

Amplitude modulation methods and applications1. AM (amplitude modulation): AM radio, short wave

radio broadcast, 2. DSBSC (double sideband suppressed carrier AM):

data modem, Color TV’s color signals 3. SSB (single sideband AM): telephone4. VSB (vestigial sideband AM): TV picture signal

Angle modulation methods and applications1. FM (frequency modulation): FM radio broadcast, TV

sound signal, analog cellular phone2. PM (phase modulation): not widely used, except in

digital communication systems (but that is different)

DIT

Amplitude Modulation (AM)AM (conventional amplitude modulation)Amplitude Modulation (AM) is the one which the amplitude of a sinusoidal carrier is varied in accordance with an incomingmessage signal

Modulated signalCarrier: Message signal: m(t)AM modulated signal

where ka, is a constant called the amplitude sensitivity of the modulator responsible forthe generation of the modulated signal s(t).

DIT

Time-Domain descriptionThe standard form of an AM wave is defined by

The amplitude of the time function multiplying is called the envelope of AM wave s(t). The envelope of s(t) has essentially the same shape as the baseband signal m(t) provided that two requirements are satisfied:

1. The amplitude of is always less than unity, that is,for all t

2. The carrier frequency fc, is much greater than the highest frequency component W (message bandwidth) of the message signal m(t), that is

(a) Baseband signal m(t) (b) AM wave for (c) AM wave for

( )tf cπ2cos

DIT

Frequency-Domain descriptionThe Fourier transform of the AM wave s(t) is given by

(a) Spectrum of baseband signal

(b) Spectrum of AM wave

DIT

Generation of AM WavesMultipliers difficult to build in hardware AM waves typically generated using a nonlinear device to obtain the desired multiplicationSquare law modulator sums carrier c(t) and information m(t) signals, then squares them using a nonlinear device. Unwanted terms are filtered out with a bandpass filter.Switched modulation sums c(t) and m(t) then passes sum through a switch, which approximately multiplies it by a periodic square wave. This generates the desired signal plus extra terms that are filtered out.

m(t)

+

Accos(2πfct+φ)

Squareor Switch BPF

s(t)

DIT

Modulation IndexThe degree of modulation is an important parameter and is known asthe modulation index. It is the ratio of the peak amplitude of the modulating signal, Am to the peak amplitude of the carrier signal, Ac

(a) Under Modulation (ka < 1)

(b) Ideal Modulation (ka = 1)

(c) Over Modulation (ka > 1)

c

ma A

Ak =

DIT

Over Modulation

http://www.williamson-labs.com/480_am.htm

DIT

Detection of AM wavesThere are two devices for the detection of AM waves, namely, thesquare-law detector and the envelope detector

Square law detector, squares signal and then passes it through aLPF

Residual distortion proportional to m2(t)Non-coherent (carrier phase not needed in RX) Envelope detection simple alternative method

( )φπ +tfA cc 2cos

DIT

Explanation

Diode D1 cut the negativeportion of AM signal s(t)

When signal after D1 is positive,C is charged.When signal after D2 is 0,C is discharged.

Overall effect: y(t) remains approximatelyas the envelope of s(t)

m(t) can be detected from y(t)using capacitor to remove d.c.1.

Very important: this isEnvelope Detector.

DIT

Bandwidth of AM signal BT = 2W

AM signal’s bandwidth is twice message bandwidth

This is also transmitted signal bandwidth, or required minimum channel bandwidth Bc

Negative frequency contents of m(t) becomes visible in positive frequency

Upper sideband (USB):

Lower sideband (LSB):

Transmission power: PT = PM + Pcarrier= PUSB + PLSB + Pcarrier

Wfff cc +≤≤

cc ffWf ≤≤−

DIT

Normalized Average Power of AM signals

( ) ( ) ( )[ ]

( ) ( )[ ]

( ) ( )tmAtmAA

tmtmA

tmAtgts

ccc

c

c

2222

22

2222

21

21

2121

121

21

++=

++=

+==

( ) ( ) 21

21 2222 tmAAts cc +=

The normalized average power of the AM signal is

If the modulation contains no dc level, then

The normalized power of the AM signal is

Discrete

carrier powerSideband power

( ) 0=tm

DIT

AM – Modulation EfficiencyDefinition : The modulation efficiency is the percentage of the total power of the modulated signal that conveys information.

Only “Sideband Components” – Convey information

Modulation Efficiency:( )( )

1001 2

2

×+

=tm

tmE

Highest efficiency for a 100% AM signal : 50% - square wave modulation

Voltage Spectrum of the AM signal:

Translated version of message signal

( ) ( ) ( ) ( )[ ]ccccc ffMffffMff

AfS ++++−+−= δδ

2)(

Carrier line spectral component

DIT

Major Properties of AMAdvantages

Simplicity in implementation, especially in receiver and transmitter

The major reason that AM was the first & most popular broadcasting methods during early days

DisadvantagesWaste power and bandwidth

Carrier components wastes a major portion power, but carrier does not have message informationBoth USB and LSB are transmitted, which carry the same message information

DIT

Ways for AM improvement

To enhance power efficiencyReduce/remove carrier: DSB-SCRemove one/partial sideband: SSB, VSB

To enhance bandwidth efficiencyRemove one/partial sideband: SSB, VSBMultiplex two message signals together: QAM

Cost for the improvementMore expensive implementationThe simple envelope detector is no longer applicable

DIT

Double-Sideband Suppressed-carrier (DSB-SC)

In the standard form of Amplitude Modulation (AM), the carrier wave c(t) is completely independent of the message signal m(t), which means that the transmission of the carrier wave represents a waste of power.

To overcome this shortcoming , we may suppress the carrier component from the modulated wave, resulting in double-sideband suppressed carrier (DSB-SC) modulation.

Thus, by suppressing the carrier, we obtain a modulated wave that is proportional to the product of the carrier wave and the message signal.

DIT

Time-Domain DescriptionThe standard form of a DSB-SC wave is defined by

This modulated wave undergoes a phase reversal whenever the message signal m(t) crosses zero, as illustrated in figure below

(a) Baseband signal (b) DSB-SC modulated wave

( ) ( ) ( )tmtcts =

( ) ( ) ( )tmtfAts cc π2cos=

DIT

The Fourier transform of the DSB-SC wave s(t) is given by

(a) Spectrum of message signal

(b) Spectrum of DSB-SC modulated wave

Frequency-Domain Description

DIT

Generation of DSB-SC WavesA DSB-SC modulated wave consists simply of the product of the message signal and the carrier wave. A device achieving this requirement is called a Product Modulator.

Remove inefficient constant term

Modulated signal is

Can also use ring modulator: diodes and inductors

DIT

Coherent Detection of DSB-SC Modulated Wave The baseband signal m(t) can be uniquely recovered from a DSB-SC wave s(t) by first multiplying s(t) with a locally generated sinusoidal wave and then low-pass filtering the productIt is assumed that the local oscillator output is exactly coherent or synchronized, in both frequency and phase, with the carrier wavec(t) used in the product modulator to generate s(t).This method of demodulation is known as coherent detection or synchronous detection.

DIT

Coherent Detection of DSB-SC Modulated Wave-2We find that the product modulator output is:

The first term represents a DSB-SC modulated signal with a carrier frequency 2fc, whereas the second term is proportional to the baseband signal m(t).the first term is removed by the low-pass filter, this requirement is satisfied by choosing fc > W. At the filter output we then obtain asignal given by

The demodulated signal is therefore proportional to m(t) when the phase error is a constant.

DIT

Coherent Detection of DSB-SC Modulated Wave-3The amplitude of this demodulated signal is maximum whenand it is minimum (zero) when

As long as the phase error is constant, the detector provides an undistorted version of the original baseband signal m(t).

In practice, however, we usually find that the phase error varies randomly with time, due to random variations in the communication channel. The result is that at the detector output, the multiplying factor also varies randomly with time, which is obviously undesirable.

Therefore, provision must be made in the system to maintain the local oscillator in the receiver in perfect synchronism, in both frequency and phase, with the carrier wave used to generate the DSB-SC modulated signal in the transmitter.

The resulting system complexity is the price that must be paid for suppressing the carrier wave to save transmitter power.

φcos

DIT

Costas Loop (DSB-SC Demodulator)Goal: Maintain οφ ≈∆

( ) ( )tmtfA cc π2cos

( )φπ +tfc2cos

( )φπ +tfc2sin

( )tmφsin21

( )tmφcos21

DIT

Costas LoopOne method of obtaining a practical synchronous receiver system, suitable for demodulating DSB-SC waves, is to use the Costas loop.

This receiver consists of two coherent detectors supplied with the same input signal, namely, the incoming DSB-SC wave Accos(2πfct)m(t), but with individual local oscillator signals that are in phase quadrature with respect to each other.

The frequency of the local oscillator is adjusted to be the same as the carrier frequency fc, which is assumed known a priori.

The detector in the upper path is referred to as the in-phase coherent detector or I-channel, and that in the lower path is referred to as the quadrature-phase coherent detector or Q-channel.

These two detectors are coupled together to form a negative feedback system designed in such a way as to maintain the local oscillator synchronous with the carrier wave.

DIT

Double Side Band Suppressed CarrierPower in a AM signal is given by

( ) ( ) 21

21 2222 tmAAts cc +=

Discrete carrier power Sideband power

Discrete carrier power can be eliminated (Suppressing carrier )if m(t) is assumed to have a zero DC level

Then ttmAts cc ωcos)()( =

Spectrum

( ) ( )[ ]ccc ffMffMAfS ++−=

2)(

Since no power is wasted in carrier the efficiency is

Power

( ) ( ) 21 222 tmAts c=

( )( )

%1001002

2

=×=tm

tmE

DIT

Noise in AM Receivers

Power in s(t) is 0.5Ac2Pm

Power in n(t) is N0BSNR=Pm/Pn= Ac

2Pm/(2N0B)= Ps/(N0B) (SNR at the receiver input)Power in m′(t) is 0.25Ac

2Pm (half the power in s(t))Power in n′(t) is 0.5N0B (PSD 0.25N0 over BW 2B)SNR=Pm´/Pn´= Ac

2Pm/(2N0B)= Ps/(N0B) (SNR at the receiver output)

ProductModulator

m´(t)+ n´(t)

Accos(2πfct+φ)

s(t)=Accos(2πfct+φ)m(t)+

n(t)LPF

1

White Gaussian noise (AWGN)

-B B

DIT

Single-SideBand (SSB) Modulation

Standard AM and DSB-SC Modulation are wasteful of bandwidth because they both require a transmission bandwidth equal to twice message the message bandwidth.This means that insofar as the transmission of information is concerned, only one sideband is necessary, and no information is lost.Thus the channel needs to provide only the same bandwidth as the message signal, a conclusion that is intuitively satisfying.When only one sideband is transmitted, the modulation is referred to as single-sideband modulation

DIT

Single Sideband Modulation(2)Only transmits upper or lower sideband of AM and DSBSCThe transmitted signal can be written in terms m(t) and the Hilbert Transform of m(t)Use same demodulator as DSBSCSSB has half the SNR of DSBSC for half the transmit power: no SNR gainSSB can introduce significant distortion at DC where the sidebands meet: not good for TV signals

USB LSBM(f)

0 fc-fcB-B

USB

LSB)]2sin()()2cos()([

2)( φπφπ +±+= tftmtftmAts chc

c

DIT

Baseband Representation of Modulated Signals

Baseband signal representation is a compact way to represent passband signals.

All passband signals at carrier frequency fc can be written as s(t) = sI(t) cos(2fct) + sQ(t) sin(2fct).

sI(t) is called the in-phase signal component; sQ(t) is called the quadrature signal component.

The sine and cosine are orthogonal signals, can be used to separate out the in-phase and quadrature components from s(t).

We define as the baseband signal representation. Then which is a compact way to represent and analyze passband signals.

DIT

Generating of SSB modulated wave by phase discrimination method

The phase discrimination method of generating an SSB modulated wave involves two separate simultaneous modulation processes andsubsequent combination of the resulting modulation products.The system uses two product modulators, I and Q, supplied with carrier waves in phase quadrature to each other.The incoming baseband signal m(t) is applied to product modulator I, producing a modulated DSBSC wave that contains reference phase sidebands symmetrically spaced about carrier frequency fc.The hilbert transform mh(t) of m(t) is applied to product modulator Q, producing DSBSC modulated wave that containssideband having identical amplitude spectra to those of modulator I, but with phase spectra such that vector addition or subtraction of the two modulator outputs results in cancellation of one setof sidebands and reinforcement of the other set.The use of plus sign yields SSB wave with only the upper sideband, whereas the use of minus sign yields SSB wave with only upper sideband.

DIT

Block diagram for generating of SSB modulated wave by phase discrimination method

( )tfA cc π2cos

( )tfA cc π2sin

DIT

Demodulation of SSB wave

To recover the baseband signal m(t) from the SSB wave s(t), we have to shift the spectrum by the amounts so as to convert the transmitted sideband back into the baseband signal.This can be accomplished using coherent detection, which involves applying the SSB wave s(t), together with locally generated carrier , assumed to be of unit amplitude for convenience, to a product modulator and then low-pass filtering the modulator output.

cf+−

( )tfcπ2cos

,

( )tfcπ2cos

DIT

Demodulation of SSB wave (2)The product modulator output is given by

The first term is the desired message signal. The second term represents an unwanted components in the product modulator output that is removed by low-pass filtering.The detection of SSB modulated waves assume perfect synchronization between the local carrier and that in the transmitter both in frequency and phase. The effect of a phase error Ф in the locally generated carrier wave is to modify the detector output as follows

( ) ( ) ( )tstftv cπ2cos=

( ) ( ) ( ) ( ) ( )[ ]

( ) ( ) ( ) ( ) ( )[ ]tftmtftmAtmA

tftmtftmtfA

cccc

cccc

ππ

πππ

4sin~4cos41

41

2sin~2cos2cos21

m+=

±=

( ) ( ) ( ) φφ sin~41cos

41 tmAtmAtv cco m=

DIT

Demodulation of SSB wave (2)Owing to the phase error Ф, the detector output vo(t) contains not only the message signal m(t) but also its Hilbert transform mh(t). Consequently, the detector output suffers from phase distortion. This phase distortion is usually not serious with voice communications because the human ear is relatively insensitive to phase distortion.In the transmission of music and video signals, on the other hand, phase distortion in the form of a constant phase difference in all components can be intolerable.

DIT

Implementation Issues and SuperheterodyneReceivers

Envelope detectors tailored to a given frequency fcIn AM radio the carrier frequency changes

In DSBSC and SSB the local oscillator can radiate out the receiver front end and cause self-interferenceFix these problems by IF processing

Downconvert the signal to an intermediate frequency (IF)Do demodulation/filtering at IFNo reradiation and envelope detector or filter can be optimized for IF rather than a variable carrierStructure is called a superheterodyne receiver (used in most analog and digital radio today)

Current technology moving to direct conversionFewer parts and less power consumption

DIT

Vestigial Side-Band (VSB) Modulation

Single-sideband modulation is well-suited for the transmission of voice because of the energy gap that exists in the spectrum of voice signals between zero and a few hundred hertz.When the message signal contains significant components at extremely low frequencies i.e. television signals, the upper and lower sidebands meet at the carrier frequency. This means SSB modulation is inappropriate for the transmission of television signals.This difficulty suggests another scheme known as vestigial sideband modulation (VSB), which is a compromise between SSB and DSBSC modulation.

DIT

Vestigial SidebandVSB is similar to SSB but it retains a small portion (a vestige) of the undesired sideband to reduce DC distortion. Transmits USB or LSBand vestige of other sideband

Reduces bandwidth by roughly a factor of 2

VSB signals are generated using standard AM or DSBSC modulation,then passing modulated signal through a band-pass filter i.e. it is the special design of the band-pass filter that distinguishes VSB modulation from SSB modulation.Demodulation uses either standard AM or DSBSC demodulation

VSB used for image transmission in TV signals

USB

DIT

Generation of VSB modulated waveThe transmission bandwidth of VSB modulation is given by

where W is the message bandwidth, and f, is the width of the vestigial sidebandTo generate a VSB modulated wave, we pass a DSBSC modulated wave through a sideband shaping filter.The exact design of this filter depends on the desired spectrum of the VSB modulated wave.the VSB modulated wave is described in the time domain as

This is the desired representation representation for a VSB modulated wave containing a vestige of the lower sideband. The component 0.5Acm(t) constitutes the in-phase component of this VSB modulated wave, and 0.5AcmQ(t) constitutes the quadrature components.

( ) ( ) ( ) ( ) ( )tftmA

tftmA

ts cQc

cc ππ 2sin

22cos

2−=

DIT

Scheme for generation and demodulation of a VSB modulated wave

Block diagram of VSB modulator

Block diagram of VSB demodulator( )tfA cc π2cos

( )tfA cc π2cos

( )tvo

DIT

Envelope detection of a VSB wave plus carrier

In commercial television broadcasting, a sizable carrier is transmitted together with the modulated wave.

This makes it possible to demodulate the incoming modulated wave by an envelope detector in the receiver.

In commercial television broadcasting, the vestigial sideband occupies a width of about 1.25 MHz, or about one-quarter of a full sideband.

This has been determined empirically as the width of vestigial sideband required to keep the distortion due to mQ(t) within tolerable limits when when the percentage modulation is nearly 100.