modulation systems

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Analog and Digital Modulations

September 2011Lectured by

Assoc Prof. Dr. Thuong Le-Tien

Slides with references from HUT Finland, La Hore uni.,Mc. Graw Hill Co., and A.B. Carlson’s Communication Systemsbook.

Textbook: A . B . Carlson, et al. "Communication Systems", third ed.,McGraw-Hill Inc., New York, 1986, 2002 ISBN: 0-07-100560-9



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Communications

Communications = Information transfer

This course is about communications Limited to information in electrical form

We will not consider delivering newspapers

We will primarily cover information transfer

at systems level We will not deal [too much] with circuits, chips,

signal processing, microprocessors, protocols,and networks



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What exactly is information?

Information is a word that is toogeneric for our purposes We will use the word message

A physical manifestation of information

What do communication systems haveto do with messages? Communication systems are responsible for

producing an “acceptable” replica of message at the destination



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Is Signal = Message? Just like information, signal is also a generic

word Derived directly from information

Scientists and Engineers use signal to denoteinformation in electrical form

We will use signal and message interchangeably



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Can we classify signals? Messages or signals can be classified:

Analog

A physical quantity that varies with “time”, usually in asmooth or continuous fashion

Fidelity describes how close is the received signal to theoriginal signal. Fidelity defines acceptability

Digital An ordered sequence of symbols selected from a finite set of

discrete elements When digital signals are sent through a communication

system, degree of accuracy within a given time defines theacceptability



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Examples Analog Signals

Values are taken from aninfinite set

Digital Signals Values are taken from a

discrete set

Binary Signals Digital signals with just

two discrete values

t

t

t

1

0 0 0

1 1

0



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Elements of Communication Systems

Transmitter Modulation Coding

Channel Attenuation Noise

Distortion Interference

Receiver Detection

(Demodulation+Decoding) Filtering (Equalization)

(Modulator)

Analogor Digital

Demodulator

m(t) s(t)h(t )

n(t )

Transmitter Channel Receiver

)(t m



What does modulation do? Encodes messages (analog) or bits (digital)

into amplitude, frequency, or phase of acarrier signal

Also makes transmitted signal robustagainst channel impairments

Coding Source coding – remove redundancy

Channel coding – add redundancy

Transmitter



Channels Channel introduces impairments

Noise

Thermal noise is the most significant Additive white Gaussian noise (AWGN)

Distortion Inter-symbol interference

Attenuation and fading Constant attenuation Variable attenuation

Interference Crosstalk



Receiver

What does demodulator do?

Extracts messages or bits from thereceived signal

Mitigates channel impairments by makinguse of equalizers

Decodes the signal, especially if channelcoding was performed at the transmitter



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Fundamental Limitations If practical implementation is not a concern and we

don’t worry about feasibility, is there something elsethat limits acceptable communications?

Bandwidth Channel must be able to allow signal to pass through

Channels usually have limited bandwidth

Can we reduce signal bandwidth? Do “something” at source

Noise Can we reduce it?

Can we reduce its effects?

Do something at the transmitter and receiver

Signal to Noise Ratio



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Performance Criterion How a “good” communication system can be

differentiated from a “sloppy” one?

For analog communications How close is to ? Fidelity!

SNR is typically used as a performance metric

For digital communications

Data rate and probability of error (BER)

No channel impairments, no error

With noise, error probability depends upon datarate, signal and noise powers, modulation scheme

m(t))(t m



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Limits on data rates Shannon obtained formulas that provide

fundamental limits on data rates (1948)

Without channel impairments, an infinite datarate is achievable with probability of errorapproaching zero

For bandlimited AWGN channels, the “capacity” of a channel is:

C = B log(1 + SNR) Bits/second



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Example: PSTN, ADSL Public Switched Telephone Network,

Asymmetrical Digital Subscriber Line

Components Phone set (analog signal is generated), MODEM

Local exchange (A/D conversion)

Long-haul exchange

Characteristics Circuit-switched network Designed for voice communications and Internet

Faxes and modems use PSTN for transmission of digital data in analog form



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Example: PSTN

Localexchange

Internationalexchange

Long distanceexchange

Localexchange

Local line

Long distance line

Long distance line

International line

Long distanceusers



ADSL Asymmetric Digital Subscriber Line

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Local Carrier End Office

Line Splitter

Customer Premises

Telephone

DSL Modem

Hub

Computer Computer

Local Loop

MainDistribution

Frame

Customer Premises

Customer Premises

VoiceTelephone

Network

DSL AccessMultiplexer

ATM Switch

ISP POP

ISP POP

ISP POP

ISP POP



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Example: CellularIslamabad

MTSO

PSTNMTSO

Lahore

MTSO: Mobile TelephoneSwitching Office



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Example: Cellular

Cellular Communication System

A cell is assigned some number of channels

Typically one channel is allocated to a user

Users communicate with a base station

Base station is connected to MTSO/PSTN

AMPS is an analog system Uses FM and frequency-division multiple access

Digital systems use digital modulation



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Example: Radio broadcast Two modes are used

AM

Amplitude modulation 600-1600kHz (MW), 1600kHz-22MHz (SW)

10kHz channels

FM

Frequency modulation

88-108MHz

Channels centered at 200kHz intervals starting at88.1MHz



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Example: Wireless LANs Various standards

IEEE 802.11a/b/g popular

IEEE 802.11b 11Mb/s data rate

2.4-2.4835GHz band

Modulation: Direct sequence spread spectrum (DSSS),

Frequency hopping spread spectrum (FHSS)

IEEE 802.11a 55Mb/s data rate

5.725-5.825GHz band (in U.S.)

Uses orthogonal frequency division multiplexing (OFDM)



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Example: LANs and WANs Local Area Networks (LANs)

Connect “closely” located computers

Data bits are transmitted in chunks (packets) forefficiency/feasibility reasons

Various LAN protocols are used in practice

Wide Area Networks (WANs) A wide area backbone network connects different

LANs

A standard protocol is needed for suchcommunication (TCP/IP)



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Example: Ad Hoc Networks Various devices connected to each other

without using an infrastructure

Sensor Networks Similar to ad hoc Networks (may be considered a special

case of ad hoc networks)

Have power constraints (Use non-rechargeable battery)

Mesh Networks

Another example of ad hoc networks

Used for provide communications to remote areas



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