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TE-7001

DIGITAL COMMUNICATION

SYSTEMS

Engr. Ghulam [email protected]

Lecture 1

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Course Objectives

This course is designed to prepare students forengineering work in the industry and for advancedgraduate work in the area of digital communications.

The course covers concepts and useful tools for designand performance analysis of transmitters and receiversin the physical layer of a communication system.

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Scope of the course

Communication is a process by which information is

exchanged between individuals through a common

system of symbols, signs, or behavior

Communication systems are reliable, economical

and efficient means of communications such aspublic switched telephone network (PSTN)

mobile telephone communication (GSM, 3G, ...)

broadcast radio or television

navigation systems, ...

The course is aiming at introducing fundamental

issues in designing a (digital) communication system

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Scope of the course ...

Example of a (digital) communication systems:

Cellular wireless communication systems

Base Station (BS)

User Equipment (UE)

UE UE

UE

BS

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Text

Digital Communications: Fundamentals and Applications,

B.Sklar, Prentice Hall, 2nded, 2001.

First Chapter of the book is available at:

http: //vig.pearsoned.com/samplechapter/0130847887.pdf

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References

1. Communication Systems, 3rdor 4thEd., Simon Haykin,John Wiley & Sons

2. Communication Systems Engineering, 2ndEdition,Prentice Hall, 2001.

3. Digital Communications, Fourth Edition, J.G. Proakis,

McGraw Hill, 2000.4. Analog and Digital Communication Systems, Leaon W.

Couch II, 6thedition, Prentice Hall, 2001.5. Modern Digital and Analog Communication Systems, B.

P. Lathi, 3rd

Ed. Oxford Univ. Press 1998.6. Lecture slides (ppt, pdf)7. Laboratory syllabus8. Set of exercises and formulae9. Home assignments and solutions

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Pre-requisites

RequiredSignals and Systems

RecommendedProbability and Stochastic Processes

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Grading of Evaluation Components

Assignments, Labs/Field Work, Quizzes 20 %

Mid Semester Exam 20 %

Course Project, Case Study, Presentation etc. 20 % Final Comprehensive Theory Exam 40 %

Total 100%

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Digital communication system

Important features of a DCS: Transmitter sends a waveform from a finite

set of possible waveforms during a limitedtime

Channel distorts, attenuates thetransmitted signal and adds noise to it.

Receiver decides which waveform wastransmitted from the noisy received signal

Probability of erroneous decision is animportant measure for the systemperformance

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Digital versus analog

Advantages of digital communications: Regenerator receiver

Different kinds of digital signal are treatedidentically.

Data

Voice

Media

Propagation distance

Original

pulse

Regenerated

pulse

A bit is a bit!

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Communication

Communication is a process of conveying messages atdistance.

If the distance involved is beyond directcommunication, then communication engineeringcomes into picture.

The branch of engineering which deals withcommunication system is known asTelecommunication Engineering.

In telecommunication, a physical message, such as

sound, word, picture, etc., is converted into anelectrical message called signal and this electricalsignal is conveyed at a distant place through somemedia, where it is reconverted into the physical

message.

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Communication

Main purpose of communication is totransfer information from a source to arecipient via a channel or a medium.

Basic block diagram of a communicationsystem:

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Modes of Communication

Broadcasting It involves the use of a single powerful

transmitter and numerous receivers.

Here information-bearing signals flow in one

direction

Point-to-point communication

Communication process takes place over a link

between a single transmitter and a receiver.

In this case, there is usually a bidirectional flowof information-bearing signals, which require theuse of a transmitter and receiver at each end of

the link.

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Communication

Thus, a communication system has three basic

components:(a) Transmitter

(b) Transmission media

(c) Receiver

Electrical Communication System

Physical message

Source

Transmitter Receiver

Physical message

Source

Communication

medium

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Communication Processes

1. The generation of amessage signal:

voice, music, picture, or computer data2. The description of that message signal with a certain

measure of precision, by a set ofsymbols:

electrical, aural, or visual.

3. The encoding of these symbols in a form that issuitable for transmission over a physical medium ofinterest.

4. The transmission of the encoded symbols to thedesired destination.

5. The decoding and reproduction of the originalsymbols.

6. The re-creationof the original message signal, with adefinable degradation in quality; the degradation is

caused by imperfections in the system.

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Channel

User of

Information

Source of

Information

Communication System

Elements of a Communication system

Transmitted

SignalReceived

Signal

Message

Signal

Estimate of

message

Signal

Communication Process

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Digital Communication Process

Source

encoder

Channel

encoder

Modulator

Source

decoder

Channel

decoder

Demodulator

Channel

Source of

Information

User of

information

Message Signal

Source

code word

Channel

code word

Waveform Received Signal


Estimate of

message signal

Estimate of

Source code

word

Estimate of

channel code

word

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DCS A Brief Description

Source:analog or digital

Transmitter:transducer, amplifier, modulator, oscillator,

power amplifier, antenna Channel:

cable, optical fiber, free space etc. Receiver:

antenna, amplifier, demodulator, oscillator,power amplifier, transducer

Recipient:

person, (loud) speaker, computer

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Types of informationVoice, data, video, music, email etc.

Types of communication systemsPublic Switched Telephone Network(voice, fax, modem)

Satellite systems RadioTV broadcastingCellular phonesComputer networks (LANs, WANs,

WLANs)

DCS A Brief Description

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Information Representation

Communication system converts information intoelectrical electromagnetic/optical signalsappropriate for the transmission medium.

Analog systems convert analog message into signals

that can propagate through the channel. Digital systems convert bits (digits, symbols) into

signals

Computers naturally generate information ascharacters/bits

Most information can be converted into bits

Analog signals converted to bits by sampling and

quantizing (A/D conversion)

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Why digital? Digital techniques need to distinguish between

discrete symbols allowing regeneration versusamplification

Good processing techniques are available for digitalsignals, such as medium.

Data compression (or source coding)

Error Correction (or channel coding) (A/D conversion)

Equalization

Security (encryption, privacy)

Easy to mix signals and data using digital techniques

Digital circuits are more reliable and can be produced

at a low cost.

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Why Digital Communication System?

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When an ideal binary digital pulse propagates along

a transmission line, the shape of the waveform is

affected by two basic mechanisms:

1) All transmission lines and circuits have some

nonideal frequency transfer function of the

medium and there is a distorting effect on the

ideal pulse.

2) Unwanted electrical noise or other interference

further distorts the pulse waveform.

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Both of these mechanisms cause the pulse shape

to degrade as a function of line length or media

length.

During the time that the transmitted pulse can still

be reliably identified, the pulse is amplified by a

digital amplifier that recovers its original ideal

shape.

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The pulse is thus rebornor regenerated.

Circuits that perform this function at regular

intervals along a transmission system are called

regenerative repeaters.

Digital circuits are less subject to distortion and

interference than are analog circuits.

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In digital communication system, two state

operation (fully on or fully off) facilitates signal

regeneration and thus prevent noise and other

disturbances from accumulating in transmission.

With digital techniques, extremely low error rates

producing high fidelity is possible through error

detection and correction.

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Communication Systems

Information

Source

Transmitter Channel Receiver Information

Destination

Keypad

Speakers Brain

IP Packet

GSM-style RF

Vocal Tract

SONET Router

Wireless RF

Acoustic

Fiber

FM Detector

Ears

Photo Diode

ATM.25 Packet

Brain

Router POTS

Analog Communications:

Information is encoded in a continuous amplitude,

continuous time signal.

Digital Communications:

Information is encoded into a discrete time sequence

with a quantized alphabet.

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Block Diagram of Digital Communication

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Basic Signal Processing Functions

1. Formatting and source coding

2. Baseband signaling

3. Band pass signaling

4. Equalization

5. Channel coding

6. Multiplexing and multiple access

7. Spreading

8. Encryption

9. Synchronization

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Signal Flow and Signal Processing Steps in DC

Upper block

1) Format

2) Source encode

3) Encrypt4) Channel encode multiplex

5) Pulse modulate

6) Bandpass modulate7) Frequency spread

8) Multiple access

Denote signal transformation

from source to transmitter

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Modulate and demodulate/detect blocks together are

called a modem.

Modem often encompasses several of the signal

processing steps;

When this the case, the modem can be thought as the

brain of the system.

The transmitter and receiver can be thought of as the

muscles of the system

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For wireless applications, the transmitter consists of a

frequency up-conversion stage to a radio frequency

(RF), a high-power amplifier, and an antenna.

The receiver portion consists of an antenna and a low-

noise amplifier (LNA).

Frequency down-conversion is performed in the front

end of the receiver and/or the demodulator.

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Figure illustrates a kind of reciprocitybetween theblocks in the upper transmitter part of the figure and

those in the lower part.

Trans side

The input information at source is converted to binary

digits (bits); the bits are then grouped to form digital

message or message symbols.

Each such symbol (mi , where I = 1, , M) can be

regarded as member of a finite alphabet set containing

M members.


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Thus, for M = 2, the message symbol mi is binary (just asingle bit).

For systems that use channel coding (error coding), a

sequence of message symbols becomes transferred to asequence of channel symbols (code symbols), where

each symbol is denoted by ui.

Because a message symbol or a channel symbol can

consist of a single bit or grouping of bits, a sequence

of such symbols is also described as bit stream.


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Only formatting, modulation, demodulation/detection,and synchronization are essential for a DCS.

Formatting transforms the source information into bits

and up to pulse modulation block, the informationremains in the form of a bit stream.

Modulation is the process by which message symbols or

channel symbols are converted to waveformsthat are

compatible with the requirements imposed by the

transmission channel.


Si l Fl d Si l P i St i DC

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Pulse modulation is an essential step because each

symbol to be transmitted must first be transformed

form a binary representation to a basebandwaveform.

The terms baseband refers to a signal whose spectrum

extends from (or near) dc up to some finite value,

usually less than a few megahertz.

The pulse-modulation block usually includes filtering

or minimizing the transmission bandwidth.

When pulse modulation is applied to binary symbols,

the resulting binary waveform is called a pulse-code-

modulation (PCM) waveform calledline codes.



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After pulse modulation, each message symbol or

channel symbol takes the form of a baseband

waveform gi(t), I = 1, , M.

For an application involving RF transmission, the next

step is bandpass modulation; the baseband waveform

gt(t) is frequency translated by a carrier wave to a

frequency si(t), that is much larger than the spectral

contents of gi(t).

As si(t) propagates over the channel, it is impacted by

the channel characteristics, described in terms of the

channelsimpulse response hc(t).



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Receive Side

At various points along the signal route, additive

random noise distorts the received signal r(t), so that

its reception must be termed as a corrupted version of

the signal si(t) that was launched at the receiver.

The received signal r(t) can be expressed as

r(t) = si(t) * hc(t) + n(t) I = 1, , M



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In the reverse direction, the receiver front end and/or

demodulator provides frequency down conversion for

each bandpass waveform r(t).

The demodulator restores r(t) to an optimally shaped

baseband pulse z(t) in preparation for detection.

Several filters associate with receiver and

demodulator, filter the baseband pulse to remove

unwanted high frequency terms, and shape the pulse

by the channel.


Signal Flo and Signal Processing Steps in DC

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Equalization can be described as the filtering option

that is used in or after the demodulatorn to reverse

any degrading effects on the signal that were caused

by the channel.

Equalization becomes essential whenever the impulse

response of the channel, hc(t), is so poor that the

received signal is badly distorted.


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Basic Digital Communication Transformations

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Communication Channels

Channel: The medium linking the transmitter and receiver.It is ALWAYS analog in nature. That is every communication

system is more or less ANALOG.

Channel TypesWire/line Channels: use a conductive medium to directtransmitted energy to the receiver:

Copper wire for telephones, xDSL

Fiber optic cable

Aluminum interconnects for ICsWireless Channels: Uses an open propagation medium

RF for cell phones

Underwater acoustic ducts for whales

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Performance Metrics

Analog Communication Systems

Metric is fidelity: want

SNR typically used as performance metric

Digital Communication Systems

Metrics are data rate (R bps) and probability of bit

error

Symbols already known at the receiver

Without noise/distortion/sync. problem, we will

never make bit errors

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Main Points

Transmitters modulate analog messages or bits in

case of a DCS for transmission over a channel.

Receivers recreate signals or bits from received

signal (mitigate channel effects)

Performance metric for analog systems is fidelity,

for digital it is the bit rate and error probability.

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Channel Impairments

As a transmitted signal propagates it loses fidelity in anumber of ways. This loss of fidelity makes the received

signal look very different from the transmitted signal.

Additive Noise: Thermal noise, multi-transmitter interference

Transmitter

Noise

Receiver+

Multiplicative Noise: Rayleigh Fading

Transmitter

Noise

Receiverx

Convolution Noise: time-delay multipath, reverberation (echo)

Transmitter ReceiverNoise

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Objective

Information

Source

Transmitter Channel Receiver Information

Destination

1. How to design

2. Taking into account

3. That will provide a system that is:

Reliable: information received is what was sent

Efficient: Not wasteful of time, power or spectrumSimple: economical for H/W and S/W and usually Robust

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Tradeoffs in Objectives

Simple H/W

Simple S/W

Spectral Use

Temporal Use Power Use

Accuracy & Robustness

Simple

Efficient

Reliable

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Digital Communications

Digital

Information

Source

Source

Encoder

Channel

Encoder

Modulator

Digital

Information

Destination

DAC

Source

Decoder

Channel

Decoder

DeModulatorADC

Channel

N

The placement of the DAC and ADC is upto the system requirements. They can be

anywhere between the Information

Sources and Destination and the

Modulator and Demodulator, respectively.

l l

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Goals in Digital CommunicationSystem Design

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Comparative Analysis

ofAnalog and Digital Communication

Analog Communication:

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Analog Communication:Transmitter and Receiver

Digital Communication:

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Digital Communication:Transmitter

Digital Communication:

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Digital Communication:Receiver

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Digital Signal Nomenclature

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Digital Signal Nomenclature

lec 1 dcs_fall 2012 (m.sc)

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