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Digital Communications Engineering 1

(COMM2108)

Introduction and Basic Concepts

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Introduction

• Lecturer: Dr. Mark Davis

• Room: 325 Kevin St.

• Email: mark.davis@dit.ie

• Web: www.electronics.dit.ie/staff/mdavis

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Introduction

• Course Code: COMM2108

• Assessment*: 60% Exam, 40% Lab

• Lectures: 2 hours/week

• Labs: 2 hours/week

*NB - you must achieve at least 35% of total available marks in each component

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Introduction

• This module is designed to give an appreciation ofthe principles of digital communications engineering.After completing this module you should:

– Be able to identify the main elements of a digitalcommunications system.

– Understand source formatting, in particular sampling,quantisation, signal-to-quantisation noise ratio.

– Be able to quantify the performance of baseband digitalsystems in terms of bandwidth requirements, intersymbolinterference, and bit-error rates.

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Introduction• The module content includes:

– Introduction and Basic Concepts

– Source Formatting (Sampling, Quantisation, PCM, Companding, and Codecs)

– Multiplexing (Multiplexing and multiple access schemes, FDM, TDM)

– Baseband Communication (Generation, Transmission, Detection)

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Introduction• Module Textbooks:

– Digital Communications: Fundamentals

and Applications (2nd Edition), Bernard

Sklar, Prentice Hall (2001).

– Communication Systems (5th Edition),

Simon Haykin and Michael Moher, Wiley

(2010).

– Principles of Communications Systems,

Modulation, and Noise (6th Edition), Wiley,

Rodger E. Ziemer and William H. Tranter

(2000).

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Introduction

• The most obvious question is: Why digitalcommunications ?

• More and more of our communicationsservices (domestic, business andentertainment) are being delivered in a digitalformat.

• For example:– Telephony in the 1970s (modern digital PSTN)

– CDs in the 1980s (music reproduction)

– Mobile telephony in the 1990s (GSM phones)

– Digital TV in 1999 (also HDTV since 2006)

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Introduction

• Also new services have been introduced,most notable of these is the Internet whichhas undergone an explosive growth since1996.

• Business - Ecommerce which is concernedwith transacting business over the Internet.

• There are obvious concerns with fraud andsecurity (cryptographic techniques have beendeveloped to address these concerns).

• Most recently, the emergence of mobilebroadband and Web 2.0

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Introduction

• Electronic and in particular digital communication underpins the“information revolution” of the 21st century.

• Wealth is now created by how we generate, process, store,manage, and communicate information.

• Here information is used in the broadest meaning of the word andincludes:

– Text

– Voice

– Video

– Multimedia

– Network applications

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Introduction

• Why the transition from analogue to digitalcommunications?

• Cheap Hardware (flexible, complex hardwareis becoming cheaper all the time, e.g.Moore’s Law).

• Demand for new services (e.g. email,ecommerce, teleworking, networkapplications etc.)

• Control of Quality (i.e. powerful error controltechniques can be employed to guaranteehigh quality communication).

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Introduction

• Compatibility and flexibility (digital signals areeasier and cheaper to store and process thananalogue signals).

• Transmission (digital signals are morespectrally efficient than analogue signalswhich means more information can be carriedin a given bandwidth).

• Security (powerful encryption techniquesexist to ensure the confidentiality and integrityof the information, critical for ecommerceapplications).

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• Some Internet stats (2008-2011)

Introduction

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• Some Internet stats (2008-2011)

Introduction

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Introduction• Some Internet stats (2008-2011)

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Introduction• Biggest growth in traffic will be in mobile data traffic.• According to Cisco Visual Networking Index (VNI) Global Mobile

Data Forecast (2011-2016)• In 2011:

– Global mobile traffic grew 2.3-fold in 2011, more than doubling forthe fourth year in a row.

– Global mobile traffic was 597 petabytes per month, over 8 times thetotal global Internet traffic in 2000.

– Smartphones represent 12% of total global handsets in use today,but they represent over 82% of total global handset traffic. In 2011,the average smart phone generated 150 MB per month

– 34 million mobile-connected tablets were connected to the mobilenetwork, each tablet generates 3.4 times more traffic than theaverage smartphone (517 MB per month compared to 150 MB permonth per smartphone)

– There were 175 million laptops on the mobile network and eachlaptop generated 22 times more traffic than the averagesmartphone (2.1 GB per month).

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Introduction• By 2016:

– Global traffic will increase 18-fold reaching 10.8 exabytes permonth.

– Mobile network connection speeds will increase 9-fold from 189kbps in 2011 to 2.9 Mbps in 2016.

– Over 100 million smartphone users will belong to the “gigabyteclub” (downloading over 1 GB per month).

– The number of mobile-connected devices will exceed the world’spopulation.

– Monthly global mobile data traffic will surpass 10 exabytes in 2016.– Monthly mobile traffic will exceed 1 exabyte per month in 2016.– Two thirds of the world’s mobile data traffic will be video by 2016.– China will exceed 10 percent of global mobile data traffic in 2016,

up from 5 percent in 2011.

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Introduction

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Introduction

• Apart from the growth of the Internet, the1990s have witnessed a number of othersignificant developments taking place withinthe communications industry, most notably:– Convergence (where voice and data services are

carried on the same network, e.g. VoIP and “tripleplay” services).

– Deregulation (the old telecommunicationsmonopolies have been broken up and competitionhas been introduced into the industry).

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Introduction• Convergence has important implications for the

telcos (i.e. the telecommunications companies) whichhave traditionally catered for voice and voice-relatedtraffic only.

• Typically telcos offered a circuit-switched channelwith a bandwidth 300-3400 Hz (narrow).

• In fact, there has been over 100 years ofdevelopment and investment in the circuit-switchedvoice network (i.e. the PSTN).

• Consequently, the modern PSTN has become a trulyubiquitous service with a very high degree ofreliability and availability.

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Introduction• However, the provision of broadband services is now

a critical part of their service offering. All of the majorplayers offer bundled services combining traditionalPSTN telephony with broadband access (usually overADSL lines).

• Over the next 5-10 years it is predicted that IP-based(i.e. the Internet Protocol) networks will replace thetraditional PSTN as the main provider oftelecommunications services.

• Skype VoIP service had 663 millionregistered users as of Sept 2011 (over40 million online at peak times). On 10May 2011, Microsoft announced it hadagreed to acquire Skype for $8.5 billion.

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Introduction• Deregulation has lead to the break up of the old

telecommunications monopolies which were usuallystate owned.

• The first steps in the deregulation of thecommunications industry began in the USA where in1984 the US government broke up the Bell telephonecompany into 22 independent Bell OperatingCompanies (BOCs) known as “baby bells”.

• The intention was to encourage competition, drivedown costs to the consumer, and to encourageinvestment and development.

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Introduction

• In Europe, the EU Commission forced member statesto deregulate their telecommunications marketplaceson 1st January 1997.

• Originally Ireland had a derogation for 3 years, butintroduced deregulation in December 1998.

• The Commission for Communciations Regulation(ComReg) is the statutory agency that oversees allaspects of the telecommunications industry in Ireland(www.comreg.ie).

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Introduction

• Deregulation has lead to modern businesspractices being introduced into thetelecommunications industry, these include:

– Competition

– Acquisitions

– Mergers

– Floatations/IPOs (Initial Public Offering)

– Strategic alliances

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Basic Concepts

• Signals are the manifestations of messagesexchanged at a distance.

• The generation, transmission and detection ofsignals are fundamental tasks performed byany communications system (applies to bothdigital and analogue signals).

• Signals can be dealt with in terms ofvariations of the value of some quantity withtime.

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Basic Concepts• Analogue signals are continuous signals where

changes in value occur smoothly and the rate ofchange is finite. Analogue signals are the naturalforms of signals generated by many physicalprocesses.

• Discrete signals are non-continuous signals whosevalues form a discrete set and occur at isolatedpoints in the time continuum.

• Digital signals can assume only a limited set ofvalues. Changes in signal value are instantaneousand the rate of change at that instant is infinite. Themost common class of digital signals are binarysignals where at any given instant of time, the signalcan have one of only two possible values.

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Basic Concepts

• Signals may be classified by theirdegree of certainty:

• Deterministic signals where at everyinstant the value can be related tovalues at neighbouring times in a waythat can be expressed exactly, i.e.deterministic signals can be describedby classical mathematical functions.

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Basic Concepts

• Probabilistic signals where future values canbe described only in statistical terms. Futurevalues are estimated on the basis of thestatistics associated with past values and theassumption that the behaviour will remain thesame in the future.

• Random signals are an important subclass ofprobabilistic functions whose values arelimited to a given range.

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Basic Concepts

• Signals may be classified in terms oftheir duration:

• Transient signals are signals that existfor a limited period of time only. Theyrepresent the class of real-world signalsthat are turned on and turned off. If thesignal is deterministic it can be analysedusing Fourier Transforms.

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Basic Concepts

• Infinite (or eternal) signals are signals thatexist for all time. These are used to representcommunication systems that are operating insteady-state.

• Periodic signals are an important subclass ofinfinite signals whose values repeat at regularintervals. Periodic signals can be analysedusing Fourier Series.

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Basic Concepts• With a few exceptions, the signals found in

real-world communications are probabilisticsignals.

• They cannot be represented accurately byprobabilistic transient or periodic functions.

• Future signal values are estimated on thebasis of the statistics associated with knownpast values (i.e. measurements) and theassumption that the statistical behaviour willremain unchanged (important property knownas stationarity).

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Basic Concepts

• There are two approaches used torepresent signals:

• Time Domain representation where thesignal values are given as a function oftime.

• Frequency Domain representationwhere the signal values are given asfunction of frequency.

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Basic Concepts

• The piece of laboratory test equipmentused to analyse signals in the timedomain is an oscilloscope.

• In the frequency domain, thecorresponding piece of laboratory testequipment is a spectrum analyser whichdisplays the spectrum or frequencycomponents of a signal.

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Basic Concepts

• Some obvious questions regarding theuse of the time domain and frequencydomain representations might be:

• Which representation is the moreaccurate?

• Which representation is the more valid?

• Which representation is the morecomplete?

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Basic Concepts• The answer is simple, neither representation

is more accurate, more valid, or morecomplete than the other.

• They are just two different ways of looking atthe same thing.

• Usually it is a matter of convenience whichwill determine which representation to use.

• In general, though not always, the frequencydomain representation is the more useful forcommunications engineering.

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Basic Concepts

• Mathematical techniques have beendeveloped to enable engineers to moveback and forth between the two domains.

• These techniques come under thegeneral heading of Fourier techniquesafter the 19th century Frenchmathematician Joseph Fourier (1768-1830) who developed them.

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Basic Concepts

• Two of these techniques are ofparticular interest to communicationsengineers:

• The Fourier Series which is used toobtain the spectrum of a periodic signal.

• The Fourier Transform which is used toobtain the spectrum of a non-periodicsignal (and vice versa).

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Basic Concepts• One of the most important uses of the frequency

domain representation is that it indicates thebandwidth of a signal, i.e. the range of frequenciesoccupied by the signal energy.

• For example:

– The range of frequencies occupied by the humanvoice is approximately 300-3400 Hz resulting in abandwidth of 3 kHz approx.

– Music occupies a range of frequencies roughlybetween 30 and 15,000 Hz resulting in abandwidth of 15 kHz approx.

– By contrast, an analogue TV picture has abandwidth of 6 MHz.

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Basic Concepts

• The passband of a communications system isthe range of frequencies that can betransmitted without distortion.

• To avoid distortion the passband shouldalways be greater than the signal bandwidth.

• For example, consider what would happed ifwe had a communications system with apassband of 10 kHz and we attempted totransmit human voice, music and TV signals.

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Basic Concepts

• For voice signals, the passband is muchgreater than the signal bandwidth andtherefore the system would perform well.

• For music signals, only those frequenciesbelow 10 kHz would be transmitted which willresult in distortion being introduced. Thedegree of distortion would very much dependon the type of music but generally it would notbe too severe.

• For TV signals where the passband is verymuch less than the signal bandwidth, severedistortion would be introduced.

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Basic Concepts• Digital communications is simply the practice of

exchanging information by using finite sets of signals.

• In practice these signals are in the form of electricalwaveforms (e.g. pulses) or electromagnetic fields(e.g. radio waves).

• The five basic elements of any communicationssystem are:– Information Source

– Transmitter

– Communications Channel

– Receiver

– Information Sink

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Basic Concepts

• The information source represents themessage or the information to becommunicated.

• The message or information can have manyforms, it can be human speech, video, text, ordigital data from a PC/server.

• The signal produced by the source is oftenreferred to as the baseband signal.

• Generally the information or baseband signalneeds to be formatted (or coded) into a digitalformat.

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Basic Concepts

• The function of the transmitter is to convertthe baseband signal into a form that iscompatible with the communications channel.

• The communications channel is the mediumover which the communication takes place(e.g. coaxial cable, optical fibre, or radiowaves).

• The function of the receiver is to recover theoriginal baseband signal from thecommunications channel.

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Basic Concepts

• Digital communications (at least at thephysical layer) is all about coding.

• The main functions of coding are:– Source representation which is concerned with the

accurate and efficient representation ofinformation.

– Error control which is concerned with protectioninformation against transmission errors.

– Security which is concerned with ensuring theconfidentiality and integrity of information throughthe use of cryptographic techniques.

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Basic Concepts – Summary

• Digital communications is:

– simply the practice of exchanginginformation by using finite sets of signals.

– is all about coding (at least at the physicallayer).

– is concerned with answering the question

• Which signal was transmitted?