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Introduction to Digital Communications Engineering

Introduction to Digital Communications Engineering
Introduction to Digital Communications Engineering I Lectures No. 1 and 2 Dr. Aoife Moloney School of Electronics and Communications Dublin Institute of TechnologyLectures No. 1 and 2: Introduction to Digital Communications Engineering I Overview These lectures look at the following: • Course introduction • History of Communications • Communications system • Communication modes • Methods of data communication • Time constraints DT008/2 Digital Communications Engineering I Slide: 1Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • Transmission modes • Analogue versus digital • Baseband and bandpass • Digital communications transceiver • Conclusion • Acknowledgement DT008/2 Digital Communications Engineering I Slide: 2Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Introduction • Lecturer: Dr. Aoife Moloney • Room: 426 Kevin St. • Email: aoife.moloneydit.ie • Web: www.electronics.dit.ie/staff/amoloney DT008/2 Digital Communications Engineering I Slide: 3Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Course Introduction • Course Code: COMM2108 • Assessment: 70 Exam 30 Lab • Lectures: 2 hours/week • Labs: 2 hours per week DT008/2 Digital Communications Engineering I Slide: 4Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Module Objectives This module is designed to give an appreciation of the princi ples of digital communications engineering. After completing this module you should: • Be able to identify the main elements of a digital com munications system. • Understand source formatting, in particular, sampling, quantisation, signal to quantisation noise ratio. • Be able to quantify the performance of baseband digital DT008/2 Digital Communications Engineering I Slide: 5Lectures No. 1 and 2: Introduction to Digital Communications Engineering I systemsintermsofbandwidthrequirements,intersymbol interference and biterror rates. DT008/2 Digital Communications Engineering I Slide: 6Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Syllabus • Introduction to digital communications • Source formatting • Multiplexing • Baseband communication: generation, transmission, de tection DT008/2 Digital Communications Engineering I Slide: 7Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Textbooks Recommended Reading: • ‘PSpice for Digital Communications Engineering’, Paul Tobin, Morgan Claypool 2007. th • ‘Communications Systems’ (4 Edition), Simon Haykin, Wiley 2001. nd • ‘CommunicationSystemsEngineering’(2 Edition),John G. Proakis and Masoud Salehi, Prentice Hall 2002. • ‘Digital Communications: Fundamentals and Applica DT008/2 Digital Communications Engineering I Slide: 8Lectures No. 1 and 2: Introduction to Digital Communications Engineering I tions’, Bernard Sklar, Prentice Hall 1988. DT008/2 Digital Communications Engineering I Slide: 9Lectures No. 1 and 2: Introduction to Digital Communications Engineering I History of Communications The highlights of the inventions which have lead to commu nications as we know it today are listed below: • 1440: Printing press Gutenberg • 1826: Ohm’s law Ohm • 1837: Line telegraphy invention Gauss, Weber • 1844: Line telegraphy patent Morse st • 1858: 1 transatlantic cable (fails after 26 days) • 1864: Electromagnetic radiation predicted Maxwell DT008/2 Digital Communications Engineering I Slide: 10Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • 1866: Successful transatlantic telegraph cable (Valentia to Newfoundland) • 1875: Telephone invented Bell • 1877: Phonograph invented Edison • 1887: Detection of radio waves Hertz • 1894: Wireless communication over 150 yards Lodge • 1895: Wireless telegraphy Marconi • 1897: Automatic telephone exchange Strowger • 1901: Transatlantic radio transmission Marconi DT008/2 Digital Communications Engineering I Slide: 11Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • 1904: Diode valve Fleming • 1905: WirelesstransmissionofspeechandmusicFesseden • 1906: Triode valve de Forest • 1907: Regular radio broadcasts • 1915: Trans. USA telephone line Bell System • 1918: Superheterodyne radio receiver Armstrong • 1919: Commercial broadcast radio KDKA Pittsburg • 1920: Sampling applied to communications Carson • 1926: Television invented Baird (UK), Jenkins (USA) DT008/2 Digital Communications Engineering I Slide: 12Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • 1928: All electronic television Farnsworth • 1928: Theory of transmission of telegraph Nyquist • 1928: Information theory Hartley • 1933: FM demonstrated Armstrong • 1934: Radar Kuhnold • 1937: PCM (pulse code modulation) proposed Reeves • 1939: Commercial TV broadcasting BBC • 1943: Microwave radar used • 1944: Statistical methods to describe noise and extract DT008/2 Digital Communications Engineering I Slide: 13Lectures No. 1 and 2: Introduction to Digital Communications Engineering I signals Rice • 1945: Geostationary satellites proposed Clarke • 1946: ARQ (automatic repeat request) proposed Du uren • 1948: Mathematical theories of communication Shan non • 1948: Invention of transistor Shockley, Bardeen, Brat tain • 1953: Transatlantic telephone cable DT008/2 Digital Communications Engineering I Slide: 14Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • 1955: Invention of laser Townes, Schawlow • 1961: Stereo FM transmission • 1962: Satellite communication TELSTAR • 1963: Touch tone telephone Bell System • 1963: Geostationary communications satellite SYN COM II • 1963: Error correction codes developed • 1964: First electronic telephone exchange • 1965: CommercialcommunicationssatelliteEarlyBird DT008/2 Digital Communications Engineering I Slide: 15Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • 1966: optical fibre proposed Kao, Hockman • 1968: Cable TV • 1970: Medium scale data networks ARPA/TYMNET • 1970: LAN, MAN, WAN • 1971: ISDN proposed CCITT • 1972: First cellular mobile phone • 1974: The Internet Cerf, Kahn • 1978: Cellular radio • 1978: Navstar GPS (global positioning system) DT008/2 Digital Communications Engineering I Slide: 16Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • 1980: Fibre optic communications system developed Bell System • 1980: OSI 7 layer reference model ISO • 1981: HDTV (highdefinition television) demonstrated • 1985: ISDN basic rate access introduced UK • 1986: SDH introduced (SONET in USA) • 1991: GSM (global system for mobile communications) Europe • 1999: WAP (wireless application protocol) DT008/2 Digital Communications Engineering I Slide: 17Lectures No. 1 and 2: Introduction to Digital Communications Engineering I There have been many many more inventions since 1999. As an exercise use the Internet to find as many recent telecom munications inventions as you can. DT008/2 Digital Communications Engineering I Slide: 18Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Communications System In its simplest form a telecommunications system consists of a transmitter, a channel, a receiver and two transducers. Channel Receiver Transmitter Estimate Message of message and input and output transducer transducer DT008/2 Digital Communications Engineering I Slide: 19Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Transducer • Converts the input message into an electrical signal. Ex amples of transducers include: – Microphone – converts sound to electrical signal – Camera – converts image to electrical signal • A transducer is also used to convert electrical signals to an output message (or approximation of the input mes sage), e.g., sound, images etc. DT008/2 Digital Communications Engineering I Slide: 20Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Transmitter • Converts electrical signal to a form that is suitable for transmission through the transmission medium or chan nel. • Generally matching of signal to channel is done bymod ulation. • Modulationusestheinformation(messagesignal)tovary the amplitude, frequency or phase of a sinusoidal carrier, e.g. amplitude/frequency modulation AM/FM. • The transmitter also filters and amplifies the signal. DT008/2 Digital Communications Engineering I Slide: 21Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Receiver • Recovers the message contained in the received signal • Receiver demodulates the message signal • Receiver filters signal and suppresses noise DT008/2 Digital Communications Engineering I Slide: 22Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Communication Modes There are a few basic modes of communication: • PointtoPoint: where one user wishes to communicate with one other user, or with a small group of nominated users. Examples include the telephone network or email. Communication is normally twoway. • Broadcast: Where one sender communicates with all capable receivers who cannot respond. the communica tion is therefore normally oneway. • Multicast: Onesendercommunicateswithanominated DT008/2 Digital Communications Engineering I Slide: 23Lectures No. 1 and 2: Introduction to Digital Communications Engineering I set of receivers who cannot respond. DT008/2 Digital Communications Engineering I Slide: 24Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Methods of Data Transmission There are a few basic methods of data transmission: • Simplex: Dataistransmittedinonedirectiononly. The receiver cannot communicate with the sender. • Duplex: Data transmission can take place in both di rections simultaneously. • HalfDuplex: Data transmission can take place in both directions but not at the same time. DT008/2 Digital Communications Engineering I Slide: 25Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Time Constraints There are generally two sets of time restraints; realtime or timelapse: Time Lapse Real Time Data In Data In Data Out Data Out • RealTime: Realtime communication is instant and data must be sent and received simultaneously. An ex ample of this is the telephone network or twoway radio DT008/2 Digital Communications Engineering I Slide: 26Lectures No. 1 and 2: Introduction to Digital Communications Engineering I communications. If a conversation is to be maintained theremustbeimmediateinteractionbetweenthetalkers. Delays will make the conversation difficult or impossible. • TimeLapse: Data may be received at any time after having been sent. Examples include email, radio and TV broadcasts. The time of receipt is not important. Consider the case of radio and TV in more detail. It does not matter when a particular program is transmit ted time lapse is possible. However, once transmission begins it must be continuous and at a constant rate DT008/2 Digital Communications Engineering I Slide: 27Lectures No. 1 and 2: Introduction to Digital Communications Engineering I during reception it appears as realtime. There are also cases where time delay is not critical un less it is excessive e.g. downloading a file from a central server or from the Internet. A delay of a few seconds or even minutes is acceptable, but a delay of several hours is not acceptable. In addition, components of a message should be received in the sequence in which they are sent (otherwise speech will be garbled). This may require that packets of data DT008/2 Digital Communications Engineering I Slide: 28Lectures No. 1 and 2: Introduction to Digital Communications Engineering I must be resequenced at the receiver end. DT008/2 Digital Communications Engineering I Slide: 29Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Transmission Modes All transmission is analogue, in the sense that physical quan tities (voltage, current, electromagnetic radiation) must vary in a smooth way. However, the representation of the under lying signals may be either analogue or digital. Analogue Digital 8,9,7,4,2,3,… Digital Analogue DT008/2 Digital Communications Engineering I Slide: 30Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Analogue versus Digital Analogue In the past most signals were generated, transmitted and re ceived in analogue form i.e. as a sine wave or as a more complex signal which could be made up from a series of sine waves. This was done because speech is an analogue signal and it was easier to implement analogue electronic circuitry than digital. In a very simple system it is still easier to build in analogue. However, analogue has the following disadvan tages: DT008/2 Digital Communications Engineering I Slide: 31Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • Itisinflexible, inthattomakeanychangestothesystem all of the changes have to be made in hardware. This becomesmoredifficultandexpensiveasthesystemgrows in size. • It is prone to noise and distortion. • Control and manipulation of signals is difficult. The mathematical treatment of analogue signals is relatively straightforward. An analogue signal is considered to have the form of a sine wave, or a combination of sine waves, the treatment of which is well established. DT008/2 Digital Communications Engineering I Slide: 32Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Digital Computersdealin‘1s’and‘0s’. Thereforecommunicationbe tween computers is a matter of transferring digital sequences between machines. The next step is to convert speech and other analogue signals into a digital format to permit a com binednetwork. Thesedaysdigitalelectroniccircuitryischeaper than analogue circuitry for the implementation of complex functions. Digital has the following advantages: • Normally large scale digital systems are software con trolledsothatitispossibletomakechangestothesystem DT008/2 Digital Communications Engineering I Slide: 33Lectures No. 1 and 2: Introduction to Digital Communications Engineering I in software and remotely. • It is less prone to noise or distortion, a ‘1’ remains a ‘1’ and will not be mistaken for a ‘0’, unless there is an extreme level of distortion. • If noise or distortion does occur, methods exist to de termine that this has happened, and if appropriate to correct the error which has occurred. • It is relatively easy to manipulate signals. The mathematical treatment is not as straight forward as that for analogue. DT008/2 Digital Communications Engineering I Slide: 34Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Baseband and Bandpass Bit representation can be: • Sent directly e.g. voltage pulses • Modulated in some way first e.g. amplitude/frequency modulation, AM/FM In the first instance we are dealing with baseband commu nication, in the second case bandpass communication. DT008/2 Digital Communications Engineering I Slide: 35Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Digital Communications Transceiver The components of a hypothetical digital communications transceiver (transmitter/receiver) are shown below. For ex planation purposes, the transceiver includes all the elements commonlyfoundindigitaltransceivers,however,notalltrans ceivers will contain all of these elements. DT008/2 Digital Communications Engineering I Slide: 36Lectures No. 1 and 2: Introduction to Digital Communications Engineering I CODEC MODEM Line Error coding/ ADC Source control pulse Multiple Sampling coder coder shaping Modulation accessing PCM encoder Anti Encryption Quantisation aliasing filter Error DAC Decision Source control Multiple circuit accessing Reconstruction decoding decoding Equalisation PCM decoder Audio Deciphering Matched Demodulation frequency filtering amplifier DT008/2 Digital Communications Engineering I Slide: 37 Deltiplexing MultiplexingLectures No. 1 and 2: Introduction to Digital Communications Engineering I CODECs At its simplest a transceiver CODEC (coder/decoder) con sists of an ADC (analogue to digital converter) in the trans mitter, which converts an analogue signal into digital pulses, and a DAC (digital to analogue converter) in the receiver, which converts these digital pulses back into an analogue sig nal. ADCs will generally consist of a sampling circuit, a quantiser and a pulse code modulator. The sampling circuit provides DT008/2 Digital Communications Engineering I Slide: 38Lectures No. 1 and 2: Introduction to Digital Communications Engineering I discrete voltage samples taken, at regular intervals of time, from the analogue signal. The quantiser approximates these voltages to the nearest one of an allowed set of voltage lev els. Indeed, it is the quantisation process that converts the analogue signal to a digital one. The PCM encoder converts each quantised level to a binary codeword, i.e., digital ones and zeros. An antialiasing filter is sometimes included prior to sampling in order to reduce distortion that can occur due to the sampling process. In the receiver’s DAC received binary voltages are converted DT008/2 Digital Communications Engineering I Slide: 39Lectures No. 1 and 2: Introduction to Digital Communications Engineering I to quantised voltage levels by a PCM decoder which is then smoothedbyalowpassfiltertoreconstructtheoriginal, ana logue, signal. Source, Security and Error Control Coding In addition to PCM encoding and decoding a CODEC may have up to 3 additional functions: • Firstly, in the transmitter it may reduce the number of digitalpulses(bits)requiredtoconveyamessage. Thisis called source coding and can be thought of as removing redundant or surplus bits. DT008/2 Digital Communications Engineering I Slide: 40Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • Secondly, it may encrypt the source coded digits using a cipher for security. This ensures security when passing private information. • Finally,theCODECmayaddextradigitstothe(possibly source coded and encrypted) PCM signal which can be usedatthereceivertodetect,andpossiblycorrect,errors made during signal detection. This is known as channel coding. The source, security and error control decoding operations in the receiver are the inverse of those in the transmitter. DT008/2 Digital Communications Engineering I Slide: 41Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Multiplexers Indigitalcommunications,multiplexing,toaccommodatesev eral simultaneous transmissions, usually means time division multiplexing(TDM).Timedivisionmultiplexersinterleaveei ther PCM codewords, or individual PCM bits, to allow more than one information link to share the same physical trans mission medium. This can be cable, optical fibre or a radio frequency channel. Demultiplexers split the received composite bit stream back DT008/2 Digital Communications Engineering I Slide: 42Lectures No. 1 and 2: Introduction to Digital Communications Engineering I into its component PCM signals. MODEMs MODEMs (modulators/demodulators) change digital pulse streamssothattheycanbetransmittedoveragivenphysical medium, at a given rate, in a specified or allocated frequency band. Typically the modulator shapes, or filters, the pulses to restrict their bandwidth. The input to a modulator is thus a baseband signal, while the output is often a bandpass waveform. DT008/2 Digital Communications Engineering I Slide: 43Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Multiple Accessing Multiple accessing refers to those techniques, and/or rules, which allow more than one transceiver pair to share a com montransmissionmedium(e.g. oneopticalfibre,onesatellite transponder or one piece of coaxial cable). Several different types of multiple accessing are currently in use, each type having its own advantages and disadvantages. The multiple accessing problem is essentially one of efficient and equitable sharing of the limited resource represented by the transmis sion medium. DT008/2 Digital Communications Engineering I Slide: 44Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Signal Transmission The communications path from transmitter to receiver may uselinesorfreespace. Examplesoftheformerarewirepairs, coaxial cables and optical fibres. The most important use of the latter is radio, although in some situations infrared and opticalfreespacelinksarealsopossible. (e.g. remotecontrols for TV, video and hifi equipment and also some security systems). Whatever the transmission medium, it is at this point that much of the attenuation, distortion, interference and noise is encountered. DT008/2 Digital Communications Engineering I Slide: 45Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Line Transmission: The essential advantages of line trans mission are: 1. Path loss is modest. 2. Signal energy is confined and interference between sys tems is therefore negligible. 3. Path characteristics (e.g. attenuation and distortion) are usually stable and relatively easy to compensate for. The disadvantages of line transmission include: 1. Laying cables in the ground or constructing overhead is expensive. DT008/2 Digital Communications Engineering I Slide: 46Lectures No. 1 and 2: Introduction to Digital Communications Engineering I 2. Planning permission may be needed for underground ca bles and overhead wires. 3. A physical connection to the network is required for each subscriber. 4. Mobile communications cannot be provided. 5. Networks cannot easily be added to or subtracted from. The table below summarises the nominal frequency range of selected types of line and typical repeater (used to com pensate for attenuation) spacings. The useful bandwidths of lines, which determine the maximum information transmis DT008/2 Digital Communications Engineering I Slide: 47Lectures No. 1 and 2: Introduction to Digital Communications Engineering I sion rate they can carry are often determined by their atten uation characteristics. Twisted wire pairs, for example, are normallylimited to(linecodedPCM)dataratesof2Mbit/s. Coaxial cables generally carry 140 or 155 Mbit/s PCM sig nalsbutcanhandleratesseveraltimesgreater. Opticalfibres have very large bandwidth potential but may be limited to a fraction of this by factors such as the spectral characteristics ofopticalsourcesanddispersioneffects. Nevertheless,optical fibre PCM bitrates of Gbit/s are possible. DT008/2 Digital Communications Engineering I Slide: 48Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Frequency Repeater Range Spacing Overhead Line 0160 kHz 40 km Twisted Pairs 01 MHz 2 km Coaxial Cables 0500 MHz 19 km Optical Fibres λ=1610−810 nm 100s of km Radio Transmission: The advantages of radio transmission are: DT008/2 Digital Communications Engineering I Slide: 49Lectures No. 1 and 2: Introduction to Digital Communications Engineering I 1. It is cheap and quick to implement. 2. Planning permission is only needed for the erection of towers to support repeaters and terminal stations. 3. It has an inherent broadcast potential. 4. It has an inherent mobile communications potential. 5. Networks can be quickly configured and extra terminals or nodes easily introduced or removed. The principal disadvantages of radio are: 1. Path loss is generally large due to the tendency of the DT008/2 Digital Communications Engineering I Slide: 50Lectures No. 1 and 2: Introduction to Digital Communications Engineering I transmitted signal energy to spread out, most of this en ergy effectively missing the receive antenna. 2. The spreading of signal energy makes interference be tween systems a problem. 3. Pathcharacteristics(i.e. attenuationanddistortion)tend to vary in time, often in an unpredictable way making equalisation more difficult 4. The time varying nature of the channel can result in anomalouspropagationofsignalstolocationswelloutside their normal range. This may cause unexpected interfer DT008/2 Digital Communications Engineering I Slide: 51Lectures No. 1 and 2: Introduction to Digital Communications Engineering I ence between widely spaced systems. DT008/2 Digital Communications Engineering I Slide: 52Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Conclusion These lectures have looked at the following: • Course introduction • History of communications • Communications system • Communication modes • Methods of data communication • Time constraints DT008/2 Digital Communications Engineering I Slide: 53Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • Transmission modes • Analogue versus digital • Baseband and bandpass • Digital communications transceiver DT008/2 Digital Communications Engineering I Slide: 54Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Acknowledgement These lecture notes refer to material obtained from Mr. Lor can MacManus and Mr. Sean O’Fearghail. DT008/2 Digital Communications Engineering I Slide: 55