Digital Communication Systems

1. Digital Communication Systems International Master Studies in Electrical Engineering and Information Technology Prof. Dr. A.S. Omar a.omar@ieee.org Tariq Jamil Saifullah,Khanzada Khanzada@ovgu.de, Khanzada@ieee.org FET,IESK, Unversity of Magdeburg , Germany http://www.iesk.uni-magdeburg.de/hf_technik/hauptmenue/mitarbeiter_hf/prof__omar.html Omar http://www.iesk.uni-magdeburg.de/en/microwave_eng_-p-1903/Hauptmen%C3%BC/Mitarbeiter+HF/tariq_j__s__khanzada.htmlKhanzada

2. Outline • Objectives • Contents • Course Details • Communication Systems & Basic Concepts • Introduction to OFDM and its concept

3. Objectives In order to comprehend learning • Digitize and Code Information Signals • Transmit Digital Signals over different types of Channels • De-noise and Decode received Digital Signals • Characterize Communication Channels

4. Contents • Sampling and Source Coding • Base-Band Techniques (DM, ADM, PCM, DPCM) • Pass-Band Techniques (ASK, PSK, FSK, QAM, MSK, GMSK) • Wideband Techniques (SS-DS, SS-FH, CDMA, WCDMA, OFDM) • Noise Reduction Techniques • Terrestrial, Mobile and Satellite Communication Networks

5. CourseDetails Teaching Lecture and exercises Prerequisites Bachelor in Electrical Engineering or related studies Probability and Random Processes Weight Compulsory module for the Master Course “Electrical Engineering and Information Technology” Exam Written test at the end of the course Credit points 4 Credit points = 120 h (42 h time of attendance and 78 h autonomous work) Work load 2 hours/week - lecture 1 hours/week - exercises Autonomous work Post processing of lectures, Preparation of exercises and exam Responsibly Lectures Prof. Dr. A.S. Omar a.omar@ieee.org Exercises M. Eng. Tariq J.S. Khanzada Khanzada@ovgu.de Location & Time Lectures (Every Tuesday 9:15-10:45 Building 22a, Room 04) Exercises (Every Alternate Thursday 9:15-10:45 Building 05, Room 313)

6. Useful literature Textbook Textbook Digital Communication, 4th Ed. John Proakis, MCGraw Hill 2000 Textbook websitewww.mhhe.com/engcs/electrical/proakis Additional Books Introduction to Digital Communication, Rodger E. Zeimer and Roger L. Peterson, Second Edition, Prentice Hall, 2001. Communication Systems (4th ed.), A. B. Carlson Digital Communications, by Bernard Sklar, Second Edition, Prentice Hall, 2001 Communication Systems , Simon Haykin, 4th Ed. Wiley, 2001, ISBN 0-471-17869-1

7. Excercises & Resources Exercises Theoritical problems about the topics covered in lectures Implementation of concepts in programming language of the choice Recommonded tool Matlab, C++, Java Some Learning Web Resources http://www.mathworks.com/moler/intro.pdfhttp://www.mathworks.com/access/helpdesk/help/techdoc/matlab.htmlhttp://jdsp.asu.edu/jdsp.html www.complextoreal.com

8. Communication Device transfer information from one location (time) to another location (time) Digital: Smoke, Morse Code Telegraph Analog: Commercial Radio, TV Digital: Data, Computer, HDTV

9. Communication Systems • Systems communicate in order to share information. • To communicate means to pass information from one place to another. • It is more convenient to convert information into a signal. Your concern as a communication engineer is with the transmission and reception of these signals.

10. Source Transmitter Channel (distortion) Receiver Destination Noise Components of Communication System Block diagram of Communication System

11. Types of Communication Systems • Point to Point: Telephone, Fax • Point to Multipoint: Broadcast (Radio, TV) • Simplex: One Way • Duplex: Two Ways

12. Design Consideration Cost/Performance Trade Off Cost Performance Data Rate Power Bit Error Probability Transmission Range Bandwidth Fault Tolerance Adaptive to Environment Complexity Security Anti Jamming Capability Low Probability of Interception

14. Digital Communication System • Source are converted into a sequence of binary digits which iscalled information sequenceRepresent the source by an efficient number of binary digits • Efficiently converting the source into a sequence of binary digitsis a process, which is called source encoding of datacompression • Channel encoder adds some redundancy into binary informationsequence that can be used for handle noise and interferenceeffects at the receiver. • Digital modulator maps the binary information sequence intosignal waveforms. • Communication channel is used to send the signal from thetransmitter to the receiver. Physical channels: the atmosphere,wireless, optical, compact disk,…. • Digital demodulator receives transmitted signal contains theinformation which is corrupted by noise • Cannel decoderattempts the reconstruct the originalinformation sequence from knowledge of the code used bychannel encoder. • Source decoder attempts the reconstruct the original signalfrom the binary information sequence using the knowledge ofthe source encoding methods. • The difference between the original signal and thereconstructed signal is measured of the distortion introduced bythe digital communication system • Estimate what was send, aiming at the minimum possibleprobability of making mistakes

15. Communication Channels and their Characteristics • Physical Channel Media • magnetic-electrical signaled wire channel • modulated light beam optical (fiber) channel • antenna radiated wireless channel • acoustical signaled water channel • • Virtual Channel • magnetic storage media • • Noise Characteristic • thermal noise (additive noise) • signal attenuation • phase distortion • multi-path distortion • • Limitation of Channel Usage • transmitter power • receiver sensitivity • channel capacity (such as bandwidth)

16. Communication Channels and their Characteristics Additive Noise Channel where α is the attenuation factor, s(t) is the transmittedsignal, and n(t) is the additive random noise process. • Called Additive Gaussian noise channel if n(t) is aGaussian noise process.

17. Overview of Wireless Systems • Guglielmo Marconi invented the wireless telegraph in 1896 • Communication by encoding alphanumeric characters in analog signal • Sent telegraphic signals across the Atlantic Ocean • Communications satellites launched in 1960s • Advances in wireless technology • Radio, television, mobile telephone, communication satellites • More recently • Satellite communications, wireless networking, cellular technology

18. Broadband Wireless Technology • Higher data rates obtainable with broadband wireless technology • Graphics, video, audio • Shares same advantages of all wireless services: convenience and reduced cost • Service can be deployed faster than fixed service • No cost of cable plant • Service is mobile, deployed almost anywhere

19. Limitations and Difficulties of Wireless Technologies • Wireless is convenient and less expensive • Limitations and political and technical difficulties inhibit wireless technologies • Lack of an industry-wide standard • Device limitations • E.g., small LCD on a mobile telephone can only displaying a few lines of text • E.g., browsers of most mobile wireless devices use wireless markup language (WML) instead of HTML

20. Electromagnetic Signal • Function of time • Can also be expressed as a function of frequency • Signal consists of components of different frequencies

21. Time-Domain Concepts • Analog signal - signal intensity varies in a smooth fashion over time • No breaks or discontinuities in the signal • Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level • Periodic signal - analog or digital signal pattern that repeats over time • s(t +T ) = s(t ) -< t < + • where T is the period of the signal • Aperiodic signal - analog or digital signal pattern that doesn't repeat over time • Peak amplitude (A) - maximum value or strength of the signal over time; typically measured in volts • Frequency (f ) • Rate, in cycles per second, or Hertz (Hz) at which the signal repeats

22. Time-Domain Concepts • Period (T ) - amount of time it takes for one repetition of the signal • T = 1/f • Phase () - measure of the relative position in time within a single period of a signal • Wavelength () - distance occupied by a single cycle of the signal • Or, the distance between two points of corresponding phase of two consecutive cycles

23. Sine Wave Parameters • General sine wave • s(t ) = A sin(2ft + ) • Figure 2.3 shows the effect of varying each of the three parameters • (a) A = 1, f = 1 Hz,  = 0; thus T = 1s • (b) Reduced peak amplitude; A=0.5 • (c) Increased frequency; f = 2, thus T = ½ • (d) Phase shift;  = /4 radians (45 degrees) • note: 2 radians = 360° = 1 period

24. Sine Wave Parameters

25. Time vs. Distance • When the horizontal axis is time, as in Figure 2.3, graphs display the value of a signal at a given point in space as a function of time • With the horizontal axis in space, graphs display the value of a signal at a given point in time as a function of distance • At a particular instant of time, the intensity of the signal varies as a function of distance from the source

26. Frequency-Domain Concepts • Fundamental frequency - when all frequency components of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequency • Spectrum - range of frequencies that a signal contains • Absolute bandwidth - width of the spectrum of a signal • Effective bandwidth (or just bandwidth) - narrow band of frequencies that most of the signal’s energy is contained in

27. Frequency-Domain Concepts • Any electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phases • The period of the total signal is equal to the period of the fundamental frequency

28. Relationship between Data Rate and Bandwidth • The greater the bandwidth, the higher the information-carrying capacity • Conclusions • Any digital waveform will have infinite bandwidth • BUT the transmission system will limit the bandwidth that can be transmitted • AND, for any given medium, the greater the bandwidth transmitted, the greater the cost • HOWEVER, limiting the bandwidth creates distortions

29. Data Communication Terms • Data - entities that convey meaning, or information • Signals - electric or electromagnetic representations of data • Transmission - communication of data by the propagation and processing of signals

30. Examples of Analog and Digital Data • Analog • Video • Audio • Digital • Text • Integers

31. Analog Signals • A continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency • Examples of media: • Copper wire media (twisted pair and coaxial cable) • Fiber optic cable • Atmosphere or space propagation • Analog signals can propagate analog and digital data

32. Digital Signals • A sequence of voltage pulses that may be transmitted over a copper wire medium • Generally cheaper than analog signaling • Less susceptible to noise interference • Suffer more from attenuation • Digital signals can propagate analog and digital data

33. Analog Signaling

34. Digital Signaling

35. Reasons for Choosing Data and Signal Combinations • Digital data, digital signal • Equipment for encoding is less expensive than digital-to-analog equipment • Analog data, digital signal • Conversion permits use of modern digital transmission and switching equipment • Digital data, analog signal • Some transmission media will only propagate analog signals • Examples include optical fiber and satellite • Analog data, analog signal • Analog data easily converted to analog signal

36. Analog Transmission • Transmit analog signals without regard to content • Attenuation limits length of transmission link • Cascaded amplifiers boost signal’s energy for longer distances but cause distortion • Analog data can tolerate distortion • Introduces errors in digital data

37. Digital Transmission • Concerned with the content of the signal • Attenuation endangers integrity of data • Digital Signal • Repeaters achieve greater distance • Repeaters recover the signal and retransmit • Analog signal carrying digital data • Retransmission device recovers the digital data from analog signal • Generates new, clean analog signal

38. About Channel Capacity • Impairments, such as noise, limit data rate that can be achieved • For digital data, to what extent do impairments limit data rate? • Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions

39. Concepts Related to Channel Capacity • Data rate - rate at which data can be communicated (bps) • Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz) • Noise - average level of noise over the communications path • Error rate - rate at which errors occur • Error = transmit 1 and receive 0; transmit 0 and receive 1

40. Spread-Spectrum • Spread-spectrum techniques are methods by which energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, and to prevent detection • Direct-sequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. • Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. • Code division multiple access (CDMA) describes a communication channel access principle that employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code). By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of "spread-spectrum" signaling, since the modulated coded signal has a much higher bandwidth than the data being communicated.

41. Introduction to OFDM • Orthogonal Frequency Division Multiplexing; • Part of xDSL, IEEE 802.11a standards • Improves Data rates, such as 56Mbps in IEEE 802.11a

42. rs OFDM Concept The information bit stream of high data rate r is Subdivided into M bit blocks that are mapped onto symbols of a lower transmission rate rs=r / M Time Domain Freq Domain Bit stream M Each Symbol has Duration Ts and separated by guard intervals of durationTg Tg Ts

43. OFDM Concept • OFDM as multicarrier system uses Discrete Fourier Transform/Fast Fourier Transform (DFT/FFT) • Sin(x)/x spectra for subcarriers • Available bandwidth is divided • into very many narrow bands • ~2000-8000 for digital TV • ~48 for Hiperlan 2 • Data is transmitted in parallel • on these bands

44. How are Signals transmitted inparallel without interference? • Each subcarrier has adifferent frequency • Frequencies chosen sothat an integral numberof cycles in a symbolperiod • Signals aremathematically Orthogonal First three Subcarriers

45. How is data carried on the Subcarriers? • Data is carried byvarying the phase oramplitude of eachsubcarrier • QPSK, 4-QAM, 16-QAM, 64-QAM Two possible subcarrier values

46. Chnnel Impulse Response = = Path Gain Time delay of path l = What is Multipath? • More than onetransmission pathbetween transmitter and receiver • Received signal is thesum of many versionsof the transmittedsignal with varyingdelay and attenuation

47. nth Sample of ith OFDM Symbol = Trnsmitted Symbol on mth Sub carrier = Complex Random variable for lth path of channel = Additive White Guassian Noise at time n = Symbol Generation in MultiPath i = Symbol Iindex N = no of SubCarriers l = multipath index

48. Effect of Multipath on received Baseband Signal • Received signal at any time depends on anumber oftransmitted bits • Inter Symbol Interference (ISI) • Need equalizer to recover data Overlapping the delayed multipath signal with the following symbols causes Inter-Symbol-Interference ISI.

49. ISI gets Worse as Data Rate Increases • ISI covers more symbol periods • Equalizer becomes too complicated

50. Symbol duration GI FFT Interval Dealing with ISI in OFDM • OFDM is the most powerful technique to combatISIbecause of the long symbol duration • ISIis almost completelyeliminatedusing a guard interval • Extract a portion of an OFDM symbol at the end and append it to the beginning to maintain the subcarriers orthogonal.