Lecture 11 Codes comparison Spread Spectrum 19th January.pptx

Advance Communications

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2. Grading Policy Mid Term Exam weightage 25 % Quizzes 15% Assignments 10 % Terminal Exam 50 % Total 100 2

3. Instructor: Dr. Mustafa Shakir Advanced Digital Communication Systems Code: TCW705310-F24-PB-GCL-MSEEW 3

4. Course Material Access All the lecture slides and helping material, and course information would be available in following yahoo or gmail group. (email your name and class section to [email protected]) Group home page: https://groups.google.com/forum/#!forum /fa24-advanced-digital-communication-systems-superior.engineering 4

5. Course Catalog Description Power Spectral Density, Formatting , Modulation and Demodulation /detection techniques, link, channel coding , probability of bit error performance, bandwidth efficiency, synchronization, phase locked loop implementation, multiplexing and multiple access, encryption/decryption, fading channels OTHER TOPICS ACCORDING TO CLOs of the course 5

6. 6 Advanced Digital Communication Systems

7. Text: Digital Communications: Fundamentals and Applications, By “ Bernard Sklar ” , Prentice Hall,   Probability and Random Signals for Electrical Engineers, Neon Garcia References: Digital Communications, Fourth Edition, J.G. Proakis , McGraw Hill, 2000.  Relevant Course Books 7

8. Line Coding 8

9. Encoding Techniques, Line codes 9

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11. 11 Comparison of Line Codes Self-synchronization Manchester codes have built in timing information because they always have a zero crossing in the center of the pulse Polar RZ codes tend to be good because the signal level always goes to zero for the second half of the pulse NRZ signals are not good for self-synchronization Error probability Polar codes perform better (are more energy efficient) than Unipolar or Bipolar codes Channel characteristics We need to find the power spectral density (PSD) of the line codes to compare the line codes in terms of the channel characteristics

12. 12 Comparisons of Line Codes Different pulse shapes are used to control the spectrum of the transmitted signal (no DC value, bandwidth, etc.) guarantee transitions every symbol interval to assist in symbol timing recovery 1. Power Spectral Density of Line Codes (see Fig. 2.23, Page 90) After line coding, the pulses may be filtered or shaped to further improve there properties such as Spectral efficiency Immunity to Intersymbol Interference Distinction between Line Coding and Pulse Shaping is not easy 2. DC Component and Bandwidth DC Components Unipolar NRZ, polar NRZ, and unipolar RZ all have DC components Bipolar RZ and Manchester NRZ do not have DC components

13. Eeng 360 13 Comparison of Line Codes Self-synchronization: Manchester codes have built in timing information because they always have a zero crossing in the center of the pulse. Polar RZ codes tend to be good because the signal level always goes to zero for the second half of the pulse. NRZ signals are not good for self-synchronization. Error probability: Polar codes perform better (are more energy efficient) than Unipolar or Bipolar codes. Channel characteristics: We need to find the PSD of the line codes to answer this ...

14. Communication Systems

15. Definition of Spread Spectrum Spread spectrum is a means of transmission in which the signal occupies a bandwidth in excess of the minimum necessary to send the information; the band spread is accomplished by means of a code which is independent of the data, and a synchronized reception with the code at the receiver is used for de-spreading and subsequent data recovery. Under this definition, standard modulation schemes such as FM and PCM which also spread the spectrum of an information signal do not qualify as spread spectrum. 15

16. What is Spread Spectrum? A transmission technique in which a pseudo-noise code, independent of the information data, is employed as a modulation waveform to “spread” the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is “de-spread” using a synchronized replica of the pseudo-noise code. 16

17. Why Spread Spectrum? Hide a signal below the noise floor Resistance to narrowband jamming and interference Mitigate performance degradation due to inter-symbol and narrowband interference Allow multiple users to share the same signal bandwidth Wide bandwidth of SS signals is useful for location and timing acquisition 17

18. Spreading 18

19. Classification of SSS 19

20. 20 Spreading methods Frequency Hopping (FH) The bandwidth spreading is achieved by hopping the carrier frequency over a large set of frequencies Applied in GSM, Military, ISM bands, Blue tooth Time Hopping (TH) A block of bits is compressed and transmitted intermittently in one or more time slots within a frame that consists of a large number of time slots. Direct sequence (DS) The spreading is achieved by multiplying the data waveform with a pseudo-random signal. Applied in IS-95 IS-136 Cellular CDMA, GPS, UMTS, W-CDMA, Military

21. A Short History the technology has become increasingly popular for applications that involve radio links in hostile environments. Typical applications for the resulting short-range data transceivers include satellite-positioning systems (GPS), 3G mobile telecommunications, W-LAN (IEEE® 802.11a, IEEE 802.11b, IEEE 802.11g), and Bluetooth®. Spread spectrum techniques also aid in the endless race between communication needs and radio-frequency availability situations where the radio spectrum is limited and is, therefore, an expensive resource. 21

22. Bandwidth effects of the Spreading Operation 22

23. Effect of de-spreading 23 Here a spread-spectrum demodulation has been made on top of the normal demodulation operations. One can also demonstrate that signals such as an interferer or jammer added during the transmission will be spread during the despreading operation!

24. Waste of Bandwidth Due to Spreading Is Offset by Multiple Users 24

25. Anti Jamming Gaussian noise by definition has infinite power spread uniformly over all frequencies. For a typical narrowband signal, this means that only the noise in the signal bandwidth can degrade performance. For signals of bandwidth W and duration T, the number of signaling dimensions can be shown to be approximately 2WT. Against white Gaussian noise, with infinite power, the use of spreading (large 2WT) offers no performance improvement. However, when the noise originates from a jammer with a fixed finite power and with uncertainty as to where in the signal space the signal coordinates are located, the jammer’s choices are limited to following: Jam all the spectrum with an equal amount of power in each one, with the result that little power is available in each coordinate. Jam a few signal coordinates with increased power in each of the jammer coordinates (or more generally, jam all the coordinates with various amounts of each. 25

26. Anti Jamming 26

27. Anti Jamming Jamming is not always the result of an intentional act. Sometimes jamming is caused by natural phenomenon. Sometimes it is the result of self interference caused by multipath, in which delayed versions of the signal interfere with direct path signal. 27

28. Resistance to interception Resistance to interception is the second advantage provided by spread-spectrum techniques. Without the right key, the spread-spectrum signal appears as noise or as an interferer. (Scanning methods can break the code, however, if the key is short.) Even better, signal levels can be below the noise floor, because the spreading operation reduces the spectral density. (Total energy is the same, but it is widely spread in frequency.) The message is thus made invisible, an effect that is particularly strong with the direct sequence spread-spectrum (DSSS) technique. (DSSS is discussed in greater detail below.) Other receivers cannot "see" the transmission; they only register a slight increase in the overall noise level! 28

29. Resistance to Fading Wireless channels often include multiple-path propagation which are caused by atmospheric reflection or refraction, and by reflection from the ground or from objects such as buildings. The reflected path (R) can interfere with the direct path (D) in a phenomenon called fading. Because the despreading process synchronizes to signal D, signal R is rejected even though it contains the same key. Methods are available to use the reflected-path signals by despreading them and adding the extracted results to the main one. 29

30. Principles of SS – Block Diagrams 30 Digital Communication System

31. Principles of SS – Block Diagrams 31 Spread Spectrum System

32. Direct Sequence Spread Spectrum (DSSS) Each bit represented by multiple bits using spreading code Spreading code spreads signal across wider frequency band In proportion to number of bits used 10 bit spreading code signal across 10 times bandwidth of 1 bit code One method: Combine input with spreading code using XOR Input bit 1 inverts spreading code bit Input zero bit doesn’t alter spreading code bit Data rate equal to original spreading code Performance similar to FHSS 32

33. Direct Sequence Spread Spectrum (DSSS) each bit is represented by multiple bits using a spreading code this spreads signal across a wider frequency band has performance similar to FHSS

34. Direct Sequence Spread Spectrum Example