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Unit IV_SS_MMS.ppt

  1. 1. Introduction to 1
  3. 3. Text Books  1. Simon Haykin, “Digital Communication Systems”, John Wiley&Sons, Fourth Edition.  2. A.B. Carlson, P B Crully, J C Rutledge, “Communication Systems”, Fourth Edition, McGraw Hill Publication. Reference Books  1. P Ramkrishna Rao, Digital Communication, McGrawHill Publication  2. Ha Nguyen, Ed Shwedyk, “A First Course in Digital Communication”, Cambridge University Press.  3. B P Lathi, Zhi Ding “Modern Analog and Digital Communication System”, Oxford University Press, Fourth Edition.  4. Bernard Sklar,Prabitra Kumar Ray, “Digital Communications Fundamentals and Applications” Second Edition,Pearson Education  5. Taub, Schilling, “Principles of Communication System”, Fourth Edition, McGraw Hill.
  4. 4. Topic Books lectures Teaching Method Introduction T1, R1 1 PPT Pseudo noise sequences T1, T2 2 Conventional A notion of spread spectrum T1 1 PPT Direct sequence spread spectrum with coherent BPSK T1, R1 Conventional Signal space dimensionality & processing gain T1 1 Conventional Probability of error T1 Conventional & simulation Concept of Jamming T1 http://nptel.ac.in/co urses/117105136/4 1 Conventional & PPT Frequency hop spread spectrum R3 2 Conventional & PPT
  5. 5. Subjects 1 Signals and Systems 2 Analog Communication 3 Digital Electronics 4 Engineering Mathematics Concepts 1 Shift Register 2 Power Spectral Density & Auto-correlation 3 Random Variables and Central Limit Theorem, Mean and Variance 4 BPSK/MFSK 5 Coherent & non-coherent Detection
  6. 6. In communication systems we focused on: 1)How to utilize bandwidth? & 2)How to minimize transmitted power?  Problems in communication systems 1) Unauthorized user can uses data. 2) Interference of another user 3)Hostile transmitter can Jam the transmitter 4) Military application problems. 6
  7. 7.  In combating the intentional interference (Jamming)  In rejecting the unintentional interference from users  To avoid interference due to Multipath propagation.  In low probability of Intercept (LPI) signals  In obtaining the message privacy. 7
  8. 8.  Spread Spectrum a) Averaging type systems eg. DS-SS b) Avoidance type Systems eg. FH-SS Time Hopping Chirp Hybrid 8
  10. 10. 1 ■ Feedback shift register becomes “linear” if the feedback logic consists entirely of modulo-2 adders. ■ From implementation standpoint, the most convenient way to generate a pseudo-noise sequence is to employ several shift-registers and a feedback through combinational logic. Pseudo-Noise (PN) sequence generator Fig. Block diagram of Pseudo-Noise (PN) sequence generator
  11. 11.  A Pseudo Noise (PN) Sequence is defined as a coded sequence of 1’s and 0’s with certain autocorrelation properties.  It consists of a shift register made up of m flip-flops and a logic circuit to form a multiloop feedback circuit as a Combinational Logic.  It generally uses D type of FF & Logic circuit as a Modulo 2 Adder.  A feedback resister having ‘m’ flip-flops can generate a maximum-length sequence with a period of N = 2m – 1  PN is a Periodic binary sequence with noise like waveform.  Becomes periodic after 2m – 1 states  zero state is not permitted. 1-Feb-23 11
  12. 12. □ The Maximum Length sequence is a type of cyclic code which represents a commonly used Periodic PN sequence. □ A PN sequence generated by a linear feedback shift register must eventually become periodic with period at most 2m  1, where m is the number of shift registers. □ A PN sequence whose period reaches its maximum value is named the maximum-length sequence or simply m-sequence 12
  13. 13. 13  A shift register of length ‘m’ consists of ‘m’ flip flops and all of them operated on the same clock. At each clock pulse, the state of each Flip Flop is shifted to the next one.  To prevent the shift register from getting empted, we have to apply an input continuously to the first FF. This input is called as a Feedback, is computed by using a logical function of the states of all the flip flops.  For a linear type of Feedback Shift Register, a feedback function is obtained by using Modulo 2 addition (i.e. EX-OR gate) of the outputs of various flip flops.  For m= 3, the Maximum Length sequence at the generator output will always be periodic with a period of, N = 2m  1 , N = 7  With increase in value of m, the sequence length increases and the sequence becomes random in true sense.
  14. 14. 14 1. Balance property 2. Correlation property 3. Run property 1. Balance property: In each period of maximal-length sequence, the number of “ 1’s “ is always one more than the number of “ 0’s ". 2. Correlation property: The Autocorrelation function of maximal-length sequence, is periodic & binary valued. This property is called the Correlation Property.
  15. 15. 15 3. Run property:  The period of a Maximal-length sequence is given by,  By Runs we mean subsequence of identical symbols ( 1’s & 0’s) within one period of Sequence.  Among the runs of 1’s and 0’s in each period of a maximum length sequence, one half the runs of each kind are of length 1, one fourth are of length two, one eighth are of length three and so on.  For a maximum length sequence generated with on shift registers has (N+1)/2 runs where, N =2m – 1.  e.g. 000 1111 0 1 0 11 00 1
  16. 16. 16 A PN sequence is an NRZ type signal with logic ‘1’ represented by +1 and binary ‘0’ is represented by -1. 4. Chip Duration (Tc): The duration of every bit is known as the Chip Duration. It denoted as Tc.
  17. 17. 17 5. Chip rate ( Rc ): The chip rate is defined as the number of bits (Chips) per second. We know, Tc = 1/Rc Rc = 1/Tc 6. Period of PN ( Pseudo-noise) Sequence: The period of PN sequence is given by, Where Tc is bit duration.
  18. 18. 18 Shift Register Stages M Feedback Taps 2 (2,1) 3 (3,2) 4 (4,1) 5 (5,2) (5,4,3,2) (5,4,2,1) 6 (6,1) (6,5,2,1) (6,5,3,2) 7 (7,1) (7,3) (7,3,2,1)(7,4,3,2) (7,6,4,2) (7,6,3,1) (7,6,5,2) (7,,6,5,4,2,1) (7,5,4,3,2,1) 8 (8,4,3,2) (8,6,5,3) (8,6,5,2) (8,5,3,1) (8,6,5,1) (8,7,6,5,2,1) (8,6,4,3,2,1)
  19. 19. 19 S1 S2 S3 CLK O/P sequence + PN with m= 3 (N=7) Example1: For the PN sequence generator shown in figure obtain the output PN sequence. If chip rate is 107 chips/sec, calculate: i) Chip, (ii) PN duration, (iii) Period of output Sequence.
  20. 20. 20 Assuming initial states S3 S2 S1 = 0 0 1. The output sequence is 0010111 which repeats with period 23 – 1 = 7 CLK Shift Register Output S3 S2 O/P S3 S3 S2 S1 0 0 0 1 0 0 1 0 1 0 1 0 2 1 0 1 1 1 3 0 1 1 1 0 4 1 1 1 0 1 5 1 1 0 0 1 6 1 0 0 1 1 7 0 0 1 0 0 8 0 1 0 1 0 9 1 0 1 1 1 10 0 1 1 1 0 +
  21. 21. 21 Rc = 107 chips/sec  i) Chip (Duration): Chip duration Tc = 1/ Rc = 1 / 107 chips/sec = 0.1 µsec  ii) Duration of PN Sequence (N): N = 2m -1 N= 7 bits  Iii) Period of Output Sequence (Tb): Tb = N x Tc = 7 x 0.1 µsec = 0.7 µsec
  22. 22. 22 PN Sequence: PN Sequence Examples 2: Answer: Output Sequence
  23. 23. 23 PN Sequence Examples 2 …..
  24. 24. 24 PN Sequence Examples 3: Answer: Output Sequence
  25. 25. 25 PN Sequence Examples 3 :
  26. 26. Spread spectrum is a modulation method applied to digitally modulated signals that increases the transmit signal bandwidth to a value much larger than is needed to transmit the underlying information bits. 26
  27. 27. 27 1. They are difficult to intercept for unauthorized person. 2. They are easily hidden, it is difficult to even detect their presence in many cases. 3. They are resistant to jamming. 4. They have an asynchronous multiple-access capability. 5. They provide a measure of immunity to distortion due to multipath propagation.
  28. 28. 28 • The signal occupies a bandwidth much larger than is needed for the information signal. • The spread spectrum modulation is done using a spreading code, which is independent of the data in the signal. • Dispreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code.
  29. 29. 29
  30. 30. 30
  31. 31. 31 Information signal b(t) is narrowband signal.PN signal c(t) is a wideband signal. The baseband transmission, the product signal m(t) represented the transmitted signal. Received signal r(t) is a m(t) & additive interference i(t) To recover the original signal b(t), received signal r(t) is applied to a demodulator that applied to the multiplier followed by a integrator.
  32. 32. 32 There are several forms of spread Spectrums 1. Direct sequence spread spectrum (DSSS) 2. Frequency hopping spread spectrum (FHSS) 1. Direct sequence spread spectrum (DSSS):-
  33. 33. 33 Block diagram of DSSS Transmitter system.
  34. 34. 34 Block diagram of DSSS Transmitter first converts the incoming binary data sequence bk into a polar NRZ waveform b(t), which is followed by two stages of modulation. - First stage consists of a product modulator or multiplier with data signal b(t) & the PN signal c(t) i.e. PN sequence as input. -The second stage consists of a binary PSK modulator. The transmitted signal x(t) is thus Direct Sequence Spread binary phase- shift keyed (DS/BPSK) signal. -The phase modulation Ө(t) of x(t) has one of two values, 0 & π depending on the polarities of the message signal b(t) and PN signal c(t) at time t, as in Table.
  35. 35. 35
  36. 36. 36
  37. 37. 37 Block diagram of DSSS Receiver system.
  38. 38. 38 Block diagram of DSSS Receiver system consist of two stages of demodulation. 1. In First stage received signal y(t) and a locally generated carrier are applied to a product modulator followed by a low pass filter whose bandwidth is equal to that of the original message signal m(t). This stage of the demodulation process reverse the phase-shift keying applied to the transmitted signal. 2. In the second stage of demodulation performs spectrum dispreading by multiplying the low pass filter output by locally generated replica of the PN signal c(t), followed by integration over a bit interval 0 ≤ t ≤ T Tb and finally decision make as received signal is 0 or 1 bit
  39. 39. 39 The channel output given by: y(t) = x(t) + j(t) = c(t) s(t)+ j(t) The Coherent detector input u(t) : u(t) =c(t) y(t) = s(t)+ c(t) j(t) = 1 Where : for all t
  40. 40. 40 1. Processing Gain 2. Probability of Error 3. Jamming margin 1. Processing Gain(PG): The processing Gain of a DS-SS system presents the gain achieved by processing a spread spectrum signal over an Unspread signal. processing Gain can also be defined as that the ratio of the Bandwidth of the spread signal to the Bandwidth of the Unspread signal. Spread code signal is m(t)= b(t) c(t) ……(1)
  41. 41. 41 -One bit period of the spread signal m(t) is given by Tc. The bandwidth of the NRZ signal is equal to the reciprocal of its one bit period. Bandwidth of the Signal= 1/ Tc ……(2) - The Unspread signal b(t) is an NRZ signal & BW of NRZ is reciprocal of the bit period Tb. BW of Unspread Signal= 1/ Tb ……(3) - Processing Gain PG= (1/Tc) / (1/Tb) - PG= Tb / Tc ……(4)
  42. 42. 42 2. Probability of Error:
  43. 43. 43 3. Jamming margin:
  44. 44. 44 -The ability of spread-spectrum system to combat the effect of Jammers is determined by the processing gain of the system, which is a function of the PN sequence period. -The processing gain can be made larger by employing PN sequence with narrow chip duration, which, in turn, Permits a greater transmission bandwidth & more chips per bit. -However, the capabilities of physical devices used to generate the PN spread- spectrum signals impose a practical limit on the attainable processing gain. Also processing gain so attained is still not large enough to overcome the effects of some jammers of concern.
  45. 45. 45 - One such alternative method is to force the Jammer to cover a wider spectrum by randomly hopping the data modulated carrier from one frequency to the next. - Then the spectrum of the transmitted signal is spread sequentially rather than instantaneously. - The type of spread spectrum in which the carrier hops randomly from one frequency to another is called Frequency-Hop (FH) spread spectrum. -The common modulation format for FH systems is that of M-ary Frequency-shift keying (MFSK). The combination of these two techniques is referred to simply as FH/MFSK.
  46. 46. 46 - Since frequency hopping does not cover the entire spread spectrum instantaneously, we are considering the rate at which the Hops occur. We have two basic technology characterizations of frequency hopping. A) Slow-Frequency Hopping: In which is the symbol rate Rs of the MFSK signal is an Integer multiple of the Hop rate Rb. That is, several symbols are transmitted on each frequency hop. B) Fast-Frequency Hopping: In which the hop rate Rb is an integer multiple of the MFSK symbol rate Rs. That is, the carrier frequency will Change or hop several times during the transmission of one symbol.
  47. 47. 47 Slow Frequency hopping spread spectrum TX:
  48. 48. 48 - Block diagram of an FH/MFSK transmitter, which involves frequency modulation followed by Mixing. First , the incoming signal binary data are applied to an M-ary FSK modulator. - The resulting modulated wave and the output from a digital Frequency Synthesizer are then applied to a mixer that consist of a multiplier followed by a Band-Pass Filter. - The filter is designed to select the sum frequency component resulting from the multiplication process as the transmitted signal. - In particular, successive k-bit segment of a PN sequence drive the frequency synthesizer, which enables the carrier frequency to hop over d distinct values.
  49. 49. 49 - On a single hop, the bandwidth of the transmitted signal is the same as that resulting from the use of conventional MFSK with an alphabet of orthogonal signals. - For a complete range of frequency hops, the transmitted FH/MFSK signal occupies a much larger bandwidth (GHz) which is larger than that achievable with DSSS. - An implication of these large FH bandwidths is that coherent detection is possible only within each hop, because frequency synthesizers are unable to maintain phase coherence over successive hops. - Accordingly, most frequency hop spread-spectrum communication systems use noncoherent M-ary modulation schemes.
  50. 50. 50
  51. 51. 51 - In the receiver shown in above figure the frequency hopping is first removed by mixing (Down- Converting) the signsl with the output of a local frequency synthesizer that is synchronously controlled in the same manner as that in the transmitter. - At the output of the multiplier we get the input signals, their sum & difference. Out of these difference frequency components, is selected by the bandpass filter that follows the multiplier. Difference signal is the MFSK signal. - The resulting output is then band-pass filtered and subsequently processed by a noncoherent M-ary FSK detector. To implement this M-ary detector, we may use a bank of M noncoherent matched filters. - An estimate of the original symbol transmitted is obtained by selecting the largest filter output.
  52. 52. 52 - In an individual FH/MFSK tone of shortest duration is referred to as a chip. The chip rate Rc for an FH/MFSK system is defined as, Rc = max(Rb, Rs) where Rb is the hop rate & Rs is the symbol rate. A slow FH/MFSK signal is characterized by having multiple symbols transmitted per hop. Hence each symbol of a slow FH/MFSK signal is a Chip. Rc = Rs =(Rb/K) ≥ Rb where K = log2 (M) Processing Gain PG= (Wc / Rs ) = -
  53. 53. 53
  54. 54. 54
  55. 55. 55
  56. 56. 56
  57. 57. 57
  58. 58. 58 Wireless Telephone Systems -In last few years Wireless telephone systems haveundergone a tremendous transformation and have becomes a necessity now. -Now Wireless telephone systems over comes the landline telephone system. -Two primary Wireless systems are: i) Cellular Telephones System ii) Personal Communication Systems OR Services (PCS )
  59. 59. 59 I Cellular Telephones System -Cellular telephone system is a wireless telephone system. -It is a multiuser system. Basic Concept: -It is wireless communication just like cordless. -Distance is not restricted to within home but one can travel in the city or even outside the city without interruption in communication. -In this city is divided in to small areas called “CELLS” of near about 10 square KM.
  60. 60. 60 Cellular Network:
  61. 61. 61 Cellular Network:
  62. 62. 62 Cell: -The basic geographic unit of a Cellular Communication system is called as Cell. -Its shape is Hexagonal as shown. -The size of cell is not fixed. Practically the shape of the cell is not be perfect Hexagon. A B C A C A C
  63. 63. 63 Cluster: - Agroup of a cells is called as a Cluster. -Its size is not fixed, it is according to requirements of perticular area. A B C A F E G D E F D E
  64. 64. 64 Frequency Reuse: -Frequency reuse refers to the use of radio channels operating on the frequency to cover different areas, that are physically separate from each other -must manage reuse of frequencies -power of base transceiver controlled allow communications within cell on given frequency limit escaping power to adjacent cells allow re-use of frequencies in nearby cells typically 10 – 50 frequencies per cell K
  65. 65. 65 Frequency Reuse: Frequency Reuse Pattern :
  66. 66. 66 Cell Splitting: Concept :
  67. 67. 67 Hand off: Concept : When you move to another cell within the same system, you get a handof You are transferred automatically to that cell’s cellsite
  68. 68. 68 Hand off:
  69. 69. 69 PCS: The term Personal Communication Service (enabling communication with a person at anytime, at any place, and in any form) include Various Wireless Access Personal Mobility Services Two of the most popular cellular systems High Tier Digital Cellular Systems Lower Tier Cordless Telecommunication Systems
  70. 70. 70 PCS:
  71. 71. 71 PCS:
  72. 72. 72
  73. 73. 73 Positive 1. Signal hiding (lower power density, noise-like) , non interference. 2. Secure communications (Privacy). 3. Code division multiple access CDMA. 4. Mitigation of multi path effect. 5. Protection to international interference (jamming) 6. Rejection of unintentional interference (narrow band)
  74. 74. 74 Negative 1. No improve in performance in the presence of Gaussian noise. 2. Increase bandwidth (frequency usage, wideband receiver). 3. Increase complexity and computational load.
  75. 75. 75 References: • Simon Haykin “Communication Systems” , John Wily & Sons 2001. • Emmanuel C. Ifeachor “Digital Signal Processing”, Prentice Hall 2002. • ir. J. Meel “Spread Spectrum introduction” DE NAYER INSTITUTE. Belgium (www.denayer.be). • B. P. Lathi “Modern Digital and Analog Communication Systems”, Oxford University Press 1998.