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Digital Communication 1


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Digital Communication - Pulse Modulation

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Digital Communication 1

  1. 1. DIgital Communication ECE 422L I. Pulse Modulation 2013
  2. 2. Elements of Digital Communication System 2
  3. 3. Elements of Digital Communication System • In Source and Input Transducer: Digital Source: • Source alphabet • Symbol rate • Source alphabet probabilities • Probabilistic dependence of symbols in a sequence Analog source: • audio • video signal 3
  4. 4. Elements of Digital Communication System • Source Encoder – use as few binary digits as possible to represent the signal. – This sequence of binary digits is called information sequence. – Source Encoding or Data Compression: the process of efficiently converting the output of wither analog or digital source into a sequence of binary digits – Codeword – a group of bits used to represent symbols – Blocksize – maximum number of distinct codewords – Codeword length –number of bits used to represent each codeword – Average data rate – – Effeciency of the encoder 4
  5. 5. Elements of Digital Communication System • Channel Encoder: – to introduced, in controlled manner, some redundancy in the binary information sequence that can be used at the receiver to overcome the effects of noise and interference encountered in the transmission on the signal through the channel. Parameters: – Coding rate that depends upon the number the redundant bit added – Coding method used – Coding efficiency – Error control capabilities – Feasibility of the encoder and decoder 5
  6. 6. Elements of Digital Communication System • Digital Modulator: – The binary sequence is passed to digital modulator which in turns convert the sequence into electric signals so that we can transmit them on channel – Parameters: • • • • Transmission bandwidth Probability of symbol Synchronous or asynchronous method of detection Complexity of implementation 6
  7. 7. Elements of Digital Communication System • Channel: – is the physical medium that is used for transmitting signals from transmitter to receiver. • Digital Demodulator: – processes the channel corrupted transmitted waveform and reduces the waveform to the sequence of numbers that represents estimates of the transmitted data symbols. 7
  8. 8. Elements of Digital Communication System • Channel Decoder: – attempts to reconstruct the original information sequence from the knowledge of the code used by the channel encoder and the redundancy contained in the received data • Source Decoder – At the end, if an analog signal is desired then source decoder tries to decode the sequence from the knowledge of the encoding algorithm. And which results in the approximate replica of the input at the transmitter end 8
  9. 9. Merits of Digital Communication 1. Digital signals are very easy to receive. 2. In digital signals, the original signal can be reproduced accurately. 3. digital signals can be cleaned up to restore the quality and amplified by the regenerators. 9
  10. 10. Merits of Digital Communication 4. The noise may change the shape of the pulses but not the pattern of the pulses. 5. But digital signals can be coded so that only the person, who is intended for, can receive them. 6. digital signals can be stored at the receiving end. 7. The digital signals can be processed 10
  11. 11. Data and Signal • Analog data – Takes on continuous values. Ex. Voice or video • Digital data – Takes on discrete values. Ex. Text and integers • Analog Signal – Continuously varying electromagnetic wave representing data carried over a variety of medium • Digital Signal – Sequence of voltage pulses representing data transmitted over a wire medium 11
  12. 12. Remember! Analog or Digital Data Can Be Represented By Either Analog or Digital Signals. These Signals Can Then Be Propogated (Moved Along a Medium). Optical Fiber Only Propogates Analog Signals 12
  13. 13. Data and Signal • Analog Data, Analog Signals – radio • Digital Data, Analog Signals (modem) – broadband & wireless • Analog Data, Digital Signals [codec] • Frequency Division Multiplexing (FDM) • Wave Division Multiplexing (WDM) [fiber] • Time Division Multiplexing (TDM) • Pulse Code Modulation (PCM) • Delta Modulation • Digital Data, Digital Signals (baseband) • wired LAN, (e.g., Ethernet) 13
  14. 14. Data and Signal • Digital data  Digital Signal – Easy and simple to implement • Analog data  Digital Signal – Allows the use of digital transmission and switching equipment • Digital data  Analog Signal – Allows us of the public telephone system – Allows use of optical fiber • Analog Data  Analog Signal – Easy – Telephone system was primarily analog 14
  15. 15. Remember! • Short distance transmissions, baseband modulation is usually used. • Baseband modulation is often called line coding • For long distance and wireless transmissions, bandpass modulation is usually used. • Bandpass modulation is also called carrier modulation 15
  16. 16. Consist essentially of sampling analog information signals Then converting those samples into discrete pulses Transporting the pulses from a source to destination over a physical transmission medium. 16
  17. 17. SAMPLING Sampling is the process of taking samples of the analogue signals at given interval of time. Only samples are being transmitted. • If sufficient samples are sent and sampling theorem are met the original signal can be reconstructed at the receiver 17
  18. 18. SAMPLING THEOREM Sampling theorem states that, if the sampling rate in any pulse modulation system exceeds twice the maximum information signal frequency, the original signal can be reconstructed in the receiver with minimum distortion. • This is called Nyquist Rate, fs ≥ 2fmax • fs – sampling frequency, • fmax – maximum freq of the modulating signal 18
  19. 19. The process of transmitting signals in the form of pulses by using special techniques. This slides includes: • Pulse Amplitude Modulation • Pulse Width Modulation • Pulse Position Modulation • Pulse Code Modulation 19
  20. 20. Analog Pulse Modulation Digital Pulse Modulation Pulse Amplitude (PAM) Pulse Code (PCM) Pulse Width (PWM) Delta (DM) Pulse Position (PPM) 20
  21. 21. Analog pulse modulation • A periodic pulse train is used as the carrier wave • Some characteristic feature of each pulse is varied in a continuous manner in accordance with the corresponding sample value of the message signal • Analog pulse-modulation systems rely on the sampling process to maintain continuous amplitude representation of the message signal 21 Digital pulse modulation • The message signal is represented in a form that is discrete in both time and amplitude • Its transmission in digital form as a sequence of coded pulse • Digital pulse-modulation system use not only the sampling process but also the quantization process. • Digital modulation makes it possible to exploit the full power of digital signal-processing techniques.
  22. 22. * amplitude of discrete carrier signal changes in accordance with the instantaneous amplitude of modulating signal(message signal) keeping width and position of carrier constant *The signal is sampled at regular intervals such that each sample is proportional to the amplitude of the signal at that sampling instant. This technique is called “sampling”. * For minimum distortion, the sampling rate should be more than twice the signal frequency. 22
  23. 23. Analog Signal AND Gate PAM Pulse Shaping Network FM Modulator PAM - FM HF Carrier Oscillator Pulses at sampling frequency 23
  24. 24. Analog Signal Amplitude Modulated Pulses 24
  25. 25. Types of PAM • There are 2 types of PAM : 1. Natural sampling 2. Flat top sampling 25
  26. 26. Natural Sampling 26
  27. 27. Flat Top Sampling 27
  28. 28. Merits and Demerits of PAM • Merits – Generation and detection is easy. • Demerits – Added noise cannot be removed easily as it has impact on amplitude which carries information. – Transmission bandwidth is too large. 28
  29. 29. Pulse Width Modulation * the amplitude is maintained constant but the duration or length or width of each pulse is varied in accordance with instantaneous value of the analog signal keeping amplitude and position of carrier constant * The negative side of the signal is brought to the positive side by adding a fixed d.c. voltage. 29
  30. 30. Pulse Width Modulation Analog Signal Width Modulated Pulses 30
  31. 31. PWM Waveform 31
  32. 32. Merits and Demerits of PWM • Merits – Very good noise immunity. – Its possible to separate out signal from noise. • Demerits – Bandwidth requirement is large as compared to PAM. 32
  33. 33. Pulse Position Modulation * In this type, the sampled waveform has fixed amplitude and width whereas the position of each pulse is varied as per instantaneous value of the analog signal. * PPM signal is further modification of a PWM signal. It has positive thin pulses (zero time or width) corresponding to the starting edge of a PWM pulse and negative thin pulses corresponding to the ending edge of a pulse. 33
  34. 34. Pulse Width Modulation PWM PPM 34 * This wave can be further amended by eliminating the whole positive narrow pulses. The remaining pulse is called clipped PPM.
  35. 35. Pulse Position Modulation • The modulation system in which position of the discrete carrier signal changes in accordance with the instantaneous amplitude of modulating signal(message signal) keeping amplitude and Width of carrier constant is called as PPM. 35
  36. 36. PPM Generator 36
  37. 37. PPM Waveform 37
  38. 38. Merits and Demerits of PPM • Merits – High noise immunity. • Demerit – Generation and detection is complex. 38
  39. 39. PAM, PWM and PPM at a glance: Analog Signal Amplitude Modulated Pulses Width Modulated Pulses Position Modulated Pulses 39
  40. 40. lets move on to digital pulse modulation. 40
  41. 41. ANALOG-TO-DIGITAL CONVERSION •A digital signal is superior to an analog signal because it is more robust to noise and can easily be recovered, corrected and amplified. • For this reason, the tendency today is to change an analog signal to digital data. •Generally used two techniques are : pulse code modulation and delta modulation . 41
  42. 42. Pulse Code Modulation • It is the type of pulse modulation in which the group of pulses or codes are transmitted which represent binary numbers corresponding to modulating signal voltage. • They are a primary building block for advanced communication systems 42
  43. 43. Pulse Code Modulation PCM is the most commonly used technique in digital communications Used in many applications: • Telephone systems • Digital audio recording • CD laser disks • voice mail • digital video etc. 43
  44. 44. Trivia! PCM was invented by the British engineer Alec Reeves in 1937 in France. It was not until about the middle of 1943 that the Bell Labs people became aware of the use of PCM binary coding as already proposed by Alec Reeves. 44
  45. 45. PCM Encoder 45
  46. 46. Pulse Code Modulation PCM consists of three steps to digitize an analog signal: 1. Sampling: • The process of generating pulses of zero width and of amplitude equal to the instantaneous amplitude of the analog signal. • The no. of pulses per second is called “sampling rate”. • Nyquist theorem 46
  48. 48. Pulse Code Modulation 2. Quantization: • • The process of dividing the maximum value of the analog signal into a fixed no. of levels in order to convert the PAM into a Binary Code. The levels obtained are called “quanization levels”. • quantizing process will produce errors called quantizing errors or quantizing noise 48
  49. 49. Two types of quantization. (a) midtread (b) midrise 49
  50. 50. Illustration of the quantization process 50
  51. 51. Nonuniform Quantizing • Voice analog signals are more likely to have amplitude values near zero than at the extreme peak values allowed. • For signals with nonuniform amplitude distribution, the granular quantizing noise will be a serious problem if the step size is not reduced for amplitude values near zero and increased for extremely large values. This is called nonuniform quantizing since a variable step size is used. 51
  52. 52. Nonuniform Quantizing 52
  53. 53. Quantization Error and SNQR • When a signal is quantized, we introduce an error the coded signal is an approximation of the actual amplitude value. • The difference between actual and coded value (midpoint) is referred to as the quantization error. • Signals with lower amplitude values will suffer more from quantization error as the error range: /2, is fixed for all signal levels. 53
  54. 54. Quantization Error and SNQR • Non linear quantization is used to alleviate this problem. Goal is to keep SNQR fixed for all sample values. • Two approaches: – The quantization levels follow a logarithmic curve. Smaller ’s at lower amplitudes and larger ’s at higher amplitudes. – Companding: The sample values are compressed at the sender into logarithmic zones, and then expanded at the receiver. 54
  55. 55. Quantization Error and SNQR • Qe=Resolution/2 • SNQR = minimum voltage / quantization noise voltage • SNQR = 10 log (average signal power/average quantization noise power) 55
  56. 56. Pulse Code Modulation PCM consists of three steps to digitize an analog signal: 3. Binary encoding: Note: A digital signal is described by its „bit rate‟ whereas analog signal is described by its „frequency range‟. 56
  57. 57. 57
  58. 58. Encoding • Encoding is the process of representing the sampled values as a binary number in the range 0 to n. • The value of n is chosen as a power of 2, depending on the accuracy required. • Increasing n reduces the step size between adjacent Quantization levels and hence reduces the Quantization noise. • The down side of this is that the amount of digital data required to represent the analog signal increases. 58
  59. 59. PCM Decoder 59
  60. 60. PCM Decoder • To recover an analog signal from a digitized signal we follow the following steps: – We use a hold circuit that holds the amplitude value of a pulse till the next pulse arrives. – We pass this signal through a low pass filter with a cutoff frequency that is equal to the highest frequency in the pre-sampled signal. 60
  62. 62. PCM Parameter • • • • • Dynamic Range Resolution Maximum allowable input amplitude Coding efficiency Ratio of the largest possible magnitude to the smallest possible magnitude that can be decoded 62
  63. 63. PCM Parameter • Dynamic Range, DR  Ratio of the largest possible magnitude to the smallest possible magnitude that can be decoded  DR=Vmax/Vmin = Vmax/Resolution  2n – 1 >=DR 63
  64. 64. Dynamic Range •Is the ratio of the strongest possible signal that can be transmitted and the weakest discernible signal •In a linear PCM system, the maximum dynamic range is found by: DR = (1.76 + 6.02m) dB 64
  65. 65. Companding • Sometimes called compassion • used to improve dynamic range • Compression is used on the transmitting end and expanding is used on the receiving end • Keep the bit rate and bandwidth low 65
  66. 66. A LAW & µ- LAW 66
  67. 67. A LAW & µ- LAW 67
  68. 68. Mu Law 68
  69. 69. Speech Companding • The human auditory system is believed to be a logarithmic process in which high amplitude sounds do not require the same resolution as low amplitude sounds. • The human ear is more sensitive to quantization noise in small signals than large signals. • A-law and µ-law coding apply a logarithmic quantization function to adjust the data resolution in proportion to the level of the input signal. 69
  70. 70. Speech Companding • quantises the difference between the original and the predicted signals, i.e. the difference between successive values. • Leads to reduction in the number of bits used per sample over that used for PCM. Using DPCM can reduce the bit rate of voice transmission down to 48 kbps. 70
  71. 71. Inter Symbol Interference • If the system impulse response h(t) extends over more than 1 symbol period, symbols become smeared into adjacent symbol periods • Known as inter symbol interference (ISI) 71
  72. 72. Modulator input Binary ‘1’ amplitude amplitude Inter Symbol Interference Time (bit periods) Slicer input Binary ‘1’ Time (bit periods) 72
  73. 73. Noise in PCM Systems • The performance of a PCM system is influenced by two noise sources: • (1) channel noise • introduce bit errors into the received signal. The presence of this noise can be measured in terms of probability of symbol error or bit error rate (BER). • can be made practically negligible by using high signal energy-to-noise density ratio through short spacing between regenerative repeaters. 73
  74. 74. Noise in PCM Systems (2) quantization noise. • can be made negligible by increasing the number of levels L • selecting a compressor-expander pair that is matched to the message signal characteristics. 74
  75. 75. Limitations of PCM systems • Choosing a discrete value near the analog signal for each sample leads to quantization error • Between samples no measurement of the signal is made; • Accurate clock is required for accurate reproduction 75
  76. 76. Merits and Demerits of PCM • Merits – – – – Secured. Encoding is possible. Very high noise immunity. Convenient for long distance communication. – Good signal to noise ratio. 76
  77. 77. Merits and Demerits of PCM • Demerits – Complex circuitry. – Requires large bandwidth. – Synchronization is required between transmitter & receiver. 77
  78. 78. Delta Modulation (DM) • only one bit is transmitted per sample • That bit is a one if the current sample is more positive than the previous sample, and a zero if it is more negative • Since so little information is transmitted, delta modulation requires higher sampling rates than PCM for equal quality of reproduction 78
  79. 79. Delta Modulation (DM) 79
  80. 80. Delta Modulation (DM) Let m n m(nTs ) , n 0, 1, 2,  whereTs is the sampling period and m(nTs ) is a sample of m(t ). The error signal is e n m n mq n 1 eq n mq n sgn(e n ) mq n 1 eq n where mq n is the quantizer output , eq n is the quantized version of e n , and is the step size 80
  81. 81. DM System 81
  82. 82. Slope overload distortion And Granular noise 82
  83. 83. Merits and Demerits of DM • Merits – – – – One bit code word for output. Low signaling rate. Low channel bandwidth. No ADC is required • Demerits 1. 2. Slope overload present. Granular noise present. 83
  84. 84. Delta-Sigma Modulation • Alternatively known as Pulse Density modulation or Pulse Frequency modulation • Modification of the delta modulation 84
  85. 85. Delta-Sigma Modulation • Conventional delta modulation - Quantizer input is an approximation of the derivative of the input message signal m(t). • Results in the accumulation of error (noise) – accumulated noise (transmission disturbances) at the receiver (cumulative error). • Possible solution: integrating the message before delta modulation – called delta sigma modulation 85
  86. 86. Delta-Sigma Modulation • The message signal is defined in its continuous form – so pulse modulator contains a hard limiter and a pulse generator to produce a 1-bit encoded signal • integration at the tx requires differentiation at the rx side. • But: As in conventional DM the message has to be integrated at the final stage this eliminates the need of differentiation here. 86
  87. 87. Delta-Sigma Modulation System Figure 3.25 87
  88. 88. Merits and Demerits of DSM • Merits – Low frequency component of input signal is boosted – Correlation between adjacent samples of delta modulator is increased – Simplifies the receiver design • Demerits – Requires sampling rate far in excess of the Nyquist rate 88
  89. 89. Adaptive Delta Modulation • This is the advanced version of DM. • Avoid the problem on slope over load error and granular noise problem. • step size is adapted to the slope (variation) of the message signal. 89
  90. 90. Adaptive Delta Modulation • If successive errors are of opposite polarity, then the delta modulator is operating in the granular mode; in such a case it is advantageous to use reduced step size. • If successive errors are of the same polarity, then the delta modulator is operating in its slope-overload mode; in this case, the step size should be increased. • . 90
  91. 91. ADM System 91
  92. 92. ADM Waveform 92
  93. 93. Merits and Demerits of ADM • Merits – – – – – Improved SNR. Low signaling rate. Wider dynamic range Better bandwidth utilization Reduction in slope overload and granular noise. 93
  94. 94. Differential Pulse Code Modulation (DPCM) • Voice and video signals represented in PCM exhibit high correlation, which means that PCM signals contain redundant information. The result is an inefficient coding. • By removing the PCM information redundancy a more efficient coded signal may be obtained. 94
  95. 95. DPCM System 95
  96. 96. DPCM System • If the prediction is well performed, then the variance of e(k) will be much smaller than the variance of m(k), which results into a smaller number of levels to quantize e(k). • DPCM can be described as a predictive coding scheme. 96
  97. 97. Type of Predictor • One-tap predictor • N-tap predictor 97
  98. 98. Merits and Demerits of DPCM • Merits 1. Less signaling rate. 2. Less bandwidth. 3. Requires less quantization levels • Demerits 1. 2. High bit rate. Needs the predictor circuit to be used which is complex. 98
  99. 99. Reference • Digital Communication – by Sanjay Sharma • Advance Electronic Communication – by Robert Tomasi • World Wide Web 99
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