Digital modulation


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Digital modulation

  1. 1. Introduction to Digital Communication 1
  2. 2.  Frequency The frequency refers to the number of cycles of the wave oscillates in a second that is measured in hertz. This variation in direction is known as a cycle, and the term frequency refers to the number of cycles in a second
  3. 3. periodic time period T in seconds represents the time of one complete cycle: 4
  4. 4.  Bandwidth The range of frequencies is of the signal the range of human voice signal from 300 to 3,400 Hz. This means that the bandwidth of the telephone channel through the network is 3,400 – 300 Hz = 3.1 kHz,
  5. 5. The bandwidth is normally measured from thepoints where the signal power drops to half from itsmaximum power. 6
  6. 6. Wavelength λ represents the propagation distance in one cycle time: 7
  7. 7.  Digital Communication Systems Are communication systems that uses such a digital sequence as an interface between the source and the channel input and similarly between the channel output and final destination. A digital communication system involves the transmission of information in digital form from one point to another point
  8. 8.  The source encoder converts the source output to a binary sequence and designed to make the source information rate approach the channel capacity (data compression). The channel encoder modulator processes the binary sequence for transmission over the channel and perform coding process.
  9. 9.  The channel decoder demodulator recreates the incoming binary sequence and error correction. The source decoder recreates the source output. 11
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  13. 13. Sampling The more samples per second there are, the more representative of the analog signal. After sampling, the signal value is known only at discrete points in time, called sampling instants. If these points have a sufficiently close spacing, a smooth curve drawn through them 15
  14. 14.  The number of samples per second is called the sampling frequency or sampling rate., and it depends on the highest frequency component present in the analog signal. The relation of sampling frequency and the highest frequency of the signal to be sampled is stated as follows: 16
  15. 15.  If the sampling frequency, fs, is higher than two times the highest frequency component of the analog signal, fm, the original analog signal is completely described by these instantaneous samples alone; that is, fs > 2 fm Nyquist rate. Ts=1/ fs= (1/2 fm ) 17
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  17. 17. Quantizing To transmit the sample values via a digital system, we have to represent each sample value in numerical form. This requires quantizing where each accurate sample value is rounded off to the closest numerical value in a set of digital words in use 19
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  19. 19.  The quality of the coding depends on the number of quantum levels that is defined to provide the required performance. The more quantum levels we use, the better performance we get. In the case of binary coding, the number of quantum levels is q = 2n, where q denotes the number of quantum levels and n is the length in bits of the binary code words that describe the sample values. 21
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  21. 21.  Finally, encoding process each sample is represented as one in the set of eight-bit binary words. 23
  22. 22.  Bit 1, the most significant bit (MSB): The MSB is the first bit and it reveals the polarity of the sample. Value 1 represents positive polarity and 0 represents negative polarity. The sample value zero may create two different code words depending on whether it has a positive or negative polarity. 24
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  24. 24. Line coding 26
  25. 25.  NRZ Is the most common form of digital signal used internally in digital systems. Each symbol has a constant value corresponding to binary symbol values 1 and 0. 27
  26. 26.  RZ Each symbol is cut into two parts. The first half of the symbol represents the binary value and the rest of the symbol is always set to zero. 28
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  28. 28. Multiplexing Multiplexing is a process that combines several signals from different users for simultaneous transmission on one transmission channel. The main principles of multiplexing are 30
  29. 29.  Frequency-Division Multiplexing (FDM) FDM modulates each message to a different carrier frequency. The modulated messages are transmitted through the same channel and a bank of filters separates the messages at the destination. 31
  30. 30.  The frequency band of the system is divided into several narrowband channels, one for each user. Each narrowband channel is reserved for one user all the time. 32
  31. 31.  The same principle is also used in analog cellular systems in which each user occupies one FDM channel for the duration of the call. In such a case, we call the process 33
  32. 32.  Frequency division multiple access (FDMA) because the frequency-division method is now used to allow multiple users to access the network at the same time. 34
  33. 33.  Time-Division Multiplexing (TDM)TDM puts different messages, from different users, in nonoverlapping time slots. Each user uses a all frequency band but only a small fraction of time. to the user channels, framing information is needed for the switching 35
  34. 34.  Circuit at the receiver that separates the user (time slots) in the demultiplexer. When the demultiplexer detects the frame synchronization word, it knows that this is the start of a new frame and the next time slot contains. 36
  35. 35.  in digital cellular networks where we call it time-division multiple access (TDMA). One user occupies one time slot of a frame, and the time-division principle allows multiple users to access the network at the same time using the same carrier frequency. 37
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  37. 37. PCM The analog signal is a 4-kHz. fm Sampling rate fs =2 fm =8 Ksample/sec Each sample is quantized into 1 of 256 levels Then encoded into digital eight-bit words. fs =8 Ksample/sec * 8 bit/sample = 64 Kbit/sec 39
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  40. 40. Channel Coding 42
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  42. 42.  A way of encoding data in a channel that adds patterns of redundancy into the transmission path in order to lower the error rate. Channel coding is often called forward error correction FEC 44
  43. 43.  The key idea of FEC is to transmit enough redundant data to allow receiver to recover from errors all by itself. No retransmission sender required. The major categories of FEC codes are Cyclic codes, Reed-Solomon codes (Not covered here), Convolutional codes, Block codes, Turbo codes, etc. 45
  44. 44. Digital Modulation 46
  45. 45.  the objective of a digital communication system is to transport digital data between two or more nodes. In radio communications this is usually achieved by adjusting a physical characteristic of a sinusoidal carrier, either the frequency, phase, amplitude or a combination thereof. 47
  46. 46.  This is performed in real systems with a modulator at the transmitting end to impose the physical change to the carrier and a demodulator at the receiving end to detect the resultant modulation on reception. 48
  47. 47.  In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passband signal 49
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  49. 49.  In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation can be considered as digital-to-analog and the corresponding demodulation or detection as analog-to- digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet). 51
  50. 50.  Fundamental digital modulation methods|: PSK (phase-shift keying), a finite number of phases are used. FSK (frequency-shift keying) finite number of frequencies are used. 52
  51. 51.  ASK (amplitude-shift keying), finite number of amplitudes are used. QAM (quadrature amplitude modulation), a finite number of at least two phases, and at least two amplitudes are used. 53
  52. 52. )Phase Shift Keyed (PSK Is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Each pattern of bits forms the symbol that is represented by the particular phase. 54
  53. 53. Binary Phase Shift Keyed)(BPSK The simplest form of phase modulation is binary that use two carrier phases modulation. With theoretical BPSK the carrier phase has only two states, +/- p/2. Obviously the transition from a one to a zero, or vice versa, 55
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  55. 55. Quadrature Phase Shift Keyed)(QPSK QPSK uses four carrier phases, each representing two bits of data. Higher order modulation schemes, such as QPSK, are often used in preference to BPSK when improved spectral efficiency is required. 57
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  57. 57. )PSK- 8( using eight different carrier phases. All methods occupy the same frequency band but the bit rate of 8-PSK is three times that of BPSK 59
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  59. 59. )Frequency Shift Keyed (FSK One of the simplest, and widest used method is frequency modulation. The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information. With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency. 61
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  62. 62.  FSK has the advantage of being very simple to generate, simple to demodulate and due to the constant amplitude can use a non-linear PA. Significant disadvantages, however, are the poor spectral efficiency 64
  63. 63. )Amplitude-shift keying (ASK is a form of modulation that represents digital data as variations in the amplitude of a carrier wave. The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. 65
  64. 64.  The level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given. 66
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  67. 67. Quadrature amplitude)modulation (QAM This combination of phase and amplitude modulations. is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK) 69
  68. 68.  In the digital QAM case, a finite number of at least two phases and at least two amplitudes are used. 16QAM & 64QAM most using types of QAM . 16QAM use 4 amplitude level and 4 phase levels. 16QAM use 8 amplitude level and 8phase levels. 70
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  72. 72. Multiple Access Methods 74
  73. 73.  Spectrum can be impossible to get and is very expensive when it is available. multiple access method allows several terminals connected to the same multi- point transmission medium to transmit over it and to share its capacity. The wireless industry uses three distinct techniques to allow multiple users to use the same spectrum. 75
  74. 74. Frequency Division Multiple)Access (FDMA the available spectrum is divided into channels with a given bandwidth. Each user is given only one channel , not certain one, from available range of channel and the receiverand transmitter are tuned to that frequency. 76
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  76. 76. Time Division Multiple Access)(TDMA the process by which each user is given a any one time slot , not certain one, from available range of time slots. The same frequency used for all the users. 78
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  78. 78. Code Division Multiple Access)(CDMA Rather than divide users in time or frequency, each user gets all of the spectrum all of the time. the process by which each user is given a unique code. 80
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  80. 80. Duplex 82
  81. 81.  A duplex communication system is a system composed of two connected devices that can communicate with one another in both directions. Simplex Half-Duplex Full-Duplex 83
  82. 82.  The unidirectional systems that transmit in one direction only are called simplex. the bidirectional systems that are able to transmit in both directions are called duplex systems. We can implement bidirectional information transfer with half- or full-duplex transmission. 84
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