Digital communication


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Digital communication

  1. 1. Digital Communication Baseband Shaping for Data Transmission
  2. 2. Discrete PAM Signals PAM Unipolar Polar Bipolar Manchester
  3. 3. Factors:  DC Component  Transmission Bandwidth  Bit Synchronization  Error Detection
  4. 4. Polar Quaternary format
  5. 5. Differentially Encoded Polar Waveform
  6. 6. Inter Symbol Interference (ISI) Intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbol. Causes: 1. Multipath propogation. 2. Bandlimited channels
  7. 7. Pulse generator HT(f) Hr(f) Hc(f) Y(t) Transmission filter Channel Receiving filter Decision device Baseband Binary Data Transmission System Hr(f)
  8. 8. Nyquist’s Criterion for zero ISI  The pulse shaping function p(t) with a fourier transform P(f) is given by  ∑P(f-nRb)=Tb has  p(iTb-kTb)=1 for i=k  P(iTb-kTb)=0 for i≠k this condition is knows as Nyquist criterion for zero ISI. Ideal solution:
  9. 9. PRACTICAL SOLUTIONS  The frequency response of an ideal low pass filter decreases towards zero abruptly which is practically unrealizable.  To overcome this problem raised cosine-roll off Nyquist filter is used.  The amplitude response is as shown:
  10. 10.  P(f) =
  11. 11.  Specifically, α governs the bandwidth occupied by the pulse and the rate at which the tails of the pulse decay.  A value of α = 0 offers the narrowest bandwidth, but the slowest rate of decay in the time domain.  When α = 1, the transmission bandwidth is 2B0, which is twice that of ideal solution.  Conversely, inverse when α = 0, the bandwidth is reduced to B0, implying a factor-of-two increase in data rate for the same bandwidth occupied by a rectangular pulse. But there is a slow rate of decay in the tails of the pulse.  Thus, the parameter α gives the system designer a trade-off between increased data rate and the tail suppression.
  12. 12.
  13. 13. Time Response
  14. 14. Transmission Bandwidth Requirement  From the earlier figure we know that the transmission bandwidth is: B = 2B0 - f1 ………………(1) where B0 =1 / 2Tb Also, a = 1- (f1/B0) » f1= B0(1-a) Therefore (1) becomes, B= 2B0- B0(1-a) » B= B0(1+a) When a=1 B = 2B0 When a=0 B=B0
  15. 15. Eye Diagram
  16. 16. Information given by Eye Diagram  The width of the eye opening defines the time interval over which the received wave can be sampled without error from ISI. It is apparent that the preferred time for sampling is the instant of time at which the eye is open widest.  The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied.  The height of the eye opening, at a specified sampling time, defines the margin over noise.