Timing Recovery in Baseband Transmission

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Timing Recovery in Baseband Transmission
2008. 02. 11.
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Contents

• Baseband Digital Communication Model
• Modulator
• Pulse shaping Filter
• Channel
• Timing Recovery
• Nonlinear Spectral Line (NSL) Method
• Gardners Method
• Symbol Decision
• Simulation Tools
• Prelab report
• Reference

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Baseband Digital Communication Model

• Complex value
• Real value

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Modulator in Digital Communication System

• Modulator
• Modulator maps a sequence of arbitrary binary
  bits into a set of corresponding symbols
• Modulation methods BPSK,QPSK (considered in
  this experiment), QAM etc.

• BPSK Modulation Modulator

• BPSK Modulation

• BPSK Modulation

Im
1
0
Re
(1,0)
( -1,0)
( , ) symbol x bit
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Modulator in Digital Communication System

• QPSK Modulation Modulator

• QPSK Modulation

• QPSK Modulator

Each modulation method has the pros and cons
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Pulse Shaping Filter 3, pp 548-561

• Design of Band-Limited signals
• Communication channels is band-limited,
  therefore, transmitted signal has the common form
• If noise corrupted is sampled at times
  ,

ISI

• The Condition for no Intersymbol Interference
  (ISI) is

Raised Cosine Filter is satisfied with this
condition, see experimental materials
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Pulse Shaping Filter (Contd)

• Raised Cosine Signal Spectrum

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Channel 3, pp 231-232

• AWGN Channel is considered in this experiment
• Model for received signal passed through as AWGN
  channel is as follows
• Noise signal denotes additive white Gaussian
  noise (AWGN) process with power spectral density
• means normal distribution with mean
  value 0 and variance

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Timing Recovery 1, pp. 741-746

• Nonlinear Spectral Line (NSL) Method
• Often, even when the mean value of received
  signal is zero, the second and higher moments are
  non-zero and periodic
• This can be observed by passing the received
  signal through memoryless nonlinearity (ex. f(x)
  x2)
• The resulting waveform often has a deterministic
  mean value that is periodic in the symbol rate,
  and a timing tone can be derived from it using a
  bandpass filter

• Block Diagram Timing Tone

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Timing Recovery (Contd) 2, pp. 391-397

• Gardners Method
• The output of TED (Timing Error Detector) is
• S-Curve is the expectation of error signal for

• Block Diagram
• TP/TI Controller tracked the stable points
  (minimal point of timing error)
• See the experimental materials

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Symbol Decision

• Timing tone (or timing phase) is obtained through
  timing recovery
• We sample the received signal corresponding to
  the estimated timing phase
• Symbol Decision is hard decision
• Hard decision maps received signals into close
  set of symbols

Hard decision
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Simulation Tools

• Matlab Tools (strongly recommended)
• Useful functions for simulation are mentioned in
  the experimental materials
• Simulation results are easy to observe
• C/C

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Reference

• 1 E.A.Lee and D.G.Messershmitt, Digital
  Communication, Kluwer Academic Publishers, 1994,
  2nd ed.
• 2 U. Mengali and Aldo N. DAndrea,
  Synchronization Techniques for Digital Receivers.
• 3 J.G.Proakis, Digital Communications, Prentice
  Hall, 2001, 4th ed.