EE 615 Lecture 8

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EE 615 Lecture 8

• Timing Synchronization
• Frequency Offset Errors
• Effect of Frequency Offset error
• Frequency Offset Estimation

Dr. Uf Tureli Department of Electrical and
Computer Engineering Stevens Institute of
Technology Hoboken NJ 07030
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Timing Synchronization

• Packet synchronization
• Random Multiple Access control, receiver does not
  know packet position
• Find start of preamble
• Received signal energy detection
• Double sliding windows packet detection
• Structure of Preamble

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Packet Preamble

• Received signal energy detection moving sum
• A similar repetitive structure exists for the
  cyclic prefix, van de Beek

Double sliding window Packet detection algorithm
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OFDM Timing Estimation Imp.
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OFDM Timing Synchronization Circuitry

• Implement in real hardware.
•  An OFDM timing synchronization ASIC Johansson,
  S. Nilsson, M. Nilsson, P. Electronics,
  Circuits and Systems, 2000. ICECS 2000. The 7th
  IEEE International Conference on , Volume 1 ,
  17-20 Dec. 2000 Page(s) 324 -327 vol.1

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Challenges to timing offset estimation

• Most timing offset estimation assumes AWGN, no
  channel convolution.

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Synchronization

• Before OFDM/MC receiver demodulates, it needs to
  synchronize in time and frequency with the
  transmitter
• Effects of symbol timing misalignment and carrier
  frequency offset, as well as clock offset
• Major synchronization algorithms

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Symbol Timing Offset

• Before performing channel estimation and
  demodulatoin, correct timing for the OFDM/MC
  signal needs to be acquired
• Cyclic prefix eases syncronization errors but
  severe penalties for violation of the cyclic
  prefix

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Carrier Frequency Offset

• Carrier frequency offset (CFO), destroys
  orthogonality and causes intercarrier
  interference (ICI)
• Bit error rate (BER) degradation
• SC transmission BER degradation is dominated by
  the reduction of the useful signal component
• For MC transmission, BER degradation is
  determined by the ICI.

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Receiver Window

• Effect of symbol timing
• Offset on a subcarrier
• Phase rotation ii) ICI
• and
• iii) ISI

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Sampling Clock Frequency Offset

• The timing circuits at the transmitter and
  receiver could have different frequencies
• Most standards make assumptions, typically 20
  ppm, I.e. a sample shift in 2500 samples, for a
  burst of 1000 samples, a shift of 4 samples. In
  systems like DVB much larger

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Carrier Frequency Offset

• Due to carrier frequency difference (offset)
  between transmitter and reference oscillators
• The received samples experience a phase rotation
• Dfc Carrier Frequency Offset
• Sampling clock offset

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Timing and Carrier Frequency Offset Estimation
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Correlation
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Timing With Cyclic Prefix
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Timing Synchronization
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Frequency Synchronization
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Using Cyclic Prefix
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Carrier Frequency Estimation

• Moose,Li technique to estimate frequency offset
  using a repeated data symbol.
• van de Beek, et. Al exploits both the
  redundancy in the cyclic prefix and the pilot
  symbols used for channel estimation. .
• Tureli, Liu exploit known signal structure,
  invariance and orthogality of subspace.

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Diversity Systems

• Tureli, Czylwick multiple antenna diversity
• Borallo, Tureli oversampling diversity.

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Multiple Access Systems

• Cao, Tureli OFDMA Uplink Interleaved Structure
  based high performance, moderate cost
• Bolsckei OFDMA Uplink, Subspace based, low
  performance, high cost
• van de Beek, Barborossa, Tonello banded, cyclic
  redundancy, low performance, low cost

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Signal Model

• Received signal is observed in noise
• The nonlinear effect needs to be compensated at
  the receiver
• Pilots are located on a subset of all (k,l)
  pairs, or in preamble or as repetition

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Case Study Ctnued

• Define normalized MSE
• L no. of data carriers, K no. of OFDM blocks in
  packet, MC no. of random runs, with channel
  generated randomly in each run.

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Case Study Ctnued

• 802.11a pilot pattern is on a rectangular grid.
• How to use the pilots for carrier phase tracking?

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HW

• Implement timing and frequency offset estimation
• J.J van de Beek et. Al
• IEEE Transactions on Signal Processing Vol. 45,
  No 7, July 1997
• Due Nov. 10