Orthogonal Frequency Division Multiplexing (OFDM)

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Orthogonal Frequency Division Multiplexing (OFDM)
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OFDM Overview

• OFDM is a combination of modulation and
  multiplexing
• OFDM is a special case of FDM where K orthogonal
  (sub)carriers are used.
• Also called Multi-Carrier Modulation
• OFDM is suitable for channels with time varying
  characteristics
• E.g. multi-path fading channel commonly
  encountered in wireless applications
• OFDM minimizes the impact of inter-symbol
  interference by extending the symbol duration (T
  KTs)

Single Carrier
Multi-Carrier
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Multi-Path Fading Channel
Different multi-path components may interact in a
destructive or constructive way
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• The spectral width of c(t) is called the Doppler
  frequency spread (Bd)
• The time span between the first and the last
  arriving multi-path components in a transmitted
  signal is called multi-path time spread (Tm)
• Coherence bandwidth of the channel Bcb1/Tm
• If the signal bandwidth is less than Bcb ?
  frequency non-selective channel
• If the signal bandwidth is greater than Bcb ?
  frequency selective channel
• Fading channels have frequencies that do not
  allow anything to pass ? deep fade frequencies

C(f)
deep fade frequencies
f
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OFDM-Basic Operation

• Instead of using a single carrier for information
  transmission the channel bandwidth is divided
  into a number of equal bandwidth creating K
  sub-channels
• KW/Df
• Different information symbols can be transmitted
  simultaneously in K sub-channels ? hence FDM
• Carriers used at the different sub-channels are
  orthogonal ? hence orthogonal
• Symbol rate T 1/Df and fk fi n/T, for
  n1,2,

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• Consider the case of 4 sub-carriers (C22C1,
  C33C1, and C44C1)
• The information to be transmitted is 1 1 -1 -1 1
  1 1 -1 1 -1 -1 -1 -1 1 -1 -1 -1 1 1
• A serial to parallel converter distributes the
  information bits to the four carriers

C1 C2 C3 C4
1 1 -1 -1
1 1 1 -1
1 -1 -1 -1
-1 1 -1 -1
-1 1 1 -1
-1 -1 1 1
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Assume the use of BPSK to modulate each carrier
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A Time-Frequency View of OFDM
Time
tn/T
T
2T
C1
C2
C3
Frequency
C4
Time
Frequency
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Modulation and Demodulation in OFDM System
Assume each sub-carrier is modulated by M-ary
QAM, then the signal on the kth sub-carrier is
given by
The OFDM transmitted signal is given by
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The received signal on the kth sub-channel is
At the receiver the correlation receiver
structure needs to be repeated K times, one for
each sub-carrier.
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OFDM Implementation Using FFT
DFT
IDFT
Set fk k/T and x(t) ? x(nT) sampling the
timexis), then the OFDM signal can be
constructed by performing IDFT (IFFT).
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OFDM Transmitter/Receiver Structure
Modulation
Serial to Parallel conversion
IFFT
Parallel to Serial Conversion
D/A
Demodulation
Parallel to Serial Conversion
FFT
Serial to Parallel conversion
A/D