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Windowing
 Looking at an example OFDM signal, sharp phase transitions
caused by the modulation can be seen at the symbol
boundaries.
 As a result, the out-of-band spectrum decreases rather slowly,
according to a Sinc function.
 To make the spectrum go down more rapidly, windowing can
be applied to the individual OFDM symbols.
 Windowing an OFDM symbol makes the amplitude go
smoothly to zero at the symbol boundaries.
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Windowing
 A commonly used window type is the raised cosine window,
which is defined as
t
1 1

cos(

(
1

)) 0  t  Ts
2 2
Ts

w(t )   1
Ts  t  Ts
1 1
 (t  Ts )
  cos(
) Ts  t  (1   )Ts
 2 2
Ts
where α is the rolloff factor.
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Windowing
 The time structure of the OFDM signal looks like following
Figure.
 Note that Ts is the symbol interval, which is shorter than the
total symbol duration because we allow adjacent symbols to
partially overlap in the roll-off region.
Ts  T  TG
T prefix
T
T poatfix
Ts
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