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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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