FFT window spread generating method
View Patent ↗The present invention provides an FFT window spread generating method used in the modulation of an OFDM system. When OFDM-modulated received signals each having a time length greater than or equal to an effective symbol length are demodulated by FFT processing using a spread FFT window, the spread FFT window makes use of received signals each having the time length greater than or equal to the effective symbol length. Spreading windows spread before and/or after the effective symbol length are provided. Received signals lying within the spreading windows are added to their corresponding received signals lying within the effective symbol length and different in time position by the effective symbol length, and the amplitudes of the added received signals are reduced to half respectively, thereby generating the spread FFT window.
1. An FFT window spread generating method comprising:
when OFDM-modulated received signals each having a time length greater than or equal to an effective OFDM symbol length are demodulated by FFT processing using a spread FFT window, combining the received signals located before and/or after the effective OFDM symbol length by a predetermined arithmetic operation in a FFT window generating arithmetic unit to generate the spread FFT window, and
using the received signals each having the time length greater than or equal to the effective OFDM symbol length, providing spreading windows spread before and/or after the effective OFDM symbol length, adding together the received signals lying within the spreading windows to the received signals which correspond to the received signals lying within the effective OFDM symbol length and are different in time position by the effective OFDM symbol length, and reducing the amplitudes of the added received signals to half respectively, thereby generating the spread FFT window.
2. The FFT window spread generating method according to claim 1 , further including storing the OFDM-modulated received signals each having the time length greater than or equal to the effective OFDM symbol length in a memory and generating the spread FFT window by the FFT window generating arithmetic unit using the stored received signals.
3. The FFT window spread generating method according to claim 1 , wherein the predetermined arithmetic operation is:
fft_in
(
T
)
=
{
(
rx_sig
(
T
)
+
T
≤
spr_win
_size
rx_sig
(
T
+
fft_t
_length
)
}
/
2
;
rx_sig
(
T
)
;
spr_win
_size
<
T
≤
(
fft_t
_length
-
spr_win
_size
)
(
rx_sig
(
T
-
fft_t
_length
)
+
fft_t
_length
-
spr_win
_size
<
T
rx_sig
(
T
)
}
/
2
;
where
fft_in(T) is an FFT input signal,
T is a sampling time of a received signal,
rx_sig(T) is the received signal,
fft_t_length is an FFT input signal length identical to the effective symbol length W 0 , and
spr_win_size is spreading windows W 1 and W 2 .
4. An FFT window spread generating method comprising:
when OFDM-modulated received signals each having a time length greater than or equal to an effective OFDM symbol length are demodulated by FFT processing using a spread FFT window, combining the received signals located before and/or after the effective OFDM symbol length by a predetermined arithmetic operation in a FFT window generating arithmetic unit to generate the spread FFT window, and
using the received signals each having the time length greater than or equal to the effective OFDM symbol length, providing spreading windows spread before and/or after the effective OFDM symbol length, multiplying the received signals which correspond to received signals lying within the effective OFDM symbol length and are different in time position by the effective OFDM symbol length, by window functions which are taken as coefficients not greater than 1 by the lengths of the spreading windows and have tilts with respect to time respectively, multiplying the received signals lying within the spreading windows by other coefficients obtained by subtracting 1 from the previous coefficients respectively, and adding together the two received signals multiplied by drift coefficients by the effective OFDM symbol length, thereby generating the spread FFT window.
5. The FFT window spread generating method according to claim 4 , wherein the predetermined arithmetic operation is:
fft_in
(
T
)
=
{
rx_sig
(
T
)
*
α
(
T
)
+
T
≤
spr_win
_size
rx_sig
(
T
+
fft_t
_length
)
*
{
1
-
α
(
T
)
}
;
rx_sig
(
T
)
;
spr_win
_size
<
T
≤
(
fft_t
_length
-
spr
rx_sig
(
T
-
fft_t
_lenght
)
*
fft_t
_length
-
spr_win
_size
<
T
{
1
-
α
(
fft_t
_length
-
T
)
}
+
rx_sig
(
T
)
*
α
(
fft_t
_length
-
T
)
;
where
fft_in(T) is an FFT input signal,
T is a sampling time of a received signal,
rx_sig(T) is the received signal,
fft_t_length is FFT input signal length an FFT input signal length identical to the effective symbol length W 0 , and
spr_win_size is spreading windows W 1 and W 2 .
6. The FFT window spread generating method according to claim 4 , further including storing the OFDM-modulated received signals each having the time length greater than or equal to the effective OFDM symbol length in a memory and generating the spread FFT window by the FFT window generating arithmetic unit using the stored received signals.