Multiplex communications – Generalized orthogonal or special mathematical techniques – Fourier transform
Reexamination Certificate
2000-07-25
2004-09-28
Vu, Huy D. (Department: 2665)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Fourier transform
C375S354000
Reexamination Certificate
active
06798738
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an orthogonal frequency division multiplexing (OFDM) system, and particularly, to a fast Fourier transform (FFT) window position recovery apparatus and method in an OFDM receiver.
BACKGROUND ART
FIG. 1
is a block diagram showing the configuration of a general OFDM system receiver. First, a symbol of an OFDM signal, when N subcarriers are used in the OFDM system, comprises N useful data samples output by transmitting inverse fast Fourier transform (IFFT) and a guard interval having the lengths of G samples inserted before a useful data interval to prevent interference between the symbols. Here, the guard interval copies the end portion of the useful data interval. A transmitter (not shown) adds G complex values to N complex values output by an inverse fast Fourier transformer (IFFT), and sequentially transmits a symbol comprised of a total of (G+N) samples. Here, the guard interval is generally longer than a delay spread time of a channel. For example, a European digital TV broadcast standard defines guard intervals having lengths of {fraction (1/4, 1/8, 1/16)} and {fraction (1/32)} of an actual symbol length, and a transmission side selects and uses one among the above-described guard intervals. Time synchronization must be accurately performed to allow a receiver to recover a received OFDM signal. Time synchronization is comprised of FFT window position recovery for accurate parallel processing of a signal, and sampling clock recovery for sampling a signal having a maximum signal-to-noise ratio (SNR). A j-th symbol comprised of a useful interval and a guard interval which are output by an IFFT (not shown) of a transmitter is expressed by the following Equation 1:
s
j
=
∑
n
=
-
G
N
-
1
x
j
,
n
=
∑
n
=
-
G
-
1
∑
k
=
0
N
-
1
X
j
,
k
ⅇ
j
2
π
(
N
+
n
)
/
N
+
∑
n
=
0
N
-
1
∑
k
=
0
N
-
1
X
j
,
k
ⅇ
j2
π
k
n
/
N
(
1
)
wherein j denotes a symbol number, k is a carrier index, N is the number of effective data samples, and n indicates a sampling time. In the right side of Equation 1, the first term is a guard interval portion, and the second term is a useful data portion.
As shown in
FIG. 1
, an analog-to-digital converter (ADC)
110
samples a received OFDM signal. A symbol start detector
120
receives the sampled OFDM signal and detects a start portion of a symbol. A FFT window controller
130
designates a FFT window point in time to activate a useful data portion of a FFT
140
using the symbol start information detected by the symbol start detector
120
. As shown in
FIG. 2
, the FFT window transmits only the useful data interval excluding the guard interval in a received signal to the FFT. Generally, the symbol start detector
120
of
FIG. 1
detects a symbol start portion using either a correlation value between received signals or the quantized values of the input signals. However, the first way requires a maximum position of the correlation value between the received signals, resulting in complicated system realization. The second way provides a simple structure because of the 2-bit quantization. However, when a frequency offset defined as a carrier frequency synchronization error, i.e., a phase difference of e
j(&ohgr;
0
t+&phgr;)
, exists, phase rotation of the input signal occurs to thus change the phases of a guard interval and the end portion of a useful data interval. Thus, a position having a maximum correlation value cannot be found, and symbol starting position recovery is thus difficult.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a method for recovering a phase error of a FFT window using a power difference between received signals in a predetermined section in an orthogonal frequency division multiplexer (OFDM) receiver.
It is another object of the present invention to provide an apparatus for recovering a phase error of a FFT window using a power difference between received signals in a predetermined section in an OFDM receiver.
To accomplish the first object, there is provided a fast Fourier transform (FFT) window position recovery method in an orthogonal frequency division multiplexing (OFDM) system receiver which receives an OFDM symbol comprised of a useful data interval and a guard interval and recovers a FFT window position, the method comprising the steps of: (a) converting a received OFDM signal into a digital complex sample; (b) obtaining a power difference between the digital complex sample output by the step (a) and a complex sample value delayed by predetermined samples, and detecting a position having a maximum power value between the samples, as a symbol starting position; and (c) activating FFT while moving the FFT window position using symbol start information detected by the step (b).
To accomplish the second object, there is provided a fast Fourier transform (FFT) window position recovery apparatus in an orthogonal frequency division multiplexing (OFDM) system receiver which receives an OFDM symbol comprised of a useful data interval and a guard interval and recovers a FFT window position, the apparatus comprising: an analog-to-digital converter (ADC) for converting a received OFDM signal into a digital complex sample; a symbol start detector for detecting a power difference between the digital complex samples output by the ADC and detecting a position having a minimum absolute value of the power difference between the samples, as a symbol starting position; and a FFT window controller for activating FFT by moving the FFT window position using symbol start information detected by the symbol start detector.
REFERENCES:
patent: 5761190 (1998-06-01), Yamauchi et al.
patent: 5812523 (1998-09-01), Isaksson et al.
patent: 6226337 (2001-05-01), Klank et al.
patent: 2003/0142764 (2003-07-01), Keevill et al.
patent: 0 730 357 (1998-09-01), None
patent: 2 318 953 (1998-05-01), None
patent: 6-244818 (1994-09-01), None
patent: 9-200176 (1997-07-01), None
patent: 97/07 620 (1997-02-01), None
Cho Hong-bae
Choi Hyung-jin
Do Sang-hyun
Mattis Jason
Sughrue & Mion, PLLC
Vu Huy D.
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