Patent Grant
7577168
This application claims priority to and the benefit of Korean Patent Application 10-2004-0104356 filed in the Korean Intellectual Property Office on Dec. 10, 2004, the entire content of which is incorporated herein by reference.
(a) Field of the Invention
The present invention relates to a packet communication system, and more particularly to a method for detecting symbol synchronization and an apparatus thereof in the packet communication system.
(b) Description of the Related Art
A method for detecting symbol synchronization that detects timing of a received packet, that is, a boundary between a preamble and a payload or a boundary between preambles in different patterns, is a very important technology in a packet-based transmission system.
In particular, a symbol synchronization method is essential to find a start point of a Fast Fourier Transform (FFT) window for a modulation process by a receiver in an Orthogonal Frequency Division Multiplexing (OFDM) packet.
The symbol synchronization method includes a cross-correlation and an auto-correlation. The cross-correlation correlates a received signal with a known preamble signal, and the auto-correlation correlates a received signal with a delayed received signal. However, the auto-correlation may cause performance degradation due to an interference signal, and the cross-correlation may experience difficulties in detecting end portions of peak values when a large delay spreading value of a channel occurs.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.
The present invention has been made in an effort to provide a method for detecting symbol synchronization in a packet communication system and an apparatus of the same having advantages of high reliability in symbol synchronization detection.
In one aspect of the present invention, an apparatus for detecting symbol synchronization in a received signal includes a first correlator, a second correlator, a third correlator, and a detector. The received signal includes a first preamble and a second preamble which is consecutive to the first preamble. The first correlator calculates a first correlation value between the received signal and a first pattern of a first period among iterative patterns of the first preamble. The second correlator calculates a second correlation value from the received signal and a second pattern of a second period among iterative patterns of the second preamble. The third correlator calculates a third correlation value from the first correlation value and the second correlation value. The detector detectes symbol synchronization from the third correlation value.
In another aspect of the present invention, a method for detecting symbol synchronization in a received signal includes calculating a channel power value from a first pattern of a first period among iterative patterns of the first preamble and a second pattern of a second period among iterative patterns of the second preamble, low-pass filtering the channel power value, and detecting symbol synchronization when an output value of the low-pass filtering exceeds a threshold value or when the output of the low-pass filtering reaches a peak value.
In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.
A method for detecting symbol synchronization and an apparatus thereof will now be described in detail with reference to the accompanying drawings.
For better comprehension and ease of description, a method for detecting a boundary between two preambles in different iterative patterns as shown in
As shown in
In
The matched filter 10 outputs a correlation value xk between a received signal rk and one period signal ak of the preamble 11 by multiplying a received signal rk by a matched filter coefficient. Herein, the matched filter coefficient corresponds to a value a*-k which is obtained by inverting an order of the one period signal ak of the preamble 11 on the time axis, and complex-conjugating the inverted order. In other words, an output xk of the matched filter 10 is given by Equation 1.
xk=rk*a*−k [Equation 1]
Herein, the received signal rk may be expressed by a result of adding a transmission channel condition hk and noise nk to a preamble ak transmitted from a transmission terminal, as shown in Equation 2.
Thus, the correlation value xk between the received signal and the preamble 11 may be expressed as given in Equation 3.
rk=ak*hk+nk [Equation 2]
xk=rk*a*−k=(ak*hk+nk)*a*−k=ak*hk*a*−k+nk*a*−k [Equation 3]
If we assume that the auto-correlation of a preamble sequence is a delta function δk, which is an ideal characteristic of a preamble, the correlation value xk between the received signal and the preamble 11 may be expressed as Equation 5. If we assume that interference and noise do not exist, the correlation value xk between the received signal and the preamble 11 become equivalent to a channel response coefficient hk.
ak*a*−k≈δk [Equation 4]
xk≈δk*hk+wk=hk+wk≈hk [Equation 5]
The matched filter 20 outputs a correlation value yk between a received signal rk and a signal bk corresponding to a portion of the preamble 12 by multiplying the received signal rk by the matched filter coefficient, the portion bk corresponding to the length of T2 of one period signal of the preamble 12. In this instance, the matched filter coefficient corresponds to a value b*−k which is obtained by inverting an order of the signal bk corresponding to the portion of T2 of one period of the preamble 12 on the time-axis and complex-conjugating the signal of the inverted order. In other words, an output yk of the matched filter 10 is given by Equation 6.
yk=rk*b*−k [Equation 6]
As above-described, if we assume that the auto-correlation of the preambles is the delta function, the correlation value yk between the received signal and the preamble 12 becomes the channel response coefficient hk. In addition, if we assume that the preamble 12 starts when k=0, Equation 6 may be expressed as shown in Equation 7.
yk≈hk−T2 [Equation 7]
The delaying unit 30 delays the correlation value xk of the matched filter 10 by the length of T2, and the complex conjugator 40 outputs a complex conjugate value (xk−T2)* of a delayed correlation value xk−T2. The correlator 50 outputs a correlation value zk between the output (xk−T2)* of the complex conjugator 40 and the output yk of the matched filter 20, and this correlation value zk is obtained by Equation 8. In other words, a power value of a channel is obtained by delaying output values of the matched filters 10 and 20 on the time-axis, rearranging the delayed output values, and correlating the rearranged output values.
zk=yk·(xk−T2)*≈hk−T2)*=|hk−T2|2 [Equation 8]
The moving average unit 60 calculates a moving average value mk having a window size that corresponds to one period T1 of the preamble 11 with respect to the correlation value zk. A frequency response characteristic of such a moving average unit 60 becomes a sinc function (i.e., a function proportional to sin(T1*f/(T1*f)). Herein, the sinc function is operated as a low-pass filter. A bandwidth of the low-pass filter is inversely proportional to a window size T1 of the moving average unit 60. In other words, when the window size of the moving average unit 60 is increased, the bandwidth is reduced and thus noise is remarkably reduced.
The real-number generator 70 takes an absolute value |Re{mk}| of a real number value of the moving average value mk as shown in Equation 10, and the determiner 80 detects symbol (or frame) synchronization when the absolute value exceeds a threshold value. The threshold value is determined to be equivalent to a half of the channel power value, and a channel power may be measured by channel estimation. The determiner 80 may detect symbol synchronization when the absolute value |Re{mk}| reaches a peak value, in addition to the case when the absolute value |Re{mk}| exceeds the threshold value. Herein, durations of start and end points for detecting a peak value may be predetermined to enhance performance of symbol synchronization detection.
In addition, the symbol synchronization detecting apparatus may further include a frequency corrector 90 correcting a frequency offset when the frequency offset is generated due to a carrier. The frequency corrector 90 corrects the frequency offset by multiplying the moving average value mk by a frequency correction value e−T2πΔf. Herein, Δf is a correction frequency.
With reference to
According to the first embodiment of the present invention, a cross-correlation value of the preamble and the received signal is obtained by using the matched filter, but it may be obtained by using other methods.
In the first embodiment of the present invention, when preambles different in patterns are arranged consecutive to a starting part of a packet, a power value of a channel detected by cross-correlating cross-correlation values which have been obtained between the received signal and the respective preambles is used to detect the symbol synchronization. Accordingly, the symbol synchronization may be reliably detected in a channel where interference and noise are unavoidable.
The symbol synchronization detecting apparatus of
As shown in
The reason that an output of the moving average unit 60′ of
As described, the moving average unit 60′ outputs a moving average value which corresponds to the sum of T1 number of correlation values zk. Similar to the moving average unit 60 of the first embodiment, the moving average unit 60′ requires (T1+1) number of delaying units (registers). However, the number of adders may be remarkably reduced compared to the moving average unit 60 of the first embodiment since the moving average unit 60′ requires one adder 62 and one subtracter 63.
A method for reducing the number of delaying units will be described in comparison with the first and second embodiments of the present invention, with reference to
As shown in
In more detail, the low-pass filter 60″ includes multipliers 65 and 68, an adder 66, and a delaying unit 67. The multiplier 65 multiplies the correlation value by a first coefficient (e.g., 0.25), and the multiplier 68 multiplies an output m′k−1 of the delaying unit 67 by a second coefficient (e.g., 0.75). The adder 66 adds outputs of the multipliers 65 and 68 and outputs an adding result, and the delaying unit 67 delays an output of the adder 66 and outputs a delayed output as an output m′k of the low-pass filter 60″ Herein, the first and second coefficients are positive numbers less than 1, and the second coefficient is greater than the first coefficient.
Thus, the output m′k of the low-pass filter 60″ is given by Equation 11. When the output m′k is converted into a frequency-domain value, the output m′k is given in a form which is similar to the above-described moving average.
m′k=0.75m′k−1+0.25zk [Equation 11]
The determiner 80′ detects a peak value of an absolute value of a real number part in the output m′k of the low-pass filter 60″, and finds symbol synchronization (or frame synchronization) with reference to the absolute value at the peak value.
In addition, similar to the determiner 60 in the first and second embodiments, the determiner 80′ may detect symbol synchronization when the absolute value |Re{m′k}| of the real number part exceeds the threshold value.
As described, the number of delaying units may be reduced compared to that of the second embodiment since the moving average may be calculated using one delaying unit according to the third embodiment of the present invention.
According to the embodiments of the present invention, cross-correlation values are obtained between a received signal and each preamble pattern, and a channel power value is obtained by cross-correlating the cross-correlation values to thereby detect symbol synchronization using the channel power value. Accordingly, the symbol synchronization may be reliably detected in a channel where interference and noise are unavoidable.
While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2004-0104356 | Dec 2004 | KR | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5815541 | Fukushi | Sep 1998 | A |
| 5883929 | Wang et al. | Mar 1999 | A |
| 5909462 | Kamerman et al. | Jun 1999 | A |
| 6259724 | Esmailzadeh | Jul 2001 | B1 |
| 6526035 | Atarius et al. | Feb 2003 | B1 |
| 6549564 | Popovic | Apr 2003 | B1 |
| 6563856 | O'Shea et al. | May 2003 | B1 |
| 6567482 | Popovic′ | May 2003 | B1 |
| 6859899 | Shalvi et al. | Feb 2005 | B2 |
| 7020218 | Arnesen | Mar 2006 | B2 |
| 7023928 | Laroia et al. | Apr 2006 | B2 |
| 7099422 | Hoctor et al. | Aug 2006 | B2 |
| 7145955 | Bohnke et al. | Dec 2006 | B1 |
| 7251282 | Maltsev et al. | Jul 2007 | B2 |
| 7251288 | Imamura | Jul 2007 | B2 |
| 7274757 | Zhou et al. | Sep 2007 | B1 |
| 7349461 | Glazko et al. | Mar 2008 | B2 |
| 7391828 | Liu et al. | Jun 2008 | B2 |
| 20050047384 | Wax et al. | Mar 2005 | A1 |
| 20070211835 | Inagawa et al. | Sep 2007 | A1 |
| Number | Date | Country |
|---|---|---|
| 1049302 | Nov 2000 | EP |
| 2000-0077075 | Dec 2000 | KR |
| 2001-0007391 | Jan 2001 | KR |
| WO 0054424 | Sep 2000 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20060126670 A1 | Jun 2006 | US |
