EP2092708A1

EP2092708A1 - Apparatus and method for receiving orthogonal frequency division multiplexing-based digital signal

Info

Publication number: EP2092708A1
Application number: EP07833482A
Authority: EP
Inventors: Yeonsu Kang; Ho-Jin Lee; Do-Seob Ahn; Kunseok Kang
Original assignee: Electronics and Telecommunications Research Institute ETRI
Current assignee: Electronics and Telecommunications Research Institute ETRI
Priority date: 2006-12-05
Filing date: 2007-10-22
Publication date: 2009-08-26
Also published as: WO2008069437A1; KR20080052192A; KR100867999B1; EP2092708A4

Abstract

A method and apparatus for receiving a digital signal that is modulated in an orthogonal frequency division multiplexing (OFDM) method are provided. The apparatus includes: a signal receiver receiving a digital signal which is modulated in an OFDM method that uses a preamble including a constant amplitude zero autocorrelation (CAZAC) sequence; a time synchronization estimator calculating a time when inter-symbol interference (ISI) is minimized by measuring a degree of cross-correlation between the received digital signal and the preamble and a power value of the received digital signal and estimating the time when the ISI is minimized to be a time synchronization point; a channel estimator estimating a channel compensation value through a first fast Fourier transform (FFT) using the time synchronization point and the degree of the cross-correlation; and a data recoverer recovering data by using the time synchronization point and the channel compensation value.

Description

APPARATUS AND METHOD FOR RECEIVING ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING- BASED DIGITAL SIGNAL

Technical Field

[1] The present invention relates to a method and apparatus for receiving an orthogonal frequency division multiplexing (OFDM) based digital signal, and more particularly, to a method and apparatus for receiving an OFDM-based digital signal capable of estimating time synchronization and a channel in which inter-symbol interference (ISI) is minimized by using a preamble including a constant amplitude zero autocorrelation (CAZAC) sequence in an OFDM-based transceiving system. Background Art

[2] An orthogonal frequency division multiplexing (OFDM) transmission method is designed to transmit data with a long symbol period by using a plurality of subcarriers in a parallel manner. The OFDM transmission method performs a modulation/demodulation process at high speed by using an inverse fast Fourier transform (IFFT) and a fast Fourier transform (FFT). The OFDM transmission method is robust to inter- symbol interference (ISI) that is a main problem of high speed cαrmunication. In addition, the OFDM transmission method has an advantage in that frequency selective fading is shown as non-selective fading. Due to this advantage, a fast data transmission system, for example, an OFDM wireless cαrmunication system such as a digital audio broadcasting (DAB) system, a digital video broadcasting (DVB) system, a Digital Terrestrial Television Broadcasting (DTTB) system, a wireless local area network (WLAN) system, a broadband wireless access system of IEEE 802.16, and the like has been developed. Recently, the OFDM wireless communication system has come to be regarded as an essential technique of the next generation mobile carmunication and is being actively researched.

[3] In the OFDM method, a transmitter generates a signal to be transmitted by performing IFFT. At this time, a receiver sets a suitable FFT section by performing time synchronization during a preamble section so as to recover the transmitted signal. In addition, a channel compensation value is estimated so as to compensate for distortion of a value after FFT is performed, which is caused by a channel.

[4] It is possible to maintain all the advantages of the OFDM transmission method, when accurate time synchronization for performing the FFT and accurate estimation of a channel based on the accurate time synchronization are assumed. However, in a multi- path channel environment, accuracy of time synchronization decreases. Accordingly, a time synchronization offset caused by the inaccuracy of the time synchronization may generate a serious ISI and inter-carrier interference (ICI), thereby causing serious deterioration in performance of the system. The deterioration in performance becomes serious as the maximum or mean delay time of the multi-path channel becomes long. Specifically, when the delay of the channel is longer than a protection section, large ISI is generated in seme cases even for an accurate time synchronization point. Disclosure of Invention Technical Problem

[5] The present invention provides a method and apparatus for receiving an orthogonal frequency division multiplexing (OFDM) based digital signal of high and stable performance by detecting a time synchronization point by searching, not for an accurate FFT execution time, but for a FFT execution time of a location where inter- symbol interference (ISI) is minimized in a multi-path channel environment and calculating a channel compensation value in a frequency domain in which the ISI and inter-carrier interference (ICI) are minimized by using a result value from a series of operations needed for performing time synchronization. Technical Solution

[6] According to an aspect of the present invention, there is provided an apparatus for receiving a digital signal, the apparatus comprising: a signal receiver receiving a digital signal which is modulated in an OFDM (orthogonal frequency division multiplexing) method that uses a preamble including a CAZAC (constant amplitude zero autocorrelation) sequence; a time synchronization estimator calculating a time when ISI (inter-symbol interference) is minimized by measuring a degree of cross- correlation between the received digital signal and the preamble and a power value of the received digital signal and estimating the time when the ISI is minimized to be a time synchronization point; a channel estimator estimating a channel compensation value by performing a first FFT (fast Fourier transform) using the time synchronization point and the degree of the cross-correlation; and a data recoverer recovering data by using the time synchronization point and the channel compensation value.

[7] According to another aspect of the present invention, there is provided a method of receiving a digital signal comprising: (a) receiving a digital signal which is modulated in an OFDM (orthogonal frequency division multiplexing) method that uses a preamble including a CAZAC (constant amplitude zero autocorrelation) sequence; (b) calculating a time when ISI (inter-symbol interference) is minimized by measuring a degree of cross-correlation between the received digital signal and the preamble and a power value of the received digital signal and estimating the time when the ISI is minimized to be a time synchronization point; (c) estimating a channel compensation value by performing a first FFT (fast Fourier transform) using the time synchronization point and the degree of the cross-correlation; and (d) recovering data by using the time synchronization point and the channel compensation value. Advantageous Effects

[8] As described above, since a time when the ISI is minimized is set as an FFT execution point in the OFDM system according to an embodiment of the present invention, it is possible to improve and stabilize the performance of the OFDM receiving apparatus by minimizing the ISI. In addition, it is possible to embody a receiving apparatus with low complexity and high performance by minimizing the ISI by using a result of an operation performed for time synchronization and estimating an accurate channel in which the ICI caused by the ISI is minimized, at the same time. Description of Drawings

[9] The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

[10] FIG. 1 is a schematic diagram illustrating a structure of an apparatus for receiving an orthogonal frequency division multiplexing (OFDM) based digital signal according to an embodiment of the present invention;

[11] FIG. 2 illustrates a preamble structure of an OFDM based digital signal according to an embodiment of the present invention;

[12] FIG. 3 A and 3B illustrate a preamble structure of an OFDM based digital signal according to another embodiment of the present invention;

[13] FIG. 4 illustrates a structure of an apparatus for receiving an OFDM based digital signal according to an embodiment of the present invention; and

[14] FIG. 5 is a flowchart of a method of receiving an OFDM based digital signal according to an embodiment of the present invention. Best Mode

[15] FIG.l is a schematic diagram illustrating a structure of an apparatus for receiving an orthogonal frequency division multiplexing (OFDM) based digital signal according to an embodiment of the present invention.

[16] Referring to FIG. 1, the apparatus for receiving the OFDM based digital signal includes a signal receiver 100, a time synchronization estimator 101, a channel estimator 102, and a data recoverer 103.

[17] The signal receiver 100 receives a digital signal that is modulated in an OFDM method using a preamble including a sequence of which an autocorrelation degree is one.

[18] The time synchronization estimator 101 calculates a time when inter-symbol interference (ISI) is minimized by measuring a degree of cross-correlation between the digital signal received by the signal receiver 100 and the preamble included therein and a power of the received digital signal and estimates the time when the ISI is minimized to be a time synchronization point.

[19] The channel estimator 102 estimates a channel compensation value by a first fast

Fourier transform (FFT) using the time synchronization point estimated by the time synchronization estimator 101 and the measured degree of cross-correlation.

[20] The data recoverer 103 recovers data by using the time synchronization point estimated by the time synchronization estimator 101 and the channel compensation value estimated by the channel estimator 102.

[21] FIG. 5 is a flowchart of a method of receiving an OFDM based digital signal according to an embodiment of the present invention.

[22] Referring to FIG. 5, an OFDM modulated digital signal that includes a preamble including a CAZAC sequence is received (operation S510).

[23] Next, a time when the ISI is minimized is calculated by measuring a degree of the cross-correlation between the received digital signal and the preamble included therein and a power value of the received digital signal, and the time when the ISI is minimized is estimated to be a time synchronization point (operation S520).

[24] Thereafter, the channel compensation value is estimated by performing the first FFT by using the estimated time synchronization point and the measured degree of cross- correlation (operation S530).

[25] Finally, data is recovered by using the estimated time synchronization point and the estimated channel compensation value (operation S540). Mode for Invention

[26] FIG. 2 illustrates a preamble structure of an OFDM based digital signal according to an embodiment of the present invention.

[27] Referring to FIG. 2, the preamble of the OFDM based digital signal includes a constant amplitude zero auto correlation (CAZAC) sequence in a time area. Structurally, the preamble of the OFDM based digital signal includes repeated values so as to maintain a zero auto-correlation (ZAC) characteristic during a predetermined section in both ends of the CAZAC sequence. As shown in FIG. 2, sequences A and B with lengths Gl and G2 are located in both ends of the CAZAC sequence having a length N.

[28] In the CAZAC sequence, the amplitude of each sample has a predetermined value.

The amplitude of each sample has a value of 1 with respect to accurate autocorrelation and has a value of 0 with respect to cyclic autocorrelation mathematically. This feature is represented as a noise power or ISI in sections except the accurate autocorrelation section when correlation between the CAZAC sequence and the received signal is obtained in a receiving apparatus of communication.

[29] That is, the autocorrelation value of the CAZAC sequence has an impulse shape.

Because of this characteristic, the CAZAC sequence is frequently considered so as to detect and synchronize signals of a communication system.

[30] When the CAZAC sequence is referred to as C, the characteristic of the preamble is summarized as shown in Equation 1.

[31] [Equation 1]

[32] C = [C₁, C₂, ..., C_N]

[34] B = [c , ..., c ]

1 G2

[35] FIG. 3A and 3B illustrate a preamble structure of an OFDM based digital signal according to another embodiment of the present invention.

[36] Referring to FIG. 3A, the preamble of the digital signal to which the present invention is applied has a structure in which a CAZAC sequence with the same value is repeated three times.

[37] Referring to FIG. 3B, the preamble of the digital signal to which the present invention is applied has a structure which concurrently includes a cyclic prefix(CP) and a cyclic suffix(CS).

[38] FIG. 4 illustrates a structure of an apparatus for receiving an OFDM based digital signal according to an embodiment of the present invention. In FIG. 4, the time synchronization estimator 101, the channel estimator 102, and the data recoverer 103 which are included in the receiver of FIG. 1 are illustrated in detail. Referring to FIG. 4, the receiver includes a preliminary time synchronization estimator 410, a cross- correlation unit 420, a timing metric calculator 430, a moving total power calculator 440, a final time synchronization estimator 450, an impulse response output unit 460, a first fast Fourier transformer 470, a second fast Fourier transformer 480, and an equalizer 490.

[39] The preliminary time synchronization estimator 410 estimates a location index i for roughly performing FFT, which can be obtained by performing a simple operation to be the preliminary time synchronization point. A typical method of estimating the preliminary time synchronization uses a correlation characteristic between a preamble and a cyclic prefix. For example, P(i) calculated in Equation 2 may be normalized by a power of the received signal.

[40] [Equation 2]

[41] [Math.l]

The cross-correlation unit 420 measures a degre e of cross-correlation between the received signal and the preamble C during an arbitrary section with respect to a location index i that is the preliminary time synchronization point estimated by the preliminary time synchronization estimator as shown in Equation 3. At this time, a length (V = a + b) of the arbitrary section in which the cross-correlation is detected may be arbitrarily set in consideration of the performance of a method of obtaining the preliminary time synchronization and lengths Gl and G2 of protection sections.

[42] [Equation 3]

[43] [Math.2]

N-I m=0

[44] Here, C(m) indicates an m-th value of the CAZAC sequence, and N indicates a length of the CAZAC sequence.

[45] The timing metric calculator 430 calculates a timing metric M(i) by using the degree of cross-correlation measured by the cross-correlation unit 420. The timing metric is obtained by normalizing information on the cross-correlation between the received signal and the preamble by the power of the received signal. A method of obtaining the timing metric is shown in Equation 4.

[46] [Equation 4] [47] [Math.3]

PQ

M(I)

R(I)

[48] At this time, the absolute value of P(i) may be used instead of in [49] [Math.4]

a numerator so as to obtain the timing metric. R(i) in the denominator is used to normalize the timing metric M(i). R(i) uses the power of the estimation section. A method of calculating R(i) may be represented by Equation 5.

[50] [Equation 5] [51] [Math.5]

m=0

[52] The moving total power calculator 440 calculates the moving total power or moving mean power during a moving section that is arbitrarily set for the timing metric M(i) calculated by the timing metric calculator 430. This process has an objective of obtaining the total power of the received signal. The process may be represented by Equation 6 or 7.

[53] [Equation 6] [54] [Math.6]

n=0

[55] [Equation 7] [56] [Math.7]

[57] Here, a size W of the moving section that is used to obtain the moving total power or moving mean power may be arbitrarily determined in consideration of features of a system. When ISI and inter-carrier interference (ICI) are considered, the length of the cyclic prefix of the OFDM may be the most preferable value of the size W of the moving section.

[58] The final time synchronization estimator 450 detects an index value i when the

FFT moving total power AP(i) calculated by the moving total power calculator 440 is maximized from the moving total power AP(i) and estimates the index value i to be

FFT the final time synchronization point. This may be represented by Equation 8. [59] [Equation 8]

[60] [Math.8] i_FFT = MAX AP(i) i

[61] The second fast Fourier transformer 480 recovers a signal in a frequency domain by selecting N numbers of samples (that is, [r(i ), ..., r(i + N - 1)]) from the time syn-

FFT FFT chronization index i and performing the second FFT with respect to the selected

FFT samples. However, since the recovered signal in the frequency domain is distorted due to a channel, a channel compensation value in the frequency domain is needed so as to compensate for the distortion.

[62] The impulse response output unit 460 can simply obtain the channel compensation value in the frequency domain by using a value obtained through an operation performed in the time synchronization process. The process of obtaining the channel compensation value is described as follows. As shown in FIG. 9, L numbers of values are selected from an output value i of the final time synchronization estimator 450

FFT among the output values P(i) of the cross-correlation unit 420. [63] [Equation 9]

[64] h = [P(i ), ..., P(i +L-I)]

FFT FFT

[65] Here, L becomes ideally a value of a maximum delay tap of the channel. In general,

L may be set so as to be the same as the length of the cyclic prefix (CP). Finally, h that is defined in Equation 9 becomes an impulse response of the channel.

[66] The first fast Fourier transformer 470 performs the first FFT with respect to the impulse response that is output by the impulse output unit 460. In this case, the channel compensation value that is estimated by performing the first FFT minimizes the ISI. Finally, the channel compensation value that is estimated by performing the first FFT becomes the channel compensation value which minimizes the ICI generated by the ISI.

[67] The equalizer 490 recovers data by compensating a signal in the frequency domain which is output by the second fast Fourier transformer 480 with a channel value in the frequency domain which is output by the first fast Fourier transformer 470.

[68] As described above, since a time when the ISI is minimized is set as an FFT execution point in the OFDM system according to an embodiment of the present invention, it is possible to improve and stabilize the performance of the OFDM receiving apparatus by minimizing the ISI. In addition, it is possible to embody a receiving apparatus with low complexity and high performance by minimizing the ISI by using a result of an operation performed for time synchronization and estimating an accurate channel in which the ICI caused by the ISI is minimized, at the same time.

[69] The invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. Also, functional programs, codes, and code sections for accomplishing the present invention can be easily construed by programmers of ordinary skill in the art to which the present invention pertains.

[70] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing frαn the spirit and scope of the present invention as defined by the appended claims.

Claims

[1] An apparatus for receiving a digital signal, the apparatus comprising: a signal receiver receiving a digital signal which is modulated in an OFDM (orthogonal frequency division multiplexing) method that uses a preamble including a CAZAC (constant amplitude zero autocorrelation) sequence; a time synchronization estimator calculating a time when ISI (inter-symbol interference) is minimized by measuring a degree of cross-correlation between the received digital signal and the preamble and a power value of the received digital signal and estimating the time when the ISI is minimized to be a time synchronization point; a channel estimator estimating a channel compensation value by performing a first FFT (fast Fourier transform) using the time synchronization point and the degree of the cross-correlation; and a data recoverer recovering data by using the time synchronization point and the channel compensation value.

[2] The apparatus of claim 1, wherein the preamble includes a left sequence located at the left end of the CAZAC sequence and a right sequence located at the right end of the CAZAC sequence, and wherein the left sequence has the same value as a predetermined section corresponding to a back-end of the CAZAC sequence, and the right sequence has the same value as a predetermined section corresponding to a front-end of the CAZAC sequence.

[3] The apparatus of claim 2, wherein the preamble has a structure in which the

CAZAC sequence with the same value is repeated three times.

[4] The apparatus of claim 2, wherein the preamble includes a prefix with a cyclic structure and a suffix with a cyclic structure on both ends of the CAZAC sequence.

[5] The apparatus of claim 1, further comprising a preliminary time synchronization point calculator calculating a preliminary time synchronization point that is an approximate time synchronization point obtained by performing a simple operation from the received digital signal, wherein the degree of the cross-correlation is a degree of cross-correlation between a signal corresponding to a predetermined section with respect to the calculated preliminary time synchronization point and the preamble.

[6] The apparatus of claim 1, wherein in the estimation of the time synchronization point, a total power value of a moving section or a mean power value of the moving section is obtained by calculation using the measured degree of the cross-correlation and a power value of the section of which cross-correlation is measured, and a time when the total power value of the moving section or the mean power value of the moving section is maximized is estimated to be the time synchronization point.

[7] The apparatus of claim 6, wherein the preamble includes a prefix with a cyclic structure and a suffix with a cyclic structure on both ends of the CAZAC sequence, and wherein the moving section has the same size as the prefix with the cyclic structure.

[8] The apparatus of claim 1, wherein the first FFT is performed with respect to the degree of the cross-correlation of a section with a predetermined size that starts from the estimated time synchronization point.

[9] The apparatus of claim 8, wherein the preamble includes a prefix with a cyclic structure and a suffix with a cyclic structure on both ends of the CAZAC sequence, and wherein a section having the degree of the cross-correlation which performs the first FFT has the same size as the prefix with the cyclic structure.

[10] The apparatus of claim 1, wherein in the recovery of the data, a signal in a frequency domain is recovered by performing a second FFT with respect to the received digital signal in a range of the CAZAC sequence size, and the signal in the frequency domain is compensated with the estimated channel compensation value.

[11] A method of receiving a digital signal comprising:

(a) receiving a digital signal which is modulated in an OFDM (orthogonal frequency division multiplexing) method that uses a preamble including a CAZAC (constant amplitude zero autocorrelation) sequence;

(b) calculating a time when ISI (inter-symbol interference) is minimized by measuring a degree of cross-correlation between the received digital signal and the preamble and a power value of the received digital signal and estimating the time when the ISI is minimized to be a time synchronization point;

(c) estimating a channel compensation value by performing a first FFT (fast Fourier transform) using the time synchronization point and the degree of the cross-correlation; and

(d) recovering data by using the time synchronization point and the channel compensation value.

[ 12] The method of claim 11 , wherein the preamble in (a) includes a left sequence located at the left end of the CAZAC sequence and a right sequence located at the right end of the CAZAC sequence, and wherein the left sequence has the same value as a predetermined section corresponding to a back-end of the CAZAC sequence, and the right sequence has the same value as a predetermined section corresponding to a front-end of the CAZAC sequence.

[13] The method of claim 11, wherein the preamble in (a) has a structure in which the

CAZAC sequence with the same value is repeated three times.

[14] The method of claim 11, wherein the preamble in (a) includes a prefix with a cyclic structure and a suffix with a cyclic structure on both ends of the CAZAC sequence.

[15] The method of claim 11, wherein (b) further comprises:

(b-1) calculating a preliminary time synchronization point that is an approximate time synchronization point obtained by performing a simple operation frcm the received digital signal; and

(b-2) calculating a degree of cross-correlation between a signal corresponding to a predetermined section with respect to the calculated preliminary time synchronization point and the preamble.

[16] The method of claim 11, wherein in the estimation of the time synchronization point in (b), a total power value of the moving section or a mean power value of the moving section is obtained by calculation using the measured degree of the cross-correlation and a power value of the section of which cross-correlation is measured, and a time when the total power value of the moving section or the mean power value of the moving section is maximized is estimated to be the time synchronization point.

[17] The method of claim 16, wherein the preamble in (a) includes a prefix with a cyclic structure and a suffix with a cyclic structure on both ends of the CAZAC sequence, and wherein the moving section in (b) has the same size as the prefix with the cyclic structure. [18] The method of claim 11, wherein the first FFT in (c) is performed with respect to the degree of the cross-correlation of a section with a predetermined size that starts from the estimated time synchronization point. [19] The method of claim 18, wherein the preamble in (a) includes a prefix with a cyclic structure and a suffix with a cyclic structure on both ends of the CAZAC sequence, and wherein a section having the degree of the cross-correlation which performs the first FFT in (c) has the same size as the prefix with the cyclic structure. [20] The method of claim 11, wherein (d) comprises:

(d-1) recovering a signal in a frequency domain by performing a second FFT with respect to the received digital signal in a range of the CAZAC sequence size; and

(d-2) compensating the signal in the frequency domain with the estimated channel compensation value. [21] A computer-readable recording medium having embodied thereon a computer program for executing the method of any one of claims 11 through 20.