WO2008066270A1

WO2008066270A1 - Iterative reception method and iterative receiver

Info

Publication number: WO2008066270A1
Application number: PCT/KR2007/005791
Authority: WO
Inventors: Jun-Young Nam; Seong-Rag Kim; Hyun Kyu Chung
Original assignee: Electronics And Telecommunications Research Institute
Priority date: 2006-12-01
Filing date: 2007-11-16
Publication date: 2008-06-05

Abstract

An iterative reception method and iterative receiver in a mobile communication system are provided. In order to remove inter-cell interference of a signal that is iteratively received in a multi-cell environment, a hard determination value of different cells excluding a specific cell is used. In addition, in order to remove interference of different user in the specific cell caused by inaccurate channel estimation, the hard determination value of the specific cell signal is used. Further, in order to improve reception performance of the iterative receiver that performs the interference by using the hard determination value, channel estimation is iteratively performed on the iteratively received signal by using a hard determination value of the specific cell to update a channel estimation value.

Description

[DESCRIPTION] [Invention Title]

ITERATIVE RECEPTION METHOD AND ITERATIVE RECEIVER [Technical Field]

The present invention relates to an iterative reception method and an iterative receiver in a mobile communication system. More particularly, the present invention relates to an iterative reception method and an iterative receiver for removing interference from a received signal that is iteratively received in a multi-cell environment.

This work was supported by the IT R&D program of MIC/IITA[200β-S-001- 01, Development of Adaptive Radio Access and Transmission Technologies for 4th Generation Mobile Communications] [Background Art]

In a mobile communication system such as a multicarrier-code division multiple access (MC-CDMA) system, intra-cell inter-user-symbol interference can be effectively removed or avoided due to orthogonality of spread codes. However, in a multi-cell environment, inter-cell interference cannot be effectively removed or avoided. The inter-cell interference greatly deteriorates mobility and stability of the mobile communication system in a cell boundary region. Particularly, in a downlink of the MC-CDMA system, a terminal having a multiple receiving antenna can relatively easily alleviate the inter-cell interference by using space-time diversity, but there is a problem in that a terminal having a single receiving antenna cannot easily alleviate the inter-cell interference.

Recently, approaches for removing the inter-cell interference in the MC-CDMA system have not been actively researched. As an approach for removing the inter-cell interference in the MC-CDMA system, there is proposed an iterative reception scheme based on minimum mean squared error (MMSE) multiuser detection (MUD). However, in the approach, since the number of multi-carriers (for example, 1024) or an inverse of a matrix having a dimension of arbitrary spread factors need to be calculated for each symbol, there is a problem in that relatively large complexity occurs.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. [Disclosure] [Technical Problem]

The present invention has been made in an effort to provide an iterative reception method and an iterative receiver having advantages of removing intra-cell interference and inter-cell interference in a multi-cell environment

In addition, the present invention has been made in an effort to provide an iterative reception method and an iterative receiver having advantages of efficiently removing intra-cell interference and inter-cell interference and providing low complexity to a terminal using a single antenna in a downlink of a mobile communication system. [Technical Solution]

An embodiment of the present invention provides an iterative reception method in which a receiver iteratively receives a signal including a first cell signal and at least one different-cell signal in a multi-cell environment, wherein the iterative reception method includes performing hard determination on the cell signals included in the received signal and outputting hard determination values corresponding to the cell signals, and estimating an inter-cell interference signal by using remaining hard determination values excluding the hard determination value corresponding to the first cell signal from the hard determination values and removing the inter-cell interference signal from the received signal.

Another embodiment of the present invention provides an iterative receiver for iteratively receiving a signal including a first cell signal and at least one second cell signal in a multi-cell environment, including a first hard determination unit that performs hard determination on the second cell signal and outputs a first hard determination value, and an inter-cell parallel interference remover that estimates an inter-cell interference signal by using the first hard determination value and removes the inter-cell interference signal from the received signal.

[Advantageous Effects]

According to the present invention, an iterative reception method and an iterative receiver in a mobile communication system can remove inter-cell interference in a multi-cell environment by using a hard determination value corresponding to signals received from remaining cells excluding a specific cell. In addition, it is possible to remove the intra-cell interference to the user signal in the specific cell by using the hard determination value of the signal received from the specific cell. Accordingly, it is possible to reduce complexity of an implementation method and effectively remove the intra-cell interference and the inter-cell interference in comparison with a conventional method of removing the interference in which an inverse of a matrix having a dimension of arbitrary spread factors needs to be calculated for every symbol .

In addition, the channel estimation value is iteratively updated by using a hard determination value of a received signal of a previous order, so that a more accurate channel estimation value is obtained. Accordingly, it is possible to improve reception performance of the iterative receiver. [Description of Drawings]

FIG. 1 is a diagram illustrating a configuration of an iterative receiver in an MC-CDMA system according to a first embodiment of the present invention.

FIG. 2 is a diagram i l lustrating a conf iguration of an equalizer of the iterative receiver according to the first embodiment of the present invention.

FIG. 3 is a f lowchart i l lustrat ing an iterative recept ion method including an interference removal process in the MC-CDMA system according to the first embodiment of the present invention. FIG. 4 is a flowchart illustrating an example of a de-spreading process, a hard determination process, and a re-spreading process according to the first embodiment of the present invention.

FIG. 5 is a diagram illustrating a configuration of a channel estimator according to the first embodiment of the present invention.

FIG. 6 is a flowchart illustrating a channel estimation method employing an EM algorithm in an iterative reception process according to an embodiment of the present invention.

FIG. 7 is a diagram illustrating a configuration of an iterative receiver in an MC-CDMA system according to a second embodiment of the present invention.

FIG. 8 is a flowchart illustrating an iterative reception method including interference removal processes in the MC-CDMA system according to the second embodiment of the present invention.

FIG. 9 illustrates intra-cell interference and inter-cell interference removal processes according to the second embodiment of the present invention. [Mode for Invention]

Hereinafter, embodiments of the present invention are described in detail with reference to the accompanying drawings so that the ordinarily skilled in the related art can easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification.

In the specification, it should be noted that a phrase that a portion "includes" an element means that the other element is not excluded but it can be further included therein if a particularly contrary phase is not disclosed. In addition, it should be noted that terms "unit", "member", and "block" disclosed in the specification denote a unit for performing at least one function or operation, and it can be implemented as a combination of hardware, software, or hardware and software.

Now, an iterative reception method and an iterative receiver in an MC- CDMA system as an example of a mobile communication system according to an embodiment of the present invention, in which a terminal using a signal receiving antenna removes inter-cell interference, are described with reference to the accompanying drawings. Although the MC-CDMA system is described in the embodiment of the present invention, the present invention can be adapted to other mobile communication systems such as a spread orthogonal frequency division multiplexing (OFDM) system.

,In the mobile communication system according to the embodiment of the present invention, a receiver that is located in a boundary of a cell receives a signal from multiple cells, and a subcarrier corresponding to a specific receiver that is to remove the inter-cell interference by using iterative reception is allocated to the same positions of all the cells. In addition, the present invention is employed to only the receiver located in the cell boundary where the inter-cell interference needs to be removed, but is not employed to the receiver located at the center of a cell where the inter-cell interference does not need to be removed. Further, in the mobile communication system according to the embodiment of the present invention, the receiver is allocated with a control channel including information on modulation and decoding schemes of all cell signals so that the receiver can perform the modulation and decoding on a signal transmitted from a specific cell and signals transmitted from other cells.

Firstly, in an MC-CDMA system model used in the embodiment of the present invention, a transmitting signal s_q of a q-th cell in the MC-CDMA system having Q cells, L subcarriers, L spread factors, and Kq users in the multi-cell environment is expressed by Equation 1, as follows. (Equation 1)

Here, c_q>k denotes a product of a spread code of a k-th user of the q-th cell and a scramble code for cell identification, and b_q>k denotes a transmitting symbol. If an orthogonal spreading matrix of the q-th cell is

set to

is satisfied. In addition, it is assumed that energy of an 1-th subcarrier signal s_q,i of a transmitting

E[\s A²]=l signal of the q-th cell satisfies ^?' . Further, a received signal vector r in a frequency domain, which is received by the receiver of the terminal, is expressed by Equation 2 as follows. (Equation 2)

Here, H_q denotes a channel matrix that can be expressed by

H =Diag(Λ „,&_!,...,A^₁) _{u A >} ■ , _A

In addition, b_q denotes a user signal and n denotes a noise vector.

Intra-cell inter-user-symbol interference can be effectively removed from the received signal expressed by Equation 2 by recovering orthogonality of a channel and performing de-spreading using a single-tap equalizer in the frequency domain. However, since scramble codes of cells may not have orthogonality, the inter-cell interference cannot be removed from the received signal .

Now, an iterative receiver that removes the inter-cell interference in the terminal using a single antenna in the MC-CDMA system according to the first embodiment of the present invention is described in detail based on the aforementioned MC-CDMA system model.

In addition, in the first embodiment of the present invention, a single-tap MMSE equalizer based on minimum mean squared error (MMSE) channel equalization is used an example of the aforementioned single-tap equalizer for removing the intra-cell inter-user-symbol interference.

FIG. 1 is a diagram illustrating a configuration of an iterative receiver in the MC-CDMA system according to the first embodiment of the present invention, in a case where signals are removed from cells.

The iterative receiver according to the embodiment of the present invention has interference removal paths for cells in order to remove the inter-cell interference. For example, as shown in FIG. 1, when two cell signals are received, the iterative receiver performs interference removal on the signals of the first and second cells through the first and second cell paths, respectively. A hard determination value of a different cell signal used for removing the inter-cell interference from a cell signal received by the iterative receiver is calculated by using a signal obtained by removing the inter-cell interference from the different cell signal. For this reason, there is also a need to remove the inter-cell interference from the different cell signal.

As described above, in FIGS. 1, two interference removal paths for the signals received from the two cells are exemplified for the convenience of description of the first embodiment of the present invention. However, the present invention is not limited thereto, and the number of interference removal paths may be varied depending on the number of cell signals received by the iterative receiver of the terminal in the multi-cell environment.

In addition, well-known components of the iterative receiver such as a discrete Fourier transformation (DFT) unit and a cyclic prefix remover can be easily implemented as initial-stage apparatuses by the ordinarily skilled in the related art, and thus description thereof is omitted in the description of the iterative receiver of the MC-CDMA system.

Referring to FIG. 1, since the two cell signals are received by the iterative receiver of the terminal in MC-CDMA system, there are two interference removal paths, and the interference removal paths include two inter-cell parallel interference removers 110 and 210, two equalizers 120 and 220, two de-spreaders 130 and 230, two hard determination units 140 and 240, two hard determination spreaders 150 and 250, and two channel estimators 160 and 260.

The channel estimators 160 and 260 iteratively update channel estimation values by using pilot symbols of iteratively-received signals and output the channel estimation values. Here, the pilot symbols corresponding to the cells are individually input to the channel estimators 160 and 260 through the interference removal paths corresponding to the cells, and are individually processed. A channel estimation method used by the channel estimators 160 and 260 are described in detail later.

The inter-cell parallel interference removers 110 and 210 receive data symbols of the iteratively-received signals and iteratively remove the inter- cell interference. More specifically, the received signal is input to the inter-cell parallel interference removers 110 and 210 through the individual interference removal paths corresponding to the cell signals, and each of the inter-cell parallel interference removers 110 and 210 removes the inter-cell interference of the remaining cell signals excluding a specific cell signal from the data symbol input through a specific interference removal path. Here, "specific cell signal" denotes a cell signal input through the interference removal path of a specific one of the parallel interference removers 110 and 210 in the received signal. Namely, referring to FIG. 1, a first cell signal is the specific cell signal corresponding to a first cell path among the interference removal paths, and a second cell signal is the specific cell signal corresponding to a second cell path among the interference removal paths.

On the other hand, in order to remove the inter-cell interference of the different cell signal from the specific cell signal, each of the parallel interference removers 110 and 210 uses a spread value of a hard determination value output from one of the hard determination units 240 and 140 corresponding to the different cell signal. For example, in order to alleviate the interference of the second cell signal to the first cell signal, the spread hard determination value corresponding to the second cell signal is used, and in order to alleviate the interference of the first cell signal to the second cell signal, the spread hard determination value corresponding to the first cell signal is used. Namely, the interference signals corresponding to the different cell signals can be estimated based on the spread hard determination values, and the. interference signals are removed from the signals input to the inter-cell parallel interference removers 110 and 210 so that the inter-cell interference can be alleviated and output .

The equalizers 120 and 220 receive the signals of which inter-cell interference is alleviated by the inter-cell parallel interference removers 110 and 210 and perform channel equalization on subcarrier signals included in the cell signals. The equalizers 120 and 220 use the channel estimation values generated by the channel estimators 160 and 260 for the channel equalization. As a result, the orthogonality of the channel-equalized signals can be recovered. As described above, in the embodiment of the present invention, the single-tap MMSE equalizer based on MMSE is used for the channel equalization, and one equalizer includes single-tap MMSE equalizers of which number corresponds to the number of subcarrier signals in the specific cell signal.

The de-spreaders 130 and 230 receive the orthogonality-recovered signals from the equalizers 120 and 220 and perform de-spreading.

The hard determination units 140 and 240 perform the hard determination on the de-spread signals output from the de-spreaders 130 and 230 and output the hard determination values. Each hard determination value is a value obtained by performing hard determination on a specific user symbol to which interference of different user symbols in the specific cell is alleviated. The term "specific user symbol" denotes a user symbol corresponding to an iterative receiver in the specific cell signal.

The hard determination spreaders 150 and 250 perform re-spreading on the hard determination values received from the hard determination units 140 and 240 and output the re-spread hard determination values to the inter-cell parallel interference removers 110 and 210 of the different cells excluding the specific cell. The re-spread hard determination values of the cells are used for removing the interference of the specific cell signal from the different cell signals excluding the specific cell.

Although a case where the iterative receiver receives two cell signals is exemplified in FIG. 1 for convenience of description of the embodiment of the present invention, the present invention is not limited thereto, and the iterative receiver may receive two or more cell signals. In this case, the iterative receiver includes interference removal paths (inter-cell parallel interference removers, channel estimators, equalizers, de-spreaders, hard determination units, and hard determination spreaders) corresponding to the cell signals, and each of the inter-cell parallel interference removers corresponding to each cell signal may receive and use at least one hard determination value for removing inter-cell interference.

FIG. 2 is a detail diagram illustrating a configuration of the equalizer 120 of the iterative receiver according to the embodiment of the present invention, in which an example of the aforementioned single-tap MMSE equalizer corresponding to the I-th subcarrier signal in the equalizer corresponding to the equalizer 120 corresponding to the q-th cell signal is illustrated.

Referring to FIG. 2, the equalizer 120 may include single-tap MMSE equalizers of which number corresponds to the number of subcarrier signals included in the cell signal, and one single-tap MMSE equalizer may include an equalization coefficient generation block 121 and an equalizer 122.

The equalization coefficient generation block 121 receives the channel estimation value output from the channel estimator 160 and a background noise variance value, and generates an equalization coefficient. The background noise variance value is empirically obtained. A method of generating the equalization coefficient in the equalization coefficient generation block 121 is described later in detail.

The equalizer 122 performs the channel equalization on an input signal by using the equalization coefficient output from the equalization coefficient generation block 121.

Now, an iterative reception method in which a terminal performs removal of interference in an MC-CDMA system according to the first embodiment of the present invention is described in detail with reference to the accompanying drawings.

A pilot symbol used for channel estimation is a general pilot symbol without additional restriction conditions. In addition, it is assumed that accurate channel estimation is performed by the channel estimator 160 in the following description of a method of removing interference. The channel estimation method is described later in detail. In addition, in the following equations, a superscript denotes the sequential order of iterative reception performed by the iterative receiver.

FIG.3 is a flowchart illustrating an iterative reception method of the terminal in the MC-CDMA system according to the first embodiment of the present invention.

Here, a method of removing inter-cell interference to the first cell signal is described with reference to FIG. 1 by exemplifying a case where a cell signal that is to be received by the iterative receiver is the first cell signal. However, the iterative receiver according to the embodiment of the present invention can remove the interference to other different cell signals as well as the specific cell signal that is to be received by the iterative receiver, and the same method of removing the interference used for the first cell signal is used for the different cells.

Referring to FIG. 3, the iterative receiver receives a signal that is iteratively transmitted from multiple cells in the multi-cell environment, and the channel estimator 160 performs channel estimation by using the pilot symbols of the received signal and outputs the channel estimation value (SlOO) .

If the sequential order of iterative reception is not the first sequential order (SIlO), after the channel estimation, the inter-cell parallel interference remover 110 alleviates the inter-cell interference by using the spread value of the hard determination value of the remaining cell (second cell) signal excluding the specific cell signal, that is, the first cell signal from the received signal (S120). Namely, the interference signal is estimated by using the hard determination value corresponding to the remaining cell (the second cell), and the estimated interference signal is removed from the received signal so that the interference is alleviated. If the sequential order of iterative reception is the first sequential order, the process for alleviating the inter-cell interference by using the spread hard determination value may be omitted.

Next, the signal of which inter-cell interference is alleviated is subject to channel equalization corresponding to the subcarrier signals in the equalizer 120 by using the channel estimation value output from the channel estimator 160, and the orthogonality thereof is recovered (S130).

The signal of which orthogonality is recovered is subject to de- spreading in the de-spreader 130 (S140), and the de-spread signal is input to the hard determination unit 140 to be used to calculate the hard determination value obtained by performing the hard determination on the user symbol of which interference of different users in the specific cell is alleviated (S150) .

Next, the hard determination value is subject to re-spreading in the hard determination spreader 150 and input to the parallel interference remover 210 of the different cell (second cell), and the re-spread value of the hard determination value is used to remove the interference of the remaining cell (second cell) signal excluding the specific cell (first cell). Now, a method of removing inter-cell interference shown in FIG. 3 according to the first embodiment of the present^' invention is described in more detail with reference to equations.

Firstly, a method of generating the equalization coefficient in the equalizer 120 is described.

In a case where the sequential order of iterative reception of the iterative receiver is the first sequential order, since the hard

determination value of the different cell (hereinafter, the specific cell is denoted by q and all the different cells excluding the specific cell are denoted by m) does not exist, the parallel interference remover 110 for the first sequential order is not operated. Therefore, the input of the equalizer 120 becomes r instead of r_q .

In the first embodiment of the present invention, as described, the MMSE is used for the channel equalization. Therefore, in the case of the first sequential order, an equalization coefficient vector G_q of the equalizer 120

is expressed by the following Equation 3. (Equation 3)

G^argmin -G^r

According to the first embodiment of the present invention, the equalizer 120 performs the channel equalization in units of a subcarrier, so that separate equalization coefficients for the subcarriers are obtained. For example, the equalization coefficient of the 1-th subcarrier in the q-th cell signal for the first sequential order is calculated by the following Equation 4 in the equalization coefficient generation block 121. (Equat ion 4)

£[K, |²]=1

Here, H_q;i denotes a channel matrix, which satisfies ' as described above.

On the other hand, if the sequential order of iterative reception of the iterative receiver is not the first sequential order, the iterative receiver performs the spreading of the hard determination value of the different cell and. uses the spread hard determination value to remove the inter-cell interference from the specific cell signal. For example, from the second sequential order to the final I-th sequential order, the removal of the interference is performed by using the hard determination value of the different cells excluding the q-th cell, that is, the specific cell, and the resulting signal r_q is expressed by the following Equation 5.

(Equation 5)

m≠q

Here, the spread signal of the hard determination value vector ^m of

all users in the m-th cell is expressed by T^m =C^mb^m1 , and an interference

error is expressed by . Therefore, in the second equation in

Equation 5, the first term corresponds to the specific cell signal of the q- th cell, and the second and third terms correspond to error terms, that is, the interference signals of the different cells. As described above, the signal of which interference is removed by the inter-cell parallel interference remover 110 is input to the equalizer 120 and subjected to the channel equalization. Here, the equalization coefficient obtained from the following Equation 6 is used. Equation 6 expresses the coefficient vector of the equalizer 120 with respect to the signal r_q obtained by removing the interference of the q-th cell using

Equation 5.

(Equation 6)

Since the channel equalization is performed in units of a subcarrier of each cell, one equalizer 120 includes a plurality of single-tap MMSE equalizers of which number is the number L of subcarriers. Therefore, the above equation can be obtained based on the equalization coefficients for the subcarriers by the equalization coefficient generation block 121 of the single-tap MMSE equalizer. The following Equation 7 expresses the equalization coefficient corresponding to the 1-th subcarrier signal in the q-th cell signal, and the same equation is used for other subcarrier signals.

(Equat ion 7)

By comparing the above coeff icient wi th Equat ion 4, the vari ance

of residual inter-cell interference signals of the 1-th subcarrier signal of other different cells (hereinafter, other different cells are denoted by m) excluding the q-th cell can be omitted in the sequential order of iterative reception where residual inter-cell interference is sufficiently reduced under the assumption that the parallel interference can be completely removed. Therefore, the equalization coefficient of the 1-th subcarrier of the q-th cell after the specific j-th. sequential order of iterative reception can be obtained by using the following Equation 8.

(Equat ion 8)

In a case where a high sequential order of iterative reception is performed, as the inter-cell interference of different cells becomes sufficiently small, the equalization coefficient is calculated in the state

that the variance term ^m' of the residual inter-cell interference signal can be omitted from the equalization coefficient calculation equation as shown in Equation 8, so that the complexity can be reduced in comparison with the equalization coefficient calculation method using Equation 4. Accordingly, additional improvement in performance of the equalizer 120 can be expected.

FIG. 4 is a flowchart illustrating an example of a de-spreading process, a hard determination process, and a re-spreading process according to the first embodiment of the present invention. Hereinafter, a flow of a signal after the channel equalization is described.

Since the channel equalization is performed in units of a subcarrier, one equalizer 120 includes L single-tap MMSE equalizers in order to perform the channel equalization on the L subcarriers. The following Equation 9

expresses a signal ^q that is obtained in the de-spreader 130 by de-

spreading L-dimension signal vectors s¹ *^' equal ized by the L single-tap MMSE

equal izers and a hard determination value obtained by performing the

L l hard determinat ion on the signal ^q . (Equation 9)

sgn(») Here, denotes a hard determination method performed by the hard determination unit 140. In addition, a k-th column vector of a code matrix C_q shown in FIG.4 is constructed with a product of a spread code of a k-th user and a scramble. code of a q-th cell. The calculated hard determination value

is used for the next (i+l)-th sequential order of iterative reception. Next, the iterative receiver allows the hard determination spreader 150

to re-spread the hard determination value by using the following Equation 10.

(Equat ion 10)

The signal ^q obtained by spreading the hard determination value

^~i+l is input to the inter-cell parallel interference remover 210 of the remaining different cell excluding the specific cell to be used to remove the inter-cell interference. As described above, in the method of removing the inter-cell interference by using the hard determination value of the different cell excluding the specific cell, it is possible to effectively alleviate the inter-cell interference, and since the inter-cell interference is removed by using the hard determination value, it is possible to reduce complexity in comparison with a conventional method of removing the interference in which an inverse of a matrix having a dimension of arbitrary spread factors needs to be calculated every symbol. In the aforementioned first embodiment, the method of removing the inter-cell interference is described under the assumption that the accurate channel estimation is performed. However, in the orthogonal frequency division multiplexing (OFDM) system including a downlink MC-CDMA system, if the channel estimation is not accurate, the reception performance of the iterative receiver that is based on the inter-cell interference may be lowered in comparison with a conventional iterative receiver that is not based on the inter-cell interference.

Therefore, a method of improving the performance by using an expectation value maximization (EM) algorithm in the channel estimation method according to the embodiment of the present invention is described.

As described above, the channel estimation method using the pilot symbol can be divided into methods of inserting the pilot symbol in a time domain and a frequency domain. In the embodiment of the present invention, the method of inserting the pilot symbol in the time domain is exemplified.

FIG. 5 is a diagram illustrating a configuration of a channel estimator 160 according to the first embodiment of the present invention, in which the channel estimation is performed by using the hard determination value.

Referring to FIG. 5, the channel estimator 160 iteratively performs the channel estimation every time that the iterative receiver iteratively receives the signal, in which the hard determination value of a received signal of a previous sequential order is used. In the channel estimation method, the channel estimation value is iteratively updated, so that the more accurate channel estimation value can be calculated.

FIG. 6 is a flowchart illustrating the channel estimation method employing the EM algorithm in the iterative reception process according to the embodiment of the present invention, in which the hard determination value is used so as to perform more accurate channel estimation.

Firstly, in order to perform the channel estimation using the EM algorithm, an initial value of the symbol vector used for the channel estimation is needed. If the sequential order of iterative reception is the first sequential order (S200), the iterative receiver constructs an initial symbol vector (S210). After that, every time the signal is iteratively received, the channel estimator 160 updates the symbol vector by using the signal obtained by re-spreading the hard determination value that is generated from the received signal of the previous sequential order (S220).

Next, the initial symbol vector or the updated symbol vector is used to obtain the later-described channel impulse response estimation value (S230) . Since the generated channel impulse response estimation value is a time domain value, the channel impulse response estimation value is transformed into a frequency domain value in order to obtain the final value, that is, the channel estimation value. The channel impulse response estimation value is transformed into a channel frequency reaction estimate in the frequency domain (S250), so that the channel frequency reaction estimate becomes the channel estimation value.

In order to obtain the accurate channel estimation value, the channel estimation method is iteratively performed until the sequential order of iterative reception is the final sequential order (S250).

Now, the channel estimation method shown in FIG. 6 according to the first embodiment of the present invention is described in detail by using equations.

In the following description, the channel estimation of the specific cell is exemplified, but the same channel estimation can be used for different cells. Therefore, in the following description, the subscript q denoting the sequential order of cell is omitted. In addition,

is used instead of a channel frequency reaction matrix Η=Diagtfr_o,ff₁,...,fr_w)

Therefore, if an N-dimensional vector corresponding to the channel h = [h h ... h _ f impulse response est imat ion value is set to °' ^"' ^{' "}' ^N~li , the channel frequency react ion est imate and the channel impulse response est imation value have a relationship . Here, an LxN matrix F_ch is a discrete Fourier transform (DFT) matrix used to calculate the channel frequency reaction estimate, which is expressed as follows.

On the other hand, a maximum likelihood channel estimation method using a maximum likelihood estimation value of a received signal r expressed by Equation 2 is performed by using the following Equation 11.

(Equation 11)

Here, f(r|h) is a likelihood function of the received signal r to the channel impulse response estimation value h, in which the maximum likelihood estimation value is very difficult to calculate due to non-linearity of the function. However, according to the embodiment of the present invention, the maximum likelihood estimation value can be easily calculated by using the EM algorithm.

In order to use the EM algorithm, the received signal r of Equation 2 is expressed by the following Equation 12.

(Equation 12) r =SH+n =SF_c;,h+ii

Here, a symbol matrix is expressed by °' ^{1^'"}' ^L~l , and a

symbol vector of the symbol matrix is expressed by

At this time, in order to use the EM algorithm, the received signal r expressed by Equation 12 is set to an observed incomplete data, and the to- be-detected symbol s is set to an unobserved data. As a result, {r, s} can be set to a complete data. The EM algorithm is configured as follows.

Firstly, under the assumption that the to-be-detected symbol s as well as the received signal r are given, an expectation process of obtaining a

£?(h|h^(?)) likelihood function used to estimate the channel impulse response estimation value h is expressed by the following Equation 13. (Equation 13)

Q(hIh^Cp))=£[log/(s,r]h)I r,h^(?)]

⁼-i^(H²-^2m(^r"^^[slr'^ήCp)]F-^h)⁺il^F-^h»²)

Next, a maximization process of maximizing the likelihood function

Q(h \ h^M) obtained in the expectation process is expressed by the fol lowing

Equation 14.

(Equation 14)

h^⁺¹> = argmaxg(h | h^(?))

Ii

- argmax{-2^(r^HS⁽^F_c;jh) + ||F_c,h||²}

Here, the symbol vector is , which is updated based on the hard determination value of the previous (p-th) received signal.

The channel impulse response estimation value in the frequency domain obtained by transforming the channel impulse response estimation value into a frequency domain value can be expressed by the following Equation 15. (Equation 15)

Here, denotes a diagonal matrix constructed with the re-spread symbols of the hard determination value obtained in the previous (p-th) order iterative reception process. The channel frequency reaction estimate

is obtained based on the aforementioned channel impulse response estimation value by using the following Equation 16. (Equation 16)

As a result, the channel frequency reaction estimate becomes the channel estimation value, that is, the output of the channel estimator 160, which is input to the inter-cell parallel interference remover 110.

As described above, the initial symbol vector used for the channel estimation is needed in order to perform the channel estimation using the EM algorithm. The initial symbol vector can be obtained by using the following Equation 17.

(Equation 17)

S⁽¹⁾-Cb⁽¹⁾

Here, is an initial vector in which pilot symbols are allocated to the positions of the pilot subcarriers and zero is allocated to the positions of other subcarriers. The performance of the EM algorithm is greatly dependent on the initial vector. Therefore, in order to obtain a higher channel estimation value, the pilot symbols needs to be more closely arranged. The channel estimator 160 iteratively performs a series of processes including the process of configuring the initial symbol vector and the channel estimation process until the final sequential order, so that the channel frequency reaction estimation value, that is, the channel estimation value, can be more accurately obtained.

In the aforementioned first embodiment, the iterative reception method and iterative receiver for removing only the inter-cell interference are described. However, in the later-described second embodiment of the present invention, an iterative reception method and an iterative receiver for removing intra-cell inter-user-symbol interference and inter-cell interference in a mobile communication system will be described. Hereinafter, the intra-cell inter-user-symbol interference is referred to as intra-cell interference.

In the second embodiment of the present invention, the iterative receiver for removing the intra-cell interference and the inter-cell interference in the mobile communication system is described by exemplifying an MC-CDMA system.

The iterative receiver according to the second embodiment of the present invention includes the same elements as those of the iterative receiver according to the first embodiment, and the same elements are denoted by the same reference numerals. In addition, in the second embodiment of the present invention, descriptions of the same elements as in the first embodiment are omitted.

FIG. 7 is a diagram illustrating a configuration of an iterative receiver in an MC-CDMA system according to the second embodiment of the present invention, in which the intra-cell interference as well as the inter- cell interference are removed.

Referring to FIG.7, the iterative receiver in a multi-cell environment includes at least one of inter-cell parallel interference removers 110 and 210, equalizers 120 and 220, de-spreaders 130 and 230, hard determination units 140 and 240, hard determination spreaders 150 and 250, and channel estimators 160 and 260. The iterative receiver may further include intra- cell parallel interference removers 170 and 270.

The channel estimators 160 and 260 perform channel estimation by using a pilot symbol of a specific cell signal. In order to perform the more accurate channel estimation, the channel estimators 160 and 260 iteratively perform the channel estimation every time a signal is iteratively received.

The inter-cell parallel interference removers 110 and 210 perform the same functions as those of the inter-cell parallel interference removers 110 and 210 according to the aforementioned first embodiment, and outputs of the inter-cell parallel interference removers 110 and 210 are input to the intra- cell parallel interference removers 170 and 270.

The intra-cell parallel interference removers 170 and 270 receive signals of which inter-cell interference is removed from the inter-cell parallel interference removers 110 and 210, and remove the intra-cell interference from the signals of which inter-cell interference is removed by using a spread hard determination value corresponding to the specific cell signal. Namely, the intra-cell interference signal corresponding to remaining users excluding a user corresponding to the iterative receiver is estimated by using the spread hard determination value corresponding to the specific cell signal, and the estimated intra-cell interference signal is removed from the specific cell signal of which inter-cell interference is removed.

The signals output from the intra-cell interference removers 170 and 270 are input through the equalizers 120 and 220 and the de-spreaders 130 and 230 to the hard determination units 140 and 240. Next, the hard determination units 140 and 240 calculate the hard determination values based on the signals, and the hard determination spreaders 150 and 250 re-spread the hard determination values and output the re-spread hard determination values to the inter-cell parallel interference removers 210 and 110 and the intra-cell parallel interference removers 170 and 270. Here, the spread signals of the hard determination values are input to the intra-cell parallel interference removers 170 and 270 corresponding to the specific cell and the inter-cell parallel interference removers 210 and 110 corresponding to the remaining cells excluding the specific cell.

Now, the iterative reception method of a terminal including an intra- cell interference removal process and an inter-cell interference removal process according to the second embodiment is described with reference to the accompanying drawings.

FIG. 8 is a flowchart illustrating the iterative reception method in the MC-CDMA system according to the second embodiment of the present invention, in which the intra-cell interference process and the inter-cell interference process are included.

Referring to FIG. 8, the iterative receiver allows the channel estimator 160 to iteratively perform the channel estimation on the signal that is iteratively received in the multi-cell environment and output the channel estimation value (S300).

If the sequential order of iterative reception is the first sequential order, the hard determination value does not exist. Therefore, only if the sequential order of iterative reception is not the first sequential order (S310) does the iterative receiver allow the inter-cell parallel interference remover 210 to remove the inter-cell interference (S320). At this time, the inter-cell parallel interference remover 210 removes the interference of the different cell signal by using the spread value of the hard determination value of the different cell signal. The signal of which inter-cell interference is alleviated is input to the intra-cell parallel interference remover 170, and the interference of other user signals in the specific cell signal is removed by using the spread signal of the hard determination value of the specific cell (S330).

If the sequential order of iterative reception is the first sequential order, since the hard determination value does not exist, the aforementioned intra-cell interference and inter-cell interference removal processes are omitted.

Next, as described above, the signal of which intra-cell interference and inter-cell interference are alleviated by the inter-cell parallel interference remover 110 and the intra-cell parallel interference remover 170 are subject to channel equalization in the equalizer 120, and the orthogonality thereof is recovered (S340).

The signal of which orthogonality is recovered by the channel equalization in the equalizer 120 is subject to de-spreading in the de- spreader 130 (S350). Next, the de-spread signal is input to the hard determination unit 140 and used to calculate the hard determination value obtained by performing the hard determination on the user symbol of which interference of different users in the specific cell is alleviated (S360) .

Next, the hard determination value is subject to re-spreading in the hard determination spreader 150 and output to the inter-cell parallel interference remover 210 of the different cell and intra-cell parallel interference remover 170 of the specific cell, and the re-spread value of the hard determination value of each cell is used to remove the intra-cell interference and the inter-cell interference.

The iterative reception method shown in FIG. 8 according to the second embodiment of the present invention is described in more detail by using equations.

The interference removal process according to the second embodiment of the present invention is similar to the inter-cell interference removal process according to the aforementioned first embodiment, and descriptions of portions similar to the inter-cell interference removal process according to the first embodiment are omitted.

Firstly, if the sequential order of iterative reception is the first sequential order, neither the intra-cell interference removal process or the inter-cell interference removal process are performed. Therefore, similar to the aforementioned first embodiment, the data symbol is input to the equalizer 120 without removal of the interference.

For this reason, hereinafter, only the removal of the interference of the i-th sequential orders excluding the first sequential order is described. In the aforementioned first embodiment, only the inter-cell interference removal in the signal r_q input to the equalizer 120 is

considered, as shown in Equation 5. However, in the second embodiment of the present invention, parallel interference removal of the i-th sequential order of the k-th user in the q-th cell is performed by using the following Equation 18 based on the hard determination value of the different cell excluding the q-th cell and the hard determination value of the different user excluding the k-th user in the cell. (Equat ion 18)

e

Here, , and the second and third terras and the fourth term are error terms corresponding to the intra-cell interference and the inter-cell interference, respectively. More specifically, the second term of the second equation in Equation 18 is the error term corresponding to the interference of the signal of the different user excluding the k-th user signal, which is removed by the intra-cell parallel interference remover 172.

FIG. 9 illustrates intra-cell interference and inter-cell interference removal processes according to the second embodiment of the present invention.

As described above, in the iterative receiver for removing the intra- cell interference and the inter-cell interference, since the hard

determination values R*■ ^m}> _man_Λd ΦΛ^' do not exist in the first sequential order, the parallel interference removers for the first sequential order are not operated. Therefore, the equalization coefficient vector of the 1-th subcarrier of the q-th cell signal for the first sequential order is equal to that of Equation 4.

On the other hand, the iterative reception operations after the second sequential order are as follows.

In the iterative receiver according to the aforementioned first embodiment, the same equalization coefficient in the channel equalization is used for different users in the specific cell. However, in the iterative receiver according to the second embodiment of the present invention in which the intra-cell interference removal process and the inter-cell interference removal process are simultaneously performed, as shown in FIG. 7, different single-tap MMSE equalizers are used for different users. More specifically,

the signal ^q* in which intra-cell interference and inter-cell interference are removed is a signal in which intra-cell interference, that is, interference of different users in the cell, is removed, as expressed by Equation 11, so that the channel equalization for the signal is varied according to the users. Therefore, the equalization coefficient vector g^! , r _k

^9> of the equalizer 120 for the signal ^* in which the intra-cell interference and the inter-cell interference are removed by using the hard determination value of the ϊ-th sequential order can be obtained from the following Equation 19. (Equation 19)

In

addition, the equalization coefficient of the single-tap MMSE equalizer corresponding to the 1-th subcarrier of the k-th user in the q-th cell is expressed by the following Equation 20 in the equalization coefficient generation block 121.

(Equation 20)

Simi lar to the aforementioned first embodiment , the variance

of residual intra-cel l interference signals of the 1-th subcarrier corresponding to the k-th user in the q-th cel l and the variance E\\ S~m,l I² ] of residual inter-cell interference signals of the 1-th subcarrier corresponding to the m-th cell may be excluded in the sequential order of iterative reception in which the inter-cell interference becomes sufficiently small. The equalization coefficient after the specific j-th sequential order of iterative reception given empirically is obtained by using the following Equation 21 in the equalization coefficient generation block 121.

(Equation 21)

According to the equalization coefficient generation method using Equation 21, in a high sequential order of iterative reception, additional improvement in performance of the equalizer 120 can be expected in comparison with the equalizer using the coefficient of Equation 20.

The channel estimation method, the de-spreading of the signal of which the intra-cell interference and the inter-cell interference are removed and the re-spreading of the hard determination value are performed in the same manner, and thus description thereof is omitted. On the other hand, in the first embodiment of the present invention, the channel estimation value obtained by performing the channel estimation in the channel estimator is input to only the inter-cell parallel interference remover 210. However, in the second embodiment of the present invention, the channel estimation value is input to the intra-cell parallel interference remover 170 as well as the inter-cell parallel interference remover 210.

In the iterative reception method and the iterative receiver in the multi-cell environment according to the second embodiment of the present invention, the intra-cell interference and the inter-cell interference are removed by using the hard determination value of the specific cell signal and the hard determination value of the different cells, so that it is possible to effectively remove the intra-cell interference or the inter-cell interference. In addition, the channel estimation value is updated by using the hard determination value of the specific cell signal for the channel estimation, so that it is possible to obtain the more accurate channel estimation value.

Exemplary embodiments of the present invention can be implemented not only through the aforementioned method and/or apparatus but also through computer programs executing functions in association with the structures of the exemplary embodiments of the present invention or through a computer readable recording medium having the computer programs embodied thereon. The present invention can be easily implemented by those skilled in the art by using the above descriptions according to the exemplary embodiments.

Although the exemplary embodiments and the modified examples of the present invention have been described, the present invention is not limited to the embodiments and examples, but may be modified in various forms without departing from the scope of the appended claims, the detailed description, and the accompanying drawings of the present invention. Therefore, it is natural that such modifications belong to the scope of the present invention.

Claims

[CLAIMS] •[Claim 1]

An iterative reception method in which a receiver iteratively receives a signal including a first cell signal and at least one different-cell signal in a multi-cell environment, the iterative reception method comprising: performing hard determination on the cell signals included in the received signal and outputting hard determination values corresponding to the cell signals; and estimating an inter-cell interference signal by using hard determination values excluding a hard determination value corresponding to the first cell signal from the hard determination values and removing the inter-cell interference signal from the received signal. [Claim 2]

The iterative reception method of claim 1, further comprising: estimating an intra-cell interference signal corresponding to users excluding a user corresponding to the receiver by using a hard determination value corresponding to the first cell signal and removing the intra-cell interference signal from the received signal of which inter-cell interference is removed. [Claim 3]

The iterative reception method of claim 1, further comprising^'- performing channel estimation based on the cell signals included in the received signal and outputting channel estimation values corresponding to the cell signals; and performing channel equalization on the cell signals included in the received signal by using the channel estimation values, wherein the outputting of the hard determination value is performing the hard determination on the channel-equalized cell signals and outputting the hard determination value. [Claim 4]

The iterative reception method of claim 3, wherein the performing of the channel equalization comprises•' generating equalization coefficients corresponding to subcarrier signals included in the cell signals by using a minimum mean squared error (MMSE) equalization algorithm; and performing channel equalization on the subcarrier signals corresponding to the equalization coefficients by using the equalization coefficients and outputting the channel-equalized subcarrier signals. [Claim 5]

The iterative reception method of claim 3, wherein the outputting of the channel estimation value comprises: updating a symbol vector used for the channel estimation by using the hard determination value; updating a channel impulse response estimation value by using the symbol vector; and transforming the channel impulse response estimation value into a frequency domain and outputting the channel estimation value. [Claim 6]

The iterative reception method of claim 5, wherein the updating of the channel impulse response estimation value comprises: calculating a likelihood function used for updating the channel impulse response estimation value by using an expectation-value maximization (EM) algorithm; and updating the channel impulse response estimation value by using the hard deterination value and the likelihood function. [Claim 7]

The iterative reception method of claim 4, wherein the equalization coefficient is calculated by using a signal of which inter-cell interference for each cell signal is removed as the received signal is iteratively received. [Claim 8] The iterative reception method of claim 1, wherein a subcarrier corresponding to the receiver is allocated to the same positions in the fist cell signal at least one different cell signal. [Claim 9]

The iterative reception method of claim 1, wherein the received signal includes a control channel through which a modulation scheme of the first cell signal and at least one different cell signal is broadcasted. [Claim 10]

An iterative receiver for iteratively receiving a signal including a first cell signal and at least one second cell signal in a multi-cell environment , comprising- a first hard determination unit that performs hard determination on the second cell signal and outputs a first hard determination value; and an inter-cell parallel interference remover that estimates an inter-cell interference signal by using the first hard determination value and removes the inter-cell interference signal from the received signal. [Claim 11]

The iterative receiver of claim 10, further comprising: a second hard determination unit that performs hard determination on the first cell signal and outputs a second hard determination value; and an intra-cell parallel interference remover that estimates an intra-cell interference signal corresponding to a remaining user excluding a user corresponding to the iterative receiver by using the second hard determination value, and removes the intra-cell interference signal from the signal output from the inter-cell parallel interference remover. [Claim 12]

The iterative receiver of claim 10, further comprising'- a channel estimator that performs channel estimation based on the first cell signal and outputs a channel estimation value; and an equalizer that performs channel equalization on the first cell signal by using the channel estimation value and outputs the channel-equalized first cell signal to the second hard determination unit. [Claim 13]

The iterative receiver of claim 12, wherein the equalizer includes single-tap minimum mean squared error (MMSE) equalizers of which a number thereof is the number of subcarrier signals included in the first cell signal, wherein the single-tap MMSE equalizer comprises: an equalization coefficient generation block that generates an equalization coefficient corresponding to one of the plurality of subcarrier signals included in the first cell signal; and an equalizer that performs channel equalization on the subcarrier signal corresponding to the equalization coefficient by using the equalization coefficient and outputs the channel-equalized subcarrier signal. [Claim 14]

The iterative receiver of claim 12, wherein the channel estimator performs channel estimation by using the second hard determination value every time the signal is received, and wherein an expectation-value maximization (EM) algorithm is used for the channel estimation.