WO2010025760A1

WO2010025760A1 - Device and method for iterative interference compensation for mobile reception of ofdm signals in fast varying multipath propagation channels

Info

Publication number: WO2010025760A1
Application number: PCT/EP2008/061546
Authority: WO
Inventors: Karsten Schmidt
Original assignee: Universität Ulm
Priority date: 2008-09-02
Filing date: 2008-09-02
Publication date: 2010-03-11
Also published as: DE112008003971T5

Abstract

A receiver and receiving method for Inter-Carrier Interference (ICI) compensation in an orthoghonal frequency division multiplexing (OFDM) system is provided. The receiver comprises an OFDM-demodulator (302) for demodulating a received OFDM signal r[l] into an input signal Y[l], first estimation means (304) for estimating and equalizing estimated transmit values X⁽⁰⁾ [l] and a channel matrix H⁽⁰⁾ [l] from said input signal Y[l], ICI reduction means (308) for reducing ICI of said input signal Y[l] by using said estimated transmit values X⁽⁰⁾ [l] and said channel matrix H⁽⁰⁾ [l], de-mapper and error correction means (306) for demapping and correcting errors of the ICI-reduced signals Y[l] and forming an output signal X[l]. The receiver (300) further comprises re-encoding means (307) for iteratively re-encoding and mapping said output signal X[l] as new estimated transmit values X^(j)[l] and said ICI reduction means (308) is adapted to reduce an ICI of said input signal Y[l] by using said new estimated transmit values X^(j)[l].

Description

Device and method for Iterative Inference Compensation for Mobile Reception of OFDM Signals in Fast Varying Multipath Propagation Channels

Technical Field

The present invention relates to a device and method for receiving signals in an OFDM (orthogonal frequency division multiplexing) based digital communication system. In particular, the present invention relates to an interference compensation receiver and an interference compensation receiving method for mobile reception in fast varying multipath propagation channels.

Background Art

The vast majority of today's digital communication systems employ OFDM (orthogonal frequency division multiplexing) for transmission to cope with the effects of multipath fading channels. In contrast to a single carrier system, the use of a large amount of (orthogonal) sub-carriers within the transmission channel bandwidth can be used to mitigate the effects of channel dispersion (multipath propagation). In a static environment, like the digital subscriber line (DSL) system, the impulse response and hence the frequency response changes slowly. In such a situation, the transmission channel is almost static and can be considered as time invariant. In this case a simple one tap equalizer can be used to revert the effects of the propagation channel, which results in (almost) error free reception of the transmitted information.

In mobile communications the situation is quite different, since either the transmitter, the receiver or surrounding objects can move around. This results in a time variant impulse response and a time variant frequency response of the communication channel. In the case of digital transmission based on OFDM, the situation will result in inter-carrier interference (crosstalk, ICI) among the sub- carriers of the OFDM symbol. The strength of this interference depends on the ratio between the maximum Doppler shift f_d and the carrier-spacing /_Δ . In general an OFDM system can tolerate a ratio f_d /f_A < 0,l wherein the Doppler shift is related

to the receiver speed by f_d = /_c - , where f_c is the carrier frequency, v the receiver c speed and c the speed of light.

A basic OFDM transmission system 100 as shown in Fig. 1 consists of the following signal processing steps.

At the transmitter (50) side, each OFDM symbol / carries a data vector z[/] . The complex values of this vector will be the result of a digital modulation scheme such as QAM or PSK. The elements ofx[/] will be mapped to the sub-carriers of the OFDM symbols in frequency domain and the time domain signal will be obtained by applying an inverse Discrete Fourier Transform (IDFT) tox[/] at IDFT means 51. After the insertion of a Guard Interval (cyclic prefix) at guard interval adder 52 between the time domain representation of the OFDM symbols, the signal s[l] will be transmitted over a radio channel 60. In a mobile environment, the channel is linear and time variant. This will result in a cross talk between the sub-carriers (intercarrier interference) and degrades the receiver performance in mobile environment.

An OFDM receiver 100 will in general remove the guard interval fromr[/] at the guard interval remover 101 and compute the received data Y[l] on each sub-carrier by using a Discrete Fourier Transform (DFT) at DFT means 102. After channel estimation and equalization at EQ means 103, an estimate of the transmitted data

Jt[l] will be forwarded to the next stage. In practical applications the OFDM transmission will always be used in combination with an error correction coding, thus the estimated transmission data will be forwarded to the error correction block (not shown in Fig. 1 ).

The whole system as shown in Fig. 1 can be described by the following equations: Transmitted signal:

where G_a and F^~x are two matrices used to model the operation of the guard interval adder 52 and the IDFT means 51 , respectively. Received signal before equalization:

Y[I] = F_NG₁ H₀ [/>[/] + N₁ [/] = H[I]X[I]+ N M (²) where G_a and F_N are two matrices used to model the operation of the guard interval remover 101 and the DFT means 102, respectively.

It can be shown that the transmission over a static (time invariant) channel will result in a diagonal matrix //[/]. A time variant channel will cause intercarrier interference and lead to a dense channel matrix.

For a static channel the equalizer can be implemented quite simple (one tap EQ), whereas the mobile receiver must also consider the full channel matrix.

Depending on the type of equalizer, the estimated values can be calculated by X = H^~]Y (Least Squares Estimator, LS-EQ) or X = H^H (HH^H + σ_N ² l)γ (Minimum Mean Square Estimator/ Wiener Filter, MMSE-EQ), where σ_NI is the noise variance matrix of the channel and receiver front end.

Several other approaches for equalizer design for mobile reception exist. Fischer, Volker et. al. "ICI Reduction Method for OFDM Systems", Institute for Communication Technology, Darmstadt University of Technology, Germany or Schmidt, Karsten et. al. "Low Complexity Inter-Carrier Interference Compensation for Mobile Reception of DVB-T", Microelectronics Department, University of UIm, Germany disclose a low complexity interference subtraction scheme 200 such as shown in Fig. 2. It is to be noted that the second channel estimation means 205 shown in Fig. 2 is optional. Each individual element Y₁ of the ICI reduced receive vector F [/]can be calculated by ι+q k=ι-q i≠k wherein H_{1 k} is the value from the Mh row and k -th column of the channel matrix H[l], and q denotes the number of relevant sub-carriers on each side of carrier i .

The ICI reduced receive vector Ϋ [/] is fed into the EQ means 203 to obtain X'[l] by equalization. The vector X'[l] is then de-mapped and error corrected by the de- mapper and error correction means 206.

The advantage of this solution compared to the conventional equalizer 100 is the computational efficiency. Equation (3) does not require the inversion of the channel matrix H[Ϊ] (e.g. for DVB-T in 8k mode, H[l] contains 8192x8192 elements). In general this is far too complex to be processed in a state of the art hardware receiver.

Equation (3) requires an estimate X_k of the transmitted values which can be obtained by an initial estimation based on a LS or MMSE equalizer 204 on the ICI perturbed receive values. Obviously the reliability of X_k has a significant impact on the receiver's ICI reduction performance and the quality of the estimated transmission values will degrade significantly at higher Doppler frequencies.

Summary of the invention

It is an object of the present invention to provide an improved interference compensation device for use in an OFDM-based digital communication system which provides more reliable estimates to the ICI reduction algorithm such that higher Doppler frequencies could be coped with. To achieve this object, the present invention provides a receiver for Inter-Carrier Interference (ICI) compensation in an orthogonal frequency division multiplexing (OFDM) system which comprises an OFDM-demodulator for demodulating a received OFDM signal r[l] into an ICI disturbed input signal Y[l] , first estimation means for estimating and equalizing estimated transmit values X⁽⁰⁾ [l] and estimating a channel matrix H⁽⁰⁾[l] from said input signal Y[l] , ICI reduction means for reducing ICI of said input signal γ[l] by using said estimated transmit values

X⁽⁰⁾ [l] and said channel matrix H^(o)[/] , de-mapper and error correction means for demapping a digital modulation scheme and correcting the remaining errors of the ICI-reduced signals Ϋ[l] and forming an output signal x[l] , wherein said receiver further comprises re-encoding means for iteratively re-encoding and mapping said output signal x[l] as new estimated transmit values X^(l)[l] and wherein said ICI reduction means for reducing an ICI of said input signal γ[l] by using said new estimated transmit values X⁰⁾[/]. The receiver of the present invention achieves a better performance in terms of the required carrier to noise ratio for a specific bit error rate and thus operates at larger Doppler shifts. Furthermore, the receiver speed is improved as a result.

An embodiment of the present invention further comprises a second estimation means for estimating an updated channel matrix H_d'[l] of said ICI reduced signals γ[l] . It is advantageous to provide said second estimation means since the detection of the OFDM values in the ICI reduced signals Y[l] may be improved.

In an embodiment of the present invention said first estimation means further uses said channel matrix /-/⁽⁰⁾[/] for estimating and equalizing said estimated transmit values Z⁽⁰⁾ [/], wherein said channel matrix H⁽⁰⁾[l] is preferably a tridiagonal matrix.

It is advantageous to use a tridiagonal channel matrix H⁽⁰⁾ [l] to estimate and equalize said estimated transmit values X⁽⁰⁾[l] since the estimated transmit values may be further improved. In another embodiment of the present invention clipping means for clipping said estimated transmit values Xⁱ⁰⁾ [l] of said first estimation means to the modulation constellation are further provided in the receiver. It is advantageous to limit said estimated transmit values X⁽⁰⁾ [/] in order to avoid excessive error propagation.

In a further embodiment of the present invention said de-mapper and error correction means uses a forward error correction scheme. It is advantageous to use forward error correction since the reliability of the estimated transmit values X^(l)[l] may be improved.

In yet another embodiment of the present invention, said de-mapper and error correction means of said receiver is a soft-decision de-mapper.

In another embodiment of the present invention, said re-encoding means of the receiver comprises an error correction coding means and a mapping means corresponding to said error correction coding means and said mapping means of a transmitter in said OFDM system. It is advantageous to use re-encoding means which correspond to the encoding means of the transmitter side since the values of the new estimated transmit values Z^(/)[/] may be further improved and made more reliable.

Furthermore, the present invention provides a method for use in an Inter-Carrier Interference (ICI) compensating receiver in an orthoghonal frequency division multiplexing (OFDM) system comprising the steps of demodulating a received OFDM signal into an input signal, estimating and equalizing estimated transmit values from said input signal, reducing said ICI of said input signal by using said estimated transmit values and demapping and correcting errors of the ICI-reduced signals and forming an output signal, wherein said method further comprises the steps of iteratively re-encoding and mapping said output signal as new estimated transmit values and reducing said ICI of said input signal by using said new estimated transmit values.

Brief description of the drawings

Various aspects of an OFDM receiver for iterative interference compensation according to the present invention are illustrated by way of example, and not by way of limitation, in the accompanying drawings, wherein:

Fig. 1 shows a basic OFDM system of the prior art,

Fig. 2 shows an ICI reduction scheme based OFDM receiver of the prior art,

Fig. 3 shows an OFDM receiver according to an embodiment of the present invention,

Fig. 4 shows a flowchart of the method according to an embodiment of the present invention, and

Fig. 5 shows a performance comparision between prior art receivers having a single and a dual antenna and a receiver according to an embodiment of the present invention.

Detailed description

The detailed description set forth below in connection with the accompanying drawings is intended as a description of various preferred embodiments of the invention and is not intended to represent the only embodiments in which the invention may be praticed. The detailed description includes specific details for the purpose of providing a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without theses specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the invention.

It is previously be noted that the present invention and the preferred embodiment described hereinafter apply to all present and future wireless network standards based on coded OFDM such as Worldwide Interoperability for Microwave Access

(WiMAX) and present and future broadcast standards and general communication systems such as, for example, Digital Audio Broadcast (DAB), Digital Radio

Mondial (DRM), Digital Media Broadcast (DMB) and any further extension (DxB), Digital Video Broadcast Terrestrial (DVB-T, DVB-H, DVB-SH), Integrated Services

Digital Broadcasting Terrestrial (ISDB-T) of Association of Radio Industries and

Businesses (ARIB) in Japan or Qualcomm's propriatary standard MediaFLO.

Furthermore, the present invention is applicable to single and multiple receiver paths (i.e., antenna diversity receivers).

The receiver according to the present invention is applicable as part of an access terminal or a base station. The access terminal may be any fixed but preferable a mobile radio device, such as a mobile telephone, a personal digital assistant (PDA), a personal or laptop computer, a television receiver, a game console, a camera, a MP3 player, or any other video, audio, or data device capable of radio communications. The access terminal may further be a receiver installed in a vehicle (e.g., in a car, train or bus). The base station may be a fixed but preferable a mobile transceiver that serves one or more access terminals in its geographic region. The base station may be used to provide multimedia broadcasts, enable access terminals to communicate with one another, or serve as a gateway to wired packet-based and/or circuit-switched networks.

The first embodiment of a receiver 300 will now be described with reference to Fig. 3 wherein a configuration is shown as a high-level block diagram. Referring to Fig. 3, the receiver 300 comprises at least an OFDM signal receiving means (not shown), an OFDM demodulator including a guard interval remover 301 and a discrete Fourier transformer (DFT) 302, ICI reduction means 308, first estimation means 304, de-mapper and error correction means 306 and re-encoding means 307.

The OFDM signal receiving means (not shown in Fig. 3) receives an OFDM signal r[l] transmitted over a radio channel send by an OFDM transmitter side (not shown) corresponding to the OFDM receiver 300 according to the embodiment. The received OFDM signal r[l] is fed into the guard interval remover 301 wherein the guard interval added by a corresponding guard interval adder 52 on the transmitter side (Fig. 1) is removed from the received OFDM signal r[l] . The added guard interval may be any one of the well known schemes such as a CP scheme or a ZP scheme. For instance, in case of the CP scheme, a cyclic prefix added by the guard interval adder 52 of the transmitter side is discarded, and the remaining signals (vectors) are outputted to the DFT 302. In case of the ZP scheme, the vectors obtained by adding the plurality of zeros added by the guard interval adder 52 of the transmitter side to the beginning portion of the vector is outputted to the DFT 302. The DFT 302 computes the input signal Y[l] on each sub-carrier by using a discrete Fourier transform corresponding to the inverse discrete Fourier transformation 51 of the transmitter side.

The input signal Y[l] is fed into the ICI reduction means 308 to reduce the inter- carrier interference by using an estimated channel matrix H^m[l] and estimated transmit values X^ϋ) [l] wherein j denotes a consecutive number which indicates the actual number of iteration steps. The channel matrix H⁽⁰⁾[/] is determined by linear approximating of the radio channel whereby the channel estimation uses, for example, at least three consecutive OFDM symbols of the input signal γ[l] to determine the diagonal elements of the channel matrix H^{0)[l] wherein the current OFDM symbol is used to determine the diagonal elements of the channel matrix H⁽⁰⁾ [l] and wherein the preceeding and succeeding OFDM symbols are used to estimate the off-diagonal elements of the channel matrix H^m [l] by linear approximation. Furthermore, the estimated transmit values X⁽⁰⁾ [l] are estimated by using an estimation method such as least square estimation (LS-EQ), minimum mean square estimation (MMSE-EQ) or by solving a reduced system of linear equations (based on a tridiagonal matrix approximation of the channel matrix f/^({)) [/]). The approximation of the channel matrix H⁽⁰⁾[l] is preferably a diagonal matrix or a tridiagonal matrix. The channel matrix H ⁽⁰⁾[/] and the estimated transmit values X^{0) [l] are initially computed by the first estimation means 304. The ICI reduction means 308 outputs an ICI reduced signal ?[/] .

The ICI reduced signal Y[ϊ] is fed into the de-mapper and error correction means

306 wherein an equalizer 303 forms an output signal Jt[l] . The equalizer uses least square estimation (LS-EQ), minimum mean square estimation (MMSE-EQ) or any other estimation method as known in the art. The de-mapper calculates an reliability of the bits representing the estimated transmit values X'[l] of the ICI reduced signal ?[l] which are the basis for the following error correction. In a preferred embodiment the error correction is a forward error correction (FEC) which is known in the art.

The output signal x[l] of the de-mapper and error correction means 306 is fed to re-encoding means 307 which re-encodes the output signal Jt[l] . The re-encoding means 307 operates preferably similar to the encoding of a signal x[l] on the transmitter side. The output of the re-encoding means 307 is fed back to the ICI reduction means 308 as new estimated transmit values Z^ω[/] wherein j = \ indicates the first iteration. The ICI reduction means 308 reduces the inter-carrier interference of the input signal Y[l] by using the new estimated transmit values

Jf^(/)[/]. The iteration number j is increased by 1 in each iteration step. The output signal Jt[l] is fed after a number of iterations to the next processing means (not shown) of the receiver. It is obvious to the person skilled in the art that the iteration stops when j reaches a maximum number of iterations which may be a predetermined value or set during operation of the receiver 300 in accordance with a system state such as a specific reliability parameter. The number of iterations may be fixed or optimized depending in the OFDM system parameters. In a DVB-T system, for example, is the number of iterations set equal to 3 or less.

A second embodiment of the receiver is based on the first embodiment and further comprises a second estimation means 305 for estimating an updated channel matrix H_d [l] based on the ICI reduced signals ?[/] . The second estimation means 305 is similar to the first estimation means 304 but preferably only determines the diagonal elements of the updated channel matrix H_d[l] . The updated channel matrix H_d [l] is used by equalizer 303 to improve the equalization of the output signal X [l].

A third embodiment of the receiver 300 further comprises clipping means (not shown in Fig. 3) for clipping said estimated transmit values X⁽⁰⁾[l] of the first estimation means 304 or the second estimation means 305. Since the estimated transmit values X⁽⁰⁾ [/] are normally not identical to the signal x[l] , it may be advantageous to limit the values to the maximum value of the corresponding digital modulation scheme (e.g. QPSK/QAM for DVB-T) instead of simply mapping to valid transmit symbols. Thus, excessive error propagation in the ICI reduction is avoided. For example, the 16 QAM modulation of DVB-T implies that the absolute value of the real and imaginary part ofX⁽⁰⁾ [/], respectively, should not be exceeding an amount of 3 (or 0,95 for normalized data). If the real or imaginary part exceeds this value it is limited to this value. In another preferred embodiment, the de-mapper and error correction means 306 of the receiver 300 is preferably a soft-decision de-mapper since known soft-decision de-mappers may result in better performance after error correction than a hard- decision de-mappers known in the art.

Furthermore as shown in Fig. 4, the present invention also provides a method which may be used in an Inter-Carrier (ICI) compensating receiver 300 for an orthoghonal frequency division multiplexing (OFDM) system. The method comprises the steps S1 , S2 of demodulating a received OFDM signal r[l] into an input signal γ[l] , estimating and equalizing estimated transmit values X⁽⁰⁾ [l] from said input signal Y[l] at S3, reducing said ICI of said input signal Y[l] at S4 by using said estimated transmit values X⁽⁰⁾[/] and at step S5 of demapping and correcting errors of the ICI-reduced signals Ϋ[l] and forming an output signal x[l] . The method further comprises the step S6 of iteratively re-encoding and mapping said output signal Jt[l] as new estimated transmit values X⁽¹⁾[l] and S4 of reducing said ICI of said input signal Y[l] by using said new estimated transmit values X^ω[/].

Fig. 5 contains a performance comparison of the method according to an embodiment of the present invention (solid line with rhombuses, -—#-—) _with several prior art techniques wherein the solid line with crosses ( ' ) denotes the performance of a receiver for static channels, the solid line with circles (—-©-—) denotes the performance of a mobile receiver and the solid line with points (— •— ) denotes the performance of a dual diversity receiver. The example is based on the transmission parameters of a DVB-T system. Fig. 5 shows the required carrier to noise ratio (C/N) for a bit error rate (BER) equal or lower than 2 - lCr⁴ after the convolutional decoder (8k mode, 16 QAM and code rate 0,5). The number of receiver paths (antennas) is denoted by M and the number of adjacent carriers used for ICI compensation by 2q = 32 . It is shown that the dual-diversity [M - I ) receiver results in an improvement of approx. 6 dB in terms of the required C/N and effectively doubles the maximum attainable Doppler shift. Hence the receiver speed can be doubled.

It can be seen in Fig. 5 that the iterative ICI reduction scheme according to the present invention results in a significant enhancement of the receiver capabilities in terms of required C/N and maximum attainable Doppler shift. The maximum Doppler shift can be extended by 30% compared to a receiver of the ICI reduction scheme and by 70% compared to a stationary receiver. The required C/N is reduced accordingly.

It is furthermore be noted that the number of adjacent carriers can be used to adjust the performance of the ICI reduction and the computational complexity of the receiver. More carriers will further improve the quality and the complexity.

The method according to embodiments of the present invention may be performed by a dedicated digital signal processor (DSP) and in software. Alternatively, all or part of the method steps may be performed in hardware or combinations of hardware and software, such as ASIC's (Application Specific Integrated Circuit), ASSP (Application Specific Standard Products), reconfigurable logic devices such as FPGA (Field Programmable Gate Array), etc.

It is mentioned that the expression "comprising" does not exclude other elements or steps and that "a" or "an" does not exclude a plurality of elements. Moreover, reference signs in the claims shall not be construed as limiting the scope of the claims.

Several embodiments of the present invention have been described with reference to the drawings herein above. A skilled person reading this description will contemplate several other alternatives and such alternatives are intended to be within the scope of the present invention. Also other combinations than those specifically mentioned herein are intended to be within the scope of the present invention. The present invention is only limited by the appended claims.

Claims

1. A receiver for Inter-Carrier Interference (ICI) compensation in an orthoghonal frequency division multiplexing (OFDM) system comprising: an OFDM-demodulator (302) for demodulating a received OFDM signal r[l] into an input signal γ[l] ; first estimation means (304) for estimating and equalizing estimated transmit values X⁽⁰⁾[l] and a channel matrix H⁽⁰⁾[l] from said input signal Y[l] ;

ICI reduction means (308) for reducing ICI of said input signal Y[l) by using said estimated transmit values X⁽⁰⁾ [l] and said channel matrix H⁽⁰⁾[l] ; de-mapper and error correction means (306) for demapping and correcting errors of the ICI-reduced signals f[l] and forming an output signal x[l] ; characterized in that said receiver (300) further comprises re-encoding means (307) for iteratively re-encoding and mapping said output signal Jt[l] as new estimated transmit values Z^(/)[/]; wherein said ICI reduction means (308) is adapted to reduce an ICI of said input signal γ[l] by using said new estimated transmit values X^(/)[/].

2. The receiver according to claim 1 , further comprising: a second estimation means (305) for estimating an updated channel matrix

H_d'[l] of said ICI reduced signals Ϋ[l] .

3. The receiver according to claim 1 or claim 2, wherein said first estimation means further uses said channel matrix H ⁽⁰⁾ [l] for estimating and equalizing said estimated transmit values X⁽⁰⁾ [/].

4. The receiver according to claim 3, wherein said channel matrix H⁽⁰⁾[l] is a tridiagonal matrix.

5. A receiver according to any one of the preceding claims, further comprising: clipping means for clipping said estimated transmit values X⁽⁰⁾ [l] of said first estimation means (304).

6. The receiver according to any one of the preceding claims, wherein said de- mapper and error correction means (306) uses forward error correction (FEC) for correcting errors of said ICI-reduced signals ?[/] .

7. The receiver according to any one of the preceding claims, wherein said de- mapper and error correction means (306) is a soft-decision de-mapper.

8. The receiver according to any one of the preceding claims, wherein said re- encoding means (307) comprises an error correction coding means and a mapping means corresponding to an error correction coding and a mapping means of a transmitter (50) in said OFDM system.

9. A method for use in an Inter-Carrier (ICI) compensating receiver in an orthoghonal frequency division multiplexing (OFDM) system comprising the steps of: demodulating a received OFDM signal r[l] into an input signal Y[l] ; estimating and equalizing estimated transmit values X⁽⁰⁾[l] and a channel matrix H⁽⁰⁾ [l] from said input signal γ[l] ; reducing said ICI of said input signal Y[l] by using said estimated transmit values X⁽⁰⁾ [l) and said channel matrix H⁽⁰⁾[l] ; and demapping and correcting errors of the ICI-reduced signals Y[l) and forming an output signal Jt[l] ; characterized in that said method further comprises the steps of iteratively re-encoding and mapping said output signal Jt[l] as new estimated transmit values X^(/)[/]; and reducing said ICI of said input signal Y[l) by using said new estimated transmit values X^o) [/].

10. The method according to claim 9, further comprising the step of: estimating an updated channel matrix ///[/] of the ICI reduced signals

Ϋ\l} .

11. The method according to claim 9 or claim 10, wherein said step of estimating and equalizing further uses said channel matrix H⁽⁰⁾ [l] to estimate and equalize said estimated transmit values X⁽⁰⁾ [/j.

12. The method according to claim 9 to 11 , further comprising the step of: clipping the estimated transmit values X⁽⁰⁾ [/].

13. The method according to any one of claims 9 to 12, wherein said step of demapping and correcting errors uses a forward error correction method.

14. The method according to any one of claims 9 to 13, wherein the ICI reduced signals γ[l] are demapped in said step of demapping and correcting errors based on a soft-decision.

15. The method according to any one of claims 9 to 14, wherein said re- encoding and mapping step utilizes an error correction coding method and a mapping method corresponding to an error correction coding method and a mapping method of a transmitter (50) in said OFDM system.