Abstract:
A method and apparatus are provided where a neighbor cell interference signal is canceled upon receiving a signal transmitted from a serving base station (BS) in a receiver of an Orthogonal Frequency Division Multiple Access (OFDMA) system. The interference signals are detected from signals received from neighbor BSs and a serving BS. The strength of the detected interference signals is measures, and a determination is made whether the measured strength is higher than a threshold value. A list of neighbor BSs satisfying the determination result is reordered, and interference signals in order of the reordered neighbor BSs are selected and detected. An interference/noise variance for the detected interference signal is calculated and normalized. The normalized interference signal is decoded and regenerated, and the regenerated interference signal is subtracted from the received signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application Serial No. 2005-117300 filed in the Korean Intellectual Property Office on Dec. 3, 2005 and Korean Patent Application Serial No. 2005-125556 filed in the Korean Intellectual Property Office on Dec. 19, 2005, the entire contents of both of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a method and apparatus for receiving data in a broadband wireless communication system. In particular, the present invention relates to a method and apparatus for canceling interference signals during data reception in an Orthogonal Frequency Division Multiple Access (OFDMA) communication system.  
         [0004]     2. Description of the Related Art  
         [0005]     Wireless communication systems employing a multi-carrier transmission scheme were first applied to a military radio in the late 1950s, and an Orthogonal Frequency Division Multiplexing (OFDM) scheme, which is a typical multi-carrier transmission scheme that overlaps a plurality of orthogonal sub-carriers, has been developing since the 1970s. The OFDM scheme is a scheme that converts a serial input symbol stream into parallel symbol streams and modulates the parallel symbol streams with a plurality of orthogonal sub-carriers before transmission. The OFDM scheme can be applied to such digital transmission technologies as Digital Audio Broadcasting (DAB), Digital Television, Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (ATM).  
         [0006]     Conventionally, OFDM scheme, which is a system suitable for the wireless communication environment where Line of Sight (LOS) is not guaranteed in the multi-path environment, can provide an efficient platform for high-speed data transmission using its advantage of robustness in multi-path fading. That is, the OFDM scheme can efficiently overcome selective fading of frequency because it divides all channels into a plurality of orthogonal narrow-band sub-channels before transmission.  
         [0007]     In addition, the OFDM scheme is effective for high-speed data transmission because it can cancel inter-symbol interference (ISI) by inserting, in front of a symbol, a cyclic prefix (CP) which is longer than a delay spread of the channel. Due to these advantages, an IEEE 802.16a standard, the entire disclosure of which is hereby incorporated by reference, has been established for the OFDM scheme, and the 802.16a standard supports Single-Carrier System, OFDM system, and OFDMA system.  
         [0008]     The OFDMA is a multiple access scheme that divides a frequency domain into sub-channels each composed of a plurality of sub-carriers, divides a time domain into a plurality of time slots, and then allocates the sub-channels for individual users to thereby perform resource allocation taking both the time and frequency domains into consideration, making it possible to accommodate a plurality of users with the limited frequency resources.  
         [0009]     Because the OFDMA system can allocate the sub-channels composed of different sub-carriers to different users, the OFDMA system can configure the sub-channels with adjacent sub-carriers if it applies an Adaptive Antenna System (AAS), which is a scheme for increasing the system capacity based on the 802.16a standard. With the use of such points, the OFDMA system allocates the best channels to the users experiencing different channel environments, thereby obtaining multi-user diversity gain.  
         [0010]     Presently, a WiBro system, which is an IEEE 802.16-based OFDMA system, supports two modes of a frequency reuse factor=1 and a frequency reuse factor=1/3. Because the use of the frequency reuse factor=1 is high in frequency efficiency, this scheme is more advantageous in terms of the cell capacity. Therefore, the current OFDMA system basically uses the frequency reuse factor=1. The frequency reuse factor=1 is remarkably advantageous in terms of the frequency efficiency, but it is disadvantageous in that all sub-carriers in use overlap with sub-carriers of neighbor base stations (BSs), thus serving as interference to each other. That is, because the same frequency is used between neighbor cells, the signals output by the neighbor cells serve as interference signals to each other. A cell currently in communication with a terminal will be called a serving cell, and a neighbor cell causing major interference to the terminal will be called an interference cell. When the terminal is located in the boundary between the serving cell and the interference cell, only the signal coded at a very low coding rate can be normally transmitted/received, because the signals that the terminal receives from the two cells are similar to each other in power. In particular, when the terminal is experiencing a time-varying fading channel, even though a signal from the serving cell is higher in average power than an interference signal, there is a case where a power ratio of the interference signal to the serving cell signal is very high. In this case, errors cannot be corrected even with QPSK with coding rate=1/12, which is the most robust coding.  
         [0011]     Due to the interference signals from the neighbor BSs, the terminal located in the cell boundary deteriorates in reception performance, and also deteriorates in handover performance, causing call drop.  
         [0012]      FIG. 1  is a diagram illustrating an exemplary terminal located in the region where 3 BSs overlap each other in an OFDMA system. As illustrated in  FIG. 1 , in order to overcome the interference problem at cell boundaries  101 ,  103 ,  105  and  107 , the IEEE 802.16 standard is specified so as to modulate a BS transmission signal with low-order modulation like QPSK, apply a low forward error correction (FEC) coding rate, and use 6-repetition coding. However, in spite of the efforts, to the existing terminal receiver in the fading channel, outage probability in the cell boundary appears high and handover performance also deteriorates. In order to basically overcome such problems, it is necessary to set the frequency reuse factor to 3. In this case, however, the frequency reuse factor=3, compared with the frequency reuse factor=1, decreases to 1/3 in terms of the frequency efficiency and increases in complexity of cell planning, so it is not preferred by the service providers. For example, the frequency reuse factor is 1 even in the existing CDMA mobile communication system.  
         [0013]     To increase reception performance of the receiver, several schemes can be considered. For example, there is a scheme for obtaining reception diversity using more than two antennas at the receiver. In this scheme, the reception performance increases by 3 dB or more, even though only 2 reception antennas are used. In this case, however, complexity of the receiver considerably increases and basically, performance degradation due to the interference signals is not remarkably improved. In addition, one of the more important factor in the reception performance is to detect reception of a DL-MAP. Because the DL-MAP is a signal that is broadcasted to all terminals linked to a BS, it is difficult to improve DL-MAP reception performance even with the use of the smart antenna technology applied to individual terminals or the Multiple-In-Multiple-Out (MEMO) technology. In addition, the reception performance deterioration problem in the cell boundary causes another problem of deterioration in handover performance.  
       SUMMARY OF THE INVENTION  
       [0014]     An aspect of certain exemplary embodiments of the present invention is to address at least the above problems and/or disadvantages and to provide at least the exemplary advantages described below. Accordingly, an aspect of exemplary embodiments of the present invention is to provide a method for increasing reception performance by canceling interference signals in an OFDMA system, and a receiver using the same.  
         [0015]     Another aspect of certain exemplary embodiments of the present invention is to provide an interference signal cancellation method for improving handover performance by canceling interference signals in an OFDMA system, and a receiver using the same.  
         [0016]     Further another aspect of certain exemplary embodiments of the present invention is to provide an interference signal cancellation method for improving decoding performance even after canceling interference signals in an OFDMA system, and a receiver using the same.  
         [0017]     According to one aspect of certain exemplary embodiments of the present invention, there is provided a method for canceling a neighbor cell interference signal upon receiving a signal transmitted from a serving base station (BS) in a receiver of an Orthogonal Frequency Division Multiple Access (OFDMA) system. The method includes detecting interference signals from signals received from neighbor BSs and the serving BS; measuring strength of the detected interference signals, and determining whether the measured strength is higher than a predetermined threshold, reordering a list of neighbor BSs satisfying the determination result, selecting and detecting interference signals in order of the reordered neighbor BSs, calculating and normalizing ari-interference/noise variance for the detected interference signal, decoding and regenerating the normalized interference signal and subtracting the regenerated interference signal from the received signal.  
         [0018]     According to another aspect of certain exemplary embodiments of the present invention, there is provided a method for canceling a neighbor cell interference signal upon receiving a signal transmitted from a serving base station (BS) in a receiver of an Orthogonal Frequency Division Multiple Access (OFDMA) system. The method includes detecting interference signals from signals received from neighbor BSs and the serving BS, measuring strength of the detected interference signals, and determining whether the measured strength is higher than a predetermined threshold, reordering a list of neighbor BSs satisfying the determination result, selecting and detecting interference signals in order of the reordered neighbor BSs, calculating and normalizing an interference/noise variance for the detected interference signal, slicing and regenerating the normalized interference signal, and subtracting the regenerated interference signal from the received signal.  
         [0019]     According to yet another aspect of certain exemplary embodiments of the present invention, there is provided an apparatus for canceling a neighbor cell interference signal in a receiver for receiving a signal transmitted from a serving base station (BS) in an Orthogonal Frequency Division Multiple Access (OFDMA) system. The apparatus includes a controller for performing a control operation of measuring strength of signals received from the serving BS and neighbor BSs, and regenerating an interference signal received from the neighbor BS if a difference between the signal strengths satisfies a predetermined threshold, and performing a control operation of outputting a variance of a particular interference/noise signal among the signals received from the serving BS and the neighbor BSs, an interference canceller for regenerating the interference signal under the control of the controller, an interference/noise variance estimator for outputting interference/noise variance information under the control of the controller, a normalizer for normalizing an input metric based on the information received from the interference/noise variance estimator, and a subtractor for subtracting the regenerated interference signal from the signal received from the serving BS.  
         [0020]     According to yet another aspect of certain exemplary embodiments of the present invention, there is provided an apparatus for canceling a neighbor cell interference signal in a receiver for receiving a signal transmitted from a serving base station (BS) in an Orthogonal Frequency Division Multiple Access (OFDMA) system. The apparatus includes a controller for performing a control operation of measuring strength of signals received from the serving BS and neighbor BSs, and regenerating an interference signal received from the neighbor BS if a difference between the signal strengths satisfies a predetermined threshold, and performing a control operation of outputting a variance of a particular interference/noise signal among the signals received from the serving BS and the neighbor BSs, an interference canceller for regenerating the interference signal under the control of the controller, an interference/noise variance estimator for outputting interference/noise variance information of the received signal from which the interference signal is canceled, under the control of the controller, a normalizer for normalizing an input metric based on the information received from the interference/noise variance estimator, and a subtractor for subtracting the regenerated interference signal from the signal received from the serving BS.  
         [0021]     According to still another exemplary embodiment of aspect of the present invention, there is provided a method for canceling a neighbor cell interference signal upon receiving a signal transmitted from a serving base station (BS) in an Orthogonal Frequency Division Multiple Access (OFDMA) system. The method includes scanning neighbor BSs from which interference signals are received, measuring a Carrier to Interference plus Noise Ratio (CINR) of the interference signals received from the neighbor BSs, determining the highest CINR among the measured CINRs which are higher than a predetermined threshold; regenerating an interference signal having the highest CINR, and subtracting the regenerated interference signal from the received signal.  
         [0022]     According to still another exemplary embodiment of aspect of the present invention, there is provided an apparatus for canceling a neighbor cell interference signal in a receiver for receiving a signal transmitted from a serving base station (BS) in an Orthogonal Frequency Division Multiple Access (OFDMA) system. The apparatus includes a controller including scanning neighbor BSs from which interference signals are received; measuring a Carrier to Interference plus Noise Ratio (CINR) of the interference signals received from the neighbor BSs, and determining the highest CINR among the measured CINRs which are higher than a predetermined threshold, an interference canceller for regenerating an interference signal having the highest CINR under the control of the controller, and a subtractor for subtracting the regenerated interference signal from the received signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0024]      FIG. 1  is a diagram illustrating an exemplary terminal located in the region where 3 BSs overlap each other in an OFDMA system;  
         [0025]      FIG. 2  is a block diagram of a transmitter in a general OFDMA system;  
         [0026]      FIG. 3  is a block diagram of a receiver in a general OFDMA system;  
         [0027]      FIG. 4  is a diagram illustrating an exemplary frame structure of an OFDMA system using a Time Division Duplexing (TDD) scheme;  
         [0028]      FIG. 5  is a flowchart illustrating a signal processing process for DL burst decoding in an OFDMA receiver;  
         [0029]      FIG. 6  is a block diagram of a receiver including a repetitive interference canceller for canceling an interference signal in an OFDMA system according to an exemplary embodiment of the present invention;  
         [0030]      FIG. 7  is a block diagram illustrating a receiver in an OFDMA system according to an exemplary embodiment of the present invention;  
         [0031]      FIG. 8  is a diagram illustrating QPSK, 16QAM and 64QAM constellations;  
         [0032]      FIG. 9  is a block diagram of an apparatus for normalizing an input metric to a channel decoder in an OFDMA receiver including a repetitive interference canceller according to an exemplary embodiment of the present invention;  
         [0033]      FIG. 10  is a block diagram of an apparatus for normalizing an input metric to a channel decoder in an OFDMA receiver including a repetitive interference canceller according to an exemplary embodiment of the present invention;  
         [0034]      FIG. 11  is a block diagram of an apparatus for normalizing an input metric to a channel decoder in an OFDMA receiver including a repetitive interference canceller according to an exemplary embodiment of the present invention;  
         [0035]      FIG. 12  is a block diagram of an apparatus for normalizing an input metric to a channel decoder in an OFDMA receiver including a repetitive interference canceller according to an exemplary embodiment of the present invention;  
         [0036]      FIG. 13  is a block diagram of an apparatus for normalizing an input metric to a channel decoder in an OFDMA receiver including a repetitive interference canceller according to an exemplary embodiment of the present invention;  
         [0037]      FIG. 14  is a block diagram of an apparatus for normalizing an input metric to a channel decoder in an OFDMA receiver including a repetitive interference canceller according to an exemplary embodiment of the present invention;  
         [0038]      FIG. 15  is a block diagram of an apparatus for normalizing an input metric to a channel decoder in an OFDMA receiver including a slicer method-based interference canceller according to an exemplary embodiment of the present invention;  
         [0039]      FIG. 16  is a flowchart illustrating a method for normalizing an input metric to a channel decoder in a receiver including an interference canceller based on a channel decoding feedback method according to an exemplary embodiment of the present invention; and  
         [0040]      FIG. 17  is a flowchart illustrating a method for normalizing an input metric to a channel decoder in a receiver including an interference canceller based on a slicer method according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0041]     Exemplary embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals throughout the drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for clarity and conciseness.  
         [0042]     An exemplary embodiment of the present invention proposes a method and apparatus for canceling interference signals to improve reception performance in a receiver of an OFDMA system. The interference signal cancellation method and apparatus proposed in certain exemplary embodiments of the present invention can be applied to both of the synchronous system and the asynchronous system. For a better understanding of the present invention, the OFDMA system will first be described before a description of the method and apparatus according to exemplary embodiments of the present invention is given.  
         [0043]      FIG. 2  is a block diagram of a transmitter  200  in a general OFDMA system.  
         [0044]     Referring to  FIG. 2 , a channel encoder (for example, an interleaver)  201 , like a forward error correction (FEC) encoder, channel-encodes transmission data d(n) including control information and generates a sequence w(n) composed of codewords. Thereafter, a symbol mapper  203  performs mapping with a preset modulation method, and outputs a QAM symbol x(m) such as Quadrature Phase Shift Keying (QPSK), 16-ary Quadrature Amplitude Modulation (16QAM), and 64QAM symbols. A symbol repeater  205  performs repetition according to the number of repetitions (hereinafter referred to as “repetition number”) set by a BS for robust communication. A signal s(l) symbol-repeated by the symbol repeater  205  is converted into a rearranged signal s(k) through an allocator  207 . The symbol-repeated signal s(l) includes not only the rearranged coded symbol s(k) but also a pilot symbol for synchronization and channel estimation of a receiver. A scrambler  209  multiplies the s(k) by a scrambling sequence q(k) applied to each BS. An Inverse Fast Fourier Transformer (IFFT)  211  converts a received signal q(k)s(k) into a time-domain signal X(i), and transmits the time-domain signal X(i) via an antenna.  
         [0045]      FIG. 3  is a block diagram of a receiver  300  in a general OFDMA system.  
         [0046]     Referring to  FIG. 3 , a signal Y(i) received via an antenna is converted into a frequency-domain signal sequence y(k) by a Fast Fourier Transformer (FFT)  301 . The y(k) is descrambled into a complex conjugate pair q(k)* of the scrambling sequence q(k) by a descrambler  303 , outputting frequency-domain data z(k). A channel estimator  305  performs channel estimation using pilot sub-carriers in the descrambled frequency-domain signal z(k). The channel estimator  305  uses a pilot in a preamble and a pilot in a data region in the channel estimation process. Thereafter, a deallocator  307  deallocates estimated channel value and data according to allocation information of control information previously known by a terminal, or allocation information given in a DL-MAP.  
         [0047]     In the deallocator  307 , rearrangement of data is achieved in the allocation process specified in the transmitter  200 . The data deallocated by the deallocator  307  is expressed as p(l), and the p(l) is a signal obtained by multiplying the signal carried on each sub-carrier by a channel response. The deallocator  307 , in the process of deallocating the p(l), deallocates a channel-estimated value c(l) corresponding thereto. A channel compensator  309  outputs the QAM symbol finally channel-compensated by Equation (1). 
 
 r ( l )= p ( l )× c ( l )*  (1) 
 
         [0048]     The QAM symbol, which is a compensated signal r(l) obtained using Equation (1), is input to a repetition combiner  311 . If 1, 2, 4 and 6 repetitions are applied in a coding and modulation process, the repetition combiner  311  combines the repeated symbols as many times as the repetition number, thereby obtaining the combined result of the x(m) and the channel response coefficient. A symbol demapper  313  obtains, from an input, an input metric Λ(n) corresponding to each bit necessary for channel decoding. Herein, positions of the repetition combiner  311  and the symbol demapper  313  are replaceable, and the exemplary structure of the present invention covers the possible replacement. A channel decoder  315 , like an FEC decoder, decodes the obtained input metric, and outputs the decoded data d(n).  
         [0049]      FIG. 4  is a diagram illustrating an exemplary frame structure of an OFDMA system using a Time Division Duplexing (TDD) scheme.  
         [0050]     Referring to  FIG. 4 , in the frame, a vertical axis  401  represents a frequency band of sub-carriers, and a horizontal axis  403  represents OFDM symbols, or a time axis. In the frame, as illustrated, a downlink (DL) interval  410  and an uplink (UL) interval  430  are divided in the time axis. The uplink frame  430  includes, for example, a control channel  431 , and UL burst  433 ,  435 ,  437  and  439 . A first symbol of the downlink frame is a preamble  411 . A terminal uses the preamble  411  for synchronization acquisition, BS ID acquisition, and channel estimation.  113  BS IDs used for identifying BSs are specified in IEEE 802.16e, and because they are used as seed values for scrambling and sub-carrier permutation, the BS ID acquisition is necessarily needed for decoding DL data bursts  419 ,  421 ,  423 ,  425  and  427 . The preamble  411  is followed by a frame control header (FCH)  413 , and the FCH  413  includes information necessary for decoding a DL-MAP  415 . That is, the FCH  413  contains information on a DL-MAP length and a coding method of the DL-MAP. The DL-MAP  415  contains therein the information necessary for decoding DL data bursts in this frame. In other words, the DL-MAP  415  includes position and size information of each individual burst, and Modulation and Coding Scheme (MCS) information. Therefore, in order to decode the DL data bursts  417 ,  419 ,  421 ,  423  and  425 , the signals are processed as shown in  FIG. 5 .  
         [0051]      FIG. 5  is a flowchart illustrating a signal processing process for DL burst decoding in an OFDMA receiver. As illustrated in  FIG. 5 , the receiver acquires a BS ID based on a preamble in step  501 , and decodes an FCH corresponding to the acquired BS ID in step  503 . Thereafter, the receiver decodes the corresponding DL-MAP using DL-MAP decoding information in the received FCH in step  505 . The receiver detects positions of DL bursts  417 ,  419 ,  421 ,  423  and  425  from the DL-MAP, and decodes the DL bursts in step  507 .  
         [0052]     In order to cancel the interference signal, the OFDMA system requires a series of signal processing processes of correctly estimating and detecting an interference signal, regenerating a signal by converting the detected interference signal into the form of a transmission signal, and subtracting the regenerated signal from the received signal.  
         [0053]     This interference signal cancellation scheme is most suitable for the DL-MAP  415 . The DL-MAP  415  suffers considerable inter-BS interference because it is received at the same position from all BSs, thereby causing deterioration of the handover performance. The core of the interference signal cancellation technique is to correctly detect an interference signal. Because the DL-MAP  415  is generally QPSK-modulated and uses a high repetition coding rate, it is easy to correctly detect an interference signal from the DL-MAP  415 . In addition, because the DL-MAP  415  immediately follows a preamble  411 , which is less affected by the interference signal in the time axis, it is also easy to perform channel estimation on the interference signal using the preamble signal.  
         [0054]     A description will now be made of an interference signal cancellation technique capable of improving reception performance of a receiver by efficiently canceling interference signals according to an embodiment of the present invention. In brief, the interference signal cancellation method according to the present invention is characterized by receiving a signal transmitted from a transmitter, regenerating an interference signal after detecting the interference signal from the received signal, and canceling the regenerated interference signal from the reception signal.  
         [0055]     A description will now be made of a structure of a receiver to which the interference signal cancellation method according to the present invention is applied, and of an exemplary signal processing process for interference signal cancellation.  
         [0056]      FIG. 6  is a block diagram of a receiver including a repetitive interference canceller  620  for canceling an interference signal in an OFDMA system according to an exemplary implementation of an embodiment of the present invention. A receiver  600  according to an exemplary embodiment of the present invention generates an interference signal for a received signal by channel coding, and subtracting the interference signal from the received signal in the frequency domain, thereby improving reception performance. In the following description, a method for decoding even the DL bursts of the generated interference signal will be referred to as a channel decoding feedback method, this reference is made simply for convenience of explanation and does not in any way limit the scope or applicability of the present invention.  
         [0057]     Referring to  FIG. 6 , the receiver  600  according to an exemplary embodiment of the present invention includes a controller  610  for controlling each of function blocks to use feedback of the channel decoding result, and an interference canceller  620  for regenerating an interference signal using feedback of the channel decoding result.  
         [0058]     A description of the receiver  600  in  FIG. 6  will be focused on the controller  610  and the interference canceller  620  that regenerates an interference signal using feedback of the channel decoding result, because the process of receiving signals from a BS in service (or serving cell) has been described above.  
         [0059]     The receiver  600  includes the interference canceller  620  for regenerating a received interference signal, a subtractor  602  for subtracting the signal regenerated by the interference canceller  620  from the signal of the serving BS, a switch  604  for switching the subtraction, and the controller  610  for controlling whether to use the regenerated signal from the interference canceller  620  and also controlling each of function blocks, in addition to the general reception blocks. In an exemplary implementation, regenerating an interference signal is equivalent to generating the same signal as the signal output from the transmitter that transmits the interference signal.  
         [0060]     A detailed description of the receiver  600  will be made herein below. In order to detect an interference signal, the controller  610  first detects an interference signal using an ID of a neighbor BS that generates interference. In an exemplary implementation, the interference signal is detected by a CINR measurer (not shown). If interference signals of the neighbor BSs, measured by the CINR measurer, satisfy a specific condition, the controller  610  controls the interference canceller  620  and the switch  604  to thereby control reception of interference signal of an interfering BS. The method for detecting the interference signal is equal to the method used by the OFDMA receiver described in  FIG. 3 . That is, the received signal is detected through a descrambler  603 , a deallocator  607 , a channel compensator  609 , a repetition combiner  611 , a symbol demapper  613 , and a channel decoder  615 . Although the receiver  600  uses herein the CINR to detect an interference signal, it can also detect the interference signal by measuring strength of a signal received in another method, for example by measuring strength of a pilot signal. In this case, the controller  610  may include a pilot strength measurer (not shown).  
         [0061]     The interference canceller  620  regenerates the interference signal transmitted over a channel, using the interference signal detected by the receiver. Herein, generating the interference signal means equally generating the signal generated by the BS that transmitted the interference signal, in order to cancel the interference signal at the receiver.  
         [0062]     A process of regenerating the interference signal follows the process used in the OFDMA transmitter described in  FIG. 2 . Accordingly, the interference canceller  620  includes a channel encoder  617 , a symbol mapper  619 , a symbol repeater  621 , an allocator  623 , a scrambler  625 , and a multiplier  627 . That is, a method for generating the interference signal includes, for example, channel encoding, symbol mapping, repetition coding, allocating, and scrambling processes.  
         [0063]     The multiplier  627  multiplies the channel-estimated result of a channel estimator  605  by the generated interference signal. In this way, the receiver  600  can obtain the interference signal received over the channel. Finally, the receiver  600  subtracts the regenerated interference signal from the received signal, thereby obtaining the pure signal from which the interference signal is canceled. The receiver  600  according to an exemplary implementation the present invention can detect its own signal with a BS ID of a serving cell in the existing OFDMA reception method, using the interference-canceled reception signal. The controller  610  controls such a signal flow. For example, the controller  610  provides a BS ID of the interference signal to the scrambler  625 , the descrambler  603 , the deallocator  607  and the allocator  623  in the process of detecting or regenerating the interference signal, and provides a BS ID of its serving BS to the above blocks in the process of detecting its own signal.  
         [0064]      FIG. 7  is a block diagram illustrating a receiver  700  in an OFDMA system according to an exemplary embodiment of the present invention. Although the receiver  600  of  FIG. 6  can detect a correct interference signal through channel coding, the channel encoder  617  is greater than other blocks in terms of the latency and complexity. Therefore, in order to cancel the interference signal, the latency and complexity increases considerably. However, the receiver  700  of  FIG. 7 , although it may be slightly lower in performance than the receiver  600  of  FIG. 6 , can reduce the latency and complexity.  
         [0065]     Referring to  FIG. 7 , the receiver  700  includes an interference canceller  720  for regenerating a received interference signal, a subtractor  703  for subtracting the regenerated signal from the interference canceller  720  from the signal of the serving BS, a switch  604  for switching the subtraction, and a controller  710  for controlling whether to use the regenerated interference signal from the interference canceller  720  and also controlling each of function blocks, in addition to the general reception blocks.  
         [0066]     The interference canceller  720  in the receiver  700 , unlike the channel decoding feedback method, does not include the channel encoder  617 , and includes a slicer  721 , a symbol mapper  722 , a symbol repeater  723 , an allocator  725 , a scrambler  727 , and a multiplier  729 . The slicer  721  performs a series of processes such as symbol mapping, using the signal output from a symbol demapper  613 . That is, the slicer  721  performs hard decision on the metric output from the symbol demapper  613 , and outputs the hard decision result back to the symbol mapper  722 , thereby generating a signal in a method different from the channel decoding feedback method. In  FIG. 7 , although the slicer  721  is connected to a chain between the symbol demapper  613  and the symbol mapper  722 , it can also be connected between the output of a repetition combiner  611  and the symbol repeater  723 . In this case, because the output of the repetition combiner  611  does not undergo the mapping and demapping processes, the receiver  700  can cancel the interference signal at higher speed.  
         [0067]     Because the interference canceller  720  regenerates an interference signal using the received signal, it does not perform Log Likelihood Ratio (LLR) calculation and channel encoding. For example, the interference canceller  720  regenerates an interference signal using the symbol-demapped signal. For a more reliable delivery of more important information such as control information, the communication system may achieve a low valid coding rate using a combination of channel coding and repetition coding. According to an exemplary implementation, this method is a technique of regenerating an interference signal using only the symbol combining corresponding to the repetition coding.  
         [0068]     According to an exemplary implementation, this method may lose out on a CINR gain as it does not perform channel decoding, but it can reduce the latency and complexity. In the following description, an exemplary implementation of this method will be referred to as a slicer method.  
         [0069]     Of the foregoing exemplary methods, the channel decoding feedback method is more complex, but shows better performance in the interference signal cancellation process. The repetitive interference signal cancellation apparatus and method performs decoding on the interference signal using the phenomenon where decoding is performed successfully even though the amount of the interference signal is large to some extent in the case where the signals transmitted by the interference cell and the serving cell are located in the same position in the time and frequency domains, or in the case where a low coding rate is set for low-order modulation, and then cancels the interference signal. The interference signal cancellation method can show acceptable performance when it uses robust modulation/coding with low modulation order and low coding rate. Generally, because the robust modulation/decoding with low modulation order and low coding rate is applied to the FCH  413  and the DL-MAP  415  of  FIG. 4 , the FCH  413  and the DL-MAP  415  can be advantageous for interference signal cancellation.  
         [0070]     Although the interference cancellers  620  and  720  of  FIGS. 6 and 7  generate signals, they can also be used for regenerating and canceling pilot signals in a data region, and using channel-estimated value of a preamble for interference signal cancellation. In addition, the controllers  610  and  710  of  FIGS. 6 and 7 , after completely canceling the interference signals, can use the pilot signal in the preamble and the pilot signal in the data region for channel estimation.  
         [0071]     The FCH  413  and the DL-MAP  415  of  FIG. 4 , to which coding of convolutional code and turbo code is applied at QPSK with coding rate 1/2, can be repeated 8 or 6 times, having valid coding rates of 1/16 and 1/12.  FIG. 6  shows a repetitive interference signal cancellation apparatus using such a channel decoder.  
         [0072]     A detailed description will now be made of the interference cancellation technique using an exemplary implementation of the channel decoding feedback method among the foregoing interference cancellation techniques.  
         [0073]     In  FIG. 6 , the received signal output from an FFT  601  can be modeled as Equation (2). 
 
 y ( k )= c   0 ( k ) q   0 ( k ) s   0 ( k )+ c   1 ( k ) q   1 ( k ) s   1 ( k )+ c   2 ( k ) q   2 ( k ) s   2 ( k )+ n ( k )+ I ( k )  (2) 
 
         [0074]     In Equation (2), s o (k) denotes a signal before scrambling by a serving cell, and c o (k) denotes a channel that s o (k) experiences. The s o (k) means a signal received from a cell with segment ID=0, and a serving sector of a serving cell does not lose generality even though the segment ID is assumed to be 0. The segment ID is provided to distinguish sub-carriers used for a preamble without overlapping positions thereof, and the IEEE 802.16e standard defines a total of 3 segment IDs. The above-stated 113 BS IDs can be sub-classified into 3 groups for each individual segment ID. The BS IDs and the segment IDs used in certain exemplary embodiments of the present invention are all defined in the IEEE 802.16e standard.  
         [0075]     According to exemplary embodiments of the present invention, a cell with segment ID=0 is assumed as a serving cell and s o (k) is assumed as a frequency-domain signal before scrambling, transmitted by the serving cell. Equation (2) shows an exemplary case in which a signal received at a receiver is modeled after being divided into a signal of a serving cell, an interference-cancelable signal, an interference-noncancelable signal, and an interference signal. In this case, there are two interference-cancelable signals.  
         [0076]     S 1 (k) denotes an interference signal received from a cell with segment ID=1, and S 2 (k) denotes a signal received from an interference cell with segment ID=2. In addition, c 1 (k) and c 2 (k) denote the channels that the interference signals experience before they are received at the receiver.  
         [0077]     The interference signals received from the cell with segment ID=1 are denoted by c 1 (k)q 1 (k)s 1 (k), and the interference signals received from a cell with segment ID=2 are denoted by c 2 (k)q 2 (k)s 2 (k).  
         [0078]     Therefore, in exemplary embodiments of the present invention, c 1 (k)q 1 (k)s 1 (k) and c 2 (k)q 2 (k)s 2 (k) are the interference signals desired to be canceled. Further, n(k) denotes a noise, and I(k) is a sum of the interference signals which are not included in the interference cancellation target, or for which interference cancellation is impossible.  
         [0079]     Generally, a criterion for including interference signals received at the receiver in the interference cancellation target can be a level of a Carrier to Interference plus Noise Ratio (CINR) of each interference signal. The foregoing cancelable interference signal generally has a preamble ID distinguishable from the serving cell, and is limited to the interference signals having a CNIR which is higher than or equal to a threshold. Although an exemplary embodiment of the present invention presents a method for canceling interference signals by measuring a level of the CINR, it can also cancel the interference signals based on strength of the signals received from the serving cell and the neighbor cell, instead of the CINR. However, in the following exemplary embodiments of the present invention, a method for performing interference cancellation using the CINR will be described.  
         [0080]     The reason why an exemplary embodiment of the present invention is described for the case where the CINR is higher than or equal to a threshold is because when the interference cancellation is attempted on the interference signal, whose CINR is lower than the threshold, it may be difficult to correctly regenerate the interference signal, causing performance degradation. According to exemplary embodiments of the present invention c 1 (k)q 1 (k)s 1 (k) and c 2 (k)q 2 (k)s 2 (k) are defined as an interference cancellation target, and c 2 (k)q 2 (k)s 2 (k) is canceled after canceling c 1 (k)q 1 (k)s 1 (k), by way of example.  
         [0081]     A final signal that may be obtained from the received signal of Equation (2) by interference cancellation is given as Equation (3). 
 
 y ′( k )= c   0 ( k ) q   0 ( k ) s   0 ( k )+ n ( k )+ l ( k )  (3) 
 
         [0082]     For example, the interference signals i 1 (k)=c 1 (k)q 1 (k)s 1 (k) and i 2 (k)=c 2 (k)q 2 (k)s 2 (k) will be canceled from Equation (2) through interference cancellation.  
         [0083]     An OFDMA receiver  600  according to an exemplary implementation of an embodiment of the present invention assumes S 1 (k) as a signal to be decoded, and performs a general reception operation. Then, the receiver  600  estimates c 1 (k) by a channel estimator  605 , regenerates i 1 (k) by decoding and symbol mapping, and subtracts i 1 (k) from y(k) by a subtractor  602 , thereby canceling the interference signal from a cell with segment ID=1 from y(k).  
         [0084]     Similarly, in order to cancel the interference signal from a cell with segment ID=2, the receiver  600  regenerates i 2 (k) and cancels the interference signal from the i 1 (k)-canceled signal. A more ideal form of the final signal obtained through this process is given as Equation (4). 
 
 y ′( k )= c   0 ( k ) q   0 ( k ) s   0 ( k )+ n ( k )+ I ( k )  (4) 
 
         [0085]     Because i 1 (k) and i 2 (k) may not be perfectly regenerated, the finally obtained signal is expressed as Equation (5). 
 
 y ′( k )= c   0 ( k ) q   0 ( k ) s   0 ( k )+ i   1 ( k ) i   2 ( k )+ n ( k )+ I ( k )− î   1 ( k )− î   2 ( k )  (5) 
 
         [0086]     After the interference cancellation process, the interference cancellation receiver decodes the data transmitted from the serving cell through the final decoding process. In Equation (5), î 1 (k) denotes an approximation of i 1 (k), and î 2 (k) denotes an approximation of i 2 (k).  
         [0087]     In the receiver of the OFDM/OFDMA system, a signal channel-compensated by a channel compensator  609  is generated as an input metric to a channel decoder  615  by a symbol demapper  613 . In the metric generation process, there is a need for appropriate normalization to show optimal performance at the channel decoder  615 . This process will be described in detail with reference to FIGS.  9  to  17 , and frequency-domain data p(l) can be expressed as Equation (6). 
 
 p ( l )= c ( l ) s ( l )+ n   xl   +j·n   yl   =c   l ( a   l   +j·b   l )+ n   yl   +j·n   yl   =A   l   +jB   l   +n   xl   +j·n   yl   (6) 
 
         [0088]     The data p(l), which is the data that has passed through a channel with channel response=c(l), can be considered as the data obtained after performing descrambling and deallocation on the frequency domain of the actually received data. For example, p(l) is multiplied by a channel response c(l), appearing in the form in which a noise signal is added. For example, n xl  and n yl  denote channels including interference signals. The c(l)s(l) can be expressed as a complex number A l +jB l . Here, the s(l) can be obtained by m-bit QAM modulation in accordance with Equation (7).  
               u     l   ,     m   -   1         ,     u     l   ,     m   -   2         ,   ⋯   ⁢           ,       u     l   ,   0       ⁢     ⟶     M   -   QAM       ⁢     a   l       ,     b   l             (   7   )             
 
         [0089]     Equation (7) indicates that information bits are mapped to a QAM symbol. One QAM symbol is generated with m bits. In IEEE 802.16, m is 2 for QPSK,  4  for 16QAM, and 6 for 64QAM.  
         [0090]      FIG. 8  is a diagram illustrating QPSK, 16QAM and 64QAM constellations. A relationship between input information bits and output symbols can be understood from  FIG. 8 .  
         [0091]     For channel decoding, a communication system using QAM modulation calculates metrics including reliabilities of the bits constituting a codeword, using a received QAM symbol p(l) and a channel response c(l). Generally, the reliability can be expressed as an LLR. An LLR for the bits u l,i  constituting the received QAM symbol p(l) is defined as Equation (8).  
                     Λ   ⁡     (     u             ⁢     l   ,           ⁢   i         )       =       ⁢     log   ⁢           ⁢       Pr   ⁢     {         u     l   ,   i       =     0   |     A   l         ,     B   l       }         Pr   ⁢     {         u     l   ,   i       =     1   |     A   l         ,     B   l       }                       =       ⁢     log   ⁢           ⁢         ∑     z   l       ⁢     exp   ⁢     {         -   1       σ   n   2       ⁢                c   ⁡     (   l   )       ⁢           ⁢     s   ⁡     (   l   )         -       z   l     ⁡     (       u     l   ,   i       =   0     )              2       }             ∑     z   l       ⁢     exp   ⁢     {         -   1       σ   n   2       ⁢                c   ⁡     (   l   )       ⁢           ⁢     s   ⁡     (   l   )         -       z   l     ⁡     (       u     l   ,   i       =   1     )              2       }                         =       ⁢         -   1       σ   n   2       ⁢   log   ⁢           ⁢         ∑     z   l       ⁢     exp   ⁢     {                c   ⁡     (   l   )       ⁢           ⁢     s   ⁡     (   l   )         -       z   l     ⁡     (       u     l   ,   i       =   0     )              2     }             ∑     z   l       ⁢     exp   ⁢     {                c   ⁡     (   l   )       ⁢           ⁢     s   ⁡     (   l   )         -       z   l     ⁡     (       u     l   ,   i       =   1     )              2     }                         =       ⁢       1     σ   n   2       ⁢     L   ⁡     (     -     u     l   ,   i         )                       (   8   )             
 
         [0092]     In Equation (8), σ n   2  denotes a variance of interference and noise signals, and it is generated by an interference/noise variance estimator  902  and then output to an LLR normalizer  904 . Therefore, the LLR normalizer  904  requires a value σ n   2  or its corresponding processed value for calculation of an appropriate LLR value to be used as an input of a channel decoder  615 . In calculation of the LLR, L(−u l,i ) excluding  
       1     σ   n   2         
 
 corresponding to normalization can be calculated in various methods. For example, an approximation of the LLR value can be obtained with the very simple linear calculation, using a dual minimum metric technique disclosed in the paper of Y. Xu, H.-J. Su, E. Geraniotis, “Pilot symbol assisted QAM with interleaved filtering and turbo decoding over Rayleigh flat-fading channel,” in Proc. MICOM &#39;99, pp. 86-91, the entire disclosure of which is hereby incorporated by reference. 
 
         [0093]     Decoding is performed using the obtained LLR by the channel decoder  615 . In performing fixed point calculation, in order to prevent performance degradation, normalization corresponding to  
       1     σ   n   2         
 
 can be performed to obtain the optimal channel decoding performance and to minimize hardware complexity. For the normalization, there may be a need for an interference/noise variance σ n   2  for the signal to be decoded in the channel decoder  615 , or its corresponding normalization parameter. By performing normalization using the parameter, it is possible to increase the efficiency in implementing a receiver with an interference canceller  620 . Normalization corresponding to  
       1     σ   n   2         
 
 (that is, process of multiplying  
       1     σ   n   2         
 
 when calculating the metric in according with Equation (8)) can be applied to bit metrics output after the calculation of L(−u l,i ), dual minimum metric technique, or demapping is performed. Alternatively, the normalization can also be applied before the calculation of L(−u l,i ), dual minimum metric technique, or demapping is performed. Exemplary embodiments of the present invention may include various normalization methods using σ n   2  or its corresponding information in any process that should be performed after deallocation. 
 
         [0094]     According to exemplary embodiments of the present invention, a normalizer performs normalization on the metric to be input to the channel decoder  615  by multiplying the metric output from a symbol demapper  613  by  
         1     σ   n   2       .       
 
 For example, in Equation (8), the normalizer performs normalization by multiplying L(−u l,i ) by a reciprocal of σ n   2  received from an interference/noise variance estimator  1002 . 
 
         [0095]     An OFDMA receiver including the interference cancellers  620  and  720  described in  FIGS. 6 and 7  can improve reception performance of the terminal in the case where inter-cell interference is considerable. In the case where the interference cancellation technique using the channel decoding feedback method described in  FIG. 6  is applied to the terminal and a general automatic gain control (AGC) loop is operated, the AGC loop can control a gain according to the sum of a level of a self signal and a level of an interference signal.  
         [0096]     In an exemplary implementation of the present invention, if the interference signals are sufficiently lower in strength than the signal received from the serving BS, performance degradation may occur in a decoding process of a first interference signal, because normalization of a channel decoding input is inappropriate in the decoding process of the first interference signal. The reason is as follows. There is a need for appropriate normalization, in order for a value of an input metric to show the optimal performance in a channel decoding process where fixed point calculation is performed. However, there is a possible case where a level of the first interference signal exceeds a general operation range given by an interworking process of an AGC loop, a channel compensator  609 , a repetition combiner  611 , and a symbol demapper  613 .  
         [0097]     The AGC loop performs an operation in which while the receiver performs signal processing, an expression range of the values process in the receiver should appropriately fall within an expression range provided by hardware. However, when the interference signals are sufficiently lower in strength than the signal received from the serving BS, the AGC loop will be appropriately set according to the signal received from the serving BS. Therefore, the interference signals are expressed with much lower values, and as a result, if channel compensation and symbol demapping are performed thereon, a value of the input metric to the channel decoder  615  is expressed with a value much lower than an appropriate expression range, causing deterioration of channel decoding performance.  
         [0098]     For example, it will be assumed that input metrics of the channel decoder  615  can express a range of 127 to −127, and an average of the metrics should be about 32, in order for the channel decoder  615  to show sufficient performance. If an input average is 5 because the interference signal is a value much lower in strength than the signal received from the serving BS, only the much smaller range is expressed even though the actual expression range is 127 to −127. As a result, performance degradation may occur in actual channel decoding.  
         [0099]     In this example, reliability of the feedback value will decrease, and in the interference cancellation process, error propagation may occur, thereby causing performance degradation of the overall interference cancellation technique. That is, when the signal from which the interference is to be canceled is not normally decoded, a difference between the original interference signal and the estimated interference signal is large. In this case, many errors occur even when the signal received from the serving BS is decoded.  
         [0100]     In an exemplary implementation, if a level of the interference signal is sufficiently lower than a level of the signal received from the serving BS, the interference cancellation is not performed.  
         [0101]     If a level of the interference signal is higher than a level of the signal received from the serving BS, the performance degradation may be more considerable for the following reasons. For the decoded interference signal, even if normal channel decoding is achieved, a level of the self signal remaining after the interference cancellation is too low. Therefore, a value of an input metric to the channel decoder  615 , generated in the symbol demapper  613 , becomes a small value lower than an appropriate level, thereby causing performance degradation.  
         [0102]     In embodiments of the present invention, a description will be made of a method and apparatus in which an OFDMA receiver including the interference canceller  620  prevents performance deterioration during channel decoding by implementation of fixed point calculation in the interference cancellation process.  
         [0103]     When a receiver according to an exemplary embodiment of the present invention implements the repetitive channel decoding process two or more times, the receiver prevents the performance degradation from occurring in the channel decoder  615 . To this end, a description will now be made of exemplary embodiments for performing normalization on the input metric to the channel decoder  615  using an interference/noise variance corresponding to the signal to be decoded during channel decoding.  
         [0104]      FIG. 9  is a block diagram of an apparatus for normalizing an input metric to a channel decoder  615  in an OFDMA receiver  900  including a repetitive interference canceller  620  according to a first embodiment of the present invention. That is, in  FIG. 9 , the OFDMA receiver  900  for canceling interference signals using the channel decoding feedback method normalizes the input metric to the channel decoder  615 . A normalization process according to an exemplary implementation of the present invention will first be described, and then, elements constituting the receiver of  FIG. 9  will be described.  
         [0105]     As described above, for the cancellation of the interference signals, exemplary embodiments of the present invention sequentially cancel interference signals i 1 (k) and i 2 (k) from Equation (1).  
         [0106]     For interference cancellation, in the process of decoding a signal s 1 (k) from an interference cell with segment ID=1 and obtaining d 1 (n) by the receiver  900 , a channel estimator  605  estimates c 1 (k), a repetition combiner  611  outputs a signal obtained by multiplying y(k) by q(k) and c 1 (k)* (complex conjugate of c 1 (k)) to a symbol demapper  613 , and the symbol demapper  613  calculates an input metric to the channel decoder  615  using the signal received from the repetition combiner  611 .  
         [0107]     In an exemplary implementation, the signals obtained by subtracting i 1 (k) from y(k) are specified as interference/noise signals, thereby defining interference/noise variance. When data d 1 (n) with sector [D=1 is decoded, interferences and noises for the data d 1 (n) with sector ID=1 are c 0 (k)q 0 (k)s 0 (k)+i 2 (k)+n(k)+I(k), and a variance for the interferences and noises are given as Equation (9). 
 
σ n1   2   =V[c   0 ( k ) q   0 ( k ) s   0 ( k )+ i   2 ( k )+ n ( k )+ I ( k )]  (9) 
 
         [0108]     In Equation (9), V[.] denotes a variance function. In a decoding process of d 1 (n), σ n1   2  is reflected in normalization of an input metric to the channel decoder  615 . After channel decoding, the channel decoder  615  can regenerate a signal i 1 (k) and cancel i 1 (k) from y(k). The signal after interference cancellation, the signal output from a subtractor  602  is given as Equation (10). 
 
 y ′( k )= c   0 ( k ) q   0 ( k ) s   0 ( k )+ i   1 ( k )− î   1 ( k )+ î   2 ( k )+ n ( k )+ I ( k )  (10) 
 
         [0109]     Thereafter, decoding and interference cancellation are performed on the signal d 2 (n) transmitted from an interference cell with sector ID=2. The interference/noise variance used in a process of performing interference cancellation on the i 2 (k) is a variance of the interference signals c 0 (k)q 0 (k)s 0 (k)+î 1 (k)+n(k)+I(k) for the i 2 (k) in the remaining signal, and is defined as Equation (11). 
 
σ n2   2   V[c   0 ( k ) q   0 ( k ) s   0 ( k )+ i   1 ( k )− î ( k )+ n ( k )+ I ( k )]  (11) 
 
         [0110]     According to an exemplary implementation, a decoding process for a signal i 2 (k) is performed in the method used for decoding the i 1 (k). In the decoding process, σ n2   2  of Equation (11) is used.  
         [0111]     If decoding on the signal i 2 (k) is successfully performed, d 2 (n) can be obtained, and i 2 (k) can be effectively calculated. In this decoding process, for the noises contributing to normalization of the metric, approximation can be made as given by Equation (12), on the assumption that decoding and interference cancellation on d 1 (n) were completely performed. 
 
σ n2   ′2   =V[c   0 ( k ) q   0 ( k ) s   0 ( k )+ n ( k )+ I ( k )]  (12) 
 
         [0112]     In a decoding process of i 2 (k), σ n2   2  is replaceable with σ n2   ′2 , because, for example, in the situation where the interference cancellation is normally performed, V[i 2 (k)−î 2 (k)] is close to 0 and σ n2   2  will be almost similar to σ n2   ′2 . Exemplary embodiments of the present invention cover various possible cases where σ n2   2  and σ n2   ′2  are used.  
         [0113]     The signal left after the interference cancellation for the interference signals i 1 (k) and i 2 (k) is performed is given as Equation (13). 
 
 c   0 ( k ) q   0 ( k ) s   0 ( k )+ i   1 ( k )− î ( k )+ î   2 ( k )+ n ( k )+ I ( k )  (13) 
 
         [0114]     From Equation (13), the signal d 0 (n) transmitted from the serving cell is decoded. When an input metric to the channel decoder  615  is normalized, interferences and noises for s 0 (k) can be expressed as Equation (14) for the finally left signal. 
 
 i   1 ( k )− î   1 ( k )+ i   2 ( k )− î   2 ( k )+ n ( k )+ I ( k )  (14) 
 
         [0115]     In addition, a variance of Equation (14) is given as Equation (15). 
 
σ n3   2   =V[i   1 ( k )− î ( k )+ i   2 ( k )− î   2 ( k )+ n ( k )+ I ( k )]  (15) 
 
         [0116]     On the assumption that the interference cancellation was almost perfectly performed, Equation (16) is given for Equation (15). 
 
σ n3   ′2   =V[n ( k )+ I ( k )]  (16) 
 
         [0117]     In the normalization process of the final self signal (that is, signal received from the serving BS), σ n3   2  or σ n3   ′2  given in Equation (15) can be used. For example, σ n3   2  or σ n3   ′2  can be used as a value for normalizing an input to the channel decoder  615 .  
         [0118]     For example, when the interference cancellation was successfully performed, values of V[i 1 (k)−î 1 (k)] and V[i 2  (k)−î 2 (k)] are sufficiently small values, so that σ n3   ′2  can be used as an approximation of σ n3   2 . Exemplary embodiments of the present invention include a process of performing normalization using σ n3   ′2  or a σ n3   2 , and a processed value of the value corresponding thereto.  
         [0119]     For the IEEE 802.16e-based OFDMA system, the interference/noise variances of Equation (9), Equation (11), Equation (12), Equation (15) and Equation (16) can be calculated in various methods using pilot sub-carriers in a preamble or a data region. The interference/noise variances σ n3   2  and σ n3   2  given in Equation (11) and Equation (15) are calculated using pilot sub-carriers deallocated from a signal y′(k) whose interference was canceled in each interference cancellation step. These can be calculated in various methods.  
         [0120]     Approximations σ n2   2  and σ n3   2  of the interference/noise variances given in Equation (12) and Equation (16) can be estimated using pilots in a preamble. Because the preamble uses only the non-overlapping pilots for each individual segment ID, σ n2   ′2  and σ n3   ′2  given in Equation (12) and Equation (16) can be calculated in the following method. This is because in the IEEE 802.16e standard, the pilots having different segment IDs do not overlap.  
         [0121]     Generally, for the IEEE 802.16e-based OFDMA system, variances of Equation (17) can be calculated through power measurement based on the fact that the pilots having different segment IDs of a preamble do not overlap. 
 
{circumflex over (σ)} 0   2   =V[c   0 ( k ) q   0 ( k ) s   0 ( k )]
 
{circumflex over (σ)} 1   2   =V[c   1 ( k ) q   1 ( k ) s   1 ( k )]
 
{circumflex over (σ)} 2   2   =V[c   2 ( k ) q   2 ( k ) s   2 ( k )]
 
{circumflex over (σ)} 3   2   =V[n ( k )+ I ( k )]  (17) 
 
         [0122]     Using the variances calculated in Equation (17), the calculation of the interference/noise variances given in Equation (12) and Equation (16), required in the interference cancellation process, can be performed. For example, {circumflex over (σ)} 0   2  as given in Equation (17), denotes an estimate of received power for the signal s 0 (k) from the serving cell. This can be estimated using pilot sub-carriers in a preamble. However, although power of the interference signals, all of which use a segment ID=0, can be added to the actual estimate, this value will be a negligibly small value if the cell planning was ideally performed. In Equation (17), {circumflex over (σ)} 1   2  and {circumflex over (σ)} 2   2 , as given in the equation, are estimates of received power for the signals s 1 (k) and s 2 (k) from the interference cells. A signal power value of another cell having the same segment ID can be added to the actual estimate. The interference/noise variances given in Equation (9), Equation (12) and Equation (16) can be expressed as Equation (18), using the values given in Equation (17). 
 
σ n1   2   =V[c   0 ( k ) q   0 ( k ) s   0 ( k )+ i   2 ( k )+ n ( k )+ I ( k )]=σ 0   2 +σ 2   2 +σ 3   2  
 
σ n2   ′2   =V[c   0 ( k ) q   0 ( k ) s   0 ( k )+ n ( k )+ I ( k )]=σ 0   2 +σ 3   2  
 
σ n3   ′2   =V[n ( k )+ I ( k )]=σ 3   2   (18) 
 
         [0123]     The interference/noise variances enumerated in Equation (18) can be expressed as a linear combination of the variances {circumflex over (σ)} 0   2 , {circumflex over (σ)} 1   2 , {circumflex over (σ)} 2   2 , {circumflex over (σ)} 3   2  calculated in Equation (17).  
         [0124]     For example, while the receiver is performing interference cancellation, for normalization of an input metric to the channel decoder  615 , an LLR normalizer  904  uses σ n1   2 , σ n2   2 , σ n3   2  in each interference cancellation step. For example, the LLR normalizer  904  performs normalization using σ n1   2 , in order to cancel the interference signal received from the BS with segment ID=1, and performs normalization using σ n2   2  in order to cancel the interference signal received from the BS with segment ID=2. Alternatively, the LLR normalizer  904  can use σ n1   2 , σ n2   ′2 , σ n3   ′2  as the interference/noise variances necessary for normalization of an input metric to the channel decoder  615  in each interference cancellation step. In the former case, however, in calculating the interference/noise variances, the LLR normalizer  904  should repetitively calculate noise variances using the interference-canceled signal y′(k). In the latter case where σ n1   2 , σ n2   ′2 , σ n3   ′2  are used, the LLR normalizer  904  can use the previously calculated interference/noise variances of Equation (18). This is because in the preamble, {circumflex over (σ)} 0   2 , {circumflex over (σ)} 1   2 , {circumflex over (σ)} 2   2 , {circumflex over (σ)} 3   2  are already calculated and σ n1   2 , σ n2   ′2 , σ n3   ′2  can be simply expressed as a linear combination of {circumflex over (σ)} 0   2 , {circumflex over (σ)} 1   2 , {circumflex over (σ)} 2   2 , {circumflex over (σ)} 3   2 . In an exemplary implementation, the use of only the y(k) of the preamble is sufficient for calculation of the interference/noise variances.  
         [0125]     As described above,  FIG. 9  illustrates an example of the methods for normalizing an input metric to the channel decoder  615  for the receiver  900  including the interference canceller  620 .  
         [0126]     In case of  FIG. 9 , σ n1   2 , σ n2   ′2 , σ n3   ′2  are used as the interference/noise variances in the interference cancellation process. In  FIG. 9 , the LLR normalizer  904  multiplies the LLR value output from the symbol demapper  613  by a reciprocal  
       1     σ   n   2         
 
 of an interference/noise variance, or takes an operation corresponding thereto, thereby performing normalization on the input metric to the channel decoder  615 . That is, in  FIG. 9 , the symbol demapper  613  first calculates L(−u i,j ) using Equation (8), and thereafter, the LLR normalizer  904  performs an operation of multiplying the metric value calculated by the symbol demapper  613  by  
         1     σ   n   2       .       
 
 The interference/noise variances are calculated from y(k), and from the previously calculated interference/noise variances {circumflex over (σ)} 0   2 , {circumflex over (σ)} 1   2 , {circumflex over (σ)} 2   2 , {circumflex over (σ)} 3   2 , the interference/noise variances σ n1   2 , σ n2   ′2 , σ n3   ′2  used for repetitive interference cancellation are calculated. This operation is performed by an interference/noise variance estimator  902 , and a controller  910  provides the interference/noise variance estimator  902  with information (IC Info) indicating which of σ n1   2 , σ n2   ′2 , σ n3   ′2  it will output according to each interference cancellation step. 
 
         [0127]     The interference/noise variance estimator  902  provides the LLR normalizer  904  with interference/noise variances corresponding to the information received from the controller  910 , or with information (i.e. processed information) corresponding thereto. That is, when the interference/noise variance estimator  902  delivers σ n   2 | an to the LLR normalizer  904 , the LLR normalizer  904  multiplies the symbol-demapped signal L(−u l,i ) by a reciprocal of the σ n   2 |, and when the interference/noise variance estimator  902  delivers  
         1     σ   n   2       ,       
 
 the LLR normalizer  904  multiplies the symbol-demapped signal L(−u l,i ) by  
         1     σ   n   2       ,       
 
 and outputs the resulting signal to the channel decoder  615 . 
 
         [0128]     For a better understanding of the foregoing exemplary embodiment of the present invention, an additional description will now be made of a process of normalizing an input to the channel decoder. For example, assume that an ID of the serving BS is ‘1’, and an ID of a neighbor BS having the highest CINR among the neighbor BSs (with ID=2 and ID=3) is ‘2’, and an ID of a neighbor BS having the next highest CINR is ‘3’.  
         [0129]     In this case, the receiver for canceling the interference signals according to the present invention first normalizes an input metric to the channel decoder  615  with an interference/noise variance of the signal received from the BS with ID=2, and then normalizes an input metric to the channel decoder  615  with an interference/noise variance of the signal received from the BS with ID=3. After the signals from the neighbor BSs are all canceled, the receiver normalizes the input metric with an interference/noise variance of the signal received from the serving BS. That is, the receiver performs normalization on the input to the channel decoder with the interference/noise variance in the state where the interference signal from the BS with ID=2 is canceled in a process of calculating an interference/noise variance for the BS with ID=3, and the state where interference signals from the BSs with ID=2 and 3 are canceled in a process of decoding the signal from the BS with ID=1.  
         [0130]      FIG. 10  is a block diagram of an apparatus for normalizing an input metric to a channel decoder in an OFDMA receiver  1000  including a repetitive interference canceller  620  according to a second exemplary embodiment of the present invention. An interference/noise variance estimator  1002  is also provided.  
         [0131]     The apparatus of  FIG. 10  according to the second exemplary embodiment of the present invention is almost equal in operation to the apparatus of  FIG. 9 , but is different in that normalization on an input metric to a channel decoder  615  is performed in a different position. In the structure of  FIG. 10 , a symbol normalizer  1004  previously performs normalization on the output of a channel compensator  609  so that an input to the channel decoder  615  operates in an appropriate region. That is, the apparatus of  FIG. 10  realizes normalization by previously multiplying the input to a repetition combiner  611  by  
         1     σ   n   2       .       
 
         [0132]      FIG. 11  is a block diagram of an apparatus for normalizing an input metric to a channel decoder  615  in an OFDMA receiver  1100  including a repetitive interference canceller  620  according to a third exemplary embodiment of the present invention.  
         [0133]     In  FIG. 11 , normalization on the metric is performed at the output of a deallocator  607 , and a symbol normalizer  1104  normalizes the output of the deallocator  607 . An interference/noise variance estimator  1102  is also provided.  
         [0134]     Other elements can be analogous to the corresponding elements of the foregoing exemplary embodiments, so a description thereof will be omitted.  
         [0135]      FIG. 12  is a block diagram of an exemplary apparatus for normalizing an input metric to a channel decoder  615  in an OFDMA receiver  1200  including a repetitive interference canceller  620  according to a fourth exemplary embodiment of the present invention. An LLR normalizer  1204  and an interference/noise variance estimator  1202  are also provided. The apparatus according to the fourth exemplary embodiment of the present invention is almost similar in operation to the apparatus of  FIG. 9 , but is different in that σ n1   2 , σ n2   2 , σ n3   3  are used as interference/noise variances. In this case, the apparatus should use a value of the repetitively obtained y′(k) in estimating the interference/noise variances. When calculating σ n1   2 , σ n2   ′2 , σ n3   ′2 , the apparatus uses pilot sub-carriers deallocated from the y′(k). In  FIG. 12 , normalization is applied to the output of a symbol demapper  613 .  
         [0136]      FIG. 13  is a block diagram of an apparatus for normalizing an input metric to a channel decoder  615  in an OFDMA receiver  1300  including a repetitive interference canceller  620  according to a fifth exemplary embodiment of the present invention. A symbol normalizer  1304  and an interference/noise variance estimator  1302  are also provided. The apparatus according to the fifth exemplary embodiment, like the exemplary apparatus of  FIG. 12 , uses σ n1   2 , σ n2   2 , σ n3   2  as interference/noise variances. In  FIG. 13 , in estimating the interference/noise variances, the receiver  1300  should repetitively perform the calculation using y′(k). In calculating σ n1   2 , σ n2   2 , σ n3   2 , the receiver  1300  uses pilot sub-carriers deallocated from the y′(k). Using the calculated interference/noise variances, the receiver  1300  applies a normalization operation of multiplying the output of a channel compensator  609  by a reciprocal of the interference/noise variance, thereby implementing normalization on the input to the channel decoder  615 .  
         [0137]      FIG. 14  is a block diagram of an apparatus for normalizing an input metric to a channel decoder  615  in an OFDMA receiver  1400  including a repetitive interference canceller  620  according to a sixth exemplary embodiment of the present invention.  
         [0138]     In  FIG. 14 , an interference/noise variance estimator  1402  obtains interference/noise information from controller  1410  in the manner similar to that of  FIG. 13 , and a symbol normalizer  1404  performs normalization on the output of the symbol demapper  613 . Other elements are equal to the corresponding elements of the foregoing embodiments, so a description thereof will be omitted.  
         [0139]      FIG. 15  is a block diagram of an apparatus for normalizing an input metric to a channel decoder  615  in an OFDMA receiver  1500  including an slicer method-based interference canceller  720  according to a seventh exemplary embodiment of the present invention. A controller  1510 , a symbol normalizer  1504  and an interference/noise variance estimator  1502  are also provided.  
         [0140]      FIG. 15  shows an exemplary method for normalizing an input metric to a channel decoder in a slicer method-based interference cancellation receiver. Herein, slicing means hard decision. This method does not perform normalization when performing interference cancellation, and performs the normalization only when decoding the last self signal. Although the receiver  1500  of  FIG. 15  uses σ n1   2 , σ n2   2 , σ n3   2  for metric normalization for a channel decoder  615  and normalizes the output of a symbol demapper  613 , it can also normalize an input metric to the channel decoder  615  using a normalizer as done in the first to sixth exemplary embodiments where the channel decoding feedback method is used.  
         [0141]      FIG. 16  is a flowchart illustrating an exemplary method for normalizing an input metric to a channel decoder  615  in a receiver including an interference canceller  620  based on a channel decoding feedback method according to an embodiment of the present invention. The flowchart provides an exemplary method for canceling an interference signal to which a low coding rate is applied, for the control information to which a low coding rate is applied.  
         [0142]     Referring to  FIG. 16 , for noise cancellation, a receiver scans in step  1601  an ID of a neighbor BS from which an interference signal is received, and measures a CINR corresponding to the BS ID. The CINR is measured by a CINR measurer included neighbor BSs with a CINR of a serving BS to which the terminal currently belongs thereby to measure an interference level of the neighbor BS, and then compares the CNR of the serving BS with the CINR of the neighbor BS. For the comparison, the receiver may determine whether a difference between the two CINRs is greater than a specific threshold. In step  1605 , the receiver selects and enumerates (or reorders) the neighbor BSs, the comparison result for which satisfies a specific condition. For example, the receiver enumerates the neighbor BSs in order of a high-strength interference signal, and performs decoding no longer after decoding of the self signal.  
         [0143]     In step  1607 , the receiver first selects a BS having the highest CINR in order to cancel the interference signal received from an interference BS having the highest interference level among the selected BSs. That is, the receiver enumerates BS signals to be decoded, in step  1605 , and determines an interference BS signal to be decoded in the enumerated order in step  1607 . For the CINR estimation, a pilot signal of a preamble or a pilot signal in a data region is used, and because the pilot signal in the data region is greatly affected by the interference signal as in the channel estimation scheme, the result of estimating a CINR using the pilot signal in the data region is inferior in performance to the result of estimating a CINR using the pilot signal in a preamble. Therefore, a method of using the pilot signal in the preamble can be advantageous for CINR estimation for an operation of an interference canceller. In an exemplary embodiment of the present invention, the receiver performs interference cancellation in order of a high-CINR interference BS, if there are two or more interference BSs that generate an interference signal higher than a threshold. For example, the receiver sequentially cancels the interference signals from the interference signal having a higher level.  
         [0144]     The reason for canceling the interference signals from the highest-strength interference signal is, for example, because it may be easier to detect the high-strength interference signal rather than the low-strength interference signal, thus increasing interference cancellation performance. After selecting the signal from the BS having the highest-strength interference signal in step  1607 , the receiver calculates an interference/noise variance of the corresponding BS in step  1609 , and performs normalization on the interference signal of the corresponding BS in step  1611 . The interference signal normalization in step  1611  can be applied to any one of an output of a symbol demapper  613 , an output of a channel compensator  609 , and an output of a deallocator  607 . After the normalization on the interference signal, the receiver decodes the signal from the corresponding BS in step  1613 . An operation of step  1613  means the start of decoding an OFDMA frame. Step  1613  includes all steps that should be completed before the start of the interference cancellation, including reception of a preamble.  
         [0145]     In step  1615 , the receiver regenerates the decoded interference signal using an interference signal canceller. Based on the data decoded after the decoding in step  1613 , the receiver regenerates the interference signal through channel coding, modulation, sub-channel repetition, and allocation processes.  
         [0146]     Thereafter, in step  1617 , the receiver subtracts the regenerated interference signal from the signal received from the serving BS using a subtractor  602 , thereby performing an interference cancellation process. In an exemplary implementation, the process of regenerating and canceling the interference signal regenerates and cancels the signal after channel decoding as done in the channel decoding feedback method.  
         [0147]     After canceling the highest-strength interference signal in step  1617 , the receiver determines in step  1619  whether the interference cancellation is completed. The receiver may determine that the interference cancellation is completed, if there is no BS satisfying a specific condition. If the interference cancellation has not been completed yet, the receiver returns to step  1607  to cancel the interference signal from a BS having the next highest interference signal strength. The receiver repeats the process of selecting a BS having the next highest interference/noise signal strength in step  1617 , calculating an interference/noise variance of the selected BS in step  1609 , and normalizes the interference signal in step  1611 .  
         [0148]     However, if it is determined in step  1619  that cancellation of the interference signals for the serving BS has been completed, the receiver proceeds to step  1621  where it calculates an interference/noise variance of the serving BS signal. The receiver normalizes the serving BS signal in step  1623 , and then decodes the signal from the serving BS in step  1625 .  
         [0149]      FIG. 17  is a flowchart illustrating an exemplary method for normalizing an input metric to a channel decoder  615  in a receiver including an interference canceller  720  based on a slicer method according to an exemplary embodiment of the present invention.  
         [0150]     Referring to  FIG. 17 , for noise cancellation, a receiver scans in step  1701  an ID of a neighbor BS from which an interference signal is received, and measures a CINR corresponding to the BS ID. The CINR is measured by a CINR measurer included in the receiver. Thereafter, in step  1703 , the receiver compares the measured CINRs of neighbor BSs with a CINR of a serving BS to which the terminal currently belongs thereby to measure an interference level of the neighbor BS, and then compares the CINR of the serving BS with the CINR of the neighbor BS. For the comparison, the receiver may determine whether a difference between the two CINRs is greater than a specific threshold. In step  1705 , the receiver selects the neighbor BSs, the comparison result for which satisfies a specific condition. The BS satisfying the specific condition indicates the neighbor BSs, interference from which is considerable.  
         [0151]     In step  1707 , the receiver first selects a BS having the highest CINR in order to cancel the interference signal received from the interference BS having the highest interference level among the selected BSs. For example, the receiver enumerates the BS signals to be decoded, in step  1705 , and determines an interference BS signal to be decoded in the enumerated order in step  1707 .  
         [0152]     The reason for canceling the interference signals from the highest-strength interference signal is, for example, because it may be is easier to detect the high-strength interference signal rather than the low-strength interference signal, thus increasing interference cancellation performance. After selecting the signal from the BS having the highest-strength interference signal in step  1707 , the receiver performs slicing (or hard decision) on the signal from the corresponding BS in step  1709 .  
         [0153]     In step  1711 , the receiver regenerates the interference signal from the corresponding BS using the interference canceller  720 . After the slicing in step  1709 , the receiver regenerates the interference signal through symbol mapping, sub-channel repetition and allocation processes in step  1711 .  
         [0154]     Thereafter, in step  1713 , the receiver subtracts the regenerated interference signal from the signal received from the serving BS using a subtractor  703 , thereby performing an interference cancellation process. For example, the process of regenerating and canceling the interference signal regenerates and cancels the signal-before channel decoding as done in the slicer method.  
         [0155]     After canceling the highest-strength interference signal in step  1713 , the receiver determines in step  1715  whether the interference cancellation is completed. The receiver may determine that the interference cancellation is completed, if there is no BS satisfying a specific condition. If the interference cancellation has not been completed yet, the receiver returns to step  1707  to cancel the interference signal from a BS having the next highest interference signal strength. The receiver repeats the process of selecting a BS having the next highest interference/noise signal strength in step  1707 , slicing the next highest-strength BS signal in step  1709 , regenerating the interference signal in step  1711 , and subtracting the interference signal in step  1713 .  
         [0156]     However, if it is determined in step  1715  that cancellation of the interference signals for the serving BS has been completed, the receiver proceeds to step  1717  where it calculates an interference/noise variance of the serving BS signal. The receiver normalizes the serving BS signal in step  1719 , and then decodes the signal from the serving BS in step  1721 .  
         [0157]     In an exemplary implementation, the procedure of  FIG. 17  may be similar to the procedure of  FIG. 16 . In the example of  FIG. 17 a  slicer method rather than the channel decoding feedback method is used for interference signal processing, so that metric normalization for the interference signals is not necessary.  
         [0158]     As can be understood from the foregoing description, in a receiver of an OFDMA system according to exemplary embodiments of the present invention, in performing a repetitive channel decoding process two or more times in the interference cancellation process, the interference cancellation receiver based on the channel decoding feedback method performs normalization on an input metric to a channel decoder using an interference/noise variance corresponding to the signal to be decoded during channel decoding, thereby reducing performance degradation at the channel decoder.  
         [0159]     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.