Abstract:
A receiving apparatus and a receiving method for interference cancellation in a wireless communication system are provided. The receiving apparatus includes a multiple-input, multiple-output (MIMO) detector for estimating a desired signal and an interference signal based on received signals in accordance with a MIMO detection scheme; a first decoder for iteratively decoding the interference signal output from the detector; a second decoder for iteratively decoding the desired signal output from the decoder; and an LLR updater for updating an interval LLR value of the second decoder using an interval LLR value of the first decoder.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
     This application claims priority under 35 U.S.C. §119(a) to an application filed in the Korean Intellectual Property Office on Jan. 4, 2007 and assigned Serial No. 2007-0000905, the disclosure of which is herein incorporated by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to a receiving apparatus and a method in a wireless communication system. More particularly, the present invention relates to an apparatus and a method for reducing a processing latency of an interference cancellation scheme at a receiver. 
     BACKGROUND OF THE INVENTION 
     Interference is a factor as important as noise in a wireless communication environment. For example, interference from a neighbor cell in a cell boundary, interference from other users concurrently accessing the same frequency band in the same cell, and interference between different data streams when a multiple-input, multiple-output (MIMO) system sends multiple data to a single user not only deteriorate a reception performance of a receiver but also degrade a system capacity. 
     There are two major methods for the efficient interference cancellation. 
     One method is a linear interference cancellation such as Minimum Mean Square Error (MMSE) and Zero Forcing (ZF). The linear interference cancellation features the low complexity by linearly canceling the interference by multiplying the received signal by an adequate weight. 
     Another method is a non-linear interference cancellation such as Serial Interference Cancellation (SIC) and Parallel Interference Cancellation (PIC). The non-linear interference cancellation detects a signal without considering the interference, regenerates the interference signal using the detected signal and a channel coefficient, removes the regenerated interference signal from the received signal, and then redetects the signal. Compared to the linear interference cancellation, the non-linear interference cancellation has the high complexity and the better performance. In specific situations, it is known that the non-linear interference cancellation is optimal in terms of information theory. 
     The non-linear interference cancellation is divided to two methods based on how to regenerate the interference signal. 
     One method regenerates the interference using a signal passing through only a detector, and the other method regenerates the interference using a signal passing through up to a decoder. While the latter method can regenerate the more accurate interference signal, its latency is increased. 
     Typically, a Forward Error Correction (FEC) coding scheme is used as the alternative method to raise reliability of a radio channel. A transmitter sends information data by adding redundancy using a FEC code, and a receiver corrects error merely with the received data. The more amount of the redundancy information, the more amount of the correctable error. Instead, the amount of data transmittable using the same resource is reduced. 
     Currently, a turbo code and a Low Density Parity Check (LDPC) code, which are known as among the best FEC codes, employ an iterative decoder in the receiver. To decode two codes generated by two encoders, the turbo coding iteratively exchanges respective information and the LDPC coding iteratively exchanges information between a variable node and a check node, to thus maximize the error-correction capability. As the number of the iterations increases, the latency and the complexity increase but the error correction capability is enhanced. 
       FIG. 1  illustrates a conventional receiver structure using the SIC. 
     The receiver of  FIG. 1  includes a MIMO detector  100 , a decoder  102 , a hard decision part  104 , an interference signal generator  106 , a subtracter  108 , a MIMO detector  110 , a decoder  112 , and a hard decision part  114 . Hereafter, it is assumed that x 1  is an interference signal and x 2  is a desired signal to receive. 
     The MIMO detector  100  outputs an estimated interference signal {circumflex over (x)} 1  by demodulating a received signal, y, received on a plurality of antennas according to a predetermined MIMO detection scheme. The decoder  102  decodes the estimated interference signal output from the MIMO detector  100  using a certain demodulation scheme. The hard decision part  104  outputs decoded data by hard-deciding soft values output from the decoder  102 , while the soft values are information bits, each with an estimated degree of certainty. 
     The interference signal generator  106  generates an interference signal h 1 {tilde over (x)} 1  with the decoded data output from the hard decision part  104  and channel information. The subtracter  108  removes the interference signal of the interference signal generator  106  from the received signal y. 
     The MIMO detector  110  outputs an estimated desired signal {circumflex over (x)} 2  by demodulating the interference-free signal output from the subtracter  108  using a certain MIMO detection scheme. The decoder  112  decodes the estimated desired signal fed from the MIMO detector  110  using a certain decoding scheme. The hard decision part  104  outputs decoded data {tilde over (x)} 2  by hard-deciding soft values from the decoder  112 . 
     As discussed above, according to the non-linear interference cancellation, the performance with the decoding process is better than the performance without the decoding process. However, since it takes a considerable time to generate the interference signal as shown in  FIG. 1 , the latency is likely to increase until the desired signal is decoded. Particularly, when the iterative decoding is used as in the turbo coding or the LDPC coding, the latency is deteriorated to thus degrade the entire system performance. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary aspect of the present invention to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an aspect of the present invention is to provide an apparatus and a method for reducing a processing latency of an interference cancellation at a receiver. 
     Another aspect of the present invention is to provide an apparatus and a method for using Log Likelihood Ratios (LLRs) of an interference signal to decode a desired signal at a receiver. 
     Yet another aspect of the present invention is to provide an apparatus and a method for updating an LLR of a desired signal using an LLR of an interference signal at a receiver. 
     The above aspects are achieved by providing a receiving apparatus in a wireless communication system. The receiving apparatus includes a detector for estimating a desired signal and an interference signal by multiple-input, multiple-output (MIMO)-detecting received signals; a first decoder for iteratively decoding the interference signal output from the detector; a second decoder for iteratively decoding the desired signal output from the decoder; and an LLR updater for updating an internal LLR value of the second decoder using an internal LLR value of the first decoder. 
     According to one aspect of the present invention, a receiving apparatus in a wireless communication system includes a detector for estimating at least two streams by MIMO detecting received signals; decoders for iteratively decoding the corresponding streams output from the detector, the decoders detecting a sign-inverted LLR during the iterative decoding; and an LLR updater for, when the sign-inverted LLR is detected from one of the decoders, updating internal LLR values of the other decoders. 
     According to another aspect of the present invention, a receiving method in a wireless communication system includes estimating a desired signal and an interference signal by MIMO-detecting received signals; iteratively decoding the interference signal and the desired signal respectively; and updating an LLR value generated during the iterative decoding of the desired signal using an LLR value generated during the iterative decoding of the interference signal. 
     According to yet another aspect of the present invention, a receiving method in a wireless communication system includes estimating at least two streams by MIMO-detecting received signals; iteratively decoding the estimated streams respectively; detecting a sign-inverted LLR during the iterative decoding; and updating, when the sign-inverted LLR is detected, corresponding LLR of the other streams being iteratively decoded. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
         FIG. 1  illustrates a conventional receiver structure using a SIC; 
         FIG. 2  illustrates a receiver structure in a wireless communication system according to an embodiment of the present invention; 
         FIG. 3  illustrates operations of the receiver to perform a SIC according to an embodiment of the present invention; and 
         FIG. 4  illustrates operations of a receiver to perform a PIC according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 2 through 4 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communication system. 
     The present invention provides a method for reducing a processing latency of an interference cancellation at a receiver using an iterative decoding. Particularly, the present invention provides a method for updating a Log Likelihood Ratio (LLR) of a desired signal using an LLR of an interference signal during the decoding at a receiver. 
     To ease the understanding of the present invention, it is assumed that the receiver uses two antennas and there are two transmit signals. In this case, the signal can be modeled as expressed as Equation 1: 
     
       
         
           
             
               
                 
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     In Equation 1, y is a 2×1 receive signal vector, n is a 2×1 noise vector, H=[h 1  h 2 ] is a 2×2 channel matrix, and 
             x   =     [           x   1               x   2           ]           
is a 2×1 transmit signal. If Equation 1 models a case where a signal x 2  of a neighbor cell acts as the interference in a cell boundary, x 2  is eliminated through the interference cancellation and then x 1  is detected. If Equation 1 models a case where a signal x 2  of a neighbor sector acts as the interference in a multi-sector system, x 2  is eliminated through the interference cancellation and then x 1  is detected. If Equation 1 models a Multi-User Detection (MUD) or a Spatial Multiplexing (SM), x 1  is detected by regarding x 2  as the interference and x 2  is detected by regarding x 1  as the interference.
 
     Hereafter, it is assumed that x 1  is the interference signal and x 2  is the desired signal to receive. 
       FIG. 2  illustrates a receiver structure in a wireless communication system according to an embodiment of the present invention. 
     The receiver of  FIG. 2  includes a channel estimator  200 , a MIMO detector  202 , a first decoder  204 , a first hard decision part  206 , a second decoder  208 , a second hard decision part  210 , and an LLR updater  212 . 
     The channel estimator  200  constructs a receive vector y with signals received on a plurality of antennas and constructs a channel matrix H by estimating a channel using a received specific signal (e.g., pilot signal). The channel estimator  200  provides the receive vector and the channel matrix to the MIMO detector  202  and the LLR updater  212 . 
     The MIMO detector  202  estimates a transmit vector by demodulating the receive vector fed from the channel estimator  200  according to a certain MIMO detection scheme. The MIMO detector  202  outputs an LLR corresponding to the interference signal among LLRs constituting the transmit vector to the first decoder  204 , and outputs an LLR corresponding to the desired signal to the second decoder  208 . The MIMO detection scheme can include an MMSE scheme, an ML scheme, and so forth. The MIMO detection scheme is not limited to a specific scheme, and it is assumed that the MMSE scheme is adopted. Using the MMSE scheme, the MIMO detector  202  can estimate the transmit vector using Equation 2.
 
 x{circumflex over (x)} =( H   H   H+σ   2   I ) −1   H   H   y    [Eqn. 2]
 
     In Equation 2, x is a transmit vector, H is a channel matrix, σ 2  is a noise power, I is an identity matrix, y is a receive vector, and the superscript H is a Hermitian transpose. 
     The first decoder  204  outputs soft decision values by iteratively decoding the estimated interference signal x, output from the MIMO detector  202 . In doing so, the first decoder  204  examines whether there is a bit of the inverted sign during the iterative decoding. Detecting the bit (LLR value) of the inverted sign, the first decoder  204  provides the LLR of the detected bit to the LLR updater  212 . 
     The LLR updater  212  updates the LLR value of the second decoder  208  with the LLR value output from the first decoder  204  and provides the updated LLR value to the second decoder  208 . Herein, the LLR can be updated in various manner based on the MIMO detection scheme. For example, using the MMSE scheme, the LLR value can be updated using Equation 3:
 
 x   2   =h   2   H (( h   2   h   2   H   +h   1 (1− {circumflex over (x)}   1   {circumflex over (x)}   1 ) h   1   H +σ 2   I ) −1 ) H ( y−h   1   {circumflex over (x)}   1 )   [Eqn. 3]
 
     In Equation 3, {circumflex over (x)} 1  is an LLR value generated in the process of the iterative decoding of the first decoder  204 . Equation 3 can be simplified as Equation 4. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     Concurrently with the first decoder  204 , the second decoder  208  outputs soft decision values by iteratively decoding the estimated desired signal {circumflex over (x)} 2  output from the MIMO detector  202 . When the LLR is updated by the LLR updater  212  in the process of the iterative decoding, the second decoder  208  continues the iterative decoding with the updated LLR value. 
     The first hard decision part  206  outputs decoded data {tilde over (x)} 1  by hard-deciding the soft decision values acquired through the iterative decoding of the first decoder  204 . The second hard decision part  210  outputs decoded data {tilde over (x)} 2  by hard-deciding the soft decision values acquired through the iterative decoding of the second decoder  208 . 
     In this embodiment of the present invention, the signal is detected by regarding one of two received signals as the interference. 
     To detect all of the two signals as in the MUD or the SM, the LLR updater  212  can update the internal LLR value of the second decoder  208  using the internal LLR value of the first decoder  204  and update the internal LLR value of the first decoder  204  using the internal LLR value of the second decoder  208 . 
     As such, two signals can be decoded at the same time using a Parallel Interference Cancellation (PIC) scheme. If there are two or more signals to be decoded using the PIC scheme, Equation 3 can be generalized as Equation 5:
 
 x   k   =h   k   H (( h   k   h   k   H   +H   k (1−diag( {circumflex over (x)}   k   {circumflex over (x)}   k   H )) H   k   H +σ 2   I ) −1 ) H ( y−H   k   {circumflex over (x)}   k )   [Eqn. 5]
 
     In Equation 5, H k =[h 1  . . . h k−1  h k+1  . . . h N     T   ] indicates a matrix excluding h k  and {circumflex over (x)} k =[{circumflex over (x)} 1  . . . {circumflex over (x)} k−1  {circumflex over (x)} k+1  . . . {circumflex over (x)} N     T   ] T  indicates an interference signal vector constituted with LLR values excluding {circumflex over (x)} k . Equation 5 can be simplified as Equation 6:
 
 x   k =( h   k   H   h   k +σ 2 ) −1   h   k   H ( y−H   k   {circumflex over (x)}   k )   [Eqn. 6]
 
       FIG. 3  illustrates operations of the receiver to perform a Serial Interference Cancellation (SIC) according to an embodiment of the present invention. To facilitate understanding, the signal is detected by regarding one of the two received signals as the interference. 
     In step  301 , the receiver constructs the receive vector y using the signals received on the plurality of antennas and constructs the channel matrix H. 
     In step  303 , the receiver generates soft decision values (LLR values) by demodulating the receive vector according to a certain MIMO detection scheme. The MIMO detection scheme can adopt the MMSE scheme, the ML scheme, and so forth. For example, using the MMSE scheme, the receiver estimates the desired signal and the interference signal based on Equation 2. 
     After the MIMO detection, the receiver iteratively decodes the estimated desired signal and the estimated interference signal at the same time in step  305 . After completing one iterative decoding, the receiver checks whether there is a bit of the inverted sign in the bits (LLR values) of the interference signal by comparing with the result prior to the iterative decoding in step  307 . When there is no LLR of the inverted signal, the receiver checks whether the iterative decoding is performed by a preset number of times in step  313 . 
     When there is the LLR of the inverted signal, the receiver calculates the LLR value of the desired signal corresponding to the sign-inverted LLR in step  309 . For example, using the MMSE detection scheme, the receiver calculates the LLR value of the desired signal based on Equation 3 or 4. In step  311 , the receiver updates the corresponding LLR value of the desired signal being iteratively decoded with the calculated value. 
     After updating the LLR value of the desired signal with the LLR value of the interference signal, the receiver examines whether the iterative decoding is performed by the preset number of times in step  313 . When the number of times of the iterative decoding does not reach the preset number of times, the receiver returns to step  305  to perform the next iterative decoding. When the number of times of the iterative decoding reaches the preset number of times, the receiver acquires the information bit stream by hard-deciding the LLR values of the desired signal acquired through the iterative decoding in step  315 . 
       FIG. 4  illustrates operations of a receiver to perform a Parallel Interference Cancellation (PIC) according to another embodiment of the present invention. To facilitate understanding, two received signals are detected at the same time. 
     In step  401 , the receiver constructs the receive vector y using the signals received on the plurality of antennas, and constructs the channel matrix H. 
     In step  403 , the receiver generates soft decision values (LLR values) by demodulating the receive vector according to a certain MIMO detection scheme. The MIMO detection scheme can adopt the MMSE scheme, the ML scheme, and so forth. For example, using the MMSE scheme, the receiver estimates the first signal and the second signal based on Equation 2. 
     After the MIMO detection, the receiver iteratively decodes the estimated first and second signals at the same time in step  405 . After completing one iterative decoding, the receiver checks whether there is an LLR of the inverted signal in each of the first signal and the second signal by comparing with the result prior to the iterative decoding in step  407 . There is no LLR of the inverted signal, the receiver checks whether the iterative decoding is performed by a preset number of times in step  413 . 
     When there is the LLR of the inverted signal, the receiver calculates the LLR value of the other signal of the sign-inverted LLR in step  409 . For example, using the MMSE detection scheme, the receiver can calculate the LLR value of the other signal based on Equation 3 or 4. In step  411 , the receiver updates the corresponding LLR value of the other signal being iteratively decoded with the calculated value. 
     After updating the LLR value, the receiver examines whether the iterative decoding is performed by the preset number of times in step  413 . When the number of times of the iterative decoding does not reach the preset number of times, the receiver returns to step  405  to perform the next iterative decoding. When the number of times of the iterative decoding reaches the preset number of times, the receiver acquires an information bit stream of the first signal by hard-deciding the LLR values of the first signal acquired through the iterative decoding in step  415 . In step  417 , the receiver acquires an information bit stream of the second signal by hard-deciding the LLR values of the second signal acquired through the iterative decoding. 
     As set forth above, the receiver, which cancels the interference using the decoding result, can reduce the latency using the decoding result. Since the SIC/PIC is applied in the process of the iterative decoding of the decoder, the latency can be eliminated by applying the SIC/PIC after the completion of the decoding. In addition, the decoding performance can be enhanced by updating the internal LLR value with the reliable value in the process of the decoding. 
     Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.