Abstract:
A method for canceling an interference signal by using a parallel multi-stage structure in a code division multiple access (CDMA) system, includes the steps of: a) delaying a receiving signal; b) receiving and predicting data and channel parameter from the individual user; c) regenerating the interference signal of the individual user by using the data and the channel parameter predicted by the multiple receiving and predicting; and d) canceling all the interference signals delayed and transferred through the delaying by the interference signal regenerated through the multiple interference regeneration.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an enhanced multi-user receiving apparatus for a code division multiple access (CDMA) system; and, more particularly, to an integrated receiving apparatus of a subtractive interference cancellation receiver and an adaptive minimum mean squared error (MMSE) receiver for a CDMA system. 
     DESCRIPTION OF THE PRIOR ART 
     In general, techniques about adaptive MMSE receivers are disclosed in articles, such as S. L. Miller and A. N. Barbosa, “A Modified MMSE Receiver for Detection of DS-CDMA Signals in Fading Channels”, Proc. MILCOM&#39;96, pp. 898-902 and M. Latvaaho and M. Juntti, “Modified Adaptive LMMSE Receiver for DS-CDMA Systems in Fading Channels”, Proc. PIMRC&#39;97, pp. 554-558. 
     An adaptive MMSE receiver shows much better performance than a conventional CDMA receiver does in a static channel. Also, the adaptive MMSE receiver can suppress out-of-cell interference in a multi-cell channel environment. However, the radio channels are not static but time varying and fading. In fading environments, specifically under the low signal to interference plus noise ratio (SINR) during the deep fading, the performance of the adaptive MMSE receiver is severely degraded. 
     And, a thesis, M. K. Varanasi and B. Aazhang, “Multistage detection in asynchronous code-division multiple-access communications “IEEETr. on Commun., vol. 38, No. 4, pp. 509-519, April 1990 and another thesis S. R. Kim, J. G. Lee and H. Lee, “Interference cancellation scheme with simple structure and better performance” Electronic Letters, November 1996, vol. 32, No. 23, pp. 2115-2117 refer to an apparatus of a subtractive interference cancellation receiver. 
     The subtractive interference cancellation receiver has better performance in comparison with an existing conventional CDMA receiver in a general wireless channel environment. However, the subtractive interference cancellation receiver cannot suppress the interference caused by other cells in multiple cell environments. 
     SUMMARY OF INVENTION 
     It is, accordingly, an object of the present invention to provide an integrated receiving apparatus of a subtractive interference cancellation receiver and an adaptive MMSE receiver that is capable of overcoming limitations of the subtractive interference cancellation receiver and the adaptive MMSE receiver and having improved performance. 
     In accordance with one aspect of the present invention, there is provided an apparatus for canceling an interference signal by using a parallel multi-stage structure in a code division multiple access (CDMA) system, comprising: a delay unit for delaying the receiving signal in regular sequence; and an interference cancellation unit for canceling the interference of the signal, which is delayed and transmitted through said delay unit. 
     In accordance with another aspect of the present invention, there is provided a method for canceling an interference signal by using a parallel multi-stage structure in a code division multiple access (CDMA) system, comprising the steps of: a) delaying a receiving signal; b) receiving and predicting data and channel parameter from the individual user; c) regenerating the interference signal of the individual user by using the data and the channel parameter predicted by the multiple receiving and predicting; and d) canceling all the interference signals delayed and transferred through the delaying by the interference signal regenerated through the multiple interference regeneration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram illustrating an integrated receiver of a parallel multi-stage interference cancellation receiver and an adaptive MMSE receiver in accordance with one embodiment of the present invention; 
     FIG. 2A is a block diagram depicting the adaptive MMSE receiver based on constrained minimum mean square error criterion shown in FIG. 1; 
     FIG. 2B is a block diagram of the channel estimating part of FIG. 2A; 
     FIG. 2C is a block diagram of the tap weight controlling part of FIG. 2A; and 
     FIG. 2D is a graph showing comparison of the convergence characteristic of a tap weight of an adaptive filtering part in processing a receiving signal in accordance with the present invention with that of the adaptive filter employing a prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an integrated receiver of a parallel multi-stage interference cancellation receiver and an adaptive MMSE receiver includes delay units  211  to  21 N and interference cancellation units  201  to  20 N. The delay units  211  to  21   n  delay the receiving signal in regular sequence. The interference cancellation units  201  to  20 N cancel interference of the receiving signal, which is delayed and transmitted through the delay units  211  to  21 N. 
     Therefore, the interference of the receiving signal delayed by the delay unit  211  is canceled by the interference cancellation unit  201 , and the interference of the receiving signal delayed by the delay unit  212  is canceled by the interference cancellation device and so on. Such an interference cancellation process is carried out until the interference of the receiving signal is canceled. 
     The interference cancellation unit  201  includes receiving devices  221  to  22 N, interference regeneration devices  231  to  23   n  and an interference cancellation device  240 . The receiving devices  221  to  22 N receive the data and the channel parameter. The interference regeneration devices  231  to  23 N regenerate the interference signal of the individual user by utilizing the data and the channel parameter predicted by the receiving devices  221  to  22 N. 
     The interference cancellation device  240  cancels all the interference signals of the receiving signal transmitted from the individual user. Of course, the interference cancellation units  201  to  20 N have the same architecture as each other. 
     The receiving devices  221  to  22 N are the same as the adaptive MMSE receiver shown in FIG. 2A, respectively. 
     The interference regeneration device  231  includes a data spreading element  251 , a pulse shaping element  252  and a multiplying element  253 . The data spreading element  251  spreads the data predicted by the receiving machine. The pulse shaping element  252  shapes the pulse-figured signal output from the data spreading element  251 . The multiplying element  253  multiplies the output signal from the pulse shaping element  252  and the channel parameter predicted through the receiving device  221  together to output the interference signal. 
     Of course, the interference regeneration devices  231  to  23 N have the same architectures, and this technique is well known to the public. 
     The interference cancellation device  240  includes an adding element  241  and adding elements  261  to  26 N. The adding element  241  adds the interference signal regenerated by the interference regeneration devices  231  to  23 N to the receiving signal transmitted through the delay unit  211 . Adding elements  261  to  26 N add the interference signal regenerated by the interference regeneration devices  231  to  23 N to the added value through the adding element  241 . 
     The operation of the integrated receiver of a parallel multi-stage interference cancellation receiver and an adaptive MMSE receiver in accordance with the present invention having the same architecture as mentioned above will be explained in detail as follows. However, thereafter the operation of the first-stage interference cancellation unit  201  will be explained for example. 
     The receiving devices  221  to  22 N transmit the predicted data and channel parameter to the interference regeneration devices  231  to  23 N, responsibly. 
     The interference generation devices  231  to  23 N regenerate the interference signals of all the users by using the data and the channel parameter predicted through the receiving devices  221  to  22 N. 
     The interference cancellation device  240  outputs only the signal component corresponding to the original signal by canceling all the interference components of the delayed signal through the delay unit  211  to be synchronized with the interference signal regenerated through the interference regeneration devices  231  to  23 N. 
     In addition, the parallel multi-stage interference cancellation process is the same as the above mentioned. 
     FIG. 2A is a block diagram depicting an adaptive MMSE receiver for detecting a receiving signal based on constrained minimum mean square error (MMSE) criterion in accordance with the present invention. 
     FIG. 2A illustrates a preferable configuration of the adaptive MMSE receiver for detecting a receiving signal, when a signal is transmitted by a CDMA sending apparatus using a pilot symbol aided binary phase shift keying (BPSK) method. 
     In the FIG. 2A,  110  indicates an adaptive filtering part,  120  indicates a channel estimating part,  130  indicates a signal restoring part,  140  indicates a selecting part,  150  indicates a reference signal generating part,  160  indicates an error calculating part, and  170  indicates a tap weight controlling part. 
     As described FIG. 2A, the adaptive MMSE receiver includes an adaptive filtering part  110  whose tap weight is controlled by an output signal of a controlling part  170  as described later for filtering a receiving signal, removing an reference signal included in the receiving signal and extracting a desired signal; a channel estimating part  120  for estimating a phase component and a amplitude component of a particular user channel by using the output signal of the adaptive filtering part  110 ; a signal restoring part for restoring an original signal from a signal transmitted from a particular user by using the channel estimated signal from the channel estimating part  120  and the filtered signal from the adaptive filtering part  110 ; a selecting part  140  for selecting and transmitting either the restored signal from the signal restoring part  130  or a known training signal; a reference signal generating part  150  for generating a reference signal by using the channel estimated signal from the channel estimating part  120  and the selected signal from the selecting part  140 ; an error calculating part  160  for calculating an error between the filtered signal from the adaptive filtering part  110  and the reference signal from the reference signal generating part  150  by comparing the signals; and a tap weight controlling part  170  for controlling the tap weight of the adaptive filtering part  110  based on constrained minimum mean square error (MMSE) criterion. 
     Hereafter, with reference to FIGS. 2B and 2C, we will describe the detailed configuration of some part and operation of the adaptive MMSE receiver. 
     A receiving signal may include a transmitted signal from multiple users, in here, we assume that a receiving signal includes the transmitted signal from the first user of the users. 
     As known to FIG. 2B, a complex receiving signal  r (m) is inputted to the adaptive filtering part  110  and is multiplied by a tap weight  w (m). The channel estimating part  120  estimates a channel by using the output signal of the adaptive filtering part  110 , then estimates a phase component {circumflex over (φ)} 1 (m) and an amplitude component {circumflex over (α)} 1 (m). 
     FIG. 2B is a block diagram of the channel estimating part  120  of FIG.  2 A. 
     As described FIG. 2B, the channel estimating part  120  includes: a pilot signal extractor  121  for extracting a known pilot signal with a predetermined period included in the filtered receiving signal from the adaptive filtering part  110 ; an operator  123  multiplying the pilot signal by a predetermined value; an operator  125  for finding a sum of the output values from the operator  123  in a predetermined period; and an operator  127  for finding and outputting the mean by dividing the sum from the operator  123  into the number of output from the operator  121 . And, the operator  127  provides the operated signal to the reference generating part  150  and provides a complex conjugate of the operated signal to the signal restoring part  130 . 
     The signal restoring part  130  includes: an operator  131  for receiving and multiplying the complex conjugated output signal (it is reverse phase information −{circumflex over (φ)} 1 (m) of the channel estimated by the channel estimating part  120 ) of the channel estimating part  120  by the filtered complex receiving signal of the adaptive filtering part  110 ; a real value extractor  132  for extracting a real component from the multiplied value from the operator  131 ; and a bit value determiner  133  for determining a bit value of the extracted real value from the real value extractor  132 . 
     The reverse phase information −{circumflex over (φ)} 1 (m) estimated by the channel estimating part  120  is multiplied by the output signal of the adaptive filtering part  110  via the operator  131  and is outputted in form of complex. As the result, a real component of the output is extracted by the real value extractor  132  and is applied to the bit value determiner  133 . 
     And, the bit value determiner  133  determines the extracted real value as “1” if the value is larger than “0”, otherwise, determines the extracted real value as “0”, then, restores the transmitted signal from the first user. 
     The selecting part  140  selects a known training data and transmits it to the reference generating part  150  in a tap weight period of a predetermined range, while it selects a bit value data outputted from the bit value determiner  133  and transmits it to the reference generating part  150  in the other periods. 
     As described above, a training data for a tap weight is generally needed in processing a receiving signal of an adaptive apparatus. However, since the present invention uses a pilot symbol transmitted from a sending apparatus as the training data of the tap weight, it need not make and provide another training data like a conventional signal processing apparatus for adaptive receiving system. 
     The reference signal generating part  150  multiplies the phase component {circumflex over (φ)} 1 (m) and an amplitude component {circumflex over (α)} 1 (m) of the estimated channel from the channel estimating part  120  by the selected signal from the selecting part  140 , outputs the result signal to the error calculating part  160  as a reference signal. 
     The error calculating part  160  calculates the error between the reference signal and the filtered complex receiving signal, provides the result as an error signal {tilde over (e)}(m). 
     The tap weight controlling part  170  controls the tap weight to minimize the size of the error signal {tilde over (e)}(m) detected by the error calculating part  160 , provides it to the adaptive filtering part  110 . 
     If the tap weight controlling part  170  changes the tap weight  w (m) of the adaptive filtering part  110  by using a prior LMS algorithm, the change of the tap weight is expressed by an equation (1): 
     
       
           w   1 ( m )= w   1 ( m− 1)+μ{tilde over (e)}( m )* r ( m ) 
       
     
     where the  w (m) indicates a tap weight vector,  r (m) indicates a receiving signal vector, and μ indicates a step size determining speed for changing the tap weight of the adaptive filtering part  110 , {tilde over (e)}(m) indicates a calculated error by the error calculating part  160 , and the superscript*indicates complex conjugate. 
     However, as described above, when a prior tap weight adaptive algorithm like the equation (1) is used to an adaptive apparatus for detecting a receiving signal, the tap weight of the adaptive filtering part  110  converges to “0”. 
     Thus, the prior LMS algorithm cannot be used at an adaptive apparatus for detecting a receiving signal, if we intend to use the prior LMS, we must input the unfiltered input signal of the adaptive filtering part  110  instead of the filtered signal of the adaptive filtering part  110 . But, such a change makes the performance of the adaptive MMSE receiver for detecting a receiving signal decreased remarkably. 
     Thus, the present invention employs the constrained MMSE criterion expressed by equation (2) as adaptive algorithm of the adaptive filtering part  110 . 
     FIG. 2C is a block diagram of the tap weight controlling part  170 . 
     As described FIG. 2C, the tap weight controlling part  170  of the present invention includes: a pseudo noise (PN) generator  171  for providing a pseudo noise (PN) signal; an operator  172  for multiplying the PN signal from the PN generator  171  by the complex receiving signal; an operator  173  for multiplying the PN signal by the output signal of the operator  172 ; an operator  174  for subtracting the output signal of the operator  173  from the receiving signal; an operator  175  for multiplying the complex conjugate of the error signal from the error calculating part  160  by the output signal of the operator  174 ; an operator  176  for multiplying the step size determining the controlling period of the tap weight by the output signal of the operator  175 ; an operator  177  for adding an operated output signal immediately before to the output signal of the operator  176 ; the operator  179  for adding the sum of the PN signals to the output signal of the operator  177 , and providing the added value to the adaptive filtering unit as a new tap weight. 
     The constrained MMSE criterion according to the present invention is expressed by an equation (2): 
     
       
           J = E[|ĉ   1 ( m )d 1 ( m )− w ( m ) H   r ( m )| 2 ] Subject to  w ( m ) H   s   1 =1 
       
     
     wherein J indicates constrained minimum mean square error criterion, and E indicates a mean value and ĉ 1 (m) of ĉ 1 (m)={circumflex over (α)} 1 (m)exp jφ     1 (m)    indicates the estimated channel from the channel estimating part  120 , d 1 (m) indicates the output signal from the selecting, part  140 ,  w (m) indicates a tap weight vector,  s   1  indicates a spreading code vector, and the subscript H indicates Hermitian operation. 
     The constrained MMSE criterion of the equation (2) may be implemented by using an orthogonal decomposition method. If the orthogonal decomposition method is used, the tap weight  w (m) of the adaptive filtering part  110  is expressed by equation (3): 
     
       
           w ( m ) =   s   1 + x ( m ) 
       
     
     Where the  s   1  indicates a spreading code vector, the  x (m) indicates an adaptive component of tap weight vector, and two vectors are orthogonal. 
     Accordingly, the multiplication of the tap weight vector by the spreading code vector ( w (m) H s 1 ) is expressed by equation (4): 
     
       
           w ( m ) H   s   1 =( s   1 + x ( m )) H   s   1 = s   1   H   s   1 =∥ s   1 ∥ 2   
       
     
     Where the multiplication ∥ s   1 ∥ 2  of the spread code vector is normalized to “1” (i.e.  w (m) H   S   1 =1). Consequently, the constrained MMSE criterion is expressed by an equation (5): 
     
       
           x ( m )= x ( m −1)+μ·{tilde over (e)}( m )*· r   x ( m ) 
       
     
     Where  x (m) indicates an adaptive component of the tap weight vector, μ indicates a step size, {tilde over (e)}(m) indicates the calculated error from the error calculating part  160 ,  r   x (m) indicates a receiving signal  r (m) projected by the adaptive component of the tap weight, and the subscript*indicates a complex conjugate operation. On the other hand, the constrained MMSE criterion may be called as “LMS algorithm of an orthogonal decomposition method”. 
     Thus, as the apparatus processes a receiving signal successively, the adaptive filtering part  110  is normally operated like FIG. 2D (see the “B” in the FIG.  2 D). The constrained MMSE criterion of the adaptive algorithm according to the present invention has the following characteristics. 
     The first, the algorithm differs from a prior art in that the signal with a simultaneously compensated phase and amplitude is used to a reference signal. Also, by the algorithm, the multiplication of the tap weight  w (m) of the adaptive filter by the spreading code vector  s   1  is limited to “1” like the equation (2) so that the tap weight does not converge to “0”, while a tap weight is controlled by the calculated error between the predetermined reference signal and the filtered receiving signal. 
     And, if the constrained MMSE criterion is implemented by using orthogonal decomposition-based LMS algorithm like the equation (5), the orthogonal decomposition-based LMS algorithm orthogonal decomposes the tap weight like the equation (3) into an adaptive component orthogonal to a spreading code vector and a spreading code vector component. Also, the algorithm uses the receiving signal projected by the adaptive component orthogonal to the spreading code vector in order to change an adaptive component orthogonal to the spreading code vector. 
     On the other hand, those skilled in the art could recognize that hardware may be implemented with the element functions of FIGS. 2A to  2 C as well as that the most functions may be replaced by software based on the above description. 
     As described above, since a tap weight of an adaptive filter does not converge to “0” even if a phase and amplitude of an estimated channel are simultaneously compensated by using the output signal of an adaptive filter, the adaptive MMSE receiver for detecting a receiving signal has an improved receiving performance in a fading channel environment. Thus, the present invention can provide a high speed and quality service in radio communication. Also, since one base station can accommodate a number of subscribers, a capacity of a system is increased. 
     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.