Abstract:
Even when the number of paths is increased, interfering noises can be effectively reduced by a rake receiver for use in a spread spectrum communication system. The rake receiver includes a timing detecting unit detecting a reception timing t i  (i=1 to N) of each of N paths when direct spread spectrum signals of the N paths are received; an inverse spreading timing setting unit setting, as a timing for inverse spreading, a timing t i,j,k  (k=1 to N, k≠j) at which an inverse spread value is obtained that has interference and correlation from the jth (j=1 to N, j≠i) path included in the inverse spread value of the ith path counted from the reception timing t i  (i=1 to N) detected by the timing detecting unit; a plurality of correlators each obtaining an inverse spread signal of the received signal corresponding to each timing set by the inverse spreading timing setting unit; and a signal composing unit composing outputs of the plurality of correlators.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is a continuation of International Application No. PCT/JP02/09414 which was filed on Sep. 13, 2002, which is herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a spread spectrum scheme, and more particularly, to a rake receiver that executes, as a reception diversity scheme in a multi-path environment, maximal-ratio composition in the time domain of signals arriving at an antenna with various differences in delay time thereof caused by multiple reflections of propagation paths of the signals. 
   2. Description of the Related Art 
   Spread spectrum or spread spectrum communication scheme is utilized extensively as a basic technique for mobile communication. In the direct spread (DS) scheme as the simplest model of the spread spectrum communication, an information signal is transmitted to the receiving side after the spectrum of the information signal is spread by modulating, that is, multiplying the information signal to be transmitted by a PN signal having the chip width Tc of 1/100 to 1/1000 of the cycle T of the information signal to be transmitted as a spread signal. 
   On the receiving side, the signal component is detected from the signal buried in noises by inverse spreading. The inverse spreading basically refers to executing demodulation by multiplying a received signal by a same PN signal having the same phase as that of the PN signal in the received signal. 
   However, in a multi-path environment for many reflected waves to be present in addition to a direct wave, it is necessary to detect a true signal component by composing appropriately signals received with various differences in delay time. 
   As one of such conventional techniques, a rake scheme can be listed. “Rake” means a rake in English and the rake scheme is a diversity scheme for executing the maximal-ratio composition by collecting power dispersed due to the delay dispersion of transmission paths, into one like a “rake”. 
   In a conventional rake receiver, a desired signal is demodulated by finding a plurality of path timings at which multiple paths arrive using a known signal, informing a demodulator of these path timings, executing inverse spreading at these timings in the demodulator and composing signals of the multiple paths. 
     FIG. 1  is a block diagram showing generally an example of the construction of a rake receiver as, for example, a mobile communication terminal. In the figure, the receiver has an antenna  100 , a wireless receiving unit  101 , an A/D converting unit  102 , a searcher  103  for detecting a plurality of timings of the multiple paths and an inverse spreading timing generating and inverse spreading unit  104  for executing an inverse spreading to the plurality of paths according to the timings of the plurality of paths detected by the searcher  103 . 
   The receiver further has a signal composing unit  105  for composing signals of the plurality of paths obtained by the inverse spreading timing generating and inverse spreading unit  104 , a signal processing unit  106  such as a channel codec for receiving an output of the signal composing unit  105  and outputting received signals to a display, speaker, etc., and a level measuring unit  107  for measuring the level of the received signals of the plurality of paths, providing reliability degree information and signal level information to the signal composing unit  105  and providing to a transmission unit  108  control information of transmission power to a base station. 
   The transmission unit  108  transmits input from a keyboard or a microphone, from the antenna  100  through a duplexer  109  in response to the control information from the level measuring unit  107 . 
     FIG. 2  is a block diagram of the detailed construction of the inverse spreading timing generating and inverse spreading unit  104  of  FIG. 1 , that is, a signal demodulating unit. In the figure, the signal demodulating unit comprises a spread code generator  110 , a plurality of delay control units  111 - 1  to  111 -n and a plurality of correlators  112 - 1  to  112 -n corresponding thereto. 
   The spread code generator  110  generates a code for inverse spreading. The plurality of delay control units  111 - 1  to  111 -n control respectively delay operations of the plurality of correlators  112 - 1  to  112 -n corresponding respectively to timings t 1  to  1 N of the multiple paths detected by the searcher  103 . Each of the correlators  112 - 1  to  112 -n executes inverse spreading on the received signals from the A/D converting unit  102  according to the inverse spread timings controlled by the corresponding delay control units  111 - 1  to  111 -n. 
   Thereby, the correlators  112 - 1  to  112 -n respectively provide inverse spread signals  1  to N to the signal composing unit  105  and the signal composing unit  105  composes these signals and outputs a demodulated signal. 
   Such an inverse spread signal includes a channel estimation signal corresponding to a propagation coefficient of each of the multiple paths. 
   As described above, for example, in  FIG. 2 , inverse spreading is executed using the timings themselves of each path of the multiple paths. When inverse spreading is executed at a timing, signals corresponding to paths other than the path of this timing are all interference. Especially, in the case where an orthogonal spread code is used for a plurality of channels in a downlink from a base station in the CDMA scheme, a problem exists that the reception property is degraded due to the multi-path interference. 
   Considering the above point, the inventor has previously proposed a rake receiver capable of suppressing multi-path interference when the spread spectrum scheme is used in a multi-path environment in Japanese Patent No. 2001-332510. 
   Here, the schematic construction of such a rake receiver as proposed previously will be described.  FIG. 3  is a block diagram of the principle construction of a rake receiver constituting a spread spectrum communication system in a multi-path environment, previously proposed. 
   In  FIG. 3 , path timing detecting  1  correspond to, for example, the path searcher  103  of  FIG. 1  and  FIG. 2  detect timings of, for example, N paths. 
   Inverse spreading timing setting  2  set the detected timings of the paths as timings for inverse spreading, that is, timings for demodulating spread encoding signal by multiplying an inverse spread code. Concurrently, settings are made to all combinations of two (2) paths such that, taking the center at a timing of one (1) path of arbitrary two (2) paths, two (2) timings at positions symmetrical to the timing of the other path on the time axis by the delayed time of the timings of the two (2) paths are timings of the inverse spreading. 
   A plurality of correlators  3 - 1  to  3 -n respectively obtains an inverse spreading signal of a signal resulted from, for example, A/D conversion of a signal sent from the transmitting side in response to each timing having been set. Signal composing  4  compose outputs of the plurality of correlators  3 - 1  to  3 -n and output a demodulated signal. 
   As described above, in the present invention proposed in the previous application, interference component contained in a desired signal is reduced using a multi-path interference correlative signal (MICS) reproduced using only information of selected two (2) paths. 
   However, as described above, in the present invention of the previous application, a drawback is recognized that the effect of the reduction of the interference component becomes smaller as the number of the paths increases because the information of only selected two (2) paths is utilized when the interference component is reproduced. That is, information that must be contained in paths other than the noted two (2) paths can not be utilized. 
   SUMMARY OF THE PRESENT INVENTION 
   It is therefore the object of the present invention to provide a spread spectrum rake receiver capable of overcoming such disadvantage of the present invention of the previous application. 
   In order to achieve the above object, according to a first aspect of the present invention there is provided a rake receiver for use in a spread spectrum communication system, comprising timing detecting operable to detect a reception timing t i  (i=1 to N) of each of N paths when direct spread spectrum signals of the N paths are received; inverse spreading timing setting operable to set, as a timing for inverse spreading, a timing t i,j,k  (k=1 to N, k≠j) at which an inverse spread value is obtained that has interference and correlation from the jth (j=1 to N, j≠i) path included in the inverse spread value of the ith path counted from the reception timing t i  (i=1 to N) detected by the timing detecting; a plurality of correlators each operable to obtain an inverse spread signal of the received signal corresponding to each timing set by the inverse spreading timing setting; and signal composing operable to compose outputs of the plurality of correlators. 
   The rake receiver may further comprise, between the correlators and the signal composing, a circuit operable to compose a multi-path interference signal (mics(i,j,k)) of a path k (k≠j) from the following Eq. (1), 
                     MICS   ⁡     (     i   ,   j     )       =       r     i   ,   j       ⁢       ∑     k   =   j       ⁢       r   k   ′     ⁢     mics   ⁡     (     i   ,   j   ,   k     )               ,           Eq   .           ⁢     (   1   )                 
to reproduce interference α j I i,j  and subtract the interference MICS(i,j) from the ith path.
 
   When the multi-path interference signal (mics(i,j,k) ) is composed, coefficients r i,j  and r′ k  of Eq. (1) may be obtained from the following Eq. (2) and Eq. (3),
 
 r   i,j =α j   I   2 /{( I/N ) j +1 }  Eq. (2),
 
 r′   k =α* k /{Σ I≠k |α I | 2   I   2   +n   2 }  Eq. (3),
 
and the multi-path interference signal (mics(i,j,k)) may be composed using the maximal ratio composition.
 
   When the multi-path interference signal (mics(i,j,k)) is composed, noises of the multi-path interference signal (mics(i,j,k)) may be approximated to be constant and the coefficients r i,j  and r′ k  of Eq. (1) may be obtained from the following Eq. (4) and Eq. (5),
 
 r   i,j =α j ( I/N ) j /{(Σ k≠j |α k | 2 ) ( ( I/N ) j +1)}  Eq. (4),
 
 r′   k =α* k    Eq. (5),
 
and the multi-path interference signal (mics(i,j,k)) may be composed using the maximal ratio composition.
 
   The circuit operable to subtract the interference MICS(i,j) may include a circuit operable to select a plurality of paths having high power, and the circuit operable to subtract the interference MICS(i,j) may subject the selected paths to processes for composing the multi-path interference signal (mics(i,j,k)) and subtracting the interference MICS(i,j). 
   The circuit operable to subtract MICS(i,j) from the ith path may select a plurality of paths i having high power and may be provided in the quantity corresponding to the number of the selected paths. 
   The inverse spreading timing setting may detect coincidence between the timing t i,j,k  for the inverse spreading and a reception timing t i  and, may not subject the paths between which the coincidence has been detected, to the processes for composing the multi-path interference signal (mics(i,j,k)) and subtracting the interference MICS(i,j). 
   The rake receiver may further comprise a level compensating circuit disposed between the circuit operable to subtract the interference MICS(i,j) and the signal composing, the level compensating circuit acting to compensate the levels of signals after reduction of the interference in the circuit operable to reduce the interference MICS(i,j) to keep the level of the noises constant. 
   The rake receiver may further comprise, at the preceding stage of the correlators, a circuit operable to compose a multi-path interference signal (mics(i,j,k) of a path k (k≠j) from Eq. (1) 
                     MICS   ⁡     (     i   ,   j     )       =       r     i   ,   j       ⁢       ∑     k   =   j       ⁢       r   k   ′     ⁢     mics   ⁡     (     i   ,   j   ,   k     )               ,           Eq   .           ⁢     (   1   )                 
to reproduce interference α j I i,j , and subtract the interference MICS(i,j) from the ith path.
 
   The circuit operable to subtract the interference MICS(i,j) may include a circuit operable to select a plurality of paths having high power, thereby subjecting the selected paths to the processes for composing the multi-path interference signal (mics(i,j,k)) and subtracting the interference MICS(i,j). 
   The circuit operable to subtract the interference MICS(i,j) from the ith path may select a plurality of paths i having high power and may be provided in the quantity corresponding to the number of the selected paths. 
   The above and other features of the present invention will become more apparent from the description of the embodiments of the invention when taken in conjunction with the accompanying drawings which follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1  is a block diagram showing generally an example of the construction of a rake receiver as a mobile communication terminal; 
       FIG. 2  is a block diagram of the detailed construction of the inverse spreading timing creating and inverse spreading unit  104  of  FIG. 1 , that is, a signal demodulating unit; 
       FIG. 3  is a block diagram of the principle construction of a rake receiver constituting a spread spectrum communication system in a multi-path environment, previously proposed; 
       FIG. 4  shows timings of signals of two (2) paths received by a CDMA (Code Division Multiple Access) mobile terminal; 
       FIG. 5  shows N path signals, and a plurality of Multi-path Interference Correlative Timings (MICTs) that can be utilized for reduction of interference of a path j contained in a path i; 
       FIG. 6  shows an example of the construction of a CDMA receiver applied with the present invention; 
       FIG. 7  shows an example of the construction of a composing unit  27  of  FIG. 6 ; 
       FIG. 8  shows another exemplary embodiment of the present invention; 
       FIG. 9  shows the detailed construction of a multi-path interference exchange reduction circuit  28 ; 
       FIG. 10  shows the details of MICS units  280 - 1  to  280 -N represented by a MICS unit  280 - i ; 
       FIG. 11  shows an example of the construction of an MRC  128 ; 
       FIG. 12  shows another example of the construction of the MICS unit  280 - i ; 
       FIG. 13  shows a construction that is provided with a selector circuit  31  at the preceding stage of a MIXR circuit  28 , as an exemplary embodiment; 
       FIG. 14  shows yet another example of the construction of the MICS unit  280 - i : 
       FIG. 15  shows a construction that is provided with a level correcting unit  32  between the MIXR unit  28  and a rake composing unit  27  that is the following stage of the level correcting unit  32 ; 
       FIG. 16  shows yet another exemplary embodiment of the present invention; and 
       FIG. 17  shows the construction of the MICS unit  280 - i  constituting the MIXR unit  28  in the construction of FIG.  16 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Here, prior to the description of an exemplary embodiments of the present invention, the principle of the previously applied invention by the present inventor described above will be further described for the full understanding of the present invention. 
   When signals of multiple paths are inversely spread at a timing, signals of paths that occur interference are determined by mutual correlation value of an inverse spreading signal, the attenuation coefficient of a propagation path, etc. The correlation value of an inverse spreading signal is a constant determined by the delay between the timing of a signal arrived through a path and the timing of inverse spreading. 
     FIG. 4  shows timings of signals of two (2) paths received byaCDMA (Code Division Multiple Access) mobile terminal. In the figure,  . .  YZABCD . .  are labels indicating signal timings of each path and “A” is assumed to be the correct inverse spreading timing. The channels of a path  1  and a path  2  are denoted respectively by α 1  and α 2 . Inverse spreading timings are respectively denoted by t 1  and t 2  and signals inversely spread at these timings are respectively denoted by x i  and x 2 . 
   Here, defining a special timing t 0 =t 1−(t   2 −t 1 ) and denoting a signal inversely spread at the timing t 0  by x 0 , x 1  and x 0  can be represented as follows.
 
 x   1 =α 1   S+α   2   I   z   +n   1  
 
 x   0 =α 1   I   z +α 2   I   Y   +n   0  
 
   α 1 S is a desired signal obtained by inversely spreading the path  1  from A and α 2 I Z  is interference obtained by inversely spreading the path  2  from Z. Furthermore, α 2 I Y  is a signal obtained by inversely spreading the path  2  from Y, and n 1  and n 0  are respectively noises thereof. 
   The above signal x 0  is a signal obtained by inversely spreading at a timing that can not be obtained, of a received signal S, and α 1 I Z  is contained therein. That is, it can be seen that α 1 I Z  has correlation with an interference component α 2 I Z  of x 1 . In this sense, a signal like x 0  is referred to a “Multi-path Interference Correlative Signal (MICS)” of the path  1  to the path  2 , and a timing like t 0  is referred to as a “Multi-path Interference Correlative Timing (MICT)” of the path  1  to the path  2 . 
   The interference component of x 1  can be reduced by multiplexing an appropriate coefficient r from x 1  to x 0  because x 0  has correction with the interference component of x 1 . 
   However, here, it should be noted that another interference component I Y  contained in x 0  is increased when the coefficientris determined such that I Z  contained in x 1  is completely cancelled. Therefore, the total magnitude of the interference may be increased instead of being decreased. Therefore, the appropriate coefficient r needs to be a coefficient that is determined such that the total power of the interference becomes minimum remaining the original interference I Z . 
   In the previously applied invention described above, the information of only two (2) paths selected when interference components are reproduced is used. Therefore, information contained in paths other than the noted two (2) paths can not be utilized. Therefore, the reduction effect of the interference components is small. 
   Therefore, the present invention solves such a drawback of the previously applied invention and improves the interference reproduction accuracy by using all the paths except interference sources in order to reproduce interference components. The principle of the present invention will be described as follows. 
     FIG. 5  shows N path signals, and a plurality of Multi-path Interference Correlative Timings (MICTs) that can be utilized for reduction of interference of a path j contained in a path i. A signal inversely spread at a timing of the path i is represented as follows. 
   
     
       
         
           
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   Here, S is a desired signal, I i,j  is an interference component by the path j contained in x i . In  FIG. 5 , a timing t i,j,i  is a multi-path interference correlative timing (MICT) used in the previous application. The timing t i,j,i , is a timing shifted from the timing t i  by the time difference Δt between the path i and the path j. A signal having a correlation with I i,j  can be obtained by inversely spreading at this timing t i,j,i . 
   Here, noting paths other than the paths i and j, similarly to the timing t i,j,i , a signal having a correlation with I i,j  can be obtained by inversely spreading at a timing t i,j,k  (k is a value from 1 to N except j.) shifted from each of the paths by Δt. The signals obtained by inversely spreading at the timings t i,j,k  and t i,j,k  are represented as follows. 
                   t     i   ,   j   ,   k       =       ⁢       t   i     -     t   j     +     t   k                     m     i   ,   j   ,   k       =       ⁢           ∑   I     ⁢       α   I     ⁢     I     i   ,   j   ,   k           +     n     i   ,   j   ,   k         =         α   k     ⁢     I     i   ,   j         +       ∑     I   =   k       ⁢       α   I     ⁢     I     i   ,   j   ,   k   ,   I           +     n     i   ,   j   ,   k                       
where I i,j,k,I =(t i −t j +t k −t I ) and, especially I i,j,k =I i,j .
 
   Next, an embodiment of the present invention will be described based on the above principle of the present invention. 
     FIG. 6  shows an example of the construction of a CDMA receiver applied with the present invention. A CDMA signal received by an antenna  20  is converted into a base-band signal by a down-converter  21 . 
   The base-band signal is inputted into an A/D converter  23  through an AGC amplifier  22 . Here, the base-band signal is converted into a digital signal and is inputted into inverse spreading circuit units  24 - 1  to  24 -n corresponding to the number of paths n and into a path searching unit  25 . 
   In the path searching unit  25 , a timing of each of the paths of the multi-paths is obtained from the received signal. Based on these path timings, inverse spreading timings t i,j,k,n  are created by a timing generating circuit  26  according to the following equation.
 
 t   i,j,k,n   =t   i −(t j −t k )n
 
   The inverse spreading timings t i,j,k,n  are sent respectively to the corresponding inverse spreading circuit units  24 - 1  to  24 -n and inverse spreading processes are executed at the respective timings. Inverse spread outputs obtained in the inverse spreading circuit units  24 - 1  to  24 -n are composed in the composing unit  27  and an inverse spread signal is obtained. 
     FIG. 7  shows an example of the construction of a composing unit  27  of  FIG. 6 . Here, the construction of an MMSE receiver is shown as an embodiment. Therefore, the composing unit  27  has an MMSE coefficient generating unit  270 . 
   The MMSE coefficient generating unit  270  obtains composition coefficients that maximizes the S/N of received signals. These coefficients are multiplied as the coefficients to multipliers  271 - 1  to  271 -n respectively corresponding to each of fingers. The MMSE coefficient generating unit  270  further comprises an adder  272  that adds the outputs of these multipliers  271 - 1  to  271 -n. Thereby, an output that maximizes the S/N of the received signals can be obtained from the adder  272 . 
   With the construction of  FIG. 6  applied with the present invention, effective timings for inverse spreading can be easily obtained and, therefore, a preferable effect can be obtained with a few inverse spread fingers. 
     FIG. 8  shows another exemplary embodiment of the present invention. A multi-path interference exchange reduction (MIXR) circuit  28  is provided between the inverse spreading circuit units  24 - 1  to  24 -n and a rake composing unit  27  compared to the exemplary embodiment of the construction of  FIG. 6 . Interference of each finger is reduced by this multi-path interference exchange reduction (MIXR) circuit  28 . 
     FIG. 9  shows the detailed construction of the multi-path interference exchange reduction circuit  28 . Interference is reduced by reproducing the interference caused by the path j contained in the path i in the MICS units  280 - 1  to  280 -N, adding all outputs of MICS units  280 - 1  to  280 -N in the adder  281  and subtracting the result of the adding from a signal of the path i. 
   Each of the above MICS units  280 - 1  to  280 -N reproduces interference entering from the path j to the path i (j≠i) . 
     FIG. 10  shows the details of the MICS units  280 - 1  to  280 -N represented by a MICS unit  280 - i . Based on timing information t i  of a rake path obtained by the path searching unit  25 , multi-path interference correlative timings (MICT) t i,j,k  are obtained in the timing generating circuit  26   i  according to the following equation.
   t   i,j,k   =t   i   −t   j   +t   k  ( j≠i ) 
   A signal mics(i,j,k) inversely spread by the respectively corresponding inverse spreading circuits  104 - i   1  to  104   i N at this obtained timing t i,j,k  is obtained and these signals are composed and outputted by an MRC unit  128 . 
     FIG. 11  shows an example of the construction of the MRC  128 . In the MRC  128 , the signal mics(i,j,k) is multiplied in a multiplier  128 - 2   i  by an appropriate coefficient r′ k  obtained by a coefficient generating unit  128 - 1  based on a channel estimation value α i (i=1 to N) obtained from a channel estimating unit  29  and the noise power n 2  obtained from a level measuring unit  30 , and the products are added by an adder  128 - 3 . Furthermore, the output of the adder  128 - 3  is multiplied by a coefficient r i,j  in an adder  128 - 4  and the MICS(i,j) is obtained. Therefore, the MICS(i,j) is represented by the following equation. 
   
     
       
         
           
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   Here, the coefficient r′ k  and r i,j  are obtained as follows.
 
 r′   k =α k */{Σ I≠k |α I | 2   I   2   +n   2 }
 
 r   i,j =α j   I   2 /{( I/N ) j +1}
 
where (I/N) j  is the ratio of interference to be reproduced and the power of interference other than that and is obtained as follows.
 
   
     
       
         
           
             
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   In the previously applied invention, the coefficient r i  is obtained as follows using only the path i as represented in Eq. 3 when the MICS(i,j) is obtained.
 
 MICS ( i,j )= r   i   mics ( i,j,i )
 
 r   i =α i *α j   I   2 /{Σ|α I | 2   I   2   +n   2 }
 
   In contrast, in the present invention, the accuracy of the MICS(i,j) can be improved and the reduction effect of interference can be improved by composing using paths other than the path i as already shown. 
   Here, in the process of the MRC unit  128  of  FIG. 11 , the coefficient r′ k  to multiply mics(i,j,k) is obtained as follows approximating the noises to be constant when the MICS(i,j) is obtained.
 
 r′   k =α* k  
 
 r   i,j =α j ( I/N ) j /{(Σ k≠j |α k | 2 )(( I/N ) j +1)}
 
   Thereby, the size of the circuitry and the amount to be processed can be reduced. 
     FIG. 12  shows another example of the construction of the MICS unit  280 - i . Compared to  FIG. 10 , this construction has a selector unit  129  as a characteristic thereof. The selector unit  129  determines the magnitude of a path based on a channel estimation value from the channel estimating unit  29  and obtains mics(i,j,k) only for large paths. Thereby, the size of the circuitry and the amount to be processed can be reduced without degrading considerably the performance. 
   In the example of the construction of  FIG. 12 , two with k=s 1  and s 2  are selected from the inverse spreading circuits  104 - ia  and  ib.    
   The MIXR process of the MIXR circuit  28  of  FIG. 8  exerts a high effect when the MIXR process is applied to large paths.  FIG. 13  shows a construction that is provided with a selector circuit  31  at the preceding stage of a MIXR circuit  28 , as an exemplary embodiment. The selector unit  31  controls such that the magnitude of the path is determined based on the channel estimation value from the channel estimating unit  29  and the MIXR processes are executed only on large paths. 
   Paths that are not targets of the MIXR process are lead directly to the rake circuit  27  to undergo a rake process without undergoing any other process before undergoing the rake processes. The example of the construction of  FIG. 13  is a construction for MIXR-processing two (2) paths and not executing any processes to other (N-2) processes. 
     FIG. 14  shows yet another example of the construction of the MICS unit  280 - i . While MIXR-processing, the inverse spreading timing t i,j,k  of the mics(i,j,k) may be different from the timing t i  of the desired signal. For example, in the case where each of t i  is lining spaced equally from each other, t i,j,k  may coincide with any one (1) of t i . Then, the case where the timing t i,j,k  coincides with t i  is detected by the timing generating circuit  26   i  and the signal of the case of the coincidence is blocked by switches  130 - 1  to  130 -N. Thereby, the degradation of the characteristic can be prevented. 
   The timing generating unit  26   i  in the exemplary embodiment of  FIG. 14  creates the timing t i,j,k  to correspond to the above operation and executes comparison with t i . Then, when coincidence or approximation almost equal to coincidence is found, the signal is prevented from being inputted into the MRC unit  128  by controlling accordingly a switch  130 - i  of an outputting unit of the mics(i,j,k) corresponding to the found timing. 
   Thereby, the mics(i,j,k) of the timing coinciding with a signal can be masked. 
   Here, in  FIG. 8 , (1) the noise level of each finger is constant in any finger in the rake composing unit  27  at the following stage of the MIXR unit  28 . Furthermore, (2) that the amplitude of the data delivered from the rake composing unit  27  to a correcting unit not shown indicates the likelihood of the signal, is a precondition for the process. 
   In many cases, in the rake composing unit  27 , in order to create appropriate signals for the correcting unit as above, the noise level of each finger is constant in any finger in the rake composing unit  27  at the following stage of the MIXR unit  28 . However, when a MIXR process has been executed to each rake finger respectively, the noise level of each finger containing interference is reduced and dispersion occurs in noise power that was at an almost same level in any finger. Consequently, effect of the rake composition and error correction may not be exerted. 
   Therefore, it is preferable to provide a level compensating unit  32  between the MIXR unit  28  and the rake composing unit  27  as a construction shown in  FIG. 15 . Thereby, the noise power can be made same as that before the MIXR process by amplifying appropriately the signal after the MIXR process. 
     FIG. 16  shows yet another exemplary embodiment of the present invention. Compared to the exemplary embodiment shown in  FIG. 8 , the embodiment is characterized in that the positions of the MIXR unit  28  and the inverse spreading circuit unit  24  are exchanged. 
   In the construction of  FIG. 16 , the MICS unit  280 - i  constituting the MIXR unit  28  has a construction shown in  FIG. 17 . Compared to  FIG. 10 , the inverse spreading circuit  104 - i  is replaced by the delay circuit  105 - i.    
   As described above, the circuit construction can be simplified by arranging the MIXR unit  28  before the inverse spreading circuit unit  24 . 
   As set forth hereinabove on the exemplary embodiments, even when the number of paths is increased, interfering noises can be effectively reduced by applying the present invention. 
   Thereby, a rake receiver can be provided, that executes the effective maximal ratio composition in the time domain of signals arriving at an antenna with various differences in delay time thereof caused by multiple reflections of propagation paths of the signals in a multi-path environment.