Abstract:
In a CDMA receiver which receives, as a reception signal ( 33 ), a signal given by subjecting a data signal comprising predetermined pattern data (PS) to spread modulation by the use of a spread code, a correlation value calculating unit ( 70   1   -70   3 ) produces correlation value data obtained by multiplying the reception signal by the spread code and the predetermined pattern data. A signal-to-interference ratio calculating portion ( 74 ) calculates a signal-to-interference ratio of the reception signal. A reception timing determining portion ( 76 ) determines a reception timing of the predetermined pattern data in response to the correlation value data and the signal-to-interference ratio. Preferably, the reception timing determining portion determines the reception timing such that the maximum value of said correlation value data exceeds a predetermined first threshold value and that said signal-to-interference ratio exceeds a predetermined second threshold value when said correlation value data have the maximum value.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a receiver of a code-division multiple access system and, in particular, to a receiver of a code-division multiple access system intended to improve a reception quality. 
     In a conventional mobile communication system such as a mobile telephone, use has been made of a multiplexing system such as a time division multiple access (TDMA) system or a frequency division multiple access (FDMA) system. However, in response to a growing demand for effective use of frequencies following an increase in number of subscribers and for multimedia communications, attention is directed to a code division multiple access (hereinafter abbreviated to CDMA) system as a multiplexing system for a next-generation mobile telephone. The CDMA system is a technique for simultaneously carrying out a plurality of communications by the use of signals in a same frequency band by means of the spread spectrum technique. In a CDMA mobile communication system using the above-mentioned technique, a plurality of users occupy a same frequency and a same time and modulate communication data by the use of spread codes assigned to the users to identify the users. The spread codes of the users are orthogonal to one another. Therefore, at a receiving side a multiplexed signal obtained by multiplexing all user&#39;s communication data is multiplied by a spread code used by each user in a same phase so as to extract communication data of a desired user. 
     In the CDMA mobile communication system, a communication quality is determined by orthogonality of communication data signals of all users multiplexed In the same frequency. Practically, however, due to variation in a propagation condition, the orthogonality can not completely be maintained. Therefore, when the signal of the desired user is demodulated a signal component of another user is undesiredly contained to result in deterioration in signal quality. In order to avoid the deterioration in signal quality, the receiving side measures a ratio between a signal reception level and an interference reception level for the desired user and requests a transmitting side to change transmission power so as to satisfy a predetermined ratio. In this approach, a transmission level is increased at the transmitting side in order to maintain a predetermined signal-to-interference ratio (hereinafter abbreviated to SIR) at a CDMA receiver in the CDMA mobile communication system. However, increase in transmission level prevents the reduction in power consumption at a terminal and the improvement in degree of multiplexing into the same frequency. In order to solve the above-mentioned problem, attention is directed to an interference removing technique. In the interference removing technique, an interference wave, i.e., a signal component other than that of a desired user is removed from a communication data signal received. Thus, it is possible to improve a reception signal quality even in a low SIR condition. 
     Hereinafter, description will be made of a CDMA receiver using the interference removing technique. Herein, it is assumed that the CDMA receiver performs an interference removing operation of a multistage type in which interference removal is repeatedly carried out in three stages for three users. 
     FIG. 1 shows the structure of a conventional CDMA receiver for carrying out interference removal in a multistage fashion. The CDMA receiver comprises a reception timing detecting section  10  for detecting reception timings of three users and, in correspondence to the reception timings, interference estimating sections in each stage. The interference estimating sections includes first- through third-stage interference estimating sections  11   11  through  11   13  corresponding to the reception timing of a first user, first through third-stage interference estimating sections  11   21  through  11   23  corresponding to the reception timing of a second user, and first- through third-stage interference estimating sections  11   31  to  11   33  corresponding to the reception timing of a third user. The CDMA receiver further comprises residual signal producing sections  12   1  and  12   2 . 
     A multiplexed signal  13  received by the CDMA receiver is supplied to the reception timing detecting section  10 , the first-stage interference estimating sections  11   11  through  11   31 , and the residual signal producing section  12   1 . The multiplexed signal  13  is a frame signal composed of a plurality of slots. At a predetermined position in the frame, a pilot symbol as predetermined pattern data is added before or after an information symbol of a predetermined length. The reception timing detecting section  10  detects the pilot symbol added to the multiplexed signal  13  to detect data reception timings of desired users. The reception timings thus detected are supplied as reception timings  14   1  through  14   3  to the first-stage interference estimating sections  11   11  through  11   31 , the second-stage interference estimating sections  11   12  through  11   32 , and the third-stage interference estimating sections  11   13  through  11   33  individually for the users, i.e., individually for the reception timings. In synchronism with the reception timings  14   1  through  14   3  detected by the reception timing detecting section  10  for the individual users, the first-stage interference estimating sections  11   11  through  11   31  multiply the multiplexed signal  13  by spread codes assigned to the individual users to extract data signals of the desired users, respectively. The data signals thus extracted are supplied as user signals  15   1  through  15   3  to the second-stage interference estimating sections  11   12  through  11   32  in a subsequent stage, respectively. In addition, the first-stage interference estimating sections  11   11  through  11   31  multiply the extracted user data signals again by the spread codes assigned to the users. Thus, signal components of the users contained in the multiplexed signal  13  are reproduced to obtain reproduction signals  16   1  through  16   3  which are supplied to the residual signal producing section  12   1 . The residual signal producing section  12   1  is supplied with the multiplexed signal  13  in addition to the reproduction signals  16   1  through  16   3  and produces a residual signal  17  obtained by subtracting the reproduction signals  16   1  through  16   3 from the multiplexed signal  13 . The residual signal  17  is used as an input signal to be subjected to interference removal in the second stage. 
     The residual signal  17  is supplied to the second-stage interference estimating sections  11   12  through  11   32  and the residual signal producing section  12   2 .In synchronism with the reception timings  14   1  through  14   3  detected by the reception timing detecting section  10  for the individual users, the second-stage interference estimating sections  11   12  through  11   32  multiply the residual signal  17  supplied thereto by the spread codes individually assigned to the users to despread the residual signal. Resultant signals (or despread signals) are weak in signal level. Therefore, in order to minimize errors produced in transmission-path estimation required upon demodulation, the user signals  15   1  through  15   3  supplied from the first-stage interference estimating sections  11   11  through  11   31  are added to the resultant signals to produce added user signals increased in ratio of the signal components of the desired users. Thus, data signals of the desired users are extracted. The data signals thus extracted are supplied as user signals  18   1  through  18   3  to the third-stage interference estimating sections  11   13  through  11   33  in a subsequent stage, respectively. In addition, the second-stage interference estimating sections  11   12  through  11   32  subtract, from the user data signals extracted thereat as demodulation signals, signal components corresponding to the user signals  15   1  through  15   3  previously added and multiply results of extraction again by the spread codes assigned to the users, respectively. Thus, signal components of the relevant users contained in the residual signal  17  are reproduced as reproduction signals  19   1  through  19   3  which are supplied to the residual signal producing section  12   2 . The residual signal producing section  12   2  is supplied with the residual signal  17  in addition to the reproduction signals  19   1  through  19   3  and produces a residual signal  20  obtained by subtracting the reproduction signals  19   1  through  19   3  from the residual signal  17 . The residual signal  20  is used as an input signal to be subjected to interference removal in the third stage. 
     Likewise, the third-stage interference estimating sections  11   13  through  11   33  extract desired user signals for the residual signal  20  and produce demodulation signals  21   1  through  21   3  of the desired users corresponding to the user signals  16   1  through  16   3  and  18   1  through  18   3  produced by the first- and the second-stage interference estimating sections  11   11  through  11   31  and  11   12  through  11   32 , respectively. In this event, the residual signal  20  approaches nearer to zero than the residual signal  17  so that the third-stage interference estimating sections  11   13  through  11   33  produce the demodulation signals  21   1  through  21   3  from the added user signals after the interference is removed at maximum, respectively. 
     The above-mentioned technique related to the CDMA receiver is disclosed, for example, in Japanese Unexamined Patent Publication (JP-A) No. H10-190494 “INTERFEERENCE CANCELLER AND CHANNEL ESTIMATION”. 
     However, in the conventional CDMA receiver already proposed, interference is not removed from the multiplexed signal itself supplied to the reception timing detecting section. Therefore, the reception timings of the desired users are detected from the reception signal containing interference waves at a great ratio. As a consequence, it is difficult to detect accurate reception timings. Furthermore, since the interference of the reception signal is removed with reference to such inaccurate reception timings, the reception quality is deteriorated to cancel the effect of interference removal. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a CDMA receiver which enables detection of accurate reception timings even in a condition that an SIR is low. 
     A CDMA receiver to which this invention is applicable is for receiving, as a reception signal, a signal given by subjecting a data signal comprising predetermined pattern data to spread modulation by the use of a spread code. 
     According to an aspect of this invention, the receiver comprises: 
     correlation value data producing means for producing correlation value data obtained by multiplying the reception signal by the spread code and the predetermined pattern data; 
     signal-to-interference ratio calculating means for calculating a signal-to-interference ratio of said reception signal; and 
     reception timing determining means for determining a reception timing of said predetermined pattern data in response to said correlation value data and said signal-to-interference ratio. 
     Preferably, the reception timing determining means determines the reception timing such that the maximum value of the correlation value data exceeds a predetermined first threshold value and that the signal-to-interference ratio exceeds a predetermined second threshold value when the correlation value data have the maximum value. 
     The signal-to-interference ratio calculating means may calculate the signal-to-interference ratio from the reception signal and the correlation value data produced by the correlation value data producing means. 
     According to another aspect of this invention, the receiver comprises: 
     correlation value data producing means for producing, at each sampling point within a predetermined time range, correlation value data obtained by multiplying the reception signal by the spread code and the predetermined pattern data; 
     correlation value data memorizing means for memorizing, in correspondence to the above-mentioned each sampling point, the correlation value data produced by the correlation value data producing means; 
     signal-to-interference ratio calculating means for calculating a signal-to-interference ratio of the reception signal; 
     signal-to-interference ratio memorizing means for producing an interpolating signal-to-interference ratio for the signal-to-interference ratio calculated by the signal-to-interference ratio calculating means for each sampling point within the time range based on a reception timing at which the signal-to-interference ratio is calculated and for memorizing the interpolating signal-to-interference ratios in correspondence to the above-mentioned each sampling point; 
     retrieving means for retrieving maximum correlation value data among the correlation value data memorized in the correlation value data memorizing means; 
     correlation value data judging means for judging whether or not the maximum correlation value data retrieved by the retrieving means exceed a predetermined first threshold value; 
     ratio judging means for judging, when the correlation value data judging means judges that the maximum correlation value data exceed the first threshold value, whether or not a particular signal-to-interference ratio memorized in the signal-to-interference ratio memorizing means in correspondence to a particular sampling point of the maximum correlation value data exceeds a predetermined second threshold value; and 
     reception timing determining means for determining, when the ratio judging means judges that the particular signal-to-interference ratio exceeds the second threshold value, a reception timing corresponding to the particular sampling point as a reception timing of the predetermined pattern data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a view showing the structure of a conventional CDMA receiver; 
     FIG. 2 is a view showing the structure of a CDMA receiver according to a first embodiment of this invention; 
     FIG. 3 is a block diagram showing a characteristic part of the structure of a first-stage interference estimating section in the first embodiment; 
     FIG. 4 is a block diagram showing a characteristic part of the structure of a second-stage interference estimating section in the first embodiment; 
     FIG. 5 is a block diagram showing a characteristic part of the structure of a reception timing detecting section in the CDMA receiver according to the first embodiment; 
     FIG. 6 is a view for describing a table structure of a correlation value table in the first embodiment; 
     FIG. 7 is a view for describing a table structure of an SIR information table in the first embodiment; 
     FIG. 8 is a flow chart showing the content of determination of a reception timing in a reception timing determining portion in the first embodiment; and 
     FIG. 9 is a block diagram showing a characteristic part of the structure of a reception timing detecting section of a CDMA receiver according to a second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, this invention will be described in detail in conjunction with several embodiments. 
     First Embodiment 
     FIG. 2 shows the structure of a CDMA receiver according to a first embodiment of this invention. The CDMA receiver in the first embodiment is a CDMA receiver utilizing a multistage-type interference removing technique of repeating interference removal in three stages for three users. However, the number of users and the number of stages are not restricted at all. The CDMA receiver in the first embodiment comprises a reception timing detecting section  30  for detecting reception timings for three users, respectively, and interference estimating sections in each stage in correspondence to the reception timings. The interference estimating sections include first- through third-stage interference estimating sections  31   11  through  31   13  corresponding to the reception timing of the first user, first- through third-stage interference estimating sections  31   21  through  31   23  corresponding to the reception timing of the second user, and first- through third-stage interference estimating sections  31   31  through  31   33  corresponding to the reception timing of the third user. The CDMA receiver further comprises residual signal producing sections  32   1  and  32   2 . 
     A multiplexed signal  33  received by the CDMA receiver is supplied to the reception timing detecting section  30 , the first-stage interference estimating sections  31   11  through  31   31 , and the residual signal producing section  32   1 . The multiplexed signal  33  is a frame signal composed of a plurality of slots. At a predetermined slot of the slots in the frame signal, a pilot symbol as predetermined pattern data is added before (or after) an information symbol (as information data) of a predetermined length. The reception timing detecting section  30  is supplied with SIR information  34   1  through  34   3  from the first-stage interference estimating sections  31   11  through  31   31 , respectively. The SIR information  34   1  through  34   3  are SIRs measured in correspondence to the reception timings. By detecting the pilot symbols added to the multiplexed signal  33 , the reception timing detecting section  30  corrects the data reception timings detected for the desired users with reference to the SIR information  34   1  through  34   3  corresponding thereto. Reception timing information  35   1  through  35   3  thus corrected are supplied to the first-stage interference estimating sections  31   11  through  31   31 , the second-stage interference estimating sections  31   12  through  31   32 , and the third-stage interference estimating sections  31   13  through  31   33 . 
     The first-stage interference estimating sections  31   11  through  31   31  are adapted to produce reception timings obtained by preliminarily compensating a processing delay in the reception timing detecting section  30  and other internal propagation delays, and to correct reception timings as demodulation timings with reference to the reception timing information  35   1  through  35   3  supplied thereto for individual users. At the reception timings thus corrected, the first-stage interference estimating sections  31   11  through  31   31  carry out demodulation of the multiplexed signal  33  in correspondence to the users by multiplying the multiplexed signal  33  by spread codes assigned to the individual users. The demodulated data are supplied as reception data  36   1  through  36   3  of the desired users to the second-stage interference estimating sections  31   12  through  31   32  as a next stage. In addition, the first-stage interference estimating sections  31   11  through  31   31  again uses the spread codes assigned to the individual users to reproduce signal components of the relevant users contained in the multiplexed signal  33 . The signal components thus reproduced are delivered as reproduction signals  37   1  through  37   3  to the residual signal producing section  32   1 . The residual signal producing section  32   1  is supplied with the multiplexed signal  33  in addition to the reproduction signals  37   1  through  37   3  and produces a residual signal  38  obtained by subtracting the reproduction signals  37   1  through  37   3  from the multiplexed signal  33 . The residual signal  38  is used as an input signal to be subjected to interference removal in the second stage. 
     The residual signal  38  is supplied to the second-stage interference estimating sections  31   12  through  31   32  and the residual signal producing section  32   2 . In synchronism with the reception timings for the individual users. The second-stage interference estimating sections  31   12  through  31   32  carry out despreading by multiplying the residual signal  38  supplied thereto by the spread codes individually assigned. Resultant signals have a small signal level. 
     Therefore, in order to minimize errors produced in transmission-path estimation required upon demodulation, the reception data  36   1  through  36   3  supplied from the first-stage interference estimating sections  31   11  through  31   31  are added thereto to produce added user signals increased in ratio of signal components of the desired users. Thus, data signals of the desired users are extracted. The data signals thus extracted are supplied as reception data  39   1  through  39   3  to the third-stage interference estimating sections  31   13  through  31   33 . In addition, the second-stage interference estimating sections  31   12  through  31   32  subtract, from the extracted reception data demodulated data corresponding to the reception data  36   1  through  36   3  previously added and then multiply user data signals again by the spread codes assigned to the users, respectively. Thus, the signal components of the relevant users contained in the residual signal  38  are reproduced. These signal components are supplied as reproduction signals  40   1  through  40   3  to the residual signal producing section  32   2 . In addition to the reproduction signals  40   1  through  40   3 , the residual signal producing section  32   2  is supplied with the residual signal  38  and produces a residual signal  41  obtained by subtracting the reproduction signals  40   1  through  40   3  from the residual signal  38 . The residual signal  41  is used as an input signal to be subjected to interference removal in the third stage. 
     In the similar manner, the third-stage interference estimating sections  31   13  through  31   33  extract desired user signals for the residual signal  41  and produce demodulation signals  42   1  through  42   3  of the desired users corresponding to the reception data  36   1  through  36   3  and  39   1  through  39   3  produced by the first- and the second-stage interference estimating sections  31   11  through  31   31  and  31   12  through  31   32 , respectively. In this event, the residual signals  38  and  41  successively approach zero so that the third-stage interference estimating sections  31   13  through  31   33  produce the demodulation signals  42   1  through  42   3  from the added user signals after the interference is removed at maximum, respectively. 
     Now, description will be made about a characteristic part of the structure of the CDMA receiver in the first embodiment. 
     FIG. 3 shows the characteristic part of the first-stage interference estimating sections of the CDMA receiver in FIG. 2 according to the first embodiment. In FIG. 2, the first-stage interference estimating sections  31   11  through  31   31  are separately illustrated in correspondence to the reception timings detected for the individual users for which simultaneous demodulation is possible. On the other hand, these sections are integrated in FIG. 3 into a first-stage interference estimating section  44   1 . The first-stage interference estimating section  44   1  has demodulation processing units  45   1  through  45   3  and re-spreading units  46   1  through  46   3  for the reception timings, respectively, and comprises a RAKE combining unit  47   1  and a reproduction signal producing unit  48   1  in common to all of the reception timings. The demodulation processing units  45   1  through  45   3  are similar in structure to one another. The re-spreading units  46   1  through  46   3  are similar in structure to one another. Although three timings are herein illustrated, the number of timings is not restricted in principle as far as the constraint in mounting is eliminated. Hereinafter, the demodulation processing unit  45   1  and the re-spreading unit  46   1  will be described among these demodulation processing units and these re-spreading units. 
     The demodulation processing unit  45   1  comprises a demodulating portion  49   1  for demodulating an input signal, a reception timing producing portion  50   1  for producing a demodulation timing of the demodulating portion  49   1 , and an SIR information producing portion  51   1  for measuring an SIR of the input signal to produce SIR information. The demodulating portion  49   1  comprises a dispreading part  53   1  and a transmission-path estimating part  54   1 . The demodulation processing unit  45   1  is supplied with the multiplexed signal  33 . The dispreading part  53   1  of the demodulating portion  49   1  multiplies the multiplexed signal by the spread code of a predetermined user to extract a desired user signal. The transmission-path estimating part  54   1  calculates transmission-path characteristic information by the use of a pilot symbol known to be preliminarily contained in the reception signal and compensates despread data with reference to the transmission-path characteristic information. Such demodulation by the demodulation portion  49   1  is carried out in synchronism with the reception timing producing portion  50   1 . The reception timing producing portion  50   1  produces the reception timing obtained by preliminarily compensating the processing delay of the reception timing detecting section  30  or other internal propagation delays, and further corrects the reception timing with reference to the reception timing information  35   1 . For example, the reception timing produced as mentioned above preliminarily considering the delay is used as a base and corrected with reference to the reception timing information  35   1 . The SIR information producing portion  51   1  calculates a signal-to-interference ratio for a signal component which is the despread data despread by the demodulating portion  49   1  and for an interference component which is a remaining component of the reception signal other than the signal component, and produces SIR information representative of the ratio. The SIR information is supplied to the RAKE combining unit  47   1  and, in the first-stage interference estimating section  44   1 , further to the reception timing detecting section  30 . 
     The RAKE combining unit  47   1  is supplied with the despread data despread by the demodulating portion  49   1  and carries out maximum ratio synthesis with reference to the SIR information produced by the SIR information producing portion  51   1  for the individual reception timings. Specifically, weighted synthesis given by “SIR 1 ×S 1 +SIR 2 ×S 2 +SIR 3 ×S 3 ” is carried out where S 1  through S 3  and SIR 1  through SIR 3  represent the despread data and the SIR information of the individual users, respectively The synthesized output of the RAKE combining unit  47   1  is supplied to the re-spreading units  46   1  through  46   3 . 
     The re-spreading unit  46   1  comprises a spreading portion  55   1 . The synthesized output of the RAKE combining unit  47   1  supplied to the re-spreading unit  46   1  is directly outputted as the reception data  36   1 . The spreading portion  55   1  multiplies the synthesized output again by the spread code corresponding to each individual user to produce a spread signal. The spread signal is supplied as the reproduction signal  37   1  to the reproduction signal producing unit  48   1 . The reproduction signal producing unit  48   1  is supplied with the reproduction signals  37   1  through  37   3  produced for the individual reception timings and combines these signals to reproduce a signal with the individual timings taken into account, as is equivalent to the multiplexed signal  33 . The reproduction signal  37  herein reproduced is delivered to the residual signal producing section  32   1 . Actually, the residual signal producing section  32   1  produces the residual signal  38  by subtracting from the multiplexed signal  33  the reproduction signal  37  with the individual timings taken into account. 
     As described above, the first-stage interference estimating section  44   1  corrects the demodulation timing with reference to the reception timing information and produces the demodulated data and the reproduced data. The second- and the third-stage interference estimating sections  44   2  and  44   3  are similar in structure and different from the first-stage interference estimating section  44   1  in that the demodulated data of the preceding stage are supplied and correction of the demodulation timings is not carried out. 
     FIG. 4 shows the characteristic part of the second-stage interference estimating sections of the CDMA receiver in FIG. 2 according to the first embodiment. In FIG. 2, the second-stage interference estimating sections  31   12  through  31   32  are separately illustrated in correspondence to the reception timings detected for the individual users for which simultaneous demodulation is possible. On the other hand, these sections are integrated in FIG. 4 into a second-stage interference estimating section  44   2 . The second-stage interference estimating section  44   2  has demodulation processing units  56   1  through  56   3  and re-spreading units  57   1  through  57   3  for the reception timings, respectively, and comprises a RAKE combining unit  58   1  and a reproduction signal producing unit  59   1  in common to all of the reception timings. The demodulation processing units  56   1  through  56   3  are substantially similar in structure to one another. The re-spreading units  57   1  through  57   3  are similar in structure to one another. The demodulation processing unit  56   1  comprises a demodulating portion  60   1  for demodulating an input signal, an SIR information producing portion  61   1  for measuring an SIR of the input signal to produce SIR information, and an adder portion  62   1 . The demodulating portion  60   1  comprises a despreading part  63   1  and a transmission-path estimating part  64   1 . The demodulation processing unit  56   1  further comprises a reception timing producing portion  65   1  for producing a demodulation timing of the demodulating portion. Although three timings are herein illustrated, the number of timings is not restricted in principle as far as the constraint in mounting is eliminated. The second-stage interference estimating section  44   2  is substantially similar in structure to the first-stage interference estimating section  44   1 . Therefore, different parts alone will be described. 
     The demodulation processing unit  56   1  is supplied with the residual signal  38  and the reception timing produced by the reception timing producing portion  65   1 . Demodulation by the demodulating portion  60   1  is carried out in synchronism with the reception timing produced by the reception timing producing portion  65   1 . The reception timing producing portion  65   1  produces the reception timing obtained by preliminarily compensating the processing delay of the reception timing detecting section  30  and other internal propagation delays, and further corrects the reception timing with reference to the reception timing information  35   1 . For example, the reception timing produced preliminarily taking the delay into account is used as a base and corrected with reference to the reception timing information  35   1 . The demodulating portion  60   1  of the demodulating processing unit  56   1  carries out despreading in synchronism with the spread code preliminarily assigned to the user to extract a desired user signal. The despread data despread by the demodulating portion  60   1  are supplied to the adder portion  62   1 . The adder portion  62   1  is supplied from the first-stage interference estimating section  44   1  with the reception data  36   1  corresponding to the reception timing and adds the reception data  36   1  to the despread data. This increases the ratio of the signal component of each individual user contained in the weak residual signal  38  supplied to the second-stage interference estimating section  44   2  as the input signal to be subjected to interference removal, and enhances the accuracy of the demodulation signal. The result of addition in the adder portion  62   1  is supplied to the RAKE combining unit  58   1 . 
     The re-spreading unit  57   1  comprises a subtracter portion  66   1  and a spreading portion  67   1 . The re-spreading unit  57   1  directly outputs, as the reception data  39   1  the synthesized output obtained by maximum ratio synthesis in the RAKE combining unit  58   1 . Supplied with the synthesized output obtained by maximum ratio synthesis by the RAKE combining unit  58   1  and with a despread signal  68   1  obtained by despreading by the despreading part  63   1  in the demodulating portion  60   1  of the demodulation processing unit  56   1 . The subtracter portion  68   1  of the re-spreading unit  57   1  subtracts the despread signal  68   1  from the maximum synthesized ratio output. The result of subtraction is supplied to the spreading portion  67   1 . The spreading portion  67   1  multiplies the subtraction result by the spread code corresponding to each individual user to produce a spread signal The spread signal is supplied as the reproduction signal  40   1  to the reproduction signal producing unit  59   1 . The reproduction signal producing unit  59   1  is supplied with the reproduction signals  40   1  through  40   3  produced for the individual reception timings and combines these signals to reproduce a signal with the individual timings taken into account, as is equivalent to the residual signal  38 . The reproduction signal  40  herein reproduced is delivered to the residual signal producing section  32   2 . Actually, the residual signal producing section  32   2  produces the residual signal  41  by subtracting, from the residual signal  38 , the reproduction signal  40  with the individual reception timings taken into account. 
     Thus, the second-stage interference estimating section  44   2  corrects the reception data  36   1  through  36   3  from the first stage and delivers the corrected data to the third stage. A combination of the third-stage interference estimating sections  31   13  through  31   33  (FIG. 2) similarly operates to obtain the reception data  42   1  through  42   3  for the individual users. 
     Next, description will be made in detail about a characteristic part of the reception timing detecting section  30  of FIG.  2 . 
     FIG. 5 shows the characteristic part of the reception timing detecting section  30  of the CDMA receiver in the first embodiment. The reception timing detecting section  30  has correlation value calculating units  70   1  through  70   3  for the individual reception timings and comprises a spread code delay generating unit  71 , a spread code producing unit  72 , and a reception timing calculating unit  73  in common to all of the reception timings. Although the correlation value calculating units  70   1  through  70   3  are provided for the three timings, the number of timings is not restricted in principle as far as the constraint in mounting is eliminated. The reception timing calculating unit  73  comprises an SIR calculating portion  74 , a correlation value data averaging portion  75 , and a reception timing determining portion  76 . The spread code producing unit  72  produces the predetermined spread codes for the individual users. The spread code delay generating unit  71  multiplies the spread codes of the individual users produced by the spread code producing unit  72  by the pilot symbol PS as the predetermined (or fixed) pattern data. For each user, the length of the pilot symbol is extracted from the spread code having a predetermined pattern length and is used in multiplication. By shifting the position of extracting the spread code over the width of a predetermined sampling period within a range of a reception timing detectable period, predetermined signal reproduction signals  77   1  through  77   3  are obtained with the spread codes delayed. The correlation value calculating units  70   1  through  70   3  multiply the multiplexed signal  33  by the predetermined signal reproduction signals  77   1  through  77   3  supplied thereto, respectively, to calculate correlation data  78   1  through  78   3  as cross-correlation values therebetween within the reception timing detectable period. The correlation data  78   1  through  78   3  are cross-correlation values corresponding in number to sampling times. The correlation value data averaging portion  75  of the reception timing calculating unit  73  carries out averaging over a predetermined time duration for each sampling and produces a correlation value table. 
     FIG. 6 shows a table structure of the correlation value table  79  produced by the correlation value data averaging portion  75 . The correlation value table  79  stores sampling times  80  and correlation value levels  81  corresponding thereto. As described above, the correlation value data averaging portion  75  carries out averaging upon the correlation data  78   1  through  78   3  produced by the correlation value calculating units  70   1  through  70   3  over the predetermined time duration for each sampling. For example, it is assumed that N times of sampling is possible within the reception timing detectable period. Then, correlation data average values LV 0  through LV N  taken over the predetermined time duration for the sampling times T 1  through T N , respectively, are stored in the correlation value table  79  in correspondence to the sampling times. 
     Turning to FIG. 5, description will continue. Since the SIR information  34   1  through  34   3  supplied from the first-stage interference estimating section  44   1  correspond in number to the reception timings, the SIR calculating portion  74  calculates, by linear interpolation and averaging, the SIR information at the sampling times within the predetermined time range between time instants before and after the reception timing. The SIR information thus calculated is stored in an SIR information table. 
     FIG. 7 shows a table structure of the SIR information table  82  produced by the SIR calculating portion  74 . For each sampling time  83  at which the above-mentioned interpolation is carried out, the SIR information table  82  stores the SIR information  84  as interpolated values corresponding thereto. It is assumed that the reception timing is located at a position T m  on a time axis. 
     Then, for a plurality of interpolation sampling times within the time range between “T m −t n ” and “T m +t n ” before and after the reception timing, the SIR information is calculated by linear interpolation and averaging from the SIR information  34   1  through  34   3  corresponding in number to the reception timings and stored as SIR 0  through SIR M . 
     The reception timing determining portion  76  of the reception timing calculating unit  73  determines the reception timing for demodulation with reference to the information stored in the correlation value table produced by the correlation value data averaging portion  75  and the SIR information table produced by the SIR calculating portion  74 . 
     FIG. 8 shows the content of reception timing determination at the reception timing determining portion  76 . At first, the reception timing determining portion  76  refers to the correlation value table  79  illustrated in FIG.  6  and retrieves a particular sampling time having a maximum correlation value level (step S 85 ). Then, judgement is made about whether or not the maximum correlation value level is not smaller than a predetermined first threshold value (step S 86 ). If it is judged that the maximum correlation value level is not smaller than the first threshold value (Y in step S 86 ), the SIR information table  82  illustrated in FIG. 7 is searched from the sampling time corresponding to the maximum correlation value level to retrieve the interpolated sampling time corresponding thereto. Since the reception timing is preliminarily known upon preparation of the SIR information table, it is easy to establish the correspondence between the interpolated sampling time  83  of the SIR information within the predetermined range and the sampling time  80  in FIG.  6 . Therefore, the interpolated sampling time  83  corresponding to the sampling time is identified and the SIR information stored in correspondence to the interpolated sampling time can be retrieved (S 87 ). If it is judged in the step S 86  that the first threshed value is not exceeded (N in step S 86 ), the operation returns to the step S 85  to retrieve the next reception timing. 
     Next, judgement is made about whether or not the SIR information retrieved in the step S 87  is not smaller than a predetermined second threshold value (step S 88 ). If it is judged that the SIR information is not smaller than the second threshold value (Y in step S 88 ), the timing in question is determined as the reception timing (step S 89 ). If it is judged in the step S 88  that the second threshold value is not exceeded (N in the step S 88 ), the operation returns to the step S 85  to retrieve the next reception timing. Finally, judgement is made about completion of retrieval, i.e., whether or not the reception timings of a required number have been determined or whether or not the correlation value table has been completely retrieved. If no further retrieval is required (Y in step S 90 ), a series of operations are finished (END). On the other hand, if any further retrieval is required, the operation returns to the step S 85  to retrieve the next reception timing. 
     As described above, in the CDMA receiver according to the first embodiment, the SIR information table including interpolation before and after the reception timing is prepared by the use of the SIR information measured by the first-stage interference estimating section  44   1  for each individual user. Furthermore, for the reception signal, the correlation value table is provided to store the correlation value with the pilot symbol to be used as the reception timing in correspondence to each sampling time within a range of the reception timing detectable period. Thus, correction into an optimum reception timing is made. Therefore, even in case where the multistage interference removal of a multi-user type is carried out, it is possible to eliminate the influence of the interference component from the reception timing and to carry out interference removal with reference to the accurate reception timing. 
     Second Embodiment 
     The CDMA receiver according to the first embodiment is applied to the multistage interference removal but this invention is not restricted thereto. In a second embodiment, illustration is made of a CDMA receiver which is not applied to the multistage interference removal. 
     FIG. 9 shows a characteristic part of a reception timing detecting section of the CDMA receiver according to the second embodiment of this invention. Similar parts are designated by like reference numerals as those of the reception timing detecting section  30  in FIG. 2 according to the first embodiment and the description thereof will appropriately be omitted. The reception timing detecting section  100  in the second embodiment has correlation value calculating units  70   1  through  70   3  for individual reception timings and comprises a spread code delay generating unit  71 , a spread code producing unit  72 , and a reception timing calculating unit  101  in common to all of the reception timings. The reception timing calculating unit  101  comprises an SIR calculating portion  102 , a correlation value data averaging portion  75 , and a reception timing determining portion  76 . The spread code producing unit  72  produces the predetermined spread codes for the individual users. The spread code delay generating unit  71  multiplies the spread codes of the individual users produced by the spread code producing unit  72  by the pilot symbol PS as the predetermined pattern data. For each user, the length of the pilot symbol is extracted from the spread code having a predetermined pattern length and is used in multiplication. By shifting the position of extracting the spread code over the width of a predetermined sampling period within a range of a reception timing detectable period, predetermined signal reproduction signals  77   1  through  77   3  are obtained with the spread codes delayed. The correlation value calculating units  70   1  through  70   3  multiply the multiplexed signal  33  by the predetermined signal reproduction signals  77   1  through  77   3  supplied thereto, respectively, to calculate correlation data  78   1  through  78   3  as cross-correlation values therebetween within the reception timing detectable period. The correlation data  78   1  through  78   3  are cross-correlation values corresponding in number to sampling times. The correlation value data averaging portion  75  of the reception timing calculating unit  73  carries out averaging over a predetermined time duration for each sampling and produces a correlation value table illustrated in FIG.  6 . 
     The SIR calculating portion  102  calculates SIRs at all sampling points from the multiplexed signal  33  and the correlation value data  78   1  through  78   3  corresponding in number to the reception timings. The SIRs are subjected to linear interpolation and averaging for a predetermined period to calculate the SIR information at sampling times in a predetermined time range before and after the reception timing. The SIR information thus calculated is stored in a SIR information table illustrated in FIG. 7, in the manner similar to the first embodiment. 
     The reception timing determining portion  76  of the reception timing calculating unit  101  determines the reception timing for demodulation with reference to the information stored in the correlation value table produced by the correlation value data averaging portion  75  and the SIR information table produced by the SIR calculating portion  102 . The operation is similar to that illustrated in FIG.  8  and will not be described. For example, the reception timing thus determined may be outputted as the reception timing information for making the reception timing producing section produce the accurate reception timing as described in the first embodiment or may be delivered directly as the reception timing to a demodulator not illustrated in the figure. Thus, the mode of output is not restricted. 
     In the CDMA receiver of the second embodiment, the reception timing detecting section produces the SIR information table from the multiplexed signal and the correlation value data. Therefore, it is unnecessary for the first-stage interference estimating section to refer to the SIR information as in the first embodiment. This contributes to a reduction in size of the receiver. The above-mentioned reception timing detecting section can notify the accurate reception timing not only to the CDMA receiver using the above-mentioned multistage interference removal but also to other types of receivers. 
     In the first embodiment, description has been directed to application to the multistage interference removing technique of repeating the interference removal for three users in three stages. However, the number of users and the number of stages are not restricted at all. 
     As described above, according to this invention, the demodulation timing of the reception multiplexed signal is corrected with reference to the correlation value detected as the reception timing and the SIR in the actual demodulation signal. Therefore, even if the SIR is low, the optimum reception timing can be detected so that the reception quality is improved. In addition, it is possible to lower the SIR required to obtain a predetermined reception quality. Therefore, the transmission power of a mobile terminal in a CDMA mobile communication system can be lowered. This contributes to a reduction in size of the terminal and to low the power consumption. Since the transmission power at each user is lowered, it is possible to increase the number of users for which multiplexing in a same frequency is possible.