Abstract:
In order to effectively reduce a size of CDMA multiuser receiver while maintaining an excellent interference cancellation, an array antenna is combined with a multiuser receiver. The multiuser receiver includes signal processing means which is supplied with incoming signals received at the array antenna. The signal processing means estimates interfering signals with respecting to each of the antenna elements and with respect to each of simultaneously accessing users.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a CDMA (code-division multiple-access) multiuser receiver which combines directivity control of an array antenna and interference canceling operations. The CDMA multiuser receiver according to the present invention features a small size and excellent interference cancellation. By way of example, the present invention may be applicable to the receiver installed in the base station of a cellular mobile communications system. 
     2. Description of the Related Art 
     It is expected that CDMA is able to markedly increase a subscriber&#39;s capacity and thus find an extensive application in a cellular mobile communications system (for example). However, the mobile communications utilizing CDMA (viz., spread-spectrum) techniques have suffered, at a receiver side, the problems of interference caused by delayed signals due to multiple transmission paths and concurrently communicating other party&#39;s signals. 
     As is known in the art, an array antenna is able to suppress and cancel interference through directivity control. On the other hand, a multiuser receiver is a receiver which demodulates all the user&#39;s signals by implementing mutual interference cancellation using all the user&#39;s spreading codes and channel characteristics. The multiuser receiver itself is known in the art. One example of such a receiver is disclosed in a paper by M. K. Varanasi and B. Aazhang, entitled “Multistage Detection in Asynchronous Code-Division Multiple-Access Communications”, IEEE Transactions on Communications, Vol. 38, No. 4, April 1990, pp. 509-519 (Prior Paper 1). Another example of a conventional multiuser receiver is disclosed in a paper by M. Sawahashi, et al., entitled “Serial Canceler Using Recursive Channel Estimation by Pilot Symbols for DS-CDMA”, Electronics Information Communications Association of Japan, Technical Report RCS95-50,July 1995 (Prior Paper 2). 
     According to the apparatus disclosed in the aforesaid Prior Paper 1, all the user&#39;s signals are demodulated at an initial stage of the apparatus, after which an interfering replica of each user becomes produced. Subsequently, interference cancellation is implemented by reducing an interference replica of each of the users other than a desired user from a received signal. At the next stage, the signal, which has been obtained through the interference cancellation, is again demodulated in connection with the desired (intended) user and therefore, the signal quality of the demodulation result at the second stage is higher than that at the first stage. Thus, the conventional technique, disclosed in Prior Paper 1, is to improve the interference cancellation by repeating a series of signal processes using multi-stage configuration. 
     Channel estimation is necessary to demodulate the signal of each user and produce an interference replica. The aforesaid Prior Paper 2 discloses that a channel (viz., transmission path) is recursively estimated at each stage thereby to prevent deterioration of the interference cancellation characteristics due to channel estimation error. 
     Another example of the multiuser receiver is disclosed in a paper by Yoshida and Ushirokawa, entitled “CDMA Multi-Stage Interference Canceler with Recursive Channel Estimation Based on Symbol Replica Processing”, the Institute of Electronics, Information and Communication Engineers, Technical Report of IEICE, A. p96-157, EMCJ96-92, RCS96-171, February 1997 (Prior Paper 3). 
     The above-mentioned Prior Paper 3 discloses a multi-stage type CDMA multiuser receiver. According to this known technique, the size of the apparatus can be reduced through the use of symbol replica processing. At the same time, it is possible to realize interference cancellation at the unit of multi-path in the case of implementing recursive channel estimation thereby to improve interference cancellation in the case of multi-path transmission. 
     FIG. 1 is a drawing showing a CDMA multiuser receiver that is based on the known techniques disclosed in Prior Paper 3. The CMDA multiuser receiver of FIG.  1  is comprised of three-stage interference cancelers  10 - 1  to  10 - 3 . At the first two stages of interference cancelers  10 - 1  and  10 - 2 , the signals of all the users, the number of which is assumed three, are demodulated and then subjected to interference cancellation. That is, this means that the multiuser interference cancellation is implemented. 
     As shown in FIG. 1, the interference canceler  10 - 1  at the first stage is provided with a delay unit  12 , three EIUs (interference estimation units)  14   a - 14   c,  an adder  16 , and another adders  18   a - 18   c.  The interference canceler  10 - 2  is configured in the same manner as the canceler  10 - 1  and is comprised of three IEUs (interference estimation units)  14   a ′- 14   c ′, an adder  16 ′, and another adders  18   a ′- 18   c′.    
     On the other hand, the interference canceler  10 - 3  at the final stage is provided with IEUs  20   a - 20   c  each of which differs from those provided at the first and second stages. 
     A received signal is directly applied to the first stage (viz., interference canceler  10 - 1 ). The interference canceler  10 - 3  at the final stage is not provided with any delay unit and any adder. The IEUs  20   a - 20   c  generate demodulated signals respectively corresponding to the first to third users. 
     The operations of the interference cancelers  10 - 1  and  10 - 2 , which are respectively provided at the first and second stages, are identical with each other and thus, there will be described the operation of the first stage. The three IEUs  14   a - 14   c  respectively output estimated interference spread signals that are applied to the adder  16 . The delay unit  12  operates such as to delay the incoming signal by the time for which each of the IEUs  14   a - 14   c  estimates the interference and outputs the result thereof, and applies the output thereof to the adder  16  and the delay unit  12 ′ of the second stage. The adder  16  subtracts the outputs of the IEUs  14   a - 14   c from the output of the delay unit  12 , and applies the output thereof to the adders  18   a - 18   c  that are respectively assigned to the users. Each of the adders  18   a - 18   c  sums the output of the adder  16  and the output of the corresponding IEU ( 14   a,    14   b,  or  14   c ), and applies the resultant sum to the second stage. 
     The IEUs  14   a - 14   c  of the first stage and the IEUs  14 ′ a - 14   c ′ of the second stage are substantially identical with each other in terms of configuration as well as operations, and accordingly there will be described only the IEU  14   a  of the first stage. 
     The IEU  14   a  of FIG. 2 is configured under the assumption that the number of paths of the incoming signal is three (3). In the drawing, the circuits prepared for first to third propagation paths are depicted by P 1 -P 3 . Since the circuits for the multiple paths are identical with each other, the description is made with reference to the circuit P 1  for the first path. The IEU shown in the drawing is generally comprised of a front section (stage) S 1 , an intermediate section S 2 , and a rear section S 3 . More specifically, the front section S 1  comprises a spread-spectrum despreader  22  and a detector  24 , while the intermediate section S 2  comprises an adder  25  and a discriminator  26 . Finally, the rear section S 3  comprises a multiplier  27 , a spread-spectrum modulator  28  and an adder  29 . Further, the detector  24  comprises a channel estimator  24   a,  a complex conjugate generator  24   b,  and a multiplier  24   c.    
     The received signal (incoming signal) is split and applied to the circuits P 1 -P 3  prepared for the three transmission paths. The despreader  22  despreads the incoming signal using the first user&#39;s spreading code at the timing in synchronism with the spreading code transmitted via the first path, and outputs the operation result. 
     The detector  24  is supplied with the output of the despreader  22 , implementing channel estimation at the channel estimator  24   a,  applying the estimated channel characteristics to the multiplier  24   c  via the complex conjugate generator  24   b,  and implementing carrier phase coherent detection. The multiplier  24   c  implements amplitude weighting on the output of the despreader  22 , using the output of the complex conjugate generator  24   b,  for the purpose of Rake combination at the subsequent block. The amplitude weighting is for implementing Rake combination (maximum ratio combination) on the output of the despreader  22 . 
     It is deemed advantageous to operate the detector  24 , in an environment of fading, using coherent detecting techniques which are disclosed in Prior Paper 2 and via which a carrier is estimated through the use of pilot symbols inserted on a time axis. 
     The adder  25  combines using Rake combination techniques, the weighted outputs of the multipliers  24   c  respectively provided in the circuits P 1 -P 3  for the three paths. The combined signal is fed to the discriminator  26  that determines the most likely transmitted symbol. 
     The output of the discriminator  26  is again split and applied to circuits P 1 -P 3  of the third section S 3 , which are respectively assigned to the thee transmission paths. The multiplier  27  multiplies the output of the discriminator  26  by the estimated channel characteristics, viz., the output of the channel estimator  24   a.  The spread-spectrum modulator  28  spreads the output of the multiplier  27  using the first user&#39;s spreading code at the timing which is in synchronism with the spreading code transmitted via the first path. 
     An adder  29  sums (synthesizes) the outputs of the circuits P 1 -P 3  which are respectively assigned to the three paths and which are the replicas of respective paths. Thus, an interference replica of the first user is generated. 
     The interference canceler  10 - 3  shown in FIG. 1 is comprised of IEUs  20   a - 20   c  which are configured in a manner identical with each other, and accordingly, only the IEU  20   a  provided for the first user will be described. 
     Referring to FIG. 3, there is shown the IEU  20   a  in block diagram form. As shown in FIG. 3, the IEU  20   a  is configured in a manner exactly identical with those of the front and intermediate sections shown in FIG.  2 . Therefore, the reference numerals already used for the blocks of FIG. 2 are attached to the counterparts of FIG.  3  and the description thereof will be omitted. 
     On the other hand, the techniques for canceling signal interference by applying an array antenna to a CDMA&#39;s single user receiver is disclosed in a paper by R. Kohno, H. Imai, M. Hatori and S. Pasupathy, entitled “Combination of an Adaptive Array Antenna and a Canceler of Interference for Direct-Sequence Spread-Spectrum Multiple-Access System”, IEEE Journal on selected areas in communications, Vol. 8, N. 4, May 1990, pp. 675-682 (Prior Paper 4). 
     According to the apparatus disclosed in the aforesaid Prior Paper 4, the array antennas is controlled such as to be directed to an arrival angle of a desired signal and acquires the same, after which the interfering signal components within the directivity are despread. The apparatus demodulates the signal components and generates a temporal symbol, after which the apparatus again spreads the signal and generates interfering signal components. In other words, the apparatus carries out interference cancellation by subtracting the interfering signal components from the signal received by the array antenna, and then demodulates the desired (intended) user&#39;s signal. Although this conventional apparatus utilizes spreading codes and channel characteristics of all users, it is understood that the apparatus implements interference cancellation for a single user and thus is classified as a single user receiver. 
     FIG. 4 shows one example of the above-mentioned conventional receiver wherein an array antenna  30  is combined with an interference canceler. In order to simplify the description and the drawing, it is assumed that an array antenna consists of two antenna elements and the number of total users is three. The receiver is a single user CDMA receiver for demodulating one user (the third user in this particular case). 
     Superimposed data of desired and interfering signals are applied to two antenna elements  30   a  and  30   b.  The signal received at the antenna elements  30   a  and  30   b  are respectively weighted, at corresponding complex multipliers  32   a  and  32   b,  by antenna weighting coefficients W 1  and W 2  and thereafter added at an adder  34 . The output of the adder  34  is applied to despreader  36   a  and  36   b  which are provided for the two users (viz., first and second users) other than the third user (whose signal is to be received in the instant case). Further, the output of the adder  34  is also applied to a delay unit  38 . The outputs of the despreader  36   a  and  36   b  are respectively applied to discriminators  40   a  and  40   b  at which temporal symbol discrimination is implemented. The outputs of the discriminators  40   a  and  40   b  (viz., signals representative of temporal symbols) are respectively applied to spreader  42   a  and  42   b  which issues interfering signal components based on the discrimination results. 
     A delay unit  38  is used to delay the output of the adder  34 , which is denoted by  34   a  and is to be applied to an adder  44 . In more specific terms, the delay unit  38  is provided to delay the signal  34   a  (viz, the output of the adder  34 ) until a signal  34   b,  applied to the despreader  36   a  and  36   b,  is outputted from spreaders  42   a  and  42   b.    
     The adder  44  subtracts the outputs of the spreaders  42   a  and  42   b  (viz., interfering signal components) from the output of the delay unit  38 , and applies the result to a despreader  46  and a delay unit  48 . 
     The output of the despreader  46  is applied to a discriminator  50  which demodulates the signal of the third user and outputs the demodulated signal to an external circuit (not shown). That is, the despreader  46  and the discriminator  50  are provided for the third user. On the other hand, the output of the discriminator  50  is also applied to a spreader  52  for the third user, via which a spread signal for the third user is obtained. 
     The delay unit  48  is to delay the output of the adder  44  (depicted by  44   b ) by a time period for which the output of the adder  44  (depicted by  44   a ) has been subjected to symbol discrimination and the spreader  52  generates the spread signal for the third user. The signal thus delayed is applied to the adder  54 . 
     The adder  54  produces an error signal  56  by subtracting the output of the spreader  52  from the output of the delay unit  48 . The error signal  56  is fed to an antenna weighting coefficient determiner (adaptively renewing means)  58 . This determiner  58  controls the directivity of the array antenna  30  using the signals received at the antenna elements  30   a  and  30   b  along with known adaptive algorithm. 
     The receiver shown in FIG. 4 is an apparatus for use in producing the demodulated signal only for the third user. In other words, in order to demodulate the signals of the other users, viz., first and second users, it is necessary to provide the receivers respectively dedicated to the first and second users. 
     There has been so far no proposal of combining an array antenna and a CDMA multiuser receiver. By way of example, if an array antenna is simply applied to the multiuser receivers shown in FIGS. 1-3, particularly the interference estimating section becomes complex thereby to be unable to simplify the overall configuration of the receiver. 
     In addition, the single user receiver shown in FIG. 4, which features a combination of an array antenna and an interference canceler, suffers from the following problem when applied to the case of simultaneously processing a plurality of users. That is, in such a case, it is absolutely necessary to prepare a plurality of identical receivers that are arranged in parallel for respective users. 
     SUMMARY OF THE INVENTION 
     Accordingly, the object of the present is to provide a CDMA multiuser receiver which is able to demodulate a plurality of user&#39;s signals, without incurring increase in the arrangement or size, by combining an array antenna and a multiuser receiver. 
     Another object of the present invention is to provide a CDMA multiuser receiver which is based on an effective combination of an array antenna and an interference canceler and features the marked reduction of size of the apparatus with excellent interference cancellation. 
     In brief, these objects are achieved by techniques wherein in order to effectively reduce a size of a CDMA multiuser receiver while maintaining an excellent interference cancellation, an array antenna is combined with a multiuser receiver coupled to said array antenna. The multiuser receiver includes signal processing means which is supplied with incoming signals received at said array antenna. The signal processing means estimates interfering signals with respect to each of the antenna elements and with respect to each of simultaneously accessing users. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which like elements are denoted by like reference numerals and in which: 
     FIG. 1 is a diagram schematically showing a conventional CDMA receiver in block diagram, having referred to in the preceding paragraphs; 
     FIG. 2 is a diagram sowing in detail an interference estimation unit (EIU) of FIG. 1; 
     FIG. 3 is a diagram sowing in detail another IEU of FIG. 1; 
     FIG. 4 is a diagram schematically showing another conventional CDMA receiver in block diagram, having referred to in the preceding paragraphs; 
     FIG. 5 is a diagram schematically showing CDMA receiver according to a first embodiment of the present invention; 
     FIGS. 6 to  8  are each diagrams showing a detailed arrangement of a block of FIG. 5; 
     FIG. 9 is a diagram schematically showing a CDMA receiver according to a second embodiment of the present invention; 
     FIGS. 10 to  12  are each diagrams showing a detailed arrangement of a block of FIG. 9; 
     FIG. 13 is a diagram schematically showing a CDMA receiver according to a third embodiment of the present invention; and 
     FIGS. 14 and 15 are each diagrams showing a detailed arrangement of a block of FIG.  13 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 5, a CDMA multiuser receiver  61  according to a first embodiment of the present invention is schematically shown in block diagram form. FIGS. 6-8 illustrate detailed arrangements of IEUs  64   a,    64   a ′, and  74   a  of FIG. 5, respectively. Incoming signals are received at an array antenna  62  that comprises two antenna elements  62   a  and  62   b  in this particular embodiment. It is assumed that the number of multiple transmission paths is three, the number of stages for interference cancellation is three, and the number of simultaneously accessing users is three. It is to be noted that the aforesaid numbers of antenna elements, multiple transmission paths, etc. are exemplary and in no way limited thereto. 
     The receiver shown in FIG. 5 comprises three interference cancels  60 - 1  to  60 - 3 , among which the interference cancelers  60 - 1  and  60 - 2  are configured in exactly the same manner except for the connection of the outputs of delay units. 
     As shown, the interference canceler  60 - 1  is comprised of three IEUs  64   a  to  64   c  which are respectively provided for first to third users, two delay units  66   a  and  66   b  whose number is the same as that of the antenna elements  62   a  and  62   b,  two adders  68   a  and  68   b  whose number is also identical with that of the antenna elements, and six (=“the number of users”×“the number of antenna elements”) adders  70   a - 1  and  70   a - 2 ,  70   b - 1  and  70   b - 2 , and  70   c - 1  and  70   c - 2 . 
     As mentioned above, the interference canceler  60 - 2  is configured in the same manner as the canceler  60 - 1 , and therefore the counterparts of the canceler  60 - 2  are depicted by like numerals with a prime. That is, the interference canceler  60 - 2  of the second stage is comprised of three IEUs (interference estimation unit)  64   a ′ to  64   c ′ which respectively correspond to the first to third users, two delay units  66   a ′ and  66   b ′ the number of which is identical with that of the antennas, two adders  68   a ′ and  68   b ′ the number of which are also identical with that of the antenna, and six (=“the number of users”×“the number of antennas”) adders respectively depicted by  70   a ′- 1  and  70   a ′- 2 ,  70   b ′- 1  and  70   b ′- 2 , and  70   c ′- 1  and  70   c ′- 2 . 
     Since the IEUs  64   a  to  64   c  are identical with each other in terms of configuration as well as operation, the IEU  64   a  will mainly be described for the sake of simplifying the disclosure. The IEU  64   a  is supplied with the signals received at the antenna elements  62   a  and  62   b,  and generates two spread “interference estimating signals” which respectively correspond to the antenna elements  62   a  and  62   b.  As shown, the outputs of the IEU 64   a  are applied to adders  68   a  and  68   b,  and adders  70   a ′- 1  and  70   b ′- 2 . Each of delay units  66   a  and  66   b  is to delay the signal applied thereto until each of the IEUs  64   a  to  64   c  produces the output thereof. The output of the delay  66   a  is applied to the adder  68   a  and a delay unit  68   a ′ of the next stage  60 - 2 , and similarly, the output of the delay  66   b  is fed to the adder  68   b  and a delay unit  68   b ′ of the next stage  60 - 2 . 
     By the way, if the first interference canceler  60 - 1  is able to completely or sufficiently remove interference of one user against the other, there is no need for providing the following canceler  60 - 2 . However, such interference can not be rejected using a signal canceler and thus, it may be typical to provide one or tow canceling stages prior to the final stage. 
     The adder  68   a  subtracts the outputs of IEUs  64   a  to  64   c,  which correspond to the antenna element  62   a,  from the output of the delay unit  66   a.  The adder  70   a - 1  adds the subtraction result outputted from the adder  68   a  and one of the outputs of the IEU  64   a,  which corresponds to the antenna element  62   a.  As mentioned later, each of the two outputs of the IEU  64   a  is a spread signal. In a similar manner, the adder  68   a  subtracts the outputs of IEUs  64   a  to  64   c,  which correspond to the antenna element  62   b,  from the output of the delay unit  66   b.  The adder  70   a - 2  adds the subtraction result outputted from the adder  68   b  and the other output of the IEU  64   a,  which corresponds to the antenna element  62   b.    
     It is understood that the IEU  64   a ′, included in the second interference canceler  60 - 2 , is supplied with a signal which includes the interfering components relating to only the first user (although ideal). 
     Antenna weighting coefficient determiners  72   a  and  72   b  are supplied with the incoming signals received at the antenna elements  62   a  and  62   b,  and respectively generate outputs W 1  and W 2  which are applied to circuits P 1  to P 3  of each of the IEUs  64   a - 64   c,    64   a ′- 64   c ′, and  74   a - 74   c.    
     The IEU  64   a  of the interference canceler  60 - 1  will be described in detail with reference to FIGS. 5 and 6. The IEU  64   a  comprises three circuits P 1  to P 3  which are respectively provided for three transmission paths. In other words, the IEU  64   a  is configured so as tom comply with the case in which the number of multiple transmission paths is three. Since the circuits P 1  to P 3  are substantially identical to each other, only the circuit P 1  is described below. As shown in FIG. 6, IEU  64   a  is generally comprised of a first section S 1 , a second section S 2 , and a final section S 3 . The section S 1  comprises two spread-spectrum despreaders (denoted by “despreading” in the drawing)  80   a  and  80   b,  whose number equals that of the antenna elements  62   a  and  62   b.  The section S 1  further comprises multipliers  82   a  and  82   b,  an adder  84 , and a detector  86 . The first section S 1  is coupled to the final section S 3  by way of an adder  88  and a discriminator  90  (viz., section S 2 ) each of which is common to all the circuits P 1  to P 3 . 
     As shown in FIG. 6, a first circuit P 1  of the final section S 3  comprises a multiplier  92 , multipliers  94   a  and  94   b  whose number equals that of the antenna elements, spread-spectrum spreaders  96   a  and  96   b,  adders  98   a  and  98   b,  and multipliers  100   a  and  100   b.  Each of the adders  98   a  and  98   b  is provided so as to add the outputs generated from the circuit P 1  to P 3  of the section S 3 . Subsequently, the outputs of the adders  98   a  and  98   b  are respectively multiplied, at the multiplier  100 , by a weighting coefficient α having a value less than unit, and then applied to the following section. 
     The detector  86  of the first section S 1  comprises a channel estimator  86   a,  a complex conjugate generator  86   b,  and multiplier  86   c.  The spread-spectrum despreaders  80   a  and  80   b  operate such as to despread the incoming signals using a despreading (viz., spreading) code previously assigned to the first user, in which the despreading code is correctly phased (synchronized) with the spreading code transmitted via the first propagation path. The multipliers  82   a  and  82   b  respectively multiply the outputs of the despreaders  80   a  and  80   b  by antenna weighting coefficients W 1  and W 2 , and apply the multiplication results to the adder  84 . As mentioned above, the coefficients W 1  and W 2  are generated from the antenna weighting coefficient determiners  72   a  and  72   b  (FIG.  5 ). 
     The detector  86  operates in exactly the same manner as the conventional detector  24  of FIG.  2 . As mentioned above, the multiplier  86   c  weights the output of the adder  84  using the output of the complex conjugate generator  86   b  in order to prepare for Rake combination (viz., maximum ratio combination) at the adder  88 . That is, the adder  88  receives the outputs from the circuits P 1  to P 3  which are assigned to three different transmission paths, and carries out Rake combination. The signal combined at the adder  88  is fed to the discriminator  90  at which most likely transmitted symbols are determined. 
     The multiplier  92  of the section  83  multiplies the output of the discriminator  90  by the output of the channel estimator  86   a  in order to estimate an interference replica. This operation is implemented at each of the circuits P 1  to P 3 . The estimated interference replica (viz., output of the multiplier  92 ) is then split into two (viz., the number of antenna elements employed) which are applied to the multipliers  94   a  and  94   b.  As shown, the multipliers  94   a  and  94   b  multiply the outputs of the multiplier  92  by complex conjugates W 1 * and W 2 * which are respectively generated by complex conjugate generators  77   a  and  77   b  using the aforesaid antenna weighting coefficients W 1  and W 2 . The outputs of the multipliers  94   a  and  94   b  are respectively applied to spread-spectrum modulators  96   a  and  96   b  and are spread thereat in a manner to be correctly phased (synchronized) with the spreading code transmitted via the first propagation path. More specifically, the modulators  96   a  and  96   b  spread respectively the estimated interfering replicas in connection with the antenna elements  62   a  and  62   b.    
     Adders  98   a  and  98   b  respectively add the spread signals issued from the modulators  96   a  and  96   b  in each of the circuits P 1  to P 3 . Thus, the adders  98   a  and  98   b  output, respectively, the spread signals indicative of the estimated interference replicas regarding the antenna elements  62  and  62   b.  The following multipliers  100   a  and  100   b  multiply respectively the outputs of the adders  98   a  and  98   b  by a weighting coefficient α with a value less than unity, and apply the multiplication results to the following section. The coefficient α is able to suppress “emphasized interference” due to a channel estimation error(s) thereby to improve the interference cancellation characteristics, which is disclosed in detail in the aforesaid Prior Paper 4. 
     If the antenna weighting coefficient utilizes a complex conjugate vector relating to a steering vector which is determined depending on signal&#39;s arrival angles and which indicates phase difference between antenna elements, the signal obtained by antenna weight composition (viz., antenna weighting coefficients) is an in-phase composed signal. In this case, it is possible to correctly reproduce the interference of each antenna element using the steering vector and the signal weighted by antenna coefficients. Further, if the interference cancellation is implemented with each antenna before the antenna weighting is carried out with each user, it is possible to effectively combine the antenna directivity control and a plurality of interference cancelers. 
     Antenna weighting coefficients are able to be generated using conventional techniques. For further details thereof, reference should be made, for example, to a paper by R. O. Schmidt, et al., entitled “Multiple Emitter Location and Signal Parameter Estimation”, IEEE Trans., Vol. AP-34, No. 3, pp. 276-280, March 1986, or a paper by R. Roy and T. Kailath, entitled “ESPRIT—Estimation of signal Parameters via Rotational Invariance Techniques”, IEEE Trans., Vol. ASSP-37, pp. 984-995, July 1989. 
     As shown in FIG. 5, the antenna weighting coefficient determiner  72   a  outputs nine independent coefficients that are applied to three IEUs of each of the interference canceling stages  60 - 1  to  60 - 3 . However, if the antenna weighting coefficients are successively renewed, the coefficients are generated, only for the first stage  60 - 1 , using an error between the demodulated result and the known symbol. In this case, it is possible that the stages following the first stage is able to utilize the same antenna weighting coefficients as those used in the first stage. 
     FIG. 7 is a block diagram showing the details of the IEU  64   a ′ that is configured in the same manner as that of the IEU  64   a  of FIG. 6, and thus, the further descriptions of FIG. 7 will be omitted for the sake of simplifying the disclosure. 
     FIG. 8 is a block diagram showing the details of the IEU  74   a.  As shown, the IEU  74   a  comprises two sections that are respectively identical with the sections S 1  and S 2  of FIG. 6 or  7 , and thus are labeled S 1  and S 2 . 
     A second embodiment will be described with reference to FIGS. 9,  10 ,  11  and  12  which respectively correspond to FIGS. 5,  6 ,  7  and  8  of the first embodiment. In connection with the second embodiment, it is assumed as in the first embodiment, that the number of multiple transmission paths is three, the number of stages for interference cancellation is three, and the number of simultaneously accessing users is three. It is to be noted that the aforesaid numbers of antenna elements, multiple transmission paths, etc. are exemplary and in no way limited thereto. 
     The second embodiment differs from the first embodiment, in terms of arrangement, as listed below: 
     (1) IEUs provided in the first stage  60 - 1  of FIG. 9 are differently configured compared with the counterparts of the first stage of FIG.  5  and therefore are denoted by  63   a,    63   b,  and  63   c  in FIG. 9; 
     (2) IEUs provided in the second stage  60 - 2  of FIG. 9 are differently configured compared with the counterparts of the second stage of FIG.  5  and therefore are denoted by  63   a ′,  63   b ′, and  63   c ′ in FIG. 9; 
     (3) IEUs provided in the third stage  60 - 3  of FIG. 9 are differently configured compared with the counterparts of the third stage of FIG.  5  and therefore are denoted by  73   a,    73   b,  and  73   c  in FIG. 9; 
     (4) the first stage  60 - 1  of FIG. 9 lacks the adders  70   a - 1 ,  70   a - 2 , . . . ,  70   c - 1  that are provided in the first stage of FIG. 5; and 
     (5) the second stage  60 - 2  of FIG. 9 lacks the adders  70   a ′- 1 ,  70   a ′- 2 , . . .  70   c ′- 1  that are provided in the second stage of FIG.  5 . 
     FIG. 10 shows the details of the IEU  63   a  (FIG. 9) in which the output of the multiplier  92  of each of the circuits P 1  to P 3  is directly applied to the IEU  63   a ′ of the next stage  60 - 2 . Other than this, the IEU  63   a  is configured in a manner that is identical with the corresponding IEU  64   a  of the first embodiment. Since the output of the multiplier  92  of the circuit P 1  is the estimated signal of the first user itself, it is understood that the first interference canceler  60 - 1  of the second embodiment requires no longer the adders  70   a - 1 ,  70   a - 2 , . . . ,  70   c - 1 , and  70   c - 2  of the IEU  64   a  of the first embodiment. 
     In order to comply with the above-mentioned modification of the IEU  63   a,  the IEU  63   a ′ of the second interference canceler  60 - 2  has the first section S 1  which includes an adder  85  in addition to the functional blocks already discussed with the first embodiment. The adder  85  is to add the estimated signal of the first user (in the illustrated case of FIG. 11) to the output of the adder  84 . The section S 3  of the IEU  63   a ′ is identical with the second  3  of the IEU  63   a  and hence, the description thereof will be omitted for the sake of simplifying the disclosure. 
     FIG. 12 is a block diagram showing the details of the IEU  73   a  of FIG.  9 . The IEU  73   a  is identical, in terms of configuration, with a combination of the first and second sections S 1  and S 2  of FIG. 10 or  11  and accordingly, further description thereof will not been given for brevity. 
     A third embodiment of the present invention will be described with reference to FIG. 13,  14  and  15  that correspond respectively to FIGS. 9,  10  and  11  of the second embodiment. In connection with the third embodiment, it is assumed, as in each of the preceding embodiments, that the number of multiple transmission paths is three, the number of stages for interference cancellation is three, and the number of simultaneously accessing users is three. It is to be noted that the aforesaid numbers of antenna elements, multiple transmission paths, etc. are exemplary and in no way limited thereto. 
     FIG. 13 shows that the outputs of delay units  66   a  and  66   b  are not directly applied to the delay units  66   a ′ and  66   b ′, instead of which the outputs of the adders  68   a and  68   b  are respectively applied to the delay units  66   a ′ and  66   b ′. That is, the first interference canceler  60 - 1  applies the error signals, produced form the adders  68   a  and  68   b,  to the delay units  66   a ′ and  66   b ′, and IEUs  65   a ′,  65   b ′, and  65   c ′. In order to meet this modification, each of IEUs  65   a ′ to  65   c ′ of the second stage  60 - 2  is slightly changed in the configuration thereof as shown in FIG.  15 . On the other hand, the IEUs (denoted by  65   a  to  65   c ) of the first stage  60 - 1  are configured in exactly the same as the counterparts  63   a  to  63   c . However, the IEU  65   a  is shown in FIG. 14 for the convenience of describing the disclosure. IEUs  75   a  to  75   c  of the third stage  60 - 3  are identical with each other and respectively identical with the IEUs  73   a  to  73   c,  and accordingly the drawing of the IEU  75   a  (for example) is not presented for simplifying the disclosure. 
     As shown in FIGS. 13,  14  and  15 , the third section S 3  of the IEU  65   a ′ is provided with an adder  93  to which the outputs of the multipliers  92  of the IEU  65   a  (FIG. 14) are directly applied. As mentioned above, according to the third embodiment, the error signals from the adders  68   a  and  68   b  are directly applied to the second interference canceler  60 - 2 . This implies that the third embodiment is able to reduce a memory capacity compared with the second embodiment. 
     It will be understood that the above disclosure is representative of only three possible embodiments of the present invention and that the concept on which the invention is based is not specifically limited thereto.