Abstract:
A multiuser interference canceler which is used in a DS-CDMA (direct sequence-code division multiple access) communication system and is capable of preventing reception quality from being degraded. In a serial multiuser interference canceler which has cascaded blocks each having an interference canceler units (ICUs) for generating interference replica signal, preliminary demodulators for measuring reception quality of the signals of the users in the received signal are provided. If the reception quality of output signals from the preceding block is more degraded than the reception quality measured by the preliminary demodulators, then each of the interference canceler units determines that the reception quality has been degraded by an interference cancellation process in the preceding block, and controls the interference canceler unit in the preceding block to turn off the outputting of the interference replica signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a DS-CDMA (Direct Sequence-Code Division Multiple Access) system which is one of CDMA communication systems, and more particularly to a multiuser interference canceler, which is used in a DS-CDMA system, for processing a received signal containing spread signals from a plurality of users to remove signals of other users from a signal of each user and thereafter decode the signal of each user. 
     2. Description of the Related Art 
     In recent years, CDMA communication systems that are resistant to interference and jamming have attracted much attention as mobile communication systems. In a CDMA communication system, a transmitting station spreads a user signal to be transmitted with a spreading code and transmits the spread user signal, and a receiving station despreads the received user signal with a spreading code which is the same as the spreading code in the transmitting station for thereby obtaining the original user signal. 
     In the CDMA communication system, a plurality of transmitting stations spread user signals with different orthogonal spreading codes, and a plurality of receiving stations select respective spreading codes for use in despreading the received user signals for thereby specifying the respective user signals. Consequently, a common frequency band can be used to transmit a plurality of the user signals from the transmitting stations to the receiving stations. 
     However, because not all spreading codes are made orthogonal to each other, the signal of a user may possibly serve as noise with respect to another user, resulting in interference. Therefore, if a plurality of users use a link with a common frequency within one cell, the signals of the users tend to interfere with each other, resulting in a reduction in the bit energy to noise ratio (Eb/No) which is the ratio of the desired reception wave power to interfering wave power. Because the Eb/No determines the quality of communications, it is necessary to attain an Eb/No having at least a certain value in order to obtain a desired quality of communications. Such an Eb/No having at least a certain value is referred to as a required Eb/No. 
     The number of links that can use a common frequency within one cell is limited to a number that can maintain the required Eb/No. For increasing a system capacity for subscribers, any interference between communication links must be lowered. 
     One proposed method of reducing any interference within cells, increasing a system capacity for subscribers, or improving the quality of communications in the DS-CDMA communication system is a multiuser interference canceling method which employs a multiuser interference canceler. 
     The multiuser interference canceling method is a method in which when a received signal containing the signals of a plurality of users is to be demodulated by a spreading code of a certain user, an interference replica component which is the same as a signal component other than the user is generated and subtracted from the received signal to be demodulated in a multiplicity of stages for thereby reducing interference from the other users. There are two types of multiuser interference cancelers, i.e., a serial multiuser interference canceler and a parallel multiuser interference canceler. The principles of the serial multiuser interference canceler are described in Technical Report (RC95-50) of the Institute of Electronics, Information and Communication Engineers, “Sequential channel estimating serial canceler using a pilot symbol in DS-CDMA”, for example. Japanese laid-open patent publication No. 09-270736 (JP, 09270736, A) entitled “DC-CDMA multiuser serial interference canceler device” discloses a type of serial multiuser interference canceler. 
     The conventional serial multiuser interference canceler disclosed in the above publication is shown in FIG.  1 . 
     The serial multiuser interference canceler shown in FIG. 1 is designed for three users and comprises first, second, and third stages for effecting an interference cancellation process with an interference replica signal and three decoders  19   a - 19   c . Each of the first, second, and third stages comprises three blocks. Therefore, the serial multiuser interference canceler has a total of nine blocks. 
     These nine blocks have respective interference canceler units (ICUs)  61   a ,  61   b ,  61   c ,  62   a ,  62   b ,  62   c ,  63   a ,  63   b ,  63   c . Each of the blocks also has, in addition to the ICU, a delay memory (D) and a subtractor and/or an adder. For example, the first block of the first stage comprises an ICU  61   a , a delay memory  3 , and a subtractor  4 , and the first block of the second stage comprises an ICU  62   a , delay memories  5  and  7 , an adder  8 , and a subtractor  6 . Each of the second and third blocks of the first stage is identical in structure to the first block of the first stage. Each of the blocks of the second and third stages, except the third block of the third stage, is identical in structure to the first block of the second stage. 
     Each of the ICUs  61   a ,  62   a ,  63   a  generates an interference replica signal which is the same as the signal component of the first user. Each of the ICUs  61   b ,  62   b ,  63   b  generates an interference replica signal which is the same as the signal component of the second user. Each of the ICUs  61   c ,  62   c ,  63   c  generates an interference replica signal which is the same as the signal component of the third user. 
     The arrangement of the ICU  61   a , as an example of the ICUs  61   a - 63   c , will be described below with reference to FIG.  2 . 
     The ICU  61   a  comprises a plurality of despreaders − 71   1 - 71   n , a rake combiner  26 , a decision unit  27 , and a plurality of respreaders  72   1 - 72   n . Each of the despreaders  71   1 - 71   n  comprises a pair of multipliers  22  and  25 , an integrator  23 , and a transmission path estimator  24 . A received signal  1  that is supplied to the ICU  61   a  is multiplied by a spreading code Ca by the multiplier  22 , and then integrated by the integrator  23 , which produces a correlated value. The transmission path estimator  24  determines a transmission path fading vector ξ from the correlated value determined by the integrator  23 . The multiplier  45  multiplies the correlated value from the integrator  23  by the inverse ξ of the transmission path fading vector ξ from the transmission path estimator  24  for thereby performing phase correction for the received signal. 
     The received signals from paths which have been corrected in phase by the multipliers  25  of the despreaders  71   1 - 71   n  are combined by the rake combiner  26 , and the combined signal is decoded into an original symbol sequence by the decision unit  27 . Since the rake combiner  26  and the decision unit  27  are of general nature in the CDMA communication system and do not have direct bearing on the operation of the multiuser interference canceler, they will not be described in detail below. However, those skilled in the art will be able to construct the rake combiner  26  and the decision unit  27  with ease. 
     Each of the respreaders  72   1 - 72   n  comprises a pair of multipliers  28  and  29 . In each of the respreaders  72   1 - 72   n , the multiplier  28  multiplies the original symbol sequence by the transmission path fading vector ξ of one of the paths to impart original transmission path characteristics to the original symbol sequence. Thereafter, the original symbol sequence is spread with the spreading code Ca by the multiplier  49 . The signals from the respreaders  72   1 - 72   n  are combined into a chip-rate interference replica signal  81   a , which is outputted from the ICU  61   a.    
     In FIG. 2, the signal supplied from the rake combiner  26  to the decision unit  27  is branched and outputted to an external circuit. Such a signal branching arrangement is included in only the ICUs  63   a ,  63   b ,  63   c  in the third stage. From the ICUs  63   a ,  63   b ,  63   c , the signals are supplied to the decoders  19   a ,  19   b ,  19   c , respectively. 
     For the sake of brevity, operation of the conventional serial multiuser interference canceler shown in FIG. 1 for demodulating the signal of the first user will be described below. The signals of the other users will be demodulated in the same manner as described below. 
     The ICU  61   a  is supplied with the received signal  1 , generates a signal which is the same as the signal component of the first user contained in the received signal  1 , and outputs the generated signal as an interference replica signal  81   a . The delay memory  3  stores the received signal  1  and then outputs the received signal  1  after having delayed it for a certain time. The time for which the delay memory  3  delays the received signal  1  is a time required for the ICU  61   a  to generate the interference replica signal  81   a . The subtractor  4  subtracts the interference replica signal  81   a  from the received signal  1  outputted from the delay memory  3 , and then outputs the differential signal. Therefore, the differential signal outputted by the subtractor  4  contains only the signal of the second user and the signal of the third user. 
     In the second block of the first stage, the subtractor  4  subtracts the signal of the second user from the signal outputted from the delay memory  3 . In the third block of the first stage, the subtractor  4  subtracts the signal of the third user from the signal outputted from the delay memory  3 . 
     Upon completion of the processing in the first stage, therefore, the interference replica signals of all the users have been subtracted from the received signal  1 , leaving a residual signal, which is supplied to the first block of the second stage. 
     In the first block of the second stage, the adder  8  adds the interference replica signal  81   a  which has been delayed by the delay memory  7  for a certain time, to the residual signal from the first stage. Therefore, the output signal from the adder  8  contains only the signal component of the first user. The ICU  62   a  is supplied with the output signal from the adder  8 , and generates and outputs an interference replica signal  82   a . The output signal from the adder  8  is also supplied to the delay memory  5 , which delays the signal for a certain time and outputs the delayed signal to the subtractor  6 . The subtractor  6  subtracts the interference replica signal  82   a  from the delayed signal from the delay memory  5 . 
     The signal outputted from the subtractor  6  after the processing in the first block of the second state is a residual signal that does not contain any signals of the users. 
     The same processing as described above is carried out in the second block of the second stage through the third block of the third stage. 
     In each of the blocks of the third stage, the ICUs  63   a - 63   c  output signals from the rake combiners  26  thereof to the respective decoders  19   a - 19   c . The decoders  19   a  through  19   c  finally decode the supplied signals. 
     The signals handled in the first to third stages of the conventional serial multiuser interference canceler are all chip-rate signals. 
     The conventional parallel multiuser interference canceler will be described below with reference to FIG.  3 . The parallel multiuser interference canceler shown in FIG. 3 is also designed for three users as with the serial multiuser interference canceler shown in FIG.  1 . Those parts of the parallel multiuser interference canceler shown in FIG. 3 that are identical to those shown in FIG. 1 are denoted by identical reference numerals. 
     As shown in FIG. 3, the parallel multiuser interference canceler comprises first, second, and third stages, and three decoders  19   a - 19   c . The first stage comprises a delay memory  51 , three ICUs  61   a - 61   c , an adder  57 , a subtractor  54 , and three adders  58   a - 58   c . The second stage comprises a delay memory  52 , three ICUs  62   a - 62   c , an adder  59 , a subtractor  55 , and three adders  60   a - 60   c . The third stage comprises a delay memory  53 , three ICUs  63   a - 63   c , an adder  64 , a subtractor  56 , and three adders  65   a - 65   c.    
     In the first stages, the ICUs  61   a - 61   c , which are connected parallel to each other, generate respective interference replica signals  81   a - 81   c  which are the same as the signal components of the first to third users. The interference replica signals  81   a - 81   c  are combined by the adder  57  into a signal that is subtracted by the subtractor  54  from a received signal  1  that has been delayed for a certain time by the delay memory  51 . Therefore, the subtractor  54  outputs a residual signal that is free of the interference replica signals of all the users. The residual signal is then added to the interference replica signals  81   a - 81   c  in the first stage by the adders  58   a - 58   c . The interference replica signals  81   a - 81   c  to which the residual signal has been added are supplied to the respective ICUs  62   a - 62   c  of the second stage. 
     The second and third stages also perform the same processing operation as the processing operation, described above, of the first stage. The decoders  19   a - 19   c  finally decode respective signals supplied from the third stage. 
     In the above conventional multiuser interference canceler, the interference replica signal is estimated and reproduced from the received signal, and the interference replica signal is subtracted from the received signal. Therefore, the accuracy with which the interference replica signal is reproduced significantly affects the characteristics of the interference canceler. Factors which are responsible for the accuracy with which the interference replica signal is reproduced are the accuracy of the transmission path fading vector ξ extracted by the transmission path estimator  24  in each ICU and the error ratio of the hard decision result carried out by the decision unit  27  after the rake combining. 
     The transmission path fading vector ξ represents an estimation of characteristics added on the transmission path of the interfering user, and the hard decision value represents an estimation of the transmitted sequence. Either one of the transmission path fading vector ξ and the hard decision value is closely related to the ratio of desired signal power to noise power of the received signal. As the ratio of desired signal power to noise power is deteriorated, the error of the transmission path fading vector ξ and the error rate of the hard decision value are rendered poorer. Therefore, the accuracy with which the interference replica signal is reproduced is also reduced. While the ratio of desired signal power to noise power may be expressed as “SN ratio”, “SIR”, “Eb/No”, etc., it will be expressed as “Eb/No” herein. 
     The conventional multiuser interference cancelers have a problem in that since interference replica signals are uniformly reproduced for interference cancellation even when the Eb/No is low, the reception quality is degraded in regions where the Eb/No of the received signal is low. 
     In the actual environments of mobile communication systems, furthermore, the reception quality is sometimes degraded by time-dependent changes due to fading of individual signals and external noise when the multiuser interference canceler is operated in regions where the Eb/No of the received signal is not significantly low. 
     Consequently, the conventional multiuser interference cancelers have suffered the following disadvantages: 
     (1) The reception quality is degraded when interference cancellation is performed in regions where the Eb/No of the received signal is low. 
     (2) If the received signal suffers time-dependent changes due to fading of individual signals and external noise, then the reception quality may be also degraded when the multiuser interference canceler is operated. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a multiuser interference canceler which is capable of preventing reception quality from being degraded in all operating environments. 
     According to a first aspect of the present invention, there is provided a DS-CDMA multiuser interference canceler for processing a received signal containing spread signals from a plurality of users to remove signals of other users from a signal of each user and thereafter decode the signal of each user, comprising a first stage comprising a plurality of series-connected blocks having respective first interference canceler units associated respectively with the users, for generating and outputting interference replica signals which are identical to signal components of the users, and respective first subtracting means for subtracting the interference replica signals from the received signal, a plurality of second stages comprising a plurality of series-connected blocks having respective adding means associated respectively with the users, for adding interference replica signals of the users generated in a preceding stage to signals from a preceding block, respective second interference canceler units for generating again interference replica signals which are identical to the signal components of the users from output signals from the adding means, and respective second subtracting means for subtracting the interference replica signals which have been generated again from the output signals from the adding means and outputting differential signals to a next block, a plurality of decoding means for decoding symbols generated in a final one of the second stages, and a plurality of preliminary demodulating means for measuring reception quality of the signals of the users contained in the received signal, each of the first and second interference canceler units comprising means for controlling an interference canceler unit in a preceding block to turn off the outputting of the interference replica signal if the reception quality of output signals from the preceding block is more degraded than the reception quality of the signals of the users which has been measured by the preliminary demodulating means. 
     In the above DS-CDMA multiuser interference canceler, which is of the serial type, if the reception quality of output signals from the preceding block is more degraded than the reception quality of the signals of the users which has been measured by the preliminary demodulating means, then each of the interference canceler units determines that the reception quality has been degraded by an interference cancellation process in the preceding block, and controls the interference canceler unit in the preceding block to turn off the outputting of the interference replica signal. The DS-CDMA multiuser interference canceler is therefore capable of preventing the reception quality from being degraded by an interference cancellation process. 
     According to a second aspect of the present invention, there is provided a DS-CDMA multiuser interference canceler for processing a received signal containing spread signals from a plurality of users to remove signals of other users from a signal of each user and thereafter decode the signal of each user, comprising a first stage comprising a plurality of series-connected blocks having respective first interference canceler units associated respectively with the users, for generating and outputting interference replica signals which are identical to signal components of the users, and measuring reception quality of the signals of the users, and respective first subtracting means for subtracting the interference replica signals from the received signal, a plurality of second stages comprising a plurality of series-connected blocks having respective adding means associated respectively with the users, for adding interference replica signals of the users generated in a preceding stage to signals from a preceding block, respective second interference canceler units for generating again interference replica signals which are identical to the signal components of the users from output signals from the adding means and measuring reception quality of the signals of the users, and respective second subtracting means for subtracting the interference replica signals which have been generated again from the output signals from the adding means and outputting differential signals to a next block, a plurality of decoding means for decoding symbols generated in a final one of the second stages, a plurality of preliminary demodulating means for measuring reception quality of the signals of the users contained in the received signal, and cancellation on/off central control means for controlling an interference canceler unit in an either one of the stages preceding the one of the second stages to turn off the outputting of the interference replica signal if the reception quality measured by the second interference canceler units in one of the second stages is more degraded than the reception quality of the users measured by the preliminary demodulating means, until the reception quality measured by all the interference canceler units in said stage becomes better than the reception quality of the users measured by the preliminary demodulating means. 
     In the above DS-CDMA multiuser interference canceler, which is of the serial type, all the information of the reception quality measured by the interference canceler units is supplied to the cancellation on/off central control means, which makes an appropriate decision to determine an interference canceler unit which is to turn off an interference cancellation process. Therefore, the turning on and off of the interference cancellation process in the interference canceler units in each block can adequately be controlled. The interference canceler unit controlled to turn off the outputting of the interference replica signal should preferably be one of the interference canceler units, whose reception quality is worst, in a stage preceding said stage. 
     According to a third aspect of the present invention, there is provided a DS-CDMA multiuser interference canceler for processing a received signal containing spread signals from a plurality of users to remove signals of other users from a signal of each user and thereafter decode the signal of each user, comprising a first stage comprising a plurality of first interference canceler units associated respectively with the users, for generating and outputting interference replica signals which are identical to signal components of the users, and measuring reception quality of the signals of the users, first adding means for adding the interference replica signals, and first subtracting means for subtracting an output signal of the first adding means from the received signal, a plurality of second stages comprising a plurality of second adding means associated respectively with the users, for adding interference replica signals of the users generated in a preceding stage to signals from a preceding stage, respective second interference canceler units for generating again interference replica signals which are identical to the signal components of the users from output signals from the second adding means and measuring reception quality of the signals of the users, respective third adding means for adding the interference replica signals generated again by the second interference canceler units, and respective second subtracting means for subtracting output signals of the third adding means from the received signal and outputting differential signals to a next stage, a plurality of decoding means for decoding symbols generated in a final one of the second stages, a plurality of preliminary demodulating means for measuring reception quality of the signals of the users contained in the received signal, and cancellation on/off central control means for controlling an interference canceler unit in an either one of the stages preceding the one of the second stages to turn off the outputting of the interference replica signal if the reception quality measured by the second interference canceler units in one of the second stages is more degraded than the reception quality of the users measured by the preliminary demodulating means, until the reception quality measured by all the interference canceler units in said stage becomes better than the reception quality of the users measured by the preliminary demodulating means. 
     In the above DS-CDMA multiuser interference canceler, which is of the parallel type, all the information of the reception quality measured by the interference canceler units is supplied to the cancellation on/off central control means, which makes an appropriate decision to determine an interference canceler unit which is to turn off an interference cancellation process. Therefore, the turning on and off of the interference cancellation process in the interference canceler units in each block can adequately be controlled. 
     Rather than simply turning off the interference cancellation process which is liable to degrade the reception quality, the output level of the interference replica signal may be reduced for making the reception quality more uniform. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional serial multiuser interference canceler; 
     FIG. 2 is a block diagram of an interference canceler unit (ICU) in the conventional serial multiuser interference canceler shown in FIG. 1; 
     FIG. 3 is a block diagram of a conventional parallel multiuser interference canceler; 
     FIG. 4 is a block diagram of a serial DS-CDMA multiuser interference canceler according to a first embodiment of the present invention; 
     FIG. 5 is a block diagram of an ICU in the serial multiuser interference canceler shown in FIG. 4; 
     FIG. 6 is a block diagram of a serial DS-CDMA multiuser interference canceler according to a second embodiment of the present invention; 
     FIG. 7 is a block diagram of a parallel DS-CDMA multiuser interference canceler according to the second embodiment of the present invention; 
     FIG. 8 is a flowchart of an operation sequence of the multiuser interference canceler shown in FIG. 7; and 
     FIG. 9 is a block diagram of an ICU in a DS-CDMA multiuser interference canceler according to a third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     FIG. 4 shows in block form a serial DS-CDMA multiuser interference canceler according to a first embodiment of the present invention. Those parts of the serial multiuser interference canceler shown in FIG. 4 which are identical to those shown in FIG. 1 are denoted by identical reference numerals. 
     The multiuser interference canceler shown in FIG. 4 differs from the conventional multiuser interference canceler shown in FIG. 1 in that it has an additional preliminary demodulating stage comprising three preliminary demodulators  10   a ,  10   b ,  10   c  and a plurality of ICUS  11   a - 13   c  in place of the ICUs  61   a - 63   c . Each construction of ICUs  11   a - 13   c  is different from that of the ICUs  61   a - 63   c.    
     The preliminary demodulators  10   a ,  10   b ,  10   c  measure Eb/No and bit error rates (BERS) of the user signals contained in the received signal  1 , and output the measured results as reception quality signals  14   a ,  14   b ,  14   c . The reception quality signal  14   a  is supplied to the ICUs  11   a ,  12   a ,  13   a , the reception quality signal  14   b  to the ICUs  11   b ,  12   b ,  13   b , and the reception quality signal  14   c  to the ICUs  11   c ,  12   c ,  13   c . The reception quality signals  14   a ,  14   b ,  14   c  may be supplied to the ICUs  11   a - 13   c  in any of various ways. For example, they may be combined with the received signal  1  according to time-division multiplexing process, and the multiplexed signals may be supplied to the ICUs  11   a - 13   c , or may be supplied along other lines to the ICUs  11   a - 13   c . 
     The arrangement of the ICU  12   a , as an example of the ICUs  11   a - 13   c , will be described below with reference to FIG.  5 . 
     As shown in FIG. 5, the ICU  12   a  differs from the conventional ICU  61   a  shown in FIG. 2 in that it has a plurality of despreaders  73   1 - 73   n  in place of the despreaders  71   1 - 71   n , a plurality of respreaders  74   1 - 74   n  in place of the respreaders  72   1 - 72   n , and additionally has a bit error rate (BER) measuring unit  33  and a quality comparison controller  32 . 
     Each of the despreaders  73   1 - 73   n  is similar in structure to the despreaders  71   1 - 71   n  shown in FIG. 2 except that it additionally has an Eb/No measuring unit  34 . The Eb/No measuring unit  34  determines an Eb/No from the correlated value determined by the integrator  23 , and outputs the determined Eb/No. 
     Each of the respreaders  74   1 - 74   n  is similar in structure to the respreaders  72   1 - 72   n  shown in FIG. 2 except that it additionally has an on/off controller  30 . The on/off controller  30  outputs the signal from the multiplier  29  as an interference replica signal  82   a  if a control signal  15   4  is not applied thereto, and does not output the signal from the multiplier  29  as an interference replica signal  82   a  if a control signal  15   4  is applied thereto. 
     The BER measuring unit  33  serves to measure a bit error rate of a pilot symbol (PL) of the correlated value outputted from the decision unit  27  by comparing the pilot symbol with a symbol pattern of a known pilot symbol. 
     The quality comparison controller  32  compares the Eb/No measured by the Eb/No measuring unit  34  with the Eb/No of the received signal  1  which has been supplied as the reception quality signal  14   a , and outputs a control signal  15   3  if the Eb/No measured by the Eb/No measuring unit  34  is more degraded than the Eb/No of the received signal  1 , as determined from the result of the comparison. The quality comparison controller  32  also compares the BER measured by the BER measuring unit  33  with the BER of the received signal  1  which has been supplied as the reception quality signal  14   a , and outputs a control signal  15   3  if the BER measured by the BER measuring unit  33  is more degraded than the BER of the received signal  1 , as determined from the result of the comparison. Stated otherwise, if the Eb/No measured by the Eb/No measuring unit  34  is more degraded than the Eb/No of the received signal  1  which has been supplied as the reception quality signal  14   a  or if the BER measured by the BER measuring unit  33  is more degraded than the BER of the received signal  1  which has been supplied as the reception quality signal  14   a , then the quality comparison controller  32  determines that the reception quality has been made poorer by the interference cancellation, and outputs a control signal  15   3 . 
     As shown in FIG. 4, the ICUs  11   a - 13   b  are supplied with respective control signals  15   1 - 15   8  from the ICUs  11   b - 13   c  in the next blocks. The ICU  13   c  in the final block of the third stage does not need to have an on/off controller  30  because the ICU  13   c  is supplied with no control signal. The ICU  11   a  in the first block of the first stage may be not have the quality comparison controller  32 , the BER measuring unit  33 , and the Eb/No measuring unit  34  because it may output no control signal. 
     Operation of the multiuser interference canceler according to the first embodiment will be described below with reference to FIGS. 4 and 5. 
     The preliminary demodulators  10   a ,  10   b ,  10   c  preliminarily demodulate the received signal  1 , measure the Eb/No of the received signal  1  and the BER of the pilot symbol, and output the measured Eb/No and BER as reception quality signals  14   a ,  14   b ,  14   c  to the ICUs  11   a - 13   c  which correspond to the users. 
     At the same time, the received signal  1  is processed for interference cancellation in each of the first, second, and third stages in the same manner as with the conventional multiuser interference canceler shown in FIG.  1 . Finally, processed signals are decoded by the decoders  19   a ,  19   b ,  19   c.    
     The quality comparison controllers  32  in the respective ICUs  11   a - 13   c  compare the reception quality represented by the Eb/No and the BERs of pilot symbols that are supplied as the reception quality signals  14   a ,  14   b ,  14   c  with the Eb/No supplied from the Eb/No measuring units  34  and the BERs of pilot symbols supplied from the BER measuring units  33 . If the reception quality represented by the Eb/No supplied from the Eb/No measuring units  34  and the BERs of pilot symbols supplied from the BER measuring units  33  is more degraded than those supplied as the reception quality signals  14   a ,  14   b ,  14   c , then the quality comparison controllers  32  output control signals  15   1 - 15   8  depending on the respective ICUs  11   a    13   c . The control signals  15   1 - 15   8  outputted from the ICUs  11   b - 13   c  are supplied to the ICUs in preceding blocks. In this manner, the interference cancellation process in those blocks which precede the blocks whose ICUs have detected a degradation of the reception quality is interrupted. 
     In this embodiment, the reception qualities of the signal before and after the interference cancellation process are compared, and if the reception quality is degraded by the interference cancellation process, then the interference cancellation process in the preceding block is interrupted. Therefore, the reception quality is prevented from being degraded by the interference cancellation process that is performed as with the conventional multiuser interference cancelers. 
     In this embodiment, the Eb/No of the signal and the BER of the pilot symbol before and after the interference cancellation process are compared. However, only the Eb/No of the signal or the BER of the pilot symbol may be compared for controlling the turning on and off of the interference cancellation process. However, since changes in the reception quality can be detected in greater detail by detecting both the Eb/No and the BER, the turning on and off of the interference cancellation process can be controlled with higher accuracy when both the Eb/No and the BER are detected. 
     Second Embodiment 
     FIG. 6 shows in block form a serial DS-CDMA multiuser interference canceler according to a second embodiment of the present invention. The serial multiuser interference canceler according to the second embodiment differs from the serial multiuser interference canceler shown in FIG. 4 in that it additionally has a cancellation on/off central controller  66  and a plurality of ICUs  41   a - 43   c  in place of the ICUs  11   a - 13   c . Those parts of the serial multiuser interference canceler shown in FIG. 6 which are identical to those shown in FIG. 4 are denoted by identical reference numerals. The reception quality signals  14   a ,  14   b ,  14   c  are supplied to the cancellation on/off central controller  66 , rather than the ICUs  41   a    43   c.    
     The ICUs  41   a - 43   c  are different from the ICU  12   a  shown in FIG. 5 in that the quality comparison controller  32  is dispensed with, and the Eb/No measured by the Eb/No measuring units  34  and the BER of the pilot symbol measured by the BER measuring units  33  are outputted as reception quality signals  45   1 - 45   8 . 
     The cancellation on/off central controller  66  compares the reception quality signals  14   a - 14   c  with the reception quality signals  45   1 - 45   8  from the ICUs  41   b    43   c , determines which blocks suffer a degradation in the reception quality, and outputs control signals  16   1 - 16   8  to determine those ICUs whose interference cancellation process is to be interrupted. 
     The turning on and off of the interference cancellation process in the ICUs  41   a - 43   c  is not controlled by control signals from the ICUs in preceding blocks, but controlled by control signals  16   1 - 16   8  from the cancellation on/off central controller  66 . The ICU  43   c  in the final block of the third stage does not need to have an on/off controller  30  because the ICU  43   c  is supplied with no control signal. The ICU  41   a  in the first block of the first stage does not have the quality comparison controller  32  and the BER measuring unit  33  because it outputs no reception quality signal. 
     FIG. 7 shows in block form a parallel DS-CDMA multiuser interference canceler according to the second embodiment of the present invention. The parallel multiuser interference canceler according to the second embodiment differs from the conventional parallel multiuser interference canceler shown in FIG. 3 in that it additionally has three preliminary demodulators  14   a ,  14   b ,  14   c , a cancellation on/off central controller  66 , and a plurality of ICUs  41   a - 43   c  in place of the ICUs  61   a - 63   c . Those parts of the parallel multiuser interference canceler shown in FIG. 7 which are identical to those shown in FIG. 3 are denoted by identical reference numerals. 
     Operation of the multiuser interference cancelers according to the second embodiment will be described below. Since basic operating principles of the serial and parallel multiuser interference cancelers are the same as each other, operation of only the parallel multiuser interference canceler according to the second embodiment will be described below. FIG. 8 shows an operation sequence of the parallel multiuser interference canceler according to the second embodiment shown in FIG.  7 . 
     In step  101 , the ICUs  41   a - 43   c  supply information of Eb/No and BER in the ICUs  41   a - 43   c , as represented by the reception quality signals  45   1 - 45   8 , to the cancellation on/off central controller  66 . Then, in step  102 , if the reception quality of a block whose interference cancellation process has been interrupted is improved beyond a certain level, then the interruption of the interference cancellation process is canceled. However, since there is initially no block whose interference cancellation process has been interrupted, nothing occurs at this time. 
     Then, the cancellation on/off central controller  66  decides, in step  103 , whether the reception quality of any one of the ICUs  42   a - 42   c  in the second stage is more degraded than the reception quality represented by the reception quality signals  14   a - 14   c . If more degraded, then the cancellation on/off central controller  66  outputs, in step  104 , control signals  16   1 - 16   3  for turning off an interference replica signal of the ICU whose reception quality is worst among the ICUs  41   a - 41   c  in the first stage. The processing in steps  103  and  104  is repeated until the second stage contains no blocks in which reception quality is degraded. 
     If the reception quality of any one of the ICUs  42   a - 42   c  in the second stage is not more degraded than the reception quality represented by the reception quality signals  14   a - 14   c  in step  103 , then the interference cancellation process in the second stage is finished. The cancellation on/off central controller  66  decides, in step  105 , whether the reception quality of any one of the ICUs  43   a - 43   c  in the third stage is more degraded than the reception quality represented by the reception quality signals  14   a - 14   c . If more degraded, then the cancellation on/off central controller  66  outputs, in step  106 , control signals  16   4 - 16   6  for turning off an interference replica signal of the ICU whose reception quality is worst among the ICUs  42   a - 42   c  in the first stage. The processing in steps  105  and  106  is repeated until the third stage contains no blocks in which reception quality is degraded. 
     If the third stage contains no blocks in which reception quality is degraded, then control goes from step  105  back to step  101 . The reception quality by Eb/No and BER is thus supplied again from the ICUs  41   b - 43   c  of all the blocks to the cancellation on/off central controller  66  in a given periodic cycle. 
     If the monitored reception quality of the each user is improved beyond a certain level with time, then it is considered that the reception quality is improved also after the interference cancellation process has started. Therefore, the interruption of the interference cancellation process in a block which has been started in steps  104  and  106  is canceled in step  102 . 
     The processing in steps  101 - 106  is repeated to prevent the interference cancellation process that tends to degrade the reception quality from being performed. 
     In the second embodiment, the cancellation on/off central controller  66  first obtains information indicative of which block suffers poor reception quality based on the reception quality signals  45   1 - 45   8  outputted from the ICUs  41   b - 43   c , and then decides whether the interference cancellation process in the ICUs in the block is to be interrupted or not. Therefore, the cancellation on/off central controller  66  attains all information indicative of which block suffers poor reception quality and then makes an appropriate decision based on the information for controlling the turning on and off of the interference cancellation process in the ICUs in each of the blocks. 
     In the illustrated second embodiment, if the reception quality of either one of the ICUs in a certain stage is more degraded than the reception quality measured by the preliminary demodulating stage, then the cancellation on/off central controller  66  turns off the interference replica signal in the ICU whose reception quality is worst among the ICUs in the stage that precedes the certain stage. However, since the interference cancellation processes performed in the respective blocks are related to each other, the above control process of the multiuser interference canceler may not necessarily be most effective in preventing the reception quality from being degraded. Accordingly, the second embodiment is not limited to the above control process, but may be arranged such that if the reception quality of either one of the ICUs in a certain stage is more degraded than the reception quality measure by the preliminary demodulating stage, then the cancellation on/off central controller  66  turns off the interference replica signal in the ICU in any preceding stage between the certain stage and the preliminary demodulating stage. 
     Third Embodiment 
     A DS-CDMA multiuser interference canceler according to a third embodiment of the present invention will be described below. The DS-CDMA multiuser interference canceler according to the third embodiment differs from the DS-CDMA multiuser interference canceler according to the first embodiment in that the ICUs  11   a - 13   c  according to the first embodiment are replaced with ICUs  91   a - 93   c  which are structurally different from the ICUs  11   a - 13   c . The arrangement of the ICU  92   a , as an example of the ICUs  91   a  through  93   c , will be described below with reference to FIG.  9 . Those parts of the DS-CDMA multiuser interference canceler shown in FIG. 9 that are identical to those shown in FIG. 5 are denoted by identical reference numerals. 
     The ICU  92   a  differs from the ICU  12   a  shown in FIG. 5 in that it has a multiplier  35  in place of the on/off controller  30  and a quality comparison controller  86  in place of the quality comparison controller  32 . The quality comparison controller  86  outputs a control signal  85   3  for controlling the level of an outputted interference replica signal, rather than the control signal  15   3  for controlling the turning on and off of the interference cancellation process. The multiplier  35  multiplies the output signal from the multiplier  29  by a coefficient a (0≦α≦1) represented by a control signal  85   4  outputted from the ICU  92   b , and outputs the product as an interference replica signal  82   a.    
     The DS-CDMA multiuser interference canceler according to the third embodiment operates in exactly the same manner as with the DS-CDMA multiuser interference canceler according to the first embodiment by setting the coefficient α to α=1 or 0. Since the coefficient α is in the range of 0≦α≦1, when the reception quality is close to a boundary of degradation in the quality, the interference cancellation process is not simply turned off, but the coefficient a is set to an intermediate value, e.g., 0.5, between 0 and 1 for making the reception quality more uniform. 
     The control process performed by the DS-CDMA multiuser interference canceler according to the present invention is a feedback control process. Therefore, the DS-CDMA multiuser interference canceler controls the interference cancellation process immediately after the reception quality at a certain point in time, judging from the reception quality. Therefore, the control process necessarily suffers a delay. If the reception quality changes in short periodic cycles, then simply turning on and off the interference cancellation process as in the first and second embodiments may not allow the interference cancellation process to follow the change in the reception quality, but may possibly degrade the reception quality. According to the third embodiment, however, when the reception quality changes, the DS-CDMA multiuser interference canceler initially sets the coefficient a to a value close to 0 and thereafter increases coefficient α with time. In this manner, the interference cancellation process is less susceptible to the effect of the delay in the control process. 
     The above principle of the third embodiment is also applicable to the multiuser interference canceler according to the second embodiment. If the principle of the third embodiment is applied to the multiuser interference canceler according to the second embodiment, then the control signals  16   1 - 16   8  outputted from the cancellation on/off central controller  66  are made equal to the control signals  85   1 - 85   8 . 
     In the above preferred embodiments of the present invention, the number of users is three and the number of stages is also three. However, the present invention is also applicable to a DS-CDMA multiuser interference canceler where the number of users is other than three and the number of stages is also other than three. 
     According the present invention, as described above, even when the signal reception environments are poor, such as when the Eb/No of the received signal is low or the received signal is degraded by time-dependent changes due to fading and external noise, the reception quality is prevented from being degraded by the interference cancellation process. 
     It is to be understood, however, that although the characteristics and advantages of the present invention have been set forth in the foregoing description, the disclosure is illustrative only, and changes may be made in the arrangement of the parts within the scope of the appended claims.