Abstract:
A multistage detector is disclosed that maximizes computation power while minimizing system delay. The differencing multistage detector receives signals from a plurality of users in a cell of a communications system and reduces the effect of multiple access interference to a signal from a desired user caused by interference from other users in the cell. The differencing multistage detector includes a plurality of stages, each stage including an interference canceller for removing intra-cell interference caused by the other users in the cell and producing an estimation output vector, wherein except for a first stage, the estimation output vector of a current stage is based on both a decision of the interference canceller of the current stage and the output from an interference canceller of a previous stage. The estimation output vector of a current stage is produced by combining the output from an interference canceller of a previous stage and the decision of the interference canceller of the current stage. Except for the first stage each interference canceller calculates an estimate of multi-user interference by computing a product of a cross-correlation of the received signals and a difference signal thereby reducing the number of multiplication operations required.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to a multistage detector in wireless communication systems, and more particularly to a method and apparatus for providing differencing multistage detection in the reverse link of a Code Division Multiple Access communication system. 
     2. Description of Related Art 
     Cellular systems have had a direct effect on the lives of millions over the past few years. For the first time, people are able to make and receive phone calls without being tied to a specific location. Mobile phones, as part of the cellular systems, have allowed people to break the tie between location and access to communication. Mobile phones have also allowed people to reach another who is in move. With the development in cellular systems, people are allowed to reach another who is mobile in anywhere at anytime. 
     The first generation of mobile communication systems were born in the early 1980s. The marriage of radio and telephone technologies gave birth to mobile phones and triggered a turning point in telecommunications. Adding radio access to a telephone network meant that for the first time in history, the concept of a telephone being at a fixed point in the network was no longer valid. The benefits of being able to make and receive telephone calls anywhere had appeal to business people—the original market. In the first generation of cellular networks, analog wireless technology were used for the user connection (called the “air interface”). Every voice channel had its own narrow frequency band, using a technology called Frequency Division Multiple Access (FDMA). 
     However, as the demand for mobile phones grew and grew, regularly exceeding forecasts, it became obvious that the available radio spectrum would not be adequate to accommodate the expected numbers of mobile phone users. The digital technology became the solution to the problem. The answer lay in new digital wireless technologies that allow larger numbers of mobile subscribers to be supported within a given frequency allocation. Time Division Multiple Access (TDMA) technology is used in which a broader frequency channel is divided into intermittent time-slots, i.e. several calls share the same frequency channel at any one time. The digital technology also offered other important benefits. It provided better voice quality and improved security against unauthorized eavesdropping. Another technology, Code Division Multiple Access (CDMA) has also been developed subsequently to increase capacity. 
     The first and second generation mobile communication systems were mainly set to support voice communications, although today&#39;s mobile phones can also be used for data transfer at rates that are acceptable for relatively low-speed data applications such as sending and receiving of faxes and e-mail. However, these systems do not support high-speed data or video applications. The third generation mobile communication system is being developed to remove the bandwidth bottleneck and support a whole new range of voice, data, video, and multimedia services. For example, smart messaging is bringing Internet services to every mobile user&#39;s fingertips. As people become used to the freedom that mobile communications have provided, they will become more demanding about the information and services required to benefit their lives. 
     The demand by consumers all over the world for mobile communications service continues to expand at a rapid pace and will continue to do so for at least the next decade. To satisfy such demand, more and more innovative mobile telecommunications networks are being built in this growing industry. 
     Code Division Multiple Access (CDMA) is emerging as one of the main technologies for the implementation of third-generation (3G) cellular systems. In CDMA, each user is assigned a unique code sequence (spreading code) that is used to encode an information bearing signal. The receiver, knowing the code sequences of the user, decodes a received signal after reception and recovers the original data. This is possible since the cross-correlations between the code of the desired user and the codes of the other users are small. Since the bandwidth of the code signal is chosen to be much larger than the bandwidth of the information-bearing signal, the encoding process spreads the spectrum of the signal and is therefore also know as spread-spectrum modulation. 
     CDMA may be classified according to the modulation techniques. For example, the system may a direct sequence (DS) spread-spectrum CDMA system wherein the information bearing signal is multiplied directly by a high chip rate spreading code. Another modulation technique is frequency hopping spread-spectrum wherein the carrier frequency at which the information-bearing signal is transmitted is rapidly changed according to the spreading code. Time hopping spread-spectrum involves transmitting the information-bearing signal in short bursts rather than continuously wherein the timing of the short bursts are decided by the spreading code. Hybrid modulation is also possible where two or more of the above-mentioned modulation techniques are used. Moreover, it is possible to combine CDMA with other multiple access methods: TDMA, multicarrier (MC) or multitone (MT) modulation. 
     In DS-CDMA, the modulated information-bearing signal (the data signal) is directly modulated by a digital, discrete time, discrete valued code signal. The data signal may be either an analog signal or a digital one. Typically it is a digital signal. For a digital signal, the data modulation is often omitted and the data signal is directly multiplied by the code signal and the resulting signal modulates a wideband carrier. CDMA system often use a hybrid diversity scheme to capture both strong and weak signals in the same cellular region. To capture both the strong and weak signals in the same cellular region antenna-array diversity and RAKE diversity are implemented. Further, RAKE receivers for both the mobile and base stations are specified to improve reception in the cases where the delay spreads are significant. 
     Current CDMA receivers are based on the RAKE receiver principle, which considers other users&#39; signals as interference. However, in an optimum receiver al signals would be detected jointly or interference from other signals would be removed by subtracting them from the desired signal. This is possible because the correlation properties between signals are known (i.e., the interference is deterministic not random). 
     The capacity of a direct sequence CDMA system using RAKE receiver is interference limited. In practice this means that when a new user, or interferer, enters the network, other users service quality will degrade. The more the network can resist interference the more users can be served. Multiple access interference that disturbs a base or mobile station is a sum of both intra- and intercell interference. 
     Multiuser detection (MUD), also called joint detection and interference cancellation (IC), provides means of reducing the effect of multiple access interference, and hence increase the system capacity. In the first place, MUD is considered to cancel only the intra-cell interference, meaning that in a practical system the capacity will be limited by the efficiently of the algorithm and the intercell interference. 
     In addition to capacity improvement, MUD alleviates the near/far problem typical to DS-CDMA systems. A mobile station close to a base station may block the whole cell traffic by using too high a transmission power. If this user is detected first and subtracted from the input signal, the other users do not see the interference. 
     The conventional matched filter bank method in a multiuser detector experiences MAI (Multiple Access Interference) and the near-far problem. Optimal multiuser detector that have been proposed can eliminate the MAI and offer a significant improvement over the conventional multiuser detector. However, for a K-user, N-bit communication system, it requires 2 NK  times exhaustive searches to find a maximum likelihood sequence, which is computational intensive. This has lead researchers to use sub-optimum multiuser detectors, such as decorrelating detectors and minimum mean-squared error (MMSE) detectors, which require the calculation of the inverse of the cross-correlation matrix or the matrix which has the same scale. 
     The other group of multiuser detectors is based upon interference cancellation (IC). The idea is to subtract the interference generated by users other than the desired user. Lower computation demanding and hardware related structures are the major advantages of this strategy. One of the most effective IC is the parallel interference cancellation (PIC) which comes from the iterative multistage method, which was first proposed by M. K. Varanasi and B. Aazhang, in “Multistage Detection in Asynchronous Code Division Multiple Access Communications”, IEEE Transactions in Communications, Vol. 33, NO. 4: 509-519, Apr. 1990. 
     The inputs of one particular stage are the estimated solution of previous stage. After interference cancellation, the new estimations, which should be closer to the transmitted bits, come out to be fed into the next stage. Almost all existing multistage based algorithms neglect the fact that as the iterations progress, the solution becomes more and more invariant, i.e. more and more elements in the output vector turn out to be the same as the elements in the input vector. Ideally at the last iteration stage, the output and the input should be identical if the algorithm converges. Therefore in last several stages, the multistage detector will almost calculate from the same input to get the same result. This is a substantial waste of the computation power and it increases the system delay. 
     It can be seen that there is a need for a multistage detector that maximizes computation power while minimizing system delay. 
     It can also be seen that there is a need for a method and apparatus for providing differencing multistage detection in the reverse link of a Code Division Multiple Access communication system. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an improved multistage detector in wireless communication systems. 
     The present invention solves the above-described problems by providing a multistage detector that maximizes computation power while minimizing system delay. The differencing multistage detector achieves both high performance in the interference cancellation and computational efficiency, which leads to a very large scale integrated circuit (VLSI) implementation. When the iterative algorithm of the differencing multistage detector converges, the difference of the solution vectors between two consecutive stages is mostly zero. 
     A system in accordance with the principles of the present invention includes a differencing multistage detector for receiving signals from a plurality of users in a cell of a communications system, the differencing multistage detector reducing the effect of multiple access interference to a signal from a desired user caused by interference from other users in the cell, wherein the differencing multistage detector includes a plurality of stages, each stage including an interference canceller for removing intra-cell interference caused by the other users in the cell and producing an estimation output vector, wherein except for a first stage, the estimation output vector of a current stage is based on both a decision of the interference canceller of the current stage and the output from an interference canceller of a previous stage. 
     Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the estimation output vector of a current stage is produced by subtracting the output from an interference canceller of a previous stage from the decision of the interference canceller of the current stage. 
     Another aspect of the present invention is that except for the first stage each interference canceller calculates an estimate of multi-user interference by computing a product of a cross-correlation of the received signals and the difference signal. 
     Another aspect of the present invention is that the difference signal comprises 0, +2, or −2. 
     Another aspect of the present invention is that the computing of the product is omitted when the difference signal is 0, and the computing of the product is performed by storing the cross-correlation of the received signals in a register and shifting the bits one place forward when the difference signal is +2 and one place forward with a sign change when the difference is −2. 
     Another aspect of the present invention is that the interference canceller is a parallel interference canceller. 
     Another aspect of the present invention is that control between stages is handled by a handshaking protocol. 
     Another aspect of the present invention is that the input to each stage is in two&#39;s compliment form. 
     These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIG. 1 illustrates one possible embodiment of an advanced communication network; 
     FIG. 2 illustrates a detailed block diagram of a mobile communication system illustrating the details of a base station according to an embodiment of the present invention; 
     FIG. 3 illustrates a direct sequence spread spectrum transmitter and a direct sequence spread spectrum receiver according to the present invention; 
     FIG. 4 shows a model system of a multiuser communication system according to the present invention; 
     FIG. 5 illustrates the first two stages of differencing multistage detector according to the present invention; 
     FIG. 6 illustrates a single adder implementation of the differencing multistage detector according to the present invention; 
     FIG. 7 illustrates the BER versus signal-to-noise ratio (SNR) and MAI in a five-user and ten-user system; and 
     FIG. 8 a  illustrates an observation of the percentage of zeros in the differencing vector according to the present invention. 
     FIG. 8 b  illustrates the number of computations that are save with a multistage detector according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration the specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention. 
     The present invention provides a differencing multistage detector for the reverse link of wireless CDMA communication systems. The differencing multistage detector achieves both high performance in the interference cancellation and computational efficiency, which leads to a very large scale integrated circuit (VLSI) implementation. When the iterative algorithm of the differencing multistage detector converges, the difference of the solution vectors between two consecutive stages is mostly zero. 
     FIG. 1 illustrates one possible embodiment of an advanced communication network  100 , e.g., a third generation GSM evolution. Those skilled in the art will recognize that the present invention is not meant to be limited to use with GSM mobile communication systems, but is applicable to other mobile communication systems. However, the present invention will be described herein using GSM as an example. 
     As shown in FIG. 1, the first implementations of Generic Radio Access Network (GRAN) may be based on the integration of RAN and SGM/UMTS core network, which has been evolved from the GSM core network by integrating new third generation capabilities. The evolved GSM network elements are referred to as 3G MSC and 3G SGSN. 
     A mobile unit  110  receives and sends signals to a base station (BS)  112 . Base stations  112  are in turn coupled to a radio network controller (RNC)  114  in the radio access network (RAN)  116 . The RAN interfaces with GSM/UMTS core network  120  via the Iu-interface  122 , which corresponds to the GSM A-interface and GPRS Gb-interface. As can be seen, radio access  130  is isolated from the core network  120 , and the goal is that the GSM/UMTS core network would have the flexibility to support any radio access scheme. Circuit switched services are routed via the GSM MSC  140 , and the packet switched services via the GPRS part  150  of the GSM/UMTS core network. 
     FIG. 2 illustrates a detailed block diagram of a mobile communication system  200  illustrating the details of a base station according to an embodiment of the present invention. The system  200  is comprised of a plurality of base stations  202  connected to system controllers  201 , and mobile terminals  203 . A service area of the mobile communication system  200  is divided into a plurality of cells  210 - 220 . The mobile switching center  230  is connected with another mobile communication system or fixed network  232  and coordinates the setting up of calls to the mobile terminals  203 . The mobile terminal  203  can move within a service area which is formed by a plurality of base stations  202  for communication through a channel allocated to the neighboring base station  202 . 
     The base station  202  includes transceivers  240 ,  242 ,  244 . The transceivers  240 ,  242 ,  244 , which represent at least one receiver and one transmitter, provide coverage to cells  210 ,  212 ,  214  respectively, wherein each transmitter/receiver pair  240 ,  242 ,  244  comprises a channel unit. The transceivers  240 ,  242 ,  244  also receive calling signals sent from the mobile terminal  203  moving in the corresponding cell, and detect up-link carrier wave power of the received signal. 
     FIG. 3 illustrates a direct sequence spread spectrum transmitter  310  and a direct sequence spread spectrum receiver  350  according to the present invention. In the transmitter  310 , binary data  312  is multiplied  314  by a spreading code generated by the code generator  316 . The coded signal  317  is modulated onto a carrier generated by the carrier generator  318  at the wideband modulator  320 . The spread spectrum signal  322  is then transmitted. 
     At the receiver  350 , a spread spectrum signal  352  is received. The received signal  352  is used to synchronize the code used for despreading the received spread spectrum signal  322  with the received spread spectrum signal  322  at the code synchronization/tracking block  354 . The code generator  356  generates the code used for despreading the received spread spectrum signal  322 . The despread signal is then demodulated at the date demodulator  362  using a carrier generated by the carrier generator  360  to reproduce the data signal  364 . 
     Assuming that the modulator uses a K-user binary phase-shift keying (BPSK) technique for the modulated DS-CDMA communications system, the channel is a single path channel with additive white Gaussian noise (AWGN). FIG. 4 shows a model system of a multiuser communication system according to the present invention. 
     To simplify the explanation of the invention, the description of the invention will be limited to a synchronized scenario. However, those skilled in the art will readily recognize that the asynchronous scenario can be derived by simply adding the delay information to each user. 
     In FIG. 4, each user signal  410  is encoded through a channel encoder  412 . The signal from the channel encoder is spread at the spreading block  414 . These signals  415  are transmitted over a channel  416  that includes, additive white Gaussian noise (AWGN)  418 . In the receiver  420 , the received signal  422  is correlated with replicas of the user spreading codes. The received signal may be represented by the cross-correlation as follows:                r        (   t   )       =         ∑     k   =   1     K                       ∑     i   =   1     N                         ɛ   k              b   k          (   i   )              s   k          (     t   -   iT   -     τ   k       )             +     η        (   t   )                 (   1   )                                
     where K is the number of users and N is the size of data block for the multiuser detection. The estimation of the kth user&#39;s signal power {square root over (ε k )} may be obtained by the parameter estimation block. The source data bits are represented by b k (i). Here because BPSK modulation is assumed, b k (i)ε{−1, +1}. s k  is the signature sequence (spreading code) of the kth user, where T is the duration of one bit. In order to get the best performance, s k  is generated by a pseudo-random number (PN) sequence or Gold code. AWGN is represented by η(t). 
     The first stage of a multiuser detector  424  is always a bank of matched filters  430 , which is the sole component of the conventional single user like detector. Each branch of the matched filter bank  430  consists of the correlation of the received signal with one particular user&#39;s signature sequence, which is:                  y   i     =       1   T            ∫   0   T            r        (   t   )              s   i          (   t   )                          t                             
            i   =   1     ,   2   ,   …              ,   K             (   2   )                                
     Equation (2) can also be expressed in a simple matrix format: 
     
       
           y=RAd+η   (3) 
       
     
     where vector y  432  and d are the output of the matched filter bank  430  and the transmitted user bits respectively. There are K elements in each vector. In a general asynchronous system, the scale of matrix R is K×K cross-correlation. The elements in the cross-correlation matrix can be represented by:                r   ij     =     {             1   T            ∫   0   T              s   i          (   t   )              s   i          (   t   )                          t                 i   ≠   j             0         i   =   j                                (   4   )                                
     In the multistage detector  424  according to the present invention, the auto-correlation is not a factor. A is the amplitude matrix of the signal, which is represented as diag{{square root over (ε 1 )},{square root over (ε 2 )} . . . {square root over (ε k )}}. The differencing multistage detector  424  according to the present invention is based on solving linear equation (3). 
     The first three stages  502 ,  504  of a differencing multistage detector  500  according to one embodiment of the present invention is shown in FIG.  5 . Interference cancellation uses the previous estimations to generate a new vector of signals. Then all the “interfering” users are summed and subtracted from previous signal z (1) . In the end, a better estimation {circumflex over (d)} (l+1)  vector is produced. 
     In each stage of the multistage detector  500 , interference cancellation  510 ,  560 ,  570  (IC), such as a parallel interference canceller (PIC), removes the intra-cell interference of other users from the received signal to get a better estimated signal for one particular user. Because the exact bit information for any user is unknown, the estimated (hard decision) value in each stage may be used. The output of the lth iteration  540  is: 
     
       
           z   (1)   =y−RA{circumflex over (d)}   (l−1) def   =y−Î   (l−1)   (5) 
       
     
     
       
         {circumflex over (d)} (0) =sign( y ) 
       
     
     
       
         {circumflex over (d)} (l−1) =sign( z   (l−1) ) 
       
     
     The term Î is defined as the estimated interference provided by the other users to the desired user. Since {circumflex over (d)} k   (l) ε{−1, +1} and RA is pre-calculated, there are no multiplications in equation (5). After l iterations, it is greatly possible to observe {circumflex over (d)} (l) ={circumflex over (d)} (l−1) . This is the exact property of the convergence. So instead of dealing with each estimated bit vector {circumflex over (d)} (l) , as before, the difference of the bits in two consecutive stages is calculated, i.e. the input of each stage  550  becomes {circumflex over (x)} (l) ={circumflex over (d)} (l) −{circumflex over (d)} (l−1) (j=1,2, . . . ,K). {circumflex over (x)} (l)  is called the differencing vector  550 . By subtracting two consecutive stages represented by equation (5): 
     
       
           z   (l)   =z   (l−1)   =−RA{circumflex over (x)}   (l−1)   (6) 
       
     
     
       
         → z   (l)   =z   (l−1)   −RA{circumflex over (x)}   (l−1)   
       
     
     The updated estimated bit vector {circumflex over (d)} (l) , can be worked out by 
     
       
         {circumflex over (d)} (l) =sign( z   (l) )  (7) 
       
     
     Using this differencing technique, many computations are saved by calculating using equation (6) instead of calculating using equation (5), because more and more elements in the vector {circumflex over (x)} (l)  turn to zero. All the non-zero elements in {circumflex over (x)} (l)  equal to +2 or −2. Such constant multiplications in equation (6) can be implemented by arithmetic shifts, which will not in actuality introduce any multiplication operations. Further, because subtraction of two consecutive stages is a linear transformation, the bit error rate (BER) after each stage  560 / 570  will not change, as compared with the conventional multistage detection. This ensures the final BER is the exact same as the conventional multistage detection method. 
     Accordingly, the complete method for performing multistage detection according to the present invention is as follows: 
      {circumflex over (d)} (0) =sign( y ) 
     for k=1 to K            z   k     (   1   )       =         y   k     -       ∑     j   =   K                     j       =   1       ,     j   ≠     k                   R   ij          A   j            d   ^     j     (   0   )                                  
     end 
     
       
         {circumflex over (d)} (1) =sign( z   (1) ) 
       
     
     for l=1 to L 
     
       
         {circumflex over (x)} (l) ={circumflex over (d)} (l) −{circumflex over (d)} (l−1)   
       
     
     for k=1 to K            z   k     (     l   +   1     )       =         z   k     (   l   )       -       ∑             j   =   K                     j       =       1                 j     ≠     k                   R   ij          A   j            x   ^     j     (   l   )                                                 
     end 
     
       
         {circumflex over (d)} (l=1) =sign( z   (l+1) ) 
       
     
     end 
     After the first stage  510 , the differencing multistage detector  500  starts to use the differencing vector (hard decision feedforward)  550 / 580  in the input. Furthermore, the decision of the current stage, e.g.,  550 , is based not only on the current PIC output  562 / 572 , but also on the output  540  of previous PIC stage (i.e., the soft decision from the previous stage. 
     FIG. 6 illustrates a single adder implementation  600  of the differencing multistage detector according to the present invention. If the differencing vector generation and the final shift are not used, the implementation can also be used as the conventional multistage detector. The present invention represented in FIG. 6 is based on an 8-user Gold spreading code system. However, those skilled in the art will readily recognize that the present invention is not meant to be limited to the particular implementation shown in FIG. 6, but that other types of systems and number of users could be accounted for without departing from the scope of the present invention. 
     The soft decision inputs are parallel in bits for each user and time duplexing for all users. Thus, a parallel to serial converter (not shown) is placed after the matched filter. The timing of these inputs and outputs is controlled by a hand shaking mechanism  602 ,  604 . As soon as the current stage is ready, the previous stage starts transmitting hard output  610  until all the bits are sent, which is indicated by the hand-shaking&#39;s signal  604 . The input numbers  612  are in two&#39;s complement format and they are stored in the data register bank. 
     At the same time, the hard decisions  614  are obtained from the first bit of the numbers and a differencing vector  616  is generated by combinational logic. A priority encoder  618  will find the non-zero elements in the order of the first come, the highest priority. The timing for the accumulation  630  is scheduled according to the positions of the non-zero elements as directed  632  by the priority encoder  618 . If an element is not zero, the accumulator  630  will subtract its corresponding cross-correlation number (which is shift left once if the current stage is the second or later) from all the other user&#39;s registers. Loading, shifting, accumulating and writing back are organized as a simple pipeline machine, controlled by a pipelining controller  640 . The pipelining controller  640  will not stall because no data and control dependencies exist. Finally the soft  660  and hard  662  decision are generated one by one with certain hand-shaking protocols  604  to the next stage. By passing the differencing vector generation  616  and shift block  650 , this device works as the conventional multistage detector. 
     A major part of the single adder implementation of the differencing multistage detector  600  is devoted to the registers and arithmetic logic unit (ALU). The estimated number of transistors for an eight-user 12-bit fixed point system is 5K. More transistors are thus needed if more than one ALU is implemented in the system. 
     Based upon hardware simulation, three-stage system delay with the differencing algorithm is less than 100 cycles. Working at the clock rate of 20 MHz, the system delay is about 5 μs, which is much less than that of the conventional multistage detector, i.e., around 12 μs. Further, system throughput is determined by the first stage because it is the slowest block in the system. The multistage detector according to the present invention can reach a throughput up to 200 kb/s with proper buffering. This rate meets 144 kb/s requirement of the Wideband CDMA communications. 
     The differencing multistage detector has a bit error rate that is exactly the same as the conventional multistage detector. This is because the framework of the iterative method, nor the convergence speed, is changed. The BER  710 ,  750  versus signal-to-noise ratio (SNR) and MAI  714 ,  754  in a five-user  720  and ten-user  770  system is shown in FIG.  7 . FIG. 7 shows that the performance of the matched filter degrades  730 ,  780  dramatically when MAI  714  increases or the number of users increases ( 720  to  770 ). On the other hand, the differencing multistage detector performs constantly along with different MAIs. So it can be regarded as a near-far resistant multistage detector. Moreover, its performance approaches a single user communication system BER bound, which is given by P e =Q({square root over (2E h /N 0 )} 
     FIG. 8 a  illustrates an observation  810  of the percentage of zeros  812  in the differencing vector according to the present invention. In FIG. 8 a , the percentage of zeros increases as the iterations progresses from  820  to  830 . After the fourth stage, the number of zeros gets to the point of 98% in a 15-users communication system. It explicitly indicates that if the conventional multistage detector is used, almost 98% computation resource is wasted. 
     FIG. 8 b  gives a clear view  860  of how many computations we are going to save in a real system. The dotted line  870  represents the accumulated number of operations needed after each stage in the conventional multistage detector. As explained earlier, the number of computations remains constant for each stage, which makes the total floating operations per second (flops) go up linearly. On the contrary, line  880  illustrates that the number of calculations in the differencing multistage detector decreases as the iteration proceeds. Thus, the overall savings can be up to 75% in a five-stage system. And the more stages in the system, the greater the speed of the conventional multistage detector as compared to a conventional multistage detector. 
     In summary, as compared to the conventional single user detector, the multistage multiuser detector shows a great improvement in the performance in CDMA communications. However, the number of computations in the multistage detector may be greatly reduced by exploiting the convergence of the iterated algorithm. The new differencing multistage detector according to the present invention calculates the difference between two consecutive stages and saves the calculation when the difference becomes zero. This technique shows a great deal of savings in contrast to the basic multistage detector. The hardware implementation of this strategy shows that the bit error rate is the same as the multistage detector, but delay cycles for a five-stage detector will be saved by up to 75%, which meets the requirement of Wideband CDMA communications. 
     The foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the claims appended hereto.