Patent Publication Number: US-7904034-B2

Title: Method and system for providing an interference cancellation in a wireless communication system

Description:
CLAIM OF PRIORITY 
     This application makes reference to, claims all benefits accruing under 35 U.S.C. §119 from, and incorporates herein a provisional application entitled A SIMPLE TECHNIQUE FOR INTERFERENCE CANCELLATION INA WIRELESS COMMUNICATION SYSTEM filed in the U.S. Patent &amp; Trademark Office on 18 Dec. 2006, and there duly assigned Ser. No. 60/875,490. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an interference cancellation/removal system, apparatus and method which can reduce inter-user interference when simultaneous transmissions to the same base station are repeated from two or more wireless users using the same time frequency resource. 
     2. Description of the Related Art 
     In mobile communications, interfering signals that hamper reception quality can result from a number of sources. 
     In a wireless communication system when two users transmit simultaneously in the same cell to a base station, their transmissions can interfere with each other. When both users use the same time-frequency resource, their transmissions interfere with each other. 
     Moreover, if both users repeat the transmission of the same symbols in subsequent slots, the desired signals may be combined coherently by the base station, but both interference and noise will be combined non-coherently, thus causing interference. 
     Furthermore, if the channels for the two users do not change across repeated transmissions, both the desired signals and the interference will be combined coherently by the base station, but the noise will still be combined non-coherently. Therefore, the interference from the other signal will result in close to an one hundred percent error rate. 
     Accordingly, we have realized that there is a need for techniques to mitigate interference during the operation of wireless communication system. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved transceiver and an improved method for data transmission in a wireless communication system. 
     It is another object to provide a transceiver and a method where repeated signals from one of two users transmitting simultaneously using the same time-frequency resource are phase shifted with the result that each user removes interference from the other user. 
     It is still another object to provide a transceiver and a method for data transmission that is simple, reliable, and inexpensive. 
     According to one aspect of the present invention, a method for controlling data transmission from a plurality of transmitters to a receiver is provided. The method contemplates determining a first through a fourth phase shift to achieve a selected percentage of interference cancellation between first and second transmitters during transmissions by the plurality of transmitters, assigning the first phase shift and the second phase shift respectively to the first transmitter and the second transmitter for corresponding simultaneous transmissions of a first signal and a second signal in a first time-frequency slot in accordance with a selected protocol, and assigning the third phase shift and the fourth phase shift respectively to the first transmitter and the second transmitter for corresponding simultaneous re-transmissions of the first and second signal in a second time-frequency slot in accordance with the selected protocol. 
     The first phase shift may be substantially equal to the second phase shift. The third phase shift may be different from the fourth phase shift by a selected amount to achieve the selected percentage of interference cancellation between first and second transmitters. 
     Moreover, the selected amount may be π radians in order to achieve a 100% interference cancellation. 
     The first phase shift, the second phase shift and the third phase shift may be substantially equal to zero, and the fourth phase shift may be substantially equal to π radians. 
     According to another aspect of the present invention, a method for controlling data transmission from a transmitter to a receiver via a plurality of antennas contemplates converting user data into two data streams, determining a first through a fourth phase shift to achieve a selected percentage of interference cancellation between the two data streams, assigning the first phase shift to a first data stream for transmission via a first antenna in a first time-frequency slot in accordance with a selected protocol, assigning the second phase shift to a second data stream for transmission via a second antenna in the first time-frequency slot in accordance with the selected protocol, simultaneously with said transmission of the first data stream, assigning the third phase shift to the first data stream for re-transmission via the first antenna in a second time-frequency slot in accordance with the selected protocol, and assigning the fourth phase shift to the second data stream for re-transmission via the second antenna in the second time-frequency slot simultaneously with said re-transmission of the first signal in accordance with the selected protocol. 
     The first phase shift may be substantially equal to the second phase shift. The third phase shift may be different from the fourth phase shift by a selected amount to achieve the selected percentage of interference cancellation between first and second transmitters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
         FIG. 1  illustrates an interference scenario in a wireless communication system; 
         FIG. 2  is a view illustrating two users transmitting and re-transmitting simultaneously the same symbols while using the same time-frequency resource in a wireless communication system; 
         FIG. 3  is a view illustrating two users transmitting and re-transmitting simultaneously the same symbols using the same time-frequency resource while transmission channels for the two users do not change across repeated transmissions in a wireless communication system; 
         FIG. 4  illustrates a mobile system in communication with a base station in a wireless communication system; 
         FIG. 5  is a view illustrating two users transmitting repeated signals simultaneously using the same time-frequency resource according to a first embodiment of the principles of the present invention; 
         FIG. 6  is a view illustrating two users transmitting repeated signals simultaneously using the same time-frequency resource wherein said first time transmission of the symbols from the two users are π radians out of phase and the repeated symbols are in phase according to a second embodiment of the principles of the present invention; 
         FIG. 7  is a view illustrating two users transmit repeating signals for multiple times wherein the first transmission and odd repetitions thereof are π radians out of phase and the odd repeated symbols are in phase according to a third embodiment of the principles of the present invention; 
         FIG. 8  is a view illustrating a change of channel at the time of symbol repetition according to a fourth embodiment of the principles of the present invention; 
         FIG. 9  is a view illustrating two users transmitting simultaneously using the same time-frequency resource with the second user repeating symbols phase shifted by π radians according to a fifth embodiment of the principles of the present invention; 
         FIG. 10  is a view illustrating a sixth embodiment according to the principles of the present invention for cancellation of interference for multiple data streams transmitted to the same user; 
         FIG. 11  is a view illustrating a seventh embodiment according to the principles of the present invention for the cancellation of interference for four data streams transmitted to the same user; 
         FIG. 12  is a view illustrating an eighth embodiment according to the principles of the present invention for the cancellation of interference when a first data stream is the dominant source of interference to each of a second through fourth data stream; 
         FIG. 13  is a view illustrating a ninth embodiment according to the cancellation principle of the present invention to a 4-stream MIMO system; 
         FIG. 14  is a view illustrating a tenth embodiment according to the cancellation principle of the present invention when the base station sends a hybrid ARQ NACK signal after failure to decode signals from both a first and second user; and 
         FIG. 15A  illustrates a transmitter constructed according to the principles of the present invention; and 
         FIG. 15B  illustrates a receiver constructed according to the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
     In a wireless communication system as illustrated in  FIG. 1 , when two users UE- 1   101  and UE- 2   102  transmit simultaneously in the same cell to a base station  103 , their transmissions can interfere with each other. As illustrated in  FIG. 2 , assuming the two users  101  and  102  transmit symbols S 1    201  and S 2    202 , respectively, and that both users use the same time-frequency resource, their transmissions interfere with each other in slot# 1   203 . 
     Moreover, if both users repeat the transmission of the same symbols in slot# 5   204 , as illustrated in  FIG. 2  (where h 1  and h 3  represent channel gains between UE- 1   101  and base station  103  in slot# 1  and slot# 5 , respectively, while h 2  and h 4  represent channel gains between UE- 2   102  and base station  103  in slot# 1  and slot# 5 , respectively), the desired signals may be combined coherently by base station  103 , but both the interference and noise will be combined non-coherently. 
     Furthermore, if the channels for the two users do not change across repeated transmissions, i.e., h 1 =h 3  and h 2 =h 4 , as illustrated in  FIG. 3 , both the desired signals and interference will be combined coherently by base station  103 , but the noise will still be combined non-coherently. Therefore, the interference from the other signal would result in close to a 100% error rate for the transmissions in slot# 5 . 
     The techniques described herein for cancelling interference may be used in various wireless communication systems, such as Code Division Multiple Access (CDMA) systems including IS-95, CDMA 2000, Wideband Code Division Multiple Access (W-CDMA), some other CDMA standards, or a combination thereof, Personal Communication Services (PCS) systems. These standards are known in the art and therefore the description of these standards need not be repeated here. 
       FIG. 4  depicts a representative hardware of a mobile system  100  in communication with a base station  103  of a wireless (e.g., MIMO-OFDM) communication system. Note that mobile system  100  within the context of this disclosure contemplates a mobile phone and other mobile devices, such as a personal digital assistant (PDA), a personal communication assistant (PCA), and an electronic organizer, or any duplex interactive devices. As shown in  FIG. 4 , a mobile system  100  includes a data processor  10 , a modulator  20 , a controller  24 , a memory  26 , a transceiver  28 , a decoder  30 , and a demodulator  32 . 
     When mobile system  100  is in transmission mode, processor  10  codes and interleaves the data based on one or more coding schemes to provide coded data in compliance with the instructions of controller  24 . Here, the data rate and coding scheme may be determined by controller  24 . The coded data is then provided to modulator  20  to modulate using one or more modulation schemes in compliance with the instructions of controller  24 . The modulated signals from transceiver  28  is transmitted via antenna to base station  103 . Note that these modulation schemes may include various methods that are well known to those skilled in this art, e.g., multi-tone phase-shift keying (M-PSK), multi-tone quadrature amplitude modulation (M-QAM), and so on, and data may be multiplexed using time-division multiplexing (TDM), frequency-division multiplexing (FDM) or code division multiplexing (CDM) in all or a subset of the frequency subchannels and in all or a subset of the spatial subchannels used to transmit the data. 
     When mobile system  100  is in reception mode, the transmitted modulated signals are received by transceiver  28  via antenna. Demodulator  32  demodulates data from transceiver  28 , and then decoder  30  decodes the demodulated data in compliance with the directions of controller  24 . Controller  24  direct the operation of the transmission and reception modes, and memory  26  provides storage for program codes and data used by controller  24 . Here, software codes may be stored in memory  26  and executed by controller  24 . Memory  26  may be implemented within controller  24 , in which case it can be communicatively coupled to controller  24  via various means as is known in the art. 
     It should be further noted that coding and decoding techniques may be implemented by various means, e.g. hardware, software, or a combination thereof, such as specific integrated circuits (ASICs), digital signal processors (DPSs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FP-GAs), micro-controllers, microprocessors, and other electronic units designed to perform the function described herein, or a combination thereof. 
     Referring to  FIG. 5 , according to a first embodiment of the present invention, repeated signals from one of two mobile users transmitting simultaneously using the same time-frequency resource are phase shifted by π radians relative to the other user. In  FIG. 5 , UE- 1   101  and UE- 2   102  transmit symbols S 1  and S 2 , respectively, with phase shifts of Φ 1  and Φ 2  radians in slot# 1 . In slot# 5 , UE- 1   101  retransmits S 1  with Φ 3  phase shift while UE- 2   102  retransmits S 2  with Φ 4  phase shift. Here, it is assumed that the channel for the two users does not change across repeated transmissions, i.e., h 1 =h 3  and h 2 =h 4 . 
     After the transmission, base station  103  performs equalization on the received signals and combines the two received signals r 1  and r 2  at slot# 1  and slot# 5 , respectively, to recover the signals for each of UEs  101  and  102 . The signal for UE- 1   101  is recovered by Equation (1) as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     In accordance with the first embodiment of the present invention, the first transmission happens with the same phase shift for the two users while the repeated symbols from the two users are transmitted out of phase, i.e.,
 
Φ 1 =Φ 2  and Φ 4 −Φ 3 =π radians
 
and therefore the estimated symbol for UE- 1   101  is given by Equation (2) as follows:
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     The transmitted signal for UE- 1  is estimated by adding the two received signals across two transmission attempts, as shown above in (1). 
     Similarly, the transmitted symbol for UE- 2   102  is estimated by Equation (3) follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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                             ⁢ 
                             
                               
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                                 2 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
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     The transmitted signal for UE- 2  is estimated by adding the signal received at the second transmission (r 2 ) attempt from the signal received at the first transmission (r 1 ) attempt as shown above in (3). 
     Accordingly, in the first embodiment of the method of the present invention it can be seen that each user completely removes interference from the other user. Hence, inter-user interference is completely cancelled when the transmission from the two users are repeated. The only source of degradation is the background additive white Gaussian noise (AWGN) noise. 
     Referring to  FIG. 6 , according to a second embodiment of the current invention, the first time transmissions of the symbols from the two users are π radians out of phase while the repeated symbols are in phase, i.e.,
 
Φ 3 =Φ 4  and Φ 2 −Φ 1 =π radians
 
     Similar to the analysis of the first embodiment where the phase shift was made in the repeated symbols, a complete cancellation/removal of interference may be achieved. 
     Referring to  FIG. 7 , according to a third embodiment of the present invention, the symbols from both users are repeated multiple times. In  FIG. 7 , the first transmission in slot# 1  and repetitions in slot# 3  and slot# 5  for the two users are π radians out of phase while the repeated symbols in slot# 2  and slot# 4  for the two users are in phase. As a result, the cross-user interference is completely eliminated after an even number of repetitions according to the method of the present invention illustrated in  FIG. 7 . 
     Referring to  FIG. 8 , according to a fourth embodiment of the present invention, the channel changes at the time of symbol repetition. For example, suppose UE- 2  scales s 2  before repetition at slot# 5  by Equation (4) as follows: 
                     S   2   ′     =           h   1   *     ·     h   2           h   3   *     ·     h   4         ·     S   2               (   4   )               
In this case, the signal for UE- 1  is recovered by Equation (5) as follows:
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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                             ^ 
                           
                           1 
                         
                         = 
                           
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                               1 
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                                     jϕ 
                                     1 
                                   
                                 
                               
                               · 
                               
                                 r 
                                 1 
                               
                             
                           
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                               h 
                               3 
                               * 
                             
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                                   - 
                                   jϕ 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 3 
                               
                             
                             ⁢ 
                             
                               r 
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               
                                 ⅇ 
                                 
                                   - 
                                   
                                     jϕ 
                                     1 
                                   
                                 
                               
                               · 
                               
                                 ( 
                                 
                                   
                                     
                                       h 
                                       1 
                                     
                                     ⁢ 
                                     
                                       s 
                                       1 
                                     
                                     ⁢ 
                                     
                                       ⅇ 
                                       
                                         + 
                                         
                                           jϕ 
                                           1 
                                         
                                       
                                     
                                   
                                   + 
                                   
                                     
                                       h 
                                       2 
                                     
                                     ⁢ 
                                     
                                       s 
                                       2 
                                     
                                     ⁢ 
                                     
                                       ⅇ 
                                       
                                         + 
                                         
                                           jϕ 
                                           2 
                                         
                                       
                                     
                                   
                                   + 
                                   
                                     n 
                                     1 
                                   
                                 
                                 ) 
                               
                             
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             h 
                             3 
                             * 
                           
                           ⁢ 
                           
                             
                               ⅇ 
                               
                                 
                                   - 
                                   jϕ 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 3 
                               
                             
                             · 
                             
                               ( 
                               
                                 
                                   
                                     h 
                                     3 
                                   
                                   ⁢ 
                                   
                                     s 
                                     1 
                                   
                                   ⁢ 
                                   
                                     ⅇ 
                                     
                                       + 
                                       
                                         jϕ 
                                         3 
                                       
                                     
                                   
                                 
                                 + 
                                 
                                   
                                     h 
                                     4 
                                   
                                   ⁢ 
                                   
                                     s 
                                     2 
                                     ′ 
                                   
                                   ⁢ 
                                   
                                     ⅇ 
                                     
                                       + 
                                       
                                         jϕ 
                                         4 
                                       
                                     
                                   
                                 
                                 + 
                                 
                                   n 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               2 
                             
                             ⁢ 
                             
                               s 
                               2 
                             
                             ⁢ 
                             
                               ⅇ 
                               
                                 j 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       ϕ 
                                       2 
                                     
                                     - 
                                     
                                       ϕ 
                                       1 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               ⅇ 
                               
                                 - 
                                 
                                   jϕ 
                                   1 
                                 
                               
                             
                             ⁢ 
                             
                               n 
                               1 
                             
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             
                               
                                  
                                 
                                   h 
                                   3 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           + 
                           
                             
                               h 
                               3 
                               * 
                             
                             ⁢ 
                             
                               h 
                               4 
                             
                             ⁢ 
                             
                               s 
                               2 
                               ′ 
                             
                             ⁢ 
                             
                               ⅇ 
                               
                                 j 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       ϕ 
                                       4 
                                     
                                     - 
                                     
                                       ϕ 
                                       3 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           + 
                           
                             
                               
                                 ⅇ 
                                 
                                   
                                     - 
                                     jϕ 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   3 
                                 
                               
                               · 
                               
                                 h 
                                 3 
                                 * 
                               
                             
                             ⁢ 
                             
                               n 
                               2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Here, the first transmission happens with the same phase shift for the two users while the repeated symbols from the two users are transmitted out of phase, that is
 
Φ 1 =Φ 2  and Φ 4 −Φ 3 =π radians
 
Therefore, the estimated symbol for UE- 1  is given as by Equation (6) as follows:
 
                             s   ^     1     =       ⁢                h   1          2     ⁢     s   1       +       h   1   *     ⁢     h   2     ⁢     s   2     ⁢     ⅇ     j   ⁡     (       ϕ   2     -     ϕ   1       )           +       h   1   *     ⁢       ⅇ     -     jϕ   1         ·     n   1         +                     ⁢                h   3          2     ⁢     s   1       +       h   3   *     ⁢     h   4     ⁢     s   2   ′     ⁢     ⅇ     j   ⁡     (       ϕ   4     -     ϕ   3       )           +       h   3   *     ⁢       ⅇ     -     jϕ   3         ·     n   2                       =       ⁢                h   1          2     ⁢     s   1       +       h   1   *     ⁢     h   2     ⁢     s   2       +         ⅇ     -     jϕ   1         ·     h   1   *       ⁢     n   1       +              h   1          2     ⁢     s   1       -                     ⁢         h   3   *     ⁢     h   4     ⁢     s   2   ′       +       h   3   *     ⁢       ⅇ     -     jϕ   3         ·     n   2                         (   6   )               
It can be seen that substituting (4) into (6) will completely eliminate interference from UE- 2  as shown below in Equation (7):
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             s 
                             ^ 
                           
                           1 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               2 
                             
                             ⁢ 
                             
                               s 
                               2 
                             
                             ⁢ 
                             
                               ⅇ 
                               
                                 j 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       ϕ 
                                       2 
                                     
                                     - 
                                     
                                       ϕ 
                                       1 
                                     
                                   
                                   ) 
                                 
                               
                             
                             ⁢ 
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               
                                 ⅇ 
                                 
                                   - 
                                   
                                     jϕ 
                                     1 
                                   
                                 
                               
                               · 
                               
                                 n 
                                 1 
                               
                             
                           
                           + 
                           
                             
                               
                                  
                                 
                                   h 
                                   3 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             
                               h 
                               3 
                               * 
                             
                             ⁢ 
                             
                               h 
                               4 
                             
                             ⁢ 
                             
                               s 
                               2 
                               ′ 
                             
                             ⁢ 
                             
                               ⅇ 
                               
                                 j 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       ϕ 
                                       4 
                                     
                                     - 
                                     
                                       ϕ 
                                       3 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           + 
                           
                             
                               h 
                               3 
                               * 
                             
                             ⁢ 
                             
                               
                                 ⅇ 
                                 
                                   - 
                                   
                                     jϕ 
                                     3 
                                   
                                 
                               
                               · 
                               
                                 n 
                                 2 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               2 
                             
                             ⁢ 
                             
                               s 
                               2 
                             
                           
                           + 
                           
                             
                               
                                 ⅇ 
                                 
                                   - 
                                   
                                     jϕ 
                                     1 
                                   
                                 
                               
                               · 
                               
                                 h 
                                 1 
                                 * 
                               
                             
                             ⁢ 
                             
                               n 
                               1 
                             
                           
                           + 
                           
                             
                               
                                  
                                 
                                   h 
                                   3 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           - 
                           
                             
                               h 
                               3 
                               * 
                             
                             ⁢ 
                             
                               
                                 h 
                                 4 
                               
                               · 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             
                               
                                 
                                   h 
                                   1 
                                   * 
                                 
                                 · 
                                 
                                   h 
                                   2 
                                 
                               
                               
                                 
                                   h 
                                   3 
                                   * 
                                 
                                 · 
                                 
                                   h 
                                   4 
                                 
                               
                             
                             ⁢ 
                             
                               s 
                               2 
                             
                           
                           + 
                           
                             
                               h 
                               3 
                               * 
                             
                             ⁢ 
                             
                               
                                 ⅇ 
                                 
                                   - 
                                   
                                     jϕ 
                                     3 
                                   
                                 
                               
                               · 
                               
                                 n 
                                 2 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           + 
                           
                             
                               
                                  
                                 
                                   h 
                                   3 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               
                                 ⅇ 
                                 
                                   - 
                                   
                                     jϕ 
                                     1 
                                   
                                 
                               
                               · 
                               
                                 n 
                                 1 
                               
                             
                           
                           + 
                           
                             
                               h 
                               3 
                               * 
                             
                             ⁢ 
                             
                               
                                 ⅇ 
                                 
                                   - 
                                   
                                     jϕ 
                                     3 
                                   
                                 
                               
                               · 
                               
                                 n 
                                 2 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Referring to  FIG. 9 , according to a fifth embodiment of the present invention, two users are transmitting repeating symbols S 1  and S 2  respectively and simultaneously using the same time-frequency resource, with UE- 2 &#39;s repeated symbols being phase shifted by π radians. This allows complete elimination inter-user interference. In  FIG. 9 , UE- 1   101  and UE- 2   102  transmit symbols S 1  and S 2 , respectively, in slot# 1 , while in slot# 5  UE- 1   101  retransmits S 1  as it is while UE- 2   102  transmits −S 2 . The symbol −S 2  is obtained by applying a phase shift of π radians to S 2 . Further, it is assumed that the channel gain for the two users does not change across repeated transmissions, that is
 
h 1 =h 3  and h 2 =h 4  
 
     Base station  103  performs equalization on the received signals and combines the two received signals (r 1  and r 2 ) to recover the data signals received from each of the UEs  101 ,  102 . Since UE- 1   101  just repeats its symbol as it is, the transmitted signal for UE- 1   101  is estimated by just adding the two received signals across two transmission attempts as shown below in Equation (8): 
                             s   ^     1     =       h   1   *     ⁡     (       r   1     +     r   2       )                   =         h   1   *     ⁡     (         h   1     ⁢     s   1       +       h   2     ⁢     s   2       +     n   1       )       +       h   1   *     ⁡     (         h   1     ⁢     s   1       -       h   2     ⁢     s   2       +     n   1       )                     =                h   1          2     ⁢     s   1       +       h   1   *     ⁢     h   2     ⁢     s   2       +       h   1   *     ⁢     n   1       +              h   1          *     ⁢     s   1       -       h   1   *     ⁢     h   2     ⁢     s   2       +       h   1   *     ⁢     n   2                     =       2   ·     (              h   1          2     ⁢     s   1       )       +       h   1   *     ⁢     n   1       +       h   1   *     ⁢     n   2                       (   8   )               
On the other hand, since UE- 2   102  repeats the transmission of its signal with a phase shift of π radians, its received signal is estimated by subtracting the signal at the second transmission (r 2 ) attempt from the signal received at the first transmission (r 1 ) as shown below in Equation (9):
 
                             s   ^     2     =       h   2   *     ⁡     (       r   1     -     r   2       )                   =         h   2   *     ⁡     (         h   1     ⁢     s   1       +       h   2     ⁢     s   2       +     n   1       )       -       h   2   *     ⁡     (         h   1     ⁢     s   1       -       h   2     ⁢     s   2       +     n   2       )                     =                h   2          2     ⁢     s   2       +       h   2   *     ⁢     h   1     ⁢     s   1       +       h   2   *     ⁢     n   1       +              h   2          2     ⁢     s   2       -       h   2   *     ⁢     h   1     ⁢     s   2       -       h   2   *     ⁢     n   2                     =       2   ·     (              h   2          2     ⁢     s   2       )       +       h   2   *     ⁢     n   1       -       h   2   *     ⁢     n   2                       (   9   )               
It can be seen that both users completely eliminate the interference from each other. The only source of degradation is the background AWGN noise.
 
     Referring to the MIMO system of  FIG. 10 , according to a sixth embodiment, the interference cancellation principle of the present invention is applied to cancel interference for multiple data streams transmitted to the same user via different respective data streams. Stream- 1  repeats its signal S 1  at slot  5  while Stream- 2  repeats its signal out of phase as e jπ S 2  at slot  5  (e.g., phase shifted by π radians). It should be noted that when Stream- 2  retransmits (i.e., repeats) its signal phase shifted by π radians, Stream- 1  and Stream- 2  completely remove interference from each other. Inter-stream interference is completely cancelled when the transmission from the two streams are repeated. The only significant source of degradation is the background AWGN noise. The percentage of interference cancellation depends on the amount of phase shift applied. Since Stream- 1  just repeats its symbol S 1  as it is, the transmitted signal for Stream- 1  is estimated by just adding the two received signals r 1  and r 2  across two transmission attempts. And, since Stream- 2  just repeats its signal S 2  with, e.g., π radians of phase shift, its received signal is estimated by subtracting the signal r 2  received at the second transmission attempt from the signal r 1  received at the first transmission attempt. 
     Referring to  FIG. 11 , according to a seventh embodiment, the interference cancellation principle of the present invention is applied to cancel interference for four data streams transmitted to the same user. In this example of the seventh embodiment, symbols from stream- 3  and stream- 4 , S 3  and S 4  respectively, are phase shifted by π radians when transmission of the symbols is repeated. The signal for data stream- 1  is estimated as shown below in Equation (10): 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             s 
                             ^ 
                           
                           1 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             h 
                             1 
                             * 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 r 
                                 1 
                               
                               + 
                               
                                 r 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     h 
                                     1 
                                   
                                   ⁢ 
                                   
                                     s 
                                     1 
                                   
                                 
                                 + 
                                 
                                   
                                     h 
                                     2 
                                   
                                   ⁢ 
                                   
                                     s 
                                     2 
                                   
                                 
                                 + 
                                 
                                   
                                     h 
                                     3 
                                   
                                   ⁢ 
                                   
                                     s 
                                     3 
                                   
                                 
                                 + 
                                 
                                   
                                     h 
                                     4 
                                   
                                   ⁢ 
                                   
                                     s 
                                     4 
                                   
                                 
                                 + 
                                 
                                   n 
                                   1 
                                 
                               
                               ) 
                             
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             h 
                             1 
                             * 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   h 
                                   1 
                                 
                                 ⁢ 
                                 
                                   s 
                                   1 
                                 
                               
                               + 
                               
                                 
                                   h 
                                   2 
                                 
                                 ⁢ 
                                 
                                   s 
                                   2 
                                 
                               
                               - 
                               
                                 
                                   h 
                                   3 
                                 
                                 ⁢ 
                                 
                                   s 
                                   3 
                                 
                               
                               - 
                               
                                 
                                   h 
                                   4 
                                 
                                 ⁢ 
                                 
                                   s 
                                   4 
                                 
                               
                               + 
                               
                                 n 
                                 1 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               2 
                             
                             ⁢ 
                             
                               s 
                               2 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               3 
                             
                             ⁢ 
                             
                               s 
                               3 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               4 
                             
                             ⁢ 
                             
                               s 
                               4 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               n 
                               1 
                             
                           
                           + 
                           
                             
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               s 
                               1 
                             
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               2 
                             
                             ⁢ 
                             
                               s 
                               2 
                             
                           
                           - 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               3 
                             
                             ⁢ 
                             
                               s 
                               3 
                             
                           
                           - 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               4 
                             
                             ⁢ 
                             
                               s 
                               4 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               n 
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             2 
                             ⁢ 
                             
                               ( 
                               
                                 
                                   
                                      
                                     
                                       h 
                                       1 
                                     
                                      
                                   
                                   2 
                                 
                                 ⁢ 
                                 
                                   s 
                                   1 
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             2 
                             ⁢ 
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               h 
                               2 
                             
                             ⁢ 
                             
                               s 
                               2 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               n 
                               1 
                             
                           
                           + 
                           
                             
                               h 
                               1 
                               * 
                             
                             ⁢ 
                             
                               n 
                               2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     The signal for data stream- 2  is estimated as shown below in Equation (11):
 
 ŝ   2   *=h   2 *( r   1   +r   2 )=2( h   2 | 2   |s   2 )+2 h   2   *h   1   s   1   +h   2   *n   1   +h   2   *n   2   (11)
 
Here, stream- 1  and stream- 2  receive interference from each other while completely removing interference from stream- 3  and stream- 4 . Since the signal for stream- 3  and stream- 4  is repeated with a phase shift of π radians, the received signals for stream- 3  and stream- 4  are estimated as shown below in Equation (12):
 
 ŝ   3   =h   3 *( r   1   r   2 )=2(| h   3   *s   3 )+2 h   3   *h   4   s   4   +h   3   *n   1   −h   3   *n   2   (12)
 
 ŝ   4   =h   4 *( r   1   −r   2 )=2(| h   4   |s   4 )+2 h   4   *h   3   s   3   +h   4   *n   1   h   4 *n 2  
 
     Referring to  FIG. 12 , according to an eighth embodiment of the present invention, the transmission of signals for stream- 2 , stream- 3  and stream- 4  are repeated with a phase shift of π radians. In this embodiment, stream- 1  completely removes interference from stream- 2 , stream- 3  and stream- 4 . The stream- 2 , stream- 3 , and stream- 4  receive interference from each other, while completely removing interference from stream- 1  as can be seen from Equation (13) below:
 
 ŝ   1   =h   1 *( r   1   +r   2 )=2·(| h   1 | 2   s   1 )+h 1   *n   1   h   1   *n   2  
 
 ŝ   2   =h   2 *( r   1   −r   2 )=2·(| h   2 | 2   s   2 )+2 h   2   *h   3   s   3 +2 h   2   *h   4   s   4   +h   2   *n   1   −h   2   *n   2   (13)
 
 ŝ   3   =h   3 *( r   1   −r   2 )=2·(| h   3 | 2   s   2 )+2 h   3   *h   2   s   2 +2 h   2   *h   4   s   4   +h   3   *n   1   −h   3   *n   2  
 
 ŝ   4   =h   4 *( r   1   −r   2 )=2·(| h   4 | 2   s   4 )+2 h   4   *h   2   s   2 +2 h   4   *h   3   s   3   +h   4   *n   1   −h   4   *n   2  
 
This eighth embodiment is a good approach if the transmission of stream- 1  is the dominant interferer to streams- 2  through  4 .
 
     Referring to  FIG. 13 , according to a ninth embodiment of the present invention, the data symbols are first de-multiplexed  1202  into two data blocks  1203  and  1204 , and the two data blocks are coded using codewords CW 1  and CW 2 , respectively. Subsequently, each coded data blocks is further de-multiplexed by demultiplexers  1205 ,  1206  into two streams for a total of 4-stream  1207  MIMO transmission via different respective antennas ANT 1 , ANT 2 , ANT 3  and ANT 4 . In this embodiment, the signals for stream- 2  and stream- 4  are repeated and phase shifted using phase shift module  1208  with a phase shift of π radians via their different corresponding antennas ANT 2 , ANT  4 , as e jπ ·S 2  and e jπ ·S 4 , respectively, while streams  1  and  3  repeat transmission of their corresponding symbols S 1 , S 3  via different corresponding antennas ANT 1  and ANT 3 . Therefore, stream- 1  experiences interference only from stream- 3 . However, successive interference cancellation can be used between data blocks  1203  and  1204 . This eliminates the interference from stream- 3  to stream- 4  and results in a completely interference-free stream- 1 . Similar processing on other streams by using the interference cancellation method of the present invention and use of successive interference cancellation (SIC) module  1209  results in completely interference-free stream- 2  to stream- 4 . 
     The foregoing method also can be used in a MU-MIMO system where each user receives one codeword so that it is possible to get some interference cancellation gains where conventional post decoding SIC has heretofore failed. 
     Referring to  FIG. 14 , according to a tenth embodiment of the principles of the present invention, the symbols are repeated after receiving a hybrid Automatic Repeat-reQuest (ARQ) negative-acknowledgement (NACK) signal. In this embodiment of the principles of the present invention, base station  103  sends a NACK signal after receiving signals S 1  and S 2  from UE- 1   101  and UE- 2   102 , respectively, when successful decoding of both signals fails. After receiving the NACK signal, UE- 1   101  repeats its signal S 1  as it is while UE- 2   102  applies a phase shift of n radians on its signal S 2  before retransmission of that signal as e jπ ·S 2 . 
     In the present invention, repeated signals from one of the two data streams (data stream  1  or data stream  2 ) being transmitted simultaneously using the same time-frequency resource are phase shifted by a selected number of radians relative to the other stream. 
     In  FIG. 15A , transmitter  1400  is constructed so that symbols S 1  and S 2  are transmitted respectively with phase shifts of Φ 1  and Φ 2  radians in slot# 1 . Subsequently, in slot# 5 , S 1  is retransmitted with Φ 3  phase shift while s 2  is retransmitted with phase shift Φ 4 . It is assumed that the channel for the two data streams does not change across repeated transmissions. As illustrated in  FIG. 15A , component  1405  processes the two data streams in order that the two data streams are transmitted simultaneously. Component  1405  includes modules  1405 . 1 ,  1405 . 2  and  1405 . 3  according to this particular implementation of the principles of the present invention. Time frequency resource assignment module  1405 . 1  assigns the same time-frequency resource to the two data streams to be transmitted. Phase shift determination module  1405 . 2  assigns phase shifts Φ 1  through Φ 4  to achieve a pre-determined percentage of interference cancellation/removal. Combiner/Repeater module  1405 . 3  combines and repeats the transmission of the symbols S 1  and S 2 . 
     Receiver  1450  illustrated in  FIG. 15B , performs equalization on the two streams (r 1  and r 2 ) received via antennas  1456  and  1457  by equalization module  1455 . 1  and signal recovery module  1455 . 2  combines the two received streams (r 1  and r 2 ) in order to recover the signals for each of the streams. The recovered data stream  1451  for stream- 1   1401  is as below: 
     
       
         
           
             
               
                 
                   
                     
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     According to the principles of the current invention, the first transmission occurs with the same phase shift for the two data streams  1401  and  1402  while the repeated symbols from the two streams are transmitted out of phase, that is, Φ 1 =Φ 2  and Φ 4 −Φ 3 =a number of radians required to achieve a selected percentage of inter-stream interference cancellation/removal. 
     Therefore, when Φ 4 Φ 3 =π radians, the estimated symbol for stream- 1  is given as: 
                       s   ^     1     =       ⁢                h   1          2     ⁢     s   1       +       h   1   *     ⁢     h   2     ⁢     s   2     ⁢     ⅇ     j   ⁡     (       ϕ   2     -     ϕ   1       )           +       h   1   *     ⁢       ⅇ     -     jϕ   1         ·     n   1         +                     ⁢                h   1          2     ⁢     s   1       +       h   1   *     ⁢     h   2     ⁢     s   2     ⁢     ⅇ     j   ⁡     (       ϕ   4     -     ϕ   3       )           +       h   1   *     ⁢       ⅇ     -     jϕ   3         ·     n   2                       =       ⁢                h   1          2     ⁢     s   1       +       h   1   *     ⁢     h   2     ⁢     s   2       +         ⅇ     -     jϕ   1         ·     h   1   *       ⁢     n   1       +              h   1          2     ⁢     s   1       -                     ⁢         h   1   *     ⁢     h   2     ⁢     s   2       +       h   1   *     ⁢       ⅇ     -     jϕ   3         ·     n   2                       =       ⁢       2   ·     (              h   1          2     ⁢     s   1       )       +       h   1   *     ⁢       ⅇ     -     jϕ   1         ·     n   1         +       h   1   *     ⁢       ⅇ     -     jϕ   3         ·     n   2                       
Similarly, the transmitted symbol for stream- 2  is estimated as below:
 
                       s   ^     2     =       h   2   *     ⁡     (         ⅇ     -     jϕ   2         ·     r   1       +       ⅇ     -     jϕ   4         ⁢     r   2         )                   =       2   ·     (              h   2          2     ⁢     s   2       )       +       h   2   *     ⁢       ⅇ     -     jϕ   2         ·     n   1         +       h   2   *     ⁢       ⅇ     -     jϕ   4         ·     n   2                       
It can be seen that each user completely removes interference attributable to the transmission of a data stream by the other user when Φ 4 −Φ 3 =π radians.
 
     It should be noted that both the transmitter and receiver can be modified to handle simultaneous transmission in the same time-frequency slots for any number of data streams, as already discussed in the foregoing detailed description sections. 
     It should be further noted that in order to facilitate the explanation of the cancellation principle of the present invention, it was assumed that repeated symbols are either in phase or phase shifted by π radians (out of phase by π radians). The cancellation principle of the present invention, however, can readily be extended to the case where an arbitrary phase shift is applied to the repeated symbols in order to control the amount of interference rejection desired under different conditions, e.g., to achieve a selected percentage of interference cancellation/removal≦100%. 
     While the method the present invention has been shown and described with reference to certain example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Accordingly, the scope of the invention is not to be limited by the above embodiments but by the claims and the equivalents thereof.