TECHNICAL FIELD 
     The present invention relates to a field of communication technology, in particular to a method and a device for eliminating interference in a Time Division Synchronous Code Division Multiple Access (TD-SCDMA) mobile communication system. 
     BACKGROUND 
     A code division multiple access technology is used in a Direct Spreading CDMA (DS-CDMA) system. Due to different propagation delays for different signals and the existence of scramble codes, the spreading code sets adopted by the respective signal are not completely orthogonal to each other. Such an interference caused by a non-zero cross-correlation coefficient is usually called as Multiple Access Interference (MAI). 
     Usually, a Matched Filter (MF, where a traditional Rake receiver just conforms to the MF principle) or a Multi-User Detector (MUD) is used in a CDMA system to recover data before being spread and scrambled. The traditional MF device is impossible to suppress multi-access interference effectively, while the MUD can eliminate the impact of MAI in a better manner. 
     In a TD-SCDMA system, Joint Detector (JD) is adopted by the MUD. As a linear MUD, it needs to complete a system matrix inversion operation. As a result, when a large Spread Factor (SF) or a long scrambler is used by the CDMA system or there exist too many interference users, the dimensions of the system matrix will increase and the computation load of the matrix inversion will become unacceptable. 
     The existing multi-user detection technology cannot eliminate interference signals effectively, so there exists a need to improve such a technology. 
     SUMMARY 
     In order to solve the above-mentioned problem, an object of the present invention is to provide a method and a device for eliminating interference in a mobile communication system, thereby to eliminate the impact of an interference signal and improve the receptivity of a receiver for a useful signal. 
     In order to achieve the above object, the present invention provides a method for eliminating interference in a mobile communication system, comprising: 
     step 1: re-constructing signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference in accordance with an output signal {circumflex over (d)} esb  from an equalizer; and 
     step 2: performing interference elimination on the inter-symbol interference and multi-access interference in an output signal ê MF  from a matched filter in accordance with the signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference, to obtain signal estimation {circumflex over (d)} JD+IC  with the interference eliminated. 
     Preferably, in step 1, the signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference is re-constructed by using the equation {circumflex over (d)} MAI+ISI ={circumflex over (d)}{circumflex over (d)} esb )}, wherein A represents a system matrix, function  diag(.)  represents a matrix generated after a diagonal entry is set to 0. 
     Preferably, in step 2, the interference elimination is performed by using the equation {circumflex over (d)} JD+IC =(diag(A H  A)) −1 ê MF −(diag(A H  A)) −1 {circumflex over (d)} MAI+ISI , wherein function (.) − represents matrix inversion, and function diag(.) represents a matrix generated after an off-diagonal entry is set to 0. 
     Preferably, in step 2, the interference elimination is performed by using the equation {circumflex over (d)} JD+IC =(diag(A H  A)) −1 (ê MF −{circumflex over (d)} MAI+ISI ), wherein function (.) −1  represents matrix inversion, and function diag(.) represents a matrix generated after an off-diagonal entry is set to 0. 
     The present invention further provides a device for eliminating interference, comprising: 
     an inter-symbol interference and multi-access interference re-constructor configured to re-construct signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference in accordance with an output signal {circumflex over (d)} esb  from an equalizer; and 
     an interference eliminator configured to perform interference elimination on the inter-symbol interference and multi-access interference in an output signal ê MF  from a matched filter in accordance with the signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference, to obtain signal estimation {circumflex over (d)} JD+IC  with the interference eliminated. 
     Preferably, the inter-symbol interference and multi-access interference re-constructor is configured to re-construct the signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference by using the equation {circumflex over (d)} MAI+ISI ={circumflex over (d)}{circumflex over (d)} esb )}, wherein A represents a system matrix, function  diag(.)  represents a matrix generated after a diagonal entry is set to 0. 
     Preferably, the interference eliminator is configured to perform interference elimination by using the equation {circumflex over (d)} JD+IC =(diag(A H  A)) −1 ê MF −(diag(A H  A)) −1 {circumflex over (d)} MAI+ISI , wherein function (.) −1  represents matrix inversion, and function diag(.) represents a matrix generated after an off-diagonal entry is set to 0. 
     Preferably, the interference eliminator is configured to perform interference elimination by using the equation {circumflex over (d)} JD+IC =(diag(A H  A)) −1 (ê MF −{circumflex over (d)} MAI+ISI ), wherein function (.) −1  represents matrix inversion, and function diag(.) represents a matrix generated after an off-diagonal entry is set to 0. 
     According to the above technical solutions, the present invention has the following beneficial effects. Signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference is re-constructed in accordance with an output signal {circumflex over (d)} esb  from an equalizer, and then interference elimination is performed on the inter-symbol interference and multi-access interference in an output signal ê MF  from a matched filter in accordance with the signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference, to obtain signal estimation {circumflex over (d)} JD+IC  with the interference eliminated. As a non-linear multi-user detection technology, the present invention, without remarkably increasing the complexity of the existing JD method, can suppress the impact of the inter-symbol interference and multi-access interference on a useful signal in a better manner and improve the receptivity of a receiver for the useful signal, 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart of a method for eliminating interference according to embodiments of the present invention; 
         FIG. 2  is a block diagram showing a simulation platform according to embodiments of the present invention; 
         FIG. 3  is a schematic view showing the simulation result according to one embodiment of the present invention; 
         FIG. 4  is a schematic view showing the simulation result according to another embodiment of the present invention; and 
         FIG. 5  is a structural schematic view showing a device for eliminating interference according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In order to make the objects, the technical solutions and the advantages of the present invention more apparent, the present invention will be described hereinafter in details in conjunction with the embodiments and the drawings. Here, the embodiments and the descriptions thereof are merely for illustrative purpose, but cannot be regarded as limitations to the present invention. 
     The present invention may be applied to either a TD-SCDMA system or a DS-SCDMA system. For easy understanding, the TD-SCDMA system is taken hereinafter as an example, and the parameters thereof are shown in Table 1. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Number of Cells 
                 1 
               
               
                   
                 Midamble Number 
                 1 
               
               
                   
                 Modulation Mode 
                 QPSK 
               
               
                   
                 Number of Tx Antennas 
                 1 
               
               
                   
                 Number of Rx Antennas 
                 1 
               
               
                   
                 Number of Codes 
                 16 
               
               
                   
                 Code Number 
                 1~16 
               
               
                   
                 Number of Timeslots 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     Referring to  FIG. 1 , which is a flow chart of a method for eliminating interference according to an embodiment of the present invention, the method comprises the following steps. 
     Step  101 : re-constructing signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference in accordance with an output signal {circumflex over (d)} esb  from an equalizer. 
     In this step, the signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference may be re-constructed by using the following equation (1): 
                       d   ^       MAI   +   ISI       =         diag   ⁡     (       A   H     ⁢   A     )       ⁢       d   ^     esb       _             (   1   )               
wherein A represents a system matrix, and function  diag(.)  represents a matrix generated after a diagonal entry is set to 0.
 
     In this embodiment, presumed that d esb ={d 1   1 d 1   1  . . . d 1   K     vru   d 2   1  . . . d 2   K     vru   d 3   1  . . . d 3   K     Vru    . . . d 22   1  . . . d 22   K     vru   }, wherein K vru  represents the number of virtual codes and K vru =16, and 
                 A   H     ⁢   A     =     R   =       [           R   0           R   1         0       …       0       0             R   1   H           R   0           R   1         …       0       0           0         R   1   H           R   0         …       0       0           .       .       .       .       .       .           0       …       0         R   1   H           R   0           R   1             0       0       …       0         R   1   H           R   0           ]     .             
At this time,
 
                   d   ^       MAI   +   ISI       =         diag   _     ⁡     [           R   0           R   1         0       …       0       0             R   1   H           R   0           R   1         …       0       0           0         R   1   H           R   0         …       0       0           ⋮       ⋮       ⋮       ⋱       ⋮       ⋮           0       …       0         R   1   H           R   0           R   1             0       0       …       0         R   1   H           R   0           ]       ·     (           d   1   1             ⋮             d   21     K   vru                 d   22   1             ⋮             d   22     K   vru             )         ,         
and further,
 
                     d   ^       MAI   +   ISI       ⁡     (   i   )       =     [       ∑       j   =   0     ,     j   ≠   1         22   ⁢     K     vru   -   1           ⁢           ⁢       R   ⁡     (     i   ,   j     )       ⁢     d       [     i     K   vru       ]     +   1         (     i   ⁢           ⁢   mod   ⁢           ⁢     K   vru       )     +   1           ]       ,         
wherein i=0, 1, . . . , 703.
 
     Step  102 : performing interference elimination on the inter-symbol interference and multi-access interference in an output signal ê MF  from a matched filter in accordance with the signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference, to obtain signal estimation {circumflex over (d)} JD+IC  with the interference eliminated. 
     In this embodiment, the interference elimination may be performed by using the following equation (2) or (3):
 
 {circumflex over (d)}   JD+IC =(diag( A   H   A )) −1   ê   MF (diag( A   H   A )) −1   {circumflex over (d)}   MAI+ISI   (2)
 
 {circumflex over (d)}   JD+IC (diag( A   H   A )) −1 (ê MF   −{circumflex over (d)}   MAI+ISI )  (3)
 
wherein function (.) −1  represents matrix inversion, and function diag(.) represents a matrix generated after an off-diagonal entry is set to 0.
 
     In this step, if R′ 0i,i =1/R0i,i, then {circumflex over (d)} JD+IC (i)=R′ 0i %16,i %16 ×ê MF (i)−R′ 0i %16,i %16 ×{circumflex over (d)} MAI+ISI (i) (i=0, 1, . . . , 703), or {circumflex over (d)} JD+IC (i)=R′ 0i %16,i %16 ×(ê MF (i)−{circumflex over (d)} MAI+ISI (i)) (i=0, 1, . . . , 703), wherein % represents a modulo operation. 
     Means for eliminating interference is positioned after the equalizer in the joint detection. Before demodulation, the input signals into the means include the output signal {circumflex over (d)} esb  from the equalizer and the output signal ê MF  from the matched filter. The output signal from the means is the signal {circumflex over (d)} JD+IC  with the interference eliminated. 
     The present invention can suppress the impact of MAI on the useful signal based on a joint detection algorithm, and thereby the performance of the receiver may be improved. Simulation is performed hereinafter on the TD-SCDMA system using the simulation platform as shown in  FIG. 2 , and the simulation parameters are shown in Table 2. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Communication System 
                 TD-SCDMA 
               
               
                   
                 Number of Cells 
                 1 
               
               
                   
                 Midamble Number 
                 1 
               
               
                   
                 Modulation Mode 
                 QPSK 
               
               
                   
                 Number of Tx Antennas 
                 1 
               
               
                   
                 Number of Rx Antennas 
                 1 
               
               
                   
                 Spreading Factors 
                 16 
               
               
                   
                 Kcell 
                 8 
               
               
                   
                 Number of Timeslots 
                 1 
               
               
                   
                 Simulation Code Configuration 
                 Standard configuration 
               
               
                   
                   
                 (12.2K and 64K) 
               
               
                   
                 Power ratio of useful signal to 
                 1 
               
               
                   
                 interference signal 
                   
               
               
                   
                 Fading channel 
                 Case1 
               
               
                   
                   
               
             
          
         
       
     
     Under the above-mentioned simulation configuration and channel environment, the simulation result is shown in  FIGS. 3 and 4 , where the simulation code in  FIG. 3  is 12.2K while the simulation code in  FIG. 4  is 64K. The method for eliminating interference according to this embodiment (the curve with dots), with less algorithm complexity, has better performance than the traditional algorithm, in particular under a worse interference environment. “Ior/Ioc” of the horizontal axis in  FIGS. 3 and 4  represents “a ratio of a total power for transmitting signals (including user signals and interference signals) to an additive white Gaussian noise (AWGN) power (dB value)”, and “raw_BER” of the vertical axis represents “Bit Error Rate”. 
     Referring to  FIG. 5 , which is a structural schematic view showing a device for eliminating interference according to embodiments of the present invention, the device comprises: 
     an inter-symbol interference and multi-access interference re-constructor  51 , configured to re-construct signal estimation {circumflex over (d)} 2   MAI+ISI  of inter-symbol interference and multi-access interference in accordance with an output signal {circumflex over (d)} esb  from an equalizer; and 
     an interference eliminator  52 , configured to perform interference elimination on the inter-symbol interference and multi-access interference in an output signal ê m , from a matched filter in accordance with the signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference, to obtain signal estimation {circumflex over (d)} JD+IC  with the interference eliminated. 
     In an embodiment of the present invention, the inter-symbol interference and multi-access interference re-constructor may be configured to re-construct the signal estimation {circumflex over (d)} MAI+ISI  of inter-symbol interference and multi-access interference by using the equation {circumflex over (d)} MAI+ISI ={circumflex over (d)}{circumflex over (d)} esb )}, wherein A represents a system matrix, function diag(.) represents a matrix generated after a diagonal entry is set to 0. 
     In an embodiment of the present invention, the interference eliminator  52  may be configured to perform interference elimination by using the equation {circumflex over (d)} JD+IC =(diag(A H  A)) −1 ê MF −(diag(A H  A)) −1 {circumflex over (d)} MAI+ISI , wherein function (.) −1  represents matrix inversion, and function diag(.) represents a matrix generated after an off-diagonal entry is set to 0. 
     In an embodiment of the present invention, the interference eliminator  52  may be configured to perform interference elimination by using the equation {circumflex over (d)} JD+IC =(diag(A H  A)) −1 (ê MF −{circumflex over (d)} MAI+ISI ), wherein function (.) −1  represents matrix inversion, and function diag(.) represents a matrix generated after an off-diagonal entry is set to 0. 
     The above are merely the preferred embodiments of the present invention. It should be appreciated that, a person skilled in the art may make improvements and modifications without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of the present invention.
The present invention relates to the field of communication technology, and provides a method and a device for eliminating interference in a mobile communication system. The method comprises: step 1: re-constructing signal estimation {circumflex over (d)} MAI+ISI of inter-symbol interference and multi-access interference in accordance with an output signal {circumflex over (d)} esb from an equalizer; and step 2: performing interference elimination on the inter-symbol interference and multi-access interference in an output signal e MF from a matched filter in accordance with the signal estimation {circumflex over (d)} MAI+ISI of inter-symbol interference and multi-access interference, to obtain signal estimation {circumflex over (d)} JD+IC with the interference eliminated. According to the present invention, it is able to eliminate the impact of an interference signal and improve the receptivity of a receiver for a useful signal.