Abstract:
A system in an asynchronous mobile communications system and located in a mobile station for determining use of space-time block-coding-based transmit diversity encoding in a base station including transmission antennas, comprises: a cell search unit for detecting frame timing information and scrambling codes of the base station from signals output from the base station; a descrambling unit for descrambling the signals output from the base station using the frame timing information and the scrambling codes; an accumulation processor for despreading the descrambled signals; a depatternization unit for depatternizing the despread signals using pilot symbol patterns corresponding to the two transmission antennas; and an accumulator bank unit for performing accumulation and addition processes of the depatternized signals to output signals corresponding to the transmission antennas.

Description:
BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The present invention relates to a next generation mobile communications system. More particularly, the present invention relates to a system and method for determining the use of forward channel transmit diversity, that is, STTD (Space-Time block-coding-based Transmit Diversity), encoding in a base station of an asynchronous broadband CDMA (Code Division Multiple Access) system. 
   (b) Description of the Related Art 
   Next generation mobile communications systems provide various high speed (2 Mbps) and high quality services such as multimedia capabilities. 
   With the goal of providing such services, the UTRAN (Universal Terrestrial Radio Access Network) FDD (Frequency Division Duplex) standard was made at the 3GPP (3 rd  Generation Partnership Project), which was a meeting that took place to create a third generation standard. 
   In wireless communications systems, there is a reduction in performance caused by a multi-path fading channel. A variety of methods are used to minimize this loss of performance. 
   Among the various methods, an effective way to prevent this problem is through the use of an antenna diversity technology, in which many antennas are used in a receiver of a base station. The basic reason for such a configuration is economic: a single base station may be used for many mobile stations, while use of a plurality of antennas in mobile stations is costly. 
   Many methods have been proposed to receive such benefits of diversity. The technology adopted in the 3GPP standard is an open-loop diversity STTD technology. The STTD technology was first proposed in 1998 by S. M. Alamouti (see Standardization Specifications 3GPP 3G TS 25.211). 
   STTD encoding adopted in the 3GPP standard is optional for a base station. Accordingly, even with an increase in added complexities, mobile stations and terminals must provide the ability to execute STTD demodulation in receivers. 
   When the power of a mobile station is first turned on, a three-stage cell search unit of a receiving end acquires timing information of a base station cell with the largest signal and a scrambling code number. Next, before system information sent through a primary common control physical channel (PCCPCH), which is a forward channel, is demodulated in the UTRAN, the receiver must compensate frequency offset between the base station and mobile stations. However, it must be known whether STTD encoding of the base station is being used to perform such frequency offset compensation. 
   It may be determined if STTD encoding of the base station is being used through symbols contained in a synchronization channel (SCH) as shown in  FIG. 1 . 
     FIG. 1  is a drawing showing a structure of a PCCPCH according to a 3GPP radio access network (RAN) standard. There is no transmission at an initial point of each slot during a 256-chip interval, and instead a primary SCH and a secondary SCH are transmitted in this interval. 
   Symbol (a) included in the primary SCH and the secondary SCH indicates whether STTD encoding of the base station is being used. That is, if a value of (a) is +1, STTD encoding is being used, while if the value of (a) is −1, this indicates that STTD encoding is not being used. 
   However, in this method of determining whether STTD decoding is being used through the symbol included in the SCHs of the PCCPCH, in the case where one base station is using STTD encoding and another is not using STTD encoding, and slot timings of the two base stations overlap, it is not possible to determine whether STTD encoding is being used for the base station cell with the largest reception signal. 
   Further, since there is no spreading gain if the SCHs do not perform spreading, errors as a result of multi-path fading and frequency offset are significant compared to other channels, thereby increasing the likelihood of making incorrect determinations of whether STTD encoding is being used. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in an effort to solve the above problems. 
   It is an object of the present invention to provide a system and method for determining the use of STTD encoding of a base station in order to compensate a frequency offset in a multi-path fading channel environment. 
   In one aspect of the present invention, a system in an asynchronous mobile communications system and located in a mobile station for determining use of space-time block-coding-based transmit diversity encoding in a base station that includes two transmission antennas, comprises: a cell search unit for detecting frame timing information and scrambling codes of the base station from signals output from the base station; a descrambling unit for descrambling the signals output from the base station using the frame timing information and the scrambling codes, which are detected by the cell search unit; an accumulation processor for despreading the signals descrambled by the descrambling unit; a depatternization unit for performing depatternization of the signals despread by the logic processor using pilot symbol patterns corresponding to the two transmission antennas of the base station; and an accumulator bank unit for performing accumulation and addition processes of the signals that have undergone depatternization by the depatternization unit to output signals corresponding to the transmission antennas. 
   In another aspect of the present invention, a method in an asynchronous mobile communications system for determining use of space-time block-coding-based transmit diversity encoding in a base station comprises: (a) detecting scrambling codes through a common pilot channel transmitted from the base station which includes two transmission antennas; (b) generating pilot symbols using the detected scrambling codes; (c) performing depatternization of the generated pilot symbols using pilot symbol patterns corresponding to each of the two transmission antennas; (d) performing accumulation and addition processes of the two pilot symbols having undergone depatternization, and outputting the symbols as signals corresponding to each of the two transmission antennas; and (e) comparing energy values of the two output signals corresponding to the transmission antennas to determine if space-time block-coding-based transmit diversity encoding of the base station is being used. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
       FIG. 1  is a drawing showing a structure of a forward synchronization channel of an asynchronous broadband CDMA system; 
       FIG. 2  is a block diagram of a system for determining use of STTD encoding of a base station according to a preferred embodiment of the present invention; 
       FIG. 3  is a drawing showing a structure of a forward joint pilot channel of an asynchronous broadband CDMA system; 
       FIG. 4  is a detailed block diagram of a first accumulator bank and a second accumulator bank shown in  FIG. 2 ; and 
       FIG. 5  is a detailed block diagram of a final determining unit shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 2  is a block diagram of a system for determining use of STTD encoding of a base station according to a preferred embodiment of the present invention. 
   As shown in  FIG. 2 , a system for determining use of STTD encoding of a base station according to a preferred embodiment of the present invention includes a cell search unit  100  and an STTD encoding usage-determining unit  200 . 
   In the case where the cell search unit  100  is already installed in mobile to stations, the corresponding cell search unit  100  may be used. 
   In the following description, although it is assumed that two antennas are used in the base station, the present invention is not limited to this configuration and it is possible to increase the number of antennas. 
   The STTD encoding usage-determining unit  200  includes a scrambling code generator  210 ; an accumulator  220 ; multipliers  215 ,  232 , and  242 ; a CPICH Ant 1  symbol pilot generator  230 ; a CPICH Ant 2  symbol pilot generator  240 ; a first accumulator bank  250 ; a second accumulator bank  260 ; and a final determining unit  270 . 
   The cell search unit  100  searches a base station covering a corresponding mobile station through I, Q baseband signals received externally, and it obtains a frame time of base stations and corresponding scrambling codes. In the preferred embodiment of the present invention, in order to determine use of STTD encoding of a base station, a CPICH is used among the channels transmitted from the base stations, and among the CPICH transmitted from the base stations, the cell search unit  100  uses, in particular, P-CPICH to detect a frame timing of base stations and corresponding scrambling codes. 
   The scrambling code generator  210 , using a scrambling code of a base station and timing information obtained by the cell search unit  100 , generates a scrambling code that is synchronized to a frame timing of a base station. 
   The scrambling code generated by the scrambling code generator  210  is multiplied by the multiplier  215  by an I, Q baseband signal, which is input while being synchronized to correct timing, such that the scrambling code is descrambled. 
   The signal descrambled by the multiplier  215  undergoes despreading by the accumulator  220  such that a P-CPICH symbol expressed as in Equation 1 below is obtained.
 
 r ( n )=√ {square root over (E     CPICH     )}   C   CPICH   1 ( n )+ √{square root over (E     CPICH     )}   C   CPICH   2 ( n )+η( n )
 
where C 1   CPICH (n) and C 2   CPICH  (n) are despread CPICH n th  symbols of antenna  1  (Ant 1 ) and antenna  2  (Ant 2 ), respectively, and η(n) indicates interference.
 
   The CPICH Ant 1  symbol pilot generator  230  and the CPICH Ant 2  symbol pilot generator  240 , with reference also to  FIG. 3 , generate pilot symbols for antenna  1  and antenna  2 , respectively (see 3GPP standard TS 25.211). At this time, the pilot symbols for the two antennas generated by the symbol pilot generators  230  and  240  are orthogonal to one another. 
   The multiplier  232  multiplies a pilot symbol pattern generated by the CPICH Ant 1  symbol pilot generator  230  to the signal r(n) that is despread by the accumulator  220  to thereby perform depatternization. A signal depafternized by the multiplier  232  is expressed by Equation 2 below. 
   
     
       
         
           
             
               
                 
                   
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   where M is a length correlated by a factor of 2, 4, . . . , 10, m is a quotient when dividing M by n (m=M div n), I is a remainder when dividing M by n (I =M mod n), {circle around (x)} is a complex number correlation, and Φ 1 (m) represents an energy of antenna  1 . 
   Similarly, the multiplier  242  multiplies a pilot symbol pattern generated by the CPICH Ant 2  symbol pilot generator  240  and the signal r(n) that is despread by the accumulator  220  to thereby perform depatternization. A signal depatternized by the multiplier  242  is expressed by Equation 3 below. 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
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   where φ 2 (m) represents an energy of antenna  2 . 
   The first accumulator bank  250  and the second accumulator bank  260  each receive both signals Φ 1 (m) and Φ 2 (m) output by the multipliers  232  and  242 , respectively. 
   The first accumulator bank  250  and the second accumulator bank  260  receive the signals Φ 1 (m) and Φ 2 (m) output respectively by the multipliers  232  and  242 , and perform accumulation and addition processes to output signals S 2 _Ant 1 , S 2 _Ant 2 , S 10 _Ant 1 , and S 10 _Ant 2  corresponding to the antenna  1  pattern and the antenna  2  pattern. At this time, although the first accumulator bank  250  and the second accumulator bank  260  output signals corresponding to both the antenna  1  pattern and the antenna  2  pattern, since during accumulation processing accumulation lengths (i.e., depatternization lengths) are different, a size of each signal may be different. 
   In the preferred embodiment of the present invention, a depatternization length of the first accumulator bank  250  is 2, and a depatternization length of the second accumulator bank  260  is 10. However, the present invention is not limited to these parameters, and in the case where a frequency offset of a particular range is permitted, different depatternization lengths may be established. 
   The first accumulator bank  250  and the second accumulator bank  260  will now be described in more detail. 
     FIG. 4  is a detailed block diagram of the first accumulator bank  250  and the second accumulator bank  260  shown in  FIG. 2 . 
   As shown in  FIG. 4 , the first accumulator bank  250  includes four accumulators  251 ,  252 ,  255 , and  256 , and two non-coherent adders  253  and  254 . Also, the second accumulator bank  260  includes four accumulators  261 ,  262 ,  265 , and  266 , and two non-coherent adders  263  and  264 . 
   The accumulator  251  of the first accumulator bank  250  accumulates the signal Φ 1 (m) output from the multiplier  232  every 2 symbols then performs output, and the accumulator  252  accumulates the signal Φ 2 (m) output from the multiplier  242  every 2 symbols then performs output. Accumulation every 2 symbols is performed since the correlation length, that is, the depatternization length, of the first accumulator bank  250  is 2. 
   The non-coherent adder  253  receives the signals accumulated in and output from the accumulator  251 , and adds and outputs signals having in-phase and quadrature-phase elements. The non-coherent adder  254  receives the signals accumulated in and output from the accumulator  252 , and similarly adds and outputs signals having in-phase and quadrature-phase elements. 
   The accumulator  255  accumulates the signals output from the non-coherent adder  253  by as many as several slots (N slots), then performs output as an accumulation result signal S 2 _Ant 1  according to the antenna  1  pattern. The accumulator  256  accumulates the signals output from the non-coherent adder  254  by as many as several slots (N slots), then performs output as an accumulation result signal S 2 _Ant 2  according to the antenna  2  pattern. Here, a slot length N, that is, an accumulation length N, is adjusted by a controller (not shown) connected to the accumulators  255  and  256 . 
   The accumulator  261  of the second accumulator bank  260  accumulates the signal Φ 1 (m) output from the multiplier  232  every 10 symbols then performs output, and the accumulator  262  accumulates the signal Φ 2 (m) output from the multiplier  242  every 10 symbols then performs output. Accumulation every 10 symbols is performed since the depatternization length of the second accumulator bank  260  is 10. 
   The non-coherent adder  263  receives the signals accumulated in and output from the accumulator  261 , and adds and outputs signals having in-phase and quadrature-phase elements. The non-coherent adder  264  receives the signals accumulated in and output from the accumulator  262 , and similarly adds and outputs signals having in-phase and quadrature-phase elements. 
   The accumulator  265  accumulates the signals output from the non-coherent adder  263  by as many as several slots (N slots), then performs output as an accumulation result signal S 10 _Ant 1  according to the antenna  1  pattern. The accumulator  266  accumulates the signals output from the non-coherent adder  264  by as many as several slots (N slots), then performs output as an accumulation result signal S 10 _Ant 2  according to the antenna  2  pattern. Here, a slot length N may be adjusted as described above, i.e., by a controller (not shown) connected to the accumulators  265  and  266 . 
   In the case where the base station is not using STTD encoding, only a pattern corresponding to antenna  1  as shown in Equations 2 and 3 is transmitted. Since the patterns of the two antennas have the property of being orthogonal to one another, a predetermined amount of energy is measured in the S 2 _Ant 1  output by the accumulator  255  of the first accumulator bank  250 , and 0 energy is measured in the S 2 _Ant 2  output by the accumulator  256  of the first accumulator bank  250 . 
   Similarly, a predetermined amount of energy is measured in the S 10 _Ant 1  output by the accumulator  265  of the second accumulator bank  260 , and 0 energy is measured in S 10 _Ant 2  output by the accumulator  266  of the second accumulator bank  260 . 
   In the case where the base station is using STTD encoding, since a result in which the two antenna patterns are combined is transmitted, similar levels of energy are measured in the accumulators  255  and  256  of the first accumulator bank  250  and the accumulators  265  and  266  of the second accumulator bank  260 . 
   In theory, if there is no frequency offset or a multi-path fading channel, a difference in energy between the two measured outputs S 2 _Ant 1  and S 2 _Ant 2  of the first accumulator bank  250  takes on an unlimited value in the case where STTD encoding is not being used by the base station, while the difference in energy becomes 0 dB if the base station is using STTD encoding. Therefore, it may be determined that STTD encoding is not being used if the difference in energies of the outputs S 2 _Ant 1  and S 2 _Ant 2  of the first accumulator bank  250  is large, and that STTD encoding is being used if this difference in energies is small. Accordingly, the determination of whether STTD encoding is being used by the base station may be made using only the first accumulator bank  250  and not the second accumulator bank  260 . 
   However, in actual practice, the above results are unable to be obtained in a state where there exist distortions as a result of frequency offset and a multi-path fading channel. If the distortions are severe, there are instances where there is no difference in energy between the two outputs in the case where STTD encoding is not being used, and instances where the difference in energies is large in the case where STTD encoding is being used. As a result, it is extremely difficult to determine whether STTD encoding is being used by the base station using only one accumulator bank, such as the first accumulator bank  250 . 
   Through simulation, it is known that at specific frequency-offset values, the difference in energies of the above outputs is reversed. For example, when the frequency-offset value is varied in the case where STTD encoding is not used, a frequency-offset value is reached such that the energy value corresponding to antenna  1  becomes smaller than the energy value corresponding to antenna  2 . If the frequency-offset value continues to be varied in the same direction, the energy value of antenna  2  again increases to exceed the energy value of antenna  1 . This critical frequency-offset value varies according to the depatternization length of the accumulator banks. 
   As described above, since it is difficult to determine whether the base station is using STTD encoding using only a single accumulator bank, it is preferable to use two accumulator banks having different critical values, that is, different depatternization lengths. 
   In the preferred embodiment of the present invention, as shown in  FIG. 4 , the first accumulator bank  250  having a depatternization length of 2 and the second accumulator bank  260  having a depatternization length of 10 are used to determine whether the base station is using STTD encoding. The depatternization lengths of 2 and 10 are values that have been shown through experimentation to allow a frequency offset of up to 3 ppm. 
   Accordingly, determination results of the two outputs S 2 _Ant 1  and S 2 _Ant 2  of the first accumulator bank  250  and determination results of the two outputs S 10 _Ant 1  and S 10 _Ant 2  of the second accumulator bank  260  may be different. That is, depending on the particular frequency-offset value, one of either the first accumulator bank  250  or the second accumulator bank  260  makes an incorrect determination, while the other of either the first accumulator bank  250  or the second accumulator bank  260  makes a correct determination. 
   As a result, the final determining unit  270  receives the result signals S 2 _Ant 1 , S 2 _Ant 2 , S 10 _Ant 1 , and S 10 _Ant 2  of the first and second accumulator banks  250  and  260 , and correctly determines if the base station is using STTD encoding. A signal of this final determination is output by the final determining unit  270 . 
   In more detail, the final determining unit  270  performs a calculation using Equation 4 shown below on the signals S 2 _Ant 1  and S 2 _Ant 2  output by the first accumulator bank  250  and the signals S 10 _Ant 1  and S 10 _Ant 2  output by the second accumulator bank  260 , and outputs results. Following this process, the final determining unit  270  performs a logical OR calculation on values resulting from performing the calculation using Equation 4 on the output signals S 2 _Ant 1  and S 2 _Ant 2  of the first accumulator bank  250  and the output signals S 10 _Ant 1  and S 10 _Ant 2  of the second accumulator bank  260 , thereby obtaining a final result value. The final result value becomes an STTD encoding use-determination signal, and result values, processed differently depending on whether the depatternization length is 2 or 10, undergo a logical OR calculation to obtain a final determination result. Accordingly, even if an incorrect determination is made by one of either the first accumulator bank  250  or the second accumulator bank  260  because of the critical frequency-offset value, the other of either the first accumulator bank  250  or the second accumulator bank  260  makes a correct determination such that a precise final determination is calculated. 
   
     
       
         
           
             
               
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   where N is an accumulation length and K Th  is a critical value. 
   A structure for enabling the final determining unit  270  to perform this function is shown in  FIG. 5 . 
     FIG. 5  is a detailed block diagram of the final determining unit  270  shown in  FIG. 2 . 
   As shown in  FIG. 5 , the final determining unit  270  includes two comparator switches  271  and  275 , two critical value multipliers  272  and  276 , two comparators  273  and  277 , and a logical OR gate  274 . 
   The calculation using Equation 4 with respect to the signals S 2 _Ant 1  and S 2 _Ant 2  output from the first accumulator bank  250  is realized by the comparator switch  271 , the critical value multiplier  272 , and the comparator  273 . Also, the calculation using Equation 4 with respect to the signals S 10 _Ant 1  and S 10 _Ant 2  output from the second accumulator bank  260  is realized by the comparator switch  275 , the critical value multiplier  276 , and the comparator  277 . 
   The calculation with respect to the signals S 2 _Ant 1  and S 2 _Ant 2  output from the first accumulator bank  250  will first be described. 
   The comparator switch  271  compares the signals S 2 _Ant 1  and S 2 _Ant 2  output from the first accumulator bank  250  and sends the signal with the larger value to a left path (i.e., to the comparator  273 ) and the signal with the smaller value to a right path (i.e., to the critical value multiplier  272 ). 
   The critical value multiplier  272  multiplies the smaller value output from the comparator switch  271  by a pre-established critical value K Th  and then outputs a result to the comparator  273 . 
   The comparator  273  compares the larger value output from the comparator switch  271  with a multiplication result value output from the critical value multiplier  272 , and outputs a comparison result value. 
   Next, in the calculation with respect to the signals S 10 _Ant 1  and S 10 _Ant 2  output from the second accumulator bank  260 , the same processes are performed by the comparator switch  275 , the critical value multiplier  276 , and the comparator  277  as described above with respect to the signals S 2 _Ant 1  and S 2 _Ant 2  output from the first accumulator bank  250 . 
   Subsequently, the logical OR gate  274  performs a logical OR operation on the signals output by the comparators  273  and  277 , then outputs a final result, that is, a result of whether the base station is using STTD encoding. 
   Table 1 below shows a rate of acquisition of STTD encoding for different frequency offsets in a multi-path fading channel provided in 3GPP when a power ratio (Ior/Ioc) between a received home cell and another cell is −5 dB. Table 2 shows STTD encoding acquisition rates for different power ratios (Ior/Ioc). As shown in Tables 1 and 2, in the preferred embodiment of the present invention, even with variations in frequency offsets and power ratios, a precise determination of whether STTD encoding is being used by a base station may be made. 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               lor/loc = −5 dB, accumulation length (N) = 5 frames, 
             
             
               multi-path channel case 3 
             
           
        
         
             
                 
               Frequency offset (ppm) 
                 
             
           
        
         
             
                 
               0 
               1 
               2 
               3 
             
             
                 
                 
             
           
        
         
             
               STTD 
               1 
               1 
               1 
               1 
             
             
               encoding not 
             
             
               used 
             
             
               STTD 
               0.955 
               0.985 
               0.955 
               0.985 
             
             
               encoding 
             
             
               used 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Accumulation length (N) = 5 frames, frequency offset = 3 ppm, 
             
             
               multi-path channel case 3 
             
           
        
         
             
                 
               lor/loc (dB) 
             
           
        
         
             
                 
               −5 
               −3 
               −1 
               1 
               3 
             
             
                 
                 
             
           
        
         
             
               STTD not 
               1 
               1 
               1 
               1 
               1 
             
             
               used 
             
             
               STTD used 
               0.985 
               0.995 
               1 
               1 
               1 
             
             
                 
             
           
        
       
     
   
   According to the present invention, in a multi-path fading channel environment, a determination of whether STTD encoding of the base station is being used may be precisely made to allow for accurate compensation of the frequency offset, thereby improving the overall performance of a demodulator. 
   Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.