Abstract:
An apparatus and method for generating multiple scrambling codes in an asynchronous mobile communication system. In a scrambling code generating apparatus for generating a current scrambling code and a compressed mode scrambling code for compressed mode transmission in a base station device having a spreader for spreading an input data sequence with one of a plurality of OVSF codes and a scrambler for scrambling the spread data sequence with a primary scrambling code used as a default or one of a plurality of secondary scrambling codes according to the number of mobile stations in communication, a first feedback linear shift register generates an m-sequence from first predetermined initial bits, a second feedback linear shift register generates another m-sequence from second predetermined initial bits, a first adder generates the current scrambling code by adding the outputs of the first and second linear feedback shift registers, a second adder adds the output of the second linear feedback register and an m-sequence one bit delayed from the output of the first linear feedback register, and a third adder adds the output of the second linear feedback register and an m-sequence two bits delayed from the output of the first linear feedback register. Here, the compressed mode scrambling code is one of the outputs of the second and third adders and provided to the scrambler to scramble the spread data sequence.

Description:
This application claims priority to an application entitled “Apparatus and Method for Generating Multiple Scrambling Codes in Asynchronous Mobile Communication System” filed in the Korean Industrial Property Office on Sep. 22, 1999 and assigned Serial No. 99-41181, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to an apparatus and method for generating multiple scrambling codes in a mobile communication system, and in particular, to an apparatus and method for concurrently generating scrambling codes for a normal transmission mode and scrambling codes for a compressed transmission mode, using a pair of initial values without the need for modifying the initial values. 
   2. Description of the Related Art 
   The term “a mobile communication system” as used herein is a UMTS (Universal Mobile Telecommunication System) that operates according to the 3GPP (3rd Generation Partnership Provider) standards. 
   The UMTS normally implements inter-frequency handoff. The inter-frequency handoff occurs between base stations with different frequency assignments. To create an inter-frequency handoff, a mobile station discontinues communication with a serving base station for a predetermined time, in contrast to a handoff with an identical frequency assignment. The non-data transmission period is referred to as an idle period. During the idle period, the mobile station searches for the frequency of a destination base station, which is different from that of the serving base station. After detecting the frequency, the mobile station searches for a control channel at that frequency. If the mobile station succeeds in detecting the frequency and control channel of the destination base station, it resumes communication with the serving base station in the serviced frequency band and commences communication with the destination base station in the new frequency band based on the detected frequency and control channel information, thereby completing the handoff. 
   According to the 3GPP standards, the idle period is produced within a 10-ms frame. Data transmission is discontinued during the idle period and resumes during the next 10-ms frame. This is called compressed mode transmission. 
   Generally, an idle period can be generated within one frame by transmitting less data in a compressed transmission mode than in a normal mode. There are two ways of creating an idle period in one frame: transmitting data of a 10-ms frame at a decreased code rate through puncturing; and reducing the SF (Spreading Factor) of the spreading code used by half to transmit one frame data for a half frame period and designating the other half frame period as an idle period. 
   A 3GPP frame is 10 ms in duration. If the frame data is spread by a spreading code an SF of one half, the data is transmitted for 5ms. As a result, a 5-ms idle period is produced. The mobile station discontinues communication with the old base station and searches for a frequency other than the serving frequency during the idle period. The idle period may be dropped further to below 50% of one frame length through rate matching, when necessary. 
   A drawback of the above idle period generating method is that channel contention might occur since orthogonality is not guaranteed between a downlink channel transmitted at a compressed transmission mode and other downlink channels. This problem is attributed to the characteristic of OVSF (Orthogonal Variable Spreading Factor) codes for channelization in the 3GPP standards. 
   OVSF codes provide channelization to downlink channels and ensures orthogonality among downlink channels with different data rates and SFs.  FIG. 1  illustrates an OVSF code generation method. As shown in  FIG. 1 , OVSF codes are a kind of Walsh code which are generated by increasing the SF. Codes with an identical SF are mutually orthogonal as observed in codes  111  and  113  and codes  121 ,  123 ,  125 , and  127 . Codes with different SFs can also be mutually orthogonal as observed between codes  111  and  125 , codes  111  and  127 , codes  113  and  121 , and codes  113  and  123 . Thus OVSF codes may be orthogonal regardless of an identical SF or different SFs. In contrast, orthogonality is not guaranteed between codes  111  and  121 , codes  111  and  123 , codes  113  and  125 , and codes  113  and  127 . That is, OVSF codes with a higher SF are not orthogonal to their source OVSF. 
   In view of the above-described characteristic of the OVSF codes, orthogonality is not guaranteed between a downlink channel transmitted in the compressed mode and some other downlink channels when an idle period is created within one frame by decreasing a specific SF to a half at the compressed transmission mode. As a result, contention may occur among the downlink channels. Referring to  FIG. 1 , in the case where mobile stations A and B are assigned to codes  121  and  123 , respectively, the base station transmits downlink channels using the same code  111  in the compressed mode, causing contention between the downlink channels. Therefore, the above idle period producing method is viable only if there is no contention between a new OVSF code with a half of the SF used for the normal transmission mode and existing OVSF codes. 
   An OVSF code with a half of the SF of an existing OVSF code can be used without channel contention through scrambling with a different scrambling code in the base station. Scrambling codes available to base stations are numbered with 0 to 262,143 in the 3GPP standards. To identify base stations, 16×k (k=0, . . . , 511) th  codes are designated as primary scrambling codes and (16×k)+j codes (k=0, . . . , 511 and j=1, . . . , 15) as secondary scrambling codes. 15 secondary scrambling codes are designated per primary scrambling code. A total of 8192 scrambling codes are available for normal mode transmission. Besides, there are 8192 even-numbered alternative scrambling codes and 8192 odd-numbered alternative scrambling codes for compressed mode transmission. Scrambling codes labeled with numbers 8192 higher than those of the normal mode scrambling codes are assigned as the even-numbered alternative scrambling codes, and scrambling codes labeled with numbers 16384 higher than those of the normal mode scrambling codes are assigned as the odd-numbered alternative scrambling codes. 
   Prior to transmission over a channel, a base station which operates based on 3GPP spreads the channel with an OVSF code for primary identification and scrambles the channel with a scrambling code for secondary identification. In the course of the scrambling, the base station uses a primary scrambling code or a secondary scrambling code. The secondary scrambling code is used in the case where the base station has no downlink channel to assign to a mobile station due to the lack of OVSF codes used together with primary scrambling codes. No contention occurs between channels spread with an identical OVSF code but scrambled with primary and secondary scrambling codes, respectively. 
   By introducing the notion of increasing base station capacity using the secondary scrambling codes to the conventional method of creating an idle period by decreasing SF by half, a channel can be generated that is immune to contention and incurs no interference with existing channels and an intended idle period can be obtained. 
   To use a scrambling code other than that for normal mode transmission in a compressed transmission mode, the mobile station must choose a scrambling code designated for compressed mode transmission that is the pair to the normal mode scrambling code. 
   In conventional technology, a pair of scrambling codes for compressed mode transmission are assigned to one scrambling code for normal mode transmission. Each base station has 16 scrambling codes for the normal mode and 32 codes for the compressed mode. The 32 scrambling codes are divided into 16 even-numbered alternative scrambling codes and 16 odd-numbered alternative scrambling codes. To commence compressed mode transmission during communication with a mobile station in a normal mode, the base station chooses one of even-numbered and odd-numbered alternative scrambling codes according to a predetermined rule. 
   The choice between an even alternative scrambling code and an odd alternative scrambling code depends on whether the OVSF code used in the normal mode is even-numbered or odd-numbered. If the OVSF code is even-numbered, the base station selects an even-numbered alternative scrambling code that is the counterpart of a primary or secondary scrambling code used in the normal mode, and vice versa. In  FIG. 1 , codes  123  and  127  are even-numbered OVSF codes and codes  121  and  125  are odd-numbered OVSF codes. 
   In case a channel is to be scrambled with a changed scrambling code for compressed mode transmission, the base station checks whether there is an available upper OVSF code having an SF half less than that of the current OVSF code in use for normal mode transmission in an OVSF code generation tree. In the presence of an available upper OVSF code, the base station assigns the upper OVSF code for the compressed mode transmission. 
   On the other hand, if there are no such available upper OVSF codes, the base station determines whether the current OVSF code is even-numbered or odd-numbered. Then, the base station informs the mobile station of a scrambling code corresponding to the OVSF code. The base station reduces the SF of the current OVSF code used in the normal mode for channelization and transmits a frame scrambled with the scrambling code known to the mobile station in the compressed mode. 
   The base station assigns scrambling codes in the way shown in  FIG. 2  to transition from a normal mode to a compressed mode. In  FIG. 2 , reference numeral  250  denotes the indexes of scrambling codes. 16 consecutive scrambling codes are assigned to each base station and the index of the base station is identical to that of its primary scrambling code. 
   Reference numeral  201  denotes scrambling code #0 (i.e., a primary scrambling code #1) assigned to base station #1. 
   As shown in  FIG. 2 , each base station has 1 primary scrambling code and 15 secondary scrambling codes to increase base station capacity. For example, base station #1 has 15 secondary scrambling codes, scrambling code #1 to #15 as indicated by reference numbers  204  to  205 . 
   A scrambling code  202  is assigned as a primary scrambling code to base station #2. As stated above, each base station is assigned to 16 consecutive scrambling codes. 16×i (i=0, . . . , 511)-numbered scrambling codes are primary scrambling codes, and (16×i)+k (i=0, . . . , 511 and k=0, . . . , 15)-numbered scrambling codes are secondary scrambling codes. A primary scrambling code  203  (i.e., scrambling code #8175 and primary scrambling code #512) is assigned to base station #512. 
   The set of scrambling codes available for compressed mode transmission includes 8192 scrambling codes #8192 to #16383 and 8192 more scrambling codes #16384 to #24576. The scrambling code set is divided into two parts  210 (#8192 to #16383) and  220 (#16384 to #24576). Reference numeral  210  denotes even alternative scrambling codes (scrambling codes #8192 to #16383) and reference numeral  220  denotes odd alternative scrambling codes (scrambling codes #16384 to #24575). An even alternative scrambling code is chosen for compressed mode transmission if an OVSF code used for a downlink channel in a normal mode is even-numbered, and an odd alternative scrambling code is chosen for compressed mode transmission if the OVSF code is odd-numbered. 
   An even alternative scrambling code  211  is numbered with 8192, matched to the scrambling code  201 . An even alternative scrambling code  212  is numbered with 8193, matched to the scrambling code  204 . An even alternative scrambling code  213  is numbered with 8207, matched to the scrambling code  205 . 16 even alternative scrambling codes  211  to  213  are assigned to base station #1. 
   The even alternative scrambling codes in the set  210  are numbered with 8192+j (the number of a scrambling code for normal mode transmission j=0, . . . , 8191). For example, for j=1, its corresponding even alternative scrambling code is 8193(=1(j))+8192). This implies that a j th  scrambling code(#0 to #8191) is one-to-one matched with a j+8192)th even alternative scrambling code. 
   An odd alternative scrambling code  221  is numbered with 16384, matched to the scrambling code  201 . An odd alternative scrambling code  222  is numbered with 16385, matched to the scrambling code  204 . An odd alternative scrambling code  223  is numbered with 16399, matched to the scrambling code  205 . 16 odd alternative scrambling codes  211  to  213  are assigned to base station #1. The odd alternative scrambling code in the set  220  are numbered with 16384+j (the number of a scrambling code for normal mode transmission j=0, . . . , 8191) and a j th  scrambling code(#0 to #8191) is one-to-one matched with a (j+8192) th  odd alternative scrambling code. 
   As described above, 8192 scrambling codes #0 to #8191 are assigned for normal mode transmission. For compressed mode transmission, 8192 consecutive scrambling codes #8192 to 16383 are assigned as even alternative scrambling codes and 8192 more consecutive scrambling codes #16384 to 24575 are assigned as odd alternative scrambling codes. 
   The above conventional scrambling code assignment method has the instinctive drawback of increased hardware complexity in a scrambling code generator. The scrambling code generator modifies initial values to generate compressed mode scrambling codes upon every entry into a compressed transmission mode. Alternatively, scrambling codes must be separately procured to generate a normal mode scrambling code, a compressed mode even alternative scrambling code, and a compressed mode odd alternative scrambling code, respectively. 
   For example, if a base station refers to the convention scrambling code numbering method shown in  FIG. 2 , the base station must be provided with compressed mode scrambling code generators as well as a normal mode scrambling code generators to allow a mobile station within the coverage area of the base station to transmit in a compressed transmission mode. The compressed mode scrambling code generators include an even alternative scrambling code generator and an odd alternative scrambling code generator. That is, the base station should have at least three scrambling code generators to support the normal transmission mode and the compressed transmission mode. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a scrambling code assigning apparatus in which a scrambling code for normal mode transmission and scrambling codes for compressed mode transmission are simultaneously generated without changing initial values, for use in scrambling code generators of a base station and a mobile station. 
   It is another object of the present invention to provide a method for simultaneously generating a scrambling code for normal mode transmission and scrambling codes for compressed mode transmission without changing initial values in a mobile communication system. 
   The above objects can be achieved by providing an apparatus and method for generating multiple scrambling codes in an asynchronous mobile communication system. According to one aspect of the present invention, in a scrambling code generating apparatus for generating a current scrambling code and a compressed mode scrambling code for compressed mode transmission in a base station device having a spreader for spreading an input data sequence with one of a plurality of OVSF codes and a scrambler for scrambling the spread data sequence with a primary scrambling code used as a default or one of a plurality of secondary scrambling codes according to the number of mobile stations in communication, a first feedback linear shift register generates an m-sequence from first predetermined initial bits, a second feedback linear shift register generates another m-sequence from second predetermined initial bits, a first adder generates the current scrambling code by adding the outputs of the first and second linear feedback shift registers, a second adder adds the output of the second linear feedback register and an m-sequence one bit delayed from the output of the first linear feedback register, and a third adder adds the output of the second linear feedback register and an m-sequence two bits delayed from the output of the first linear feedback register. Here, the compressed mode scrambling code is one of the outputs of the second and third adders and provided to the scrambler to scramble the spread data sequence. 
   According to another aspect of the present invention, in a scrambling code generating apparatus for generating a current scrambling code and a compressed mode scrambling code for compressed mode transmission in a mobile station device having a descrambler for descrambling an input data sequence with a primary scrambling code used as a default or one of a plurality of secondary scrambling codes according to the number of mobile stations in communication, and a despreader for despreading the descrambled data sequence with one of a plurality of OVSF codes, a first feedback linear shift register generates an m-sequence from first predetermined initial bits, a second feedback linear shift register generates another m-sequence from second predetermined initial bits, a first adder generates the current scrambling code by adding the outputs of the first and second linear feedback shift registers, a second adder adds the output of the second linear feedback register and an m-sequence one bit delayed from the output of the first linear feedback register, and a third adder adds the output of the second linear feedback register and an m-sequence two bits delayed from the output of the first linear feedback register. Here, the compressed mode scrambling code is one of the outputs of the second and third adders and provided to the descrambler to descramble the input data sequence. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  illustrates a code tree from which OVSF codes are generated in a conventional mobile communication system; 
       FIG. 2  is a layout diagram of scrambling codes in the conventional mobile communication system; 
       FIG. 3  is a layout diagram of scrambling codes in a mobile communication system according to an embodiment of the present invention; 
       FIG. 4  is a block diagram illustrating a transmitting device using a scrambling code generator in the mobile communication system according to the embodiment of the present invention; 
       FIG. 5  is a block diagram illustrating the scrambling code generator according to the embodiment of the present invention; 
       FIG. 6  illustrates the structure of a Gold code generator; 
       FIG. 7  is a block diagram illustrating a receiving device using the scrambling code generator in the mobile communication system according to the embodiment of the present invention; 
       FIG. 8  is a flowchart illustrating the operation of the transmitting device shown in  FIG. 4 ; and 
       FIG. 9  is a flowchart illustrating the operation of the receiving device shown in  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention will be described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
   The present invention is intended to provide an apparatus and method for numbering a primary or secondary scrambling code for normal mode transmission and even and odd alternative scrambling codes matched with the normal mode scrambling code for compressed mode transmission with consecutive indexes and generating the scrambling codes simultaneously without modifying initial values used in a scrambling code generator. 
     FIG. 3  is a layout diagram of scrambling codes available to base stations according to an embodiment of the present invention. In  FIG. 3 , reference numeral  350  denotes the indexes of the scrambling codes. In the present invention, 48 consecutive scrambling codes are assigned to each base station. Reference numeral  311  denotes scrambling code #0, i.e., primary scrambling code #1 assigned to base station #1. Reference numeral  312  denotes scrambling code #1, i.e., an even alternative scrambling code matched with scrambling code #0, for use in compressed mode transmission. Reference numeral  313  denotes scrambling code #2, i.e., an odd alternative scrambling code matched with scrambling code #0 for use in compressed mode transmission. Reference numeral  314  denotes scrambling code #3, i.e., secondary scrambling code #1 assigned to base station #1. Reference numeral  315  denotes scrambling code #4, i.e., an even alternative scrambling code matched with scrambling code #3, for use in compressed mode transmission. Reference numeral  316  denotes scrambling code #5, i.e., an odd alternative scrambling code matched with scrambling code #3 for use in compressed mode transmission. Reference numeral  317  denotes scrambling code #45, i.e., secondary scrambling code #15 assigned to base station #1. Reference numeral  318  denotes scrambling code #46, i.e., an even alternative scrambling code matched with scrambling code #45, for use in compressed mode transmission. Reference numeral  319  denotes scrambling code #47, i.e., an odd alternative scrambling code matched with scrambling code #45 for compressed mode transmission. 
   As stated above, each base station uses 48 consecutive scrambling codes according to the present invention. The scrambling codes are arranged in the following order: normal mode scrambling code, even alternative scrambling code, and odd alternative scrambling code. Such a scrambling code layout obviates the need for modifying initial values in generating scrambling codes. An even alternative scrambling code and an odd alternative scrambling code are generated by shifting a scrambling code for normal mode transmission, once and twice, respectively. Therefore, an encoder for generating the normal mode scrambling code outputs a code one tap earlier to produce the even alternative scrambling code and two taps earlier to produce the odd alternative scrambling code. 
   In  FIG. 3 , reference numerals  321  to  329  denote scrambling codes available to base station #2. Reference numeral  321  denotes scrambling code #48, i.e., a primary scrambling code for base station #2. Reference numeral  322  denotes scrambling code #49, i.e., an even alternative scrambling code matched with scrambling code #48. Reference numeral  323  denotes scrambling code #50, i.e., an odd alternative scrambling code matched with scrambling code #48. Reference numeral  324  denotes scrambling code #51, i.e., secondary scrambling code #1 for base station #2. Reference numeral  325  denotes scrambling code #52, i.e., an even alternative scrambling code matched with scrambling code #51. Reference numeral  326  denotes scrambling code #53, i.e., an odd alternative scrambling code matched with scrambling code #51. Reference numeral  327  denotes scrambling code #93, i.e., secondary scrambling code #15 for base station #2. Reference numeral  328  denotes scrambling code #94, i.e., an even alternative scrambling code matched with scrambling code #93. Reference numeral  329  denotes scrambling code #95, i.e., an odd alternative scrambling code matched with scrambling code #93. 
   Similarly, reference numerals  331  to  339  denote scrambling codes available to base station #512. Reference numeral  331  denotes scrambling code #24528, i.e., a primary scrambling code for base station #512. Reference numeral  332  denotes scrambling code #24529, i.e., an even alternative scrambling code matched with scrambling code #24528. Reference numeral  333  denotes scrambling code #24530, i.e., an odd alternative scrambling code matched with scrambling code #24528. Reference numeral  334  denotes scrambling code #24531, i.e., secondary scrambling code #1 for base station #512. Reference numeral  335  denotes scrambling code #24532, i.e., an even alternative scrambling code matched with scrambling code #24531. Reference numeral  336  denotes scrambling code #24533, i.e., an odd alternative scrambling code matched with scrambling code #24531. Reference numeral  337  denotes scrambling code #24573, i.e., secondary scrambling code #15 for base station #512. Reference numeral  338  denotes scrambling code #24574, i.e., an even alternative scrambling code matched with scrambling code #24573. Reference numeral  339  denotes scrambling code #24575, i.e., an odd alternative scrambling code matched with scrambling code #24573. 
     FIG. 4  is a block diagram of a transmitting device using a scrambling code generator in a base station according to an embodiment of the present invention. The transmitting device is illustratively configured with a downlink channel toward one mobile station only. Referring to  FIG. 4 , a controller  420  provides overall control to the transmitting device according to the present invention. The controller  420  receives information about the mobile station from a base station controller (BSC) or a mobile switching center (MSC) (both not shown) and outputs transmission mode control information including a transmission mode control signal  415  and a scrambling code index  413  based on a transmission mode of the mobile station. A spreader  401  determines the SF of an OVSF(Orthogonal Variable Spreading Factor) code via the transmission mode control signal  415  received from the controller  420  and spreads data with an OVSF code with the determined SF(Spreading Factor). If the transmission mode control signal  415  is a compressed transmission mode signal, the spreader  401  spreads the data with an OVSF code with an SF decreased by a factor of one half, as compared to the current OVSF code. 
   While the spreader  401  generally produces OVSF codes appropriate for downlink channel and spreads the downlink channel with the OVSF code, the following description is confined to the spreading operation of the spreader  401  for better understanding of the present invention. 
   The OVSF code with the half SF is equivalent to the root of the current OVSF code used at the normal transmission mode on an OVSF code generation tree. A multiplier  417  multiplies the spread data by a scrambling code available to the base station. The multiplier  417  and a scrambling code generator  411  act as a scrambler for scrambling the transmit data with the scrambling code. 
   Control information of the controller  420  includes the type(primary/secondary/even/odd), number(scrambling code number), and transmission mode(normal mode/compressed mode) of scrambling code assigned to downlink channels in the base station. For the base station #1, the type and number of scrambling code according to transmission mode of downlink channel are listed in Table 1. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Primary 
               secondary 
               even alternative 
               odd alternative 
             
             
                 
               scrambling code 
               scrambling code 
               scrambling code 
               scrambling code 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               normal 
               1 
               0 
               0 
               0 
             
             
               transmission mode 
               1 
               a 
               0 
               0 
             
             
               normal 
               1 
               0 
               1 
               1 
             
             
               transmission mode + 
             
             
               compressed 
               1 
               a 
               1~(a + 1) 
               1~(a + 1) 
             
             
               transmission mode 
             
             
                 
             
           
        
       
     
   
   As noted from Table 1, the number and type of scrambling code used in a base station vary according to transmission mode. The ‘a’ indicates the number of secondary scrambling code required to increase base station capacity, ranging from 1 to 15. 
   The controller  420  shown in  FIG. 4  detects the number and type of scrambling code used for current downlink channel and transmits the transmission mode control signal  415  and the scrambling code index signal  413  to the scrambling code generator  411  to generate the necessary scrambling code. The controller  420  controls the spreader  401  to generate OVSF code appropriate for downlink channel in a normal transmission mode or in a compressed mode transmission. 
   Upon receipt of the scrambling code index  413  and the transmission mode control signal  415 , the scrambling code generator  411  generates a scrambling code for normal mode transmission using the scrambling code index  413  as initial values. The output of the scrambling code generator  411  may be one primary scrambling code, one to fifteen secondary scrambling codes, one to fifteen even alternative scrambling codes or one to fifteen odd alternative scrambling codes. According to the transmission mode control signal  415 , the scrambling code generator  411  generates scrambling codes for both normal mode transmission and compressed mode transmission. The scrambling codes for compressed mode transmission include an even alternative scrambling code and an odd scrambling code. The even or odd alternative scrambling code is selected respectively according to whether the OVSF code in current used for a downlink channel is even-numbered or odd-numbered. The scrambling code generator  411  generates the even alternative scrambling code by shifting the normal mode scrambling code once and the odd alternative scrambling code by shifting the normal mode scrambling code twice. The multiplier  417  scrambles the downlink channel data with a scrambling code received from the scrambling code generator  411 . The scrambled downlink channel is transmitted to the mobile station through a filter  403 , an RF (Radio Frequency) module  405 , and an antenna  407 . 
     FIG. 5  is a block diagram of the scrambling code generator according to the embodiment of the present invention. It is assumed here that the base station uses a primary scrambling code only. 
   In  FIG. 5 , the scrambling code generator is comprised of a Gold code generator  501  having two shift registers that generate m-sequences and a scrambling code generation portion for generating complex scrambling codes with I channel codes and Q channel codes out of a generated Gold code. The Gold code generator  501  generates four kinds of Gold codes for use in generating a primary scrambling code, a secondary scrambling code, an even alternative scrambling code, and an odd alternative scrambling code. Thus, the scrambling code generator  411  has a scrambling code generation portion for each downlink channel. 
   The scrambling code generation portion includes delays  513  to  517  for delaying the Gold code received from the Gold code generator  501  by predetermined chips. The number of delays is equal to the number N of scrambling codes that can be generated in the base station. 
     FIG. 6  is a block diagram of the Gold code generator that generates different Gold codes according to the embodiment of the present invention. The in Gold code generator is assumed to generate Gold codes to be used for one mobile station only, with reference to the scrambling code layout shown in  FIG. 3 . 
   In  FIG. 6 , shift registers  601  and  603  generate different m-sequences. A Gold code is produced by XOR-ing(adding) the m-sequences. An XOR gate  611  XOR-gates bits stored in register #0 and register #7 and feeds the result to register #17 of the shift register  601 . An XOR gate  612  XOR-gates bits stored in registers #0, #5, #7, and #10 and feeds the result in register #17 of the shift register  603 . 
   XOR gates  613 ,  614 , and  615  generate Gold codes to be used in generating a normal mode scrambling code (primary scrambling code or secondary scrambling code), an even alternative scrambling code, and an odd alternative scrambling code. The even and odd alternative scrambling codes being paired with the primary scrambling code. A switch  621  selects one of the scrambling codes received from the XOR gates  613 ,  614 , and  615  according to the scrambling code index  413  and the transmission mode control signal  415  received from the controller  420 . If the transmission mode control signal  415  indicates a compressed transmission mode and an even-numbered OVSF code is used, an even alternative scrambling code is selected. 
   For example, if a primary scrambling code is used for normal mode transmission and an even-numbered OVSF code is assigned to a specific mobile station, the XOR gate  613  generates a Gold code to produce the primary scrambling code (scrambling code #0). At that case, if the transmission mode is changed to compressed mode, the XOR gate  614  generates a Gold code to produce an even alternative scrambling code matched with the primary scrambling code. If an odd-numbered OVSF code is assigned, the XOR gate  615  generates a Gold code to produce an odd alternative scrambling code matched with the primary scrambling code according to the scrambling code layout shown in  FIG. 3 . The switch  621  selects one of the Gold codes received from the XOR gates  613 ,  614 , and  615  according to the transmission mode of a downlink channel to the mobile station and the number of the OVSF code assigned to the mobile station. 
     FIG. 6  shows the Gold code generator on the assumption that only one primary scrambling code is used for a downlink channel to one mobile station, the downlink channel is transmitted in an alternating normal/compressed mode, and the scrambling code layout shown in  FIG. 3  is referred to. 
   The Gold code generator shown in  FIG. 6  operates based on the Fibonacci method of generating m-sequences. The shift register  601  is 18 in length and implements an m-sequence m 1 (t) generator polynomial, f(x)=x 18 +x 7 +1. This polynomial has the feedback characteristic shown below with respect to consecutive symbols in a generated code.
 
 X (18+ i )={ x ( i )+ x ( i+ 7)} modulo 2 (0≦ i≦ 2 18 −20)  (1)
 
   An initial value can arbitrarily be selected for the shift register  601 . Scrambling codes based on 3GPP are generated by shifting a scrambling code that is produced from the initial values. That is, if a base station that operates according to the 3GPP standards uses scrambling code #125, scrambling code #125 is generated by shifting a scrambling code generated using the initial value 125 times. Therefore, an initial value of the shift register  601  for use in completing scrambling code #125 results from 125 times shifting an initial value used to generate scrambling code #0. A binary number resulting from subtracting 1 from the scrambling code used in the base station is used as an initial value. In the embodiment of the present invention, &lt;1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0&gt; is the initial value for the shift register  601 . It is assumed that the scrambling code  311  in  FIG. 3  is generated from the initial value. 
   The shift register  603  is the same length as the shift register  601  and implements an m-sequence m 2 (t) generator polynomial, f(x)=x 18 +x 10 +x 7 +x 5 +1. This polynomial has the feedback characteristic shown below with respect to consecutive symbols in a generated code.
 
 X (18+ i )={ x ( i )+ x ( i+ 5)+ x ( i+ 7)+ x ( i+ 10)} modulo 2 (0≦ i≦ 2 18 −20)  (2)
 
   An initial value of the m-sequence m 2 (t) is common in all base stations. In the embodiment of the present invention, &lt;1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1&gt; is the initial value for the shift register  603 . 
   The XOR gate  611  XOR-s the bit of register #0 and the bit of register #7 and inputs the result in register #17. This feedback satisfies Eq. 1. The XOR gate  612  XOR-s the bits of registers #0, #5, #7, and #10 and inputs the result in register #17. This feedback satisfies Eq. 2. 
   The XOR gate  613  generates a Gold sequence by XOR-ing the output of register #0 in the shift register  601  with the output of register #0 in the shift register  603 . The XOR gate  614  generates a Gold sequence by XOR-ing the output of register #1 in the shift register  601  with the output of register #0 in the shift register  603 . The XOR gate  615  generates a Gold sequence by XOR-ing the output of register #2 in the shift register  601  with the output of register #0 in the shift register  603 . The Gold sequence generated from the XOR gate  614  is equivalent to a once-shifted sequence of the Gold sequence generated from the XOR gate  613 . The Gold sequence generated from the XOR gate  615  is equivalent to a twice-shifted sequence of the Gold sequence generated from the XOR gate  613 . 
   The switch  621  selects one of the Gold sequences received from the XOR gates  613 ,  614 , and  615  according to the transmission mode of a downlink channel and the OVSF code number used at the channel. 
     FIG. 7  is a block diagram illustrating a receiving device in a mobile station according to the embodiment of the present invention. Referring to  FIG. 7 , a downlink channel is received at the mobile station through an antenna  701 . A descrambler  737  descrambles the received downlink channel signal with a scrambling code received from a scrambling code generator  731 . The scrambling code generator  731  generates an appropriate scrambling code based on a scrambling code index  733  and a transmission mode signal  735  received from a controller  750 . The scrambling code generator  731  generates a primary scrambling code continuously to receive a common channel signal from a base station. In case the base station uses a secondary scrambling code for a downlink dedicated channel, the scrambling code generator  731  generates the primary scrambling code and the secondary scrambling code at the same time. When the mobile station transmits in a compressed transmission mode and the base station transmits a downlink channel alternately between normal and compressed transmission modes, the scrambling code generator  731  alternately generates required scrambling codes. These scrambling codes can be generated at the same time by configuring the scrambling code generator  731  in the structure shown in  FIG. 6  and using the scrambling code layout shown in  FIG. 3 . The scrambling code index  733  is equal to the scrambling code index  413  shown in  FIG. 4  and used to set an initial value. The transmission mode signal  735  provides information about the type and number of scrambling code necessary to receive the downlink channel signal and information about an OVSF code necessary to despread an input frame. 
   A despreader  703  despreads the descrambled signal with a modification to an OVSF code in use according to the transmission mode signal  735 . If the transmission mode signal  735  indicates a compressed transmission mode, the despreader  703  despreads the received downlink data with an OVSF with a half SF of the OVSF used in a normal transmission mode, that is, the root of the normal mode OVSF code in an OVSF code generation tree. The despread signal is recovered to user data through a channel estimator  705 , a multiplexer  707 , a deinterleaver  709 , and a decoder  711 . 
     FIG. 8  is a flowchart illustrating the operation of the base station according to the embodiment of the present invention. 
   Referring to  FIG. 8 , the controller  420  of the base station receives a compressed mode transmission command from an upper layer in step  801 . The upper layer signal is transmitted when the mobile station is about to implement an inter-frequency hard handoff. In step  802 , the controller  420  determines whether an OVSF code with a half of the SF of the OVSF code currently used is available. The availability decision is made by obtaining the OVSF code with the half SF and checking whether there is any other mobile stations using an OVSF code with the same SF as that of the current OVSF code. 
   If the OVSF code with the half SF is available, the controller  420  generates the OVSF code with the half SF in step  803 . On the contrary, if it is not available, the controller  420  proceeds to step  811 . Steps  811  to  814  are performed to allow the OVSF code with the half SF to be used by modifying the current scrambling code. In step  811 , the controller  420  checks whether the mobile station currently uses an even-numbered or odd-numbered OVSF code in the normal transmission mode. In the case of an even-numbered OVSF code, the controller  420  designates an even alternative scrambling code set and generates an even alternative scrambling code matched with the scrambling code used for the normal transmission mode in step  812 . On the other hand, in the case of an odd-numbered OVSF code, the controller  420  designates an odd alternative scrambling code set and generates an odd alternative scrambling code matched with the scrambling code used for the normal transmission mode in step  813 . The controller  420  replaces the OVSF code for the normal transmission mode by the newly generated OVSF code (half SF OVSF code) in step  814 . The even alternative scrambling code or the odd alternative scrambling code are generated by shifting the current scrambling code used in the normal transmission mode once or twice, respectively referring to the scrambling code layout of  FIG. 3 . 
   In step  804 , the controller  420  controls the spreader  401  to spread a frame to be transmitted in the compressed transmission mode with the new OVSF code generated in step  803  or  814 . The controller  420  controls the multiplier  417  to scramble the spread signal with the current scrambling code or the newly generated even or odd alternative scrambling code in step  805 . The choice between the current scrambling code and the even or odd alternative scrambling code is made in step  802 . The decision is made in step  811  as to whether the even alternative scrambling code or the odd alternative scrambling code is to be selected. The scrambled frame is transmitted to the mobile station in step  806  and the normal transmission mode resumes in step  807 . 
     FIG. 9  is a flowchart illustrating the operation of the mobile station according to the embodiment of the present invention. 
   Referring to  FIG. 9 , the controller  750  of the mobile station checks whether a compressed mode frame receipt message for inter-frequency handoff has been received from an upper layer in step  901  and whether a message indicating which scrambling code is to be used (hereinafter referred to as a scrambling code assignment message) has been received from the base station in step  902 . Upon receipt of the scrambling code assignment message, the controller  750  determines whether the message indicates that the current scrambling code for normal mode transmission will be used. If the current scrambling code is to be used, the controller  750  replaces an OVSF code used to despread the current downlink channel by an OVSF code used to despread a compressed mode frame in step  903 . 
   On the other hand, if the scrambling code assignment message indicates that a different scrambling code will be used for the compressed transmission mode, the controller  750  determines whether the current OCSF code is even-numbered or odd-numbered in step  911 . In the case of an even-numbered OVSF code, the controller  750  proceeds to step  912  and in the case of an odd-numbered OVSF code, it goes to step  913 . The controller  750  designates an even alternative scrambling code set and generates an even alternative scrambling code matched with the scrambling code used in the normal mode in step  912 . In step  913 , the controller  750  designates an odd alternative scrambling code set and generates an odd alternative scrambling code matched with the scrambling code used in the normal mode. The even alternative scrambling code and the odd alternative scrambling code are generated by shifting the current scrambling code used in the normal transmission mode once and twice, respectively with reference to the scrambling code layout of  FIG. 3 . 
   In step  914 , the controller  750  replaces the OVSF code used to despread the current downlink channel by the OVSF code used to despread a compressed mode frame. This step is the same as step  903 . 
   The mobile station receives a compressed mode frame in step  904  and the descrambler  737  descrambles the frame with the same scrambling code that is used in the base station in step  905 . The scrambling code is a normal mode scrambling code or a compressed mode scrambling code. The base station determines whether a normal mode scrambling code or a compressed mode scrambling code is used and notifies the mobile station of the determination result. The mobile station despreads the descrambled frame with the new OVSF code generated in step  903  or  914  at the despreader  703  and recovers user data by subjecting the despread frame to channel estimation, multiplexing, deinterleaving, and decoding through the channel estimator  705 , the multiplexer  707 , the deinterleaver  709 , and the decoder  711  in step  906  and then returns to a normal reception mode in step  907 . 
   If the conventional scrambling code layout shown in  FIG. 2  is used, the base station and the mobile station that are communicating in a normal transmission mode should generate an even alternative scrambling code by shifting the current scrambling code 8192 times used for normal mode transmission or an odd alternative scrambling code by shifting the current scrambling code 16384 times in order to commence communication in the compressed transmission mode. To generate the even alternative scrambling code, a scrambling code generator for generating a normal mode scrambling code must receive a separate initial value resulting from shifting an initial value of the normal mode scrambling code 8192 times, or an even alternative scrambling code generator should be procured separately. To generate the odd alternative scrambling code, the scrambling code generator for generating a normal mode scrambling code should receive a separate initial value resulting from shifting the initial value 16384 times of the normal mode scrambling code, or an odd alternative scrambling code generator should be procured separately. 
   Meanwhile, if the scrambling code layout shown in  FIG. 3  is applied to the scrambling code generator shown in  FIG. 6 , once an initial value has been set, the scrambling code generator can generate 6 normal mode scrambling codes and 12 compressed mode scrambling codes without any additional operation. Although a base station based on the 3GPP standards is assigned to a total of 48 scrambling codes, interference between scrambling codes is increased as the base station uses more scrambling codes. Therefore, the number of scrambling codes available in reality is limited in the base station. The number of scrambling codes available to the base station in reality are estimated to be 18, the same number of scrambling codes that can be generated simultaneously in the scrambling code generator shown in  FIG. 6 . Consequently, all the scrambling codes available to the base station in reality can be generated concurrently from a set initial value by using the scrambling code generator of  FIG. 6  and the scrambling code layout of  FIG. 3 . While the initial value must be re-input to generate a compressed mode scrambling code or a separate scrambling code generator is required in the conventional technology, all scrambling codes can be generated with the initial value without changing the initial value in the present invention. Thus, hardware complexity is reduced. 
   In accordance with the present invention as described above, scrambling codes are numbered with sequential indexes in the following order: a primary scrambling code, an even alternative scrambling code matched with the primary scrambling code, and an odd alternative scrambling code matched with the primary scrambling code. Accordingly, once an initial value has been set, scrambling codes necessary for a normal transmission mode and a compressed transmission mode can be generated at the same time. 
   Furthermore, when a normal mode scrambling code and a compressed mode scrambling code are alternately used, there is no need for a separate scrambling code generator that would be required if the initial value were changed to generate the compressed mode scrambling code. Therefore, hardware complexity is decreased. 
   While the invention has been shown and described with reference to a certain preferred embodiment 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.