Abstract:
Disclosed is a base station transmission apparatus in a mobile communication system using transmit antenna diversity between a base station with a plurality of antennas and a mobile station. A modulator generates a complex symbol in response to a coded symbol. A first spreader generates a plurality of different complex symbols in response to the complex symbol, and generates a plurality of first spread complex symbols by spreading the generated complex symbols with a first orthogonal code assigned to the mobile station. A second spreader generates a plurality of same complex symbols in response to the complex symbol from the modulator, spreads the same complex symbols with a second orthogonal code, and generates a plurality of second spread complex symbols by multiplying the spread complex symbols by weights for the antennas, determined based on feedback information indicating reception status of a base station signal. A summer sums up the first complex symbols and the second complex symbols. Finally, a transmitter complex-spreads an output of the summer, shifts the complex-spread signals to a radio frequency band, and transmits the shifted signals through the antennas.

Description:
PRIORITY 
     This application claims priority to an application entitled “Transmit Antenna Diversity Apparatus and Method for Base Station in a CDMA Mobile Communication System” filed in the Korean Industrial Property Office on Dec. 21, 2000 and assigned Serial No. 2000-79713, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a communication apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a forward transmit antenna diversity apparatus and method in a CDMA mobile communication system. 
     2. Description of the Related Art 
     An existing CDMA mobile communication system that mainly supports voice service, has been developed into a future (CDMA mobile communication system which provides high-speed data service as well as voice service. The future CDMA mobile communication system supports voice, moving image and Internet search services. In the mobile communication system, communication links existing between a base station and a mobile station are classified into a forward link for transmitting a signal from the base station to the mobile station, and a reverse link for transmitting a signal from the mobile station to the base station. 
     The mobile communication system must resolve a fading problem in order to transmit high-speed data. The fading causes a reduction in the amplitude of a received signal from several dB to several tens dB. In order to solve the fading problem, a variety of diversity techniques are used. 
     One of the techniques used in the CDMA system employs a Rake receiver, which receives a signal on a diversity basis using delay spread of a channel and the Rake receiver supports a reception diversity technique for receiving a multi-path signal. However, this diversity technique is disadvantageous in that it is not operable when the delay spread is low in level. 
     Also, a time diversity technique utilizing interleaving and coding is used in a Doppler spread channel. However, this technique is not effective in a low-speed Doppler spread channel. It is possible, though, to effectually solve the fading problem using a space diversity technique, in an indoor channel with a low Doppler spread level and a pedestrian channel, a low-speed Doppler channel. 
     The space diversity technique uses two or more antennas. In this technique, even though a signal transmitted through one antenna is attenuated due to the fading, it is possible to compensate for the attenuation using a signal transmitted through the other antennas. The space antenna diversity technique is divided into a reception antenna diversity using a plurality of reception antennas and a transmit (transmission) antenna diversity using a plurality of transmission antennas. It is hard to install the reception antenna diversity in the mobile station in light of its size and cost. Thus, the use of the transmit antenna diversity for the base station is recommended. 
     The transmit antenna diversity includes a “closed loop transmit diversity” transmitting a signal based on forward channel information fed back from the mobile station, and an “open loop transmit diversity” receiving no feedback information from the mobile station. In the closed loop transmit diversity scheme, the base station applies weights to transmission signals of the respective transmission antennas based on the channel information measured and fed back by the mobile station to maximize a signal-to-noise ratio (SNR) of an antenna at the mobile station. In the open loop transmit diversity scheme, the base station transmits the same signal through two quadrature (or orthogonal) paths without using the feedback information. The quadrature paths can be provided by time division, frequency division or code division. 
     FIG. 1 illustrates a structure of a base station transmitter using an open loop transmit diversity scheme according to the prior art. Referring to FIG. 1, an input bit stream is encoded by a channel encoder  101 , and an output sequence of the channel encoder  101  is mapped into an M-ary symbol by an M-ary symbol modulator  102 . The M-ary symbol modulator  102  serves as a QPSK (Quadrature Phase Shift Keying), 8-PSK (8-ary Phase Shift Keying) or 16-QAM (16-ary Quadrature Amplitude Modulation) modulator according to its data rate, and its modulation mode can be changed in a physical layer packet unit where the data rate can be changed. I and Q sequences of the M-ary symbol output from the M-ary symbol modulator  102  are modulated into two different complex symbols by an STTD/STS (Space-Time Transmit Diversity/Space Time Spreader) modulator  103 . A detailed description of the STTD/STS modulator  103  will be made with reference to FIGS. 4 and 5. Walsh cover parts  104  and  105  orthogonally spread their input symbols using a Walsh orthogonal code W N   i  assigned to the mobile station. A detailed structure of the Walsh cover parts  104  and  105  is illustrated in FIG.  2 . The two complex symbols spread by the Walsh cover parts  104  and  105  are subject to complex spreading by their associated complex spreaders  106  and  107 , respectively. An internal operation of the complex spreaders  106  and  107  is illustrated in FIG.  3 . The output signals of the complex spreaders  106  and  107  are shifted to RF (Radio Frequency) band signals by associated RF parts  108  and  109 , and then radiated through first and second antennas ANT 1  and ANT 2 . 
     FIG. 2 illustrates a detailed structure of the Walsh cover parts  104  and  105  illustrated in FIG.  1 . Each Walsh cover part  104  and  105  spreads its input complex symbol to a transmission bandwidth, using a Walsh code assigned to a transmission channel. FIG. 3 illustrates an internal operation of the complex spreaders  106  and  107  shown in FIG.  1 . Each of the complex spreaders  106  and  107  complex-spreads its input complex signal into an I-channel (or I-arm) signal and a Q-channel (or Q-arm) signal, using a spreading sequence comprised of an I-channel spreading sequence PN I  and a Q-channel spreading sequence PN Q . 
     FIG. 4 illustrates an internal operation of the STTD/STS modulator  103  of FIG. 1 when it operates in an STTD (Space-Time Transmit Diversity) mode. In the STTD mode, the STTD/STS modulator  103  operates as shown in Table 1. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                          TABLE 1 
               
               
                   
                   
               
               
                   
                 Input to 
                 Antenna #1 
                 Antenna #2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Time t 
                 S 0   
                 S 0   
                 −S* 1   
               
               
                   
                 Time t + T 
                 S 1   
                 S 1   
                 S* 0   
               
               
                   
                   
               
             
          
         
       
     
     In Table 1, S 0  and S 1  represent complex symbols, and are represented by 
     
       
         
           S 
           0 
           =Si 
           0 
           +jSq 
           0 
         
       
     
     
       
         
           S 
           1 
           =Si 
           1 
           +jSq 
           1 
         
       
     
     If symbols S 0  and S 1  are input to the STTD modulator  103  at a specific time t and a time t+T, respectively, then the STTD modulator  103  outputs the symbol S 0  for the first antenna ANTI and a minus conjugate of the symbol S 1  for the second antenna ANT 2  at the time t, and outputs the symbol S 1  for the first antenna ANT 1  and a conjugate of the symbol S 0  for the second antenna ANT 2  at the time t+T. 
     FIG. 5 illustrates an internal operation of the STTD/STS modulator  103  of FIG. 1 when it operates in the STS (Space Time Spreader) mode. Referring to FIG. 5, a serial-to-parallel (S/P) converter  501  converts each of its input complex symbols comprised of an I-phase symbol and a Q-phase symbol into two ½-rate complex symbols comprised of an I-phase symbol and a Q-phase symbol. The two complex symbols I 1 /Q 1  and I 2 /Q 2  are provided to symbol repeaters  511 - 518 , where they are repeated. For example, the symbol I 1  is input to the symbol repeaters  511  and  515 . The symbol repeater  511  (++) repeats the input symbol I 1 , while the symbol repeater  515  (+−) repeats the input symbol I 1 . The outputs of the symbol repeaters  511 - 518  are provided to four summers  519 - 522  and then converted to two complex symbols. Herein, the STTD/STS modulator will be referred to as a “diversity modulator” for simplicity. 
     FIG. 6 illustrates a structure of a base station transmitter using a closed loop transmit diversity scheme according to the prior art. Referring to FIG. 6, an input bit stream is encoded by a channel encoder  601 , and an output sequence of the channel encoder  601  is mapped into an M-ary symbol by an M-ary symbol modulator  602 . The outputs of the symbol modulator  602  are provided to both Walsh cover parts  611  and  612 . That is, the I-phase output of the modulator  602  is provided to both the Walsh cover parts  611  and  612 , and the Q-phase output of the modulator  602  is also provided to both the Walsh cover parts  611  and  612 . The Walsh cover parts  611  and  612  orthogonally-spread by multiplying their input complex symbols by a Walsh code allocated to the mobile station. Complex spreaders  621  and  622  complex-spread the outputs of their associated Walsh cover parts  611  and  612 . A weight generator  651  generates weights C 1  and C 2  to be applied to the respective antennas, based on forward channel information fed back from the mobile station. Here, the feedback information can be either phase-related information or amplitude-related information. Complex multipliers  631  and  632  multiply the outputs of their associated complex spreaders  621  and  622  by the weights C 1  and C 2  provided from the weight generator  651 , respectively. The outputs of the complex multipliers  631  and  632  are modulated into RF band signals by RF parts  641  and  642 , respectively, and then radiated through first and second antennas ANT 1  and ANT 2 . 
     In IS-2000 Release A for the cdma2000 system, a common pilot channel is transmitted through a first antenna ANT 1 , while a diversity pilot channel is transmitted through a second antenna ANT 2 . The mobile station calculates weight information for the two antennas using the common pilot channel and the diversity pilot channel, and then transmits the calculated weight information to the base station. Then, the weight generator  651  in the base station creates the weights C 1  and C 2  based on the received weight information. 
     Comparing theoretical maximum performance of the transmit antenna diversity schemes, the closed loop transmit antenna diversity scheme of FIG. 6 is superior to the open loop transmit antenna diversity scheme of FIG. 1 by 3 dB in terms of SNR (signal-to-noise ratio) required to attain a given bit error rate (BER). However, in the case of a non-ideal, normal Doppler channel, the closed loop transmit diversity scheme cannot properly perform the transmit diversity due to delay of the feedback information in a high-speed fading channel environment where the mobile station moves at high speed, so it has lower performance than the open loop transmit diversity scheme. That is, in the environment where the mobile station moves at high speed, it is never possible to obtain a gain of the closed loop transmit diversity. Therefore, there is a demand for a transmit antenna diversity method capable of obtaining a diversity gain over the whole speed range, regardless of the moving speed of the mobile station. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a base station transmission apparatus and method for obtaining a transmit antenna diversity gain over the whole speed range, regardless of a moving speed of a mobile station in a CDMA mobile communication system. 
     It is another object of the present invention to provide a base station transmission apparatus and method for enabling the combined use of a closed loop antenna diversity scheme and an open loop antenna diversity scheme in a CDMA mobile communication system. 
     It is yet another object of the present invention to provide a base station transmission apparatus and method for obtaining a gain of a closed loop transmit antenna diversity scheme in a fading channel environment where a mobile station has a low speed, and obtaining a gain of an open loop transmit antenna diversity scheme in a channel environment where the mobile station has a high speed, in a CDMA mobile communication system. 
     According to a first object of the present invention, a base station transmission apparatus in a mobile communication system using transmit antenna diversity between a base station with a plurality of antennas and a mobile station, comprises a modulator for generating a complex symbol in response to a coded symbol; a first spreader for generating a plurality of different complex symbols in response to the complex symbol from the modulator, and generating a plurality of first spread complex symbols by spreading the plurality of the generated complex symbols with a first orthogonal code assigned to the mobile station; a second spreader for generating a plurality of same complex symbols being different from the plurality of the complex symbols in response to the complex symbol from the modulator, spreading the plurality of the same complex symbols with a second orthogonal code being different from the first orthogonal code, and generating a plurality of second spread complex symbols by multiplying the spread complex symbols by weights for the antennas, determined based on feedback information, received from the mobile station, indicating reception status of a base station signal; a summer for summing up the first complex symbols from the first spreader and the second complex symbols from the second spreader; and a transmitter for complex-spreading an output of the summer, shifting the complex-spread signals to a radio frequency band, and transmitting the shifted signals through the antennas. 
     According to a second object of the present invention, a base station transmission apparatus in a mobile communication system using transmit antenna diversity between a base station with a plurality of antennas and a mobile station, comprises a modulator for generating a complex symbol in response to a coded symbol; a serial-to-parallel converter for outputting two complex symbols with a reduced symbol rate by demultiplexing the complex symbol from the modulator; a first spreader for generating a plurality of different complex symbols in response to one complex symbol from the serial-to-parallel converter, and generating a plurality of first spread complex symbols by spreading the plurality of the complex symbols with a first sub-orthogonal code created from one orthogonal code assigned to the mobile station; a second spreader for generating a plurality of same complex symbols being different from the plurality of the complex symbols in response to another complex symbol from the serial-to-parallel converter, spreading the plurality of the same complex symbols with a second sub-orthogonal code being different from the first sub-orthogonal code, and generating a plurality of second spread complex symbols by multiplying the spread complex symbols by weights for the antennas, determined based on feedback information, received from the mobile station, indicating reception status of a base station signal; a summer for summing up the first complex symbols from the first spreader and the second complex symbols from the second spreader; and a transmitter for complex-spreading an output of the summer, shifting the complex-spread signals to a radio frequency band, and transmitting the shifted signals through the antennas. 
     According to a third object of the present invention, a base station transmission apparatus in a mobile communication system using transmit antenna diversity between a base station with a plurality of antennas and a mobile station, comprises a modulator for generating a complex symbol in response to a coded symbol; a diversity modulator for generating a plurality of different complex symbols in response to the complex symbol from the modulator; a Walsh cover part for generating a plurality of spread complex symbols by spreading the plurality of the complex symbols with an orthogonal code assigned to the mobile station; and a plurality of transmitters, a number of the transmitters being equal to a number of the complex symbols output from the Walsh cover part, for generating a plurality of complex symbols by multiplying one complex symbol from the Walsh cover part by weights for the antennas, determined based on feedback information, received from the mobile station, indicating reception status of a base station signal, complex-spreading the plurality of the complex symbols, shifting the complex-spread signals to a radio frequency band, and transmitting the shifted signals through the antennas associated with the weights. 
     According to a fourth object of the present invention, a base station transmission apparatus in a mobile communication system using transmit antenna diversity between a base station with a plurality of antennas and a mobile station, comprises a modulator for generating a complex symbol in response to a coded symbol; a diversity modulator for generating a plurality of different complex symbols in response to the complex symbol from the modulator; a spreader for generating a plurality of spread complex symbols by spreading the plurality of the complex symbols from the diversity modulator with an orthogonal code assigned to the mobile station; a switch for sequentially selecting the plurality of the complex symbols from the spreader in a given period; a complex multiplier for generating a plurality of weighted complex symbols by multiplying the complex symbol output from the switch by weights for the antennas, determined based on feedback information, received from the mobile station, indicating reception status of a base station signal; and a complex spreading and RF part for complex-spreading the plurality of the complex symbols from the complex multiplier, shifting the complex-spread signals to a radio frequency band, and transmitting the shifted signals through the antennas. 
    
    
     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 structure of a base station transmitter using an open loop transmit diversity scheme according to the prior art; 
     FIG. 2 illustrates a detailed structure of the Walsh cover part shown in FIG. 1; 
     FIG. 3 illustrates a detailed operation of the complex spreader shown in FIG. 1; 
     FIG. 4 illustrates an operation of a general STTD (Space-Time Transmit Diversity) modulator according to the prior art; 
     FIG. 5 illustrates an operation of a general STS (Space Time Spreader) modulator according to the prior art; 
     FIG. 6 illustrates a structure of a base station transmitter using a closed loop transmit diversity scheme according to the prior art; 
     FIG. 7 illustrates a transmit antenna diversity apparatus with two antennas according to an embodiment of the present invention, in which two Walsh codes are assigned to a mobile station, and an open loop antenna diversity apparatus is connected in parallel with a closed loop antenna diversity apparatus; 
     FIG. 8 illustrates a transmit antenna diversity apparatus with two antennas according to another embodiment of the present invention, in which one Walsh code is assigned to the mobile station, and the open loop antenna diversity apparatus is connected in parallel with the closed loop antenna diversity apparatus; 
     FIG. 9 illustrates a detailed operation of the serial-to-parallel converter shown in FIG. 8; 
     FIG. 10 illustrates a transmit antenna diversity apparatus with four antennas according to another embodiment of the present invention, in which the open loop antenna diversity apparatus is connected in series with the closed loop antenna diversity apparatus; and 
     FIG. 11 illustrates a transmit antenna diversity apparatus with two antennas according to another embodiment of the present invention, in which the open loop antenna diversity apparatus is connected in serial with the closed loop antenna diversity apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described herein below 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 combines a closed loop antenna diversity apparatus with an open loop antenna diversity apparatus capable of obtaining constant performance regardless of a speed of the mobile station. In a preferred embodiment of the present invention, the open loop antenna diversity apparatus compensates for performance deterioration of the closed loop antenna diversity apparatus due to high-speed movement of the mobile station, thereby to prevent abrupt performance deterioration. 
     FIG. 7 illustrates a base station transmission apparatus with two antennas according to an embodiment of the present invention, in which an open loop antenna diversity apparatus is connected in parallel with a closed loop antenna diversity apparatus so as to obtain both a gain of the open loop antenna diversity and a gain of the closed loop antenna diversity. As illustrated in FIG. 7, a channel transmitted by the open loop antenna diversity scheme is separated from a channel transmitted by the closed loop antenna diversity scheme by two different Walsh codes assigned to the mobile station. 
     Referring to FIG. 7, a channel encoder  701  encodes an input bit stream. A modulator  702  maps the coded symbols output from the channel encoder  701  into an M-ary complex symbol. Here, the modulator  702  serves as a QPSK, 8-PSK or 16-QAM modulator according to its data rate. An STTD/STS modulator (or diversity modulator)  703  modulates a complex symbol (I and Q sequences) output from the modulator  702  into two different complex symbols. The detailed operation of the STTD/STS modulator  703  has been described with reference to FIGS. 4 and 5. A Walsh cover part  704  spreads one complex symbol from the STTD/STS modulator  703  by multiplying it by a Walsh orthogonal code assigned to the mobile station. A Walsh cover part  705  spreads another complex symbol from the STTD/STS modulator  703  by multiplying it by the Walsh orthogonal code. 
     A weight generator  716  generates weights C 1  and C 2  to be applied to the two antennas, based on forward channel information fed back from the mobile station. Walsh cover parts  712  and  713  spread the complex symbol (I and Q sequences) output from the modulator  702  by multiplying them by a Walsh orthogonal code being different from the above Walsh orthogonal code assigned to the mobile station. A complex multiplier  714  multiplies the outputs of the Walsh cover part  712  by the weight C 1  from the weight generator  716 , and a complex multiplier  715  multiplies the outputs of the Walsh cover part  713  by the weight C 2  from the weight generator  716 . 
     A first summer (or Walsh chip level summer)  706  sums up the output of the Walsh cover part  704  and the output of the complex multiplier  714  in a Walsh chip level, and a second summer  707  sums up the output of the Walsh cover part  705  and the output of the complex multiplier  715  in a Walsh chip level. Complex spreaders  708  and  709  complex-spread the outputs of the first and second summers  706  and  707 , respectively. The detailed operation of the complex spreaders  708  and  709  has been described with reference to FIG.  3 . RF parts  710  and  711  shift the outputs of the associated complex spreaders  708  and  709  to an RF band. The shifted RF band signals are transmitted to the mobile station through their associated antennas. 
     An operation of the structure illustrated in FIG. 7 will be described below The coded symbols output from the channel encoder  701  are modulated into a complex symbol by the modulator  702 . The complex symbols (I and Q sequences) output from the modulator  702  is simultaneously provided to a first spreading part and a second spreading part. The first spreading part is comprised of the STTD/STS modulator  703  and the Walsh cover parts  704  and  705 , while the second spreading part is comprised of the Walsh cover parts  712  and  713  and the complex multipliers  714  and  715 . The signals provided to the first and second spreading parts are multiplied by two Walsh codes W N   i  and W N   j  assigned to the mobile station, respectively, so that they are separated into two interference-free channels. Thereafter, the first summer  706  sums up the signals to be transmitted through a first antenna ANT 1  among the output signals of the first and second spreading parts, while the second summer  707  sums up the signals to be transmitted through a second antenna ANT 2  among the output signals of the first and second spreading parts. The outputs of the first and second summers  706  and  707  are subject to complex spreading by the complex spreader  708  and  709 , and then shifted to an RF band through the RF parts  710  and  711 . The shifted RF band signals are transmitted through the first and second antennas ANT 1  and ANT 2 . 
     FIG. 8 illustrates a structure of a base station transmission apparatus with two antennas according to another embodiment of the present invention, in which an open loop antenna diversity apparatus is connected in parallel with a closed loop antenna diversity apparatus to obtain both a gain of the open loop antenna diversity and a gain of the closed loop antenna diversity. As illustrated in FIG. 8, a channel transmitted by the open loop antenna diversity scheme is separated from a channel transmitted by the closed loop antenna diversity scheme, using two sub-Walsh codes created from one Walsh code assigned to the mobile station. 
     Referring to FIG. 8, a channel encoder  801  encodes an input bit steam into coded symbols. A modulator  802  maps the coded symbols from the channel encoder  802  into an M-ary complex symbol. Here, the modulator  802  serves as a QPSK, 8-PSK or 16-QAM modulator according to its data rate. A serial-to-parallel (S/P) converter  803  separates the complex symbol output from the modulator  802  into two complex symbols. 
     More specifically, as illustrated in FIG. 9, the serial-to-parallel converter  803  repeats twice a symbol Si 0  received through an I channel and provides the repeated symbols to the first spreading part through the I channel, and also repeats twice a symbol Si 1  received through the I channel and provides the repeated symbols to the second spreading part through the I channel. Further, the serial-to-parallel converter  803  repeats twice a symbol Sq 0  received through a Q channel and provides the repeated symbols to the first spreading part through the Q channel, and also repeats twice a symbol Sq 1  received through the Q channel and provides the repeated symbols to the second spreading part through the Q channel. That is, the symbol streams provided to the respective spreading parts have ½ symbol rate, as compared with the input stream of the serial-to-parallel converter  803 . 
     An STTD/STS modulator  804  modulates the complex symbol (I and Q sequences) output from the serial-to-parallel converter  803  into two different complex symbols. A detailed operation of the STTD/STS modulator  804  has been described with reference to FIGS. 4 and 5. A Walsh cover part  805  orthogonally spreads one complex symbol output from the STTD/STS modulator  803  by multiplying it by a first Walsh code out of two sub-Walsh codes created from one Walsh code assigned to the mobile station. A Walsh cover part  806  orthogonally spreads another complex symbol output from the STTD/STS modulator  803  by multiplying it by the first Walsh code. 
     For example, if it is assumed that a Walsh code W N   i  with a length N is assigned to the mobile station, then two sub-Walsh orthogonal codes W N   i W N   i  and W N   i {overscore (W N   i )} with a length 2N are created from the Walsh code W N   i . Here, for a binary symbol xε{−1,1}, {overscore (x)}=−x. 
     A weight generator  817  generates weights C 1  and C 2  to be applied to the respective antennas, based on forward channel information fed back from the base station. Walsh cover parts  813  and  814  orthogonally spread the complex symbol (I and Q sequences) output from the serial-to-parallel converter  803  by multiplying it by a second Walsh code out of the two sub-Walsh codes. Here, the first Walsh code used in the Walsh cover parts  805  and  806  is different from the second Walsh code used in the Walsh cover parts  813  and  814 . A complex multiplier  815  multiplies the output of the Walsh cover part  813  by the weight C 1  from the weight generator  817 , and the complex multiplier  816  multiplies the output of the Walsh cover part  814  by the weight C 2  from the weight generator  817 . 
     A first summer (or Walsh chip level summer)  807  sums up the output of the Walsh cover part  805  and the output of the complex multiplier  815  in a Walsh chip level, and a second summer  808  sums up the output of the Walsh cover part  806  and the output of the complex multiplier  816  in a Walsh chip level. Complex spreaders  809  and  810  complex-spread the outputs of their associated summers  807  and  808 . A detailed operation of the complex spreaders  809  and  810  has been described with reference to FIG.  3 . RF parts  811  and  812  shift the outputs of the associated complex spreaders  809  and  810  to an RF band, and the shifted RF band signals are transmitted to the mobile station through the associated antennas ANT 1  and ANT 2 . 
     An operation of the structure shown in FIG. 8 will be described below. The coded symbols output from the channel encoder  801  are modulated into a complex symbol by the modulator  802 . The complex symbol output from the modulator  802  is divided into two complex symbols by the serial-to-parallel converter  803 , reducing the symbol rate to ½. The two complex symbols are simultaneously provided to the first and second spreading parts. Here, the first spreading part includes the STTD/STS modulator  804  and the Walsh cover parts  805  and  806 , while the second spreading part includes the Walsh cover parts  813  and  814  and the complex multipliers  815  and  816 . The complex symbols provided to the first and second spreading parts are multiplied by the above stated Walsh codes, so that they are separated into interference-free two channels. Thereafter, the first summer  807  sums up the signals to be transmitted through the first antenna ANT 1  among the outputs of the first and second spreading parts, and the second summer  808  sums up the signals to be transmitted through the second antenna ANT 2  among the outputs of the first and second spreading parts. The outputs of the first and second summers  807  and  808  are subject to complex spreading by the complex spreaders  809  and  810 , and then shifted to an RF band through the RF parts  811  and  812 . The shifted RF band signals are transmitted through the first and second antennas ANT 1  and ANT 2 . 
     FIG. 10 illustrates a structure of a base station transmission apparatus with four antennas according to another embodiment of the present invention, in which an open loop antenna diversity apparatus is connected in series with a closed loop antenna diversity apparatus so as to obtain both a gain of the open loop antenna diversity and a gain of the closed loop antenna diversity. 
     Referring to FIG. 10, a channel encoder  1001  encodes an input bit stream into coded symbols. A modulator  1002  maps the coded symbols from the channel encoder  1001  into an M-ary complex symbol. Here, the modulator  1001  serves as a QPSK, 8-PSK or 16-QAM modulator according to its data rate. An STTD/STS modulator  1003  modulates the complex symbol (I and Q sequences) output from the modulator  1002  into two different complex symbols. A detailed operation of the STTD/STS modulator  1003  has been described with reference to FIGS. 4 and 5. A weight generator  1011  generates weights C 1 , C 2 , C 3  and C 4  to be applied to the respective antennas, based on forward channel information fed back from the mobile station. 
     A Walsh cover part  1004  spreads one complex symbol from the STTD/STS modulator  1003  by multiplying it by a Walsh orthogonal code assigned to the mobile station. A complex multiplier  1005  multiplies the output of the Walsh cover part  1004  by the weight C 1  from the weight generator  1011 , and a complex multiplier  1006  multiplies the output of the Walsh cover part  1004  by the weight C 2  from the weight generator  1011 . Complex spreaders  1007  and  1008  complex-spread the outputs of the associated complex multipliers  1005  and  1006 , respectively. A detailed operation of the complex spreaders  1007  and  1008  has been described with reference to FIG.  3 . RF parts  1009  and  1010  shift the outputs of their associated complex spreaders  1007  and  1008  to an RF band. The shifted RF band signals are transmitted to the mobile station through their associated antennas ANT 1  and ANT 2 . 
     A Walsh cover part  1012  spreads another complex symbol from the STTD/STS modulator  1003  by multiplying it by the Walsh code assigned to the mobile station. A complex multiplier  1013  multiplies the output of the Walsh cover part  1012  by the weight C 3  from the weight generator  1011 , and a complex multiplier  1014  multiplies the output of the Walsh cover part  1012  by the weight C 4  from the weight generator  1011 . Complex spreaders  1015  and  1016  complex-spread the outputs of the associated complex multipliers  1013  and  1014 , respectively. A detailed operation of the complex spreaders  1015  and  1016  has been described with reference to FIG.  3 . RF parts  1017  and  1018  shift the outputs of their associated complex spreaders  1015  and  1016  to an RF band. The shifted RF band signals are transmitted to the mobile station through their associated antennas ANT 3  and ANT 4 . 
     An operation of the structure shown in FIG. 10 will be described below. The coded symbols output from the channel encoder  1001  are modulated into a complex symbol by the modulator  1002 . Further, the complex symbol is modulated into two different complex symbols by the STTD/STS modulator  1003 . Here, one complex symbol out of the two complex symbols output from the STTD/STS modulator  1003  is provided to a first transmission part, while another complex symbol is provided to a second transmission part. The first transmission part includes the Walsh cover part  1004 , the complex multipliers  1005  and  1006 , the complex spreaders  1007  and  1008 , and the RF parts  1009  and  1010 . The second transmission part includes the Walsh cover part  1012 , the complex multipliers  1013  and  1014 , the complex spreaders  1015  and  1016 , and the RF parts  1017  and  1018 . The two complex signals output from the first transmission part are transmitted through the first and second antennas ANT 1  and ANT 2 , while the two complex signals output from the second transmission part are transmitted through the third and fourth antennas ANT 3  and ANT 4 . That is, the base station transmits a signal to the mobile station through a total of 4 antennas. 
     In IS-2000 Release A for the cdma2000 system, a common pilot channel is transmitted through a first antenna ANT 1 , while a diversity pilot channel is transmitted through a second antenna ANT 2 . The mobile station calculates weight information for the two antennas ANT 1  and ANT 2  using the common pilot channel and the diversity pilot channel, and then transmits the calculated weight information to the base station. If the structure of FIG. 10 is applied to a system supporting the IS-200 Release A, an auxiliary pilot channel is assigned to the third antenna ANT 3  and an auxiliary diversity pilot channel is assigned to the fourth antenna ANT 4 . The mobile station calculates weight information for the third and fourth antennas ANT 3  and ANT 4  using the auxiliary pilot channel and the auxiliary diversity pilot channel, and then transmits the calculated weight information to the base station. The weight generator  1011  shown in FIG. 10 generates the weights C 1 , C 2 , C 3  and C 4  to be applied to the respective antennas, based on the weight information for the first to fourth antennas ANT 1 -ANT 4 . The base station structure shown in FIG. 10 transmits each of the two channels separated by the STTD/STS modulator  1003  through two associated antennas, thereby guaranteeing a higher gain than when only the open loop antenna diversity is used. 
     FIG. 11 illustrates a structure of a transmit antenna diversity apparatus with two antennas according to another embodiment of the present invention, in which an open loop antenna diversity apparatus is connected in serial with a closed loop antenna diversity apparatus so as to obtain both a gain of the open loop antenna diversity and a gain of the closed loop antenna diversity. 
     Referring to FIG. 11, a channel encoder  1101  encodes an input bit stream into coded symbols. A modulator  1102  maps the coded symbols output from the channel encoder  1101  into an M-ary complex symbol. Here, the modulator  1102  serves as a QPSK, 8-PSK or 16-QAM modulator according to its data rate. An STTD/STS modulator  1103  modulates the complex symbol (I and Q sequences) output from the modulator  1102  into two different complex symbols. A detailed operation of the STTD/STS modulator  1003  has been described with reference to FIGS. 4 and 5. A weight generator  1113  generates weights C 1  and C 2  to be applied to the respective antennas, based on forward channel information fed back from the mobile station. 
     A switch  1106  selects one of the complex symbols output from a Walsh cover part  1104  and a Walsh cover part  1105 , and provides the selected complex symbol to complex multipliers  1107  and  1108 . The switch  1106 , under the control of an upper layer controller (not shown), performs a switching operation at a 1× or 2× Walsh chip rate. When the switch  1106  operates at the 1× Walsh chip rate, only half of the output symbols of the STTD/STS modulator  1103  are transmitted. However, when the switch  1106  operates at the 2× Walsh chip rate, all of the output symbols of the STTD/STS modulator  1103  are transmitted. Further, the switch  1106  can also perform the switching operation in a unit of a predetermined number of chips (e.g., in a symbol unit). 
     The complex multiplier  1107  multiplies the complex symbol provided from the switch  1106  by the weight C 1  output from the weight generator  1113 . The complex multiplier  1108  multiplies the complex symbol received from the switch  1106  by the weight C 2  provided from the weight generator  1113 . Complex spreader  1109  and  1110  complex-spread the outputs of their associated complex multipliers  1107  and  1108 . A detailed operation of the complex spreaders  1109  and  1110  has been described with reference to FIG.  3 . RF parts  1111  and  1112  shift the outputs of their associated complex spreaders  1109  and  1110  to an RF band. The shifted RF band signals are transmitted to the mobile station through the associated antennas ANT 1  and ANT 2 . 
     An operation of the structure shown in FIG. 11 will be described below. The coded symbols output from the channel encoder  1101  are modulated into a complex symbol (I and Q sequences) by the modulator  1102 . Further, the complex symbol is modulated into two different complex symbols by the STTD/STS modulator  1103 , and the two complex symbols subjected to orthogonal spreading through the Walsh cover parts  1104  and  1105 , respectively. The two spread complex symbols are alternately provided to the complex multipliers  1107  and  1108  through the switch  1106 . The switch  1106  can either perform the switching operation at a 1× or 2× Walsh chip rate or perform the switching operation in a symbol unit. Thereafter, the outputs of the switch  1106  are multiplied by the weights C 1  and C 2  by the complex multipliers  1107  and  1108 . The weighted complex signals are subject to complex spreading, and then shifted to an RF band. The shifted RF band signals are transmitted through the first and second antennas ANT 1  and ANT 2 . The structure of FIG. 11 provides a serial connection-type antenna diversity technique capable of obtaining both a gain of the open loop antenna diversity and a gain of the closed loop antenna diversity, while minimizing the hardware complexity. 
     As described above, when the CDMA mobile communication system uses a combined antenna diversity apparatus of the closed loop antenna diversity apparatus and the open loop antenna diversity apparatus, it is possible to prevent abrupt performance deterioration in spite of an increase in a moving speed of the mobile station. Therefore, compared with a system not using the antenna diversity apparatus, the system according to the present invention shows superior performance over the whole speed rage of the mobile station, and can prevent performance degradation at a specific speed, which may be caused in a system using only the open loop or closed loop antenna diversity apparatus. As a result, compared with when only the open loop or closed loop antenna diversity apparatus is used, it is possible to increase data throughput of the system and expand an available service area. 
     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.