Abstract:
There is provided a radio station and a radio network controller performing communication by CDMA, able to change transmission speed in response to the propagation environment, and carry on communications without communication quality degradation. The radio station comprises a signal quality reception unit for receiving information of quality of the signals having been transmitted by the radio station, received by a party thereof, and sent to the radio station by the party, in which the information is measured from the received signals by the party, and a code multiplicity determination unit configured to determine a code multiplicity of signals transmitted from the radio station to the party based on the received information of signal quality. The radio station transmits the signals to the party using the determined code multiplicity.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a radio network controller and a radio station able to control radio resources assignment of spread codes according to the propagation environment in communications using CDMA (Code Division Multiple Access).  
           [0003]    2. Description of the Related Art  
           [0004]    W-CDMA (Wideband Code Division Multiple Access) is a well known mobile communication scheme using CDMA. In W-CDMA, FDD (Frequency Division Duplex) is adopted for duplex communications. In addition to FDD, there is also a duplex system known as TDD (Time Division Duplex). In TDD, transmission frequencies are the same with reception frequencies, and these frequencies are divided by time and are used for interactive uplink and downlink communications.  
           [0005]    In the IMT-2000 mobile communication system that is being standardized by the 3GPP (3rd Generation Partnership Project), which is a standards organization for the third generation mobile and wireless communication systems, in addition to the W-CDMA that utilizes the FDD, there is also a TD-CDMA (Time Division-Code Division Multiple Access) system utilizing the TDD. Since the TD-CDMA has the TDMA structure, it is able to assign users slot by slot, and hence able to assign each of a number of users a different time slot. Further, since code multiplexing is enabled at each slot, high-speed data transmission can be achieved by multiplexing codes at a time slot used by one user. Therefore, the TD-CDMA system enables flexible assignment of resources in response to traffic asymmetry in the downlink and uplink channels, and is considered to be superior to the FDD in respect of asymmetric traffic capacity.  
           [0006]    In a CDMA system, because signals from other users act as interference, transmission power control is performed by keeping constant the ratio of the power of signals desired to be received at a mobile station or a base station to the power of interference signals emitted from other mobile stations or base stations. This ratio is known as SIR, standing for Signal to Interference Ratio. Therefore, in the CDMA system, the signal transmission power becomes low when the interference power becomes low, and the signal transmission power becomes high when the interference power becomes high.  
           [0007]    In the CDMA system, the transmission data sequence is multiplied with spread codes (just abbreviated as “codes” below) and is spread to wideband signals for transmission. The high-rate data sequence in the spread bandwidth is called a chip, and the speed of variation of the spread data is called chip rate. The ratio of the chip rate to the symbol rate is called spreading factor, and is abbreviated as SF. If the chip rate after spreading is a constant, the amount of data able to be transmitted increases when the spreading factor decreases. Whereas, since the spreading factor deceases, the SIR needed to satisfy the desired quality requirements becomes larger.  
           [0008]    In other words, under the transmission power control, the signal transmission power decreases when the interference power becomes low, so the interference to other users is reduced. On the other hand, under the condition that the transmission power control is not performed, if the signal transmission power is kept to be high even if the interference power becomes low, as a result, SIR increases. In this case, a larger amount of data can be transmitted by reducing the spreading factor SF.  
           [0009]    In the TD-CDMA system as shown above, because users are divided by time slots, the interference between users can be relatively well suppressed. So, the amount of data to be transmitted (or the transmission speed) can be increased by keeping the signal transmission power constant when the interference power is low, and reducing the spreading factor SF while not performing transmission power control. This technique has been disclosed, for example, in the Japanese Unexamined Patent Publication (Kokai) No.2000-31884.  
           [0010]    In the technique disclosed in the above publication, however, a base station needs to notify a mobile station of a change of the spreading factor, so overhead increases, and the data transmission speed decreases. Further, when a mobile station in communication with a base station moves and the interference power changes, the spreading factor in communication changes constantly, so the change of the spreading factor has to be processed in a short time. But to do that, the processing system, such as modulation and demodulation devices, becomes complicated, and the scale of the devices increases.  
         SUMMARY OF THE INVENTION  
         [0011]    Accordingly, it is a general object of the present invention to solve the above problems of the related art.  
           [0012]    A more specific object of the present invention is to provide a radio network controller and a radio station able to change a transmission speed in response to the propagation environment, and carry on communications without communication quality degradation.  
           [0013]    To attain the above object, according to a first aspect of the present invention, there is provided a radio station configured to transmit signals to and receive signals from a party thereof through a radio link by Code Division Multiple Access, comprising a signal quality reception unit for receiving information of quality of the signals that have been transmitted by the radio station, received and sent to the radio station by the party, said information being measured from the received signals by the party, and a code multiplicity determination unit for determining a code multiplicity of signals transmitted from the radio station to the party based on the received information of signal quality, the radio station transmitting signals to the party by using the determined code multiplicity.  
           [0014]    Preferably, in the above radio station, the code multiplicity determination unit comprises a storage unit for storing a plurality of code multiplicities in correspondence with data of said quality of signals.  
           [0015]    Preferably, in the above radio station, the quality of signals includes one of a ratio of signal to noise, a ratio of signal to interference, and a signal error rate.  
           [0016]    According to the first aspect of the present invention, at a radio station, such as a mobile station or a base station, the code multiplicity of downlink or uplink transmission signals is determined based on the downlink or uplink SIR or other signal quality information sent by a party of the radio station. That is, the information of SIR or others of one radio station is provided by its party, and the code multiplicity is determined in response to the condition of propagation environment. Therefore, for a radio station located in a good propagation environment, a code multiplicity is assigned to allow higher-speed communication, and for a radio station located in a poor propagation environment, a code multiplicity is assigned to allow communications at relatively lower speed to secure communication quality.  
           [0017]    To attain the above object, according to a second aspect of the present invention, there is provided a radio station configured to transmit signals to and receive signals from a party thereof through a radio link by Code Division Multiple Access, comprising a propagation loss calculation unit for calculating loss of power of signals in propagation from the radio station to the party, a reception power estimation unit for estimating signal reception power of the party using the signal transmission power of the radio station and the measured loss of power, and a code multiplicity determination unit for determining a code multiplicity of signals transmitted from the radio station to the party based on the estimated signal reception power of the party, the radio station transmitting signals to the party by using the determined code multiplicity.  
           [0018]    Preferably, in the above radio station, the propagation loss calculation unit comprises a first unit configured to receive signals from the party and measure signal reception power of the radio station, a second unit configured to receive data of signal transmission power of the party, and a third unit for calculating the loss of power of signals in propagation using the signal reception power of the radio station and the signal transmission power of the party.  
           [0019]    Alternatively, in the above radio station, the propagation loss calculation unit comprises a fourth unit for receiving data of a distance between the radio station and the party, and said propagation loss deduction unit deduces the loss of power from the distance between the radio station and the party.  
           [0020]    Preferably, in the above radio station, the code multiplicity determination unit comprises a storage unit for storing a plurality of code multiplicities in correspondence with data of said signal reception power of the party.  
           [0021]    According to the second aspect of the present invention, a radio station, such as a mobile station or a base station, predicts the propagation environment between itself and its party from the reception power of the received signals, and determines the code multiplicity of the downlink or uplink transmission signals. That is to say, the propagation environment is predicted from the amplitudes (power) of the received signals, and the code multiplicity is determined in response to the propagation environment. Therefore, code multiplicity may be increased in a good propagation environment, and may be decreased in a poor propagation environment. As a result, it is possible to provide communication service of sufficiently high quality constantly and a communication speed fit for the propagation environment.  
           [0022]    To attain the above object, according to a third aspect of the present invention, there is provided a radio network controller configured to control signal transmission and signal reception between radio stations through a radio link by Code Division Multiple Access, comprising a signal quality reception unit for receiving information of quality of signals having been transmitted by a first radio station, received and sent to the radio network controller by a second radio station, said information being measured by the second radio station from the received signals, a code multiplicity determination unit for determining a code multiplicity of signals transmitted from the first radio station to the second radio station based on the received information of signal quality; and a unit for sending the determined code multiplicity to the first radio station.  
           [0023]    According to the third aspect of the present invention, at the radio network controller, the code multiplicity of uplink or downlink transmission signals is determined based on the uplink or downlink signal quality sent between two radio stations. That is, the information of signal quality of one radio station is provided by its party, and the code multiplicity is determined by the radio network controller in response to the condition of interference signals. Therefore, for a radio station located in a good propagation environment, a code multiplicity is assigned by the radio network controller to allow higher-speed communication, and for a radio station located in a poor propagation environment, a code multiplicity is assigned by the radio network controller to allow communications at relatively lower speed to prevent communication quality from being degraded.  
           [0024]    These and other objects, features, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments given with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a schematic view showing a mobile communication system according to embodiments of the present invention;  
         [0026]    [0026]FIG. 2 is a block diagram showing an example of a configuration of the mobile communication system according to a first embodiment of the present invention;  
         [0027]    [0027]FIG. 3 is an example of a code assignment table;  
         [0028]    [0028]FIG. 4 is a sequence chart showing an example of the procedure of determining the code multiplicity of downlink transmission signals from the base station to the mobile station;  
         [0029]    [0029]FIG. 5 is a sequence chart showing an example of the procedure of determining the code multiplicity of uplink transmission signals from the mobile station to the base station;  
         [0030]    [0030]FIG. 6 is a sequence chart showing another example of the procedure of determining the code multiplicity of downlink transmission signals from the base station to the mobile station;  
         [0031]    [0031]FIG. 7 is a sequence chart showing another example of the procedure of determining the code multiplicity of uplink transmission signals from the mobile station to the base station;  
         [0032]    [0032]FIG. 8 is a schematic view showing an example of operation of a mobile communication system for determining a code multiplicity of uplink transmission signals from a mobile station to a base station, according to a second embodiment of the present invention;  
         [0033]    [0033]FIG. 9 is a sequence chart showing an example of the operation of the mobile communication system shown in FIG. 8 for determining the code multiplicity of the uplink transmission signals from a mobile station to a base station;  
         [0034]    [0034]FIG. 10 is a sequence chart showing an example of the procedure of determining a code multiplicity of downlink transmission signals from the base station to the mobile station, according to the second embodiment of the present invention; and  
         [0035]    [0035]FIG. 11 is a block diagram showing another example of the configuration of the mobile communication system according to the second embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    Below, preferred embodiments of the present invention will be explained with reference to the accompanying drawings.  
         [0037]    [0037]FIG. 1 is a schematic view showing a configuration of a mobile communication system according to embodiments of the present invention.  
         [0038]    For example, the mobile communication system shown in FIG. 1 is a TD-CDMA system, and is comprised of a base station  11   a , a base station  11   b , a mobile station  12 , and a radio network controller  21 . The base station  11   a  and base station  11   b  have service areas (called “cell”)  13   a  and  13   b , respectively. In this example, the mobile station  12  is located in the cell  13   a  of the base station  11   a , and receives signals from or transmits signals to the base station  11   a  by CDMA through radio links. The radio network controller  21  is connected with a number of base stations (base stations  11   a  and  11   b  in this example), and controls the radio connections with them.  
         [0039]    Below, explanations are made of examples of embodiments of the present invention applied to the above system. Note that in the following explanations, the same reference numerals are used to represent the same elements.  
       The First Embodiment  
       [0040]    [0040]FIG. 2 is a block diagram showing an example of a configuration of the mobile communication system according to a first embodiment of the present invention.  
         [0041]    As shown in FIG. 2, the mobile communication system shown in FIG. 1 includes the radio network controller  21 , the base station  11   a  and the mobile station  12 .  
         [0042]    The radio network controller  21  is comprised of a controller (abbreviated as CPU)  24 , a memory (abbreviated as MEM)  25  connected to the controller  24  for storing, for example, a code assignment table (abbreviated as TBL)  22  as described below, an interface  26  for connection with the base station  11   a . The interface  26  is connected to the controller  24 .  
         [0043]    The base station  11   a  is comprised of a controller (abbreviated as CPU)  10 , a memory (abbreviated as MEM)  9  connected to the controller  10  for storing data, an interface  7  for connection with the radio network controller  21 , a radio set  6  connected to the controller  10 , and an antenna  5  for receiving and transmitting radio signals, for example, by the TDD transmission method through radio links. The interface  7  is connected to the controller (abbreviated as CPU)  10 . Optionally, as described later, a code assignment table (abbreviated as TBL)  8  may be stored in the memory  9 .  
         [0044]    The radio network controller  21  and the base station  11   a  are connected to each other through the interface  26  and the interface  7 .  
         [0045]    The mobile station  12  is comprised of a controller (abbreviated as CPU)  13 , a memory (abbreviated as MEM)  14  connected to the controller  13  for storing data, a radio set  16  connected to the controller  13 , and an antenna  17  for receiving and transmitting radio signals, for example, by the TDD transmission method through radio links. Optionally, a code assignment table (abbreviated as TBL)  15  may be stored in the memory  14  as described below.  
         [0046]    Controlled by the controller  10 , the radio set  6  performs necessary processing, such as receiving and transmitting, encoding and decoding, modulating and demodulating, interleaving and deinterleaving radio signals. Similarly, radio set  16  performs similar processing under the control of the controller  13 .  
         [0047]    The radio network controller  21  is connected to a number of base stations, such as the base station  11   a  and  11   b , and controls their radio connections.  
         [0048]    In this example, the radio network controller  21  has a code assignment table  22 , which records the code multiplicity k associated with an SIR provided by the mobile station  12  through the base station  11   a.    
         [0049]    [0049]FIG. 3 shows an example of the code assignment table  22 .  
         [0050]    As shown in FIG. 3, the code assignment table  22  has the two major fields of SIR and code multiplicity k, listing the calculated optimal code multiplicity k for a given SIR. In the table shown in FIG. 3, the desired code multiplicity k, which is expressed as “modified code multiplicity”, is calculated for each SIR on the basis of the code multiplicity being used presently, which is expressed as “present code multiplicity” in FIG. 3.  
         [0051]    Below, a method for calculating the modified code multiplicity k is explained in detail.  
       Method of Calculating Code Multiplicity k  
       [0052]    In this example, a quantity SIR_req is defined to represent the required SIR per code for desired communication quality at the time when a signal is received, and a quantity SIR_rx is defined to represent the actual SIR for each code when a signal is received. If the code multiplicity before modification (that is, the present code multiplicity) is denoted as k′, the modified code multiplicity k can be determined on condition that the following formula is satisfied. 
           k ′*SIR —   rx=k *SIR_req 
         [0053]    That is, the modified code multiplicity k is determined as below. 
           k=k ′*(SIR —   rx /SIR_req)  (1) 
         [0054]    From equation (1), the optimal code multiplicity k can be calculated if the present code multiplicity k′, SIR-req, and SIR_rx are known. The value of SIR_req is determined by simulations or by experimental measurements.  
         [0055]    Next, following the sequence chart in FIG. 4, explanations are made of the operation for determining the code multiplicity k when signals are transmitted from the base station  11   a  to the mobile station  12 .  
         [0056]    [0056]FIG. 4 is a sequence chart showing an example of the procedure of determining the code multiplicity of downlink transmission signals from the base station  11   a  to the mobile station  12 .  
         [0057]    In the following explanations referring to the flow chart in FIG. 4, it is assumed that the present code multiplicity k′ is a known quantity.  
         [0058]    In FIG. 4, the base station is abbreviated as BS, the mobile station is abbreviated as MS, and the radio network controller is abbreviated as RNC.  
         [0059]    In the following explanation, the transmission power for transmitting signals from the base station  11   a  to the mobile station  12  is represented by Ptx 11 .  
         [0060]    In step S 11  as shown in FIG. 4, in the course of communication or at the beginning of communication between the mobile station  12  and the base station  11   a , the mobile station  12  measures the power Prx 11  of the received signals from the base station  11   a  and the power of the interference signals, and calculates the SIR.  
         [0061]    In step S 12 , the mobile station  12  notifies the radio network controller  21  of the calculated value of SIR (that is, the SIR_rx) through the base station  11   a.    
         [0062]    In step S 13 , after receiving the calculated value of SIR from the base station  11   a , the radio network controller  21  makes reference to code assignment table  22  stored in memory  25 , and determines the code multiplicity k corresponding to the specified SIR.  
         [0063]    In step S 14 , the radio network controller  21  sends information of the code multiplicity k to the base station  11   a.    
         [0064]    In step S 15 , the base station  11   a  uses the code multiplicity k sent from the radio network controller  21  to transmit signals to the mobile station  12  with the transmission power Ptx.  
         [0065]    Note that, since the value of the SIR_req depends on the type of services, the SIR_req may have several values. In this case, in order to determine the code multiplicity k, the SIR_req needs be determined first, and then the code multiplicity matching the determined SIR_req can be found by referring to the code assignment table  22  in FIG. 3.  
         [0066]    In the above example, an explanation is made of a case in which the downlink transmission signal code multiplicity is determined. The same method is applicable to determination of the signal code multiplicity for uplink transmission, that is, signal transmission from the mobile station  12  to the base station  11   a.    
         [0067]    Next, following the sequence chart in FIG. 5, explanations are made of the operation for determining the code multiplicity k when signals are transmitted from the mobile station  12  to the base station  11   a.    
         [0068]    [0068]FIG. 5 is a sequence chart showing an example of the procedure of determining the code multiplicity of uplink transmission signals from the mobile station to the base station;  
         [0069]    [0069]FIG. 5 is basically the same as FIG. 4, except that the roles of the base station  11   a  and the mobile station  12  are reversed compared with FIG. 4. In this case, the code assignment table to be used should be modified relative to the code assignment table  22 .  
         [0070]    In step S 21  as shown in FIG. 5, in the course of communication or at the beginning of communication between the mobile station  12  and the base station  11   a , the base station  11   a  measures the power Prx 12  of the received signals from the mobile station  12  and the power of the interference signals, and calculates the SIR.  
         [0071]    In step S 22 , the base station  11   a  notifies the radio network controller  21  of the calculated value of SIR (that is, the SIR_rx)  
         [0072]    In step S 23 , after receiving the calculated value of SIR from the base station  11   a , the radio network controller  21  makes reference to code assignment table  22  stored in memory  25 , and determines the code multiplicity k corresponding to the specified SIR.  
         [0073]    In step S 24 , the radio network controller  21  sends information of the code multiplicity k to the mobile station  12  through the base station  11   a.    
         [0074]    In step S 25 , the mobile station  12  uses the code multiplicity k sent from the radio network controller  21  to transmit signals to the base station  11   a  with a specified transmission power.  
         [0075]    In the above examples, explanations are made of cases in which the downlink or uplink transmission signal code multiplicity is determined in the radio network controller  21 . The signal code multiplicity k may also be determined by the base station  11   a  or the mobile station  12 . In this case, the base station  11   a  or the mobile station  12  needs to have a code assignment table stored in its memory, as done above by the radio network controller  21 .  
         [0076]    Next, following the sequence chart in FIG. 6, an explanation is made of the operation for determining the code multiplicity k by the base station  11   a  when signals are transmitted from the base station  11   a  to the mobile station  12 . In this case, the code assignment table  8  similar with the code assignment table  22  stored in the memory  9  of the base station  11   a  is used for determining the code multiplicity.  
         [0077]    In step S 31  as shown in FIG. 6, in the course of communication or at the beginning of communication between the mobile station  12  and the base station  11   a , the mobile station  12  measures the power of the received signals from the base station  11   a  and the power of the interference signals, and calculates the SIR.  
         [0078]    In step S 32 , the mobile station  12  notifies the base station  11   a  of the calculated value of SIR (that is, the SIR_rx) through the radio line.  
         [0079]    In step S 33 , after receiving the deduced value of SIR from the mobile station  12 , the base station makes reference to the code assignment table  8  stored in the memory  9 , and determines the code multiplicity k corresponding to the specified SIR.  
         [0080]    In step S 34 , the base station  11   a  uses the determined code multiplicity k to transmit signals to the mobile station  12  with a specified transmission power.  
         [0081]    Next, following the sequence chart in FIG. 7, an explanation is made of the operation for determining the code multiplicity k by the mobile station  12  when signals are transmitted from the mobile station  12  to the base station  11   a . In this case, the code assignment table  15  similar to the code assignment table  22  stored in the memory  14  of the mobile station  12  is used for determining the code multiplicity.  
         [0082]    In step S 41  as shown in FIG. 7, in the course of communication or at the beginning of communication between the mobile station  12  and the base station  11   a , the base station  11   a  measures the power of the received signals from the mobile station  12  and the power of the interference signals, and calculates the SIR.  
         [0083]    In step S 42 , the base station  11   a  notifies the mobile station  12  of the calculated value of SIR (that is, the SIR_rx) through the radio line.  
         [0084]    In step S 43 , after receiving the calculated value of SIR from the base station  11   a , the mobile station makes reference to the code assignment table  15  stored in the memory 14, and determines the code multiplicity k corresponding to the specified SIR.  
         [0085]    In step S 44 , the mobile station  12  uses the determined code multiplicity k to transmit signals to the base station  11   a  with a specified transmission power.  
         [0086]    In the above embodiment, the communication system is a TD-CDMA system. In a TD-CDMA system, the TDD is used in which the frequencies of the downlink and uplink channels are the same, so, the propagation environment of the downlink channels is similar to that of the uplink case. Therefore, from the uplink signals received by a base station, it is possible to predict SIR or amplitudes of the downlink signals from the base station when the downlink signals are received by a mobile station. In other words, when applying the method of the present embodiment to a system in which TDD is used, the mobile station  12  may use the code multiplicity k of the signals received from the base station  11   a  as the code multiplicity of the uplink signals to be transmitted from the mobile station  12  to the base station  11   a.    
         [0087]    Furthermore, in the above embodiment, as for the quality of signals, in addition to SIR (Signal to Interference Ratio), use can also be made of signal to noise ratio, or the signal error rate.  
         [0088]    Summarizing the first embodiment, according to the present embodiment, at the radio network controller  21  or the base station  11   a , the code multiplicity of the downlink transmission signals is determined based on the downlink SIR provided by the mobile station  12 . On the other hand, at the mobile station  12 , the code multiplicity of the uplink transmission signals is determined based on the uplink SIR provided by the base station  11   a . That is to say, in this embodiment, the information of SIR of one radio station is informed by its party, and the code multiplicity is determined in response to the condition of interference signals. Therefore, for a mobile station located in a good propagation environment, a code multiplicity is assigned to allow higher-speed communication, and for a mobile station located in a poor propagation environment, a code multiplicity is assigned to allow communications at relatively lower speed to prevent communication quality from being degraded. So it is possible to provide communication service of sufficiently high quality constantly and a communication speed fit for the propagation environment. Furthermore, because it is the code multiplicity but not the spreading factor that is used as the parameter varied in response to the propagation environment, a complicated modulation and-demodulation device is not necessary.  
       The Second Embodiment  
       [0089]    [0089]FIG. 8 is a schematic view showing an example of operation of a mobile communication system for determining a code multiplicity of uplink transmission signals from a mobile station to a base station, according to a second embodiment of the present invention. The mobile communication system in FIG. 8 is the same as that in FIG. 1.  
         [0090]    That is, the mobile communication system shown in FIG. 8 is a TD-CDMA system, and is comprised of a base station  11   a , a base station  11   b , and a mobile station  12 . In this embodiment, the radio network controller  21  in FIG. 1 is not explicitly shown. The base station  11   a  and base station  11   b  have service areas  13   a  and  13   b , respectively, and the mobile station  12  is located in the cell  13   a  of the base station  11   a , receiving signals from or transmitting signals to the base station  11   a  by CDMA through radio links.  
         [0091]    In addition, the configurations of the mobile station  12  and the base station  11   a  are the same as those shown in FIG. 2.  
         [0092]    Furthermore, as shown in FIG. 8, the mobile station  12  has a code assignment table  23 , which records the code multiplicity k associated with an estimated reception power P at the time when a base station receives the signals transmitted from the mobile station  12 . The code assignment table  23  has two major fields for the estimated reception power P and the code multiplicity k, including n records. In this table, the desired code multiplicity k is calculated in correspondence with the estimated reception power P.  
         [0093]    Next, an explanation is made of a method for calculating the code multiplicity k from the estimated reception power P.  
         [0094]    In this example, a quantity Prx_req is defined to represent the required reception power for each code for desired communication quality, a quantity Ptx is defined to represent the transmission power related to all codes of the transmission signals, a quantity Prx is defined to represent the reception power related to all codes of the reception signals at the time of signal reception, and a quantity L is defined to represent the propagation loss between a mobile station and a base station.  
         [0095]    The code multiplicity k is the ratio of the reception power related to all codes of the reception signals over the required reception power for each code. So, the code multiplicity k is expressed by the following equation.  
                   k   =            Prx   /   Prx_req                   =              (     Pt   ×   L     )     /   Prx_req       )                 (   2   )                               
 
         [0096]    In equation (2), if Prx and Prx_req are known, the code multiplicity k can be calculated easily. The propagation loss L between a mobile station and a base station can be calculated from the distance D between them. So if the distance D is known, the propagation loss L is calculated from the distance D, and the obtained value is assigned to L in equation (2), the code multiplicity k can be calculated easily. The value of Prx_req is determined by simulations or by experimental measurements. Since the code multiplicity k is an integer, the calculated k is reduced to an integer.  
         [0097]    Next, following the sequence chart in FIG. 9, an explanation is made of the operation for determining the code multiplicity k when signals are transmitted from the mobile station  12  to the base station  11   a.    
         [0098]    [0098]FIG. 9 is a sequence chart showing an example of the operation of the mobile communication system shown in FIG. 8 for determining the code multiplicity k of the uplink transmission signals from the mobile station  12  to the base station  11   a . In FIG. 9, the base station is abbreviated as BS, and the mobile station is abbreviated as MS.  
         [0099]    In the following explanation, the transmission powers of the mobile station  12  and the base station  11   a  are represented by Ptx 12  and Ptx 11 , respectively; the reception power at the mobile station  12  for receiving signals from the base station  11   a  is represented by Prx 11 ; the reception power at the base station  11   a  for receiving signals from the mobile station  12  is represented by Prx 12 ; and L is the propagation loss between the mobile station  12  and the base station  11   a.    
         [0100]    In step S 51  as shown in FIG. 9, in the course of communication or at the beginning of communication between the mobile station  12  and the base station  11   a , the base station  11   a  transmits signals to the mobile station  12  with a transmission power Ptx 11 .  
         [0101]    In step S 52 , the mobile station  12  receives the signals transmitted from the base station  11   a , and measures the reception power Prx 11  at the mobile station  12 .  
         [0102]    In step S 53 , the mobile station  12  obtains data of the transmission power Ptx 11  of the base station  11   a  sent by the base station  11   a.    
         [0103]    In step S 54 , the mobile station  12  calculates the propagation loss L between itself and the base station  11   a  from the reception power Prx 11  at the mobile station  12  and the transmission power Ptx 11  of the base station  11   a . The propagation loss L is expressed as below, 
           L =Ptx 11 /Prx 11   (3) 
         [0104]    The transmission power Ptx 11  of the base station  11   a  is a known quantity, so if the transmission power Ptx 11  of the base station  11   a  is stored in advance, the propagation loss L can be easily calculated from the above equation (3). Note that the base station  11   a  may also inform the mobile station  11   a  of its transmission power Ptx 11 . In this case, the same as the above, since the transmission power Ptx 11  of the base station  11   a  is a known quantity, the propagation loss L can be easily calculated from equation (3).  
         [0105]    In step S 55 , after obtaining the propagation loss L from the reception power Prx 11  at the mobile station  12  and the transmission power Ptx 11  of the base station  11   a , from the following equation, the mobile station  12  estimates the reception power Prx 12  at the base station  11   a  when the uplink transmission signals from the mobile station  12  are received at the base station  11   a.   
         Prx 12 =Ptx 12 / L   
         [0106]    In step S 56 , from the estimated reception power Prx 12 , the mobile station  12  makes reference to cede assignment table  23 , and determines the code multiplicity k corresponding to the estimated reception power Prx 12 .  
         [0107]    In step S 57 , the mobile station  12  uses the determined code multiplicity k to transmit signals to the base station  11   a  with the transmission power Ptx 12  at the mobile station  12 .  
         [0108]    In the above example, an explanation is made of a case in which the uplink transmission signal code multiplicity is determined. The same method is applicable to determination of the signal code multiplicity for downlink transmission.  
         [0109]    Next, following the sequence chart in FIG. 10, explanations are made of the operation for determining the code multiplicity k when signals are transmitted from the base station  11   a  to the mobile station  12 .  
         [0110]    [0110]FIG. 10 is a sequence chart showing an example of the operation for the mobile communication system shown in FIG. 8 for determining the code multiplicity k of the downlink transmission signals from the base station  11   a  to the mobile station  12 . FIG. 10 is basically the same as FIG. 9, except that the roles of the base station  11   a  and the mobile station  12  are reversed compared with FIG. 9. In this case, the code assignment table to be used is stored in the base station  11   a , and is modified relative to the code assignment table  23 .  
         [0111]    In step S 61  as shown in FIG. 10, in the course of communication or at the beginning of communication between the mobile station  12  and the base station  11   a , the mobile station  12  transmits signals to the mobile station  12  with a transmission power Ptx 12 .  
         [0112]    In step S 62 , the base station  11   a  receives the signals transmitted from the mobile station  12 , and measures the reception power Prx 12  at the base station  11   a.    
         [0113]    In step S 63 , the base station  11   a  obtains data of the transmission power Ptx 12  of the mobile station  12  sent by the mobile station  12 .  
         [0114]    In step S 64 , the base station  11   a  calculates the propagation loss L between itself and the mobile station  12  from the reception power Prx 12  at the base station  11   a  and the transmission power Ptx 12  of the mobile station  12 .  
         [0115]    In step S 65 , after obtaining the propagation loss L from the reception power Prx 12  at the base station  11   a  and the transmission power Ptx 12  of the mobile station  12 , the base station  11   a  estimates the reception power Prx 11  at the mobile station  12  when the downlink transmission signals from the base station  11   a  are received at the mobile station  12 .  
         [0116]    In step S 66 , from the estimated reception power Prx 11 , the base station  11   a  makes reference to the code assignment table in the base station  11   a , and determines the code multiplicity k corresponding to the estimated reception power Prx 11 .  
         [0117]    In step S 67 , the base station  11   a  uses the determined code multiplicity k to transmit signals to the mobile station  12  with the transmission power Ptx 11  at the base station  11   a.    
         [0118]    In the second embodiment, it is explained that the propagation loss L between the mobile station  12  and the base station  11   a  is calculated at the mobile station  12  (or at the base station  11   a ) from the reception power Prx 11  (or Prx 12 ) at the mobile station  12  (or the base station  11   a ) and the transmission power Ptx 11  of the base station  11   a  (or the transmission power Ptx 12  of the mobile station  12 ), and from the calculated propagation loss L, the reception power of the mobile station  12  at the base station  11   a  (the reception power of the base station  11   a  at the mobile station  12 ) is estimated, and further the code multiplicity is determined.  
         [0119]    Nevertheless, the propagation loss L can also be deduced from the distance D between the mobile station  12  and the base station  11   a , and this distance D can be determined by the signal arrival time. For example, if the mobile station is equipped with positioning means, such as a GPS (Global Positioning System), the distance D can be calculated from the positions of the mobile station  12  and the base station  11   a.    
         [0120]    [0120]FIG. 11 is a schematic block diagram showing an example of a configuration of such a mobile communication system.  
         [0121]    As shown in FIG. 11, the basic configuration of the mobile communication system shown in FIG. 11 is the same as that shown in FIG. 2, except for a position detector  31  in the mobile station  12  and a position detector  32  in the base station  11   a.    
         [0122]    The position detector  31  is used to measure the position of the mobile station  12 , for example, it may be a GPS (Global Positioning System) receiver for measuring positions. The position detector  31  computes the position (for example, latitude and longitude) of the mobile station  12  according to the propagation time and the angle of arrival of radio waves from a number of base stations. From the point of view of improving precision, it is preferable to use a GPS receiver, whereas from the point of view of simplicity, it is preferable to use measurement methods based on trigonometric relationships other than GPS.  
         [0123]    The position detector  32  is used for measuring the position of the base station  11   a . The same as the position detector  31  in the mobile station  12 , the position detector  32  may be any device capable of measuring the position by radio waves.  
         [0124]    Furthermore, the propagation loss may also be calculated by the radio network controller  21 . The details of the method may be easily formulated from the above descriptions, so specific explanations are omitted.  
         [0125]    Summarizing the second embodiment, the mobile station  12  predicts the propagation environment between the mobile station  12  and the base station  11   a  from the reception power of the received signals, and determines the code multiplicity of the uplink transmission signals. On the other hand, the radio network controller  21  or the base station  11   a  predicts the propagation environment with the mobile station  12  from the reception power of the received signals, and determines the code multiplicity of the downlink transmission signals. That is to say, in this embodiment, the propagation environment is predicted from the amplitudes of the received signals, and the code multiplicity is determined in response to the propagation environment.  
         [0126]    Therefore, code multiplicity may be increased in good propagation environment, and may be decreased in poor propagation environment. As a result, it is possible to provide communication service of sufficiently high quality constantly and a communication speed fit for the propagation environment. Furthermore, because it is the code multiplicity but not the spreading factor that is used as the parameter varied in response to the propagation environment, a complicated modulation and demodulation device is not necessary.  
         [0127]    While the present invention has been described with reference to specific embodiments chosen for the purpose of illustration, it should be apparent that the invention is not limited to these embodiments, but numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.  
         [0128]    Summarizing the effect of the present invention, a mobile station located in a good propagation environment is capable of high-speed communications, and a mobile station located in a poor propagation environment is capable of communications at relatively lower speed but without quality degradation.  
         [0129]    Furthermore, because the code multiplicity is used as a parameter adjusted in response to the propagation environment, additional special modulation and demodulation devices are not necessary, so lower price and simplicity of devices is achievable.  
         [0130]    This patent application is based on Japanese priority patent application No. 2002-113828 filed on Apr. 16, 2002, the entire contents of which are hereby incorporated by reference.