Abstract:
The present invention has an object to secure a satisfactory reception environment efficiently regardless of a degree of the traffic intensity. The object is achieved by the following. A pilot power control unit is provided to a base station radio unit. The pilot power control unit controls a desired wave power-to-interference wave power ratio of a pilot signal to stay at a preset value. More specifically, upon receipt of a multiplex wave from a multiplexing unit, the pilot power control unit computes a desired wave power-to-interference wave power ratio of a pilot signal. Then, the pilot power control unit transmits a command signal to a coder to change a transmission power value of the pilot signal in an amount corresponding to a difference between the computed desired wave power-to-interference wave power ratio and a set desired wave power-to-interference wave power ratio.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a radio communication system using the CDMA (Code Division Multiple Access) communication method abide by the IS-95 standard, and more particularly to control of the transmission power of a base station radio unit in a radio communication system.  
           [0002]    A method of connecting a plurality of mobile stations to a base station in the same frequency band is referred to as multiplex access or multiple access. With the CDMA (Code Division Multiple Access) communication method, multiple access is performed by the spread spectrum communication. The spread spectrum communication is a communication means for transmitting spectrum an information signal by spreading a spectrum thereof to a sufficiently wide bandwidth compared with the original information bandwidth.  
           [0003]    Each information signal is assigned with a spread code sequence orthogonal to each other. Each information signal is multiplied with the assigned spread code sequence to be spread. This makes it possible to share one particular frequency with a plurality of communications, and therefore, all the desired wave signals and interference wave signals contained in a multiplex wave transmitted from one particular base station reach mobile stations via a single transmission channel. Then, at the end of each mobile station, a desired wave signal is extracted by using the same spread code sequence.  
           [0004]    In the radio communication system using the CDMA communication method, each service area allocated to one base station is referred to as a cell. Each cell covers a circular area with the base station at the center. However, such a circular area is susceptible to geographical conditions, such as buildings, mountains, etc., in practice. Hereinafter, to locate a base station in a service area is refer to as “BS (Base Station) location”. The BS location in consideration with a size of the allocated service area is referred to as “cell design”. By aligning each cell so as to slightly overlap the adjacent one without leaving any space by cell design, a region is filled with the service areas.  
           [0005]    A down link information signal transmitted from the base station to each mobile station includes, besides a speech frame signal containing speech data or the like, a base station selection signal referred to as a pilot signal. Each mobile station judges its currently located cell (cell in service) based on the reception pilot signal. Each base station multiplexes the speech frame signal and pilot signal by spreading spectrum with the spreading code sequence uniquely assigned to each base station. Each base station transmits the multiplex wave thus generated while controlling the transmission power.  
           [0006]    If a mobile station travels during a call, the reception level of the pilot signal that has been received is lowered. Hence, the mobile station is re-located in another cell in which a pilot signal with a higher reception level is transmitted. As a result, the base station that communicates with the mobile station switches to another base station. This is referred to as handover (hereinafter, referred to as H/O), and in the CDMA communication system, smooth H/O is performed such that permits the mobile station to continue a call without interruption by allowing the mobile station to receive multiplex waves from a plurality of base stations simultaneously. This is referred to as soft H/O. The soft H/O is one of the important characteristics of the CDMA communication method.  
           [0007]    As to the cell design, in general, a radius of cell indicating the size of the cell is determined by setting a limit level necessary to perform the soft H/O in a desired wave power-to-interference wave power ratio (hereinafter, referred to as Ec/lo) per chip (time width of the smallest change in the spread signal) of the pilot signal. A certain Ec/lo set in this manner is referred to as the set Ec/lo.  
           [0008]    The following description will describe a conventional radio communication system using the CDMA communication method with reference to the accompanying drawings. In the first place, explanation is given to an arrangement of the radio communication system using the CDMA communication method.  
           [0009]    [0009]FIG. 2 is a view showing an arrangement of the radio communication system using the CDMA communication method.  
           [0010]    As shown in the drawing FIG. 2, the radio communication system using the CDMA communication method comprises: a base station control unit (hereinafter, referred to as BSC)  10  for providing a CDMA communication service; a plurality of base station radio units (hereinafter, referred to as BTSs)  20 - 1  through  20 - n  each connected to the BSC  10  via a special line  1 ; and mobile station terminal units (hereinafter, referred to as MSs)  30 - 1  through  30 - m  for communicating with the BTSs  20 .  
           [0011]    Next, the following description will describe a structure of each signal frame handled in the present system with reference to FIGS. 4 through 6. FIG. 4 is a view showing a structure of a speech frame signal  50 . As shown in the drawing, the speech frame signal  50  is composed of speech data  51  and speech data signal pass/fail information  52 .  
           [0012]    [0012]FIG. 5 is a view showing a structure of transmission power report information  60 . As shown in the drawing, the transmission power report information  60  is composed of electric field intensity  61 , a measured frame number  62 , and an error frame number  63 .  
           [0013]    [0013]FIG. 6 is a view showing a structure of a speech control frame signal  70 . As shown in the drawing, the speech control frame signal  70  is composed of speech data  64 , transmission power report information  60 , and speech data signal quality information  65 .  
           [0014]    Next, the following description will describe an arrangement of each MS  30  used in the radio communication system using the CDMA communication method. FIG. 8 is a view showing an arrangement of the MS  30  used in the radio communication system using the CDMA communication method.  
           [0015]    As shown in the drawing, the MS  30  comprises: a mixing unit (hereinafter, referred to as M unit)  35  for receiving and outputting a radio signal  2  a receiving unit (hereinafter, referred to as RX unit)  31  for receiving the radio signal  2  via the M unit  35  and demodulating the same; a decoder  33  for decoding the demodulated signal into the speech frame signal  50 ; a speech frame signal mixing unit  36  for judging usable speech data  51  based on the speech data signal pass/fail information  52  contained in the speech frame  50  and composing the usable speech data  51  by assigning weights with the reception signal electric field intensity; a speech coder  37  for converting the speech data  51  into a speech signal; a coder  34  for encoding the speech data  64 ; and a transmitting unit (hereinafter, referred to as TX unit)  32  for modulating the encoded signal and transmitting the resulting signal.  
           [0016]    Next, the following description will describe an operation of the radio communication system using the CDMA communication method.  
           [0017]    The RX unit  31  receives a radio signal  2  via the M unit  35 . The radio signal  2  referred to herein is a multiplex wave generated by multiplexing the speech frame signal  50  shown in FIG. 4 and an unillustrated pilot signal. Then, the reception signal is decoded by the decoder  33 , and reproduced as the speech frame signal  50 . The speech frame signal mixing unit  36  composes a certain number of speech frame signals  50  presumed to be in the same frame by assigning weights for each reception signal electric field intensity. Subsequently, the speech frame signal mixing unit  36  decides whether the speech data  51  should be transmitted to the speech coder  37  or not based on the speech data signal pass/fail information  52  contained in the speech frame signal  50 .  
           [0018]    Further, the speech frame signal mixing unit  36  stores the speech data signal pass/fail information  52  for a certain period notified through notification information from the BTS  20 . Then, the speech frame signal mixing unit  36  generates the transmission power report information  60  as shown in FIG. 5. The transmission power report information  60  is composed with the transmission speech data  64  and sent to the coder  34 . The composed signal is encoded by the coder  34  and outputted as the speech control frame signal  70  as shown in FIG. 6. The speech control frame signal  70  is modulated by the TX unit  32  and outputted to the BTS  20  via the M unit  35 .  
           [0019]    Next, the following description will describe, with reference to FIG. 1, an arrangement and an operation of the conventional base station of the CDMA communication method.  
           [0020]    As shown in FIG. 1, the conventional BTS  20  comprises: an M unit  28  for receiving and outputting a radio signal  2 ; an RX unit  21  for receiving and demodulating the radio signal  2 ; a decoder  23  for decoding the demodulated signal into the speech control frame signal  70 ; a coder  24  for encoding the speech frame signal  50  sent via the special line  1 ; a multiplexing unit  26  for composing the encoded signals; a TX unit  22  for modulating the multiplexed signal and transmitting the resulting signal; and a transmission power control unit  25  for varying transmission power of the speech frame signal  50  by computing the electric field intensity  61  and a reference value sent via the special line  1 .  
           [0021]    On the other hand, the BSC  10  comprises a speech frame signal selecting unit  11 , a speech coder  12 , and a control processing unit  13 .  
           [0022]    Next, the following description will describe operations of the BSC  10  and BTS  20 . In the first place, at the end of the BTS  20 , the RX unit  21  receives a radio signal  2  via the M unit  28 . The reception radio signal  2  is decoded by the decoder  23 , and reproduced as the speech control frame signal  70 . The speech control frame signal  70  is sent to the BSC  10  via the special line  1 . The speech control frame signal  70  sent to the BSC  10  is inputted into the speech frame signal selecting unit  11 . The speech frame signal selecting unit  11  transmits the speech data  64  contained in the speech control frame signal  70  to the speech coder  12 . The speech coder  12  converts the reception speech data  64  into a speech signal.  
           [0023]    The speech frame signal selecting unit  11  also transmits the transmission power report information  60  contained in the speech control frame signal  70  to the control processing unit  13 . The control processing unit  13  generates a reference value  80  based on the measured frame number  62  and error frame number  63  contained in the transmission power report information  60 .  
           [0024]    The control processing unit  13  lowers the reference value  80 , if the error frame number  63  contained in the transmission power report information  60  is lower than a predetermined threshold value. The control processing unit  13  maintains the reference value  80 , if the error frame number  63  is equal to the predetermined threshold value. The control processing unit  13  increases the reference value  80 , if the error frame number  63  is greater than the predetermined threshold value.  
           [0025]    The BSC  10  sends out the electric field intensity  61  and reference value  80  to the BTS  20  via the special line  1 .  
           [0026]    At the end of the BTS  20 , based on the electric field intensity  61  and reference value  80  sent from the BSC  10  via the special line  1 , the transmission power control unit  25  computes an optimal transmission power value of the speech frame signal  50 . The transmission power value thus computed is sent to the coder  24 . The coder  24  encodes the speech frame signal  50  and pilot signal, and at the same time, determines the transmission power according to a down link transmission power value. Here, the pilot signal is used in selecting the base station. The down link transmission power value is defined as a sum value of the transmission power value of the speech frame signal  50  and the transmission power value of the pilot signal determined by the cell design. The multiplexing unit  26  generates a multiplex wave by multiplexing the encoded speech frame signal  50  and pilot signal, which is sent to the TX unit  22 . The TX unit  22  outputs the multiplex wave thus generated to the MS  30  via the M unit  28  as the radio signal  2 .  
           [0027]    Incidentally, if traffic intensity per cell increases, so does the power allocated to the speech frame signal. Likewise, if the power allocated to the speech frame signal increases, so does the down link transmission power value. On the other hand, the larger the power of the multiplex wave, the larger an output from an amplifier circuit becomes from the physical viewpoint. Hence, the TX unit  22  has the maximum output power allowable standard. Accordingly, a limit with respect to a physical size is set for an output from the amplifier circuit in the TX unit  22 . This is done, so that output power exceeding the maximum output power allowable standard value will not be outputted. More specifically, the transmission power of the multiplex wave sent from the multiplexing unit  26  is suppressed by providing an ATT (attenuator) at the input within the TX unit  22 . Consequently, it is possible to control the output power of the radio signal  2  outputted from the TX unit  22 .  
           [0028]    The prior art discussed above has the following problems.  
           [0029]    That is, the pilot signal contained in the multiplex wave received by the MS will not be attenuated in the TX unit if the down link transmission power value is within the maximum output power allowable standard of the TX unit. Hence, the pilot signal is kept transmitted continuously from the base station while maintaining the level corresponding to the set Ec/lo.  
           [0030]    However, if the traffic intensity per cell (or per sector) increases, the transmission power value of the speech frame signal increases in trying to establish more connections. Accordingly, the down link multiplex wave transmission power exceeding the maximum output power allowable standard value is sent from the multiplexing unit to the TX unit. Hence, the down link multiplex wave transmission power is attenuated by the ATT. This attenuation reduces a ratio of the transmission power value of the pilot signal contained in the multiplex wave even if the transmission power value of the pilot signal remains the same. In short, the Ec/lo of the pilot signal becomes smaller than the set Ec/lo value. This causes problems as described below.  
           [0031]    The following description will describe the above-mentioned problems with reference to FIG. 3 and FIGS. 9A through 9D. FIG. 3 is a view showing comparison of the cells for the base stations to which the BS location has been performed. FIGS. 9A through 9D are views showing comparison between a normal BS location and a BS location given with a shorter distance between the base stations.  
           [0032]    In the first place, as shown in FIG. 3, the Ec/lo of the pilot signal becomes smaller than the set Ec/lo as the traffic intensity increases, and the radius of the cell becomes smaller than the area of the cell design. When the radius of the cell becomes smaller, either the overlapping area of the cells is diminished or lost. This raises a problem that the soft H/O function unique to the CDMA communication is not performed smoothly or cannot be performed at all.  
           [0033]    On the other hand, Solving the above problem by the cell design would cause a problem as follows. That is, suppose that the BS location is performed by giving a shorter distance between the base stations as shown in FIG. 9B to the cell design given with a normal distance as shown in FIG. 9A, so that the cells overlap in a larger area. This cell design, however, requires a greater number of base stations, which increases the construction cost and imposes a burden. Also, as discussed above, if the traffic intensity is low, the size of the cell changes, thereby expanding the area where the soft H/O is performed. This will become a problem in that the utilization of a radio channel is increased.  
           [0034]    In practice, the normal cell design such that every two cells overlap as shown in FIG. 9C will become the cell design such that generates areas where three or four cells overlap as shown in FIG. 9D. When this happens, the interference power lo increases, and as a result, the desired wave power Ec of the pilot signal is reduced relatively. In short, a pilot pollution state occurs. The pilot pollution state is defined as a state where an effective pilot signal with a high Ec/lo is absent and many pilot signals with a low (the limit level necessary to perform the soft H/O) Ec/lo are present. In the pilot pollution state, the soft H/O is performed so frequently that the sound quality is deteriorated. Also, such frequent soft H/O causes a problem that the system becomes too tight.  
         SUMMARY OF THE INVENTION  
         [0035]    The present invention is devised to solve the above problems without changing the cell design. The present invention therefore has an object to secure a satisfactory reception environment efficiently regardless of a degree of the traffic intensity. More concretely, the present invention has an object to secure the soft H/O function, which is one of the important characteristics of the CDMA communication method, throughout the areas even when the traffic intensity per cell is high.  
           [0036]    The present invention has another object to provide a communication service with a satisfactory sound quality by avoiding the pilot pollution caused when the traffic intensity per cell is low and thereby preventing the system from becoming too tight.  
           [0037]    A first aspect of the invention provides a radio communication system using a CDMA communication method, comprising a base station radio unit, a base station control unit, and a mobile station terminal unit, characterized in that the base station radio unit comprises a pilot power control unit for controlling a desired wave power-to-interference wave power ratio of a pilot signal to stay at a preset value.  
           [0038]    A second aspect of the invention provides a radio communication system using a CDMA communication method according to the first aspect of the invention, characterized in that the pilot power control unit receives a multiplex wave from a multiplexing unit, and controls the desired wave power-to-interference wave power ratio of the pilot signal to stay at the preset value only when multiplex wave power exceeds a predetermined value.  
           [0039]    A third aspect of the invention provides a radio communication system using a CDMA communication method according to the first aspect of the invention, characterized in that the base station radio unit comprises a coder, a multiplexing unit, and the pilot power control unit, and that the pilot power control unit receives a multiplex wave from the multiplexing unit, computes a desired wave power-to-interference wave power ratio of the pilot signal, and transmits a command signal to the coder to change a power transmission value of the pilot signal in an amount corresponding to a difference between the desired wave power-to-interference wave power ratio computed and a set desired wave power-to-interference wave power ratio.  
           [0040]    A fourth aspect of the invention provides a radio communication system using a CDMA communication method according to the first aspect of the invention, characterized in that the base station radio unit comprises a coder, a multiplexing unit, and the pilot power control unit, and that the pilot power control unit receives a multiplex wave from the multiplexing unit, and transmits a command signal to the coder to change a transmission power value of the pilot signal only when multiplex wave power exceeds a predetermined value.  
           [0041]    A fifth aspect of the invention provides a radio communication system using a CDMA communication method according to the first aspect of the invention, characterized in that the base station radio unit comprises a coder, a multiplexing unit, and the pilot power control unit, and that the pilot power control unit receives a multiplex wave from the multiplexing unit, and only when multiplex wave power exceeds a predetermined value, the pilot power control unit computes a desired wave power-to-interference wave power ratio of the pilot signal, and transmits a command signal to the coder to increase a transmission power value of the pilot signal set by the coder, so that the desired wave power-to-interference wave power ratio computed is equal to a set desired wave power-to-interference wave power ratio. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0042]    [0042]FIG. 1 is a view showing an arrangement of a conventional radio communication system using the CDMA communication method;  
         [0043]    [0043]FIG. 2 is a view showing an arrangement of a radio communication system using the CDMA communication method to explain an embodiment of the present invention or a conventional example;  
         [0044]    [0044]FIG. 3 is a view showing comparison of cells of base stations to which the BS location has been performed to explain one embodiment of the present invention or a conventional example;  
         [0045]    [0045]FIG. 4 is a view showing a structure of a speech frame signal  50  to explain one embodiment of the present invention or a conventional example;  
         [0046]    [0046]FIG. 5 is a view showing a structure of transmission power report information  60  to explain one embodiment of the present invention or a conventional example;  
         [0047]    [0047]FIG. 6 is a view showing a structure of a speech control frame signal  70  to explain one embodiment of the present invention or a conventional example;  
         [0048]    [0048]FIG. 7 is a view showing an arrangement of a radio communication system using the CDMA communication method in accordance with one embodiment of the present invention;  
         [0049]    [0049]FIG. 8 is a view showing an arrangement of an MS  30  used in the radio communication system using the CDMA communication method to explain one embodiment of the present invention or a conventional example;  
         [0050]    [0050]FIGS. 9A through 9D are views showing comparison between a normal BS location and a BS location given with a shorter distance between the base stations to explain one embodiment of the present invention or a conventional example;  
         [0051]    [0051]FIG. 10 is a flowchart detailing one embodiment of the present invention;  
         [0052]    [0052]FIG. 11A is a graph showing a relation of multiplex wave power versus transmission power of a pilot signal;  
         [0053]    [0053]FIG. 11B is a graph showing a relation of multiplex wave power versus Ec/lo of a pilot signal; and  
         [0054]    [0054]FIG. 11C is a graph showing a relation of multiplex wave power versus output power from a TX unit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0055]    The following description will describe a control method of the transmission power of a pilot signal in the CDMA communication method in accordance with one embodiment of the present invention. It should be appreciated, however, that the following embodiment is for illustrative purpose only, and the present is not limited thereto.  
         [0056]    In the first place, with reference to FIGS. 7 and 8, explanation is given to an arrangement of a control system of the transmission power of a pilot signal in the CDMA communication method in accordance with one embodiment of the present invention.  
         [0057]    [0057]FIG. 7 is a view showing an arrangement of the control system of the transmission power of a pilot signal in the CDMA communication method in accordance with one embodiment of the present invention.  
         [0058]    As shown in the drawing, a BTS  20  comprises: an RX unit  21  for receiving and demodulating a radio signal  2 ; a decoder  23  for decoding the demodulated signal into a speech control frame signal  70 ; a coder  24  for encoding a speech frame signal  50  sent via a special line  1 ; a multiplexing unit  26  for composing the encoded signals; a pilot power control unit  27  for computing Ec/lo of a pilot signal contained in the multiplexed signal, and controlling transmission power of the pilot signal so as to be increased if the computed Ec/lo is lower than a set Ec/lo value; a TX unit  22  for modulating the multiplexed signal and transmitting the resulting signal; an M unit  28  connected to both the RX unit  21  and TX unit  22 ; and a transmission power control unit  25  for varying transmission power of the speech frame signal  50  by computing the electric field intensity  61  and a reference value sent via the special line  1 .  
         [0059]    Also, as shown in the drawing, a BSC  10  comprises: a speech frame signal selecting unit  11  for selecting a signal with the highest signal quality based on speech data signal quality information  65  contained in the speech control frame signal  70  sent via the special line  1 , and comparing the quality of the selected signal with a predetermined reference value; a speech coder  12  for converting speech data  64  into a speech signal; and a control processing unit  13  for extracting the electric field intensity  61  from transmission power report information  60  and generating a reference value.  
         [0060]    [0060]FIG. 8 is a view showing an arrangement of an MS  30  in accordance with one embodiment of the present invention.  
         [0061]    As shown in the drawing, the MS  30  comprises: an M unit  35  for receiving and outputting a radio signal  2 ; an RX unit  31  for receiving the radio signal  2  via the M unit  35  and demodulating the same; a decoder  33  for decoding the demodulated signal into the speech frame signal  50 ; a speech frame signal mixing unit  36  for judging usable speech data  51  based on speech data signal pass/fail information  52  contained in the speech frame signal  50  and composing the usable speech data  51  by assigning weights with the reception signal electric field intensity; a speech coder  37  for converting the speech data  51  into a speech signal; a coder  34  for encoding the speech frame signal  50 ; and a TX unit  32  for modulating the encoded signal and transmitting the resulting signal.  
         [0062]    Next, the following description will describe an operation in accordance with the present embodiment with reference to FIG. 10. FIG. 10 is a flowchart detailing a control method of the transmission power of the pilot signal in the present embodiment. At the end of the BTS  20 , the transmission power control unit  25  computes an optimal transmission power value of the speech frame signal  50  based on the speech frame signal  50 , electric field intensity  61  and reference value sent from the BSC  10  via the special line  1 , and transmits the transmission power value data to the coder  24  (Step  1 ).  
         [0063]    The pilot power control unit  27  computes a transmission power value of the pilot signal and transmits a command signal to the coder  24  to change the transmission power value of the pilot signal. In case that no command is issued, an initial set value is used (Step  2 ).  
         [0064]    The coder  24  encodes the speech frame signal  50  and pilot signal. These encoded signals are sent to the multiplexing unit  26  (Step  3 ).  
         [0065]    The multiplexing unit  26  composes each of the encoded signals and generates a multiplex wave (Step  4 ).  
         [0066]    The pilot power control unit  27  computes the Ec/lo of the pilot signal from the multiplex wave generated by the multiplexing unit  26  (Step  5 ).  
         [0067]    In case that the transmission power of the multiplex wave is lower than the maximum output power allowable standard value, the computed Ec/lo is equal to or greater than the set Ec/lo. In this case, no command is issued to the coder  24  to change the transmission power of the pilot signal. In case that the transmission power of the multiplex wave is greater than the maximum output power allowable standard value, the computed Ec/lo is smaller than the set Ec/lo, and the transmission power of the multiplex wave increases further above the maximum output power allowable standard value, then a command is issued to the coder  24  to change the transmission power value of the pilot signal, so that the computed Ec/lo is maintained equal to the set Ec/lo. Likewise, in case that the transmission power of the multiplex wave is equal to the maximum output power allowable standard value, the computed Ec/lo is smaller than the set Ec/lo, and the transmission power of the multiplex wave is lowered to the maximum output power allowable standard value, then a command is issued to the coder  24  to change the transmission power value of the pilot signal, so that the computed Ec/lo is maintained equal to the set Ec/lo (Step  6 ).  
         [0068]    Further, the following description will describe the control method of the power transmission of the pilot signal of the present embodiment with more concrete numerical example values.  
         [0069]    That is, given 20 W as the maximum output power allowable standard value of the TX unit in the BTS  20  determined by the cell design, and 4 W as the initial set value of the pilot signal. Then, the set Ec/lo is −7 dB.  
         [0070]    At the end of the BTS  20 , the transmission power control unit  25  receives return data from the BSC  10  via the special line  1 . The transmission power control unit  25  computes an optimal transmission power value of the speech frame signal  50  based on the electric field intensity  61  and reference value, and transmits the transmission power value thus computed to the coder  24 .  
         [0071]    The pilot power control unit  27  designates 4 W given as the initial set value of the pilot signal as the transmission power value and notifies the coder  24  of the designation. The coder  24  encodes the speech frame signal  50  having undergone the power control and the pilot signal at the initial set value having not undergone the power control. The coder  24  determines the transmission power according to a down link transmission power value, which is a sum of the transmission power value of the speech frame signal  50  and 4 W given as the initial set value of the pilot signal, and transmits the transmission power data thus determined to the multiplexing unit  26 .  
         [0072]    The multiplexing unit  26  composes the speech frame signal  50  and pilot signal, and transmits the resulting multiplex wave to the TX unit  22 .  
         [0073]    Then, in case that the set Ec/lo computed from the Ec/lo of the pilot signal contained in the multiplex wave is lower than −7 dB, the pilot power control unit  27  transmits a command signal to the coder  24  to change the transmission power value of the pilot signal, so that the transmission power of the pilot signal is increased until the Ec/lo of the pilot signal contained in the multiplex wave reaches −7 dB. In case that the Ec/lo of the pilot signal is larger than −7 dB, no command is issued to the coder  24  to change the transmission power value of the pilot signal.  
         [0074]    The pilot power control unit  27  computes the set value (−7 dB) of the set Ec/lo based on 20 W given as the maximum output power allowable standard value of the TX unit  22  and 4 W given as the initial set value of the pilot signal.  
         [0075]    [0075]FIG. 11A is a view showing a relation of the multiplex wave power versus the pilot signal power. FIG. 11B is a view showing a relation of the multiplex wave power versus the Ec/lo of the pilot signal.  
         [0076]    [0076]FIG. 11A indicates that the transmission power is controlled in such a manner that, when the multiplex wave power is 20 W or lower, the transmission power of the pilot signal is maintained at 4 W, and when the multiplex wave power exceeds 20 W, the Ec/lo is maintained at −7 dB.  
         [0077]    [0077]FIG. 11B indicates that the transmission power is controlled in such a manner that, when the multiplex wave power is 20 W or lower, a state where the Ec/lo of the pilot signal is greater than −7 dB is maintained, and when the multiplex wave power exceeds 20 W, the Ec/lo is maintained at −7 dB. The larger the allowable output power of the multiplex wave, the larger the TX unit  22  becomes in physical size. This is the reason why the maximum output power allowable standard is provided. A phenomenon such that the Ec/lo of the pilot signal contained in the multiplex wave drops below the set Ec/lo in trying to allocate higher traffic intensity for each cell (or sector) occurs when the input is increased above the maximum output allowable standard of the TX unit  22 . As shown in FIG. 11C, the amplifier circuit in the TX unit  22  has a limit of 20 W at the maximum output in the linear characteristics. Thus, the output value is maintained at or below the maximum output power allowable value by providing the ATT at the input within the TX unit  22  to control in such a manner that output power equal to or greater than the maximum output power allowable standard value will not be outputted.  
         [0078]    As has been discussed, the present invention achieves the following effects.  
         [0079]    According to the control method of the transmission power of the pilot signal of the present invention, even when the traffic intensity per cell is high, it is possible to control the transmission power by increasing the transmission power of the pilot signal, so that the Ec/lo of the pilot signal will not drop below the set Ec/lo. Consequently, it is possible to prevent the radius of the cell from becoming smaller the one set by the cell design. As a result, the soft H/O function unique to the CDMA communication method can be performed smoothly. Also, it is possible to prevent generation of an area where the soft H/O cannot be performed.  
         [0080]    In addition, in case that the traffic intensity per cell is low, inconvenience caused by the pilot pollution can be lessened by maintaining the Ec/lo at a constant level by controlling the pilot signal. Consequently, it is possible to prevent the system from becoming too tight, and therefore, a satisfactory sound quality can be provided without any interruption.