Patent Publication Number: US-2011051609-A1

Title: Base station apparatus, mobile station apparatus, and mobile station control method in mobile communication system

Description:
TECHNICAL FIELD 
     The present invention generally relates to a technical field of mobile communications, and more particularly to a base station apparatus, a mobile station apparatus, and a base station control method in a mobile communication system. 
     BACKGROUND ART 
     In the CDMA (Code Division Multiple Access) mobile communication system and in the LTE (Long Term Evolution) system having been studied and standardized by the 3GPP (3rd Generation Partnership Project) lately, when a radio link is connected, after establishing a downlink radio link by performing the cell search, a mobile station transmits an uplink Random Access Channel (hereinafter simplified as RACH) to perform random access (see Non-Patent Documents 1 through 3). As known methods of multiplexing the RACH with a data channel, there are the time-multiplexing method in which the RACH and the data channel are transmitted in different time frames, the frequency-multiplexing method in which the RACH and the data channel are transmitted in different frequencies, and the code-multiplexing method in which the RACH and the data channel are transmitted using different codes. When the RACH is transmitted, the mobile stations perform contention-based access. Therefore, based on the number of mobile stations within a coverage area of each base station and the call probabilities, it is required to allocate minimally required radio resources that satisfy a required success probability or a required time period. In this case, the more radio resources are allocated to the RACH, the slower the throughput of the data channel. On the other hand, the less radio resources are allocated, the more difficult to satisfy the required success probability and the required time period. The resources (radio resources) herein include time resources, frequency resources, code resources, and space resources. 
     In the W-CDMA (Wideband Code Division Multiple Access) mobile communication system, the RACH and the data channel are code-multiplexed, and a preamble power ramping scheme are employed. The preamble is a short signal transmitted before an RACH message is transmitted and is spread using a predetermined spreading code. By receiving the preamble in advance, the base station can specify the receiving timings and the using spreading code of the RACH message (message part) to be transmitted later. Because of this feature, the load in receiving process of the message part may be reduced and the receiving performance in the base station may be improved. In the preamble power ramping scheme, the preamble is repeatedly transmitted plural times from the mobile station until the mobile station receives a reception response (Acknowledgement or Ack) from the base station, the reception response (Ack) indicating the detection of the preamble by the base station. Further, in this case, the transmission power of the preamble is gradually increased as the number of repeated transmission is increased. Upon receiving the reception response (Ack), the mobile station stops the transmission of the preamble and transmits the message part at the same transmission power level as that of the last preamble transmitted to the base station. The preamble is 4096 chip-long signal sequence in which a 16 chip-long signature is repeated 256 times. There are 16 kinds of signatures. If the number of kinds of the signatures is increased, the repeated number is decreased, and therefore, the decoding characteristics are degraded and a complexity in decoding process is increased. On the other hand, if the number of kinds of the signature is decreased, when plural mobile stations perform the random accesses at the same time, the collision probability of using the same signature is increased. Therefore, the number of kinds of the signatures is determined by assuming (considering) the number of mobile stations within the coverage areas of the typical base stations and the uplink interference amounts from the mobile stations. 
     The timings when the mobile station can transmit the preamble are divided in a random access sub-channel.  FIG. 1  shows usable access slots in the random access sub-channel. In  FIG. 1 , the vertical direction represents System Frame Number (SFN) modulo 8 corresponding to Primary Common Control Physical Channel (P-CCPCH), and the lateral direction represents random access sub-channel numbers. In  FIG. 1 , the numbers in the sections specified by the SFN modulo 8 and the random access sub-channel numbers represent access slots. The random access sub-channel is a sub-set of combinations of all the uplink access slots in the W-CDMA mobile communication system. The access slots refer to 15 predetermined time offsets provided at 5120 chip interval in two radio frames. When the number of the random access sub-channels is increased, the collision probability between mobile stations is decreased. However, in this case, the transmission interval of the preamble is elongated, and therefore, longer time is required to perform the random access. Because of the feature, the number of random access sub-channels is determined by assuming (considering) the number of mobile stations within the coverage areas of the typical base stations. 
     In the LTE system, as the method of multiplexing the RACH and the data channel, the time-multiplexing method, the frequency-multiplexing method, and the code-multiplexing method are studied. Further, similar to the W-CDMA mobile communication system, the preamble power ramping scheme and the spreading of the preamble using the signatures are also studied. As the number of kinds of the signatures, an adequate number is determined based on the number of mobile stations within the coverage areas of the typical base stations, the uplink interference amount from the mobile stations, and an amount of radio resources to be allocated to the RACH. The preamble length, which is shorter than the length of several sub-frames, is determined based on a balance with a guard time determined by considering uncertainty of receiving time of uplink signals. 
     Non-Patent Document 1: Keiji TACHIKAWA, W-CDMA mobile communication system, Maruzen K.K., issued on Jun. 25, 2001 
     Non Patent Document 2: 3GPP TSG RAN1#44-bis, R1-060885, Mar. 27-Mar. 30, 2006 
     Non Patent Document 3: 3GPP TSG RAN1#44-bis, R1-061041, Mar. 27-Mar. 30, 2006 
     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     In a mobile communication system, due to a large number of existing base stations, hard work may be required to design and modify radio parameters for the system. In other words, it may be difficult to optimize the radio parameters in real time in response to the changes of traffic distribution, propagation environments, and peripheral environment due to such as installation and removal of a surrounding base station. In a case where the radio parameters are not adequately set, a cell throughput and a call-blocking rate may be degraded, and a non-service area may also be generated. To overcome these problems, methods of performing automatic initial setting and automatic optimization of the radio parameters have been studied. However, no method has been proposed so far to perform automatic initial setting and automatic optimization of the parameters with regard to the RACH. 
     The present invention is made in light of the above circumstances and may provide a base station apparatus, a mobile station apparatus, and base station control method capable of performing automatic initial setting and automatic optimization of the radio parameters for the RACH, the parameters including an allocation amount of the radio resources, initial transmission power level, increase of transmission power in power ramping scheme, the maximum retransmission number, the number of kinds of signatures, the number of the random access sub-channels and the like. 
     Means for Solving the Problems 
     According to an aspect of the present invention, a base station apparatus in a mobile communication system includes a receiving unit receiving a random access channel transmitted from a mobile station apparatus, an acquisition unit acquiring random access channel quality based on a measurement of the received random access channel or a report from the mobile station apparatus, a calculation unit calculating random access channel parameters based on the acquired random access channel quality, and a broadcast unit broadcasting the calculated random access channel parameters to all the mobile stations within a coverage area of the base station. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     In a mobile communication system, a base station apparatus, a mobile station apparatus, a base station control method according to an embodiment of the present invention may be capable of performing automatic initial setting and automatic optimization of RACH parameters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a (first) drawing illustrating usable access slots in random access sub-channels; 
         FIG. 2  is a block diagram illustrating exemplary configurations of a base station apparatus and a mobile station apparatus according to an embodiment of the present invention; 
         FIG. 3  is a drawing illustrating an preamble power ramping operation; 
         FIG. 4  is a sequence diagram illustrating an exemplary process of RACH resource control; 
         FIG. 5  is a flowchart illustrating an exemplary process of observing RACH quality and calculating RACH parameters based on a report on the number of retransmissions performed at the maximum transmission power level, the report being transmitted from the mobile station apparatus; 
         FIG. 6  is another (second) drawing illustrating the usable access slots in the random access sub-channels; 
         FIG. 7  is another (third) drawing illustrating the usable access slots in the random access sub-channels; 
         FIG. 8  is a flowchart illustrating an exemplary process of observing the RACH quality and calculating the RACH parameters based on a report on the number of retransmission, the report being transmitted from the mobile station apparatus; 
         FIG. 9  is a flowchart illustrating an exemplary process of observing the RACH quality and calculating the RACH parameters based on a report on success probability, the report being transmitted from the mobile station apparatus; and 
         FIG. 10  is a flowchart illustrating an exemplary process of observing the RACH quality and calculating the RACH parameters in the base station apparatus. 
     
    
    
     EXPLANATION OF REFERENCES 
     
         
         
           
               100 : BASE STATION APPARATUS 
               101 : RACH RECEIVING SECTION 
               102 : RACH QUALITY RECEIVING SECTION 
               103 : RACH QUALITY ACQUIRING SECTION 
               104 : RACH PARAMETER CALCULATION SECTION 
               105 : RACH PARAMETER CHANGE INSTRUCTION BROADCASTING SECTION 
               200 : MOBILE STATION APPARATUS 
               201 : RACH PARAMETER CHANGE INSTRUCTION RECEIVING SECTION 
               202 : RACH PARAMETER CONTROL SECTION 
               203 : RACH TRANSMISSION SECTION 
               204 : RACH QUALITY MEASUREMENT SECTION 
               205 : RACH QUALITY TRANSMISSION SECTION 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     According to an embodiment of the present invention, a base station apparatus in a mobile communication system includes a receiving unit receiving a random access channel transmitted from a mobile station apparatus, an acquisition unit acquiring random access channel quality based on a measurement of the received random access channel or a report from the mobile station apparatus, a calculation unit calculating random access channel parameters based on the acquired random access channel quality, and a broadcast unit broadcasting the calculated random access channel parameters to all the mobile stations within a coverage area of the base station. 
     Preferably, the random access channel quality may include at least one of a retransmission number of the random access channel, a retransmission number of the random access at a maximum transmission power level under a control of increasing a transmission power level of the random access channel when a receipt of the random access channel is failed in the base station apparatus, a success probability of the random accesses, a detection probability of the random access channel, a number of mobile station apparatuses having attempted to perform the random accesses per unit time, a number of mobile station apparatuses having succeeded to perform the random access per unit time, and a number of mobile station apparatuses having failed to perform the random access per unit time. 
     Further, preferably, the random access channel parameters may include at least one of time, frequency, an allocated amount of code or space, initial transmission power level, increase of transmission power, a maximum retransmission number, a number of signatures, and a number of sub-channels. 
     According to another aspect of the present invention, a mobile station apparatus in a mobile communication system includes a measurement unit measuring random access channel quality, a reporting unit reporting the measured random access channel quality to a base station apparatus, a receiving unit receiving random access channel parameters broadcasted from the base station apparatus, a control unit controlling the random access channel parameters, and a transmission unit transmitting a random access channel based on the controlled random access channel parameters. 
     Preferably, the random access channel quality may include at least one of a retransmission number of the random access channel, a retransmission number of the random access at a maximum transmission power level under a control of increasing a transmission power level of the random access channel when a receipt of the random access channel is failed in the base station apparatus, a success probability of the random accesses, and a detection probability of the random access channel. 
     According to another aspect of the present invention, a control method used in a base station apparatus mobile in a mobile communication system includes the steps of receiving a random access channel transmitted from a mobile station apparatus, acquiring random access channel quality based on a measurement of the received random access channel or a report from the mobile station apparatus, calculating random access channel parameters based on the acquired random access channel quality, and broadcasting the calculated random access channel parameters to all the mobile stations within a coverage area of the base station. 
     In the following preferred embodiments of the present invention are described with reference to the accompanying drawings. 
       FIG. 2  is a block diagram illustrating exemplary configurations of a base station apparatus  100  and a mobile station  200  apparatus according to an embodiment of the present invention. For simplification purposes,  FIG. 2  describes only two mobile station apparatuses within a coverage area of the base station apparatus; however, there may be many more mobile station apparatus existing within the coverage area of the base station apparatus. 
     As shown in  FIG. 2 , the base station apparatus  100  includes an RACH (Random Access Channel) receiving section  101 , an RACH quality receiving section  102 , an RACH quality acquiring section  103 , an RACH parameter calculation section  104 , and an RACH parameter change instruction broadcasting section  105 . Further, the mobile station apparatus  200  includes an RACH parameter change instruction receiving section  201 , an RACH parameter control section  202 , an RACH transmission section  203 , an RACH quality measurement section  204 , and an RACH quality transmission section  205 . 
     In the base station apparatus  100 , the RACH receiving section  101  receives a random access channel (RACH) transmitted from the mobile station apparatus  200 . 
     The RACH quality receiving section  102  receives data indicating the quality of RACH (RACH quality), the data having been transmitted from the mobile station apparatus  200 . 
     The RACH quality acquiring section  103  acquires the data of the RACH quality received by the RACH quality receiving section  102 , and measures the RACH quality by observing the RACH quality for a predetermined time period. 
     The RACH parameter calculation section  104  acquires either the measurement result of the RACH quality in the RACH quality acquiring section  103  or the RACH quality that is measured by the mobile station apparatus  200  and that is received by the RACH quality receiving section  102 . Further, the RACH parameter calculation section  104  detects a parameter for the RACH (RACH parameter) to be modified (changed), and performs calculations on the detected RACH parameter. 
     The RACH parameter change instruction broadcasting section  105  broadcasts an instruction to change the RACH parameter (RACH parameter change instruction) to all the mobiles station terminals  200  within coverage areas of the base station apparatus  100 . 
     In the mobile station apparatus  200 , the RACH parameter change instruction receiving section  201  receives the RACH parameter change instruction broadcasted by the RACH parameter change instruction broadcasting section  105  of the base station apparatus  100 . 
     The RACH parameter control section  202  controls the designated RACH parameter based on the RACH parameter change instruction received by the RACH parameter change instruction receiving section  201 . 
     The RACH transmission section  203  transmits the RACH based on the controlled RACH parameter. 
     The RACH quality measurement section  204  measures at least one of the number of retransmission (retransmission number) of the RACH, the retransmission number at the maximum transmission power level under a control of increasing the transmission power level of the RACH when the base station apparatus  100  fails to receive the RACH, a success probability of the random accesses, and a detection probability of RACH, as the RACH quality. 
     The RACH quality transmission section  205  transmits the RACH quality measured by the RACH quality measurement section  204  to the base station apparatus  100  having the coverage area where the mobile station apparatus  200  is located. 
     First Embodiment 
       FIG. 3  illustrates a preamble power ramping operation. In the preamble power ramping operation shown in  FIG. 3 , upon normally receiving the RACH from the mobile station apparatus  200 , the base station apparatus  100  transmits a reception response (Acknowledgement or Ack) to the mobile station apparatus  200 . On the other hand, the mobile station apparatus  200  gradually increase the transmission power level of the preamble and retransmits the preamble until the reception response (Ack) is received. In a case where even when the mobile station apparatus  200  transmits the preamble at the maximum transmission power level, if the base station apparatus  100  cannot normally receive the preamble, the probability of collision with an RACH of another mobile station apparatus  200  is high. 
     When the received power level of the preamble in the random access is low or when an interference power level is high at the base station apparatus  100 , the base station apparatus  100  may not normally recognize the transmission power level of the preamble nor the retransmission number of the preamble. Therefore, according to an embodiment of the present invention, the mobile station apparatus  200  notifies the base station apparatus  100  of data indicating the number of retransmission of the preamble, the number of retransmission of the preamble at the maximum transmission power level, a success probability of the random accesses, a detection probability of the RACH, the number of attempt to perform the random access, and the number of failed to perform the random access. Then, and based on the above notified data, the base station apparatus  100  calculates the RACH parameter. 
       FIG. 4  is a sequence diagram illustrating an exemplary process of RACH resource control according to an embodiment of the present invention. 
     As shown in  FIG. 4 , in the process of the RACH resource control, the base station apparatus  100  starts a timer to measure the predetermined time period (step S 1 ). 
     Then, the mobile station apparatus  200  transmits the RACH to perform the random access (step S 2 ), and the base station apparatus  100  receives the RACH (step S 3 ). 
     Further, the mobile station apparatus  200  observes and measures various amounts related to the RACH quality (step S 4 ). As specific examples, the various amounts related to the RACH quality may include the retransmission number of the preamble, the retransmission number of the preamble at the maximum transmission power level, a success probability of the random accesses, a detection probability of the RACH, the number of attempt to perform the random access, the number of failed to perform the random access and the like. 
     The base station apparatus  100  also observes and measures various amounts related to the RACH quality (step S 5 ). As specific examples, the various amounts related to the RACH quality may include a decoding probability of the preambles, an SIR (Signal to Interference power Ratio: in this case, a ratio of received power level of the RACH to interference power level and noise power level) of known symbols transmitted by using resource blocks for the RACH, the number of reception of the preambles and the like. The decoding probability of the preamble is a value obtained by dividing the number of times of successful decoding of the preambles by the number of reception of the preambles in a predetermined time period. In cases other than cases of successful decoding of the preambles, it cannot be determined whether the mobile station apparatus  200  transmitted the preambles. However, as long as the received power level of the signal received by using the resource blocks for the RACH is greater than a threshold value, by regarding (assuming) that the preambles are received, the number of reception of the preambles are calculated. Otherwise, the mobile station apparatus  200  may transmit known symbols by using resource blocks for the RACH, so that the base station apparatus  100  may measure the SIR of the resource blocks; then, the measured SIR may be treated as the RACH quality. 
     After that, similarly, the mobile station apparatus  200  transmits the RACH (step S 6 ), and the base station apparatus  100  receives the RACH (step S 7 ); then, the RACH quality is observed in the mobile station apparatus  200  and the base station apparatus  100  (steps S 8  and S 9 ). Then, the base station apparatus  100  transmits the reception response (Ack) indicating successful reception of the preamble to the mobile station apparatus  200 . The above process is repeated until the mobile station apparatus  200  receives the reception response (Ack). 
     After that, the mobile station apparatus  200  transmits a report on the RACH quality to the base station apparatus  100  (step S 10 ). Then, the base station apparatus  100  receives the report on the RACH quality (step S 11 ). 
     The above process is repeated until the timer is stopped (step S 12 ). 
     When the timer is stopped, the base station apparatus  100  receives a measurement result of the RACH quality from the mobile station apparatus  200 . Further, the base station apparatus  100  calculates the RACH parameters based on the RACH quality measured by the base station apparatus  100  (step S 13 ). In this case, when determining that the RACH quality is better than a predetermined quality, the base station apparatus  100  reduces the resources to be allocated to the RACH. The reduce of the resources for the RACH may be performed by adequately increasing the transmission rate per symbol in modulation and demodulation so as to reduce time, frequency, code, or space resource to be allocated to the RACH. Further, the resources for the RACH may be reduced by reducing the number of kinds of signatures and the number of random access sub-channels, by reducing the initial transmission power level of the RACH, or by reducing the increase of the transmission power in power ramping. Any of the above methods of reducing the resources for the RACH may be performed alone or in any combination thereof. 
     On the other hand, when determining that the RACH quality is worse than the predetermined quality, the base station apparatus  100  increases the resources to be allocated to the RACH. The increase of the resources for the RACH may be performed by adequately reducing the transmission rate per symbol in modulation and demodulation so as to increase time, frequency, code, or space resources to be allocated to the RACH. Further, the resources for the RACH may be increased by increasing the number of kinds of signatures and the number of random access sub-channels, by increasing the initial transmission power level of the RACH, or by increasing the increase of the transmission power in power ramping. Any of the above methods of increasing the resource for the RACH may be performed alone or in any combination thereof. 
     Then, the base station apparatus  100  broadcasts an instruction to change the RACH parameter (RACH parameter change instruction) to the mobile station apparatus  200  (step S 14 ). The mobile station apparatus  200  receives the RACH parameter change instruction (step S 15 ). 
     The mobile station apparatus  200  changes the RACH parameter based on the received RACH parameter change instruction (step S 16 ). 
     After that, the mobile station apparatus  200  transmits the RACH using the changed RACH parameter (step S 17 ), and the base station apparatus  100  receives the RACH (step S 18 ). The above process goes back to step  1  to start the timer to similarly repeat the process. 
     Second Embodiment 
       FIG. 5  is a flowchart illustrating an exemplary process of observing RACH quality and calculating RACH parameters based on a report on the number of retransmissions performed at the maximum transmission power level, the report being transmitted from the mobile station apparatus. Upon starting the random access, the mobile station apparatus  200  initializes the number of retransmission (retransmission number) R by setting its value to zero (step S 101 ), and transmits the preamble (step S 102 ). Then, the mobile station apparatus  200  determines whether the transmission power level of the preamble is the maximum transmission power level (step S 103 ). When determining that the transmission power level of the preamble is the maximum transmission power level (YES in step S 103 ), the process goes to step S 104 , where the retransmission number R is increased by one. Then, the mobile station apparatus  200  determines whether the reception response (Ack) is received from the base station apparatus  100  within a predetermined time period (step S 105 ). When determining that the reception response (Ack) is not received within the predetermined time period (NO in step S 105 ), the process goes back to step S 102 , where unless the transmission power level of the preamble is equal to the maximum transmission power level, the transmission power level of the preamble is increased, and the mobile station apparatus  200  retransmits the preamble (step S 102 ). 
     On the other hand, when determining that the reception response (Ack) is received within the predetermined time period (YES in step S 105 ), the process goes to step S 106 , where mobile station apparatus  200  transmits the message part of the random access to the base station apparatus  100 . Next, the mobile station apparatus  200  transmits the measured retransmission number R to the base station apparatus  100  by including the retransmission number R in the message part or by using another signal (step S 107 ), and initializes the retransmission number R (step S 101 ). 
     The base station apparatus  100  starts a timer (step S 108 ) and then, receives reports on the retransmission number R from the mobile station apparatuses  200  (step S 109 ). Next, the base station apparatus  100  determines whether the timer is stopped (step S 110 ). When determining that the timer is not stopped (NO in step S 110 ), the process goes back to step S 109 , where the base station apparatus  100  further receives the reports on the retransmission number R from the mobile station apparatuses  200 . On the other hand, when determining that the timer is stopped (YES in step S 110 ), the process goes to step S 111 , where the base station apparatus  100  calculates an average value or a median value Rave (value Rave) of the retransmission number at the maximum transmission power level per random access (step S 111 ). Then, the base station apparatus  100  compares the value Rave with predetermined threshold values (step S 112  and S 113 ). 
     When determining that the value Rave is equal to or greater than the minimum value (Rmin) of a predetermined target range and equal to or less than the maximum value (Rmax) of the predetermined target range, the process goes back to steps S 108  and S 109  to start the timer and receives the reports on the RACH quality from the mobile station apparatuses  200 . 
     On the other hand, when determining that the value Rave is less than the value Rmin, it is supposed that the frequency of collisions of the RACH between the mobile station apparatuses  200  is low and that the resources for the RACH are excessively allocated. Therefore, the base station apparatus  100  reduces the resources for the RACH by reducing the time, frequency, code, or space resources to be allocated to the RACH or by reducing the number of the kinds of the signatures and the number of the random access sub-channels (step S 114 ). Then, the base station apparatus  100  broadcasts the RACH parameter change instruction to the mobile station apparatuses  200  (step S 116 ). Then, the process goes back to steps S 108  and S 109  to start the timer and receive the reports on the RACH quality from the mobile station apparatuses  200  again. 
     The time resources may be reduced by, for example, reducing the number of the time slots and the number of the sub-frames to be allocated to the RACH. 
     In the ODFMA (Orthogonal Frequency Division Multiple Access) system or the SC-FDMA (Single-Carrier Frequency Division Multiple Access) system, the frequency resources may be reduced by, for example, reducing the number of the sub-carriers or the number of the frequency blocks which are groups of plural sub-carriers. 
     In a system where the RACH and the data channel are multiplexed using the codes, the code resources may be reduced by, for example, reducing the number of codes to be allocated to the RACH. 
     The number of kinds of the signatures may be reduced by, for example, reducing the length of the code sequence. In this case, however, the repeated number of the signature is accordingly increased. 
     Any of the above methods of reducing the resources for the RACH may be performed alone or in any combination thereof. 
       FIG. 6  shows a case where the number of the random access sub-channel is reduced to 11. The case where the number of the random access sub-channel is 12 is described in  FIG. 1 . When the number of the random access sub-channels is reduced as shown in  FIG. 6 , the positions of the access slots allocated in the random access sub-channels are shifted. Due to the shift, when the number of the random access sub-channels is 12 as shown in  FIG. 1 , each random access sub-channels has eight radio frames (in one cycle) and five access slots are included in the eight radio frames. As a result, 0.625 access slots per radio frame are allocated to the RACH. On the other hand, when the number of the random access sub-channels is 11 as shown in  FIG. 6 , each random access sub-channels has 22 radio frames and 15 access slots are included in the 22 radio frames. As a result, 0.682 access slots per radio frame are allocated to the RACH. As described above, when the number of the random access sub-channels is reduced, the collision probability between the mobile station apparatuses  200  is increased. However, the time interval between the transmissions of the preambles in the mobile station apparatuses  200  is reduced; and therefore, the time period required to perform the random access is decreased. 
     On the other hand, in  FIG. 5 , when determining that the value Rave is greater than the value Rmin, it is supposed that the frequency of collisions of the RACH between the mobile station apparatuses  200  is high and that the resources for the RACH are insufficiently allocated. Therefore, the base station apparatus  100  increases the resources for the RACH by increasing the time, frequency, code, or space resources to be allocated to the RACH or by increasing the number of the kinds of the signatures and the number of the random access sub-channels (step S 115 ). Then, the base station apparatus  100  broadcasts the RACH parameter change instruction to the mobile station apparatuses  200  (step S 116 ). Then, the process goes back to steps S 108  and S 109  to start the timer and receive the reports on the RACH quality from the mobile station apparatuses  200  again. 
     The time resources may be increased by, for example, increasing the number of the time slots and the number of the sub-frames to be allocated to the RACH. 
     In the ODFMA (Orthogonal Frequency Division Multiple Access) system or the SC-FDMA (Single-Carrier Frequency Division Multiple Access) system, the frequency resource may be increased by, for example, increasing the number of the sub-carriers and the number of the frequency blocks which are groups of plural sub-carriers. 
     In a system where the RACH and the data channel are multiplexed using the codes, the code resource may be increased by, for example, increasing the number of codes to be allocated to the RACH. 
     The number of kinds of the signatures may be increased by, for example, increasing the length of the code sequence. In this case, however, the repeated number of the signature is accordingly reduced. 
       FIG. 7  shows a case where the number of the random access sub-channel is increased to 13. The case where the number of the random access sub-channel is 12 is described in  FIG. 1 . When the number of the random access sub-channels is increased as shown in  FIG. 7 , the positions of the access slots allocated in the random access sub-channels are shifted. Due to the shift, when the number of the random access sub-channels is 12 as shown in  FIG. 1 , each random access sub-channels has eight radio frames (in one cycle) and five access slots are included in the eight radio frames. As a result, 0.625 access slots per radio frame are allocated to the RACH. On the other hand, when the number of the random access sub-channels is 13 as shown in  FIG. 7 , each random access sub-channels has 26 radio frames, and 15 access slots are included in the 26 radio frames. As a result, 0.577 access slots per radio frame are allocated to the RACH. 
     As described above, when the number of the random access sub-channels is increased, the collision probability of the RACH between the mobile station apparatuses  200  is decreased. However, the time interval between the transmissions of the preambles in the mobile station apparatuses  200  is increased; and therefore, the time period required to perform the random access is increased. 
     Any of the above methods of increasing the resources for the RACH may be performed alone or in any combination thereof. 
     Third Embodiment 
       FIG. 8  is a flowchart illustrating an exemplary process of observing RACH quality and calculating RACH parameters based on a report on the number of retransmissions, the report being transmitted from the mobile station apparatus. Upon starting the random access, the mobile station apparatus  200  initializes the retransmission number R by setting its value to zero (step S 201 ), and transmits the preamble (step S 202 ). Then, the mobile station apparatus  200  increases the retransmission number R by one (step S 203 ). Next, the mobile station apparatus  200  determines whether the reception response (Ack) is received from the base station apparatus  100  within a predetermined time period (step S 204 ). When determining that the reception response (Ack) is not received within the predetermined time period (NO in step S 204 ), the process goes back to step S 202 , where unless the transmission power level of the preamble is equal to the maximum transmission power level, the transmission power level of the preamble is increased, and the mobile station apparatus  200  retransmits the preamble. On the other hand, when determining that the reception response (Ack) is received within the predetermined time period (YES in step S 204 ), the process goes to step S 206 , where the mobile station apparatus  200  transmits the message part of the random access to the base station apparatus  100  (step S 206 ), and initializes the retransmission number R (step S 201 ). 
     On the other hand, the base station apparatus  100  starts a timer (step S 207 ) and then, receives reports on the retransmission number R from the mobile station apparatuses  200  (step S 208 ). Next, the base station apparatus  100  determines whether the timer is stopped (step S 209 ). When determining that the timer is not stopped (NO in step S 209 ), the process goes back to step S 208 , where the base station apparatus  100  further receives the reports on the retransmission number R from the mobile station apparatuses  200 . On the other hand, when determining that the timer is stopped (YES in step S 209 ), the process goes to step S 210 , where the base station apparatus  100  calculates an average value or a median value Rave (value Rave) of the retransmission number at the maximum transmission power level per mobile station apparatus (step S 210 ). Then, the base station apparatus  100  compares the value Rave with predetermined threshold values (step S 211  and S 212 ). When determining that the value Rave is equal to or greater than the minimum value (Rmin) of a predetermined target range and equal to or less than the maximum value (Rmax) of the predetermined target range, the process goes back to steps S 207  and S 208  to start the timer and receives the reports on the RACH quality. 
     On the other hand, when determining that the value Rave is less than the value Rmin, the RACH quality (RACH received quality) is supposed to be in excessively good condition. The reasons for such a situation are supposed to be that, for example, there are few mobile stations attempting the random accesses at the same time, other-cell interference amount is small, and the received power level at the base station apparatus  100  is high. Therefore, the base station apparatus  100  reduces the resources for the RACH by reducing the time, frequency, code, or space resources to be allocated to the RACH by increasing the transfer rate per symbol in modulation and demodulation (step S 213 ), and broadcasts the RACH parameter change instruction to the mobile station apparatuses  200  (step S 215 ). In this case, to reduce the resources for the RACH, the number of the kinds of the signatures and the number of the random access sub-channels may be reduced, or an RACH received quality initial transmission power level may be reduced. Otherwise, the increase of the transmission power in power ramping may be reduced. Any of the above methods of reducing the resources for the RACH may be perfomed alone or in any combination thereof. Then, the process goes back to steps S 207  and S 208  to start the timer and receive the reports on the RACH quality from the mobile station apparatuses  200  again. 
     On the other hand, when determining that the value Rave is greater than the value Rmax, the RACH received quality is supposed to be in bad condition. The reasons for such a situation are supposed to be that, for example, there are many mobile stations attempting the random accesses at the same time, the other-cell interference amount is large, and the received power level at the base station apparatus  100  is low. Therefore, the base station apparatus  100  increases resources for the RACH by increasing the time, frequency, code, or space resources to be allocated to the RACH by reducing the transfer rate per symbol in modulation and demodulation (step S 214 ), and broadcasts the RACH parameter change instruction to the mobile station apparatuses  200  (step S 215 ). 
     In this case, to increase the resources for the RACH, the number of the kinds of the signatures and the number of the random access sub-channels may be increased, or the RACH received quality initial transmission power level may be increased. Otherwise, the increase of the transmission power in power ramping may be increased. Any of the above methods of increasing the resources for the RACH may be performed alone or in any combination thereof. Then, the process goes back to steps S 207  and S 208  to start the timer and receive the reports on the RACH quality from the mobile station apparatuses  200  again. 
     Fourth Embodiment 
       FIG. 9  is a flowchart illustrating an exemplary process of observing RACH quality and calculating RACH parameters based on a report on the success probability of the random accesses, the report being transmitted from the mobile station apparatus. Upon starting the random access, the mobile station apparatus  200  initializes the number of successes S and the number of failures F in the random accesses by setting their value to zero (step S 301 ), and starts a timer (step S 302 ). Then, the mobile station apparatus  200  initiates the random access (step S 303 ). Next, the mobile station apparatus  200  determines whether the random access is succeeded or failed (step S 304 ). When determining that the random access is failed (NO in step S 304 ), the process goes to step S 305 , where the number of failures F is increased by one. Then, the process goes back to step S 303 , where, when necessary, the mobile station apparatus  200  initiates the random access again. 
     The trigger for initiating the random access depends on the system. On the other hand, when determining that the random access is succeeded (YES in step S 304 ), the process goes to step S 306 , where the number of successes S is increased by one. Then, the mobile station apparatus  200  determines whether the timer is stopped (step S 307 ). When determined that the timer is not stopped yet (NO is step S 307 ), the process goes back to step S 303 , where, when necessary, the mobile station apparatus  200  initiates the random access again. On the other hand, when determining that the timer is stopped (YES is step S 307 ), the process goes to step S 308 , where the mobile station apparatus  200  transmits a report on the number of successes S and the number of failures F to the base station apparatus  100 . Then process goes back to step S 301 , where the mobile station apparatus  200  initializes the number of successes S and the number of failures F again. 
     On the other hand, the base station apparatus  100  starts a timer (step S 309 ) and then, receives the reports on the number of successes S and the number of failures F from the mobile station apparatuses  200  (step S 310 ). Next, the base station apparatus  100  determines whether the timer is stopped (step S 311 ). When determining that the timer is not stopped (NO in step S 311 ), the process goes back to step S 310 , where the base station apparatus  100  further receives the reports on the number of successes S and the number of failures F from the mobile station apparatuses  200 . On the other hand, when determining that the timer is stopped (YES in step S 311 ), the process goes to step S 312 , where the base station apparatus  100  calculates the success probability (value Rave) of the random accesses by dividing a sum of the numbers of successes S of each of the mobile station apparatuses  200  by a sum of the random accesses (i.e. a sum of the number of successes S and the number of failures F) of each of the mobile station apparatuses  200 . In this case, instead of using the success probability (value Rave) of the random accesses, the decoding probability of the preambles may be used as the value Rave. The decoding probability of the preambles is a value obtained by multiplying the success probability (value Rave) of the random accesses by a sum of the retransmission numbers of each of the random accesses. Further, as the value Rave, a sum of the numbers of the random accesses performed by each of the mobile station apparatuses  200  in a predetermined time period, a sum of the numbers of failures F of each of the mobile station apparatuses  200  in a predetermined time period, an average value of the numbers of the random accesses performed by each of the mobile station apparatuses  200  in a predetermined time period, or an average value of the numbers of failures F of each of the mobile station apparatuses  200  in a predetermined time period may be used. 
     Then, the base station apparatus  100  compares the value Rave with predetermined threshold values (step S 313  and S 314 ). When determining that the value Rave is equal to or greater than the minimum value (Rmin) of a predetermined target range and equal to or less than the maximum value (Rmax) of the predetermined target range, the process goes back to steps S 309  and S 310  to start the timer and receives the reports on the RACH quality. On the other hand, when determining that the value Rave is less than the value Rmin, the RACH quality (RACH received quality) is supposed to be in excessively good condition. The reasons for such a situation are supposed to be that, for example, there are few mobile stations attempting the random accesses at the same time, the other-cell interference amount is small, and the received power level at the base station apparatus  100  is high. Therefore, the base station apparatus  100  reduces the resources for the RACH by reducing the time, frequency, code, or space resources to be allocated to the RACH by increasing the transfer rate per symbol in modulation and demodulation (step S 315 ), and broadcasts the RACH parameter change instruction to the mobile station apparatuses  200  (step S 317 ). In this case, to reduce the resource for the RACH, the number of the kinds of the signatures and the number of the random access sub-channels may be reduced, or the RACH received quality initial transmission power level may be reduced. Otherwise, the increase of the transmission power in power ramping may be reduced. Any of the above methods of reducing the resources for the RACH may be performed alone or in any combination thereof. Then, the process goes back to steps S 309  and  5310  to start the timer and receive the reports on the RACH quality from the mobile station apparatuses  200  again. 
     On the other hand, when determining that the value Rave is greater than the value Rmax, the RACH received quality is supposed to be in bad condition. The reasons for such a situation are supposed to be that, for example, there are many mobile stations attempting the random accesses at the same time, the other-cell interference amount is large, and the received power level at the base station apparatus  100  is low. Therefore, the base station apparatus  100  increases resources for the RACH by increasing the time, frequency, code, or space resources to be allocated to the RACH by reducing the transfer rate per symbol in modulation and demodulation (step S 316 ), and broadcasts the RACH parameter change instruction to the mobile station apparatuses  200  (step S 317 ). In this case, to increase the resources for the RACH, the number of the kinds of the signatures and the number of the random access sub-channels may be increased, or the RACH received quality initial transmission power level may be increased. Otherwise, the increase of the transmission power in power ramping may be increased. Any of the above methods of increasing the resources for the RACH may be performed alone or in any combination thereof. Then, the process goes back to steps S 309  and S 310  to start the timer and receive the reports on the RACH quality from the mobile station apparatuses  200  again. 
     Fifth Embodiment 
       FIG. 10  is a flowchart illustrating an exemplary process of observing RACH quality and calculating RACH parameters. The base station apparatus  100  starts a timer to measure a predetermined observation period (step S 401 ), and observes the RACH quality (step S 402 ). Then, the base station apparatus  100  determines whether the timer is stopped (step S 403 ). While determining that the time is not stopped yet (NO in step S 403 ), the process goes back to step S 402  and base station apparatus  100  further observes the RACH quality while determining the timer is not stopped yet. Then, when determining the time is stopped (YES in step S 403 ), the process goes to step S 404 , where the base station apparatus  100  calculates the RACH quality. 
     The RACH quality includes the decoding probability of the preambles (Rave), the SIR of known symbols transmitted using the resources blocks for RACH, and the number of receiving the preambles. The decoding probability of the preambles (Rave) is a value obtained by dividing the number of successfully decoding the preambles in a predetermined time period by the number of received preambles in the predetermined time period. In cases other than cases of successfully decoding of the preambles, it cannot be determined whether the mobile station apparatus  200  transmitted the preambles. However, as long as the received power level of the signal received by using the resource blocks for the RACH is greater than a threshold value, by regarding (assuming) that the preambles are received, the number of reception of the preambles are calculated. Otherwise, the mobile station apparatus  200  may transmit known symbols by using resource blocks for the RACH, and the base station apparatus  100  may measure the SIR of the resource blocks; then, the measured SIR may be treated as the RACH quality. In the following, it is assumed that the decoding probability of the preambles (Rave) is used as the RACH quality. 
     Then, the base station apparatus  100  compares the value Rave with predetermined threshold values (step S 405  and S 406 ). When determining that the value Rave is equal to or greater than the minimum value (Rmin) of a predetermined target range and equal to or less than the maximum value (Rmax) of the predetermined target range, the process goes back to steps S 401  and  5402  to start the timer and then, observes the RACH quality again. 
     On the other hand, when determining that the value Rave is less than the value Rmin, the RACH quality (RACH received quality) is supposed to be in excessively good condition. In a case where the SIR of the resource blocks is used as the RACH quality, if the SIR is greater than a predetermined target value, the RACH received quality is supposed to be in excessively good condition. The reasons for such situations are supposed to be that, for example, there are few mobile stations attempting the random accesses at the same time, the other-cell interference amount is small, and the received power level at the base station apparatus  100  is high. Therefore, the base station apparatus  100  reduces the resources for the RACH by reducing the time, frequency, code, or space resources to be allocated to the RACH by increasing the transfer rate per symbol in modulation and demodulation (step S 407 ), and broadcasts the RACH parameter change instruction to the mobile station apparatuses  200  (step S 409 ). In this case, to reduce the resource for the RACH, the number of the kinds of the signatures and the number of the random access sub-channels may be reduced, or an RACH received quality initial transmission power level may be reduced. Otherwise, the increase of the transmission power in power ramping may be reduced. Any of the above methods of reducing the resources for the RACH may be performed alone or in any combination thereof. Then, the process goes back to steps S 401  to start the timer again. 
     On the other hand, when determining that the value Rave is greater than the value Rmax, the RACH received quality is supposed to be in bad condition. In the case where the SIR of the resource blocks is used as the RACH quality, if the SIR is less than the predetermined target value, the RACH received quality is supposed to be in bad condition. The reasons for such situations are supposed to be that, for example, there are many mobile stations attempting the random accesses at the same time, the other-cell interference amount is large, and the received power level at the base station apparatus  100  is low. Therefore, the base station apparatus  100  increases resources for the RACH by increasing the time, frequency, code, or space resources to be allocated to the RACH by reducing the transfer rate per symbol in modulation and demodulation (step S 408 ), and broadcasts the RACH parameter change instruction to the mobile station apparatuses  200  (step S 409 ). In this case, to increase the resources for the RACH, the number of the kinds of the signatures and the number of the random access sub-channels may be increased, or the RACH received quality initial transmission power level may be increased. Otherwise, the increase of the transmission power in power ramping may be increased. Any of the above methods of increasing the resources for the RACH may be performed alone or in any combination thereof. Then, the process goes back to steps S 401  to start the timer again. 
     General Overview 
     As described above, according to an embodiment of the present invention, it may become possible to perform automatic initial setting and automatic optimization of the RACH parameters in a mobile communication system. Further, the radio parameters may be optimized in real time in response to the changes of traffic distribution, propagation environments, and peripheral environment due to such as installation and removal of a surrounding base station. Furthermore, it may become possible to prevent the degradation of the cell throughput and the call-blocking rate and the generation of a non-service area. 
     The present invention is described by referring to preferred embodiments of the present invention. However, a person skilled in the art may understand that the above embodiments are described for illustrative purpose only and may think of examples of various modifications without departing from the wide scope and spirit defined in claims of the present invention. That is, the present invention should not be construed to be limited to the descriptions of specific embodiments and accompanying figures. 
     The present application is based on and claims the benefit of priority of Japanese Patent Application No. 2007-220257, filed on Aug. 27, 2007, the entire contents of which are hereby incorporated herein by reference.