Patent Publication Number: US-2009238126-A1

Title: Communication System, Mobile Station, And Communication Method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-074696, filed on Mar. 21, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to a technology for communication between a base station and a mobile station using a random access procedure. 
     2. Description of the Related Art 
     A mobile communication system of the next generation (e.g., mobile communication system based on LTE (Long Term Evolution)) enables communication of packets and various types of data including digital voice, image, etc., between mobile stations. The mobile station is called a user equipment (UE) too. 
     This mobile communication system performs an initial access using a RACH (Random Access Channel) to cope with an access failure, etc., that occur when a plurality of mobile stations access a base station (eNB) at one time (see, e.g., Japanese Translation of PCT International Patent Application Laid-open Publication No. 2002-524990 and Japanese Laid-open Patent Publication Nos. 2001-326978 and 2004-274794). 
       FIG. 12  illustrates one example of a sequence up to completion of the initial access by the RACH. As illustrated in  FIG. 12 , the mobile station (UE 1 ) selects a preamble number at random and transmits “Message  1 ” to the base station (eNB). 
     The base station receives the signal of the mobile station, performs power detection, calculates timing information (hereinafter, “TA (Timing Advance) information”), and notifies the mobile station of this TA information by means of “Message  2 ”. The mobile station adjusts transmitting timing, using the TA information notified by the base station and transmits information unique to the mobile station by means of “Message  3 ” to the base station. 
     The base station decodes “Message  3 ” and notifies the mobile station of information necessary for connection of a call by means of “Message  4 ”. As seen above, after transmission and receipt of “Message  1 ” to “Message  4 ” between the mobile station and the base station, the mobile station may perform the connection of the call. 
     The TA information will now be explained.  FIG. 13  is a diagram for explaining one example of the TA information. The above mobile communication system requires that the signal of the mobile station arrive in synchronization with a reference timing of the base station. As in the example of  FIG. 13 , if the signal of the mobile station arrives at the base station in timing  1 , the base station transmits the timing information to the mobile station to expedite transmitting timing by a difference between the reference timing and the timing  1  so that the signal of the mobile station may arrive in the reference timing. This timing information is referred to as TA information. 
     However, in the above conventional technology, since only a small number of preambles are used in the RACH, there is a high probability of a plurality of mobile stations transmitting “Message  1 ” to the base station at the same preamble number and the initial access takes time.  FIG. 14  is a diagram for explaining the conventional technology.  FIG. 14  illustrates as example the case where two mobile stations (mobile station A and mobile station B) transmit “Message  1 ” with the same preamble number. 
     As illustrated in  FIG. 14 , if the mobile stations A and B transmit “Message  1 ” with the same preamble number, it arrives at the base station like a multipath and the base station performs the power detection, calculates the TA information, and transmits the TA information by means of “Message  2 ” to the mobile stations A and B. 
     Each of the mobile stations A and B continues to process the TA information as the information addressed to itself and transmits the information unique to the mobile station by means of “Message  3 ” to the base station. At this point, collision of “Message  3 ” takes place. At the base station, since the information unique to the mobile station A and the information unique to the mobile station B contained in respective “Message  3 ” differ, respective signals interfere with each other and “Message  3 ” may not be demodulated. 
     As a result, with no “Message  4 ” being transmitted to the mobile stations A and B, the mobile stations A and B retransmit “Message  3 ” to the base station. Until receipt of “Message  4 ”, the mobile stations A and B continue to retransmit “Message  3 ” for the preset maximum number of retransmission times. 
     Only after retransmission of “Message  3 ” for the maximum number of retransmission times, the mobile stations A and B recognize the collision and restart the initial access by transmitting “Message  1 ” and accordingly, the initial access takes time. 
     Accordingly, it is an object in one aspect of the invention to provide a communication system and a mobile station capable of reducing the time taken for initial access. 
     SUMMARY 
     According to an aspect of an embodiment, a communication system includes a base station and a mobile station that communicates with the base station. The mobile station includes: a first transmitting unit that transmits a first signal containing a preamble number to the base station; a receiving unit that receives from the base station first timing information indicating a difference between reference time of the base station and time the first signal has arrived at the base station; a calculating unit that calculates second timing information indicating a difference between the reference time of the base station and time the first signal is expected to arrive at the base station; and a second transmitting unit that transmits to the base station any one of the first signal and a second signal containing individual information unique to the mobile station based on the first timing information and the second timing information. 
     According to another aspect of an embodiment, a mobile station that communicates with a base station includes: a first transmitting unit that transmits a first signal containing a preamble number to the base station; a receiving unit that receives from the base station first timing information indicating a difference between reference time of the base station and time the first signal has arrived at the base station; a calculating unit that calculates second timing information indicating a difference between the reference time of the base station and time the first signal is expected to arrive at the base station; and a second transmitting unit that transmits to the base station any one of the first signal and a second signal containing individual information unique to the mobile station based on the first timing information and the second timing information. 
     According to another aspect of an embodiment, a communication method applied to a mobile station that communicates with a base station, includes: transmitting a first signal containing a preamble number to the base station; receiving from the base station first timing information indicating a difference between reference time of the base station and time the first signal has arrived at the base station; calculating second timing information indicating a difference between the reference time of the base station and time the first signal is expected to arrive at the base station; and transmitting to the base station any one of the first signal and a second signal containing individual information unique to the mobile station based on the first timing information and the second timing information. 
     Additional objects and advantages of the invention (embodiment) will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of one example of a configuration of a communication system according to a first embodiment; 
         FIG. 2  is a diagram for explaining one example of exchange of signals in the communication system according to the first embodiment; 
         FIG. 3  is a diagram of one example of a configuration of a mobile station according to the first embodiment; 
         FIG. 4  is a diagram for explaining a method of estimating a position of a base station; 
         FIG. 5  is a flowchart of the operation of the mobile station according to the first embodiment; 
         FIG. 6  is a diagram of one example of a configuration of the mobile station according to a second embodiment; 
         FIG. 7  is a diagram of a relationship between a height of received power and a weighting coefficient k; 
         FIG. 8  is a flowchart of the operation of the mobile station according to the second embodiment; 
         FIG. 9  is a diagram of one example of a configuration of the mobile station according to a third embodiment; 
         FIG. 10  is a diagram of a relationship between a moving velocity and a weighting coefficient v; 
         FIG. 11  is a flowchart of the operation of the mobile station according to the third embodiment; 
         FIG. 12  is a diagram of a sequence up to completion of an initial access by RACH; 
         FIG. 13  is a diagram for explaining TA information; and 
         FIG. 14  is a diagram for explaining a conventional technology. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
       FIG. 1  is a diagram of one example of a configuration of the communication system according to a first embodiment. As illustrated in, this communication system has a base station  100  and a mobile station  200  and performs an initial access, using a RACH (Random Access Channel). Although  FIG. 1  illustrates only the base station  100  and the mobile station  200 , this communication system includes other base stations and mobile stations. 
     The base station  100  includes an RF unit  101 , a demodulating unit  102 , a modulating unit  103 , and a RACH signal processing unit  104 . The RF unit  101  receives a signal transmitted by the mobile station  200 , outputting the received signal to the demodulating unit  102 , and of transmitting the signal input from the modulating unit  103  to the mobile station  200 . 
     The demodulating unit  102  demodulates the signal input from the RF unit  101  and outputting the demodulated signal to the RACH signal processing unit  104 . The modulating unit  103  modulates the signal input from the RACH signal processing unit  104  and outputs the modulated signal to the RF unit  101 . 
     The RACH signal processing unit  104  responds to a request to connect a call from the mobile station  200  (or other mobile stations). Specifically, the RACH signal processing unit  104 , upon receipt of “Message  1 ” from the mobile station  200 , performs power detection, calculates TA information, and transmits the TA information by means of “Message  2 ” to the mobile station  200 . 
     As explained in  FIG. 14 , the TA information contains the information on a difference between a reference timing preset at the base station  100  and the timing of receipt of “Message  1 ”. The mobile station  200  adjusts the timing of transmitting the signal referring to the TA information. 
     Thereafter, the RACH signal processing unit  104 , upon receipt of “Message  3 ” containing inherent information from the mobile station  200 , transmits information necessary for the connection of the call by means of “Message  4 ” to the mobile station  200 . In the case of simultaneous receipt of “Message  3 ” from a plurality of mobile stations which have transmitted “Message  1 ” at the same preamble number, the RACH signal processing unit  104 , due to occurrence of the collision of “Message  3 ”, may not transmit “Message  4 ” to the applicable mobile station. 
     The mobile station  200  includes an RF unit  201 , a demodulating unit  202 , a modulating unit  203 , and a RACH signal processing unit  204  (detailed configuration of the mobile station  200  will be explained later). The RF unit  201  receives a signal transmitted from the base station  100 , outputting the received signal to the demodulating unit  202 , and transmitting the signal input from the modulating unit  203  to the base station  100 . 
     The demodulating unit  202  demodulates the signal input from the RF unit  201  and outputting the demodulated signal to the RACH signal processing unit  204 . The modulating unit  203  modulates the signal input from the RACH signal processing unit  204  and outputting the modulated signal to the RF unit  201 . 
     The RACH signal processing unit  204  requests the base station  100  to connect the call using the RACH. Specifically, the RACH signal processing unit  204  first selects the preamble number at random and transmits “Message  1 ” to the base station. 
     The RACH signal processing unit  204 , in the case of acquiring the TA information by means of “Message  2 ” from the base station  100 , determines whether the acquired TA is transmitted thereto (mobile station  200 ). 
     The RACH signal processing unit  204  calculates the TA information based on a distance from the base station  100  to the mobile station  200  and a propagation path delay amount (amount of delay of the radio wave from the mobile station  200  to the base station  100 ). The TA information calculated by the mobile station  200  (RACH signal processing unit  204 ) itself is hereinafter referred to as estimated TA information. 
     The RACH signal processing unit  204  compares the TA information received from the base station  100  and the estimated TA information and determines, based on results of comparison, whether the TA information received from the base station is transmitted thereto (mobile station  200 ). If the TA information is addressed thereto, then the RACH signal processing unit  204  transmits “Message  3 ” containing the individual information to the base station  100 . On the other hand, if the TA information is not addressed thereto, then the RACH signal processing unit  204  retransmits “Message  1 ” to the base station  100 . 
     As seen above, the communication system according to the first embodiment, in which the mobile station changes the signal to be transmitted to the base station (“Message  3 ” or “Message  1 ”), based on the TA information and the estimated TA information, is capable of avoiding the collision of “Message  3 ” at the base station  100  and shortening the initial access time for connection of the call. 
       FIG. 2  is a diagram for explaining one example of exchange of signals in the communication system according to the first embodiment. The example of  FIG. 2  assumes that mobile stations ( 200  and  300 ) transmit “Message  1 ” containing the same preamble number. Then, the base station  100  calculates the TA information and transmits “Message  2 ” to the mobile stations  200  and  300  (it is assumed that the mobile station  300  is of the same configuration as the mobile station  200 ). 
     If the TA information contained in “Message  2 ” is for the mobile station  300 , the mobile station  200  determines that the TA information is not addressed thereto, and retransmits “Message  1 ” to the base station  100 . On the other hand, the mobile station  300  determines that the TA information is addressed thereto, and transmits “Message  3 ” to the base station  100 . 
     The base station  100  demodulates “Message  3 ” transmitted from the mobile station  300  and transmits the information necessary for the call connection by means of “Message  4 ” to the mobile station  300  and the mobile station  300 , by receiving and demodulating “Message  4 ”, completes the initial access. 
     The base station  100 , in the case of repeated receipt of “Message  1 ” from the mobile station  200 , calculates the TA information and transmits “Message  2 ” to the mobile station  200 . The mobile station  200 , determining that the received TA information is addressed thereto, transmits “Message  3 ” to the base station  100 . 
     The base station  100  demodulates “Message  3 ” transmitted from the mobile station  200  and transmits the information necessary for the call connection by means of “Message  4 ” to the mobile station  200  and the mobile station  200 , by receiving and demodulating the signal, completes the initial access. 
     As seen above, with each mobile station comparing the TA information and the estimated TA information and determining whether the TA information is addressed thereto, the collision of “Message  3 ” may be avoided. Accordingly, there is no transmission of “Message  3 ” for the maximum number of retransmission times and the initial access time is shortened. 
       FIG. 3  is a diagram of one example of a configuration of the mobile station  200  according to the first embodiment. As illustrated in  FIG. 3 , this mobile station  200  includes an RF unit  210 , a demodulating unit  220 , a modulating unit  230 , a TA detecting unit  240 , a TA information estimating unit  250 , a TA appropriateness determination processing unit  260 , a RACH signal control unit  270 , and a RACH signal generating unit  280 . 
     Out of these, the RF unit  210  receives the signal transmitted from the base station  100 , outputting the received signal to the demodulating unit  220 , and transmitting the signal input from the modulating unit  230  to the base station  100 . 
     The demodulating unit  220  demodulates the signal input from the RF unit  210 . The demodulating unit  220  outputs user data contained in the demodulated signal to other processing unit (not shown) and outputs the demodulated signal to the TA detecting unit  240  and the RACH signal control unit  270 . 
     The modulating unit  230  generates (modulating) the signal to be transmitted to the base station  100 , based on the signal input from the RACH signal generating unit  280 , the user data input from other processing unit, and a pilot signal for estimating the channel. 
     The TA detecting unit  240  detects the TA information from the signal input from the demodulating unit  220  and outputting the detected TA information to the TA appropriateness determination processing unit  260 . 
     The TA information estimating unit  250  estimates the position of the mobile station  200  (position of its own) from the distance between the base station  100  and the mobile station  200  and the propagation path delay amount and calculating the estimated TA information. While any technique of known technologies may be used for calculation of the position of its own, the use of, for example, AFLT (Advanced Forward Link Trilateration) system, etc., enables calculating the position of its own. 
       FIG. 4  is a diagram for explaining a method of estimating the position of the base station. In  FIG. 4 , the positions of the base stations A, B, and C are known and their positional coordinates are given as (x 1 , y 1 ), (x 2 , y 2 ), and (x 3 , y 3 ), respectively. The positional coordinates of the mobile station are given as (x, y) and the distance between the base station A and the mobile station is given as r 1 , the distance between the base station B and the mobile station as r 2 , and the distance between the base station C and the mobile station as r 3 . 
     The relationship of the distance between the base station and the mobile station, the positional coordinates of the base station and the positional coordinates of the mobile station may be expressed by the following equations (1) to (3): 
         r 1 2 =( x−x 1) 2 +( y−y 1) 2    (1) 
         r 2 2 =( x−x 2) 2 +( y−y 2) 2    (2) 
         r 3 2 =( x−x 3) 2 +( y−y 3) 2    (3) 
     The time at which the base station A transmits a broadcast signal is given as tx 1 , the time at which the mobile station receives the signal is given as tr 1 , the time at which the base station B transmits a broadcast signal is given as tx 2 , the time at which the mobile station receives the signal is given as tr 2 , the time at which the base station C transmits a broadcast signal is given as tx 3 , and the time at which the mobile station receives the signal is given as tr 3 . 
     Since the base stations A, B, and C and the mobile station are in synchronization on a predetermined timing as a prerequisite of the AFLT system, the mobile station may treat the times tx 1  to tx 3  at which the base stations A, B, and C transmit the broadcast signal as known information. 
     The relationship of the time at which the base station transmits the broadcast signal, the time at which the mobile station receives the broadcast signal, and the distance between the base station and the mobile station may be expressed by the following equations (4) to (6), where “c” contained in the equations (4) to (6) represents the speed of light. 
         r 1= c ( tr 1− tx 1)   (4) 
         r 2= c ( tr 2− tx 2)   (5) 
         r 3= c ( tr 3− tx 3)   (6) 
     The TA information estimating unit  250  calculates the positional coordinates (x, y) of the mobile station, using the above equations (1) to (6). The TA information estimating unit  250  then calculates the distance between the mobile station  200  and the base station  100  based on the positional coordinates of its own and the positional coordinates of the base station, estimates the time at which the radio wave reaches the base station, based on the calculated distance and the speed of light, and calculates the estimated TA time by obtaining a difference between the estimated time and the reference timing of the base station. 
     The TA information estimating unit  250  calculates the propagation delay amount by communicating with the base station  100  and corrects the estimated TA time according to the calculated propagation delay amount. While the positional coordinates of the mobile station  200  were obtained, by way of example, as two-dimensional coordinates, they may also be obtained as three-dimensional coordinates. The position of the mobile station  200  may also be calculated using techniques other than the AFLT system (e.g., GPS &lt;Global Positioning System&gt;). 
     The TA appropriateness determination processing unit  260  compares the TA information and the estimated TA information, determining whether the TA information from the base station  100  is addressed thereto, and outputting results of determination to the RACH signal control unit  270 . 
     Specifically, the TA appropriateness determination processing unit  260  calculates a difference value between the TA information and the estimated TA information (hereinafter, TA difference information) and, by comparing the TA difference information with a first threshold and a second threshold (first threshold&gt;second threshold), outputs n (n represents a natural number of two or over; in the first embodiment, description will be made with n=3 for the convenience of description) kinds of determination results to the RACH signal control unit  270 . 
     The TA appropriateness determination processing unit  260  outputs first determination results to the RACH signal control unit  270  if the TA difference information is greater than the first threshold. The TA appropriateness determination processing unit  260  outputs second determination results to the RACH signal control unit  270  if the TA difference information is equal to or smaller than the first threshold but greater than the second threshold. The TA appropriateness determination processing unit  260  outputs third determination results to the RACH signal control unit  270  if the TA difference information is equal to or smaller than the second threshold. 
     The probability of the TA information being the TA information addressed to the mobile station  200  becomes greater in the order of the first determination results, the second determination results, and the third determination results. In other words, when the TA appropriateness determination processing unit  260  outputs the first determination results, it indicates an extremely low possibility of the TA information being the TA information addressed to the mobile station  200 . On the other hand, when the TA appropriateness determination processing unit  260  outputs the third determination results, it indicates an extremely high possibility of the TA information being the TA information addressed to the mobile station  200 . 
     The RACH signal control unit  270  controls the RACH signal generating unit  280  to transmit various signals based on the RACH to the base station  100 . First, in the case of making the request to connect the call to the base station  100 , the RACH signal control unit  270  selects a preamble number at random and controls the RACH signal generating unit  280  to transmit “Message  1 ” to the base station  100 . 
     Then, upon receipt of “Message  2 ” from the base station  100 , the RACH signal control unit  270  determines whether to retransmit “Message  1 ” or transmit “Message  3 ” to the base station  100 , based on the determination results of the TA appropriateness determination processing unit  260 . 
     (Case of Acquiring First Determination Results) 
     When the first determination results are acquired, the RACH signal control unit  270  transmits “Message  3 ” to the base station  100  for a number of times (hereinafter, a first number of times) obtained by subtracting a threshold x 1  from the maximum number of retransmission times m (maximum number of times “Message  3 ” is to be retransmitted; m is a natural number). 
     In the case of not receiving “Message  4 ” from the base station  100  even after transmission of “Message  3 ” for the first number of times, the RACH signal control unit  270  transmits “Message  1 ” to the base station  100 . Since, in the case of acquiring the first determination results, there is a low possibility that the TA information is addressed thereto, the RACH signal control unit  270  may retransmit “Message  1 ” without first transmitting “Message  3 ”. 
     (Case of Acquiring Second Determination Results) 
     When the second determination results are acquired, the RACH signal control unit  270  transmits “Message  3 ” to the base station  100  for a number of times (hereinafter, a second number of times) obtained by subtracting a threshold x 2  (provided x 1 &gt;x 2 ) from the maximum number of transmission times m. In the case of not receiving “Message  4 ” from the base station  100  even after transmission of “Message  3 ” for the second number of times, the RACH signal control unit  270  transmits “Message  1 ” to the base station  100 . 
     (Case of Acquiring Third Determination Results) 
     When the third determination results are acquired, the RACH signal control unit  270  transmits “Message  3 ” to the base station  100  for a number of times (hereinafter, a third number of times) obtained by subtracting a threshold x 3  (provided x 1 &gt;x 2 &gt;x 3 ) from the maximum number of transmission times m. In the case of not receiving “Message  4 ” from the base station  100  even after transmission of “Message  3 ” for the third number of times, the RACH signal control unit  270  transmits “Message  1 ” to the base station  100 . 
     In the case of receiving “Message  4 ” from the base station  100 , the RACH signal control unit  270  makes the call connection with the base station  100 , based on the necessary information for the call connection, contained in “Message  4 ”. The RACH signal generating unit  280  generates “Message  1 ”, “Message  3 ”, etc., under control of the RACH signal control unit  270 . 
       FIG. 5  is a flowchart of the operation of the mobile station  200  according to the first embodiment. In the description of  FIG. 5 , by way of example, TA resolution is given as about 150 m (0.52 μsec), the first threshold th 1  as “1.04 μs”, the second threshold th 2  as “0.52 μs”, the maximum number of transmission times m as “4”, the threshold x 1  as “4 (value equal to the maximum number of transmission times m)”, the threshold x 2  as “2”, and the threshold x 3  as “0”. 
     As illustrated in  FIG. 5 , the mobile station  200  selects the preamble number (Step S 101 ), transmits “Message  1 ” to the base station  100  (Step S 102 ), calculates the position thereof from the distance estimation and the propagation path delay amount, and generates the estimated TA information (Step S 103 ). 
     The mobile station  200  receives “Message  2 ” from the base station  100  (Step S 104 ). The mobile station  200  compares the TA information and the estimated TA information, and calculates the TA difference information (ΔTA) (Step S 105 ). If the TA difference information is equal to or smaller than the first threshold “1.04 μs” (No at Step S 106 ), then it is determined whether the TA difference information satisfies a condition of th 1 (1.04 μs)≧ΔTA&gt;th 2 (0.52 μs) (Step S 107 ). 
     If the TA difference information does not satisfy the condition of th 1 (1.04 μs)≧ΔTA&gt;th 2 (0.52 μs) (No at Step S 107 ), then the mobile station  200  substitutes a value of m(4)−x3(0) into a variable N (Step S 108 ), and the process moves to Step S 111 . 
     On the other hand, if the TA difference information satisfies the condition of th 1 (1.04 μs)≧ΔTA&gt;th 2 (0.52 μs) (Yes at Step S 107 ), the mobile station  200  substitutes a value of m(4)−x2(2) into the variable N (Step S 109 ), and the process moves to Step S 111 . 
     If the TA difference information is greater than the first threshold “1.04 μs” (Yes at Step S 106 ), then the mobile station  200  substitutes a value of m(4)−x1(4) into the variable N (Step S 110 ) and determines whether the value of the variable N is “0” (Step S 111 ). 
     If the value of the variable N is “0” (Yes at Step S 111 ), the process moves to Step S 102 . On the other hand, if the value of the variable N is other than “0” (No at Step S 111 ), the mobile station  200  transmits “Message  3 ” to the base station  100  (Step S 112 ) and, upon receipt of “Message  4 ” from the base station  100  (Yes at Step S 113 ), terminates the process (the initial access is completed). On the other hand, in the case of not receiving “Message  4 ” from the base station  100  (No at Step S 113 ), the mobile station  200  subtracts 1 from the value of the variable N (Step S 114 ), and the process moves to Step S 111 . 
     The process of Step S 103  is not required to be performed between Step S 102  and Step S 104  but is only required to be completed before Step S 105 . 
     As explained above, the communication system according to the first embodiment, in which the mobile station  200  calculates the estimated TA information based on the position of the mobile station  200  and the propagation path delay amount, determines, based on the TA information transmitted from the base station  100  and the estimated TA information, whether the TA information is addressed thereto, and determines whether to transmit “Message  3 ” to the base station  100  depending on the determination results, is capable of avoiding a collision between the signal transmitted from other mobile station and the signal transmitted therefrom and shortening the time required for the initial access. 
     Described below is a mobile station of a communication system according to a second embodiment. The mobile station according to the second embodiment detects the received power of the radio wave transmitted from the base station  100 , corrects the TA difference information based on detection results, and determines whether the TA information from the base station  100  is addressed thereto using the corrected TA difference information. 
     As seen above, the mobile station, which corrects the TA difference information based on the results of detection of the received signal power, is capable of more accurately determining whether the TA information transmitted from the base station  100  is addressed thereto. Since the configuration of the communication system according to the second embodiment is the same as that of the communication system according to the first embodiment, description thereof is omitted. 
     The configuration will then be explained of a mobile station  400  according to the second embodiment.  FIG. 6  is a functional block diagram of one example of the configuration of the mobile station  400  according to the second embodiment. As illustrated in  FIG. 6 , the mobile station  400  includes an RF unit  410 , a demodulating unit  420 , a modulating unit  430 , a TA detecting unit  440 , a TA information estimating unit  450 , a TA appropriateness determination processing unit  460 , a RACH signal control unit  470 , and a RACH signal generating unit  480 . 
     Description is omitted of the RF unit  410 , the demodulating unit  420 , the modulating unit  430 , the TA detecting unit  440 , the TA information estimating unit  450 , the RACH signal control unit  470 , and the RACH signal generating unit  480  since the description thereof is the same as that of the RF unit  210 , the demodulating unit  220 , the modulating unit  230 , the TA detecting unit  240 , the TA information estimating unit  250 , the RACH signal control unit  270 , and the RACH signal generating unit  280  illustrated in  FIG. 3 . 
     The TA appropriateness determination processing unit  460  determines whether the TA information from the base station  100  is addressed thereto based on the TA information, the estimated TA information, and the received power of the radio wave. Specifically, the TA appropriateness determination processing unit  460  calculates the TA difference information by obtaining the difference between the TA information and the estimated TA information. 
     The TA appropriateness determination processing unit  460  also detects the height of the received power of the radio wave received from the base station  100  and calculates a weighting coefficient k, based on the received power detected.  FIG. 7  is a diagram of a relationship between the height of the received power and the weighting coefficient k. As illustrated in  FIG. 7 , the weighting coefficient k assumes a value of 0&lt;k≦1.0 and the weighting coefficient k comes closer to the value 1 as the received power becomes higher and comes closer to the value 0 as the received power becomes lower. 
     The TA appropriateness determination processing unit  460 , by dividing the value of TA difference information by the weighting coefficient k, calculates a probability coefficient Pk and, by comparing the probability coefficient Pk with a first threshold and a second threshold (first threshold&gt;second threshold), outputs n (n represents a natural number of two or over; in the second embodiment, description will be made with n=3 for the convenience of description) kinds of determination results to the RACH signal control unit  470 . 
     The TA appropriateness determination processing unit  460  outputs first determination results to the RACH signal control unit  470  if the probability coefficient Pk is greater than the first threshold. The TA appropriateness determination processing unit  460  outputs second determination results to the RACH signal control unit  470  if the probability coefficient Pk is equal to or smaller than the first threshold but greater than the second threshold. The TA appropriateness determination processing unit  460  outputs third determination results to the RACH signal control unit  470  if the probability coefficient Pk is equal to or smaller than the second threshold. 
     The probability of the TA information being the TA information addressed to the mobile station  400  becomes greater in the order of the first determination results, the second determination results, and the third determination results. In other words, when the TA appropriateness determination processing unit  460  outputs the first determination results, it indicates an extremely low possibility of the TA information being the TA information addressed to the mobile station  400 . On the other hand, when the TA appropriateness determination processing unit  460  outputs the third determination results, it indicates an extremely high possibility of the TA information being the TA information addressed to the mobile station  400 . 
     Described below is the operation of the mobile station  400  according to the second embodiment.  FIG. 8  is a flowchart of the operation of the mobile station  400  according to the second embodiment. In the description of  FIG. 8 , by way of example, TA resolution is given as about 150 m (0.52 μsec), the first threshold th 1  as “1.04 μs”, the second threshold th 2  as “0.52 μs”, the maximum number of transmission times m as “4”, the threshold x 1  as “4 (value equal to the maximum number of transmission times m)”, the threshold x 2  as “2”, and the threshold x 3  as “0”. 
     As illustrated in  FIG. 8 , the mobile station  400  selects the preamble number (Step S 201 ), transmits “Message  1 ” to the base station  100  (Step S 202 ), calculates the position thereof from the distance estimation and the propagation path delay amount, and generates the estimated TA information (Step S 203 ). 
     The mobile station  400  receives “Message  2 ” from the base station  100  (Step S 204 ), compares the TA information and the estimated TA information, and calculates the TA difference information (ΔTA) (Step S 205 ). 
     The mobile station  400  detects the received power, calculates the probability coefficient Pk (Step S 206 ), and, in the case of the probability coefficient Pk being equal to or smaller than the first threshold “1.04 μs” (No at Step S 207 ), determines whether the probability coefficient Pk satisfies a condition of th 1 (1.04 μs)≧Pk&gt;th 2 (0.52 μs) (Step S 208 ). 
     If the probability coefficient Pk does not satisfy the condition of th 1 (1.04 μs)≧Pk&gt;th 2 (0.52 μs) (No at Step S 208 ), then the mobile station  400  substitutes a value of m(4)−x3(0) into a variable N (Step S 209 ), and the process moves to Step S 212 . 
     On the other hand, if the probability coefficient PK satisfies the condition of th 1 (1.04 μs)≧Pk&gt;th 2 (0.52 μs) (Yes at Step S 208 ), then the mobile station  400  substitutes a value of m(4)−x2(2) into the variable N (Step S 210 ), and the process moves to Step S 212 . 
     If the probability coefficient Pk is greater than the first threshold “1.04 μs” (Yes at Step S 207 ), then the mobile station  400  substitutes a value of m(4)−x1(4) into the variable N (Step S 211 ) and determines whether the value of the variable N is “0” (Step S 212 ). 
     If the value of the variable N is “0” (Yes at Step S 212 ), the process moves to Step S 202 . On the other hand, if the value of the variable N is other than “0” (No at Step S 212 ), then the mobile station  400  transmits “Message  3 ” to the base station  100  (Step S 213 ) and, upon receipt of “Message  4 ” from the base station  100  (Yes at Step S 214 ), terminates the process (the initial access is completed). On the other hand, in the case of not receiving “Message  4 ” from the base station  100  (No at Step S 214 ), the mobile station  400  subtracts 1 from the value of the variable N (Step S 215 ), and the process moves to Step S 212 . 
     The process of Step S 203  is not required to be performed between Step S 202  and Step S 204  but is only required to be completed before Step S 205 . While, in the case of Yes at Step S 212 , “Message  1 ” is retransmitted, Step S 203  is not always required. 
     The detection of the received power at Step S 206  is only required to be completed before the calculation of the probability coefficient Pk. In the case of Yes at Step S 212 , if “Message  1 ” is retransmitted and if there is no change in the received power, the probability coefficient Pk is not necessarily required to be calculated. 
     As explained above, according to the second embodiment, the mobile station  400  calculates the estimated TA information based on the position of the mobile station  400  and the propagation path delay amount, determines, based on the TA information transmitted from the base station  100 , the estimated TA information, and the height of the received power of the radio wave, whether the TA information is addressed thereto, and determines whether to transmit “Message  3 ” to the base station  100  depending on the determination results. This prevents a collision between the signal transmitted from other mobile station and the signal transmitted therefrom and shortens the time required for the initial access. 
     While, in the second embodiment, whether the TA information is addressed thereto is determined by calculating the probability coefficient Pk by diving the TA difference information by the weighting coefficient k, the determination is not limited to this but the weighting coefficient k may be applied to the first threshold and the second threshold. 
     In such case, when the received power is low, it is preferable that values of the first and the second thresholds be smaller as compared with such values before application of the weighting coefficient. When the received power is high, it is preferable that the values of the first and the second thresholds be the same or greater as compared with such values before the application of the weighting coefficient. 
     For example, if the weighting coefficient is applied to the first and the second thresholds, the first and the second thresholds may be expressed by the following equations: 
       th1(after application)=th1(before application)×k 
       th2(after application)=th2(before application)×k 
     A mobile station  500  will then be explained of the communication system according to a third embodiment. The mobile station according to the third embodiment detects the moving velocity thereof, corrects the TA difference information based on detection results, and determines whether the TA information from the base station is addressed thereto using the corrected TA difference information. 
     As seen above, the mobile station, which corrects the TA difference information based on the results of detection of the moving velocity, is capable of more accurately determining whether the TA information transmitted from the base station  100  is addressed thereto. Since the configuration of the communication system according to the third embodiment is the same as the system configuration of  FIG. 1 , description thereof is omitted. 
     The configuration will then be explained of the mobile station  500  according to the third embodiment.  FIG. 9  is a diagram of one example of the configuration of the mobile station  500  according to the third embodiment. As illustrated in  FIG. 9 , the mobile station  500  includes an RF unit  510 , a demodulating unit  520 , a modulating unit  530 , a TA detecting unit  540 , a TA information estimating unit  550 , a TA appropriateness determination process unit  560 , a RACH signal control unit  570 , and a RACH signal generating unit  580 . 
     Description is omitted of the RF unit  510 , the demodulating unit  520 , the modulating unit  530 , the TA detecting unit  540 , the TA information estimating unit  550 , the RACH signal control unit  570 , and the RACH signal generating unit  580  since the description thereof is the same as that of the RF unit  210 , the demodulating unit  220 , the modulating unit  230 , the TA detecting unit  240 , the TA information estimating unit  250 , the RACH signal control unit  270 , and the RACH signal generating unit  280  illustrated in  FIG. 3 . 
     The TA appropriateness determination processing unit  560  determines whether the TA information from the base station is addressed thereto based on the TA information, the estimated TA information, and the moving velocity thereof (mobile station  500 ). The TA appropriateness determination processing unit  560  calculates the moving velocity using a known technology. 
     Specifically, the TA appropriateness determination processing unit  560  calculates the TA difference information by obtaining the difference between the TA information and the estimated TA information. The TA appropriateness determination processing unit  560  also detects the moving velocity thereof and calculates a weighting coefficient v, based on the detected moving velocity. 
       FIG. 10  is a diagram of a relationship between the moving velocity and the weighting coefficient v. As illustrated in  FIG. 10 , the weighting coefficient v assumes a value of 0&lt;v≦1.0 and the weighting coefficient v comes closer to the value 1 as the moving velocity of its own becomes lower (slower) and comes closer to the value 0 as the moving velocity of its own becomes higher (faster). 
     The TA appropriateness determination processing unit  560 , by dividing the value of TA difference information by the weighting coefficient v, calculates a probability coefficient Pv and, by comparing the probability coefficient Pv with a first threshold and a second threshold (first threshold&gt;second threshold), outputs n (n represents a natural number of two or over; in the third embodiment, description will be made with n=3 for the convenience of description) kinds of determination results to the RACH signal control unit  570 . 
     The TA appropriateness determination processing unit  560  outputs first determination results to the RACH signal control unit  570  if the probability coefficient Pv is greater than the first threshold. The TA appropriateness determination processing unit  560  outputs second determination results to the RACH signal control unit  570  if the probability coefficient Pv is equal to or smaller than the first threshold but greater than the second threshold. The TA appropriateness determination processing unit  560  outputs third determination results to the RACH signal control unit  570  if the probability coefficient Pv is equal to or smaller than the second threshold. 
     The probability of the TA information being the TA information addressed to the mobile station  500  becomes greater in the order of the first determination results, the second determination results, and the third determination results. In other words, when the TA appropriateness determination processing unit  560  outputs the first determination results, it indicates an extremely low possibility of the TA information being the TA information addressed to the mobile station  500 . On the other hand, when the TA appropriateness determination processing unit  560  outputs the third determination results, it indicates an extremely high possibility of the TA information being the TA information addressed to the mobile station  500 . 
     Described below is the operation of the mobile station  500  according to the third embodiment.  FIG. 11  is a flowchart of the operation of the mobile station  500  according to the third embodiment. In the description of  FIG. 11 , by way of example, the first threshold th 1  is given as “1.04 μs”, the second threshold th 2  as “0.52 μs”, the maximum number of transmission times m as “4”, the threshold x 1  as “4 (value equal to the maximum number of transmission times m)”, the threshold x 2  as “2”, and the threshold x 3  as “0”. 
     As illustrated in  FIG. 11 , the mobile station  500  selects the preamble number (Step S 301 ), transmits “Message  1 ” to the base station  100  (Step S 302 ), calculates the position thereof from the distance estimation and the propagation path delay amount, and generates the estimated TA information (Step S 303 ). 
     The mobile station  500  receives “Message  2 ” from the base station  100  (Step S 304 ), compares the TA information and the estimated TA information, and calculates the TA difference information (ΔTA) (Step S 305 ). 
     The mobile station  500  detects the moving velocity, calculates the probability coefficient Pv (Step S 306 ), and, in the case of the probability coefficient Pv being equal to or smaller than the first threshold “1.04 μs” (No at Step S 307 ), determines whether the probability coefficient Pv satisfies a condition of th 1 (1.04 μs)≧Pv&gt;th 2 (0.52 μs) (Step S 308 ). 
     If the probability coefficient Pv does not satisfy the condition of th 1 (1.04 μs)≧Pv&gt;th 2 (0.52 μs) (No at Step S 308 ), the mobile station  500  substitutes a value of m(4)−x3(0) into a variable N (Step S 309 ), and the process moves to Step S 312 . 
     On the other hand, if the probability coefficient Pv satisfies the condition of th 1 (1.04 μs)≧Pv&gt;th 2 (0.52 μs) (Yes at Step S 308 ), then the mobile station  500  substitutes a value of m(4)−x2(2) into the variable N (Step S 310 ), and the process moves to Step S 312 . 
     If the probability coefficient Pv is greater than the first threshold “1.04 μs” (Yes at Step S 307 ), then the mobile station  500  substitutes a value of m(4)−x1(4) into the variable N (Step S 311 ) and determines whether the value of the variable N is “0” (Step S 312 ). 
     If the value of the variable N is “0” (Yes at Step S 312 ), the process moves to Step S 302 . On the other hand, if the value of the variable N is other than “0” (No at Step S 312 ), then the mobile station  500  transmits “Message  3 ” to the base station  100  (Step S 313 ) and, upon receipt of “Message  4 ” from the base station  100  (Yes at Step S 314 ), terminates the process (the initial access is completed). On the other hand, in the case of not receiving “Message  4 ” from the base station  100  (No at Step S 314 ), the mobile station  500  subtracts 1 from the value of the variable N (Step S 315 ), and the process moves to Step S 312 . 
     The process of Step S 303  is not required to be performed between Step S 302  and Step S 304  but is only required to be completed before Step S 305 . The number of times of the process of Step S 303  may be increased depending on the moving velocity (for example, the number of times is increased in the case of a high moving velocity). 
     While, in the case of Yes at Step S 312 , “Message  1 ” is retransmitted, Step S 303  is not always required. The detection of the moving velocity at Step S 306  is only required to be completed before the calculation of the probability coefficient Pv. For example, in the case of Yes at Step S 312 , if “Message  1 ” is retransmitted and if there is no movement of the mobile station, the probability coefficient Pv is not necessarily required to be calculated. 
     As explained above, the communication system according to the third embodiment, in which the mobile station  500  calculates the estimated TA information based on the position of the mobile station  500  and the propagation path delay amount, determines, based on the TA information transmitted from the base station  100 , the estimated TA information, and the moving velocity thereof, whether the TA information is addressed thereto, and determines whether to transmit “Message  3 ” to the base station  100  depending on the determination results, is capable of avoided collision of the signal transmitted from other mobile station and the signal transmitted therefrom and shortening the time required for the initial access. 
     While, in the third embodiment, the mobile station determines whether the TA information is addressed thereto by calculating the probability coefficient Pv by diving the TA difference information by the weighting coefficient v, the determination is not limited to this but the weighting coefficient v may be applied to the first threshold and the second threshold. 
     In such case, when the moving velocity is high (fast), it is preferable that values of the first and the second thresholds be smaller as compared with such values before application of the weighting coefficient. When the moving velocity is low (slow), it is preferable that the values of the first and the second thresholds be the same or greater as compared with such values before the application of the weighting coefficient. 
     For example, if the weighting coefficient is applied to the first and the second thresholds, the first and the second thresholds may be expressed by the following equations: 
       th1(after application)=th1(before application)×v 
       th2(after application)=th2(before application)×v 
     The foregoing embodiments are susceptible to considerable variation in their practice. Accordingly, the embodiments are not intended to be limited to the specific exemplifications set forth hereinabove. 
     For example, in the first to the third embodiments, the mobile station retransmits “Message  1 ” to the base station  100  upon determining that the TA information from the base station  100  is not addressed thereto. However, in the case of retransmission of “Message  1 ” to the base station  100 , the RACH signal control units  270 ,  470 , and  570  may perform various processes. 
     For example, the RACH signal control units  270 ,  470 , and  570 , in the case of retransmission of “Message  1 ” to the base station  100 , may change the preamble number contained in “Message  1 ” or may change the timing of retransmission (transmitting timing) of “Message  1 ” based on the TA information. 
     For example, the RACH signal control units  270 ,  470 , and  570  may transmit “Message  1 ” by so arranging that the reference timing of the base station  100  and the timing of “Message  1 ” are the same or by so arranging that the timing of “Message  1 ” advances by a predetermined amount of time from the reference timing, based on the TA information. The RACH signal control units  270 ,  470 , and  570  may change the transmission power when transmitting “Message  1 ”. 
     As seen above, the RACH signal control units  270 ,  470 , and  570 , by changing the preamble number, the transmitting timing, and the transmission power in the case of retransmission of “Message  1 ”, are capable of preventing the collision with the signal transmitted from other mobile station. 
     As set forth hereinabove, according to an embodiment, the mobile station determines, based on the first timing information and the second timing information, whether to transmit the individual information thereof or the first signal to the base station. This prevents a collision between the individual information thereof and the individual information of the other mobile station, and the time required for the initial access may be shortened. 
     According to an embodiment, the mobile station calculates the second timing information based on the distance between the base station and its own and/or the amount of delay of the radio wave transmitted or received between the base station and its own. Thus, the second timing information may be calculated accurately. 
     According to an embodiment, the mobile station determines whether to transmit the individual information thereof or the first signal to the base station further using at least one of moving velocity thereof and the received signal power of the radio wave. This more accurately prevents a collision between the individual information thereof and the individual information of the other mobile station, and the time may be shortened that is required for the initial access. 
     According to an embodiment, the mobile station changes the preamble number in the case of transmitting the first signal. This prevents a collision between the individual information thereof and the individual information of the other mobile station. 
     According to an embodiment, the mobile station changes the timing of transmitting the first signal based on the first timing information. This prevents a collision between the individual information thereof and the individual information of the other mobile station. 
     According to an embodiment, the mobile station changes the transmission power when transmitting the second signal. This prevents a collision between the individual information thereof and the individual information of the other mobile station. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present invention(s) has(have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.