Patent Publication Number: US-2013244679-A1

Title: Communication system, communication method, base station device, and communication device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application PCT/JP2010/070134 filed on Nov. 11, 2010 and designated the U.S., the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The embodiments described herein are related to a wireless communication. 
     BACKGROUND 
     In order to start communication with a base station, terminal devices transmit preamble signals to a base station through a Random Access Channel (RACH). As an example, a case will be described where communication starts between a base station and a terminal device operating in accordance with Long Term Evolution (LTE), which is a service scheduled to begin service soon. A terminal device transmits a preamble signal to a base station through a Physical Random Access Channel (PRACH). The base station obtains an index value corresponding to the received preamble signal. The base station makes a RACH response message include an index value corresponding to the preamble signal received from a communication target terminal device, and broadcasts the message to terminal devices in a cell covered by the base station. Each terminal device compares the value detected from the RACH response message with the index corresponding to the preamble signal that it has transmitted to the base station. When they are equal to each other, the terminal device transmits to the base station an RRC connection request (or Radio Resource Control Connection Request) message, an RRC connection reestablishment request (or Radio Resource Control Connection Reestablishment), or an RRC connection reconfiguration complete (or Radio Resource Control Connection Reconfiguration Complete). When the terminal device receives an RRC Connection setup from the base station, an RRC connection is established between the base station and the terminal device. 
     When congestion has occurred in an event such as a disaster, many terminal devices transmit preamble signals to one base station in a short period of time, sometimes leading to a situation where some terminal devices are not allowed to establish connections with the base station. However, when a disaster has occurred, prompt communication is preferable between policemen, fire fighters, ambulance crews, etc., and accordingly it is desirable that terminal devices used by policemen, etc. be given priority as to establishing connections with base stations even under congestion. Accordingly, a method has been invented by which terminal devices having priority are allowed to establish connections with base stations preferentially even under congestion. 
     For example, a method has been invented by which whether or not each terminal device that has established an RRC connection with a base station has priority is judged, and communication is performed between the base station and a terminal device that has been judged to have priority. Also, a method is discussed by which a base station rejects accesses from terminal devices not having priority by using information for controlling accesses transmitted in advance by the base station. Further, a configuration is also discussed in which terminal devices having priority and terminal devices not having priority use separate PRACHs. 
     As a related technique, a method has been proposed in which identification information is reported to a base station by using preamble transmission when a terminal device uses a RACH, and the base station assigns wireless resources for data transmission to terminal devices in accordance with the identification information. Also, a system is known in which a base station reports to a terminal device a signature to be used for a next preamble when the base station rejects a communication-starting request from the terminal device. In this system, abase station returns a request permission signal preferentially to mobile terminals that have transmitted preamble signals including specified signatures. 
     Patent Document 1: Japanese Laid-open Patent Publication No. 2009-521892 
     Patent Document 2: Japanese Laid-open Patent Publication No. 2008-187551   
     non-Patent Document 1: 3GPP TS 36.300 V8.3.0 
     non-Patent Document 2: 3GPP TS 36.321 V8.7.0
 
non-Patent Document 3: 3GPP TS 36.211 V8.6.0
 
non-Patent Document 4: 3GPP TS 36.212 V8.3.0
 
non-Patent Document 5: 3GPP TS 36.213 V8.6.0
 
non-Patent Document 6: 3GPP TS 36.331V8.5.0
 
     All of the above methods described as methods in which terminal devices having priority are connected to a base station preferentially involve problems. According to the method in which a base station judges whether each terminal device is a terminal device having priority after RRC connection, even terminal devices having priority are permitted to establish RRC connections with a base station at only the same rate as terminal devices not having priority, leading to a situation where even terminal devices having priority are sometimes not permitted to establish connections to a base station when PRACH are congested. According to the method in which information for controlling accesses transmitted from a base station is used, terminal devices not having priority are not permitted to establish connections with a base station, with communication by users using terminal devices not having priority being ignored. The method in which terminal devices having priority and terminal devices not having priority use separate PRACHs is unable to utilize a bandwidth effectively. 
     Also, in the system in which request permission signals are preferentially returned to mobile terminals that have transmitted preamble signals including signatures specified by a base station, priority connections are only established with terminal devices that failed in an RRC connection. Accordingly, even terminal devices having priority are permitted to establish connections preferentially only after failing in an RRC connection. 
     As described above, methods that have been proposed today sometimes lead to a situation where it is not easy for even terminal devices having priority to establish connections with a base station. 
     SUMMARY 
     According to an aspect of the embodiments, in a wireless communication system, there are a first terminal device and a second terminal device covered by a base station. The first terminal device transmits a first preamble signal representing a first value to the base station. The second terminal device transmits a second preamble signal representing a second value to the base station. The base station identifies the first value and the second value when the base station has received the first preamble signal and the second preamble signal. The base station compares a threshold with the first value, and also compares the threshold with the second value. The base station gives priority to an establishment of a connection with the first terminal device over establishment of a connection with the second terminal device when the first value is greater than the threshold and the second value is equal to or smaller than the threshold. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example of preamble signals received by a base station according to an embodiment. 
         FIG. 2  explains an example of a configuration of a base station. 
         FIGS. 3A and 3B  illustrate examples of preamble index tables. 
         FIG. 4  explains an example of a configuration of a terminal device. 
         FIG. 5  illustrates an example of information elements included in an SIB. 
         FIG. 6  illustrates an example of a root sequence number table. 
         FIG. 7  illustrates an example of a cyclic shift table. 
         FIG. 8  illustrates an example of cyclic shifting. 
         FIGS. 9A and 9B  are flowcharts explaining an example of operations of a base station. 
         FIG. 10  illustrates an example of a format of a sub header of a RACH response message. 
         FIG. 11  is a sequence diagram explaining an example of transmission and reception of messages between a base station and a terminal device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, detailed explanations will be given for the embodiments by referring to the drawings. In the explanations below, a terminal device that is connected to a base station  10  preferentially even when congestion has occurred is referred to as a “priority terminal device”. Priority terminal devices may be terminal devices used by, for example, policemen, fire fighters, ambulance crews, etc. A terminal device that is not a priority terminal device may also be referred to as a “non-priority terminal device”. 
       FIG. 1  illustrates an example of preamble signals received by the base station  10  according to an embodiment. The base station  10  covers a cell  1 . There is a non-priority terminal device  2  and a terminal device  50  that is a priority terminal device in the cell  1 . Note that “terminal device” in the explanations below is intended to mean the terminal device  50  as a priority terminal device. 
     As illustrated in  FIG. 1 , each of the terminal device  50  and the non-priority terminal device  2  transmits a preamble signal  3  ( 3   a  and  3   b ) to the base station  10 . A preamble signal is the first signal that the terminal device  50  or the non-priority terminal device  2  transmits to the base station  10  when they request to start communication with the base station  10 . Also, the preamble signal  3  uniquely corresponds to a preamble index. For example, the value of the preamble index corresponding to the preamble signal  3   a  transmitted from the terminal device  50  to the base station  10  is Np. In this example, Np is assumed to be greater than the threshold Th. The non-priority terminal device  2  generates the preamble signal  3   b  having Nn as a preamble index, and transmits it to the base station  10 . Nn is assumed to be a number equal to or smaller than threshold Th. Also, the non-priority terminal device  2  is set not to generate the preamble signal  3  that corresponds to a preamble index of a value greater than threshold Th. In other words, threshold Th is equal to or greater than the greatest value among values of preamble indexes that the non-priority terminal device  2  uses for generating preambles. According to, for example, LTE, values of preamble indexes are in the range from 1 through 64, and thus threshold Th is a value equal to or greater than 64. The base station  10  and the terminal device  50  store threshold Th beforehand. 
     The base station  10  identifies the corresponding preamble index value for each of the received preamble signals  3 . Next, the base station  10  determines a terminal device that it will permit to connect. The base station  10  preferentially permits connection by a terminal device that has transmitted the preamble signal  3  corresponding to a preamble index of a value greater than threshold Th. For example, the preamble index corresponding to the preamble signal  3   a  transmitted from the terminal device  50  is greater than threshold Th, and accordingly the terminal device  50  is permitted to connect preferentially over the non-priority terminal device  2 . The base station  10  broadcasts, to terminal devices in the cell  1 , a permission message including information identifying a terminal device that will be permitted to connect. For this broadcasting, when the terminal device  50  is to be permitted to connect, the base station  10  makes the permission message include a value obtained by subtracting threshold Th from the preamble index. 
     The terminal device  50  compares a value obtained by adding threshold Th to the value included in the permission message with the preamble index value corresponding to the preamble signal  3  transmitted by the terminal device  50 . When they are equal to each other, the terminal device  50  judges that connection establishment has been permitted between the terminal device  50  and the base station  10 , and transmits a control message to the base station  10  so as to establish communication between the terminal device  50  and the base station  10 . 
     As described above, the base station  10  compares a threshold with a preamble index value represented by the preamble signal  3  so as to recognize a terminal device for which it is desired that a connection be established preferentially. In other words, the base station  10  is capable of recognizing a terminal device for which a connection is to be established preferentially by confirming a signal transmitted from the terminal device  50  through a RACH. Accordingly, the terminal device  50  is selected by the base station  10  as a target to be connected preferentially since the first preamble signal transmitted from the terminal device  50  is processed. This means that the terminal device  50  is selected as a target to be connected preferentially before an RRC connection with the base station  10  fails. Accordingly, it is easy for the terminal device  50  to establish a connection with the base station  10  even when a RACH is congested. 
     Further, the PRACH of the terminal device  50  and the PRACH of the non-priority terminal device  2  may use the same band, making it possible to save bandwidth. Also, in some cases, the non-priority terminal device  2  may establish a connection with the base station  10  when there are redundancies in bandwidth. When the base station  10  permits the non-priority terminal device  2  to connect, the base station  10  makes a permission message include a preamble index that corresponds to the preamble signal  3  transmitted from the terminal device that is to obtain the connection permission. The non-priority terminal device  2  compares the value included in the permission message with the preamble index transmitted to the base station  10 , and when they are equal to each other, a connection is established between the non-priority terminal device  2  and the base station  10 . 
     Threshold Th is a value set in accordance with implementation conditions, and is set in accordance with, for example, specifications of permission messages transmitted from the base station  10 . There is a case, for example, where the greatest value that maybe represented by a bit length used for reporting a preamble index value in a permission message is set to threshold Th. It is assumed as an example that six bits are used for reporting a preamble index value of a permission message. In such a case, values other than 1 through are not represented by the prepared bit length, and accordingly the threshold Th is set to 64. In the explanations below, a case is described where threshold Th is the greatest value that may be represented by a bit length used for a permission message for reporting a preamble index value. 
       FIG. 2  explains an example of a configuration of the base station  10 . The base station  10  includes a radio frequency (RF) circuit  11 , a digital signal processor (DSP)  12 , a central processing unit (CPU)  13 , and a memory  14 . The DSP  12 , the CPU  13 , and the memory  14  operate as a RACH processing unit  30 . The RACH processing unit  30  includes an identification unit  31 , a selection unit  32 , a permission message generation unit  33 , a permission message transmission unit  34 , and a storage unit  40 . The DSP  12  operates as the identification unit  31  and the permission message transmission unit  34 . The CPU  13  operates as a System Information Base (SIB) transmission unit  21 , a selection unit  32 , and a permission message generation unit  33 . The memory  14  operates as the storage unit  40 . Also, the storage unit  40  stores a preamble index table  41 . The RF circuit  11  performs a process of multiplying a carrier wave by a signal that has performed a transmission process by the SIB transmission unit  21  or the permission message transmission unit  34 , and transmits the resultant signal to the terminal device  50  or the non-priority terminal device  2  in the cell  1 . Also, the RF circuit  11  eliminates the carrier wave from the signal received from the terminal device  50  or the non-priority terminal device  2 . 
     The SIB transmission unit  21  transmits an SIB generated by the base station  10  to terminal devices in the cell  1 . When the terminal device  50  and the non-priority terminal device  2  are in the cell  1 , an SIB is broadcast to both the terminal device  50  and the non-priority terminal device  2 . An SIB includes information used by the non-priority terminal device  2  or the terminal device  50  for establishing connections with the base station  10 . Examples of information related to a preamble include information used for generating preamble signals, assignment information of PRACHs, and the like. Assignment information of PRACHs includes information used for determining the cyclic shift, the waveform used for representing a preamble index, and the like. Also, in information on generation of preamble signals, a timing at which the terminal device  50  or the non-priority terminal device  2  confirms a permission message is specified. 
     The identification unit  31  identifies a preamble index represented by each of the preamble signals  3  received from the terminal device  50  or the non-priority terminal device  2 . A method of identifying preamble indexes will be explained later in detail. The identification unit  31  makes identified preamble indexes correspond to the numbers of subframes to which are transmitted the preamble signals  3 , and records the resultant information in the preamble index table  41 .  FIG. 3A  illustrates an example of the preamble index table  41 . Note that  FIG. 3A  and  FIG. 3B  illustrate examples of the preamble index table  41 , and the preamble index table  41  may be modified so as to respond to implementation conditions so that it stores information other than preamble indexes and subframe numbers, in addition to such information. 
     The selection unit  32  compares preamble indexes included in the preamble index table  41  with threshold Th so as to confirm whether the base station  10  has received a preamble signal transmitted from the terminal device  50 . When the preamble signal  3  transmitted by the terminal device  50  has received, the selection unit  32  permits the terminal device  50  to connect preferentially over the non-priority terminal device  2 . Operations of the selection unit  32  will be explained later in detail. The selection unit  32  reports to the permission message generation unit  33  the value of the preamble index transmitted from the terminal device that will be permitted to connect. 
     The permission message generation unit  33  uses a preamble index reported from the selection unit  32  so as to generate a permission message. For example, a permission message may be a RACH response message. When the value of a preamble index reported from the selection unit  32  is greater than a threshold, the permission message generation unit  33  may report the difference between the value of the reported preamble index and the threshold by using a permission message. For example, it is assumed that threshold Th is 64 and the preamble index reported from the terminal device  50  that will be permitted to connect is 70. In such a case, the permission message generation unit  33  generates a permission message for reporting the difference between the preamble index and the threshold, i.e., six (6). The permission message generation unit  33  outputs the generated permission message to the permission message transmission unit  34 . The permission message transmission unit  34  transmits the permission message to the terminal device  50  or the non-priority terminal device  2  in the cell  1 . 
       FIG. 4  explains an example of a configuration of the terminal device  50 . The terminal device  50  includes a CPU  51 , a Field Programmable Gate Array (FPGA)  52 , an RF circuit  53 , and a memory  54 . The CPU  51  and the FPGA  52  operate as a RACH processing unit  70 . The RACH processing unit  70  operates as a determination unit  71 , a signal generation unit  72 , a preamble signal transmission unit  73 , a permission message reception unit  74 , and a judgment unit  75 . The CPU  51  operates as an SIB reception unit  61  and a determination unit  71 . The FPGA  52  operates as the signal generation unit  72 , the preamble signal transmission unit  73 , the permission message reception unit  74 , and the judgment unit  75 . The memory  54  operates as a storage unit  80 . The storage unit  80  includes a root sequence number table  81  and a cyclic shift table  82 . The RF circuit  53  eliminates carrier waves from signals received from the base station  10 . Further, the RF circuit  53  performs processes of multiplying carrier waves by the signals that have performed a transmission process by the preamble signal transmission unit  73 , and transmits the obtained signal to the base station  10 . 
     The SIB reception unit  61  obtains the SIB transmitted from the base station  10 , and identifies information included in the SIB. The SIB reception unit  61  reports to the signal generation unit  72  information used for generating a preamble. Further, the SIB reception unit  61  reports information included in the SIB in response to requests from the permission message reception unit  74 , etc. 
     The determination unit  71  determines a preamble index. The determination unit  71  is capable of generating random numbers. A method of determining a preamble index will be explained later. The determination unit  71  reports the determined preamble index to the signal generation unit  72  and the judgment unit  75 . 
     The signal generation unit  72  generates a preamble signal  3  that corresponds to the determination unit  71 . Upon this generation, the signal generation unit  72  obtains information used for generating a preamble signal from the SIB reception unit  61 . Further, the signal generation unit  72  refers to the root sequence number table  81  and the cyclic shift table  82 . A method of generating the preamble signal  3  will be explained later in detail. The signal generation unit  72  outputs the generated preamble signal  3  to the preamble signal transmission unit  73 . The preamble signal transmission unit  73  transmits the preamble signal  3  to the base station  10 . 
     The permission message reception unit  74  identifies information included in the permission message received from the base station  10 . For example, the permission message reception unit  74  obtains the value reported in the permission message, and outputs the value to the judgment unit  75 . The judgment unit  75  compares the value obtained by adding threshold Th to the value input from the permission message reception unit  74  with the preamble index value reported from the determination unit  71 . When they are equal, the judgment unit  75  judges that a connection has been permitted between the terminal device  50  and the base station  10 . When a connection has been permitted, the terminal device  50  transmits to the base station  10  a control signal for establishing the connection. 
     Hereinafter, detailed explanations will be given for an example of operations performed when the terminal device  50  having received an SIB generates the preamble signal  3 .  FIG. 5  illustrates an example of information elements included in an SIB. Although  FIG. 5  illustrates information elements related to a preamble, an SIB includes other information used by the terminal device  50  for performing communication with the base station  10  such as information related to the transmission power. In this example, rach-Config Common is information used for generating a preamble signal, and prach-config SIB is assignment information of a PRACH. The SIB reception unit  61  reports to the signal generation unit  72  the values of rootSequencelndex, zeroCorrelationZoneConfig, and the like included in the PRACH assignment information. 
     The determination unit  71  determines a preamble index. The determination unit  71  generate a random number equal to or greater than 1 and equal to or smaller than threshold Th, and adds the obtained random number to threshold Th so as to use the resultant value as a preamble index. For example, when threshold Th is 64, the determination unit  71  generates a random number in a range between 1 through 64, and adds 64 to the generated random number so as to generate a random number greater than threshold Th. Accordingly, when the value of the random number generated by the determination unit  71  is 1, the value of the generated preamble index is 65. The purpose of the determination unit  71  using a random number for generating a preamble index is to reduce the possibility of collisions between signals having the same preamble signals generated by different terminal devices  50 . The determination unit  71  reports the generated random number to the signal generation unit  72  and the judgment unit  75 . 
     The signal generation unit  72  uses the value of rootSequencelndex so as to determine on the basis of the root sequence number table  81  a waveform for generating a preamble signal representing the preamble index input from the determination unit  71 . 
       FIG. 6  illustrates an example of the root sequence number table  81 . The root sequence number table  81  records logical root sequence numbers and physical root sequence numbers (u) in an associated manner. A rootSequencelndex included in an SIB specifies a logical root sequence number. A logical root sequence number has the same value as the value specified by rootSequencelndex. When, for example, one value is specified from among 0 through 23 by rootSequencelndex, logical root sequence numbers are also in a range from 0 through 23. The group of physical root sequence numbers associated with 0 through 23 as logical root sequence numbers are recorded in the top record of the root sequence number table  81  in the example illustrated in  FIG. 6 . Accordingly, when one value is specified from 0 through 23 by rootSequencelndex, the signal generation unit  72  selects a physical root sequence number used for generating the preamble signal  3  from the top record of the root sequence number table  81 . Similarly, when rootSequencelndex is in a range of 24 through 29, a physical root sequence number is selected from the second record of the root sequence number table  81  of  FIG. 6 , and when rootSequencelndex is in a range of 30 through 35, a physical root sequence number is selected from the third record of  FIG. 6 . The signal generation unit  72  may generate as many preamble signals  3  as the number of times of cyclic shifting for each physical root sequence number. 
     The signal generation unit  72  uses the value of zeroCorrelationZoneConfig and the cyclic shift table  82  so as to obtain the number of times of cyclic shifting.  FIG. 7  illustrates an example of the cyclic shift table  82 . The cyclic shift table  82  records Ncs Configurations and Ncs values are recorded in an associated manner. An Ncs Configuration is the same value as that reported by zeroCorrelationZoneConfig. An Ncs value represents a time period over which the waveform of a signal is shifted when a cyclic shift value has been incremented by one. The signal generation unit  72  obtains an Ncs value by using the cyclic shift table  82  ( FIG. 7 ). When, for example, 8 is specified as zeroCorrelationZoneConfig, the Ncs Configuration is 8. The signal generation unit  72  recognizes that the Ncs value is 46Ts by referring to the cyclic shift table  82 . In other words, the signal generation unit  72  recognizes that the waveform of a signal shifts by 46Ts when the cyclic shift value has been incremented by one. In this example, Ts represents the time period of 1/(15000×2048) second. 
       FIG. 8  explains an example of cyclic shifting. In the example of  FIG. 8 , a case is illustrated in which the preamble signal  3  has a time period of 839Ts. As illustrated in  FIG. 8 , the length of the preamble signal  3  is prescribed, and accordingly an integer obtained by truncating numbers after the decimal point of the value obtained by dividing the length of a preamble signal by the Ncs value is the number of times of cyclic shifting. When, for example, the Ncs value is 46Ts, the number of times of cyclic shifting is 18 because 839/46=18.23••. Accordingly, when the Ncs value is 46Ts, the signal generation unit  72  may represent eighteen types of preamble indexes by using a waveform specified by one physical root sequence number.  FIG. 8  illustrates the preamble signals  3  generated by using the same physical root sequence numbers. The preamble signal  3 A illustrates the preamble signal  3  that has not been cyclically shifted. The preamble signal  3 B illustrates the preamble signal  3  that has been cyclically shifted by the Ncs value. The waveform of 0Ts through 45Ts in the preamble signal  3 A corresponds to the waveform of 46Ts through 91Ts in the preamble signal  3 B having one as the number of times of cyclic shifting. Similarly, in the preamble signal  3  having the cyclic shifting value of 6, the waveform is shifted as illustrated in the preamble signal  3 C. 
     The signal generation unit  72  uses the waveform specified by the physical root sequence number and the cyclic shifting so as to generate the preamble signal  3  representing the preamble index. The signal generation unit  72  selects a physical root sequence number that specifies the waveform used for representing the preamble index from the group of physical root sequence numbers associated with the logical root sequence number of the same value as that of rootSequencelndex. When it is assumed that a value obtained by rounding up the numbers after the decimal point of a value obtained by dividing the preamble index value by the number of times of cyclic shifting is x, the signal generation unit  72  uses the waveform specified by the x-th physical root sequence number corresponding to the logical root sequence number. For example, when the preamble index is 1, the signal generation unit  72  treats the waveform corresponding to the first physical root sequence number in the record specified by rootSequencelndex as the preamble signal  3 . When rootSequencelndex is 3, the logical root sequence number is 3, and accordingly the signal generation unit  72  refers to the first record of the root sequence number table  81  (illustrated in  FIG. 6 ). The signal generation unit  72  treats as the preamble signal  3  the waveform having a physical root sequence number of 129 when the preamble index is 1. 
     The signal generation unit  72  cyclically shifts the waveform specified by a physical root sequence number as the preamble index increases. When a reminder left when dividing the value of a preamble index by the number of times of cyclic shifting is assumed to be y, the signal generation unit  72  treats y as a cyclic shift value. In other words, the signal generation unit  72  treats as the preamble signal  3  the waveform obtained by cyclically shifting, by the length of the Ncs value multiplied by y, the waveform specified by the selected physical root sequence number. Accordingly, the relationship between physical root sequence numbers used for representing preamble index values and cyclic shifting is as below, where u represents a physical root sequence number. 
     PremableIndex=1: u=129, cyclic shift value=0 
     PremableIndex=2: u=129, cyclic shift value=1
 
PremableIndex=3: u=129, cyclic shift value=2
 
PremableIndex=19: u=129, cyclic shift value=18
 
PremableIndex=20: u=710, cyclic shift value=0
 
PremableIndex=39: u=140, cyclic shift value=0
 
PremableIndex=58: u=669, cyclic shift value=0
 
PremableIndex=65: u=669, cyclic shift value=7
 
PremableIndex=66: u=669, cyclic shift value=8
 
PremableIndex=67: u=669, cyclic shift value=9
 
     In the above method, the signal generation unit  72  generates a signal representing a preamble index reported by the determination unit  71 . When, for example, a preamble index is 65, the signal generation unit  72  shifts the waveform having a physical root sequence number of 669 by 322Ts (46Ts×7 times) so as to generate the preamble signal  3 . The signal generation unit  72  outputs the generated preamble signal  3  to the preamble signal transmission unit  73 . The preamble signal transmission unit  73  transmits the input signal to the base station  10 . 
       FIGS. 9A and 9B  are flowcharts explaining an example of operations of the base station  10 . Referring to  FIGS. 9A and 9B , Explanations will be given for an example of operations of the base station  10  after it has received the preamble signal  3 . Note that the base station  10  holds the value of threshold Th beforehand as described above. In the example illustrated in  FIGS. 9A and 9B , a permission message is a RACH response message. Also, variable n and constant K are used for generating a permission message. Constant K represents the number of RACH responses transmitted in one subframe. Variable n is used for counting the number of generated RACH responses. 
     When the SIB transmission unit  21  has transmitted an SIB, the identification unit  31  predicts the waveform of a preamble signal to be received for each preamble index. This prediction is based on the physical root sequence number corresponding to the waveform specified by the transmitted SIB and the amount of cyclic shifting. Upon making this prediction, the identification unit  31  also predicts the waveform of the preamble signal  3  corresponding to a preamble index having a value greater than threshold Th. When the base station  10  has received the preamble signal  3 , the identification unit  31  performs matching between the predicted waveform and the received preamble signal  3  so as to obtain the preamble index value that corresponds to the preamble signal  3  (step S 1 ). The identification unit  31  associates the obtained preamble index value with a subframe number, and records them in the preamble index table  41  ( FIG. 3 ) (step S 2 ). Thereby, the identification unit  31  and the selection unit  32  may recognize the order in which preamble indexes were received by the base station  10  by confirming subframe numbers on the preamble index table  41 . 
     When generation of permission messages for preamble signals has started, the permission message generation unit  33  sets variable n to zero. Also, the selection unit  32  deletes data that has been associated with subframe numbers that are smaller than the greatest subframe number by a difference of a certain value or greater, among pieces of data included in the preamble index table  41  (step S 11 ). For example, the selection unit  32  deletes pieces of data associated with subframe numbers smaller than the greatest subframe number recorded in the preamble index table  41  by a difference of ten or greater. By this process, old pieces of data are deleted, and only data that has been received relating to the preamble signals  3  by the base station  10  within a prescribed period of time after the present time is recorded in the preamble index table  41 . 
     Next, the selection unit  32  confirms whether or not a preamble index value greater than threshold Th is recorded in the preamble index table  41  (step S 12 ). When a preamble index value greater than threshold Th is recorded in the preamble index table  41 , the selection unit  32  preferentially selects a preamble index value greater than threshold Th (Yes in step S 12  and step S 13 ). When a plurality of preamble index values greater than threshold Th are recorded, the selection unit  32  preferentially selects the preamble index value that was received by the base station  10  earliest. Also, when there are a plurality of preamble indexes that are associated with the same subframe number, the selection unit  32  selects only one of those preamble indexes that are to be selected preferentially. When, for example, threshold Th is 64 and the selection unit  32  has referred to the preamble index table  41  illustrated in  FIG. 3A , the selection unit  32  selects one of the preamble indexes  65  and  67  associated with subframe number  2001 . In this example, it is assumed that the selection unit  32  selected the preamble index  65 . 
     When a preamble index value greater than threshold This not recorded in the preamble index table  41 , the selection unit  32  selects the preamble index having the smallest subframe number (No in step S 12  and step S 14 ). It is assumed as an example that the preamble index table  41  has been updated in step S 11  as illustrated in  FIG. 3B . In such a case, the selection unit  32  judges in step S 12  that a preamble index greater than a threshold is not recorded in the preamble index table  41 . Accordingly, the selection unit  32  selects a preamble index  18  having subframe number  1999 . When the selection of preamble index is terminated, the selection unit  32  deletes the selected preamble index value from the preamble index table  41 , and reports to the permission message generation unit  33 . 
     The permission message generation unit  33  generates a RACH response message including a Random Access Preamble Identifier (RAPID). A RAPID records a preamble index corresponding to the preamble signal  3  transmitted from a terminal device permitted by the base station  10  to connect.  FIG. 10  illustrates an example of a format of a sub header of a RACH response message. “E” (extended bit) indicates whether or not there is information following the RAPID. Because the sub header in this example does not include information following the RAPID, the bit is set to zero. “T” indicates whether or not the sub header includes a random access ID, and “1” is set for a message including a RAPID. As illustrated in  FIG. 10 , the RAPID included in a RACH response message is of six bits, and accordingly values from 1 through 64 are recorded in a RAPID. Because threshold Th is the maximum value that may be represented by the number of bits used for storing a RAPID, preamble indexes greater than threshold Th are not recorded in a RAPID. Accordingly, the permission message generation unit  33  compares the preamble index value reported from the selection unit  32  with threshold Th, determines the value of the RAPID in accordance with the comparison result, and generates a RACH response message (step S 15 ). In other words, when a selected preamble index is greater than threshold Th, the permission message generation unit  33  stores, in the RAPID, a value obtained by subtracting threshold Th from the preamble index value. It is assumed as an example that the preamble index value selected in step S 13  is 65, greater than threshold Th, 64. In such a case, the permission message generation unit  33  generates a RACH response message having as a RAPID a value obtained by subtracting threshold Th from a preamble index value (65−64=1). When a selected preamble index is smaller than threshold Th as described in step S 14 , the permission message generation unit  33  generates a RACH response message including a preamble index. 
     The permission message generation unit  33  outputs the generated RACH response message to the permission message transmission unit  34 . Also, the permission message generation unit  33  increments variable n by one (step S 16 ). Further, the permission message generation unit  33  compares variable n with constant K (step S 17 ). These processes of steps S 12  through S 17  are repeated until variable n becomes equal to or greater than constant K. 
     The permission message transmission unit  34  transmits the RACH response message input from the permission message generation unit  33 , to the terminal device  50  and the non-priority terminal device  2  in the cell  1 . Upon this transmission, the permission message transmission unit  34  uses the number of the subframe that includes the preamble signal  3 , and the RACH response window size, so as to transmit the message at a timing at which a terminal device to be permitted to connect may receive the RACH response message. 
     Next, explanations will be given for operations performed when the terminal device  50  has received a RACH response message. When the terminal device  50  has received a RACH response message, the permission message reception unit  74  obtains the value specified as the RAPID. The permission message reception unit  74  outputs the value specified as the RAPID to the judgment unit  75 . The permission message reception unit  74  also obtains information related to bandwidth assignment or the like from the RACH response message appropriately. 
     The judgment unit  75  compares a value obtained by adding threshold Th to the value specified as the RAPID with the preamble index represented by the preamble signal  3  transmitted by the terminal device  50 . When they are equal, the judgment unit  75  judges that the terminal device  50  has been given permission to connect with the base station  10 . Accordingly, the terminal device  50  transmits a connection request message to the base station  10  by using the band specified by the information that the permission message reception unit  74  obtained. 
       FIG. 11  is a sequence diagram explaining an example of transmission and reception of messages between the base station  10  and the terminal device  50 . By referring to  FIG. 11 , operations performed by the terminal device  50  and the base station  10  will be described over the course of time. 
     (1): The terminal device  50  transmits the preamble signal  3  to the base station  10  through a RACH. 
     (2): The identification unit  31  included in the base station  10  obtains the preamble index represented by the preamble signal  3  so as to record the index in the preamble index table  41 . Further, the selection unit  32  compares the obtained preamble index with threshold Th.
 
(3): The selection unit  32  preferentially selects a preamble index greater than threshold Th.
 
(4): When the preamble index selected by the selection unit  32  is greater than the threshold Th, the base station  10  broadcasts to terminal devices in the cell  1  a permission message reporting the difference between the preamble index and threshold Th.
 
(5): The terminal device  50  compares a value obtained by adding threshold Th to the value reported in the permission message with the preamble index value represented by the preamble signal  3  transmitted by the terminal device  50 . When they are equal, the terminal device  50  judges that establishment of a connection with the base station  10  has been permitted.
 
(6): When it has been judged that a connection has been permitted, the terminal device  50  transmits an RRC connection request message to the base station  10 .
 
(7): The base station  10  transmits an RRC connection set up message to the terminal device  50 . Communication will be continued between the terminal device  50  and the base station  10  also after this transmission.
 
     As described above, according to the present embodiment, the terminal device  50 , which is a priority terminal device, generates the preamble signal  3  by using the value of a preamble index that is not used by the non-priority terminal device  2 , and transmits the preamble signal  3  to the base station  10 . Accordingly, the base station  10  may recognize a priority terminal device by comparing the preamble index value represented by the preamble signal  3  with the threshold so that the base station  10  may permit a priority terminal device preferentially to connect when responding to the preamble signal. Accordingly, it is easy for the terminal device  50  as a priority terminal device to establish a connection with the base station  10 . 
     Note that when a preamble index used by the non-priority terminal device  2  is equal to the value specified as a RAPID, there is a possibility of collision between an RRC connection request message transmitted from the terminal device  50  and a message transmitted from the non-priority terminal device  2 . In such a case, the terminal device  50  transmits a preamble signal again. When a preamble signal is to be transmitted a second time or subsequent times, the determination unit  71  generates a new preamble index, and in such cases too, the determination unit  71  uses a value greater than threshold Th for a preamble index. Accordingly, the preamble signal  3  that is transmitted again is also selected preferentially by the base station  10 , and therefore there is a higher possibility for the terminal device  50  to be permitted to connect with the base station  10  than for the non-priority terminal device  2 . 
     Note that embodiments are not limited to the above described examples, and various modifications are allowed. For example, the determination unit  71  may be set beforehand to generate a random number equal to or greater than threshold Th. 
     It is made easier to establish a connection with a base station for a terminal device for which it is desired that a connection be established with abase station preferentially. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations 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 one or more embodiments of the present invention 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.