Patent Publication Number: US-2015080045-A1

Title: Base station, wireless communication system, and wireless communication method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-192284, filed on Sep. 17, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a base station, a wireless communication system and a wireless communication method. 
     BACKGROUND 
     To date, wireless communication systems of long term evolution (LTE) and so forth are known. There is also known random access processing with which, in a wireless communication system, the occurrence of collisions of signals is reduced when a plurality of mobile stations make requests for communication at the same time, so that communication channels are set with good efficiency (for example, refer to International Publication Pamphlet No. WO 2010/050497 and Japanese National Publication of International Patent Application No. 2012-526425). In the random access processing, random access channels (RACHs) are used in order for a mobile station to make a request for allocation of communication channels. 
     SUMMARY 
     According to an aspect of the invention, a base station includes a receiver configured to receive a random access signal wirelessly transmitted from a terminal, a transmitter configured to wirelessly transmit a response signal in response to the random access signal, and a processer configured to limit a transmission of the response signal based on a timing difference between a reference timing and a reception timing when the random access signal has been received. 
     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, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a block diagram illustrating an example of a communication system according to a first embodiment; 
         FIG. 1B  is a block diagram illustrating an example of the flow of signals in the communication system illustrated in  FIG. 1A . 
         FIG. 2  is a block diagram illustrating an example of a communication system according to a second embodiment; 
         FIG. 3  is a sequence diagram illustrating an example of initial access of a mobile station to a wireless base station device; 
         FIG. 4A  is a block diagram illustrating an example of a base station device according to the second embodiment; 
         FIG. 4B  is a block diagram illustrating an example of the flow of signals in the base station device illustrated in  FIG. 4A ; 
         FIG. 5A  is a block diagram illustrating an example of a random access signal processing unit; 
         FIG. 5B  is a block diagram illustrating an example of the flow of signals in the random access signal processing unit illustrated in  FIG. 5A ; 
         FIG. 6  is a sequence diagram illustrating an example of random access signal detection processing; 
         FIG. 7  is a graph illustrating an example of a delay profile; 
         FIG. 8  is a graph illustrating a first example of limitation of response processing based on a result of detection; 
         FIG. 9  is a flowchart illustrating an example of processing performed by a base station device according to the first example; 
         FIG. 10A  is a table illustrating an example of results of detection for every random access signal ID; 
         FIG. 10B  is a table illustrating an example of the number of detections for every TA command value; 
         FIG. 10C  is a table illustrating an example of a result of comparison between the number of detections for every TA command value and a threshold; 
         FIG. 10D  is a table illustrating an example of a result of discarding of random access signals for every TA command value; 
         FIG. 11  is a graph illustrating a second example of limitation of response processing based on a result of detection; 
         FIG. 12  is a flowchart illustrating an example of processing performed by the base station device according to the second example; 
         FIG. 13A  is a table illustrating an example of a comparison result between the number of detections for every TA command value and a threshold; 
         FIG. 13B  is a table illustrating an example of a result of discard of random access signals for every TA command value; 
         FIG. 14A  is a block diagram illustrating an example of a mobile station according to the second embodiment; 
         FIG. 14B  is a block diagram illustrating an example of the flow of signals in the mobile station illustrated in  FIG. 14A ; and 
         FIG. 15  is a pictorial representation illustrating an example of application of a communication system according to the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of a base station and a wireless communication system according to the present disclosure will be described in detail with reference to the accompanying drawings. 
     In the aforementioned conventional technology, even if random access processing has been performed, there are some cases where a mobile station moves past and away from a base station device in a short period of time and therefore the random access processing becomes unnecessary, which raises the problem that the throughput of the base station device decreases. 
     In one aspect, it is an object of the disclosed embodiments to provide a base station device and a communication system with which throughput may be improved. 
     First Embodiment 
       FIG. 1A  is a block diagram illustrating an example of a communication system according to a first embodiment.  FIG. 1B  is a block diagram illustrating an example of the flow of signals in the communication system illustrated in  FIG. 1A . As illustrated in  FIG. 1A  and  FIG. 1B , a communication system  100  according to the first embodiment includes a base station device  110  and a terminal device  120 . 
     The terminal device  120  is a communication device, such as a mobile station, which is capable of wirelessly communicating with the base station device  110 . The terminal device  120  may be implemented as a plurality of terminal devices. At the time of establishing wireless communication with the base band device  110 , the terminal device  120  wirelessly transmits, for example, a contention-based random access signal (RACH preamble) to the base station device  110 . 
     The base station device  110  performs wireless communication with a terminal device (for example, the terminal device  120 ) located in the cell of the base station device  110 . The base station device  110  includes a receiver  111 , a transmitter  112 , and a controller  113 . The receiver  111  receives a random access signal wirelessly transmitted from the terminal device  120 . Then, the receiver  111  outputs a result of reception of the random access signal to the controller  113 . 
     In accordance with control from the controller  113 , the transmitter  112  wirelessly transmits a response signal in response to the random access signal received by the receiver  111 . As a result, the transmitter  112  is capable of performing random access processing with the terminal device  120 . 
     The controller  113  controls wireless transmission of response signals performed by the transmitter  112 . For example, the controller  113  derives information indicating a timing difference relative to a reference point in time (reference timing) of a random access signal received by the receiver  111 , based on a result of reception (reception timing) of the random access signal output from the receiver  111 . Then, based on the derived information indicating a timing difference, the controller  113  controls limitation (or suppression) of wireless transmission of a response signal performed by the transmitter  112 . 
     In this way, with the base station device  110 , limitation of wireless transmission of a response signal may be controlled in accordance with a timing difference relative to the reference point in time of the received random access signal. This may reduce unnecessary random access processing, thereby improving throughput. 
     Specific Example of Information Indicating Timing Difference 
     For example, the controller  113  measures a timing difference relative to the reference point in time of a random access signal received by the receiver  111 . Then, the controller  113  causes a response signal containing a timing adjustment (TA), which gives an instruction for adjusting the timing of transmission of a wireless signal by the measured timing difference, to be wirelessly transmitted from the transmitter  112 . 
     Thereby, the terminal device  120  is capable of adjusting the timing of transmission of a wireless signal performed by the terminal device  120 . In this case, information indicating a timing difference relative to the reference point in time of a random access signal may be referred to as, for example, a TA command. However, the information indicating a timing difference relative to the reference point in time of a random access signal is not limited to the TA command and may be, for example, a value obtained by digitizing the measured timing difference by a predetermined resolution. The timing difference indicated by the TA command is referred to as a TA command value hereinafter. 
     Specific Example of Limitation of Wireless Transmission of Response Signal 
     For example, the controller  113  measures timing differences relative to the reference point in time of a plurality of random access signals received by the receiver  111  within a predetermined period of time. The controller  113  also classifies the plurality of random access signals into groups each in accordance with a measured timing difference. Then, the controller  113  extracts, among the groups, a group where the number of random access signals classified into that group exceeds a predetermined number. 
     If a group where the number of random access signals classified into that group exceeds the predetermined number is extracted, the controller  113  also limits (in other words, suppresses, ceases, discontinues, prevents or prohibits) wireless transmission of a response signal performed by the transmitter  112  in response to a random access signal classified into the extracted group. For example, the controller  113  controls the transmitter  112  so as not to wirelessly transmit response signals in response to at least some of the random access signals classified into the extracted group. 
     Here, all the random access signals classified into the same group may be estimated to be random access signals transmitted from terminal devices whose distances from the base station device  110  are at the same level. Therefore, each random access signal of a group containing a large number of random access signals is highly likely to be a random access signal transmitted from one of terminal devices that are located close together. In many cases, such terminal devices are, for example, terminal devices that are moving due to being on a (public) transportation vehicle, such as a train. 
     Accordingly, by limiting wireless transmission of response signals in response to random access signals transmitted from such terminal devices, it is possible to reduce random access processing of the terminal devices that are highly likely to move out of range of the base station device  110 , or to be handed over, in a short period of time. The throughput in the base station device  110  may therefore be improved. 
     Exclusion from Limitation for Timing Difference 
     The base station device  110  may also include a storage unit configured to store information indicating a predetermined timing difference. The predetermined timing difference is, for example, a timing difference relative to the reference point in time in the base station device  110 , which is calculated in the base station device  110  for a random access signal transmitted from a predetermined location through which a transportation vehicle, such as a train, passes. 
     The location through which a transportation vehicle, such as a train, passes is limited, and therefore the timing difference measured for a random access signal transmitted from a predetermined location through which a transportation vehicle, such as a train, passes is also limited. Therefore, based on an actual measurement value and so on, information indicating a predetermined timing difference may be stored in advance. 
     The information indicating a predetermined timing difference is not limited to information indicating the predetermined timing itself, and may be information by which the predetermined timing difference is identifiable. For example, the information indicating a predetermined timing difference may be information indicating each timing difference different from the predetermined timing difference. 
     The controller  113  may be configured not to limit wireless transmission of a response signal performed by the transmitter  112  for a random access signal having a timing difference different from the predetermined timing difference, based on information stored by the storage unit. Thus, wireless transmission of a response signal is not limited for a random access signal that has not been transmitted from a transportation vehicle, such as a train, so that a decrease in throughput may be limited. 
     Exclusion from Limitation for Period 
     Also, the base station device  110  may include a storage unit configured to store information indicating a predetermined period of time. The predetermined period of time is, for example, a period of time during which a transportation vehicle, such as a train, passes the cell of the base station device  110 . 
     The period during which a transportation vehicle, such as a train, passes is limited, and therefore the period during which a random access signal is transmitted from a transportation vehicle, such as a train, is also limited. Therefore, based on an actual measurement value, a time table of a transportation vehicle, and so on, information indicating a predetermined timing difference may be stored in advance. 
     The information indicating a predetermined period of time is not limited to information indicating the predetermined period of time itself, and may be information by which the predetermined period of time is identifiable. For example, the information indicating a predetermined period of time may be information indicating each period of time different from the predetermined period of time. 
     The controller  113  may be configured not to limit wireless transmission of a response signal performed by the transmitter  112  for a random access signal that has been received during a period different from the predetermined period, based on information stored by the storage unit. Thus, wireless transmission of a response signal is not limited for a random access signal that has not been transmitted from a transportation vehicle, such as a train, so that a decrease in throughput may be limited. 
     The controller  113  may also be configured not to limit wireless transmission of a response signal for a random access signal that has been received during a period of time different from the predetermined period of time or that has a timing difference different from the predetermined timing difference, based on each piece of information mentioned above and stored by the storage unit. Thus, wireless transmission of a response signal is not limited for a random access signal that has not been transmitted from a transportation vehicle, such as a train, so that a decrease in throughput may be limited. 
     Second Embodiment 
     Communication System According to Second Embodiment 
       FIG. 2  is a block diagram illustrating an example of a communication system according to a second embodiment. As illustrated in  FIG. 2 , a communication system  200  according to the second embodiment includes base station devices  211  and  212  and mobile stations  221  to  228 . 
     The mobile stations  221  to  224  are located in a cell  211   a  of the base station device  211 , and perform wireless communication with the base station devices  211 . The mobile stations  225  to  228  are located in a cell  212   a  of the base station device  212 , and perform wireless communication with the base station device  212 . 
     The base station device  211  is connected through an S1 interface  231  to a core network  240 . The base station device  212  is connected through an S1 interface  232  to the core network  240 . The base station device  211  and the base station device  212  are connected to each other through an X2 interface  250 . 
     The base station device  110  illustrated in  FIG. 1A  and  FIG. 1B  is applicable to, for example, the base station devices  211  and  212 . The terminal device  120  illustrated in  FIG. 1A  and  FIG. 1B  is applicable to, for example, the mobile stations  221  to  228 . 
     Initial Access to Wireless Base station device of Mobile Station 
       FIG. 3  is a sequence diagram illustrating an example of initial access to a wireless base station device of a mobile station. With reference to  FIG. 3 , by way of example, the case where the mobile station  221  initially accesses the base station device  211  will be described. 
     First, the mobile station  221  wirelessly transmits a random access signal (RACH preamble), as a message  1 , to the base station device  211  (step S 301 ). Thereafter, the base station device  211  and the mobile station  221  perform connection using radio resources such as a downlink physical channel and an uplink physical channel defined in LTE. 
     For example, the base station device  211  wirelessly transmits a RACH response, as a message  2 , to the mobile station  221  (step S 302 ). In response to this, the mobile station  221  wirelessly transmits a message  3  to the base station device  211  (step S 303 ). In response to this, the base station device  211  wirelessly transmits a message  4  to the mobile station  221  (step S 304 ). Through the process of these steps, the base station device  211  links the random access signal received in step S 301  with the mobile station  221 , and starts wireless transmission with the mobile station  221 . 
     Examples of the downlink physical channel include a physical downlink control channel (PDCCH) and a physical downlink shared channel (PDSCH). Examples of the uplink physical channel include a physical uplink control channel (PUCCH) and a physical uplink shared channel (PUSCH). 
     Base station device According to Second Embodiment 
       FIG. 4A  is a block diagram illustrating an example of a base station device according to a second embodiment.  FIG. 4B  is a block diagram illustrating an example of the flow of signals in the base station device illustrated in  FIG. 4A . The base station device  211  will be described here, and a similar description applies to the base station device  212 . In the example illustrated in  FIG. 4A  and  FIG. 4B , the base station device  211  is a base station device that performs wireless communication based on orthogonal frequency division multiplexing access (OFDMA). 
     The base station device  211  includes, as illustrated in  FIG. 4A  and  FIG. 4B , a transmitter  410 , a digital-to-analog (D/A) converter  421 , a transmission radio frequency (RF) unit  422 , an antenna  423 , a reception RF unit  424 , an analog-to-digital (A/D) converter  425 , a receiver  430 , and a scheduler  440 . 
     The transmitter  410  performs modulation processing of downlink signals to be transmitted. For example, the transmitter  410  includes an error correction encoder  411 , a data modulator  412 , a data-pilot signal multiplexer  413 , an inverse fast Fourier transform (IFFT) unit  414 , and a cyclic prefix (CP) insertion unit  415 . Each of the error correction encoder  411 , the data modulator  412 , the data-pilot signal multiplexer  413 , the IFFT unit  414 , and the CP insertion unit  415  operates in accordance with a control signal from the scheduler  440 . 
     A downstream transmission data signal output from a higher-level processing unit and to be transmitted by the base station device  211  is input to the error correction encoder  411 . The error correction encoder  411  performs error correction encoding on the input transmission data signal. Then, the error correction encoder  411  outputs the transmission data signal on which error correction encoding has been performed to the data modulator  412 . 
     The data modulator  412  performs modulation using a transmission data signal output from the error correction encoder  411 . Then, the data modulator  412  outputs a transmission data signal obtained by modulation to the data-pilot signal multiplexer  413 . 
     The transmission data signal output from the data modulator  412  and a pilot signal are input to the data-pilot signal multiplexer  413 . The data-pilot signal multiplexer  413  multiplexes the input transmission data signal and pilot signal. Then, the data-pilot signal multiplexer  413  outputs a transmission signal obtained by multiplexing to the IFFT unit  414 . 
     The IFFT unit  414  performs inverse fast Fourier transform (IFFT) on the transmission signal output from the data-pilot signal multiplexer  413 . Then, the IFFT unit  414  outputs the transmission signal on which IFFT has been performed to the CP insert unit  415 . 
     The CP insertion unit  415  inserts a cyclic prefix (CP) to the transmission signal output from the IFFT unit  414 . Then, the CP insertion unit  415  outputs the transmission signal to which the CP has been inserted to the D/A converter  421 . 
     The D/A converter  421  converts the transmission signal output from the transmitter  410  into an analog signal. Then, the D/A converter  421  outputs the transmission signal converted into the analog signal to the transmission RF unit  422 . 
     The transmission RF unit  422  performs radio frequency (RF) processing on the transmission signal output from the D/A converter  421 . The RF processing performed by the transmission RF unit  422  includes, for example, conversion from a baseband frequency band to a radio frequency band. Then, the transmission RF unit  422  outputs the transmission signal on which RF processing has been performed to the antenna  423 . 
     The antenna  423  wirelessly transmits transmission signals output from the transmission RF unit  422  to mobile stations such as the mobile station  221 . The antenna  423  also receives signals wirelessly transmitted from mobile stations such as the mobile station  221 . Then, the antenna  423  outputs the received signals (reception signals) to the reception RF unit  424 . 
     The reception RF unit  424  performs RF processing on a reception signal output from the antenna  423 . The RF processing performed by the reception RF unit  424  includes, for example, conversion from a radio frequency band to a baseband frequency band. Then, the reception RF unit  424  outputs the reception signal on which RF processing has been performed to the A/D converter  425 . 
     The A/D converter  425  converts the reception signal output from the reception RF unit  424  to a digital signal. Then, the A/D converter  425  outputs the reception signal converted to the digital signal to the receiver  430 . 
     The receiver  430  performs demodulation processing on the received uplink signal. For example, the receiver  430  includes a CP removal unit  431 , a fast Fourier transform (FFT) unit  432 , a data-pilot signal separation unit  433 , a random access signal processing unit  434 , a distribution creation processing unit  435 , a demodulator  436 , and an error correction decoder  437 . Each of the CP removal unit  431 , the FFT unit  432 , the data-pilot signal separation unit  433 , the random access signal processing unit  434 , the distribution creation processing unit  435 , the demodulator  436 , and the error correction decoder  437  operates in accordance with a control signal from the scheduler  440 . 
     The CP removal unit  431  removes a CP inserted into a reception signal output from the A/D converter  425 . Then, the CP removal unit  431  outputs the reception signal from which the CP has been removed to the FFT unit  432 . 
     The FFT unit  432  performs fast Fourier transform (FFT) of the reception signal output from the CP removal unit  431 . Then, the FFT unit  432  outputs the reception signal on which FFT has been performed to the data-pilot signal separation unit  433 . 
     The data-pilot signal separation unit  433  performs demultiplexing of the reception signal output from the FFT unit  432 . Then, the data-pilot signal separation unit  433  outputs upstream reception data of RACH obtained by the demultiplexing to the random access signal processing unit  434 . The data-pilot signal separation unit  433  also outputs a reception data signal of PUSCH or PUCCH obtained by the demultiplexing to the demodulator  436 . 
     The random access signal processing unit  434  performs random access signal processing based on the upstream reception data output from the data-pilot signal separation unit  433 . For example, the random access signal processing unit  434  detects random access signals from the upstream reception data and calculates TA command values (timing differences) for the detected random access signals. Then, the random access signal processing unit  434  outputs the result of detection of random access signals and TA command values calculated for the detected random access signals to the distribution creation processing unit  435 . 
     Based on the result of detection of random access signals and the TA command values both output from the random access signal processing unit  434 , the distribution creation processing unit  435  creates distribution of random access signals (mobile stations) for every TA command value. Then, the distribution creation processing unit  435  outputs the detection result and the TA command values, which have been output from the random access signal processing unit  434 , and the created distribution for every created TA command value to the scheduler  440 . 
     The demodulator  436  demodulates the reception data signals output from the data-pilot signal separation unit  433 . Then, the demodulator  436  outputs the reception data signal obtained by demodulation to the error correction decoder  437 . 
     The error correction decoder  437  performs error correction decoding on the reception data signal output from the demodulator  436 . Then, the error correction decoder  437  outputs the reception data signal obtained by error correction decoding to a higher-level processing unit. The error correction decoder  437  also outputs the result of reception processing obtained by error correction decoding to the scheduler  440 . Examples of the result of reception processing include a result of error detection (acknowledgement or negative acknowledgement) and a channel quality indicator (CQI). 
     The scheduler  440  performs scheduling for selecting a mobile station (for example, the mobile station  221 ) that wirelessly communicates with the base station devices  211 . That is, in a system like a mobile phone, one base station device  211  handles a plurality of mobile stations. Therefore, the scheduler  440  included in the base station device  211  selects a mobile station that actually performs data communication among a plurality of mobile stations for each of the uplink and the downlink. 
     By way of example, the scheduler  440  determines a mobile station to which a signal is to be transmitted and determines the transmission speed using index values calculated based on line quality and transmission data rates, and thus may allocate radio resources. Then, the scheduler  440  outputs control information in accordance with a result of scheduling, thereby controlling the transmitter  410  and the receiver  430 . 
     Also, at the timing of reception of a random access signal, for example, in order for the sequence to proceed as illustrated in  FIG. 3 , the scheduler  440  selects a mobile station for each of the uplink and downlink and allocates radio resources. At the time of reception of a random access signal, the result of detection of the random access signal from the random access signal processing unit  434  is input to the scheduler  440 . Based on the input result of detection a random access signal, the scheduler  440  allocates radio resources to the detected random access signal, and controls the transmitter  410  so as to return a response to a mobile station. 
     The response to a random access signal is made, for example, using a RACH response (message  2 ) illustrated in  FIG. 3 . Then, each time a response from a mobile station is ascertained, the scheduler  440  performs the sequence of random access while using radio resources. At this time, the communication environment and the volume of data possessed by a mobile station are unknown, and therefore the scheduler  440  allocates minimum radio resources for securing the message volume in order to perform the sequence. 
     The scheduler  440  also stores a TA command in a RACH response (message  2 ) to be transmitted from the transmitter  410  in response to a random access signal, thereby causing the mobile station  221  to perform control of a transmission timing. The scheduler  440  also controls limitation of a response, to a random access signal, performed by the transmitter  410 , based on a result of detection of random access signals output from the distribution creation processing unit  435 , TA command values, and distribution for every TA command. 
     The transmitter  410 , the receiver  430 , and the scheduler  440  may be implemented, for example, by a digital circuit  401 . A field programmable gate array (FPGA) and a digital signal processor (DSP), for example, may be used for the digital circuit  401 . 
     The receiver  111  illustrated in  FIG. 1A  and  FIG. 1B  may be implemented, for example, by the antenna  423 , the reception RF unit  424 , the A/D converter  425 , and the receiver  430 . The transmitter  112  illustrated in  FIG. 1A  and  FIG. 1B  may be implemented, for example, by the transmitter  410 , the D/A converter  421 , the transmission RF unit  422 , and the antenna  423 . The controller  113  illustrated in  FIG. 1A  and  FIG. 1B  may be implemented, for example, by the random access signal processing unit  434 , the distribution creation processing unit  435 , and the scheduler  440 . 
     Random Access Signal Processing Unit 
       FIG. 5A  is a block diagram illustrating an example of a random access signal processing unit.  FIG. 5B  is a block diagram illustrating an example of the flow of signals in the random access signal processing unit illustrated in  FIG. 5A . In LTE-based random access signal processing, modulation processing is performed at the time of transmission in the order of a discrete Fourier transform (DFT) process, a process of mapping along the frequency axis, and IFFT. Reversely, at the time of reception, modulation processing is performed in the order of FFT, a process of demapping along the frequency axis, an inverse discrete Fourier transform (IDFT) process. 
     In the configuration illustrated in  FIG. 4A  and  FIG. 4B , a demapping process is performed at a stage before the random access signal processing unit  434 , and therefore the IDFT process and the subsequent processes are performed in the random access signal processing unit  434 . For example, the random access signal processing unit  434  illustrated in  FIG. 4A  and  FIG. 4B  includes, for example, as illustrated in  FIG. 5A  and  FIG. 5B , an IDFT processing unit  501 , a correlation value calculator  502 , a power converter  503 , and a peak detector  504 . 
     The IDFT processing unit  501  performs an IDFT process on upstream reception data output from the data-pilot signal separation unit  433  (for example, refer to  FIG. 4A  and  FIG. 4B ). The IDFT processing unit  501  outputs the upstream reception data on which the IDFT process has been performed to the correlation value calculator  502 . 
     The upstream reception data output from the IDFT processing unit  501  and random access signal replicas (signals for correlation detection) are input to the correlation value calculator  502 . The random access signal replica is a pattern that serves as a candidate random access signal, and is stored in, for example, a memory of the base station device  211 . 
     The correlation value calculator  502  calculates a correlation value with the upstream reception data for every random access signal replica. Then, the correlation value calculator  502  outputs the calculated correlation value for every random access signal replica to the power converter  503 . 
     The power converter  503  converts the correlation value for every random access signal replica output from the correlation value calculator  502 , to a power value. Then, the power converter  503  outputs the correlation value converted to the power value to the peak detection unit  504 . 
     The peak detector  504  performs peak detection processing for the correlation value for every random access signal replica output from the power converter  503 . Then, the peak detector  504  detects a random access signal included in the upstream reception data, based on the peak detection processing. The peak detection unit  504  also calculates a timing difference relative to a predetermined reference point in time (for example, the head of a reception window) of reception of a random access signal detected in the peak detection processing. Then, the peak detector  504  outputs the result of detection of a random access signal and a TA command including the timing difference calculated for the detected random access signal to the distribution creation processing unit  435 . 
     In this way, the random access signal processing unit  434  performs correlation calculation (autocorrelation calculation) between a reception signal and a random access signal replica, by using the correlation value calculator  502 . Thus, a peak of autocorrelation appears if there is an actually transmitted random access signal, and therefore it is possible to determine the presence or absence of a random access signal, based on the peak of autocorrelation. 
     If there are a plurality of random access signals that may be transmitted, peaks are detected for a plurality of corresponding random access signal replicas, respectively. A matched filter, for example, may be used for the correlation calculation in the correlation value calculator  502 . In the matched filter, a correlation value between a reception signal and a random access signal replica is detected, and then the detected correlation value is squared, and is converted to electric power. 
     Random Access Signal Detection Processing 
       FIG. 6  is a sequence diagram illustrating an example of random access signal detection processing. The base station device  211  and the mobile station  221  perform, for example, the steps illustrated in  FIG. 6 . First, the base station device  211  notifies the mobile station  221  located in the cell  211   a  of a random access signal group (step S 601 ). The random access signal group is a plurality of patterns that serve as candidate random access signals. 
     Then, the mobile station  221  selects an arbitrary random access signal from the random access signal group of which the mobile station  221  has been notified in step S 601 , and transmits the selected random access signal to the base station device  211  (step S 602 ). Then, the base station device  211  calculates a correlation value between a random access signal replica corresponding to each pattern of the random access signal group of which the base station device  211  has been notified in step S 601  and a reception signal (step S 603 ). 
     Then, the mobile station  221  identifies the random access signal transmitted by the base station device  211 , based on the magnitude of the correlation value (reception level) calculated in step S 603  (step S 604 ). The base station device  211  also calculates the amount of delay (TA command value) of the random access signal transmitted by the mobile station  221  (step S 605 ). 
     In such a way, in the LTE-based random access signal processing, pattern matching is used with the correlation calculation between the random access signal group of which the mobile station  221  has been notified in advance and the reception signal. 
     Delay Profile 
       FIG. 7  is a graph illustrating an example of a delay profile. In  FIG. 7 , the horizontal axis represents time and the vertical axis represents the receiving level. In LTE, a Zadoff-Chu sequence, for example, is used as random access signals. The cross-correlation between a Zadoff-Chu sequence and the cyclically shifted version of itself is low. 
     Accordingly, only when the heads of the Zadoff-Chu sequences coincide with each other is a high correlation obtained, and as a result, the random access signal processing unit  434  may obtain, for example, a delay profile  701  as illustrated in  FIG. 7 . 
     The random access signal processing is performed in digital processing here, and therefore the delay profile  701  is a delay profile at a discretely sampled resolution. Ts [sec] denotes a period of time during which digital sampling is performed in the delay profile  701 . 
     Using the delay profile  701  obtained by correlation detection, the random access signal processing unit  434  may determine whether the mobile station  221  has transmitted a random access signal for which the correlation detection has been performed. In addition, if the mobile station  221  has transmitted the random access signal for which the correlation detection has been performed, the random access signal processing unit  434  may measure a propagation delay period of time (TA command value) between the mobile station  221  and the base station device  211 . 
     For example, if the peak of the reception level in the delay profile  701  exceeds a predetermined detection threshold  702 , the random access signal processing unit  434  may determine that a random access signal corresponding to the delay profile  701  has been transmitted. 
     In addition, the random access signal processing unit  434  may calculate the propagation delay period of time from a difference between a predetermined processing reference timing  703  and the random access signal in the base station device  211 . It is possible to measure a difference between the processing reference timing  703  and the random access signal, for example, by calculating a timing difference  705  between a timing at the highest reception level in the delay profile  701  and the processing reference timing  703 . 
     The base station device  211  transmits a TA command including the calculated timing difference  705  to the mobile station  221 , thereby causing a timing of transmission of a wireless signal to be adjusted in the mobile station  221 . 
     First Example of Limitation of Response Processing Based on Detection Result 
       FIG. 8  is a graph illustrating a first example of limitation of response processing based on a result of detection. In  FIG. 8 , the horizontal axis represents the TA command value, and the vertical axis represents the number of mobile stations corresponding to random access signals detected by the random access signal processing unit  434 . The distribution creation processing unit  435  creates a distribution  801  based on a result of detection of random access signals output from the demodulator  436  and TA command values. 
     The distribution  801  represents the number of mobile stations for every TA command value. If mobile stations located close together transmit respective random access signals to the base station device  211  at the same timing, the number of mobile stations having a specific TA command value is large as illustrated in  FIG. 8 . 
     In the first example, based on the distribution  801 , the scheduler  440  does not return a RACH response to a random access signal to each of mobile stations (a shaded area  802 ) of a TA command value at which the number of mobile stations exceeds a threshold TH1. 
       FIG. 9  is a flowchart illustrating an example of processing performed by the base station device according to the first example. In the first example illustrated in  FIG. 8 , the base station device  211  performs, for example, the process of steps illustrated in  FIG. 9 . First, the base station device  211  performs the process of the following steps S 901  to S 903  for every random access signal ID. The random access signal ID is identification information for a random access signal replica. 
     The base station device  211  first performs detection processing in the random access signal processing unit  434  using a random access signal replica identified by a random access signal ID in question (step S 901 ). The base station device  211  then determines whether the peak of a delay profile obtained by the detection processing in step S 901  exceeds a threshold (for example, the detection threshold  702 ) (step S 902 ). If the peak does not exceed the threshold (step S 902 : No), the base station device  211  completes the process for the random access signal ID in question. 
     If, in step S 902 , the peak exceeds the threshold (step S 902 : Yes), the base station device  211  stores the random access signal ID in question as a detected random access signal ID (step S 903 ). At this point, the base station device  211  also stores a TA command value in association with the detected random access signal ID. Then, the base station device  211  completes the process for the random access signal ID in question. 
     Having performed the process of steps S 901  to S 903  for every random access signal ID, the base station device  211  calculates the number of detections of random access signal IDs for every TA command value (step S 904 ). Then, based on the result of calculation in step S 904 , the base station device  211  compares the number of detections to the threshold TH1 for every TA command value (step S 905 ). 
     Then, based on the results of comparison in step S 905 , the base station device  211  transmits RACH messages  2  for random access signal IDs of each TA command value at which the number of detections is equal to or less than the threshold TH1 (step S 906 ). Also, based on the results of comparison in step S 905 , the base station device  211  does not transmit RACH messages  2  for random access signal IDs of each TA command value at which the number of detections exceeds the threshold TH1. Then, the base station device  211  completes the sequential process, and proceeds to the next random access signal processing. 
     In this way, in the first example, for random access signals detected by performing random access signal detection processing once, the base station device  211  calculates the number of detections of random access signals (mobile stations) for every TA command value. Then, if the base station device  211  detects random access signals for the same TA command the number of which exceeds the threshold TH1, at a time, the base station device  211  does not return RACH responses to random access signals from which that TA command value is calculated. Otherwise, the base station device  211  returns RACH responses to random access signals from which a TA command value at which the number of random access signals does not exceed the threshold TH1 is calculated. 
     Note that the process of steps S 901  to S 903  for every random access signal ID may be performed, for example, in the case where a plurality of configurations of the random access signal processing unit  434  illustrated in  FIG. 6  are provided. Alternatively, the process of steps S 901  to S 903  for every random access signal ID may be performed in the case where a single configuration of the random access signal processing unit  434  illustrated in  FIG. 6  is provided and the process is performed serially. 
     In LTE, there is an approach in which, by making use of the feature of cross-correlation between a Zadoff-Chu sequence and the cyclically shifted version of itself, the cyclically shifted sequence in a certain cycle is handled as another random access signal. Cross-correlation between the sequences with different amounts of cyclic shift is low and therefore these sequences may be handled as different random access signals. This may reduce the number of sequences within a group, thereby reducing the amount of processing of correlation calculation. 
     Result Obtained from First Example 
     An example of results obtained by performing the process of steps illustrated in  FIG. 9  will be described next. 
       FIG. 10A  is a table illustrating an example of results of detection for every random access signal ID. Detection results  1010  illustrated in  FIG. 10A , for example, may be obtained by performing the process of steps S 901  to S 903  for every random access signal ID illustrated in  FIG. 9 . In the detection results  1010 , “Detection” and “TA command value” are associated with every “Random access signal ID”. “Detection” indicates whether a random access signal corresponding to “Random access signal ID” is present (“Yes”) or absent (“No”). “TA command value” indicates the value of a TA command (timing difference) for the detected random access signal. 
       FIG. 10B  is a table illustrating an example of the number of detections for every TA command value. By performing the process of step S 904  illustrated in  FIG. 9 , distribution information  1020  illustrated in  FIG. 10B , for example, may be obtained. In the distribution information  1020 , “Number of detections” is associated with every “TA command value”. The “Number of detections” indicates the number of detections for the “TA command value” in the detection results  1010 . 
       FIG. 10C  is a table illustrating an example of a result of comparison between the number of detections for every TA command value and a threshold. By performing the process of step S 905  illustrated in  FIG. 9 , a comparison result  1030  illustrated in  FIG. 10C , for example, may be obtained. In the case where the threshold TH1=10, as illustrated in the comparison result  1030 , only the “Number of detections” corresponding to the “TA command value”=10 exceeds the threshold TH1. 
       FIG. 10D  is a table illustrating an example of a result of discarding of random access signals for every TA command value. By performing the process of step S 906  illustrated in  FIG. 9 , the result of discarding of random access signals is, for example, as illustrated in  FIG. 10C . In  FIG. 10D , the discard result is indicated in the “Detection” field for the detection results  1010 . “Discard” listed under “Detection” indicates that a random access signal of the corresponding “Random access signal ID” has been discarded, and a RACH message  2  has not been returned. 
     As illustrated in  FIG. 10D , the base station device  211  discards random access signals having random access signal IDs and a TA command value of  10  at which the “Number of detections” exceeds the threshold TH1, among random access signal IDs of detected random access signals. 
     Second Example of Limitation of Response Processing Based on Detection Result 
       FIG. 11  is a graph illustrating a second example of limitation of response processing based on a result of detection. In  FIG. 11 , portions similar to those illustrated in  FIG. 8  are denoted by the same reference numerals and redundant description thereof is omitted. In the second example, the scheduler  440  does not return a RACH response to a random access signal to each of mobile stations (a shaded area  1101 ), which are mobile stations above the threshold TH2 among mobile stations of a TA command value at which the number of mobile stations exceeds a threshold TH2 (for example, TH1&gt;TH2), based on the distribution  801 . 
       FIG. 12  is a flowchart illustrating an example of processing performed by the base station device according to the second example. In the second example illustrated in  FIG. 11 , the base station device  211  performs, for example, the process of steps illustrated in  FIG. 12 . The process of steps S 1201  to S 1204  illustrated in  FIG. 12  is similar to that of steps S 901  to S 904  illustrated in  FIG. 9 . Subsequent to step S 1204 , based on the result of calculation in step S 1204 , the base station device  211  compares the number of detections to the threshold TH2 for every TA command value (step S 1205 ). 
     Then, based on the result of comparison in step S 1205 , the base station device  211  transmits RACH messages  2  for random access signal IDs of each TA command value at which the number of detections is equal to or less than the threshold TH2 (step S 1206 ). The base station device  211  also transmits RACH messages  2  for a predetermined number (TH2) of random access signal IDs selected from random access signal IDs of a TA command value at which the number of detections exceeds the threshold TH2 (step S 1207 ). The order of steps S 1206  and S 1207  may be re-arranged. 
     In this way, in the second example, as in the first example, for random access signals detected by performing random access signal detection processing once, the base station device  211  calculates the number of detections of random access signals (mobile stations) for every TA command value. Then, if the base station device  211  detects random access signals the number of which exceeds the threshold TH2 for the same TA command, at a time, the number of RACH responses to the random access signals for which the TA command value is calculated is set to a number corresponding to the threshold TH2. 
     Which of the detected random access signals RACH responses are to be returned to may be selected based on, for example, the reception qualities of the detected random access signals. The reception levels or reception signal-to-interference ratios, for example, may be used as the reception qualities. 
     For example, the base station device  211  selects some of random access signals of a TA command value at which the number of detections exceeds the threshold TH2, in order of decreasing quality, such that the number of the selected random access signals corresponds to the threshold TH2, and then returns RACH responses only to the selected random access signals. Note that the number of RACH responses to be returned may be less than the threshold TH2, or larger than the threshold TH2 (provided that the number is less than the number of detections of random access signals). 
     Result Obtained from Second Example 
     An example of results obtained by performing the process of steps illustrated in  FIG. 12  will be described next. 
     The detection results for every random access signal ID in the second example are similar to, for example, the detection results  1010  illustrated in  FIG. 10A . Also, the number of detections for every TA command value in the second example is similar to, for example, the distribution information  1020  illustrated in  FIG. 10B . 
       FIG. 13A  is a table illustrating an example of a comparison result between the number of detections for every TA command value and a threshold. In the second example, by performing the process of step S 1205  illustrated in  FIG. 12 , a comparison result  1310  illustrated in  FIG. 13A , for example, may be obtained. In the case where the threshold TH2=10, as illustrated in the comparison result  1310 , only the “Number of detections” corresponding to “TA command value”=10 exceeds the threshold TH2. 
       FIG. 13B  is a table illustrating an example of a result of discarding of random access signals for every TA command value. In the second example, by performing the process of step S 1206  illustrated in  FIG. 12 , the result of discarding of random access signals is, for example, as illustrated in  FIG. 13B . In  FIG. 13B , the discard result is indicated in the “Detection” field for the detection results  1010 . “Discard” listed under “Detection” indicates that a random access signal of the corresponding “random access signal ID” has been discarded, and a RACH message  2  has not been returned. 
     As illustrated in  FIG. 13B , the base station device  211  discards seven random access signals, which are random access signals above the threshold TH2=10 among the random access signals having random access signal IDs and a “TA command value” of  10  at which the “Number of detections” exceeds the threshold TH2. 
     Mobile Station According to Second Embodiment 
       FIG. 14A  is a block diagram illustrating an example of a mobile station according to the second embodiment.  FIG. 14B  is a block diagram illustrating an example of the flow of signals in the mobile station illustrated in  FIG. 14A . The mobile station  221  will be described here, and a similar description applies to mobile stations  222  to  228 . 
     The mobile station  221  includes, for example, as illustrated in  FIG. 14A  and  FIG. 14B , a controller  1410 , a transmitter  1421 , a D/A converter  1422 , a transmission RF unit  1423 , an antenna  1430 , a reception RF unit  1441 , an A/D converter  1442 , and a receiver  1443 . 
     The controller  1410  controls data transmission of the transmitter  1421  for the uplink, based on radio resource allocation information output from the receiver  1443 . The controller  1410  also controls the timing of transmission of the transmitter  1421  for the uplink, based on a TA command output from the receiver  1443 . The controller  1410  also controls data reception of the receiver  1443  for the downlink, based on radio resource allocation information output from the receiver  1443 . 
     The transmitter  1421  performs modulation processing on an uplink signal to be transmitted. Then, the transmitter  1421  outputs the transmission signal obtained through modulation processing to the D/A converter  1422 . The D/A converter  1422  converts the transmission signal output from transmitter  1421  to an analog signal. Then, the D/A converter  1422  outputs the transmission signal converted to the analog signal to the transmission RF unit  1423 . 
     The transmission RF unit  1423  performs RF processing of the transmission signal output from the D/A converter  1422 . The RF processing performed by the transmission RF unit  1423  includes, for example, conversion from a baseband frequency band to a radio frequency band. Then, the transmission RF unit  1423  outputs the transmission signal on which RF processing has been performed to the antenna  1430 . 
     The antenna  1430  wirelessly transmits transmission signals output from the transmission RF unit  1423  to the base station device  211 . The antenna  1430  also receives signals wirelessly transmitted from the base station device  211 . Then, the antenna  1430  outputs received signals (reception signals) to the reception RF unit  1441 . 
     The reception RF unit  1441  performs RF processing of a reception signal output from the antenna  1430 . The RF processing performed by the reception RF unit  1441  includes, for example, conversion from a radio frequency band to a baseband frequency band. Then, the reception RF unit  1441  outputs the reception signal on which RF processing has been performed to the A/D converter  1442 . 
     The A/D converter  1442  converts the reception signal output from the reception RF unit  1441  to a digital signal. Then, the A/D converter  1442  outputs the reception signal converted to the digital signal to the receiver  1443 . The receiver  1443  performs detection and demodulation processing on the reception signal output from the A/D converter  1442 . The receiver  1443  also outputs radio resource allocation information and TA commands obtained through the detection and demodulation processing to the controller  1410 . 
     The controller  1410 , the transmitter  1421 , and the receiver  1443  may be implemented, for example, by a digital circuit  1401 . An FPGA and a DSP, for example, may be used for the digital circuit  1401 . 
     Application Example of Communication System According to Second Embodiment 
       FIG. 15  is a pictorial representation illustrating an example of application of a communication system according to the second embodiment. For example, the base station device  211  according to the second embodiment is applicable to the base station device  211  illustrated in  FIG. 15 . A railway line  1502  through which a train  1501  passes is constructed in the cell  211   a  of the base station device  211 . The mobile station  221  according to the second embodiment is applicable to, for example, a portable terminal device of each user who takes the train  1501 . 
     Once the train  1501  enters the cell  211   a,  portable terminal devices (for example, the mobile stations  221 ) of users who are on the train  1501  transmit random access signals to the base station device  211 . At this point, the distances between the portable terminal devices of users who are on the train  1501  and the base station device  211  are almost the same, and therefore TA command values of the random access signals received by the base station device  211  are also highly likely to be the same. For this situation, the base station device  211  limits responses to these random access signals if the number of random access signals having the same TA command value is large. 
     In this way, with the base station device  211 , responses to random access signals from portable terminal devices of users who are on the train  1501  may be limited. This may reduce unnecessary random access processing for each mobile station that moves out of range of the cell  211   a  in a short period of time as the train  1501  moves, thereby improving the throughput of wireless communication with other mobile stations in the base station device  211 . 
     While the train  1501  passes through the cell  211   a,  the response to a random access signal is sometimes limited for a portable terminal device the user of which is not on the train  1501  and whose distance to the base station device  211  is equal to the distance to the base station device  211  of the train  1501 . However, the distance to the base station device  211  of that portable terminal device is merely temporarily equal to the distance to the base station device  211  of the train  1501 , and therefore such a portable terminal device may be connected to the base station device  211  by re-transmission of a random access signal. 
     The base station device  211  also may store a predetermined timing difference relative to the reference point in time in the base station device  211 . The predetermined timing difference is calculated in the base station device  211  for a random access signal transmitted from the location of the railway line  1502 . For a random access signal having a timing difference different from the stored predetermined timing difference, the base station device  211  may be configured not to limit a response to the random access signal. Thus, the response is not limited for a random access signal that has not been transmitted from, for example, the train  1501 , so that a decrease in throughput of a portable terminal device of the user who is not on the train  1501  may be limited. 
     The base station device  211  also may store a predetermined period of time during which the train  1501  passes the cell  211   a  of the base station device  211 . For a random access signal received during a period of time different from (namely, out of, or not within) the stored, predetermined period of time, the base station device  211  may be configured not to limit a response to the random access signal. Thus, the response is not limited for a random access signal that has not been transmitted from the train  1501 , so that a decrease in throughput of a portable terminal device of the user who is not on the train  1501  may be limited. 
     In this way, with the base station device  211  according to the second embodiment and so forth, responses of random access may be limited in accordance with TA commands for the received random access signals. This may reduce unnecessary random access processing, thereby improving the throughput. 
     For example, depending on the environment surrounding mobile stations and a radio base station device, there are some cases where random access signals are simultaneously transmitted from a plurality of mobile stations. For example, in the case where a vehicle that accommodates many mobile stations, like a crowded train, has moved into the area of a wireless base station device, many random access signals are transmitted from the mobile stations for the purposes of location registration and handover processing for the mobile stations. 
     If the wireless base station device tries to perform the processing in response to this by increasing the amount of processing, it is assumed that such a train as mentioned above immediately moves to another area such that the train is outside the current area. Particularly, it is assumed that such a case will become increasingly common with the recent trend toward smaller cells. 
     A wireless base station device tries to start random access processing (for example, refer to  FIG. 2 ) if a random access signal is detected. Accordingly, when receiving a plurality of random access signals simultaneously, the wireless base station device tries to transmit RACH messages  2  through the downlink and receive RACH messages  3  through the uplink, using radio resources, to a plurality of mobile stations. 
     However, mobile stations on a moving train sometimes have moved to another area during random access processing. In this case, exchanges of random access processing made until the movement will be discarded and, as a result, unnecessary radio resources have been used. Usually, the radio resources are shared with mobile stations to which data communication services (data communication services such as Web browsing services) are provided. Therefore, unnecessary use of radio resources as mentioned above results in a reduction in allocation of radio resources to mobile stations to which data communication services are provided, which, in turn, results in a decrease in throughput of the entire system. 
     To address this, according to each embodiment described above, in a wireless base station device, when the number of the mobile stations with the same TA command value exceeds a threshold, those mobile stations are assumed to be moving due to being on a moving train or the like, and random access responses may be limited. This may reduce unnecessary random access processing, thereby improving throughput. 
     As described above, with the base station device and the communication system, improvement in throughput may be achieved. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding 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 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.