Abstract:
Provided are a wireless base station apparatus and communication control method whereby a peripheral terminal device immediately hands over to a base station apparatus when said base station apparatus is activated. In response to timer values and the number of terminal units within a coverage area, a controller ( 504 ) of an MeNB ( 500 ) selects a dormant mode, activation mode or stationary mode for the base station itself and notifies an uplink received signal determiner ( 505 ) and transmission power determiner ( 506 ) of the selected mode information. In activation mode, the uplink received signal determiner ( 505 ) sets the timing for initial connection signals more frequently than dormant mode. In activation mode, the transmission power determiner ( 506 ) determines the increase in downlink transmission power over time and notifies the downlink transmission signal processor ( 507 ) of the transmission power.

Description:
TECHNICAL FIELD 
     The present invention relates to a radio base station apparatus that controls downlink transmission power and uplink received signals when the apparatus is activated. 
     BACKGROUND ART 
     In recent years, providing small-cell base station apparatuses (hereafter referred to as HeNB: Home eNode B) each having a small cell radius in addition to macro-cell base station apparatuses (hereafter referred to as MeNB: Macro eNode B) each having a large cell radius has been considered so as to compensate a dead zone for mobile phones or to disperse data traffic. By providing HeNB, concentration of the traffic on a specific base station (hereafter referred to as eNB) is reduced, and a high user throughput as an entire network is expected. 
     However, in a time period when the number of network users is small such as midnight hours, a required throughput for each eNB declines, and thus constantly keeping a number of eNB in an active state at the same time would result in excessive energy consumption. 
     In order to solve this problem, a technique for suspending functions other than essential functions, based on the usage status of eNB, has been proposed. NPL 1 discloses a method for a shift to a power-saving mode for suspending the downlink transmission function of eNB when it is determined that no UE is present in an area covered by eNB. 
       FIGS. 1A to 1D  illustrate a technique disclosed in NPL 1. In  FIG. 1A , HeNB  102  is provided in a coverage of MeNB  101 , and two mobile station apparatuses (hereafter referred to as UE: User Equipment) UE  103  and UE  104  are further present in the coverage of MeNB  101 . In a case illustrated in  FIG. 1A , no UE is present in the coverage of HeNB  102 . Accordingly, HeNB under such a circumstance stops downlink transmission ( FIG. 1B ). Since MeNB  101  covers the area used to be covered by HeNB  102  (inside the broken line in  FIG. 1B ), the communication connection is secured even if UE is moved to the area. 
     In  FIG. 1B , when new UE  105  and UE  106  enter the area covered by HeNB  102  ( FIG. 1C ) and HeNB  102  recognizes that UE is present in the area covered by HeNB  102  by detecting uplink signals from UE  105  or UE  106 , for example, HeNB  102  starts downlink transmission ( FIG. 1D ). 
     Another technique disclosed in NPL 1 is illustrated in  FIGS. 2A to 2D .  FIG. 2A  illustrates an example in which four HeNBs, that is, HeNBs  202  to  205  are provided in the coverage of MeNB  201 . Furthermore, although UE  206  and UE  207  are present in the area covered by MeNB  201 , the UEs establish connection with HeNB  203  and HeNB  204 , respectively, instead of MeNB  201 . In this case, since there is no UE that established connection with MeNB  201 , MeNB under such a circumstance stops downlink transmission. In order to fill the dead zone due to the downlink transmission stopped by MeNB  201 , HeNB  202  to HeNB  205  increase the downlink transmission power, expanding the coverage of each of the HeNBs ( FIG. 2B ). 
     In  FIG. 2B , when new UE  208  to UE  216  enter the area covered by MeNB  201  ( FIG. 2C ), and MeNB  201  recognizes that the increase in the number of UEs in its own area, MeNB  201  starts the downlink transmission. HeNB  202  to HeNB  205  subsequently lower the downlink transmission power, setting the coverage of each HeNB back to a regular range ( FIG. 2D ). 
     CITATION LIST 
     Non-Patent Literature 
     NPL 1 
     TR36.927 “Potential solutions for energy saving for E-UTRAN” 
     SUMMARY OF INVENTION 
     Technical Problem 
     When eNB starts the downlink transmission, neighboring UEs around the eNB connected to other eNBs nearby can be a large source of interference immediately after the eNB starts transmitting the downlink signals. For this reason, it is preferable that the neighboring UEs are immediately handed over to the eNB. 
     However, handovers, that is, random access channel (hereafter referred to as RACH) transmission from the neighboring UEs occur at the same time immediately after eNB is activated. Accordingly, RACHs between the UEs tend to collide, making it difficult to complete the handovers immediately. This tendency becomes more prominent when the cell radius of a base station is larger, or when the neighboring UEs are closely located. For example, in the example in  FIG. 2D , UE  208 , UE  211 , UE  212 , UE  213 , UE  214 , and UE  216  transmit RACH at the same time for the handovers to MeNB  201 . As a result, RACHs from the UEs collide or interfere with each other, and the RACHs transmitted from all of UEs end up to be resent. In addition, in RACH, a time resource that can be allocated is set to have a long interval, unlike a regular data channel. Resending RACH takes a long time, and consequently it takes a long time for MeNB  201  to detect RACH from all of the UEs correctly and to complete the handovers. 
     It is an object of the present invention to provide a radio base station apparatus and a communication control method in which peripheral terminal apparatuses of a radio base station apparatus are immediately handed over to the radio base station apparatus when the radio base station apparatus is activated. 
     Solution to Problem 
     A radio base station apparatus configured to shift to a first state in which transmission and reception of a radio signal are stopped, a second state in which transmission and reception of a radio signal are started, and a third state in which a radio signal is transmitted in transmission power necessary for securing a predetermined communication range, the radio base station apparatus comprising: a downlink transmission signal processing section configured to instruct, in the second state and the third state, a terminal apparatus timing for transmitting an initial connection signal; an uplink received signal processing section configured to receive the initial connection signal transmitted by the terminal apparatus at the timing for transmitting the initial connection signal; and an uplink reception determining section configured to determine the timing for transmitting the initial connection signal such that the initial connection signal is sent more frequently in the second state than in the third state. 
     A method for controlling communication according to the present invention is a method for controlling communication in a radio base station apparatus that shifts to a first state in which transmission and reception of a radio signal are stopped, a second state in which transmission and reception of a radio signal are started, and a third state in which a radio signal is transmitted in transmission power necessary for securing a predetermined communication range, the method including: instructing, in the second state and the third state, a terminal apparatus timing for transmitting an initial connection signal; receiving the initial connection signal transmitted by the terminal apparatus at the timing for transmitting the initial connection signal; and determining the timing for transmitting the initial connection signal, such that the initial connection signal is sent more frequently in the second state than in the third state. 
     Advantageous Effects of Invention 
     According to the present invention, the possibility of RACH collision between the terminal apparatuses at the time when the base station apparatus is activated is reduced without the base station apparatus individually signaling the peripheral terminal apparatuses. Therefore, the handovers to the base station apparatus can be immediately conducted. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A to 1D  are diagrams illustrating a technique disclosed in NPL 1; 
         FIGS. 2A to 2D  are diagrams illustrating another technique disclosed in NPL 1; 
         FIGS. 3A to 3C  are diagrams illustrating an example of system configuration according to an embodiment of the present invention; 
         FIG. 4  is a diagram illustrating shifts of states in MeNB according to the embodiment of the present invention; 
         FIG. 5  is a block diagram illustrating configuration of MeNB according to the embodiment of the present invention; 
         FIG. 6  illustrates a table listing receipt frequency of PRACH according to the embodiment of the present invention; and 
         FIG. 7  is a flowchart illustrating an operation by the control section according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
     Embodiment 
       FIGS. 3A to 3C  illustrate an example of system configuration according to an embodiment of the present invention, and  FIG. 4  is a diagram illustrating shifts of states in MeNB  301  illustrated in  FIGS. 3A to 3C . As an example of radio communication system, an LTE (Long Term Evolution) system is used for description. 
     First, MeNB  301  is in a state immediately after placement (ST 401 ) and referred to as “dormant mode” (corresponding to the first state). This state indicates that the downlink transmission and uplink reception by MeNB  301  are stopped as illustrated in  FIG. 3A . In the dormant mode, the dormant mode is maintained until conditions for shift to the next “activation mode” (corresponding to the second state) are satisfied (ST 402 ). The conditions for the shift will be described later. 
     When MeNB  301  in the dormant mode satisfies the certain conditions, MeNB  301  shifts to an activation mode (ST 403 ). This state indicates that a downlink transmission power of MeNB  301  gradually increases, expanding coverage from  305  to  306  and  307 . In the activation mode, the activation mode is maintained until conditions for shift to the next “stationary mode” (corresponding to the third state) are satisfied (ST 404 ). The conditions for the shift will be described later. 
     When MeNB  301  in the activation mode satisfies the certain conditions, MeNB  301  shifts to the stationary mode (ST 405 ). The stationary mode indicates that MeNB  301  fixes the downlink transmission power to certain power such that the coverage is maintained at a constant level. In the stationary mode, the stationary mode is maintained until conditions for shift to the next “dormant mode” are satisfied (ST 406 ). The conditions for the shift will be described later. 
     When MeNB  301  in the stationary mode satisfies the certain condition, MeNB  301  shifts to the dormant mode (ST 407 ). 
       FIG. 5  is a block diagram illustrating the configuration of MeNB  500  according to the embodiment of the present invention. The configuration of MeNB  500  will be described as follows referring to  FIG. 5 . 
     Radio transmitting/receiving section  502  receives an uplink radio signal from a UE within its own coverage via antenna  501 , performs predetermined radio processing on the received uplink radio signal in order to convert the signal to an uplink baseband signal, and outputs the uplink baseband signal to uplink received signal processing section  503 . When MeNB  500  is in the activation mode or the stationary mode, radio transmitting/receiving section  502  performs predetermined radio processing on the downlink baseband signal provided by downlink transmission signal processing section  507  in order to convert the signal into a downlink radio signal, and outputs the downlink radio signal to antenna  501 . 
     Uplink received signal processing section  503  performs predetermined demodulation on the uplink baseband signal provided from radio transmitting/receiving section  502  so as to generate an uplink received signal. The uplink received signal includes, in addition to a PRACH (Physical Random Access Channel) corresponding to an initial connection signal, a PUCCH (Physical Uplink Control Channel) for multiplexing control information, a PUSCH (Physical Uplink Shared Channel) for mainly multiplexing data signals, an SRS (Sounding Reference Signal) for measuring quality of an uplink channel, and others. The uplink received signal generated is provided to control section  504 . 
     Control section  504  manages the number of UE (hereafter N UE ) present in the coverage of MeNB  500 , based on the uplink received signal provided from the uplink received signal processing unit  503 . More specifically, among the output from uplink received signal processing section  503 , when control section  504  detects a message indicating that UE successfully conducts a handover to MeNB  500  (RRC connection Reconfiguration Complete Message), control section  504  increments N UE . Furthermore, when control section  504  detects an uplink reception quality report (measurement report) from an output of uplink received signal processing section  503 , and when the value is smaller than a predetermined threshold, control section  504  considers the UE to be handed over to another cell, and decrements N UE . Control section  504  is capable of managing modes of MeNB  500 , and further includes a timer that manages an elapsed time since the shift to the dormant mode and a timer that manages an elapsed time since the shift to the activation mode (not illustrated). The mode information determined by control section  504  is provided to uplink reception determining section  505  and transmission power determining section  506 . 
     The uplink reception determining section  505  determines receipt frequency of the PRACH based on the mode information provided from control section  504 , and provides the result to uplink received signal processing section  503  and downlink transmission signal processing section  507 . 
     The receipt frequency of the PRACH determined by uplink reception determining section  505  will be described in detail.  FIG. 6  illustrates a table listing the receipt frequency of the PRACH held by uplink reception determining section  505 . A PRACH Configuration Index is a number representing the receipt frequency of the PRACH. Preamble Format is a number representing a physical channel structure of the PRACH, and a value from 0 to 4 is assigned. As an example, all of Preamble Formats here are set to zero. This is a channel structure assigned to a base station having a relatively small cell radius. System frame number and Subframe number represents the system frame number and the subframe number for receiving the PRACH. 
     Physical channels in LTE are configured on a per-subframe basis, and the length of one subframe is 1 [msec]. One system frame is composed of 10 subframes. A value from 0 to 1023 is cyclically assigned to the system frame number. For example, when the PRACH Configuration Index=0, the PRACH is received only when the system frame number is an even number and the subframe number is 1. As a result, the PRACH is received in an interval of 20 [msec]. When the PRACH Configuration Index=14, the PRACHs are received in all of the subframes. 
     Uplink reception determining section  505  determines the PRACH Configuration Index based on the mode information provided from control section  504 . Uplink reception determining section  505  outputs System frame number and Subframe number corresponding to the PRACH Configuration Index to uplink received signal processing section  503 , and outputs the PRACH Configuration Index to downlink transmission signal processing section  507 . 
     The transmission power determining section  506  determines downlink transmission power based on the mode information provided from control section  504 , and outputs an instruction regarding the determined transmission power to downlink transmission signal processing section  507 . 
     Downlink transmission signal processing section  507  generates a downlink baseband signal based on the instruction provided from transmission power determining section  506 , and provides the downlink baseband signal to radio transmitting/receiving section  502 . Here, the downlink baseband signal includes a PDCCH (Physical Downlink Control Channel) for multiplexing control information, a PDSCH (Physical Downlink Shared Channel) for multiplexing data signals or an SIB (System Information Block), a PBCH (Physical Broadcast Channel) for multiplexing notification information, SS (Synchronization Signal) for synchronizing a UE, and others. Note that, the PRACH Configuration Index provided from uplink reception determining section  505  is multiplexed on the PDSCH as the SIB. 
     Details of operations by control section  504 , uplink reception determining section  505 , and transmission power determining section  506  will be described with reference to the flowchart illustrated in  FIG. 7 . MeNB shifts to the dormant mode immediately after placement (ST 701 ). Here, the first timer T 1  is initialized, that is, T 1 =0. In addition, no downlink transmission is performed, that is, downlink transmission power P tx =0. 
     The first timer T 1  aims to measure an elapsed time from the point in time when MeNB shifts to the dormant mode. Until T 1  reaches the predetermined threshold T th1 , the dormant mode is maintained, and T 1  is kept incremented (ST 702 , ST 703 ). 
     When the timer for T 1  expires (T 1 ≧T th1 ), MeNB shifts to the activation mode (ST 704 ). Here, the second timer T 2  is initialized, that is, T 2 =0. Furthermore, the downlink transmission power P tx  is set to the minimum transmission power, that is, P tx =P min . Furthermore, in order to enable reception of the PRACH in all of the subframes, the PRACH Configuration Index=14. 
     The second timer T 2  aims to measure an elapsed time from the point in time when MeNB shifts to the activation mode. The activation mode is maintained until T 2  reaches T th2 , and T 2  is kept incremented (ST 705 , ST 706 ). At the same time, downlink transmission power P tx  is updated. P tx  is represented by the following equation.
 
(Equation 1)
 
 P   tx =min( P   max   ,aT+P   min )  [1]
 
     P max  denotes a maximum transmission power of MeNB, and a denotes an increase in the transmission power per unit time. 
     This process allows UEs near MeNB to conduct handovers to the MeNB easily. For example, in  FIG. 3B , suppose that coverage  305  is formed at time T 305 , coverage  306  is formed at time T 306 , and coverage  307  is formed at time T 307 . However, 0≦T 305 &lt;T 306 &lt;T 307 ≦T th2 . 
     Since UE  302  is present in coverage  305  formed at time T 305 , UE  302  transmits the PRACH for conducting a handover to MeNB  301 . Since UE  303  and UE  304  are outside of the coverage of MeNB  301 , the handover to MeNB  301  cannot be started, and consequently PRACH cannot be sent either. Since the only UE transmitting PRACH is UE  302 , PRACH transmitted from UE  302  is received by MeNB  301  in high quality, without interruption by another PRACH. As a result, the PRACH transmitted from UE  302  has increased possibility of being detected by MeNB  301 , allowing a speedy handover to MeNB  301 . 
     Even if the PRACH transmitted from UE  302  is not detected by MeNB  301  due to fading or interference by other cells, MeNB  301  receives the PRACH in all subframes. Accordingly, UE  302  can immediately resend the PRACH in the next opportunity for transmission. 
     Next, the explanation focuses on time T 306 . At time T 306 , coverage  306  is formed, and UE  302  and UE  303  are present in coverage  306 . When T 306 -T 305  is significantly longer than the time necessary for detecting the PRACH in UE  302 , UE  302  has already completed a handover to MeNB  301  at time T 306 . Accordingly, the only UE that transmits the PRACH at time T 306  is UE  303 . As a result, the PRACH transmitted by UE  303  is received by MeNB  301  at high quality without being interrupted by another PRACH. Accordingly, the PRACH transmitted by UE  303  will have higher possibility of being detected by MeNB  301 , allowing a speedy handover to MeNB  301 . 
     For the same reason, UE  304  that enters the coverage of MeNB  301  at time T 307  can be immediately handed over to MeNB  301 . 
     When the timer for T 2  expires (T 2 ≧T th2 ), MeNB shifts to the stationary mode (ST 707 ). The downlink transmission power P tx  is set to the maximum transmission power, that is, P tx =P max . Furthermore, the PRACH configuration Index=0 in order to reduce the receipt frequency of the PRACH. With this process, more resources can be allocated to the uplink data communication than in the activation mode. 
     In the stationary mode, the number of UE N UE  in the base station is monitored (ST 708 ). When N UE  is larger than a predetermined threshold (T UE ) (ST 709 ), the stationary mode is maintained. When N UE  is smaller than or equal to T UE , MeNB shifts to the dormant mode. 
     As described above, according to the embodiment, when the downlink transmission is started, by setting MeNB to gradually increase downlink transmission power as the time passes so as to enable reception of the PRACH in more subframes than in the stationary mode, neighboring UEs can perform handovers to the MeNB easily. 
     Note that, in the embodiment, although the state shift from the dormant mode to the activation mode is performed under the condition that the first timer is expired, the present invention is not limited to this example. For example, the shift to the activation mode may be conducted based on a manual activation or an activation instruction signal from a core network. 
     In addition, the state shift from the activation mode to the stationary mode is performed under the condition that the second timer is expired. However, the present invention is not limited to this example, and the shift to the stationary mode may be made based on the number of UEs successfully conducted handovers, for example. 
     Furthermore, the transmission power in the stationary mode is the maximum transmission power provided by MeNB. However, the present invention is not limited to this example, and the transmission power may be any given transmission power. 
     In addition, in the embodiment, the shift of states from the stationary mode to the dormant mode is performed under the condition that the number of users is smaller than the predetermined threshold. However, the present invention is not limited to this example, and the shift to the dormant mode may be performed based on the data traffic or time information. 
     Although MeNB is used as an example in the explanation in the embodiment, the present invention is not limited to this example, and a small base station apparatus such as HeNB may take the same steps. 
     The disclosure of Japanese Patent Application No. 2011-099184, filed on Apr. 27, 2011, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The radio base station apparatus and the communication control method according to the present invention are suitable for use in mobile communication systems, for example. 
     REFERENCE SIGNS LIST 
     
         
           101 ,  201 ,  301  MeNB 
           102 ,  202 ,  203 ,  204 ,  205  HeNB 
           103 ,  104 ,  105 ,  106 ,  206 ,  207 ,  208 ,  209 ,  210 ,  211 ,  212 ,  213 ,  214 ,  215 ,  216 ,  302 ,  303 ,  304  UE 
           305 ,  306 ,  307  Coverage 
           500  MeNB 
           501  Antenna 
           502  Radio transmitting/receiving section 
           503  Uplink received signal processing section 
           504  Control section 
           505  Uplink reception determining section 
           506  Transmission power determining section 
           507  Downlink transmission signal processing section