Patent Publication Number: US-2011051690-A1

Title: Radio communication system, base station, terminal, radio communication method, and program

Description:
TECHNICAL FIELD 
     The present invention relates to a technology for controlling the transmission power of uplink data that are transmitted to a base station from a terminal in a radio communication system. 
     BACKGROUND ART 
     Orthogonal Frequency Division Multiple Access (OFDMA) is receiving attention in recent years as a multiple access method of a radio communication system. 
     OFDMA is a method of dividing each frequency direction by subcarriers and each time directions by time slots and allotting subcarriers and time slots in which the subcarriers can be used, to a data transmission origin. OFDMA has been adopted in, for example, WiMAX (Worldwide Interoperability for Microwave Access). 
     In the case of uplink (UL) in OFDMA, the transmission power and transmission timing in a terminal and the frequency assigned to the terminal are adjusted between the base station and the terminal. This adjustment is referred to as ranging (Patent Document 1). Ranging is carried out at the time of initial entry of a terminal to a base station (for example, entry to the base station that is the handover target during handover of a terminal) and is subsequently carried out periodically. 
     In the ranging of transmission power among such rangings, a terminal uses the information of the downlink path loss to calculate the uplink data transmission power on the assumption that the path loss in uplink and downlink (DL) are substantially the same. This type of transmission power control is referred to as open-loop power control, and the present invention presupposes the implementation of open-loop power control. 
     Details of open-loop power control in uplink are next described. 
     As shown in  FIG. 1  and  FIG. 2 , base station BS in Step S 501  reports to terminal MS not only the noise power and interference power (N+I values) in its own station, but also the transmission power of downlink data in its own station and information of the required CINR (Carrier to Interference plus Noise Ratio) value of the modulation in its own station. 
     In this case, the noise power is the power of the thermal noise produced in the interior of the receiver in its own station, and the interference power is the power of disturbance arising from sources other than within its own station (for example, interference waves and radiowaves from neighboring cells). 
     In Step S 502 , terminal MS next estimates the difference in path loss between downlink and uplink based on the difference between CINR of the downlink data (burst data) received from base station BS and the required CINR, and from this path loss difference, calculates the transmission power of uplink data. As a result, the method of calculating transmission power can be represented by the following Equation 1: 
         P (dBm)= L+C/N+NI− 10 log 10( R )+Offset —   MS   perMS +Offset —   BS   perMS   [Equation 1]
 
     where L is the downlink path loss estimated in terminal MS, C/N is the required CINR value of the modulation method in base station BS. NI is noise power and interference power calculated in base station BS, 10 log 10 (R) is the correction value of repetition coding, Offset_MS perMS  is the correction value for each&#39;terminal MS, and Offset_BS perMS  is the correction value for each base station BS. 
     In Step S 503 , terminal MS then transmits to the base station BS uplink data (burst data) at the transmission power that was calculated as described above. 
     However, in open-loop power control in uplink, variations in the radio environment cause the noise power and interference power to differ for each base station BS, resulting in the problem in which terminal MS is unable to accurately calculate the initial transmission power at the time of handover. 
     This problem is next described in detail. As shown in  FIG. 3 , it is here assumed that terminal MS is undergoing handover from base station BS 1  to base station BS 2 . In addition, the N+I values that represent noise power and interference power at base stations BS 1  and BS 2  are further assumed to be “a” and “b.”, respectively. 
     As shown in  FIG. 4 , base station BS 1  carries out periodic transmission of periodically inserting the N+I value “a” into downlink frames and broadcast transmitting to terminal MS that is under its own jurisdiction. 
     Base station BS 2  similarly carries out periodic transmission of periodically inserting N+I value “b” into downlink frames and sending by broadcast to terminal MS that is under its own jurisdiction. 
     In Step S 201 , terminal MS transmits to the base station BS 1  that is providing service (Serving BS) a handover request message that takes base station BS 2  as the handover target base station (Target BS). 
     In Step S 202 , a handover preparation phase is then executed between base station BS 1  and base station BS 2 . 
     In the handover preparation phase, a process is carried out in base station BS 1  of reporting to base station BS 2  information of terminal MS that is undergoing handover, and in base station BS 2 , a process is carried out of generating a data path to terminal MS with higher-order network (in the case of WiMAX, the access service network). 
     Upon completion of the handover preparation phase, base station BS 1  transmits to terminal MS a handover request response message in response to the handover request message in Step S 203 , and terminal MS transmits to base station BS 1  a handover execution notification message in Step S 204 . 
     Terminal MS subsequently carries out switching from base station BS 1  to base station BS 2  in Step S 205 , and in Step S 206 , executes ranging with base station BS 2 . This ranging is specially referred to as handover ranging. 
     Subsequently, a network re-entry process is executed between terminal MS and base station BS 2  in Step S 207 . 
     In the network re-entry process, a process is carried out in base station BS 2  in which terminal MS re-enters the higher-order network (in the case of WiMAX, the access service network) by way of base station BS 2 . 
     However, in the interval from the implementation of switching to base station BS 2  in Step S 205  to the start of handover ranging in Step S 206 , terminal MS does not receive N+I value “b” from base station BS 2  and does not know N+I value “b”. 
     As a result, when calculating the initial transmission power of uplink data by means of Equation 1 in handover ranging, terminal MS is compelled to use N+I value “a” of base station BS 1 . 
     In this case, the initial transmission power diverges by the portion (a−b) from the transmission power that is calculated using N+I value “b” that is later received from base station BS 2  in ranging that is subsequently carried out periodically. The initial transmission power that is calculated in handover ranging is consequently not calculated correctly. 
     Patent Document 1: JP 2006-005946 A 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a radio communication system, a terminal, a radio communication method, and a program that can solve the above-described problems. 
     The radio communication system of the present invention includes a terminal and a base station that reports power information in the base station to the terminal, wherein the terminal includes: 
     a prediction unit that, at the time of handover, predicts the next timing at which power information is to be reported based on the report period for reporting power information by the base station; and 
     a switch unit that, at the time of handover, carries out switching to the handover target base station before the timing that was predicted. 
     The terminal of the present invention is a terminal to which power information in a base station is reported from the base station and includes: 
     a prediction unit that, at the time of handover, predicts the next timing at which power information is to be reported based on the report period for reporting power information by the base station; and 
     a switch unit that, at the time of handover, switches to the handover target base station before the timing that was predicted. 
     The radio communication method of the present invention is realized by a terminal to which power information in a base station is reported from the base station, and includes: 
     a prediction step of, at the time of handover, predicting the next timing at which power information is to be reported based on the report period for reporting power information by the base station; and 
     a switching step of, at the time of handover, carrying out switching to the handover target base station before the timing that was predicted. 
     The program of the present invention causes a terminal to which power information in a base station is reported from the base station to execute: 
     a prediction procedure of, at the time of handover, predicting the next timing at which power information is to be reported based on the report period for reporting power information by the base station; and 
     a switching procedure of, at the time of handover, carrying out switching to the handover target base station before the timing that was predicted. 
     According to the present invention, at the time of handover, a terminal predicts the next timing that power information is to be reported based on the report period for reporting power information by the base station, and then, before the timing that was predicted, carries out switching to the handover target base station. 
     As a result, the terminal is able to carry out switching to the handover target base station before power information is next reported from the handover target base station, whereby the effect is obtained in which power information of the handover target base station can be used to accurately calculate initial transmission power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view for explaining open-loop power control in uplink; 
         FIG. 2  is a sequence chart for explaining open-loop power control in uplink; 
         FIG. 3  is a view for explaining an example of handover in a radio communication system; 
         FIG. 4  is a sequence chart for describing an example of operations during handover in a related radio communication system; 
         FIG. 5  shows the configuration of the radio communication system of an exemplary embodiment of the present invention; 
         FIG. 6  is a sequence chart for explaining an example of the operations at the time of handover in the radio communication system of an exemplary embodiment of the present invention; 
         FIG. 7  is a flow chart for explaining an example of the operations at the time of handover in a terminal of an exemplary embodiment of the present invention; and 
         FIG. 8  is a flow chart for explaining another example of the operations at the time of handover in a terminal of an exemplary embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     A best mode of carrying out the present invention is next described with reference to the accompanying figures. 
     Although an example is described in the following exemplary embodiment in which the radio communication system is a WiMAX radio communication system, the present invention is not limited to this form. 
     As shown in  FIG. 5 , the radio communication system of the present exemplary embodiment includes base stations BS 1  and BS 2  and terminal MS. In  FIG. 5 , in the interest of simplifying the explanation, the numbers of base stations BS and terminals MS are two and one, respectively, but the present invention is not limited to this form. 
     Base station BS 1  includes radio communication unit  11 , network communication unit  12 , and base station operation unit  13 . Although not shown in the figure, base station BS 2  also has the same means as base station BS 1 . 
     Radio communication unit  11  carries out radio communication with terminal MS. 
     Network communication unit  12  carries out network communication with other base stations BS by way of a network. 
     Although not shown in the figure, base station operation unit  13  includes means equivalent to a base station that is typically used in a WiMAX radio communication system. For example, in  FIG. 4 , these are: means that periodically transmits to terminal MS information of the noise power and interference power in the base station, means that receives and transmits various messages with terminal MS, means that executes a handover preparation phase with other base stations BS, means that executes ranging with terminal MS, and means that executes a network re-entry process with a terminal MS. However, these means are not essential parts of the present invention and are well known in the art, and detailed explanation is therefore here omitted. 
     In the exemplary embodiment, it is assumed that base stations BS 1  and BS 2  periodically transmit in synchronization information of the noise power and interference power. 
     As shown in  FIG. 5 , terminal MS includes radio communication unit  21  and terminal operation unit  22 . 
     Radio communication unit  21  carries out radio communication with base stations BS 1  and BS 2 . 
     Terminal operation unit  22  includes report timing prediction unit  23 , base station switch unit  24 , and ranging control unit  25 . 
     At the time of handover, report timing prediction unit  23  predicts the next timing at which noise power and interference power are to be reported from the handover target base station BS (for example, BS 2 ) based on the report period for reporting noise power and interference power by the serving base station BS (for example, BS 1 ). 
     At the time of handover, base station switch unit  24  carries out switching to the handover target base station BS before the timing that was predicted in report timing prediction unit  23 . 
     Ranging control unit  25  controls the transmission power control unit (not shown) that executes open-loop power control in terminal operation unit  22 . 
     Although not shown in the figure, terminal operation unit  22  includes means equivalent to a terminal that is typically used in a WiMAX radio communication system. For example, in addition to the above-described means, this is means for receiving and transmitting various messages with serving base station BS in  FIG. 4 . However, these means are not essential parts of the present invention and are further well known in the art, and detailed explanation is therefore here omitted. 
     The operations of the radio communication system of the present exemplary embodiment at the time of handover are next described. It will here be assumed that terminal MS undergoes handover from base station BS 1  to base station BS 2 , as shown in  FIG. 3 . It is further assumed that the N+I values that represent the noise power and interference power are “a” and “b”, respectively. 
     As shown in  FIG. 6 , base stations BS 1  and BS 2  execute periodic transmission of N+I values “a” and “b”, respectively. 
     Terminal MS receives N+I value “a” from serving base station BS 1  during the interval until switching to base station BS 2  that is the handover target in subsequent Step S 205 . 
     As a result, report timing prediction unit  23  of terminal MS in Step S 101  learns the report period of N+I value “a” by serving base station BS 1 . 
     Because it is assumed that base stations BS 1  and BS 2  execute periodic transmission of N+I values in synchronization in the present exemplary embodiment, the report period of N+I value “b” from handover target base station BS 2  can be learned at the same time as the report period of N+I value “a” by base station BS 1 . 
     Report timing prediction unit  23  next predicts the next timing at which N+I value “b” is to be reported from base station BS 2  based on the report period that was learned. 
     Base station switch unit  24  of terminal MS then, in Step S 205 , carries out switching to base station BS 2  before the timing that was predicted in report timing prediction unit  23   
     In this way, terminal MS is able to receive N+I value “b” from base station BS 2  after switching to base station BS 2 . 
     The operations of terminal MS at the time of handover are next described in detail. 
     Operation Example 1 
     As shown in  FIG. 7 , upon receiving N+I value “a” from base station BS 1  in Step S 301 , report timing prediction unit  23  in Step S 302  stores frame number x of the downlink frame in which N+I value “a”, that was received this time, was inserted. 
     In Step S 303 , report timing prediction unit  23  next determines whether or not frame number y of the downlink frame in which was inserted N+I value “a”, that was previously received, is stored, and if not stored, in Step S 304  sets frame number x to frame number y of N+I value “a” that was previously received and returns to the process of Step S 301 . 
     On the other hand, if in Step S 303  frame number y is stored, report timing prediction unit  23  finds the frame number differential z (=x−y) in Step S 305  and learns the value obtained by multiplying differential z by the report period of downlink frames as the report period of N+I values in base stations BS 1  and BS 2 . Report timing prediction unit  23  further predicts the next timing at which N+I value “b” from handover target base station BS 2  is to be received as the timing of receiving the downlink frame of frame number x+z. 
     In Step S 306 , base station switch unit  24  then determines whether or not the HO preparation phase has been completed, and if completed, in Step S 307  carries out switching to base station BS 2  before receiving the downlink frame of frame number x+z (for example, in the time interval from receiving the downlink frame of frame number x+z−1 up to reception of the downlink frame of frame number x+z). 
     Operation Example 2 
     This operation example is similar to Operation Example 1 of  FIG. 7  with respect to the processing up to Steps S 301 -S 306 , but differs with respect to subsequent processing. 
     As shown in  FIG. 8 , base station switch unit  24  in Step S 401 , taking into consideration the possibility that N+I value “b” from base station BS 2  will be transmitted earlier than the downlink frame of frame number x+z that was predicted, carries out switching to base station BS 2  before receiving the downlink frame of frame number x+z−w (where w is a predetermined number) (for example, in the interval from the reception of the downlink frame of frame number x+z−w−1 up to the reception of the downlink frame of frame number x+z−w). Accordingly, switching to base station BS 2  in this operating example is carried out w frames earlier than in the Operation Example I in  FIG. 7 . 
     In Step S 402 , ranging control unit  25  then, after waiting in standby until reception of the downlink frame of frame number x+z+w, determines in Step S 403  whether N+I value “b” from base station BS 2  has been received. Although the standby interval is set to a portion of 2w frames after switching to base station BS 2 , the present invention is not limited to this form. 
     If N+I value “b” has been received from base station BS 2  in Step S 403 , ranging control unit  25  in Step S 405  causes the transmission power control unit (not shown) of terminal operation unit  22  to execute handover ranging of the transmission power by open-loop power control. In this case, N+I value “b” from base station BS 2  has been received and initial transmission power can therefore be correctly calculated. 
     In contrast, if N+I value “b” from base station BS 2  has not been received in Step S 403 , ranging control unit  25  in Step S 404  causes transmission power control unit in terminal operation unit  22  to execute handover ranging after invalidating open-loop power control. The invalidation of open-loop power control means that execution of open-loop power control is temporarily halted. In this case, the transmission power control unit controls the transmission power of uplink data based on instructions from base station BS 2 . This type of transmission power control is referred to as closed-loop power control. In this case, the occurrence of divergence is avoided between the initial transmission power and the transmission power that is calculated in ranging that is subsequently carried out periodically. 
     In the present exemplary embodiment as described hereinabove, terminal MS at the time of handover predicts the next timing at which noise power and interference power are to be reported based on the report period for reporting noise power and interference power by base stations BS and then carries out switching to the handover target base station BS before the timing that was predicted. 
     Terminal MS is therefore able to carry out switching to the handover target base station BS before noise power and interference power from the handover target base station BS are next reported, and can therefore use the information of the noise power and interference power of the handover target base station BS to correctly calculate the initial transmission power. 
     More specifically, terminal MS in the present exemplary embodiment predicts the frame number of the downlink frame, in which information of the noise power and interference power is inserted, and that is to be received next from base station BS and carries out switching to the handover target base station BS before receiving the downlink frame of the frame number that was predicted or of a frame number obtained by subtracting a predetermined number from that frame number. 
     Of these, carrying out switching before receiving the latter, i.e., the downlink frame of the frame number that is obtained by subtracting a predetermined number from the frame number that was predicted, enables coping with a case in which the information of the noise power and interference power from the handover target base station BS is transmitted earlier than predicted. 
     In the present exemplary embodiment, moreover, the execution of open-loop power control is temporarily halted when information of the noise power and interference power is not received from the handover target base station BS within a fixed interval after carrying out switching to the handover target base station BS. In this case, the occurrence of divergence can be avoided between the initial transmission power and the transmission power that is calculated in ranging that is subsequently carried out periodically. 
     The method carried out in terminal MS of the present invention may be applied to a program for realizing execution by a computer. This program can be stored in a memory medium, and can be provided to the outside by way of a network. 
     Although the present invention has been described with reference to an exemplary embodiment, the present invention is not limited to the above-described exemplary embodiment. The details and constitution of the present invention are open to various modifications within the scope of the present invention that will be clear to one of ordinary skill in the art. 
     The present application claims priority based on Japanese Patent Application No. 2008-134218 for which application was submitted on May 22, 2008 and incorporates all disclosures of that application.