Abstract:
The present invention relates to a method and apparatus for efficiently controlling inter-cell interference power in a wireless communication system. According to the present invention, a measurement method to be performed by a terminal in a wireless communication system comprises the following steps: receiving, from a base station, an RRC connection reestablishment message containing pattern information for measuring a serving cell or an adjacent cell; measuring, for an object to be measured indicated by said RRC reestablishment message, the serving cell or adjacent cell using the pattern information; and reporting the result of the measurement to the base station.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a wireless communication system and, in particular, to a method and apparatus for controlling inter-cell interference power amount efficiently. 
         [0003]    2. Description of the Related Art 
         [0004]    The mobile communication system has been developed for the user to communicate on the move. With the rapid advance of technologies, the mobile communication system has evolved to the level capable of providing high speed data communication service as well as voice telephony service. 
         [0005]    Recently, as one of the next generation mobile communication system, Long Term Evolution (LTE) is on the standardization by the 3 rd  Generation Partnership Project (3GPP). LTE is a technology designed to provide high speed packet-based communication of up to 100 Mbps and aims at commercial deployment around 2010 timeframe. In order to accomplish the aim, a discussion is being held on several schemes: one scheme for reducing the number of nodes located in a communication path by simplifying a configuration of the network, and another scheme for maximally approximating wireless protocols to wireless channels. 
         [0006]    Meanwhile, unlike voice service, the data service is provided on the resource determined according to the data amount to be transmitted and channel condition. Accordingly, the wireless communication system, especially cellular communication, is provided with a scheduler manages transmission resource allocation in consideration of the required resource amount, channel condition, data amount, etc. This is the fact in the LTE system as the next generation mobile communication system, and the scheduler located at the base station manages the transmission resource allocation. 
         [0007]    Recent studies are focused on the LTE-Advanced (LTE-A) for improving data rate with the adaptation of several new techniques to legacy LTE system. For such a leap in technology, Inter-Cell Interference Coordination (ICIC) is also evolving to enhanced ICIC (eICIC). The eICIC/Time-domain ICIC is a technique to mitigate inter-cell interference to the victim cell by reducing transmit power in a cell causing interference or totally muting data transmission in unit of subframe. The UE located in the neighbor cell can have an opportunity for channel measurement to maintain the radio link to the neighbor cell and transmit data only in the corresponding subframe. This subframe is referred to as Almost Blank Subframe (ABS) and appears in a predetermined pattern. The ABSs are offered in various patterns depending on the aim of Radio Resource Management (RRM/RLM) measurement in serving cell, RRM measurement in neighbor cell, or Channel Status Information (CSI) measurement in serving cell, and the pattern size varies depending on FDD/TDD. Therefore, there is a need of a method for using the various patterns appropriately according to the situation. 
       DISCLOSURE OF INVENTION 
     Technical Problem 
       [0008]    The present invention has been conceived to meet the above requirements and it is an object of the present invention to provide a method and apparatus for controlling inter-cell interference power amount efficiently using ABS pattern. 
       Solution to Problem 
       [0009]    In order to solve the above problem, a measurement method of a terminal in a wireless communication system according to the present invention includes receiving RRC connection reconfiguration message including pattern information for serving cell or neighbor cell measurement; measuring the serving cell or neighbor cell in measurement target indicated in the RRC connection reconfiguration message based on the pattern information; and reporting measurement result to a base station. 
         [0010]    The terminal for measuring a serving cell or neighbor cell in a wireless communication system according to the present invention includes a transceiver which transmits and receives signals to and from a base station and performs measurement for the serving cell or neighbor cell; and a control unit which controls receiving RRC connection reconfiguration message including pattern information for serving cell or neighbor cell measurement, measuring the serving cell or neighbor cell in measurement target indicated in the RRC connection reconfiguration message based on the pattern information, and reporting measurement result to a base station. 
       Advantageous Effects 
       [0011]    The present invention is capable of controlling inter-cell interference power amount efficiently. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a diagram illustrating an interference scenario between a macro cell and a CSG cell. 
           [0013]      FIG. 2  is a diagram illustrating an interference scenario between a macro cell and a pico cell. 
           [0014]      FIG. 3  is a diagram illustrating a pattern for use in the eICIC technique. 
           [0015]      FIG. 4  is a message flow diagram illustrating RRC signaling with pattern  1 . 
           [0016]      FIG. 5  is a message flow diagram illustrating RRC signaling with pattern  2 . 
           [0017]      FIG. 6  is a message flow diagram illustrating RRC signaling with pattern  3 . 
           [0018]      FIG. 7  is a diagram illustrating the relationship between bitmap informations and subframes. 
           [0019]      FIG. 8  is a diagram illustrating the additional operation definition in the present invention. 
           [0020]      FIG. 9  is a flowchart illustrating the operation procedure of the UE performing eICIC. 
           [0021]      FIG. 10  is a block diagram illustrating the internal structure of the UE to which the present invention is applied. 
           [0022]      FIG. 11  shows a procedure of indicating the pattern to be applied to CSI report in a specific subframe using the scheduling information of the conventional Channel Status Information Reference Signal (CSI-RS). 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0023]    Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. The same reference numbers are used throughout the drawings to refer to the same or like parts. Detailed description of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. 
         [0024]    There are various types of cells in the LTE system. For example, a Closed Subscriber Group (CSG) cell serves a constrained set of UEs. The CSG cell is mainly directed to providing indoor service and has small service coverage. A pico cell has small service coverage with a diameter of about 500 meter and mainly deployed at a hot spot area with frequent traffic of data. In the following, the cells having a short radius service area such as CSG cell and pico cell, as compared to the macro cell, may be referred to as micro base station. 
         [0025]    These cells have relatively short radius service coverage as compared to the macro cell, and plural CSG cell and pico cells may be deployed in the service area of one macro cell. These cell interfere to each other, and this is described in detail hereinafter. 
         [0026]      FIG. 1  is a diagram illustrating an interference scenario between a macro cell and a CSG cell. 
         [0027]    The UE  105  connects to the macro cell  100  to receive service. At this time it is assumed that the UE  105  moves to the service area of the CSG cell  110 . If it is not a member of the CSG cell  110 , the UE  105  is interfered significantly by the signal from the CSG cell. In this case, the CSG cell  110  becomes the aggressor cell, and the macro cell  100  becomes the victim cell. 
         [0028]      FIG. 2  is a diagram illustrating an interference scenario between a macro cell and a pico cell. 
         [0029]    The UE connects to the pico cell to receive service. At this time, it is assumed that the UE  205  moves to the service area of the macro cell  200 . In this case, the UE  205  is interfered significantly by the signal from the macro cell. 
         [0030]    If the UE  205  performs handover to the macro cell  200 , it is possible to cancel to interference from the macro cell. However, the pico cell  210  may be required to serve more users for the sake of operation efficiency and, in this case, it is necessary for the users at the cell boundary to be served by the pico cell  210 . In this case, the macro cell  200  becomes the aggressor cell, and the pico cell  201  the victim cell. 
         [0031]    The eICIC is a technique to mitigate interference form the aggressor cell to the victim cell in these inter-cell interference scenarios. In Rel-10, eICIC is restricted to be applied only in PCell. 
         [0032]    In eICIC, The aggressor cell decreases transmission power or mutes data transmission according to a predetermined pattern in unit of subframe so as to mitigate the interference to the victim cell. In contrast, the UE in the victim cell performs channel measurement in the corresponding subframe to maintain the radio link with the victim cell and be scheduled for data transmission. Such a subframe is referred to as Almost Blank Subframe and appears in a predetermined pattern. Hereinafter, such a pattern may be referred to as ABS pattern and the terms are used interchangeably according to the situation. 
         [0033]      FIG. 3  is a diagram illustrating a pattern for use in the eICIC technique. 
         [0034]      FIG. 3  shows the consecutive subframes structure  300  of the victim cell and the consecutive subframe structure  305  of the aggressor cell. In the aggressor cell, the subframe with reduced transmission power or muted is referred to as ABS and marked by  310  in the drawing. Other subframes carry the data based on the conventional power control. In the ABS subframe, there may be little or no interference to the subframe transmitted at the same time in the victim cell. In contrast, there may be significant interference to the victim cell in the other subframes  315  with the exception of the subframes. 
         [0035]    The victim cell restricts RRM measurement or CSI measurement according to the position of the ABS subframe. The measurement restricted subframe  325  and the measurement non-restricted subframe  320  carry a pattern information indicated by 0 or 1 from the eNB to the UE through RRC signaling. The subframe indicated by 1 is ABS such that the corresponding subframe in the victim cell is not restricted in channel measurement or CSI measurement. There are various patterns as follows.
       pattern  1 : indicate ABS position for RRM/RLM measurement in serving cell   pattern  2 : indicate ABS position for RRM measurement in neighbor cell on frequency indicated by carrierFreq   pattern  3 : indicate subframe position in which CSI measurement is not restricted in serving cell. Unlike patterns  1  and  2 , two patterns are provided.       
 
         [0039]    In pattern  3 , the two patterns are notified to the UE using respective csi-MeasSubframeSet1-r10 IE and csi-MeasSubframeSet2-r10 IE. The reason for using two patterns is to perform scheduling efficiently. It is not impossible absolutely to use the non-ABS subframe for data transmission in the victim cell. Accordingly, it is necessary to transmit data in the abs subframe and check channel status with the CSI report. If a relatively good CSI is received for the non-ABS subframe, the eNB may perform scheduling in the corresponding subframe. The UE stores the ABS configuration information on the two patterns and measures CSI information according to the pattern indicated by csi-MeasSubframeSet1-r10 and csi-MeasSubframeSet2-r10. 
         [0040]      FIG. 11  shows a procedure of indicating the pattern to be applied to CSI report in a specific subframe using the scheduling information of the conventional Channel Status Information Reference Signal (CSI-RS). There are two patterns  1100  and  1105  for CSI measurement. Each pattern indicates the subframes to be used for CSI measurement. If the CSI-RS report is performed at the subframe  1115 , it is determined whether the subframe  1110  considering CSI-RS matches the subframe indicating a pattern. The first pattern  1100  does not indicate the subframe matching the subframe carrying the CSI-RS to be referenced. In contrast, the second pattern  1105  indicates one subframe  1120  matching the subframe carrying the CSI-RS. Accordingly, the CSI report in the subframe  115  is performed with the application of the second pattern. Problems occur when both the two patterns indicate the subframe referencing CSI-RS as denoted by reference number  1125  or no subframe. In order to solve this, one bit indication is used to indicate the pattern to be applied for CSI report carried in a specific subframe through explicit signaling. The first ABS pattern is indicated by 0, and the second ABS pattern is indicated by 1. As another method, implicit indication can be used. For example, if both the two patterns indicates the subframe referencing the CSI-RS, the CSI information to which the first or second ABS pattern has been applied unconditionally. 
         [0041]    According to the proposal of the present invention, the pattern size varies according to Frequency Division Duplex/Time Division Duplex (FDD/TDD). The FDD uses 40-bit bitmap in which each bit corresponds to one subframe. Accordingly, 4 frames composed of total 40 subframes are expressed by one bitmap and this corresponds to 40 ms in time. In case of TDD, the bitmap size varies according to the TDD configuration. The bitmap size is 70 bits (corresponding to 70 ms) for TDD configuration 0, 20 bits (corresponding to 20 ms) for TDD configurations 1 to 5, and 60 bits (corresponding to 60 ms) for TDD configuration 6. The reason for using different-s size bitmaps is to provide the pattern information in consideration of HARQ period size.  FIG. 3  is directed to the FDD case of using 40-bit bitmap corresponding to ABS information for 4 frames (40 ms), i.e. 40 subframes. This pattern information is applied repeatedly every 40 subframes. 
         [0042]      FIG. 4  is a message flow diagram illustrating Radio Resource Control (RRC) signaling for indicating measurement with pattern  1  according to an embodiment of the present invention. 
         [0043]    The eNB  405  sends the UE  400  a UE capability report request message (UECapabilityEnquiry) to request for UE capability at step  410 . Then the UE  400  sends the eNB  405  a UE capability report message (UECapabilityInformation) to notify of eICIC capability at step  415 . 
         [0044]    Then the UE  400  receives ABS pattern information for RRM/RLM measurement in the serving cell from the eNB  405  through the RRC connection reconfiguration message at step  420 . If the measSubframePatternPCell IE of the received RRC connection reconfiguration message includes the pattern information measSubframePattern along with measurement setup indicator, the UE  400  performs RRM/RLM measurement at SBSs in the serving cell. 
         [0045]    That is, measSubframePatternPCell IE of the RRC connection reconfiguration message is configured, it can be interpreted as the indication of measurement in the serving cell using the pattern information included in the RRC connection reconfiguration message. 
         [0046]    If the UE receives a release indication in the measSubframePatternPCell IE of the RRC connection reconfiguration message at step  425 , the UE stops the ongoing eICIC. 
         [0047]      FIG. 5  is a message flow diagram illustrating RRC signaling for indicating measurement with pattern  2  according to an embodiment of the present invention. 
         [0048]    The eNB  505  sends the UE  500  a UE capability report request message (UECapabilityEnquiry) to request the UE  500  to report UE capability at step  510 . Then the UE  500  sends the eNB  505  a UE capability message (UECapabilityInformation) to report the eICIC capability to the eNB  505  at step  515 . 
         [0049]    Then the UE  500  receives the RRC connection reconfiguration message including the ABS pattern information for RRM measurement in the neighbor cell from the eNB  505  at step  520 . If the measSubframePatternConfigNeigh IE of the received RRC connection reconfiguration message includes the pattern information measSubframePattern along with the measurement setup indication, the UE  500  performs Radio Resource Management (RRM) measurement of the neighbor cell in ABSs. 
         [0050]    That is, if the measSubframePatternConfigNeigh IE of the RRC connection reconfiguration message is configured, it can be interpreted as the indication of measurement in the neighbor cell using the pattern information included in the RRC connection reconfiguration message. 
         [0051]    If the UE  500  receives a release indication in the measSubframePatternConfigNeigh IE of the RRC connection reconfiguration message at step  525 , the UE stops the ongoing eICIC. 
         [0052]      FIG. 6  is a message flow diagram illustrating RRC signaling indicating measurement with pattern  3  according to an embodiment of the present invention. 
         [0053]    The eNB  605  first sends the UE  600  a UE capability report request message (UECapabilityEnquiry) to request the UE  600  for UE quality at step  610 . Then the UE  600  sends the eNB  605  a UE capability report message (UECapabilityInformation) to report the eICIC capability to the eNB  605  at step  615 . 
         [0054]    Then the UE  600  receives the RRC connection reconfiguration message including the ABS pattern information for CSI measurement in the serving cell from the eNB  605  at step  620 . If the csi-SubframePatternConfig IE of the received RRC connection reconfiguration message includes the two pattern informations measSubframePattern along with the measurement setup indication, the UE  600  performs CSI measurement of the serving cell in ABSs. 
         [0055]    If the UE  600  receives a release indication in the csi-SubframePatternConfig IE of the RRC connection reconfiguration message at step  625 , the UE stops the ongoing eICIC. 
         [0056]    Meanwhile, in or der to use the pattern information provided to the UE through RRC signaling to restrict RRM/RLM or CSI measurement, it is necessary to define the mappings between the bits of the bitmap and the subframes. The first bit of the pattern information provided in the form of a bitmap is mapped to the first subframe of the frame with SFN (System Frame Number)=0. The following bits are mapped to the subsequent subframes one by one. The number of subframes in one SFN cycle is greater than the number of bits of the bitmap such that the bits of the bitmap are applied repeatedly at a pattern period. That is, the subframe at the start of the pattern is defined with the following formula. 
         [0000]      [SFN*10+subframe number] mod [length of the bitmap (or length of the Pattern)]−0  [formula I]
 
         [0057]      FIG. 7  is a diagram illustrating the relationship between bitmap informations and subframes. Each frame (or radio frame) is allocates an SFN value which increments by 1 as the frames increases. The SFN value is repeated cyclically. According to an embodiment of the present invention, the SFN value repeats in the range of 0 to 1023 cyclically at the period of 1024. If the frame  700  ends with the value of 1023, the next frame  710  is allocates a value of SFN=0 to start the new SFN cycle  705 . The bitmap information  720  received for eICIC from the eNB is mapped to the subframes to indicate the subframes for used in RRM/RLM or CSI measurement. 
         [0058]    The subframe number  715  is reset for every frame and has a range from 0 to 9. That is, one subframe includes 10 subframes. The first bit  730  of the bitmap  730  indicates whether perform eICIC at the first subframe of SFN=0. Then the following bits are mapped to the subsequent subframes one by one. As aforementioned, the number of subframes in one SFN cycle is greater than the number of bits of the bitmap, each bit of the bitmap repeats at the pattern period. 
         [0059]    However, there is a need of further definition in the above mapping scheme. That is, in TDD configurations 0 and 6, the pattern informations having the period of 70 ms and 60 ms are used. Since the SFN cycle has the size of 1024, if the mapping is performed with the repetition of 70 ms or 60 ms, the pattern does not restart at the next SFN=0. Accordingly, if the SFN cycle size (period) is not an integer multiple of the pattern information length, it is necessary to further define which scheme is applied to perform eICIC. 
         [0060]      FIG. 8  is a diagram illustrating the additional operation definition in the present invention. The pattern of the TDD configuration  6   805  is provided at the position  800  as the first SFN=0, and the respective bits constituting this pattern  810  are mapped from the first subframe of the frame of SFN=0 to indicate whether to perform eICIC at each subframe. The pattern repeats until the frame  815  of SFN=1020. 
         [0061]    However, the number of subframes from the frame  815  of SFN=1020 to the frame  820  of SFN=0 is not enough for mapping of one pattern. Accordingly, in the present invention, the pattern information is applied continuously even from the frame of SFN=1020 but, if the frame  802  of next SFN=0 arrives, stops mapping, although the corresponding pattern information is not mapped completely, and starts the pattern  830  newly again. 
         [0062]      FIG. 9  is a flowchart illustrating the operation procedure of the UE performing eICIC. 
         [0063]    The UE first receives the UE capability report request message from the eNB at step  900 . Then the UE reports its capability of supporting eICIC to the eNB at step  905 . 
         [0064]    The eNB instructs the UE to perform eICIC through RRC connection reconfiguration message at step  910  and provides the UE with the configuration such as ABS pattern information and measurement target (one of patterns  1  to  3 ) necessary for eICIC. The UE determines the subframe in which the pattern information starts at step  915  and the bit information of the bitmap which should be applied in the current subframe. According to an embodiment of the present invention, the first bit of the bitmap corresponds to the first subframe of the frame of SFN=0 and indicates whether to perform measurement. 
         [0065]    The UE performs RRM/RLM or CSI measurement at the current subframe based on the bit information at step  920 . The UE increments the subframe number at step  925 . The UE determines whether the end time of the SFN cycle arrives at step  930 . If the end time of the SFN cycle arrives, this may mean to determine where it is before a predetermined number of frames from the end time of the predetermined SFN repetition period. For example, if SFN has a period of 1024 and if the predetermined number is 40, this means that the determination is performed at the time where the SFN has the value of 1204−40=984. 
         [0066]    If it has not end, the UE determines whether the pattern has ended at step  950 . If the pattern has ended, the UE applies the first bit of the bitmap at step  955  and returns the procedure to step  920  to perform RRM/RLM or CSI measurement at the next subframe. 
         [0067]    If it is determined that the SFN cycle is ending at step  930 , the UE checks whether the pattern size is 60 ms or 70 ms at step  935 . If the pattern size is 60 ms or 70 ms and if the SFN cycle ends to be reset to SFN=0, the UE stops ongoing pattern at step  940 , applies the first bit of the bitmap at step  945 , and returns the procedure to step  920  to perform the RRM/RLM or CSI measurement at the next subframe of SFN=0. 
         [0068]      FIG. 10  is a block diagram illustrating the internal structure of the UE to which the present invention is applied. 
         [0069]    The UE communicates data with higher layer  1010  and transmits/receives control message through a control message processor  1015 . When transmitting control signals or data to the eNB, the UE multiplexes the control signals or data by means of the multiplexer/demultiplexer  1205  and transmits the data through the transceiver  1000  under the control of the controller  1020 . In contrast, if a signal is received by the transceiver  1000 , the UE receives the physical signal by means of the transceiver  1000  under the control of the controller  1020 . The received signal is demultiplexed by the multiplexer/demultiplexer  1005  and then transferred to the higher layer device  1010  or the control message processor  1015  according to the respective message informations. 
         [0070]    According to an embodiment of the present invention, the controller  1020  receives the RRC connection reconfiguration message including the pattern information for serving cell or neighbor cell measurement from the eNB. The controller  1020  controls to perform the serving cell or neighbor cell measurement in the measurement target indicated by the RRC connection reconfiguration message based on the pattern information and report the measurement result to the eNB. 
         [0071]    According to an embodiment of the present invention, the controller  1020  determines, when the end time of the system frame number repeating at a predetermined cyclic period is arriving, whether the cyclic period of the system frame number is an integer multiple of the pattern information size. If not any integer multiple, the controller  1020  ends the application of the current pattern information at the time when the system frame number value becomes 0 and corresponds the first bit of the pattern information to the first subframe having the frame of system frame number 0 to perform measurement.