Abstract:
The present invention relates to a cell selection method of a terminal in a heterogeneous network, and cell selection method of a terminal in a heterogeneous network according to the present invention includes receiving a measurement restriction pattern from a base station; and selecting a cell based on the measurement restriction pattern. According to an embodiment of the present invention, it is possible to select a cell accurately and efficiently.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wireless communication system and, in particular, to a cell selection method and apparatus of a terminal in a Long Term Evolution (LTE) system of heterogeneous network. 
     2. Description of the Related Art 
     Recently, wireless communication technology has advanced rapidly. With this rapid advance, the communication systems have evolved. Among the recent wireless communication technologies, Long Term Evolution (LTE) is a promising 4 th  Generation mobile communication standard. An LTE system may include various types of base stations such as macro base station, pico base station, and femto base station according to their deployment positions or purposes. A network in which the diverse types of base stations coexist is referred to as Heterogeneous Network (HetNet). The diameter pico/femto cell is small as compared to that of the macro cell. 
     In the heterogeneous network, the coverage of the pico/femto base station may be overlapped with or included in the coverage of the macro cell. In this case, it is difficult to expect accurate signal measurement due to the inter-cell interference. In order to solve this problem, inter-cell interference coordination, i.e. Enhanced Inter Cell Interference Coordination (eICIC), technology is used. 
     Almost Blank Subframe (ABS) is one of the key techniques in the eICIC technology. In LTE, transmission is performed in unit of subframe. ABS is a technique for muting transmission in the serving cell or neighboring cell at a specific subframe. ABS aims to make it possible for the terminal to measure received signal without interference at the corresponding subframe. 
       FIG. 1  is a diagram illustrating the HetNet to which ABS technique of eICIC technology is applied. 
     Referring to  FIG. 1 , a femto Closed Subscriber Group (CSG) base station  103  allowing access to restricted users is located within the coverage of the macro base station  101 . In the case that the non-CSG terminal  105  in communication with the macro base station  101  moves to approach the femto CSG base station  103 , the non-CSG terminal  105  may undergo a problem in communication due to the interference from the femto CSG base station  103 . In this case, the macro base station  101  notifies the terminal  105  of a measurement-restricted subframe pattern  107  in the form of a bitmap. If the pattern is received, the terminal  105  suspends measurement at the subframes indicated by 0 in the pattern  107  and performs measurement only at the subframes indicated by 1. The femto CSG base station  130  incurring interference does not transmit signal almost at the subframe indicated by 1 (ABS) so as to avoid interference. The terminal  105  is capable of measuring signal in the interference-mitigated environment. 
     The eICIC technologies in HetNet that are known so far are almost related to the terminal (UE) connected to the network. According to the conventional method, the base station can control the UE connected to the network directly so as to mitigate interference by controlling the UE. However, there is a need of applying the eICIC of HetNet to the UEs in idle mode that are not controlled by the network. In the HetNet system, the Reference Signal Received Quality/Reference Signal Received Power (RSRQ/RSRP) of the UEs in idle mode varies at every subframe, this influence the cell selection or reselection result. There is therefore a need of determining the subframe for performing cell reselection or reselection based on the signal strength measured thereat. Accordingly, there is a need of modification in UE operation. 
     DISCLOSURE OF INVENTION 
     Technical Problem  
     The present invention has been made in an effort to solve the above problem, and it is an object of the present invention to provide a cell selection method and apparatus of a UE for selecting a cell efficiently and accurately in case of using eICIC technology in the wireless mobile communication system of HetNet environment. 
     Solution to Problem 
     In order to accomplish the above object, a cell selection method of a terminal in a heterogeneous network includes receiving a measurement restriction pattern from a base station; and selecting a cell based on the measurement restriction pattern. 
     In order to accomplish the above object, a terminal for selecting a cell in a heterogeneous network includes a transceiver which receives a measurement restriction pattern from a base station; and a controller which selects a cell based on the measurement restriction pattern. 
     Advantageous Effects 
     According to an embodiment of the present invention, the terminal is capable of selecting a cell efficiently and accurately in a heterogeneous network environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the HetNet to which ABS technique of eICIC technology is applied. 
         FIG. 2  is a signaling diagram illustrating the measurement procedure of the cell (re)selection method according to the first embodiment of the present invention. 
         FIG. 3  is a signaling diagram illustrating the measurement procedure of the cell (re)selection method according to the second embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating the measurement procedure of the cell (re)selection method according to the third embodiment of the present invention. 
         FIG. 5  is a flowchart illustrating the measurement procedure of the UE according to the first embodiment of the present invention. 
         FIG. 6  is a flowchart illustrating the measurement procedure of the UE according to the second embodiment of the present invention. 
         FIG. 7  is a flowchart illustrating the measurement procedure of the UE according to the third embodiment of the present invention. 
         FIG. 8  is a block diagram illustrating the configuration of the UE according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. 
     Detailed description of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. This aims to omit unnecessary description so as to make the subject matter of the present invention clear. 
     For the same reason, some of elements are exaggerated, omitted or simplified in the drawings and the elements may have sizes and/or shapes different from those shown in drawings, in practice. The same reference numbers are used throughout the drawings to refer to the same or like parts. 
     Descriptions are made of the cell selection method and apparatus according to the embodiments of the present invention hereinafter with reference to accompanying drawings. 
     Detailed description of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. Exemplary embodiments of the present invention are described hereinafter with reference to the accompanying drawings in detail. 
       FIG. 2  is a signaling diagram illustrating the measurement procedure of the cell (re)selection method according to the first embodiment of the present invention. 
     The UE  201  receives information on a measurement restriction pattern, i.e. the pattern related to the measurement restriction, from a base station (eNB)  203  at step  205 . The measurement restriction pattern is the pattern indicating subframes at which the UE performs/suspends measurement. The UE  201  is capable of receiving the measurement restriction pattern through unicast, broadcast, or combination thereof. In both the connected mode and idle mode, the UE  201  is capable of receiving the measurement restriction pattern through broadcast from the eNB  203 . In the connected mode, the UE  201  is capable of receiving the measurement restriction pattern through unicast. 
     In order to transmit the measurement restriction pattern, i.e. the pattern related to the measurement restriction, at least one of following signaling schemes can be used. 
     In order to transmit the measurement restriction pattern, Radio Resource Management/Radio Link Management (RRM/RLM) resource restriction of the serving cell can be used. The RRM/RLM resource restriction is a bitmap having a size of 40 bits. Each bit of the bitmap corresponds to a subframe. In order to manage mobility, when measuring the serving cell signal strength, the UE  201  performs measurement only at the subframes indicated by 1 in the bitmap. 
     In order to transmit the measurement restriction pattern, the RRM/RLM resource restriction of a neighbor cell. The RRM/RLM resource restriction of the neighbor cell is a bitmap having the size of 40 bits. Each bit of the bitmap corresponds to a subframe. In order to manage mobility, when measuring the serving cell signal strength, the UE  201  performs measurement only at the subframes indicated by 1 in the bitmap. 
     In order to transmit the measurement restriction pattern, Channel Quality Information/Channel State Information (CQI/CSI) resource restriction can be used. The CQI/CSI resource restriction of the serving cell is a bitmap having the size of 40 bits. Each bit of the bitmap corresponds to a subframe. When measuring CQI/CSI for the serving cell, the UE  201  performs measurement only at the subframes indicated by 1 in the bitmap. 
     If the information on the measurement restriction pattern is received as described above, the UE  201  checks the subframes at which measurement is restricted in the time domain at step  207 . The UE  201  performs measurement only at the restricted subframes and uses the measurement result for cell selection or reselection at step  209 . 
       FIG. 3  is a signaling diagram illustrating the measurement procedure of the cell (re)selection method according to the second embodiment of the present invention. 
     The UE  301  receives the information on the pattern related to measurement, i.e. measurement restriction pattern, from the eNB  303  at step  305 . The UE  301  is capable of receiving the measurement restriction pattern through unicast, broadcast, or combination thereof. In both the connected mode and idle mode, the UE  301  is capable of receiving the measurement restriction pattern from the eNB  303  through broadcast. In the connected mode, the UE  301  is capable of receiving the measurement restriction pattern through unicast. If the information on the measurement restriction pattern is received, the UE  301  checks the subframes at which measurement is restricted at step  307 . The UE  301  performs measurement at both the restricted subframes and the non-restricted subframes and uses the measurement result acquired at the non-restricted subframes for cell selection or reselection at step  309 . 
     The UE  301  compares the RSRQs (or RSRPs) measured at the restricted and non-restricted subframes with each other to determine whether the difference therebetween is greater than a threshold value A at step  311 . Here, the threshold value A can be signaled through a separate message. According to a modified embodiment, the UE  301  is capable of utilizing a predetermined threshold value A. In the case that the threshold value is signaled through a separate message, the UE  301  is capable of receiving the threshold value form the eNB  303  through unicast, broadcast, or combination thereof. In both the connected mode and idle mode, the UE  301  is capable of receiving the threshold value A through broadcast from the eNB  303 . In the case that the difference between measurement results is greater than the threshold value A (or equal to the threshold value), the UE  301  uses the measurement result acquired at the restricted subframe for cell selection or cell reselection at step  313 . 
       FIG. 4  is a flowchart illustrating the measurement procedure of the cell (re)selection method according to the third embodiment of the present invention. 
     The UE  401  receives the measurement restriction pattern from the eNB  403  at step  405 . The UE  401  is capable of receiving the measurement restriction pattern through unicast, broadcast, or combination thereof. In both the connected mode and idle mode, the UE  401  is capable of receiving the measurement restriction pattern through broadcast from the eNB  403 . In the connected mode, the UE  401  is capable of receiving the measurement restriction pattern from the eNB  403  through unicast. If the information on the measurement restriction pattern is received, the UE  401  checks the subframes at which measurement is restricted at step  407 . The UE  301  performs measurement at the non-restricted subframes and uses the measurement result for cell selection or reselection at step  409 . 
     If the signal strength (e.g. RSRQ) measured at the non-restricted subframe is less than a threshold value B (or equal to the threshold value B), the UE  401  performs measurement at the restricted subframe to use the measurement result for cell selection or cell reselection. Here, the threshold value B can be signaled through a separate message. According to a modified embodiment, the UE  401  is capable of using a predetermined value as the threshold value B. In the case that the threshold value B is signaled through a separate message, the UE  401  in the connected mode or the idle mode is capable of receiving the threshold value B through broadcast from the eNB  403 . In the connected mode, the UE  401  is capable of receiving the threshold value B through unicast from the eNB  403 . 
       FIG. 5  is a flowchart illustrating the measurement procedure of the UE according to the first embodiment of the present invention. 
     The operation of the UE  201  starts at step  501 . The UE  201  receives the information on the pattern related to measurement at step  503 . Next, the UE  201  performs measurement only at the measurement-restricted subframes indicated in the pattern and uses the measurement result for cell selection or reselection at step  505 . 
       FIG. 6  is a flowchart illustrating the measurement procedure of the UE according to the second embodiment of the present invention. 
     The operation of the UE  301  starts at step  601 . The UE  301  receives the measurement restriction pattern at step  603 . The UE  301  performs measurement at both the restricted subframes and non-restricted subframes indicated by the measurement restriction pattern at step  605 . After measurement, the UE  301  calculates the difference between the measurement results (RSRPs or RSRQs) at the restricted and non-restricted subframes at step  607 . The UE  301  determines whether the measurement result difference is greater than the threshold value A (or equal to the threshold value A) at step  609 . If the measurement result difference is greater than the threshold value A, the UE  301  uses the measurement result acquired at the restricted subframe for cell selection or reselection at step  613 . Otherwise, if the measurement result difference is less than the threshold value A, the UE  301  uses the measurement result acquired at the non-restricted subframe for cell selection or reselection. 
       FIG. 7  is a flowchart illustrating the measurement procedure of the UE according to the third embodiment of the present invention. 
     The operation of the UE  401  starts at step  701 . The UE  401  receives the measurement restriction pattern at step  703 . The UE  401  performs measurement at the non-restricted subframe indicated by measurement restriction pattern and uses the measurement result for cell selection or reselection at step  705 . The UE  401  determines whether the RSRQ value of the measurement result is less than the threshold value B (or equal to the threshold value B) at step  707 . If the RSRQ value is greater than the threshold value B, the UE  401  returns the procedure to step  705  to perform measurement at the non-restricted subframes continuously and use the measurement result for cell selection or reselection. If the RSRQ value is less than the threshold value B, the UE  401  performs measurement at the restricted subframe and uses the measurement result for cell selection or reselection at step  709 . 
       FIG. 8  is a block diagram illustrating the configuration of the UE according to an embodiment of the present invention. 
     The higher layer device  805  transmits and receives data through higher layer. The control message processor  807  transmits and receives data through control messages. The control unit  809  controls the multiplexer/demultiplexer  803  to multiplex the data and transmit the data through the transceiver  801 . The control unit  809  controls the transceiver  801  to receive a physical signal and controls the multiplexer/demultiplexer  803  to demultiplex the received signal. The control unit  809  transfers the messages to the higher layer device  805  or control message processor  807  according to the information on the messages. 
     According to an embodiment of the present invention, if the information message about the measurement restriction pattern from the eNB, the control message processor  807  of the UE analyzes the message to differentiate between the restricted subframes and non-restricted subframes. The control message processor  807  notifies the controller  809  of the type of subframe at which measurement is to be performed. 
     According to the first embodiment, the control message processor  807  instructs the controller  809  to perform measurement only at the restricted subframes for cell selection and reselection. According to the second embodiment, the control message processor  807  instructs the controller  809  to perform measurement at both the restricted and non-restricted subframes. The control message processor  807  receives the measurement result from the controller  809  back and instructs the controller  809  to perform measurement at the restricted subframe or not based on the measurement result difference. According to the third embodiment, the control message processor  807  receives the measurement result from the controller  809  and instructs the controller  809  to perform measurement at the restricted subframes when the RSRQ is less than the threshold value B. 
     With the proposed method, the UE in the idle mode is capable of measuring signal according to the measurement restriction pattern of eICIC and performing cell (re)selection in consideration of the interference from the neighbor cells in the HetNet environment, resulting in improvement of cell selection efficiency and accuracy. 
     Advantages and features of the present invention and methods of accomplishing the same may be understood more readily by reference to the following detailed description of exemplary embodiments and the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art, and the present invention will only be defined by the appended claims. Like reference numerals refer to like elements throughout the specification. 
     The present invention is described with reference to accompanying drawings illustrating [tile of the invention] in the embodiment of the present invention. 
     It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     Furthermore, the respective block diagrams may illustrate parts of modules, segments or codes including at least one or more executable instructions for performing specific logic function(s). Moreover, it should be noted that the functions of the blocks may be performed in different order in several modifications. For example, two successive blocks may be performed substantially at the same time, or may be performed in reverse order according to their functions. 
     The term “module” according to the embodiments of the invention, means, but is not limited to, a software or hardware component, such as a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC), which performs certain tasks. A module may advantageously be configured to reside on the addressable storage medium and configured to be executed on one or more processors. Thus, a module may include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. The functionality provided for in the components and modules may be combined into fewer components and modules or further separated into additional components and modules. In addition, the components and modules may be implemented such that they execute one or more CPUs in a device or a secure multimedia card. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 
     Although exemplary embodiments of the present invention have been described in detail hereinabove with specific terminology, this is for the purpose of describing particular embodiments only and not intended to be limiting of the invention. While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. 
     Although the description has been made with reference to particular embodiments, the present invention can be implemented with various modification without departing from the scope of the present invention. Thus, the present invention is not limited to the particular embodiments disclosed but will include the following claims and their equivalents.