Radio base station, user terminal, cell selection method and radio communication system

Abstract

A system in which user terminal is able to aggressively select small cells in a radio communication system in which a macro cell and small cells are arranged to overlap each other is disclosed. A macro base station (M) configured to form a macro cell carries out the steps of determining the offset value to use in cell selection in a user terminal (UE) based on transmission property information of a second carrier in the small cell, and reporting the offset value to the user terminal (UE). Also, the user terminal (UE) carries out the steps of measuring the received quality of reference signals from the macro base station (M) and the small base station (S), receiving the offset value that is reported from the macro base station (M), and carrying out cell selection based on the received quality that is measured and the offset value that is received.

1 . A radio base station that forms a first cell in a radio communication system in which the first cell that uses a first carrier and a second cell that uses a second carrier of a higher frequency band than the first carrier are arranged to overlap each other, the radio base station comprising:
 a determining section that determines an offset value to use in cell selection in a user terminal, based on transmission property information of the second carrier in the second cell; and   a reporting section that reports the offset value to the user terminal.   
     
     
         2 . The radio base station according to  claim 1 , wherein the transmission property information includes at least one of the number of transmitting antenna elements provided in a radio base station forming the second cell, a transmission bandwidth used in the radio base station forming the second cells, and beamforming gain of a transmission beam used in the radio base station forming the second cell. 
     
     
         3 . The radio base station according to  claim 1 , wherein the determining section determines the offset value based on interference information from a neighboring cell of the second cell. 
     
     
         4 . A user terminal used in a radio communication system in which a first cell that uses a first carrier and a second cell that use a second carrier of a higher frequency band than the first carrier are arranged to overlap each other, the user terminal comprising:
 a measurement section that measures received quality of a reference signal from a first radio base station forming the first cell and a second radio base station forming the second cell;   a receiving section that receives an offset value that is reported from the first radio base station; and   a cell selection section that carries out cell selection based on the received quality measured in the measurement section and the offset value received in the receiving section,   wherein the offset value is determined based on transmission property information of the second carrier in the second cell.   
     
     
         5 . The user terminal according to  claim 4 , wherein the transmission property information includes at least one of the number of transmitting antenna elements provided in the second radio base station, a transmission bandwidth used in the second radio base station, and beamforming gain of a transmission beam used in the second radio base station. 
     
     
         6 . The user terminal according to  claim 4 , wherein the cell selection section applies the offset value to the received quality of a reference signal from the second radio base station. 
     
     
         7 . The user terminal according to  claim 6 , wherein, when a best received quality to which the offset value is applied fulfills a predetermined threshold, the cell selection section selects a second cell based on a comparison result of the received quality of a reference signal from the second radio base station. 
     
     
         8 . The user terminal according to  claim 6 , wherein, when the best received quality to which the offset value is applied does not fulfill a predetermined threshold, the cell selection section selects a first cell or a second cell based on a comparison result of the received quality of a reference signal from the second radio base stations and the received quality of a reference signal from the first radio base station. 
     
     
         9 . A cell selection method in a radio communication system in which a first cell that uses a first carrier and second cells that use a second carrier of a higher frequency band than the first carrier are arranged to overlap each other, the cell selection method comprising the steps of:
 in a first radio base station forming the first cell:   determining an offset value to use in cell selection in a user terminal, based on transmission property information of the second carrier in the second cell; and   reporting the offset value to the user terminal; and   in the user terminal:   measuring received quality of reference signals from the first radio base station and a second radio base station forming the second cell;   receiving an offset value that is reported from the first radio base station; and   carrying out the cell selection based on the received quality that is measured and the offset value that is received.   
     
     
         10 . A radio communication system in which a first cell that uses a first carrier and a second cell that uses a second carrier of a higher frequency band than the first carrier are arranged to overlap each other, wherein:
 a first radio base station forming the first cell comprises:   a determining section that determines an offset value to use in cell selection in a user terminal, based on transmission property information of the second carrier in the second cell; and   a reporting section that reports the offset value to the user terminal; and
 the user terminal comprises: 
   a measurement section that measures received quality of reference signals from the first radio base station and a second radio base station forming the second cell;   a receiving section that receives the offset value that is reported from the first radio base station; and   a cell selection section that carries out cell selection based on the received quality that is measured and the offset value that is received.   
     
     
         11 . The user terminal according to  claim 5 , wherein the cell selection section applies the offset value to the received quality of a reference signal from the second radio base station. 
     
     
         12 . The radio base station according to  claim 2 , wherein the determining section determines the offset value based on interference information from a neighboring cell of the second cell.