Abstract:
A method and apparatus for selecting a serving cell/Node-B in a single carrier frequency division multiple access (SC-FDMA) system are disclosed. For intra-Node-B serving cell selection, a serving Node-B measures channel quality indicators (CQIs) of each subcarrier block in an uplink of each cell controlled by the serving Node-B and selects a new serving cell based on the CQIs. The serving Node-B reports the selected new serving cell to a wireless transmit/receive unit (WTRU). For inter-Node-B serving cell selection, each of a plurality of Node-Bs measures a CQI of each of a plurality of subcarrier blocks in an uplink transmission in each cell controlled by each Node-B and forwards the CQIs to a serving cell selection entity. The serving cell selection entity selects a new serving cell/Node-B based on the CQIs. The serving cell selection entity may be a centralized access gateway, a current serving Node-B or a WTRU.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/711,592 filed Aug. 26, 2005, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention is related to a wireless communication system including at lease one wireless transmit/receive unit (WTRU), at least one Node-B and a plurality of cells. More particularly, the present invention is related to a method and apparatus for selecting a serving cell and Node-B in a single carrier frequency division multiple access (SC-FDMA) system.  
       BACKGROUND  
       [0003]     The third generation partnership project (3GPP) and 3GPP2 are currently considering a long term evolution (LTE) of the universal mobile telecommunication system (UMTS) terrestrial radio access (UTRA). Currently, SC-FDMA is being considered for the uplink of the evolved UTRA.  
         [0004]     In an SC-FDMA system, a plurality of orthogonal subcarriers are transmitted simultaneously. The subcarriers are divided into a plurality of subcarrier blocks, (also known as “resource blocks”). A subcarrier block may be a localized subcarrier block or a distributed subcarrier block. The localized subcarrier block is defined as a set of consecutive subcarriers and the distributed subcarrier block is defined as a set of non-consecutive subcarriers.  FIG. 1  illustrates two localized subcarrier blocks, each comprising four consecutive subcarriers. A subcarrier block is a basic scheduling unit for uplink transmissions in a conventional SC-FDMA system. Depending on a data rate or a buffer status, a Node-B assigns at least one subcarrier block for uplink transmission for a WTRU.  
         [0005]     For uplink macro diversity, (either inter-Node-B or intra-Node-B macro diversity), soft handover or fast cell selection may be performed. In a conventional wideband code division multiple access (WCDMA), a serving cell/Node-B selection for soft handover or fast cell selection is based on channel quality indicator (CQI) measurements of the uplink. One CQI measurement of the entire bandwidth in the uplink for each cell/Node-B is used to make the serving cell/Node-B selection. However, in the SC-FDMA system, there is one CQI per subcarrier block. Therefore, there are multiple CQIs for the uplink per cell. In addition, a WTRU usually does not transmit data using the whole bandwidth. Therefore, it is desirable to provide an improved method for serving cell/Node-B selection in SC-FDMA.  
       SUMMARY  
       [0006]     The present invention is related to serving cell and Node-B selection in a SC-FDMA system. For intra-Node-B serving cell selection, a serving Node-B measures CQIs of each subcarrier block in an uplink of each cell controlled by the serving Node-B and selects a new serving cell based on the CQIs. The serving Node-B reports the selected new serving cell to a WTRU. For inter-Node-B serving cell selection, each of a plurality of Node-Bs measures a CQI of each of a plurality of subcarrier blocks in an uplink transmission in each cell controlled by each Node-B and forwards the CQIs to a serving cell selection entity. The serving cell selection entity selects a new serving cell/Node-B based on the CQIs. The serving cell selection entity may be a centralized control-plane access gateway, a current serving Node-B or a WTRU. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  illustrates a plurality of localized subcarrier blocks associated with uplink transmissions in a conventional SC-FDMA system.  
         [0008]      FIG. 2  shows an exemplary wireless communication system configured in accordance with the present invention.  
         [0009]      FIG. 3  is a flow diagram of an intra-Node-B cell selection process implemented in the system of  FIG. 2  in accordance with a first embodiment of the present invention.  
         [0010]      FIG. 4  is a flow diagram of an inter-Node-B cell selection process implemented in the system of  FIG. 2  in accordance with a second embodiment of the present invention.  
         [0011]      FIG. 5  is a flow diagram of an inter-Node-B cell selection process implemented in the system of  FIG. 2  in accordance with a third embodiment of the present invention.  
         [0012]      FIG. 6  is a flow diagram of an inter-Node-B cell selection process implemented in the system of  FIG. 2  in accordance with a fourth embodiment of the present invention.  
         [0013]      FIG. 7  is a flow diagram of an inter-Node-B cell selection process implemented in the system of  FIG. 2  in accordance with a fifth embodiment of the present invention.  
         [0014]      FIG. 8  is a block diagram of a Node-B used in the system of  FIG. 2  in accordance with the present invention.  
         [0015]      FIG. 9  is a block diagram of a WTRU used in the system of  FIG. 2  in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     When referred to hereafter, the terminology “WTRU” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, the terminology “Node-B” includes but is not limited to a base station, a site controller, an access point (AP) or any other type of interfacing device in a wireless environment.  
         [0017]     The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components.  
         [0018]      FIG. 2  shows an exemplary wireless communication system  200  configured in accordance with the present invention. The system  200  includes at least one WTRU  202 , a plurality of Node-Bs  204   a - 204   c,  a plurality of cells  208   a - 208   i  and an optional centralized access gateway (aGW)  206 . The Node-B  204   a  controls the cells  208   a - 208   c,  the Node-B  204   b  controls the cells  208   d - 208   f,  and the Node-B  204   c  controls the cells  208   g - 208   i.  The WTRU  202  is currently connected to the cell  208   a,  (i.e., serving cell), and the Node-B  204   a,  (i.e., serving Node-B). The Node-Bs  204   a - 204   c  may be connected to each other via a high speed link  212 . The centralized aGW  206  may also be connected to the Node-Bs  204   a - 204   c  via a high speed link  210 .  
         [0019]     Where only one Node-B, such as Node-B  204   a,  is involved in a serving cell/Node-B selection and two or more cells, such as cells  208   a - 208   c,  are controlled by the Node-B, such as Node-B  204   a,  an intra-Node-B cell selection is performed. There are two possible macro diversity schemes. One is soft handover and the other is fast cell selection. In soft handover, a transmission from a WTRU  202  is received and processed by several cells, (e.g., cells  208   a - 208   c ), controlled by the same Node-B, (e.g., Node-B  204   a ). Among those cells, one cell is designated as a serving cell, (e.g., cell  208   a ). In fast cell selection, a transmission from the WTRU  202  is received and processed only by the serving cell  208   a,  and the WTRU  202  may switch from one cell to another very quickly to achieve a “best” radio link.  
         [0020]     For both handover and fast cell selection, the same serving cell selection procedure is implemented in accordance with the present invention. The major difference between the serving cell selection in soft handover and fast cell selection is the frequency that the serving cell selection is performed. The uplink serving cell selection for soft handover may be performed as fast as one per several transmission time intervals (TTIs); whereas the uplink fast cell selection may be performed as fast as one per TTI or per several TTIs, which should be faster than the intra-Node-B soft handover. The time interval that the uplink serving cell selection may be performed is called uplink intra-Node-B serving cell selection interval.  
         [0021]      FIG. 3  is a flow diagram of an intra-Node-B cell selection process  300  implemented in the system  200  of  FIG. 2  in accordance with a first embodiment of the present invention. Where two or more cells are controlled by the same Node-B, an intra-Node-B cell selection is performed. A serving Node-B  204   a  of a WTRU  202  measures CQIs of subcarrier blocks in an uplink transmission transmitted by the WTRU  202  in the serving cell  208   a  of the WTRU  202  and other cells  208   b - 208   c  controlled by the serving Node-B  204   a  (step  302 ). The serving Node-B  204   a  preferably considers CQIs of the best K subcarrier blocks of each cell  208   a - 208   c  controlled by the serving Node-B  204   a.  The K subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks within a cell  208   a - 208   c.  Based on the uplink data rate of the WTRU  202 , the WTRU  202  may be assigned to M subcarrier blocks (1≦M≦N). The value of K is a design parameter, which satisfies M≦K≦N.  
         [0022]     The serving Node-B  204   a  selects a new serving cell for the WTRU  202  based on the CQIs (step  304 ). For example, the Node-B  204   a  may simply select a new serving cell that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks. Alternatively, the Node-B  204   a  may select the new serving cell for the WTRU  202  by considering both CQIs, (i.e., CQIs of the WTRU  202  and other WTRUs in the cells  208   a - 208   c  controlled by the Node-B  204   a ), and scheduling strategy. For example, an appropriate scheduling strategy may balance the cell loads based on the number of WTRUs transmitting in the cells and their data rate and channel conditions, (e.g., uplink CQIs).  
         [0023]     The serving Node-B  204   a  then reports the selected new cell to the WTRU  202 , (preferably via a downlink shared control channel), (step  306 ). The process  300  is repeated every uplink serving cell selection interval.  
         [0024]     Where several Node-Bs are involved with the serving cell/Node-B selection, an inter-Node-B cell/Node-B selection is performed. The inter-Node-B cell/Node-B selection decision is made by a serving cell selection entity. The serving cell selection entity may be a centralized aGW  206 , a current serving Node-B  204   a,  a WTRU  202  or any other entity in the network, depending on the network architecture.  
         [0025]     There are two possible macro diversity schemes for the inter-Node-B cell/Node-B selection. One scheme is soft handover and the other is fast cell selection. In soft handover, a transmission from the WTRU  202  is received and processed by several cells  208   a - 208   i  controlled by different Node-Bs  204   a - 204   c.  Among those cells, one cell is designated as a serving cell, (e.g., cell  208   a ). A Node-B, (e.g., Node-B  204   a ), that controls the serving cell is called a serving Node-B. The WTRU  202  may receive scheduling information, (i.e., at which subcarrier blocks to transmit), only from the serving Node-B  204   a.  In fast cell selection, a transmission from the WTRU  202  is received and processed by cells  208   a - 208   c  controlled by the serving Node-B  204   a.  The WTRU  202  may switch from one Node-B to another very quickly to get the “best” radio link.  
         [0026]     For both handover and fast cell selection, the same serving cell selection procedure is implemented in accordance with the present invention. The major difference between the serving cell/Node-B selection for soft handover and fast cell selection is the frequency that the serving cell/Node-B selection may be performed. Basically, the fast cell selection may be performed faster than the serving cell/Node-B selection in soft handover. The time interval that uplink serving cell/Node-B selection is performed is called uplink inter-Node-B serving cell/Node-B selection interval.  
         [0027]      FIG. 4  is a flow diagram of an inter-Node-B cell selection process  400  implemented in the system  200  of  FIG. 2  in accordance with a second embodiment of the present invention. Each of a plurality of Node-Bs  204   a - 204   c  measures CQIs on each subcarrier block in a cell  208   a - 208   i  controlled by each Node-B  204   a - 204   c  (step  402 ). For the inter-Node-B fast cell selection, only a serving Node-B  204   a  processes data received from the WTRU  202 , while other Node-Bs  204   b - 204   c  ignore the data. However, other Node-Bs  204   b - 204   c  should process pilot signals transmitted by the WTRU  202  to measure the CQIs on uplink transmissions by the WTRU  202 .  
         [0028]     The Node-Bs  204   a - 204   c  report the CQIs to a centralized aGW  206  (step  404 ). The centralized aGW  206  connects several Node-Bs via a high-speed link  210 . Each Node-B  204   a - 204   c  preferably reports CQIs of the best K subcarrier blocks of each cell controlled by the Node-B  204   a - 204   c.  The K subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks within a cell. To reduce the signaling overhead, each Node-B  204   a - 204   c  may report CQIs of the cell that has the best K CQIs, (e.g., in terms of average CQI), among the cells controlled by the Node-B  204   a - 204   c.    
         [0029]     The centralized aGW  206  then selects a new cell/Node-B based on the CQIs (step  406 ). For example, the centralized aGW  206  may simply select the cell/Node-B that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks. Alternatively, the centralized aGW  206  may select a cell/Node-B by considering both CQIs of the WTRU  202  and other WTRUs in the cells  208   a - 208   i  controlled by the Node-Bs  204   a - 204   c  and scheduling strategy.  
         [0030]     The centralized aGW  206  sends messages to the current serving Node-B  204   a,  the new Node-B, (e.g., Node-B  204   b ), which controls the selected new cell and optionally other Node-Bs, (e.g., Node-B  204   c ), to report the selected new serving cell/Node-B (step  408 ). The current serving Node-B  204   a  sends a message to the WTRU  202  to report the selected new serving cell/Node-B, (preferably via a downlink shared control channel), (step  410 ). The process  400  is repeated every uplink inter-Node-B serving cell/Node-B selection interval.  
         [0031]      FIG. 5  is a flow diagram of an inter-Node-B cell selection process  500  implemented in the system  200  of  FIG. 2  in accordance with a third embodiment of the present invention. Each of a plurality of Node-Bs  204   a - 204   c  measures CQIs on each subcarrier block in a cell  208   a - 208   i  controlled by each Node-B  204   a - 204   c  (step  502 ). The Node-Bs  204   a - 204   c  process pilot signals transmitted by the WTRU  202  to measure the CQI on uplink transmissions by the WTRU  202 .  
         [0032]     Non-serving Node-Bs  204   b - 204   c  report the CQIs to a current serving Node-B  204   a  (step  504 ). Each non-serving Node-B  204   b - 204   c  preferably reports CQIs of the best K subcarrier blocks of each cell  208   d - 208   i  controlled by the non-serving Node-B  204   b - 204   c.  The K subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks within a cell. To reduce the signaling overhead, each non-serving Node-B  204   b - 204   c  may report CQIs of the cell that has the best K CQIs, (e.g., in terms of average CQI), among the cells controlled by the non-serving Node-B  204   b - 204   c.    
         [0033]     The current serving Node-B  204   a  then selects a new cell/Node-B based on the CQIs (step  506 ). For example, the serving Node-B  204   a  may simply select the cell/Node-B that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks. Alternatively, the serving Node-B  204   a  may select a cell/Node-B by considering both CQIs of the WTRU  202  and other WTRUs in the cells controlled by the Node-Bs  204   a - 204   c  and scheduling strategy.  
         [0034]     The current serving Node-B  204   a  sends a message to a centralized aGW  206  to report the selected new cell/Node-B (step  508 ). The centralized aGW  206  connects several Node-Bs  204   a - 204   c  via a high-speed link. The centralized aGW  206  then forwards the messages to the selected new Node-B, (e.g., Node-B  204   b ), which controls the selected new cell and optionally other Node-Bs, (e.g., Node-B  204   c ), to report the selected new serving cell/Node-B (step  510 ).  
         [0035]     The current serving Node-B  204   a  sends a message to the WTRU  202  to report the selected new serving cell/Node-B, (preferably via a downlink shared control channel), (step  512 ). The process  500  is repeated every uplink inter-Node-B serving cell/Node-B selection interval.  
         [0036]      FIG. 6  is a flow diagram of an inter-Node-B cell selection process  600  implemented in the system  200  of  FIG. 2  in accordance with a fourth embodiment of the present invention. Each of a plurality of Node-Bs  204   a - 204   c  measures CQIs on each subcarrier block in a cell  208   a - 208   i  controlled by each Node-B  204   a - 204   c  (step  602 ). The Node-Bs  204   a - 204   c  process pilot signals transmitted by the WTRU  202  to measure the CQI on uplink transmissions by the WTRU  202 .  
         [0037]     Non-serving Node-Bs  204   b - 204   c  report the CQIs to a current serving Node-B  204   a  (step  604 ). Each non-serving Node-B  204   b - 204   c  preferably reports CQIs of the best K subcarrier blocks of each cell controlled by the non-serving Node-B  204   b - 204   c.  The K subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks within a cell. To reduce the signaling overhead, each non-serving Node-B  204   b - 204   c  may report CQIs of the cell that has the best K CQIs, (e.g., in terms of average CQI), among the cells controlled by the non-serving Node-B  204   b - 204   c.    
         [0038]     The current serving Node-B  204   a  then selects a new cell/Node-B based on the CQIs (step  606 ). For example, the serving Node-B  204   a  may simply select the cell/Node-B that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks. Alternatively, the serving Node-B  204   a  may select a cell/Node-B by considering both CQIs of the WTRU  202  and other WTRUs in the cells controlled by the Node-Bs  204   a - 204   c  and scheduling strategy.  
         [0039]     The current serving Node-B  204   a  sends messages to the selected new Node-B, (e.g., Node-B  204   b ), which controls the selected new cell and optionally other Node-Bs, (e.g., Node-B  204   c ), to report the selected new serving cell/Node-B via a high-speed link  212  connecting the Node-Bs  204   a - 204   c  to each other (step  608 ). The current serving Node-B  204   a  sends a message to the WTRU  202  to report the selected new serving cell/Node-B, (preferably via a downlink shared control channel), (step  610 ). The process  600  is repeated every uplink inter-Node-B serving cell/Node-B selection interval.  
         [0040]      FIG. 7  is a flow diagram of an inter-Node-B cell selection process  700  implemented in the system  200  of  FIG. 2  in accordance with a fifth embodiment of the present invention. Each of a plurality of Node-Bs  204   a - 204   c  measures CQIs on each subcarrier block in a cell  208   a - 208   i  controlled by each Node-B  204   a - 204   c  (step  702 ). The Node-Bs  204   a - 204   c  process pilot signals transmitted by the WTRU  202  to measure the CQI on uplink transmissions by the WTRU  202 .  
         [0041]     The Node-Bs  204   a - 204   c  report the CQIs to the WTRU  202  (step  704 ). Each Node-B  204   a - 204   c  preferably reports CQIs of the best K subcarrier blocks of each cell  208   a - 208   i  controlled by the Node-B  204   a - 204   c.  The K subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks within a cell  208   a - 208   i.  To reduce the signaling overhead, each Node-B  204   a - 204   c  may report CQIs of the cell that has the best K CQIs, (e.g., in terms of average CQI), among the cells  208   a - 208   i  controlled by the Node-B  204   a - 204   c.    
         [0042]     The WTRU  202  then selects a new cell/Node-B based on the CQIs (step  706 ). For example, the WTRU  202  may simply select the cell/Node-B that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks.  
         [0043]     The WTRU  202  sends messages to the current serving Node-B  204   a,  the selected new Node-B, (e.g., Node-B  204   b ), which controls the selected new cell and optionally other Node-Bs, (e.g., Node-B  204   c ), to report the selected new serving cell/Node-B (step  708 ). The process  700  is repeated every uplink inter-Node-B serving cell/Node-B selection interval.  
         [0044]      FIG. 8  is an exemplary block diagram of a Node-B  204  configured in accordance with the present invention. The Node-B  204  includes a transceiver  802 , a CQI measurement unit  804  and a serving cell selection unit  806 . The transceiver  802  receives uplink transmissions transmitted by a WTRU  202  and sends messages to the WTRU  202 . The CQI measurement unit  804  measures CQIs of each subcarrier block in the uplink transmission in each cell controlled by the Node-B  204 . The serving cell selection unit  806  selects a new serving cell based on the CQIs and sends a message to the WTRU  202  which indicates the selected new serving cell.  
         [0045]      FIG. 9  is an exemplary block diagram of a WTRU  202  configured in accordance with the present invention. The WTRU  202  includes a transceiver  902  and a serving cell selection unit  904 . The WTRU  202  receives CQIs from the Node-Bs  204   a - 204   c  and the serving cell selection unit  904  selects a new serving cell based on the CQIs. The transceiver  902  sends messages to a current serving Node-B and a selected new Node-B and/or other Node-Bs which indicate the selected new serving cell.  
         [0046]     Although the features and elements of the present invention are described in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.