Abstract:
A wireless communication method and apparatus for selecting cells in an orthogonal frequency division multiple access (OFDMA) system are disclosed. A wireless transmit/receive unit (WTRU) measures a downlink path loss of a current serving cell and at least one other cell. The WTRU determines whether a path loss difference between the serving cell and the other cell is below a threshold. If the path loss difference is below the threshold, the WTRU measures a channel quality indicator (CQI) for each of the plurality of subcarrier blocks in the downlink of the current serving cell and the other cell, respectively. The WTRU reports the CQIs to a serving Node-B which selects a new cell based on the CQIs. Alternatively, a centralized access gateway (aGW) may select the new serving cell/Node-B, or the cell selection decision made by the serving Node-B may be forwarded to another Node-B via the centralized aGW.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/711,463 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 least 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 cells in an orthogonal frequency division multiple access (OFDMA) 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, OFDMA is being considered for the downlink of the evolved UTRA.  
         [0004]     In an OFDMA system, a plurality of orthogonal subcarriers is transmitted simultaneously. The subcarriers are divided into a plurality of localized subcarrier blocks, (also known as resource blocks).  FIG. 1  illustrates two localized subcarrier blocks, each comprising four subcarriers, as an example. Each of the localized subcarrier blocks is defined as a set of several consecutive subcarriers. The localized subcarrier block is a basic scheduling unit for downlink transmissions in a conventional OFDMA system. Depending on a data rate or a buffer status, a Node-B assigns at least one localized subcarrier block for downlink transmission for a WTRU.  
         [0005]     As with wideband code division multiple access (WCDMA) high speed downlink packet access (HSDPA), it is very difficult to implement inter-Node-B soft handover in the downlink OFDMA system since scheduling of downlink data transmission at different Node-Bs to support hybrid automatic repeat request (H-ARQ) operation is very difficult to coordinate. For soft handover, a new cell has to be selected by either the WTRU or the system. Therefore, it would be desirable to have a fast cell selection method for the OFDMA system.  
       SUMMARY  
       [0006]     The present invention is related to a method and apparatus for selecting cells in an orthogonal frequency division multiple access (OFDMA) system. A WTRU monitors downlink path loss of a current serving cell and at least one other cell. The WTRU determines whether a path loss difference between the serving cell and the other cell is below a threshold. If the path loss difference is below the threshold, the WTRU measures a channel quality indicator (CQI) for each of a plurality of subcarrier blocks in the downlink of the current serving cell and the other cell, respectively. The WTRU reports the CQIs to a serving Node-B which selects a new cell based on the CQIs. Alternatively, a centralized access gateway (aGW) may select the new serving cell/Node-B, or the cell selection decision made by the serving Node-B may be forwarded to another Node-B via the centralized aGW. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein:  
         [0008]      FIG. 1  illustrates a plurality of localized subcarrier blocks associated with downlink transmissions in a conventional OFDMA system;  
         [0009]      FIG. 2  shows an exemplary wireless communication system configured in accordance with the present invention;  
         [0010]      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;  
         [0011]      FIG. 4  is a flow diagram of an inter-Node-N cell selection process implemented in the system of  FIG. 2  in accordance with a second embodiment of the present invention;  
         [0012]      FIG. 5  is a flow diagram of an inter-Node-N cell selection process implemented in the system of  FIG. 2  in accordance with a third embodiment of the present invention;  
         [0013]      FIG. 6  is a flow diagram of an inter-Node-N cell selection process implemented in the system of  FIG. 2  in accordance with a fourth embodiment of the present invention; and  
         [0014]      FIG. 7  is an exemplary block diagram of a WTRU used in the system of  FIG. 2  in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     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 or any other type of interfacing device in a wireless environment.  
         [0016]     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.  
         [0017]      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 aGW  206 . The Node-B  204   a  controls the cells  208   a - 208   c,  the Node-B  204   b  controls the cell  208   d - 208   f,  and the Node-B  204   c  controls the cell  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), for communication. 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 .  
         [0018]     In an OFDMA system, N localized subcarrier blocks are defined and a WTRU may be assigned to M localized subcarrier blocks, where 1≦M≦N. The cell selection is performed based on the quality of the subcarrier blocks. The cell selection may be performed as fast as per transmission time interval (TTI) or per several TTIs. The time interval during which cell selection is performed is called a cell selection interval.  
         [0019]      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. A WTRU  202  measures a downlink path loss of a serving cell  208   a  of the WTRU  202  and at least one other cell  208   b,    208   c  controlled by the serving Node-B  204   a  (step  302 ). The downlink path loss is preferably measured with the downlink pilot strength of the serving cell  208   a  and the other cell  208   b,    208   c.  The WTRU  202  determines whether a path loss difference between the serving cell  208   a  and the other cell  208   b,    208   c  is below a threshold (step  304 ). Suppose that the path losses of cells  208   a  and  208   b  are PL a  and PL c  respectively. One example definition of the path loss difference between cells  208   a  and  208   b  aforementioned is |PL a −PL b |. If the path loss difference is not below the threshold, the process  300  returns to step  302  to further measure the path loss at the next cell selection interval.  
         [0020]     If the path loss difference is below the threshold, the WTRU  202  measures a CQI for each of a plurality of subcarrier blocks in the downlink of the serving cell  208   a  and the other cell  208   b,    208   c  preferably whose path loss is close to the path loss of the serving cell  208   a,  respectively (step  306 ). Based on the downlink data rate of the WTRU  202 , the WTRU  202  may occupy M subcarrier blocks (1≦M≦N).  
         [0021]     The WTRU  202  then reports the CQIs to a current serving Node-B  204   a  (step  308 ). The WTRU  202  may report CQIs of the best K subcarrier blocks of each of those cells to the serving Node-B  204   a.  The K reported subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks for each cell. The value of K is a design parameter, which satisfies M≦K≦N.  
         [0022]     The current serving Node-B  204   a  then selects a new serving cell for the WTRU  202  based on the CQIs (step  310 ). The current serving Node-B  204   a  may simply select the new cell that has the best average CQI, (or weighted average CQI), of M subcarrier blocks out of the K reported subcarrier blocks. Alternatively, the current serving Node-B  204   a  may select the new cell by considering both CQIs of the WTRU  202  and downlink transmission and CQIs of other WTRUs and a scheduling strategy. After selecting the new cell for the WTRU  202 , the current serving Node-B  204   a  sends a message to the WTRU  202 , preferably via a downlink shared control channel, which indicates the new serving cell/Node-B (step  312 ).  
         [0023]     Inter-Node-B cell selection is explained hereinafter. Where several Node-Bs are involved in the cell selection, (i.e., inter-Node-B), the cell selection maybe made at different locations. The cell selection decision may be made by a centralized aGW  206  or the current serving Node-B  204   a,  depending on the particular network architectures. The cell selection may be made as fast as per several TTIs. The actual value of the cell selection interval depends on the network architecture and interface of the evolved UTRA.  
         [0024]      FIG. 4  is a flow diagram of an inter-Node-N cell selection process  400  implemented in the system  200  of  FIG. 2  in accordance with a second embodiment of the present invention. A WTRU  202  measures a downlink path loss of a serving cell  208   a  and at least one other cell  208   b - 208   i  (step  402 ). The downlink path loss is preferably measured with the downlink pilot strength of the serving cell  208   a  and the other cell  208   b - 208   i.  The WTRU  202  determines whether a path loss difference between the serving cell  208   a  and the other cell  208   b - 208   i  is below a threshold (step  404 ). If the path loss difference is not below the threshold, the process  400  returns to step  402  to further measure the path loss at the next cell selection interval.  
         [0025]     If the path loss difference is below the threshold, the WTRU  202  measures a CQI for each of a plurality of subcarrier blocks in the downlink of the serving cell  208   a  and the other cell  208   b - 208   i  preferably whose path loss is close to the path loss of the serving cell  208   a,  respectively (step  406 ). Based on the downlink data rate of the WTRU  202 , the WTRU  202  may occupy M subcarrier blocks (1≦M≦N).  
         [0026]     The WTRU  202  then reports the CQIs to a current serving Node-B  204   a  (step  408 ). The WTRU  202  may report CQIs of the best K subcarrier blocks of each of those cells  208   a - 208   i  to the current serving Node-B  204   a.  The K reported subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks for each cell  208   a - 208   i.  Alternatively, the WTRU  202  may report K best K subcarrier blocks of the best cells controlled by each Node-B  204   a - 204   c.  The value of K is a design parameter, which satisfies M≦K≦N.  
         [0027]     Upon receiving the reported CQIs, the current serving Node-B  204   a  forwards the CQIs to a centralized aGW  206  (step  410 ). The centralized aGW  206  connects several Node-Bs  204   a - 204   c  via a high-speed link  210 . The centralized aGW  206  then selects the new serving cell/Node-B for the WTRU  202  (step  412 ). The centralized aGW  206  may simply select the new serving cell/Node-B that has the best average CQI, (or weighted average CQI), of M subcarrier blocks out of the K reported subcarrier blocks. Alternatively, the centralized aGW  206  may select the new serving cell/Node-B by considering both CQIs of the WTRU  202  and downlink transmission and CQIs of other WTRUs and a scheduling strategy.  
         [0028]     The centralized aGW  206  sends a message to the current serving Node-B  204   a  and the new Node-B, such as Node-B  204   b,  which controls the selected new cell, and optionally to other Node-Bs, such as Node-B  204   c,  to inform the Node-Bs  204   a,    204   b,    204   c  of the selected new serving cell/Node-B (step  414 ). Finally, the current serving Node-B  204   a  sends a message to the WTRU  202 , preferably via a downlink shared control channel, which indicates the new serving cell/Node-B (step  416 ).  
         [0029]      FIG. 5  is a flow diagram of an inter-Node-N cell selection process  500  implemented in the system  200  of  FIG. 2  in accordance with a third embodiment of the present invention. A WTRU  202  measures a downlink path loss of a serving cell  208   a  and at least one other cell  208   b - 208   i  (step  502 ). The WTRU  202  determines whether a path loss difference between the serving cell  208   a  and the other cell  208   b - 208   i  is below a threshold (step  504 ). If the path loss difference is not below the threshold, the process  500  returns to step  502  to further measure the path loss at the next cell selection interval.  
         [0030]     If the path loss difference is below the threshold, the WTRU  202  measures a CQI for each of a plurality of subcarrier blocks in the downlink of the serving cell  208   a  and the other cell  208   b - 208   i  preferably whose path loss is close to the path loss of the serving cell  208   a,  respectively (step  506 ). The WTRU  202  then reports the CQIs to a current serving Node-B  204   a  (step  508 ). The WTRU  202  may report CQIs of the best K subcarrier blocks of each of those cells  208   a - 208   i  to the serving Node-B  204   a.  The K reported subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks for each cell  208   a - 208   i.  Alternatively, the WTRU  202  may report K best K subcarrier blocks of the best cells controlled by each Node-B  204   a - 204   c.  The value of K is a design parameter, which satisfies M≦K≦N.  
         [0031]     Upon receiving the reported CQIs, the current serving Node-B  204   a  selects a new Node-B/cell based on the CQIs (step  510 ). The current serving Node-B  204   a  then sends a message to a centralized aGW  206  and the WTRU  202  which indicates the new serving cell/Node-B (step  512 ). The centralized aGW  206  then sends a message to the new Node-B, such as Node-B  204   b,  and optionally to other Node-Bs, such as Node-B  204   c,  which indicates the new serving cell/Node-B (step  514 ).  
         [0032]      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. A WTRU  202  measures downlink path losses of a serving cell  208   a  and at least one other cell  208   b - 208   i  (step  602 ). The WTRU  202  determines whether a path loss difference between the serving cell  208   a  and the other cell  208   b - 208   i  is below a threshold (step  604 ). If the path loss difference is not below the threshold, the process  600  returns to step  602  to further measure the path losses at the next cell selection interval.  
         [0033]     If the path loss difference is below the threshold, the WTRU  202  measures a CQI for each of the plurality of subcarrier blocks in the downlink of the serving cell  208   a  and the other cell  208   b - 208   i  preferably whose path loss is close to the path loss of the serving cell  208   a,  respectively (step  606 ). The WTRU  202  then reports the CQIs to the current serving Node-B  204   a  (step  608 ). The WTRU  202  may report CQIs of the best K subcarrier blocks of each of those cells  208   a - 208   i  to the serving Node-B  204 . The K reported subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks for each cell  208   a - 208   i.  Alternatively, the WTRU  202  may report K best K subcarrier blocks of the best cells controlled by each Node-B  204   a - 204   c.  The value of K is a design parameter, which satisfies M≦K≦N.  
         [0034]     Upon receiving the reported CQIs, the current serving Node-B  204   a  selects a new serving cell/Node-B based on the CQIs (step  610 ). The Node-Bs  204   a - 204   c  are directly connected to each other via a high speed link  212 . The current serving Node-B  204   a  then sends messages to the new Node-B, such as Node-B  204   b,  and optionally to other Node-Bs, such as Node-B  204   c,  via the high speed link  212  and to the WTRU  202  preferably via the downlink shared control channel, which indicates the new serving cell/Node-B (step  612 ).  
         [0035]      FIG. 7  is an exemplary block diagram of the WTRU  202  used in the system  200  of  FIG. 2  in accordance with the present invention. The WTRU  202  includes a path loss calculation unit  702 , a comparator  704 , a CQI measurement unit  706  and a controller  708 . The path loss calculation unit  702  calculates a downlink path loss of a serving cell  208   a  and at least one other cell  208   b - 208   i  preferably based on pilot channel strength. The comparator  704  determines whether a difference of the path losses of the serving cell  208   a  and the other cell  208   b - 208   i  is below a threshold. The CQI measurement unit  706  measures a CQI for each of the plurality of subcarrier blocks in the downlink of the serving cell  208   a  and the other cell  208   b - 208   i,  respectively, when a cell selection procedure is triggered. The controller  708  reports the CQIs to a current serving Node-B  204   a.    
         [0036]     Although the features and elements of the present invention are described in the preferred embodiments 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.