Patent Publication Number: US-2013242767-A1

Title: Cell Selection and Reselection in a Telecommunication Network

Description:
TECHNICAL FIELD 
     The present invention relates to the management of cell selection and reselection in telecommunication networks such as UMTS or LTE networks. 
     BACKGROUND OF THE INVENTION 
     In Universal Mobile Telecommunications System (UMTS) networks and Long term Evolution (LTE) networks, a terminal which tries to camp on a cell of the network performs a series of measurements for choosing a best cell to camp on. 
     Documents TS 25.304 and TS 36.304 of the 3 rd  Generation Partnership Project (3GPP) standard specification define conditions for and procedures of the selection and reselection of cell of a network. 
     PRIOR ART 
     The cell selection and reselection operations are based on the comparison of quality parameters (such as the power of the signals in the cells) with thresholds according to determined criteria. 
     When a terminal determines that the parameters of a cell fulfil the criteria (for determining that quality is acceptable) and when a user of the terminal tries to make a call for example, the terminal sends connection messages to a management unit of the network such as a base station BS (the NodeB for a UMTS network or the eNodeb for an LTE network). The management unit then accepts or rejects the connection. In case the management unit rejects the connection, the terminal may search for another cell to camp on. 
     The connection rejection may be caused by the fact that there is congestion on the cell. For example, the network may then temporarily prevent access to a cell for maintenance or traffic load reasons. The cell in this case may be barred by the network. 
     The repetition of unsuccessful connection attempts takes time and consumes uselessly power of the terminal. 
     In the prior art, the terminal has no means for avoiding trying selecting, reselecting, or connecting to congested cells. 
     SUMMARY OF THE INVENTION 
     There is still a need for improving the management of cell selection and reselection in UMTS and LTE networks. 
     To that end there is provided, according to a first aspect of the invention, a method for performing a selection or a reselection of a cell of an UMTS or LTE network, comprising the steps of:
     performing a quality measurement in a cell of the network;   reading information representing a state of load of the cell, said information being broadcasted by a management unit of the network;   computing a parameter based on a result of the quality measurement performed and taking into account the information representing the state of load of the cell; and   selecting or reselecting the cell based on the computed parameter.   

     Hence, when deciding to select or reselect a cell, the terminal has information on the state of load of the cell. In case the cell is heavily loaded, the terminal may avoid camping on the cell because it has a great probability of being already barred or to be barred in the next moments due to a congestion on the cell. The terminal then avoids initiating a connection with the base station (for performing a call for example) that will be rejected because of a congestion of the cell. 
     With the present invention, unsuccessful selections and reselections are avoided or at least significantly reduced which results in power saving in the terminal, and in a faster selection or reselection procedure. 
     For example, the parameter is compared to a threshold. 
     The information representing the state of load of the cell may be read in the broadcasted System Block Information (SIB) broadcasted by a management unit of the network. 
     For example, the traffic load is determined based on an occupation of a bandwidth available in the cell. 
     Alternatively, the traffic load is determined based on a number of ongoing communications in the cell. 
     According to other aspects of the invention, there is also provided:
     a computer program comprising instructions for executing a method according to the first aspect when the program is executed by a processor;   a computer program product for storing the computer program; and   a terminal for implementing a method according the first aspect.   

     These objects provide at least the same advantages as those associated to the method. 
     Other advantages are obtained by additional features which are recited in the dependent claims attached hereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments, with reference to the appended drawings, in which: 
         FIG. 1  is a schematic illustration of a context of implementation of embodiments of the invention; 
         FIG. 2  illustrates a cell selection (or reselection) according to an embodiment of the invention; 
         FIG. 3  is a schematic illustration of a terminal according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIG. 1  illustrates a context of implementation of embodiments of the invention. A mobile telecommunication device  10  is in the coverage of three different cells  11 ,  12 , and  13  of a telecommunication network. The network may be an UMTS or an LTE network. 
     In order to decide on which cell it will camp on, the terminal performs a series of measurements and checks the fulfilment of some criteria. The terminal camps on a cell when it has selected it and tuned to the cell&#39;s control channel. 
     The terminal may have been activated under the coverage of cells  11 ,  12 , and  13  and may then search for a cell to camp on (selection case), or it may already camp on one of the cells (for example cell  11 ) and searching for a better cell to use (re-selection case). 
     For a cell selection (when the terminal tries to camp on a cell), for an UMTS network, the terminal checks the following criteria:
         NewSrxlev&gt;0 and Squal&gt;0 for Frequency Division Duplex (FDD) cells, or   NewSrxlev&gt;0 for Time Division Duplex (TDD) cells.       where:   NewSrxLev=Qrxlevmeas−Qrxlevmin−Pcompensation−Q1NwLoad,   Squal=Qqualmeas−Qqualmin,   Squal: Cell Selection quality value (dB).   Srxlev: Cell Selection RX level value (dB).   Qqualmeas: Measured cell quality value. The quality of the received signal expressed in CPICH Ec/N0 (dB) for FDD cells.   Qrxlevmeas: Measured cell RX level value. This is received signal, CPICH RSCP for FDD cells (dBm) and P-CCPCH RSCP for TDD cells (dBm).   Qqualmin: Minimum required quality level in the cell (dB).   Qrxlevmin: Minimum required RX level in the cell (dBm).   Pcompensation: max(UE_TXPWR_MAX_RACH−P_MAX, 0) (dB).   UE_TXPWR_MAX_RACH: Maximum TX power level a terminal may use when accessing the cell on RACH (read in system information) (dBm).   P_MAX: Maximum RF output power of the terminal (dBm).   

     In an embodiment of the present invention, the state of load of the cell is taken into account. A new parameter NewSrxlev is defined as follows:
         NewSrxlev=Srxlev−Q1NwLoad, where       Q1NwLoad is a parameter measuring the state of load of the cell.   

     The parameter Q1NwLoad may be computed as follows: 
     
       
         
           
             
               
                 Q 
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                 1 
                  
                 
                   NwLoad 
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               = 
               
                 60 
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                     TRUNC 
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                           OccupiedBandwidth 
                           TotalBandwidth 
                         
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                   10 
                 
               
             
             , 
           
         
       
     
     where OccupiedBandwidth is the bandwidth currently used for service executions in the cell, and TotalBandwidth is the total bandwidth which can be provided by the cell. 
     For a cell selection, for an LTE network, the terminal checks the following criteria:
         NewSrxlev&gt;0, where       NewSrxLev=Qrxlevmeas−(Qrxlevmin−Qrxlevminoffset)−Pcompensaion−Q1NwLoad,   Qrxlevmeas: Measured cell RX level value. This is received signal, CPICH,   RSCP for FDD cells (dBm) and P-CCPCH RSCP for TDD cells (dBm).   Qrxlevmin: Minimum required RX level in the cell (dBm),   QrxlevminOffset: Offset to the signalled Qrxlevmin,   Pcompensation: max(UE_TXPWR_MAX_RACH−P_MAX, 0) (dB),   UE_TXPWR_MAX_RACH: Maximum TX power level an terminal may use when accessing the cell on RACH (read in system information) (dBm),   P_MAX: Maximum RF output power of the terminal (dBm), and   Q1NwLoad is the same as in the UMTS case.   

     For a cell re-selection (when the terminal was previously camping on a cell), for an UMTS or LTE network, the terminal ranks the neighbouring cell according the following parameters: 
       New Rs=Q   meas,s   +Q   hysts   Q 2 Nw Load, and 
       New Rn=Q   meas,n   +Q   offset,s,n   −Q 2 Nw Load, where     Q meas,s  is a quality parameter measured in the current cell (for example the RSCP parameter),   Q hyst  is a hysteresis value,   Q meas,n  is a quality parameter measured in the neighbouring cell, and   Q offset,s,n  is an offset value between the current cell and the neighbouring cell.   
     In an embodiment of the present invention, the state of load of the cell is taken into account, through the parameter Q2NwLoad. This parameter may be defined identically as Q1NwLoad. 
     In the present embodiment new parameters NewSrxlev, NewRn and NewRs that take into account a state of load of the cells have been defined. 
     The way these parameters are used in the cell selection and reselection procedures may be the same as the procedures (using the old parameters) already defined in the 3GPP standard specification to which the person with ordinary skill in the art may refer. 
       FIG. 2  illustrates a cell selection (or reselection) according to an embodiment of the invention. 
     A terminal  20 , which is under the coverage of several cells of a telecommunication network, performs a series of measurements in the cells. The measurements are performed during step  22 . The terminal performs quality measurements on the signals of the cells. During this step, the terminal also reads information broadcasted by a management unit  21  of the network such as the base station of the cell. The information broadcasted concern a state of load of the cells. This information may be read in the SIB. 
     Then, in step  23 , the terminal computes parameters for deciding on which cell it will camp on. The parameters determined have already been described above. The selection (or reselection) decision is made in step  24 , by comparing the parameters determined to thresholds. After this step, the terminal has selected a cell to camp on. 
     Optionally, the terminal may then send a connection message  25  on the selected cell, for example a registration or a connection when user tries to initiate a communication. The cell then sends a message  26  for accepting or rejecting the connection. In case the connection is rejected, the terminal may return to step  22 . 
     A computer program comprising instructions for executing the steps of the method described above may be designed based on an algorithm derived from the general flowchart depicted in  FIG. 2  and the present description. 
     Referring to  FIG. 3 , a mobile communication terminal  30  according to the present invention is described. The device comprises a processing unit  31  for implementing a method according to embodiments of the invention. To that purpose, the device comprises a memory unit  32 . The memory unit may comprise several types of memory. For example, the memory unit may comprise a memory for storing processing data. The memory unit may also comprise a memory for storing a computer program according to embodiments of the invention. The device also comprises a communication unit  33  for performing communications over the network. 
     While the invention has been illustrated and described in details in the drawings and foregoing description, such illustration and description are to be considered illustrative and exemplary only, the invention being not restricted to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. 
     In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the invention.