Scrambling code selection

Abstract

A basestation forms part of a group of basestations within a cellular communications network, and selects an identifying code for use in identifying transmissions from the basestation. The basestation receives from a management node a first list of identifying codes and a second list of identifying codes, wherein the identifying codes of the first list can appear in neighbour cell lists of basestations outside said group, and wherein the identifying codes of the second list can not appear in neighbour cell lists of basestations outside said group. The basestation determines whether there is at least one identifying code either in the first list of identifying codes or the second list of identifying codes that is not used by any other basestation in said group. If there is at least one identifying code in the first list of identifying codes and at least one identifying code in the second list of identifying codes that are not used by any other basestation in said group, an identifying code from the first list of identifying codes is selected in preference to an identifying code from the second list of identifying codes.

1 . A method, for use in a basestation forming part of a group of basestations within a cellular communications network, for selecting an identifying code for use in identifying transmissions from the basestation, the method comprising:
 receiving from a management node a first list of identifying codes and a second list of identifying codes, wherein the identifying codes of the first list can appear in neighbour cell lists of basestations outside said group, and wherein the identifying codes of the second list can not appear in neighbour cell lists of basestations outside said group;   determining whether there is at least one identifying code either in the first list of identifying codes or the second list of identifying codes that is not used by any other basestation in said group; and   if there is at least one identifying code in the first list of identifying codes and at least one identifying code in the second list of identifying codes that are not used by any other basestation in said group, selecting an identifying code from the first list of identifying codes in preference to an identifying code from the second list of identifying codes.   
     
     
         2 . A method as claimed in  claim 1 , wherein the identifying codes are scrambling codes. 
     
     
         3 . A method as claimed in  claim 1  or  2 , wherein the basestation is a femtocell access point, and the group of basestations is a group of femtocell access points connected by means of a local area network. 
     
     
         4 . A method, for use in a basestation forming part of a group of basestations within a cellular communications network, for selecting an identifying code for use in identifying transmissions from the basestation, the method comprising:
 receiving from a management node a first list of identifying codes and a second list of identifying codes, wherein the identifying codes of the first list can appear in neighbour cell lists of basestations outside said group, and wherein the identifying codes of the second list can not appear in neighbour cell lists of basestations outside said group;   determining whether it is necessary to be able to perform cell reselection between the basestation and basestations outside said group;   if it is determined that it is necessary to be able to perform cell reselection between the basestation and basestations outside said group, selecting an identifying code from the first list of identifying codes; and   if it is determined that it is not necessary to be able to perform cell reselection between the basestation and basestations outside said group, selecting an identifying code from the first list of identifying codes or from the second list of identifying codes.   
     
     
         5 . A method as claimed in  claim 4 , further comprising:
 determining whether it is necessary to be able to perform cell reselection between the basestation and basestations outside said group, based on a history of cell reselections involving the basestation.   
     
     
         6 . A method as claimed in  claim 4  or  5 , wherein the basestation is a femtocell access point, and the group of basestations is a group of femtocell access points connected by means of a local area network. 
     
     
         7 . A method, for use in a management node of a cellular communications network, wherein the network includes at least one group of basestations and further includes other basestations not within said group, the method comprising:
 dividing the available identifying codes into a first list of identifying codes and a second list of identifying codes, wherein the identifying codes of the first list can appear in neighbour cell lists of basestations outside said group, and wherein the identifying codes of the second list can not appear in neighbour cell lists of basestations outside said group; and   notifying the basestations in said group of basestations of the first list of identifying codes and the second list of identifying codes.   
     
     
         8 . A method as claimed in  claim 7 , wherein the identifying codes are scrambling codes. 
     
     
         9 . A method of allocating identifying codes to basestations forming part of a group of basestations within a cellular communications network, the method comprising, in each basestation within the group:
 selecting an identifying code in a manner that attempts where possible to avoid selecting the same identifying code as a neighbour,   and the method further comprising, in at least one basestation within the group:   determining that a clash has occurred if any neighbour basestation has selected the same identifying code as said at least one basestation, and   if a clash has occurred, determining whether the clash can be resolved by said basestation or by another basestation in said group.   
     
     
         10 . A method as claimed in  claim 9 , further comprising, if a clash has occurred:
 determining whether there are any identifying codes that are not in use by neighbours of the basestation; and   if so, selecting an identifying code that is not in use by a neighbour of the basestation.   
     
     
         11 . A method as claimed in  claim 10 , further comprising:
 if there are no identifying codes that are not in use by neighbours of the basestation, identifying a neighbour that has a high freedom of selection of an alternative identifying code; and   selecting the identifying code that is in use by that neighbour.   
     
     
         12 . A method as claimed in  claim 11 , further comprising:
 for one or more basestations, calculating a freedom of selection parameter, based on a number of identifying codes potentially available for use by that basestation and on a number of said available identifying codes that are actually in use by neighbours of that basestation.   
     
     
         13 . A method as claimed in any of  claims 9  to  12 , wherein the basestations are femtocell access points. 
     
     
         14 . A method as claimed in any of  claims 9  to  12 , wherein the identifying codes comprise scrambling codes. 
     
     
         15 . A method as claimed in  claim 14 , wherein the identifying codes further comprise MIB Value Tags. 
     
     
         16 . A basestation, for use in a cellular communications network, the basestation being configured to perform a method as claimed in any of  claim 1 ,  4 , or  9 .