At present, there is a great deal of interest in the telecommunications field regarding the possibility of providing coverage for cellular telephony systems such as, for example, GSM or WCDMA, in small areas with a limited number of users, from now on generically referred to as Mobile Station, MS, by means of Radio Base Stations, RBSs, with a limited coverage area, sometimes referred to as Femto RBSs.
A Femto RBS would provide normal GSM or WCDMA coverage for the MSs in the cell (“Femto cell”) controlled by the Femto RBS, and the Femto RBS would be connected to the Core Network in a way which is similar to a “normal” RBS, i.e. by means of a network node referred to, depending on the type of system, as Base Station Controller, BSC, or Radio Network Controller RNC, from now on generically referred to as BSC. It is at present envisioned that the link between the Femto RBS and the Femto BSC would be by means of some kind of IP based transmission.
The coverage area provided by the Femto RBS is referred to as a Femto cell to indicate that the coverage area is relatively small compared to a normal RBS, a “Macro” RBS.
A Femto BSC would suitably be based on a standard “Macro” BSC and can be provided either as a stand-alone Femto BSC node, i.e. not capable of controlling Macro RBSs, or as a function in a Macro BSC which would enable the Macro BSC to control both Macro RBSs and Femto RBSs.
A main field of application for Femto cells would be homes or offices. As such, there is a problem associated with the Femto cells, mainly caused by the definition of a Femto cell as a cell with a small or very small coverage area as compared to the area covered by “normal” cells: if, for example, a user with a MS in a home which is defined as a Femto cell steps out to mow the lawn, or simply to get some fresh air while conducting a phone call, the system might hand the call to the Macro network. When the user enters the area of the Femto cell again, e.g. moves indoors again, the user will want the call to be handed back to the original Femto cell again.
A problem with the scenario described above, i.e. handing a call back to a Femto cell in which the call originated, is the so called “neighbouring cell lists” in the Macro cells, more precisely i.e. in the Macro BSC. These lists are used by the Macro BSC to configure the MS with measurement reporting information regarding the available RBSs in the area, i.e. RBSs to which the MS might be handed over to.
The main problem is the number of future Femto cells, which is estimated to be in the magnitude of tens of thousands, as opposed to the number of cells that can be included in the neighbouring cell lists: the neighbouring cell lists in UTRAN can currently contain a total of 96 cells, with the neighbouring cell lists in GSM being limited in a similar way. Furthermore, it is not feasible to configure this information statically as the end users will be able to move the Femto RBS on their own to a new location.