1. Field
The present disclosure is directed to a method and apparatus for distinguishing cells with the same physical cell identifier. More particularly, the present disclosure is directed to distinguishing cells with the same physical cell identifier by using a radio frame timing offset.
2. Introduction
Presently, in a cellular network, cells use physical cell identifiers to distinguish themselves from each other. An operator ensures that a physical cell identifier unambiguously identifies a base station. However, Closed Subscriber Group (CSG) base stations, such as access points, may use the same physical cell identifiers, which can result in physical cell identifier confusion. For example, CSG cells can be a collection of cells used for deployment in a campus or can be individual cells used for deployment in users' homes. The CSG cells co-exist with macro cells on the same carrier frequency. CSG cells have a smaller coverage area than macro cells. Unlike macro cells, the CSG cells are un-planned, in that the operator has much less control over their placement and configuration than with macro cells. Thus, two CSG cells that are located within the coverage of the same macro cell can use the same physical cell identifiers. Unfortunately, this results in physical cell identifier confusion.
To elaborate, a mobile station uses physical cell identifiers (PCID) during synchronization and during cell ranking. The mobile station ranks cells by measuring the received signal strength and then uses the ranking to facilitate handover and reselection. If a PCID is not guaranteed to be unique within a macro cell, then PCIDs cannot be used for reselection and handover. If PCIDs cannot be used for reselection and handover, a mobile terminal would need to read system information of the target cell and acquire the cell global identity to determine if it is allowed to access the cell. Unfortunately, this requires considerable additional battery usage in idle mode and can seriously impact battery life. Another problem with using the same PCIDs is that mobile station cell handover will fail when there is more than one cell with the same PCID and a network cannot determine which cell is the right one for handover.
A range of PCIDs can be reserved for CSG cells. Also, a mobile terminal can have a list of CSG PCIDs, such as a CSG white-list of cells that it is allowed to access. These restrictions limit the problem in the reselection case to when the target cell is a CSG cell in the CSG white-list. However, PCID confusion can still frequently occur in metropolitan areas where more CSG cells are deployed. Even in cases where the spatial likelihood of PCID confusion is low, when confusion occurs, it affects the same mobile terminal repeatedly. For example, if two homes within the coverage of the same macro cell use CSG cells with the same PCID, the corresponding users will experience handover failures when entering their homes and they will have substantially higher battery drain.
In order to resolve the PCID confusion, a mobile terminal could read additional system information of a cell, which contains a unique cell identifier, which the mobile terminal could rely on to determine if the cell is suitable. Unfortunately, reading the additional system information in connected mode would cause substantial delay which negatively impacts handover performance. Also, a mobile terminal would have to read the additional system information every time it encounters a CSG PCID, because different encounters with the same PCID could correspond to different cells. Furthermore, the mobile terminal would lose data being sent through the serving cell as a result of reading the additional system information because the mobile terminal would have to synchronize to the target cell.
It is also possible to ignore a cell based on the PCID if it has been found to be unsuitable after previously reading additional system information. However, this would not resolve the PCID confusion problem because a cell encountered later may be suitable to the mobile terminal but would be ignored if it has same PCID as a previously unsuitable cell. Furthermore, in connected mode, a mobile terminal would not measure and report the ignored PCIDs and the network would not know when interference from the PCID is significant and would not be able to take measures to prevent disruption of service.
Thus, there is a need for a method and apparatus for distinguishing cells with the same physical cell identifier.