Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-06-29
2001-08-21
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S443000, C455S436000
Reexamination Certificate
active
06278881
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to cellular mobile communications systems, and in particular to a method of managing a handover of a radio connection between two base stations in a network.
DESCRIPTION OF RELATED ART
A mobile telephone system includes a network of radio base stations, which can communicate with mobile devices which are active within the network. Each base station is able to communicate with mobile devices within a particular geographical area, called a cell.
A hierarchical cell structure groups the cells into layers, and typically includes a relatively large number of microcells, which are in a lower layer and each cover a relatively small geographical area, and a smaller number of macrocells, which are in a higher layer, and each cover a relatively large geographical area. In such a hierarchy, the lower layer cells are located such that they can communicate with mobile stations in those areas where the connection requirements are greatest, but there may be relatively large areas where there is no lower level cell. The upper layer cells then handle connections with mobile devices in these areas.
When a mobile device moves, it may be necessary for a connection with one base station to be handed over to another base station.
EP-0701382A discloses a hierarchical cell structure, with macrocells and microcells.
One widespread digital cellular system, GSM, defines its handover procedure, in Technical Specification GSM 04.08 section 3.4.4. The mobile station (MS) is ordered to switch to a channel in another cell by the air interface message HANDOVER COMMAND. The message contains a detailed description of the channel to switch to as well as other information such as the power to use when accessing the base station carrying this channel. The message may also include a time indication, expressed in the target cell time frame, when to access the new channel.
The HANDOVER COMMAND message is issued by the radio network control unit i.e. the BSC (Base Station Controller) via the base station of the serving cell. The typical reason for ordering a cell change is that the BSC has detected a better cell for the connection.
If the MS is unable to decode the HANDOVER COMMAND message, it will remain on the present channel until the subscriber hangs up, or another HANDOVER COMMAND message is successfully received, or radio link failure is detected, whichever happens first.
Radio link failure is detected on the MS side by the radio link counter S. If the MS is unable to decode a SACCH message, a counter S is decreased by 1. When the MS successfully decodes a SACCH message, S is increased by 2. In any case S shall not exceed the value of the parameter RADIO_LINK_TIMEOUT, which gives the initial value of counter S. If S reaches 0 a radio link failure will be declared and the MS will return to idle mode. The algorithm described above is often referred to as a “leaky bucket” algorithm.
If the HANDOVER COMMAND message is received properly, but the MS cannot establish on the new channel, then the MS will return to the first channel after some time, controlled by a timer. If the reestablishment on that channel also fails, a radio link failure will be detected and the MS will return to idle mode.
A problem arises with the use of this handover procedure, particularly in a hierachical cell structure, in which the lower layer cells are quite small, and the over-lapping areas quite narrow. Near the cell border, the signal received from the serving base station drops very quickly. There is a substantial risk that the HANDOVER COMMAND message will not be received properly by the MS and the MS will remain on its present channel. Unless the MS reenters the serving cell, the MS will release the connection through its “leaky bucket” algorithm. The fact that the target cell, or indeed a higher level cell, could provide a high signal strength connection in the location of the MS at the time the HANDOVER COMMAND is sent will not be of any help. The MS is dependent on receiving a correct handover order from the serving base station.
SUMMARY OF THE INVENTION
The present invention seeks to provide a solution to this problem, by allocating a backup channel to a mobile station as it approaches the edge of a cell. Where the mobile device is communicating with a base station in a lower level cell, the backup channel can be in a higher level. If the mobile station fails to handover correctly to a new cell in the lower level, it can revert to the higher level backup channel. However, in the event that the handover is achieved correctly, the backup channel is not used, and the resources of the higher level cell are not occupied.
Aspects of the invention relate to methods of managing handover in a mobile communications system, to base stations for use in a mobile communications network, and to mobile devices for use in such a network.
An object of the invention, therefore, is to allow handovers to be achieved with a greater probability of success.
REFERENCES:
patent: 4829554 (1989-05-01), Barnes et al.
patent: 4955082 (1990-09-01), Hattori et al.
patent: 5164958 (1992-11-01), Omura
patent: 5175867 (1992-12-01), Wejke et al.
patent: 5265263 (1993-11-01), Ramsdale et al.
patent: 5289525 (1994-02-01), Issenmann et al.
patent: 5301356 (1994-04-01), Bodin et al.
patent: 5392453 (1995-02-01), Gudmundson et al.
patent: 5394158 (1995-02-01), Chia
patent: 5581548 (1996-12-01), Ugland et al.
patent: 5594943 (1997-01-01), Balachandran
patent: 5640677 (1997-06-01), Karlsson
patent: 5765103 (1998-06-01), Chang et al.
patent: 5822696 (1998-10-01), Bergkvist
patent: 0 369 535 (1990-05-01), None
patent: 0 526 436 (1993-02-01), None
patent: 0 568 212 (1993-11-01), None
patent: 0 660 632 (1995-06-01), None
patent: 0701382A1 (1996-03-01), None
patent: 0 701 382 (1996-03-01), None
patent: 2 220 117 (1989-12-01), None
patent: 07023449 (1995-01-01), None
patent: 07046644 (1995-02-01), None
patent: 07059145 (1995-03-01), None
patent: 07067165 (1995-03-01), None
patent: 07095648 (1995-04-01), None
patent: 0888876 (1996-04-01), None
patent: 08154269 (1996-06-01), None
patent: 93/06663 (1993-04-01), None
patent: 95/08896 (1995-03-01), None
patent: 95/35006 (1995-12-01), None
patent: 96/05708 (1996-02-01), None
patent: 96/36190 (1996-11-01), None
International Search Report dated Dec. 18, 1998.
Great Britain Patent Office Search Report dated Sep. 1, 1997.
Draft prETS 300 557: Nov. 1996 (GSM 04.08 Version 4.17.0 Sections 3.4.4-3.4.11.2) pp. 37-43.
Burns Doane Swecker & Mathis L.L.P.
Chow C.
Hunter Daniel
Telefonaktiebolaget LM Ericsson (publ)
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