Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-02-24
2001-08-14
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
Reexamination Certificate
active
06275701
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method in a mobile telecommunications system for restoration of a connection, especially a connection that risks being interrupted prematurely.
The invention also relates to a device for carrying out said method.
DESCRIPTION OF RELATED ART
A cellular mobile telecommunications system generally has to be able to handover a connection between different cells in the system, that is, an inter cell handover. Typical situations in which an inter cell handover will have to be performed are when a mobile subscriber moves from one cell to another, or in case of an overload in one cell.
In some case, for example when one or more channels are exposed to disturbances, it may be advantageous to assign a new channel to an existing connection within the same cell, that is, to perform an intra-cell handover.
If an inter- or intra-cell handover of a connection is needed, but a suitable new channel to which to hand over the connection cannot be found, there is a risk that the connection will be interrupted. The most common reason for a connection interruption is that a mobile subscriber moves to an area in which the radio coverage of the communication system is too weak or completely missing. Disturbing radio transmitters in or outside the system may also interfere with the radio channel of a connection and cause the premature interruption of the connection. A mobile subscriber may also have moved into an overloaded cell in which no idle channels are available. If the connection cannot be handed over to a channel in an adjacent cell, with which the mobile subscriber has contact, or if the mobile subscriber entirely lacks radio contact with other cells in the system, the connection is interrupted when the signal becomes too weak. A connection to a mobile subscriber may also be interrupted even if the subscriber has radio contact with a cell in which idle channels are available. This may happen if a system parameter in the communication system is exceeded. The timing advance parameter in a Time Division Multiple Access (TDMA) system is one such system parameter. The Timing advance parameter states the time by which a mobile station must bring forward its transmission in order for its information to fit into the frame structure of the serving base station. When the distance between a mobile station and the base station with which it is communicating exceeds a certain value, the timing advance parameter is unable to compensate for the propagation delay between the mobile station and the base station, and the connection is interrupted.
A threshold value maxTA, set by the operator of the mobile telecommunications system in order to mark out a cell border of a base station is another system parameter limiting the maximal distance from which a mobile station can communicate with a certain base station. If the distance between a mobile station and the base station with which it communicates exceeds maxTA, the connection is interrupted.
Reconfiguration of a cell normally implies that all traffic transmitted through the cell must cease. This constitutes another reason for the interruption of connections. Of course, a base station can also cease to serve connections, entirely or in part, on more unpredictable occasions, such as power failure or other serious problems.
The connection of a certain mobile station, having a low priority, may have to be interrupted because a new connection having a higher priority is established in the cell in which the connection is set up. If all ongoing connections in the cell have a higher priority than said first connection, when the new connection is set up, the first connection will have to be assigned to a traffic channel in another cell. If there is no idle capacity in the other cells with which the mobile station has contact, the connection is interrupted. Usually connections for transmitting data are assigned a lower priority than speech connections in which speech is transmitted in real time.
There are two principally different ways of reducing the fraction of interrupted connections in a mobile telecommunications system. It is possible to try to prevent interrupted connections, and to, to the degree possible, try to re-establish the connection that risk being interrupted soon, or that have recently been interrupted.
The preventative actions may be concentrated on increasing the radio coverage, improving the quality and/or increasing the capacity. The radio coverage may be increased by introducing more base stations, improved antennas and/or increased output power from the base stations. The quality may be improved by refining the signal encoding or by increasing the channel spacing in the system. The connection capacity of the system may be increased by increasing the frequency range of the system or by reducing the channel spacing, which of course in turn may result in a reduced quality. All of said preventative actions imply high costs for the telecommunications operator while none of the actions will guarantee that no connection is ever prematurely interrupted.
Solutions that seek to re-establish interrupted connections, or connections that will probably soon be interrupted, offer relatively inexpensive complement to the above mentioned preventative actions.
From the Patent Specification U.S. Pat. No. 4,811,380 a method of preventing the loss of a connection at handover is known. If a connection between a first base station and a mobile station is handed over to a second base station and the mobile station does not receive a handover confirmation from the second base station, the mobile station will utilize a signalling channel from the channel set of the second base station, to send a request for the re-establishment of the connection to the second base station. The second base station responds by transmitting a handover confirmation to the mobile station, and the connection continues, served by the second base station.
European Patent Application EP, A, 690 650 discloses a method in a cellular communications system in which a handover request from a mobile station is sorted into a queue until a channel becomes idle in the cell to which the connection should be handed over. According to this method, channels are reserved in the target cell (that is, the cell to which the connection is to be handed over) for the mobile stations in said queue, so that, for example, the establishment of new connections in the target cell is prevented if all idle channels in this cell have already been reserved for queuing mobile stations in the neighbouring cells. In this way, the risk of a connection being interrupted in connection with a handover is reduced.
International Patent Application WO, A, 96/07287 describes a method in which the handover of a low quality connection in a first cell, in the case of an emergency, is forced to a second cell having a higher signal strength. When a normal handover occurs, the signal strength in the target cell must exceed the signal strength in the original cell (that is, the cell from which the handover is initiated) with a certain hysteresis value. In an emergency situation, this hysteresis value may be lowered so that handover will occur at an earlier stage, reducing the risk of the connection being interrupted.
None of the above mentioned solutions indicates the possibility of temporarily parking a connection on a dedicated signalling channel while waiting for a traffic channel to become idle. The method according to U.S. Pat. No. 4,811,380 does employ a control channel, but through this channel only a request for reestablishment of an interrupted call is transmitted. The disclosed method is based on the mobile station indicating on the control channel that its speech connection has been interrupted. The mobile station then contacts the stationary system for a re-establishment of the connection. Such a reestablishment of an interrupted speech connection takes quite a long time and there is therefore a risk that at least one of the connected subscribers (the A or B subscriber) interrupts t
Burns Doane Swecker & Mathis L.L.P.
Hunter Daniel
Nguyen Thuan T.
Telefonaktiebolaget LM Ericsson (publ)
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