Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Patent
1997-06-13
1999-06-15
Eisenzopf, Reinhard J.
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
455562, H04B 100
Patent
active
059131691
DESCRIPTION:
BRIEF SUMMARY
This application is the national phase of international application PCT/F195/00676, filed Dec. 12, 1995 which was designated the U.S.
1. Field of the Invention
The invention relates to a cellular mobile station system wherein a handover decision is based on measuring downlink transmissions of a serving cell and neighboring cells by a mobile station. Adjacent cells are typically arranged to be in a partially overlapping configuration so as to ensure that there is a sufficient period of time for the reception and decoding of the downlink transmissions of the neighboring cells for a handover decision.
2. Background of the invention
In cellular mobile station systems, a mobile station, in which a call is established, that is, which is using a traffic channel, measures constantly the quality and/or the strength of the downlink signal from a serving base station. The serving base station also informs the mobile station of the neighboring cells/base stations and of their control channel frequencies. On the basis of this information, the mobile station is able to measure also the control channel frequencies of the neighboring base stations during a call. The measuring results are stored into the mobile station and/or are transmitted to the mobile station network for the handover decision depending on whether the handover decision is made by the mobile station or by the mobile station network. A handover refers to a switching of a call from one traffic channel to another during a call.
The base stations of the cellular network constantly broadcast information about themselves and their surroundings. This information includes a cell or base station identifier, based on which the mobile station is able to identify the neighboring cell to be measured. The cell or base station identifier is also transmitted to the cellular network in possible measuring reports. The mobile station has to decode the cell or base station identifier of a neighboring cell before this cell can be used as a potential target cell for a handover. In order that the mobile station would have a sufficient period of time to decode the cell or base station identifier and that way allow the handover, the adjacent cells of the cellular mobile station system must have overlapping areas. Besides the acceptable decoding time of the cell or base station identifier, the size required of the overlapping area of two adjacent cells depends, e.g., on the speed of the mobile station and the parameters of handover algorithm, such as, the size of the time window used for averaging the measured signal levels of the neighboring cell, for example.
If the overlapping area of two adjacent cells is so small that the handover cannot be made, the call is lost. One reason for a lost call is an undecoded cell or base station identifier of the neighboring cell. The overlapping requirement of cells is significant especially in microcells in which the diameter is some hundreds of meters. It is, however, difficult to arrange a sufficiently large overlapping area in places where terrain obstacles, buildings or the like obstacles obstructing the propagation of radio signals are shadowing the transmission of the neighboring cell. Such problematic places are found, for example, in tunnels, at street corners and inside buildings. For example, when a mobile station comes fast out of a tunnel, the neighboring cell outside the tunnel may develop an immediate need for a handover but the cell or base station identifier of the neighboring cell outside the tunnel is undecoded and the call is lost because a handover to the neighboring cell cannot be made. This situation is illustrated in more detail by means of an example with reference to FIG. 1.
In FIG. 1 the serving cell is indicated by the reference character S and the neighboring cell by the reference character A. The serving cell is, for example obstructed by a tunnel 3 so that cell S extends somewhat outside the tunnel at the opening of the tunnel. The neighboring cell A is a cell outside the tunnel and the radio coverage of cell A
REFERENCES:
patent: 3916311 (1975-10-01), Martin et al.
patent: 5235632 (1993-08-01), Raith
patent: 5404570 (1995-04-01), Charas et al.
patent: 5537637 (1996-07-01), Sugita et al.
patent: 5603080 (1997-02-01), Kallander et al.
Aoki Makoto
Eisenzopf Reinhard J.
Nokia Telecommunications Oy
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