Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-04-09
2001-02-20
Eisenzopf, Reinhard (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S436000
Reexamination Certificate
active
06192246
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to cellular telephone systems and, more particularly, to a method and system for increasing capacity for calls and reducing failures during call handoff and call setup in a cellular telephone network.
BACKGROUND OF THE INVENTION
A cellular telephone system serves a defined area by dividing the area into cells. Each cell is served by a single base station, or cell site, and each cell site is connected to a base station controller (“BSC”), which is connected to a message switching center (“MSC”), which is connected to the public switched telephone network (“PSTN”). A mobile unit is connected, or a call is completed, to the PSTN by establishing a radio frequency (“RF”) link with a nearby cell site.
The situation in which a mobile unit ends communication with a first cell site and begins communication with a second cell site is referred to as a handoff. A hard handoff occurs when a first RF link between the mobile unit and the first cell site is ended, or dropped, before a second RF link between the mobile unit and the second cell site is established. Alternatively, a soft handoff occurs when the second RF link to the mobile unit is established before the first RF link is dropped. In soft handoff the first cell site is considered the dominant cell while the second cell site is considered a secondary cell. A mobile unit may concurrently enter soft handoff with several different secondary cells.
Soft handoffs typically occur when both cell sites operate on a common radio frequency, such as in a code division multiple access (“CDMA”) cellular network. Typically, each cell site operating on the common radio frequency emits a pilot signal. When a mobile unit detects that the strength of a cell site's pilot signal exceeds a certain minimum threshold, it reports a pilot strength measurement (“PSM”) message to the BSC via the dominant cell site. Each cell has its own list of potential handoff candidates, or neighbor list, and if the cell site, or pilot, associated with the pilot signal is in the neighbor list for the dominant cell, then the BSC informs the mobile unit to add the pilot to the mobile unit's Active Set. An Active Set is a list of secondary cells currently in soft handoff, or Active Set Pilots, maintained by the mobile unit. As a result, the BSC can thereby select the appropriate secondary cells for the mobile unit to enter soft handoff.
A detailed description of soft handoff parameters, terms, and procedures is disclosed in “
Mobile Station
-
Base Compatibility Standard for Dual
-
Mode Wide Band Spread Spectrum Cellular System
” TIA/EIA/IS-95-A Standard, Mar. 15, 1995, which is hereby incorporated by reference.
Under this procedure, however, the mobile unit may enter into soft handoff with undesired secondary cells. For example, although the pilot signal from the secondary cell exceeds the minimum threshold, the secondary cell site may be in a location that will improperly service the mobile unit due to other factors or obstructions not yet detected by the mobile unit, which could result in a dropped call. In addition, many pilot signals may exceed the minimum threshold such that soft handoff occurs with each associated secondary cell, requiring an extensive load on the BSC which could result in lost call capacity. Also, since each cell site has a finite number of channel elements with which to link to a mobile unit, each channel element unnecessarily used for soft handoff is thereby unavailable for other calls.
SUMMARY OF THE INVENTION
The present invention, accordingly, provides a method and system for increasing capacity and improving performance of a cellular network by improving the conditions for handoff. In one embodiment the method optimizes the performance of the cellular telephone network by maintaining a matrix of performance values concerning a cell in the network and adjusting certain conditions for handoff in response to the performance values. Examples of performance values include frequency statistics of successful handoffs to a second cell of the cellular telephone network, frequency statistics of failed handoffs to the second cell because the second cell was removed from the first cell's neighbor list, and frequency statistics of failed handoffs to the second cell because the second cell had insufficient resources.
In another embodiment, the method optimizes the performance of the cellular network by maintaining a matrix of status values concerning the cell and adjusting the conditions for handoff in response to the status values. Examples of the status values include a pilot signal strength measurement for the second cell, a designation of whether the second cell is current, removed, or most recently removed, and/or round trip delay measurement from the second cell to the mobile unit. With the status values, a determination can be made to remove the second cell from the mobile unit's Active Set if either the pilot signal strength for the second cell falls below a predetermined threshold or the round trip delay value exceeds a predetermined maximum.
In yet another embodiment, the method optimizes the neighbor list for the cell by maintaining a second list of cells that are not in the neighbor list but have had their pilot signals reported by the mobile unit operating in the dominant cell. Furthermore, the pilot signal strength for the reported cells is tracked, along with the number of occurrences with which any particular cell has been reported. As a result, a determination can be made as to whether to include certain cells in the neighbor list.
A technical advantage achieved is that cost of the cellular network is minimized due to an increased call capacity.
Another technical advantage achieved is that the potential of handing off a call to an undesired cell is greatly reduced.
Another technical advantage achieved is that the cellular network dynamically adapts to its environment.
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Kang, Chang Soon et al, “CDMA Mobile Communication System Performance Analysis Tools for Network Parameter Planning,” IEEE, Apr. 28, 1996.
Aoki Makoto
Eisenzopf Reinhard
Haynes and Boone LLP
Nortel Networks Limited
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