Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-06-02
2001-10-16
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S422100, C455S429000, C455S443000, C455S446000, C455S449000, C455S457000
Reexamination Certificate
active
06304754
ABSTRACT:
TECHNICAL FIELD
This invention is related to arranging wireless cells in a wireless network to account for cell handoff. More particularly, this invention relates to a system and method for displaying the interaction between cells in a cell layout to assist in determining an optimal layout solution.
BACKGROUND OF THE INVENTION
When designing the cell layout for a wireless cellular communication system, it is necessary for the various cells to overlap enough to ensure that there are no “dead” areas in the cellular coverage area between cells. The cell overlap layout requirement generally stems from the system imposed hand-off requirements between cells. A typical requirement consists of two rules: (1) sense the occurrence of a particular trigger signal strength at the receiver, e.g., −70 dBm, and notify the system that a hand-off to a new cell is required to maintain system performance, and (2) find a suitable cell to transfer to by finding another cell with a signal strength that is at least equal to some specific target value, e.g., −65 dBm.
In a free space unobstructed, environment, the shape of a cellular radio coverage area, called a cell, is circular in a plane with a size that is easily calculated. In this environment, it is relatively easy to design a cell layout which provides complete coverage without a substantial degree of overlap between the cells. Typical layout procedures based on the ideal, free-space circularly shaped cell require the designer to either ensure that all cells are touching at a particular signal strength or to ensure that all cells are overlapping at a particular signal strength. A nearly perfect layout of circular cells
5
within a defined geographic region
10
is illustrated in FIG.
1
.
While the circular cell model is useful for some situations, in typical indoor environments, the shape of the cell becomes highly distorted due to signal reflections, scattering, and absorption caused by the surrounding structure and various objects within the environment. Sophisticated computer modeling of the indoor environment is used to determine the shape of a cell originating from a given point by calculating signal strength as a function of distance from the center of the cell, i.e., the signal source, with due regard for known environmental characteristics, such as the position and composition of walls, metal supports, wiring, etc.
Conventional site layout software tools, such as WiSE (“Wireless System Engineering” tool), generally include a graphical user interface (“GUI”) through which the calculated shape of the cells is displayed. The user is permitted to adjust the position of the cells based on the graphically displayed information to obtain good coverage while complying with handoff requirements. The cells are typically displayed in color using a graded color map or continuous color contours of a cell, where the color grade or the contours are determined as a function of signal strength away from the center of the cell. This allows the various signal strength boundaries and cell overlap to be visualized.
As is apparent, in order to guarantee good handoff operation, the closer the cells are placed together, the greater the overlap. However, a competing design requirement is to maximize cell coverage to reduce the number of cells which are required to cover a given area. While conventional color layout tools, such as WiSE, make it easier for a layout designer to achieve complete coverage, they result in relatively conservative layouts since it is difficult to determine whether the cells are too close together, and therefore, whether more cells than necessary have been used.
Accordingly, it would be advantageous to provide a graphically based wireless cell layout tool which provides an improved visualization of the cell overlap relative to the handoff requirements that both satisfies the system hand-off requirements and at the same time maximizes cell coverage.
SUMMARY OF THE INVENTION
According to the invention, an improved system and method for indicating the hand-off region between a primary wireless cell and one or more neighbor wireless cells is presented which allows a layout designer to better maximize cell coverage while still satisfying hand-off requirements. The site layout system operates on a conventional computer system running appropriate cell layout software. Details about the surrounding environment are programmed into the system and signal strength calculations are made to determine the signal strength profiles of one or more cells at defined positions in the environment. The determined signal strengths are stored in a computer memory and made available for use by other programs running on the computer system. In addition, each cell of interest is assigned a hand-off trigger threshold signal and a cell boundary threshold signal level, indicating the signal strengths at which a handoff to another cell should be initiated and where a handoff from another cell can be accepted, respectively.
According to the invention, overlap regions between a primary cell and surrounding neighbor cells are determined based on the particular hand-off and boundary threshold regions. In particular, between the primary cell and each neighbor cell, an overlap region defined by the area where the signal strength of the primary cell is at least equal to its boundary threshold signal level and the signal strength of the neighbor cell is at least equal to its hand-off trigger threshold signal level is defined. A second set of overlap regions can also be determined where the signal strength of the primary cell is at least equal to its hand-off trigger threshold and the signal strength of the neighbor cell is at least equal to its boundary threshold signal level.
Once the overlap regions between the primary and neighbor cells are determined, the positions of the cells in the defined wireless environment are graphically indicated on a suitable display and the overlap regions between the primary cell and the neighbor cells are graphically displayed. Preferably, each cell is assigned a unique display attribute, such as a unique color or cross-hatching method. A cell's position is displayed using its unique attribute and the overlap regions are displayed using a combination of the attributes for the overlapping cells.
The graphically displayed overlap regions indicate regions where the hand-off requirements between cells are met in an easy to comprehend manner. Once an operator is able to visualize the hand-off regions in this manner, the position of the cells can be adjusted using appropriate input commands to maximize the cell spacing while at the same time also maximizing the overlap regions. Preferably when a cell position is altered by the user, the overlap regions are redetermined and the graphical display updated accordingly.
It can be appreciated by those skilled in the art that when the bi-colored overlap regions form a continuous boundary around the primary cell, full hand-off coverage is guaranteed. Preferably, an indication is provided to the operator when such a continuous boundary is present.
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DeSantis Charles Michael
Klein Susan L.
Read Christine T.
Avaya Technology Corp.
Darby & Darby
Hunter Daniel
Woldetatios Yemane
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