CA2193690A1 - Method for generating a handoff candidate list - Google Patents
Method for generating a handoff candidate listInfo
- Publication number
- CA2193690A1 CA2193690A1 CA002193690A CA2193690A CA2193690A1 CA 2193690 A1 CA2193690 A1 CA 2193690A1 CA 002193690 A CA002193690 A CA 002193690A CA 2193690 A CA2193690 A CA 2193690A CA 2193690 A1 CA2193690 A1 CA 2193690A1
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- Prior art keywords
- handoff
- candidate
- list
- sector
- eliminating
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- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 238000000034 method Methods 0.000 title claims description 23
- 230000010267 cellular communication Effects 0.000 abstract 1
- 230000001419 dependent effect Effects 0.000 abstract 1
- 239000011159 matrix material Substances 0.000 description 11
- 230000001413 cellular effect Effects 0.000 description 9
- 238000004891 communication Methods 0.000 description 3
- 230000004075 alteration Effects 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- IJJWOSAXNHWBPR-HUBLWGQQSA-N 5-[(3as,4s,6ar)-2-oxo-1,3,3a,4,6,6a-hexahydrothieno[3,4-d]imidazol-4-yl]-n-(6-hydrazinyl-6-oxohexyl)pentanamide Chemical compound N1C(=O)N[C@@H]2[C@H](CCCCC(=O)NCCCCCC(=O)NN)SC[C@@H]21 IJJWOSAXNHWBPR-HUBLWGQQSA-N 0.000 description 1
- 238000012935 Averaging Methods 0.000 description 1
- 239000000969 carrier Substances 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000011179 visual inspection Methods 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/24—Reselection being triggered by specific parameters
- H04W36/32—Reselection being triggered by specific parameters by location or mobility data, e.g. speed data
- H04W36/322—Reselection being triggered by specific parameters by location or mobility data, e.g. speed data by location data
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/24—Reselection being triggered by specific parameters
- H04W36/32—Reselection being triggered by specific parameters by location or mobility data, e.g. speed data
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W16/00—Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
- H04W16/24—Cell structures
Abstract
In a cellular communication system (40) a handoff candidate list is generated for a source sector (51) by eliminating (12) from a list of potential handoff candidates (52-56) any sectors having an interference parameter exceeding a threshold value. The list is further reduced by determining (21) a handoff radius (60) and eliminating any candidate sectors (52-56) that fall outside the hadoff radius (60). Candidate sectors (52-56) that do not have overlapping coverage areas are also eliminated (23). In addition, candidate sectors (52-56) having a relative angle of sight to the source sector (51) that exceeds a maximum angle are also eliminated (24). Finally, the list is reduced (15) to a preset number of candidates dependent upon the interference parameter.
Description
~ WO96~6190 2 1 9 3 6 9 0 PCT~S96/04709 METHOD FOR GENERATING A HANDOFF ~NnTn~F LIST
Field of the Invention The present invention relates, in general, to handoffs in a communication system and, more particularly, to a method for generating a handoff candidate list.
Background of the Invention A Handoff Candidate List consists of a list of a plurality of cellular base stations (such as cells 32-34 of FIG. 3) to which a mobile transceiver may be handed-off when leaving the service region of the source cell31. The list of potential handoff ~n~ t~c for each cell is used by a switch to locate another cell which is better able to serve the mobile.
Generation of the handoff ~n~ te lists for an entire cellular system re~uires knowledge of how Radio Fre~uency (RF) propagation from each cell interacts with the propagation of its surrounding cells.
The creation of a handoff candidate list is, in general, a subjective process. E~ually skilled ~ng;n~r.~, given the same set of input parameters, would be unlikely to produce identical handoff candidate lists. A common methodology involves producing a best server map which indicates the primary coverage regions of each cell, and designating handoffs between cells whose coverage regions share a border (e.g. cells 31 and 33). Potential candidates are eliminated by visual inspection when the line-of-sight path between the source and target cell site is obstructed or when that path crosses through a significant portion of a third cell site. This manual process is time consuming and is -2 1 9 3 6 9 a PCT~S96/04709 prone to error, especial~y when a large number of cell sites are invo~ved When de~rm;n;ng which cells are preferred handoff candidate cells, distance and direction are important factors to co s ider. But distance and direction alone are not always sufficie t criteria. This is because the terrain (buildings, trees, etc.) affects how the signals travel so that the signal strength at the same distance from a cell varies with the direction. Therefore, it is important to fl~tprm;n~ how the terrain affects the propagation.
For the purposes o~ this description, the terms "relative signal strength" and ~'signal-to-noise value"
are used interchangeably, although more precisely, the relative signal strength values represent potential signal-to-noise, since the transmitting frequency is not taken into co~sideration. A signal-to-noise matrix is developed to represent how much the signal from one cell would inter~ere with the signal from another cell i~ the two cells were~to transmit on the same fre~uency. A low signal-to-noise value tends to indicate~cells which are transmitting in~close "RF proximity~, and as such, are likely to re~uire the ability to handoff to one another.
The signal-to-noise matrix is actually based on the relative amount of area which is common to the serving areas of both cells. The serving area oi a cell is the region where a mobile would be serviced by the cell, or sector of a cell in a sectorized design. This is defined by a set of all the points where a signal from the serving cell has the strongest signal strength compared to all other cells transmitting signals. This attempts to model what occurs when a mobile is ~handed off" from one cell to another cell. ~hen the signal strength from another cell becomes stronger than the signal strength from the cell to which the mobile is WO96136190 2 I g 3 ~ ~ 0 r~ . 0:709 currently linked, the mobile is ~handed off~ to the cell providing the stronger signal In actuality, a handoff does not occur when a signal is detected from another cell which is just slightly stronger than the current signal. This would cause a hysteresis effect where the mobile is constantly ~s~ t;ng between two cells with similar signal strength values. This problem is ~V~lC~llle differently in analog and digital systems. In an analog system, a handoff occurs when the current signal drops below some predetermined minimum threshold. In a digital system, the handoff occurs when another signal is detected which is stronger than the current signal by some predetermined threshold.
The signal-to-noise matrix is computed by averaging the signal-to-noise values between a source cell 31 and a target cell 33 for all points where the source cell is a potential server. This is accomplished by comparing the signal strength from the source cell 33 to that of all other cells 32-34 in the cellular system 30. If the source cell 31 has the best signal strength at that point, then it is considered to be the serving cell. If it falls below the best signal strength, then it is not considered the serving cell and a handoff to another cell will occur. In reality, the signal strength must fall below a minimum threshold in the analog case, and below a relative threshold to other carriers in the digital case. So when deciding if the source cell 31 is still the carrier we apply either or both of these criteria. If the signal strength falls below a minimum value, or if it falls outside o~ a range below the best signal strength, then it is not the carrier at that point and the signal-to-noise (S/N) ratio is not computed.
- 21g3690 WO96136190 ~ '01709 -The method to generate~the relative signal-to-noise matrix can be ~crrih~ as follows: for every possible pairing of one cell to another in the cellular system, compare the signal strength at every point common to both cell's propagation regions against the best signal strength at that point for all cells in the system. If the source cel~_~s signal strength at a point is greater than some mini~m value and is greater than or e~ual to the best signal strength at that point less some threshold value, compute the signal-to-noise value at this point. The average of all the signal-to-noise values computed for a source and target cell pair is the signal-to-noise value for that pair.
l~ Brief Description of the Drawings FIGS. l and 2 are flow charts of a process embodying the present invention;
FIG. 3 is a diagram of a communication system; and FIG. 4 is a diagram o~ a communication system utilizing the process of FIGS. l and 2.
Detailed Description of the Drawings ~he prese~t invention is implemented in the context of a method used to predict signal attenuation based on elevation and terrain data; and to produce a matrix of the relative signal strengths between each cell and every other celL in the cellular system.
In the init;al step of the handoff candidate generation process, every possible pairing of two cells is evaluated. Bach pair of cells is ranked based on their relative "RF proximity" to one another. Cells which are strong RF interferers o~ a given cell, by definition7 have coverage areas which overlap. The ~, ?~1935?0 WOg6/36190 r~ o~709 degree to which a cell's coverage area overlaps that of another cell is the first criterion used in det~rm;ni~g whether a handoff may be required between the cells.
Referring initially to FIG. 1, a flow chart, generally designated 10, illustrating a method for generating a handoff candidate list is presented. In step 11, all potential handoff pairs are ranked based on a signal-to-noise matrix. An example of a signal-to-noise matrix, developed using cells 31-34 of FIG. 3, is illustrated in Table 1, below.
Source Target Signal-Cell # Cell # to-Noise Value TAsLE 1 SIGNAL-TO-NOISE MATRIX
The matrix consists of a plurality of values representing the relative signal strength of each possible carrier/interferer pair, thus the matrix contains n~n - 1) values, where n is the number of cells being considered. For this example, we are considering 4 cells and the matrix contains 12 values. For each 21~3690 WO96136190 PCT~S96/04709 carrier, or source cell 31, the set of signal-to-noise values represe~ting the relative interference produced by each of the other cells~-~n~- 1 values) are ranked from lowest to highest magnitude: where low values indicate interferer signal strength nearly eriual to the carrier signal strength (i.e., strong ~ proximity); and higher values ~ndicate interferer signal strength less than the carrier signal strength ~i.e., weak RF
proximity).
R~tnrn;n~ now to FIG. 1, once the ranked list of potential handoff candidates is generated, step 11, cells having i~terference parameters (such as signal-to-noise values) within a predetermined threshold value -and deemed to be within a maximum RF proximity - are included as po~ential handoff candidates, step 12.
Alternatively, handoff candidates having an interference ~L '~r above a maximum value may be eliminated from the handoff candidate list. Next, in step 13, certain handofi ran~ tpc may be eliminated based upon physical location of the candidate cell. Candidates which are located a distance from the source cell which exceeds a maximum distance or whose bore angle direction relative to the line of sight from the source to target exceeds a maximum angle are eliminated from the list. This is described in ~etail below with regard to FIG. 2.
In some cellular technologies, it is nPr~qs~ry to have bi-directional handoff capability ~i.e. the target cell must also specify a handoff to the source cell).
For those systems, the handoff list may be reduced by Pli~;n~t;ng ca~didates for which there is no reciprocal handoff, step 14. For example, if a list was generated for source sector 51 of cel~ 41 of FIG. 4, having sector 52 of cell 42 -as a handoff ~candidate, but the list for sector 52 did not have sector 51 as a handoff candidate, t .
~ WO96~6190 2 1 ~ 3 ~ 9 0 PCT~S96/04709 then, sector 52 would be eliminated as a handoff candidate for sector 51.
In addition, limitations of cellular hardware and software systems make it n~rPc~ry to restrict the total number of target cells to which a given cell may handoff. In order to comply with these e~l; t limitations, it may be necessary to further reduce the number of handoff target cells chosen for a given cell.
Based on the signal-to-noise rankings computed in step 11, those handoff target cells having the highest signal-to-noise value (weakest RF proximity) are eliminated, until the total number of target cells is equal to a preset number, step 15.
In FIG. 2, a flow chart of subprocess 13 is illustrated. Subprocess 13 begins by computing a handoff radius between the source cell and the ~n~ r, cells, step 21. First, the grid coordinates of the cells are used to compute the distance between two cells according to formula (1).
d = ~( X 2 - X 1) ~ ( Y ~ - y l) ~1) where Xl, Yl specifies the grid coordinates of the source cell; and X2, Y2 specifies the grid coordinates of the target cell.
For each source cell, the average distance (D) between the source cell and each of its three nearest neighbor cells is calculated using e~uation (2), below.
D = (dl + d2 + d3)/3 (2) 2193~0 WO96/36190 PCT~S96/04709 The average distance (D) is then lt;p1;ed by a predet~rTin~ constant to provide the handoff radius ~R) in er~uation (3).
R ~ Dx (3) where x ic the predetermined constant.
This handoff radius is represented by dashed circle 60 of FIG. 4. The rationale behind choosing the three neighbors for this purpose is the realization that when a hexagonal grid is overlaid onto the geographical m.ap of a cellular system such that the centers of the hexagons roinri~p with cell sites (a standard practice in the design and modeling of cellular systems), the sides of each hexagon touch the border of three cells adjacent to any given cell. The average distance of the three nearest neighbors is multiplied by the constant value (1.5 for ex_mple) in order to compensate for cell sites located slightly off-grid. The resulting value es~Ahliq~q the maximum distance 60 from a source cell site 41 for which handoff target cells may be located.
Once handoff radius R has been determined, the candidate cells outside the handoff radius, such as cells 42 and 47, are eliminated from the candidate list, step 22. In other words, potential target cells located at a distance greater than the maximum radius 60 computed for a given cell site 41 are eliminated.
The next step in reducing the candidate list is to eliminate any handoff candidates that to do not have an overlapping cov~erage area, step 23. In FIG. 4, sector 56 of cell G7 would be ~l;m;n~t~ since the coverage areas do not overlap.
When directional antennas are deployed at a cell site, the direction that an antenna faces can be used to ~ WO96~6190 21 g 3 6 9 0 r _l/U~_ ~.'0 ~709 _g_ eliminate hardoffs to cells which are outside the antenna's "field of view'~. In most cases, these candidates will also have a high signal-to-noise value.
However, a low signal-to-noise value does not necessarily indicate that the two cells in question face one another It is desirable to ~l;m;n~ handoffs between sectors which do not face one another, because the path a mobile transceiver would travel to re~uire such a handoff can contain coverage gaps, or may cross through the coverage area of a third cell (e.g. between cells 41 and 4~). The formula for computing the relative (bore) angle (relative to the X axis) between a directional source cell and a potential target cell is:
150 = ATAN((Y2 - Y1)/(X2 - X1))(180/~) (4) for (Y2 - Y1) > 0; and 0 = hTAN((Y2 - Y1)/(X2 - X1))(180/~) + 360 (5) for (Y2 - Y1) c 0.
where Xl, Y1 specifies the grid coordinates of the source cell;
X2, Y2 specifies the grid coordinates of the candidate target cell; and ATAN represents the furction arc tangent.
If the relative angle, 0, is greater than a maximum value (threshold angle), then the source sector does not face the potential target sector, and the target sector shall be eliminated, step 24. Subprocess 13 then returns to step 14 of process 10. As an example, the angle between sector 51 of cell 41 and sector 52 of cell 52 would be great enough that, even if the signal-to-noise value between the sectors were sufficient, sector WO96136190 2 1 g 3 6 9 0 PCT~S96/04709 52 would be eliminated from the handoff candidate listof sector 51. .~
While the 'nvention has been described in conjunction with specific embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alterations, modificatlons, and variations in the appended claims.
Field of the Invention The present invention relates, in general, to handoffs in a communication system and, more particularly, to a method for generating a handoff candidate list.
Background of the Invention A Handoff Candidate List consists of a list of a plurality of cellular base stations (such as cells 32-34 of FIG. 3) to which a mobile transceiver may be handed-off when leaving the service region of the source cell31. The list of potential handoff ~n~ t~c for each cell is used by a switch to locate another cell which is better able to serve the mobile.
Generation of the handoff ~n~ te lists for an entire cellular system re~uires knowledge of how Radio Fre~uency (RF) propagation from each cell interacts with the propagation of its surrounding cells.
The creation of a handoff candidate list is, in general, a subjective process. E~ually skilled ~ng;n~r.~, given the same set of input parameters, would be unlikely to produce identical handoff candidate lists. A common methodology involves producing a best server map which indicates the primary coverage regions of each cell, and designating handoffs between cells whose coverage regions share a border (e.g. cells 31 and 33). Potential candidates are eliminated by visual inspection when the line-of-sight path between the source and target cell site is obstructed or when that path crosses through a significant portion of a third cell site. This manual process is time consuming and is -2 1 9 3 6 9 a PCT~S96/04709 prone to error, especial~y when a large number of cell sites are invo~ved When de~rm;n;ng which cells are preferred handoff candidate cells, distance and direction are important factors to co s ider. But distance and direction alone are not always sufficie t criteria. This is because the terrain (buildings, trees, etc.) affects how the signals travel so that the signal strength at the same distance from a cell varies with the direction. Therefore, it is important to fl~tprm;n~ how the terrain affects the propagation.
For the purposes o~ this description, the terms "relative signal strength" and ~'signal-to-noise value"
are used interchangeably, although more precisely, the relative signal strength values represent potential signal-to-noise, since the transmitting frequency is not taken into co~sideration. A signal-to-noise matrix is developed to represent how much the signal from one cell would inter~ere with the signal from another cell i~ the two cells were~to transmit on the same fre~uency. A low signal-to-noise value tends to indicate~cells which are transmitting in~close "RF proximity~, and as such, are likely to re~uire the ability to handoff to one another.
The signal-to-noise matrix is actually based on the relative amount of area which is common to the serving areas of both cells. The serving area oi a cell is the region where a mobile would be serviced by the cell, or sector of a cell in a sectorized design. This is defined by a set of all the points where a signal from the serving cell has the strongest signal strength compared to all other cells transmitting signals. This attempts to model what occurs when a mobile is ~handed off" from one cell to another cell. ~hen the signal strength from another cell becomes stronger than the signal strength from the cell to which the mobile is WO96136190 2 I g 3 ~ ~ 0 r~ . 0:709 currently linked, the mobile is ~handed off~ to the cell providing the stronger signal In actuality, a handoff does not occur when a signal is detected from another cell which is just slightly stronger than the current signal. This would cause a hysteresis effect where the mobile is constantly ~s~ t;ng between two cells with similar signal strength values. This problem is ~V~lC~llle differently in analog and digital systems. In an analog system, a handoff occurs when the current signal drops below some predetermined minimum threshold. In a digital system, the handoff occurs when another signal is detected which is stronger than the current signal by some predetermined threshold.
The signal-to-noise matrix is computed by averaging the signal-to-noise values between a source cell 31 and a target cell 33 for all points where the source cell is a potential server. This is accomplished by comparing the signal strength from the source cell 33 to that of all other cells 32-34 in the cellular system 30. If the source cell 31 has the best signal strength at that point, then it is considered to be the serving cell. If it falls below the best signal strength, then it is not considered the serving cell and a handoff to another cell will occur. In reality, the signal strength must fall below a minimum threshold in the analog case, and below a relative threshold to other carriers in the digital case. So when deciding if the source cell 31 is still the carrier we apply either or both of these criteria. If the signal strength falls below a minimum value, or if it falls outside o~ a range below the best signal strength, then it is not the carrier at that point and the signal-to-noise (S/N) ratio is not computed.
- 21g3690 WO96136190 ~ '01709 -The method to generate~the relative signal-to-noise matrix can be ~crrih~ as follows: for every possible pairing of one cell to another in the cellular system, compare the signal strength at every point common to both cell's propagation regions against the best signal strength at that point for all cells in the system. If the source cel~_~s signal strength at a point is greater than some mini~m value and is greater than or e~ual to the best signal strength at that point less some threshold value, compute the signal-to-noise value at this point. The average of all the signal-to-noise values computed for a source and target cell pair is the signal-to-noise value for that pair.
l~ Brief Description of the Drawings FIGS. l and 2 are flow charts of a process embodying the present invention;
FIG. 3 is a diagram of a communication system; and FIG. 4 is a diagram o~ a communication system utilizing the process of FIGS. l and 2.
Detailed Description of the Drawings ~he prese~t invention is implemented in the context of a method used to predict signal attenuation based on elevation and terrain data; and to produce a matrix of the relative signal strengths between each cell and every other celL in the cellular system.
In the init;al step of the handoff candidate generation process, every possible pairing of two cells is evaluated. Bach pair of cells is ranked based on their relative "RF proximity" to one another. Cells which are strong RF interferers o~ a given cell, by definition7 have coverage areas which overlap. The ~, ?~1935?0 WOg6/36190 r~ o~709 degree to which a cell's coverage area overlaps that of another cell is the first criterion used in det~rm;ni~g whether a handoff may be required between the cells.
Referring initially to FIG. 1, a flow chart, generally designated 10, illustrating a method for generating a handoff candidate list is presented. In step 11, all potential handoff pairs are ranked based on a signal-to-noise matrix. An example of a signal-to-noise matrix, developed using cells 31-34 of FIG. 3, is illustrated in Table 1, below.
Source Target Signal-Cell # Cell # to-Noise Value TAsLE 1 SIGNAL-TO-NOISE MATRIX
The matrix consists of a plurality of values representing the relative signal strength of each possible carrier/interferer pair, thus the matrix contains n~n - 1) values, where n is the number of cells being considered. For this example, we are considering 4 cells and the matrix contains 12 values. For each 21~3690 WO96136190 PCT~S96/04709 carrier, or source cell 31, the set of signal-to-noise values represe~ting the relative interference produced by each of the other cells~-~n~- 1 values) are ranked from lowest to highest magnitude: where low values indicate interferer signal strength nearly eriual to the carrier signal strength (i.e., strong ~ proximity); and higher values ~ndicate interferer signal strength less than the carrier signal strength ~i.e., weak RF
proximity).
R~tnrn;n~ now to FIG. 1, once the ranked list of potential handoff candidates is generated, step 11, cells having i~terference parameters (such as signal-to-noise values) within a predetermined threshold value -and deemed to be within a maximum RF proximity - are included as po~ential handoff candidates, step 12.
Alternatively, handoff candidates having an interference ~L '~r above a maximum value may be eliminated from the handoff candidate list. Next, in step 13, certain handofi ran~ tpc may be eliminated based upon physical location of the candidate cell. Candidates which are located a distance from the source cell which exceeds a maximum distance or whose bore angle direction relative to the line of sight from the source to target exceeds a maximum angle are eliminated from the list. This is described in ~etail below with regard to FIG. 2.
In some cellular technologies, it is nPr~qs~ry to have bi-directional handoff capability ~i.e. the target cell must also specify a handoff to the source cell).
For those systems, the handoff list may be reduced by Pli~;n~t;ng ca~didates for which there is no reciprocal handoff, step 14. For example, if a list was generated for source sector 51 of cel~ 41 of FIG. 4, having sector 52 of cell 42 -as a handoff ~candidate, but the list for sector 52 did not have sector 51 as a handoff candidate, t .
~ WO96~6190 2 1 ~ 3 ~ 9 0 PCT~S96/04709 then, sector 52 would be eliminated as a handoff candidate for sector 51.
In addition, limitations of cellular hardware and software systems make it n~rPc~ry to restrict the total number of target cells to which a given cell may handoff. In order to comply with these e~l; t limitations, it may be necessary to further reduce the number of handoff target cells chosen for a given cell.
Based on the signal-to-noise rankings computed in step 11, those handoff target cells having the highest signal-to-noise value (weakest RF proximity) are eliminated, until the total number of target cells is equal to a preset number, step 15.
In FIG. 2, a flow chart of subprocess 13 is illustrated. Subprocess 13 begins by computing a handoff radius between the source cell and the ~n~ r, cells, step 21. First, the grid coordinates of the cells are used to compute the distance between two cells according to formula (1).
d = ~( X 2 - X 1) ~ ( Y ~ - y l) ~1) where Xl, Yl specifies the grid coordinates of the source cell; and X2, Y2 specifies the grid coordinates of the target cell.
For each source cell, the average distance (D) between the source cell and each of its three nearest neighbor cells is calculated using e~uation (2), below.
D = (dl + d2 + d3)/3 (2) 2193~0 WO96/36190 PCT~S96/04709 The average distance (D) is then lt;p1;ed by a predet~rTin~ constant to provide the handoff radius ~R) in er~uation (3).
R ~ Dx (3) where x ic the predetermined constant.
This handoff radius is represented by dashed circle 60 of FIG. 4. The rationale behind choosing the three neighbors for this purpose is the realization that when a hexagonal grid is overlaid onto the geographical m.ap of a cellular system such that the centers of the hexagons roinri~p with cell sites (a standard practice in the design and modeling of cellular systems), the sides of each hexagon touch the border of three cells adjacent to any given cell. The average distance of the three nearest neighbors is multiplied by the constant value (1.5 for ex_mple) in order to compensate for cell sites located slightly off-grid. The resulting value es~Ahliq~q the maximum distance 60 from a source cell site 41 for which handoff target cells may be located.
Once handoff radius R has been determined, the candidate cells outside the handoff radius, such as cells 42 and 47, are eliminated from the candidate list, step 22. In other words, potential target cells located at a distance greater than the maximum radius 60 computed for a given cell site 41 are eliminated.
The next step in reducing the candidate list is to eliminate any handoff candidates that to do not have an overlapping cov~erage area, step 23. In FIG. 4, sector 56 of cell G7 would be ~l;m;n~t~ since the coverage areas do not overlap.
When directional antennas are deployed at a cell site, the direction that an antenna faces can be used to ~ WO96~6190 21 g 3 6 9 0 r _l/U~_ ~.'0 ~709 _g_ eliminate hardoffs to cells which are outside the antenna's "field of view'~. In most cases, these candidates will also have a high signal-to-noise value.
However, a low signal-to-noise value does not necessarily indicate that the two cells in question face one another It is desirable to ~l;m;n~ handoffs between sectors which do not face one another, because the path a mobile transceiver would travel to re~uire such a handoff can contain coverage gaps, or may cross through the coverage area of a third cell (e.g. between cells 41 and 4~). The formula for computing the relative (bore) angle (relative to the X axis) between a directional source cell and a potential target cell is:
150 = ATAN((Y2 - Y1)/(X2 - X1))(180/~) (4) for (Y2 - Y1) > 0; and 0 = hTAN((Y2 - Y1)/(X2 - X1))(180/~) + 360 (5) for (Y2 - Y1) c 0.
where Xl, Y1 specifies the grid coordinates of the source cell;
X2, Y2 specifies the grid coordinates of the candidate target cell; and ATAN represents the furction arc tangent.
If the relative angle, 0, is greater than a maximum value (threshold angle), then the source sector does not face the potential target sector, and the target sector shall be eliminated, step 24. Subprocess 13 then returns to step 14 of process 10. As an example, the angle between sector 51 of cell 41 and sector 52 of cell 52 would be great enough that, even if the signal-to-noise value between the sectors were sufficient, sector WO96136190 2 1 g 3 6 9 0 PCT~S96/04709 52 would be eliminated from the handoff candidate listof sector 51. .~
While the 'nvention has been described in conjunction with specific embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alterations, modificatlons, and variations in the appended claims.
Claims (10)
1. A method for generating a handoff candidate list for a source sector comprising the steps of:
generating a ranked list of a plurality of handoff candidates by ranking a plurality of potential handoff sectors in order of an interference parameter; and eliminating a candidate of said plurality of handoff candidates from said ranked list based on physical location in relation to said source sector, thereby forming said handoff candidate list.
generating a ranked list of a plurality of handoff candidates by ranking a plurality of potential handoff sectors in order of an interference parameter; and eliminating a candidate of said plurality of handoff candidates from said ranked list based on physical location in relation to said source sector, thereby forming said handoff candidate list.
2. The method of claim 1 wherein said step of forming said handoff candidate list comprises the steps of:
determining a handoff radius; and eliminating a candidate of said plurality of handoff candidates from said ranked list which falls outside said handoff radius.
determining a handoff radius; and eliminating a candidate of said plurality of handoff candidates from said ranked list which falls outside said handoff radius.
3. The method of claim 1 wherein said step of eliminating said candidate of said plurality of handoff candidates comprises the step of eliminating a target sector from said ranked list unless said target sector has a coverage area which overlaps a coverage area of said source sector.
4. The method of claim 1 further comprising the step of eliminating a target sector of said plurality of handoff candidates from said ranked list of said source sector unless said source sector is present on a handoff candidate list for said target sector.
5. The method of claim 1 further comprising the step of eliminating a target sector of said plurality of handoff candidates from said ranked list of said source sector if a relative angle between said source sector and said target sector exceeds a maximum value.
6. A method for generating a handoff candidate list for a source cell comprising the steps of:
generating a ranked list of a plurality of handoff candidates by ranking a plurality of potential handoff cells in order of an interference parameter; and eliminating a candidate of said plurality of handoff candidates from said ranked list based on physical location in relation to said source cell, thereby forming said handoff candidate list.
generating a ranked list of a plurality of handoff candidates by ranking a plurality of potential handoff cells in order of an interference parameter; and eliminating a candidate of said plurality of handoff candidates from said ranked list based on physical location in relation to said source cell, thereby forming said handoff candidate list.
7. The method of claim 6 wherein said step of eliminating said candidate of said plurality of handoff candidates to from said handoff candidate list comprises the steps of:
determining a handoff radius; and eliminating a candidate of said plurality of handoff candidates from said ranked list which falls outside said handoff radius.
determining a handoff radius; and eliminating a candidate of said plurality of handoff candidates from said ranked list which falls outside said handoff radius.
8. The method of claim 6 wherein said step of eliminating said candidate of said plurality of handoff candidates from said handoff candidate list comprises the step of eliminating a target cell from said ranked list unless said target cell has a coverage area which overlaps a coverage area of said source cell.
9. A method for generating a handoff candidate list for a source sector comprising the steps of:
generating a ranked list of a plurality of handoff candidates by ranking a plurality of potential handoff sectors in order of an interference parameter;
computing a handoff radius;
eliminating a candidate of said plurality of handoff candidates from said ranked list which falls outside said handoff radius; and eliminating a target sector from said ranked list unless said target sector has a coverage area which overlaps a coverage area of said source sector, thereby forming said handoff candidate list.
generating a ranked list of a plurality of handoff candidates by ranking a plurality of potential handoff sectors in order of an interference parameter;
computing a handoff radius;
eliminating a candidate of said plurality of handoff candidates from said ranked list which falls outside said handoff radius; and eliminating a target sector from said ranked list unless said target sector has a coverage area which overlaps a coverage area of said source sector, thereby forming said handoff candidate list.
10. The method of claim 13 further comprising the step of eliminating a target sector of said plurality of handoff candidates from said ranked list of said source sector unless said source sector is present on a handoff candidate list for said target sector.
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|---|---|---|---|
| US08/439,149 | 1995-05-11 | ||
| US08/439,149 US5640676A (en) | 1995-05-11 | 1995-05-11 | Method for generating a handoff candidate list |
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| Publication Number | Publication Date |
|---|---|
| CA2193690A1 true CA2193690A1 (en) | 1996-11-14 |
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| CA002193690A Abandoned CA2193690A1 (en) | 1995-05-11 | 1996-04-05 | Method for generating a handoff candidate list |
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| US (1) | US5640676A (en) |
| EP (1) | EP0770313B1 (en) |
| JP (1) | JP3848681B2 (en) |
| KR (1) | KR100232266B1 (en) |
| CA (1) | CA2193690A1 (en) |
| DE (1) | DE69626352T2 (en) |
| FI (1) | FI115590B (en) |
| WO (1) | WO1996036190A1 (en) |
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-
1996
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- 1996-04-05 WO PCT/US1996/004709 patent/WO1996036190A1/en active IP Right Grant
- 1996-04-05 JP JP53405696A patent/JP3848681B2/en not_active Expired - Lifetime
- 1996-04-05 KR KR1019970700149A patent/KR100232266B1/en not_active IP Right Cessation
- 1996-04-05 EP EP96913780A patent/EP0770313B1/en not_active Expired - Lifetime
- 1996-04-05 DE DE69626352T patent/DE69626352T2/en not_active Expired - Lifetime
- 1996-12-27 FI FI965221A patent/FI115590B/en not_active IP Right Cessation
Also Published As
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| US5640676A (en) | 1997-06-17 |
| KR970705317A (en) | 1997-09-06 |
| EP0770313A1 (en) | 1997-05-02 |
| KR100232266B1 (en) | 1999-12-01 |
| JP3848681B2 (en) | 2006-11-22 |
| JPH10503353A (en) | 1998-03-24 |
| WO1996036190A1 (en) | 1996-11-14 |
| DE69626352T2 (en) | 2003-09-18 |
| FI965221A (en) | 1996-12-27 |
| FI965221A0 (en) | 1996-12-27 |
| FI115590B (en) | 2005-05-31 |
| EP0770313A4 (en) | 1998-12-09 |
| DE69626352D1 (en) | 2003-04-03 |
| EP0770313B1 (en) | 2003-02-26 |
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