WO2000072618A1

WO2000072618A1 - A method for cell load sharing in a cellular mobile radio communications system

Info

Publication number: WO2000072618A1
Application number: PCT/SE2000/000984
Authority: WO
Inventors: Lars STRÖBERG; Christer ÅKESSON; Robert Karlsson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1999-05-21
Filing date: 2000-05-18
Publication date: 2000-11-30
Also published as: JP2003500952A; SE9901846D0; EP1179274A1; AU4967800A; SE515898C2; CN1351806A; SE9901846L

Abstract

In a method of load sharing between a first cell (C4) and its neighbouring cells (C1, C2, C3, C5, C6, C7) in a cellular mobile radio system, wherein each cell serves a number of mobile stations (1, A, B, C, D), it is determined if the traffic load of said first cell (C4) is higher than a first load threshold (201, 202), and if so, it is determined if the traffic load of the neighbouring cells (C1, C2, C3, C5, C6, C7) is lower than a second load threshold (203). If it is lower, at least a neighbouring cell (C5) having the highest number of mobile stations, associated with said first cell (C4), with a signal strength above a signal strength threshold (204) is selected from the neighbouring cells (C2, C5, C7). The cell coverage of said neighbouring cell (C5) is increased towards said first cell (C4) (205), and the cell coverage of said first cell (C4) is decreased a corresponding amount in a direction from said neighbouring cell (C5) (206).

Description

TITLE: A METHOD FOR CELL LOAD SHARING IN A CELLULAR MOBILE RADIO COMMUNICATIONS SYSTEM

Field of the Invention

The present invention relates generally to a method of load sharing between cells in a cellular mobile radio communications system, and more particularly to a method of load sharing, wherein the cell coverage of the cells is dynamically changed.

Description of the Prior Art

In a cellular mobile radio communications system or cellular network each cell has a set of neighbouring cells, wherein each cell serves a number of mobile stations.

When a mobile station crosses the border from one cell to another in a cellular network a handover or handoff is performed in the system. Since each cell has several neighbouring cells the system has to determine to which cell the handover should be passed. A traffic channel has to be idle in the neighbouring cell to be selected for the handover.

A common problem in cellular mobile radio communica- tions systems is that the stochastic behaviour of the traffic in some cases causes congestion even in a correctly dimensioned cellular system. Cell Load Sharing (CLS) is a feature that provides increased network capacity by distributing the traffic load between neighbouring cells at tem- porary local load peaks. The traffic load in a cell may show large statistical variations over a short period of time .

US-A-5 241 685 discloses a method for load sharing between cells in a mobile radio system, wherein load ba- lancing is achieved by dynamically moving the borders between any two cells such that an overloaded cell becomes smaller and the neighbouring cell larger. Hence, when a cell is being overloaded, the system determines whether there are any not overloaded neighbouring cells and which of the cells is least loaded. If such a cell exists, the signal strength threshold of the overloaded cell in the di- rection from this neighbouring cell is increased, and/or the signal strength threshold of the neighbouring cell in the direction from the overloaded cell is decreased. By dynamically varying the thresholds, the effective areas of the cells are either increased or decreased and the cell shapes are changed. The effective area of the overloaded cell is decreased to a point such that fewer mobile stations are handled in the overloaded cell and a higher number of mobile stations is handled in neighbouring cells.

According to US-A-5 241 685, the movement of the cell border is performed to at least one of the neighbouring cells at a time, to ensure small changes of the cell size. However, the effect of this movement is not predicted, and the method could cause a movement of several of the neighbours' cell borders before enough mobile stations are moved out from the cell having high traffic load.

Further, the cell border is only moved a small predetermined step each time, in order to avoid a considerable amount of handovers.

The algorithm is relatively slow and a risk is im- pending that the cell load sharing feature would be activated late, and, consequently, only has time to provide a marginal effect.

Summary of the Invention It is an object of the present invention to provide an improved method of cell load sharing in a cellular mobile radio communications system, handling temporary load peaks in cells without adding additional cell resources, while still maintaining the quality of service in the sy- stem. This object is accomplished by a method of load sharing according to the present invention between a first cell and its neighbouring cells in a cellular mobile radio system, , wherein each cell serves a number of mobile sta- tions. It is determined if the traffic load of a first cell is higher than a first load threshold. If so, it is determined if the traffic load of the neighbouring cell is lower than a second load threshold, if so, at least one neighbouring cell having the highest number of mobile sta- tions, associated with said first cell, with a signal strength above a signal strength threshold is selected from neighbouring cells having a traffic load lower than said second load threshold. The cell coverage of the at least one neighbouring cell is increased towards the first cell and the cell coverage of the first cell is decreased to a corresponding degree in a direction from the at least one neighbouring cell.

In another embodiment of the invention the location of mobile stations is determined for the sharing of traffic load of mobile stations located in a border region between adjacent cells in the communications system. It is determined if the traffic load of the first cell is higher than the first load threshold. If so, it is determined if the traffic load of the neighbouring cell is lower than the second load threshold, if so, at least one neighbouring cell having the highest number of mobile stations, associated with said first cell, within a distance threshold from a cell border between the first cell and the neighbouring cell is selected from the neighbouring cells having a traffic load lower than said second load threshold. The cell coverage of the neighbouring cell is increased towards the first cell and the cell coverage of the first cell is decreased to a corresponding degree in a direction from the neighbouring cell. According to another aspect of the invention it is determined how much the cell coverage of each cell should be changed in order to decrease the traffic load of the first cell to become lower than said first load threshold.

An advantage of the present invention is the effi- cient handling of fluctuations in the traffic load, wherein the cell coverage of each cell is changed in steps sufficiently small to prevent oscillation and sufficiently large to be fast .

Brief Description of the Drawings

In order to explain the invention in more detail and the advantages and features of the invention references in the following detailed description of the preferred embodiment are made to the accompanying drawings, in which FIG 1 is an illustrative view of a portion of a cellular network, and

FIG 2 is a flow chart illustrating a first embodiment of the method according to the present invention.

Detailed Description of the Invention

A cell load sharing (CLS) method according to the present invention provides increased network capacity by distributing the traffic load between neighbouring cells at temporary local load peaks. The invention relates to the field of cellular mobile radio communications systems and is exemplified with a TDMA (Time Division Multiple Access) based system, specifically GSM. It is, however, applicable to other systems such as the PDC standard etc.

Referring to FIG 1, there is illustrated a cellular network comprising 7 cells C1-C7 each cell serving a number of mobile stations (MS) 1, A, B, C, D. The mobile station interface to the cellular network is a base station (BS) , usually located in the center of each cell. In order to establish or maintain a call between two subscribers in a cellular network, the mobile station has to be in contact with the base station. As each cell has a limited area, the mobile station 1 has to be in contact with different base stations if it is moved out from one cell area to another. A mobile services switching center (MSC) is a switch- ing node controlling a number of base stations. Depending on the configuration and size of the network one or several MSCs are required.

Of course, a cellular network such as GSM (global system for mobile communications) comprises additional equipment in order to operate in a proper way. However, this is well known by the skilled man and will not be described in further detail herein.

Hence, in the following description, numerous specific details, such as the number of cells etc., are pro- vided in detail in order to give a more thorough description of the present invention. It will be obvious for those skilled in the art that the present invention may be practised without these specific details. Some well-known features such as base stations, mobile service switching cen- ters, and other system entities are not described in detail so as not to make the present invention unclear.

In a digital system such as GSM, a mobile station continuously monitors the perceived power levels of the neighbouring cells. Each base station transmits a list of base stations or channels to be measured to the mobile station. The mobile station continuously performs measurements on the quality of the transmission to/from and the power level of the serving cell, and the neighbouring cells. The results are put in a measurement report, which are periodically sent back to the base station. Based on the measurements, decisions and actions can be taken and the signal strength thresholds, constraints and/or other information parameters associated with the handover can be adjusted according to changing load or other operating conditions. Generally, the MS is continuously measuring and reporting the signal strength from the cell within which it is located, and from its neighbouring cells. If the MS receives a better signal strength from a neighbouring cell than from the current serving cell, a handover is performed.

Hence, it is possible to determine the location of a particular MS between a serving cell BS and each neighbouring cell BS, by using the signal strength measurements. The higher the signal strength is from the neighbouring cell, the closer the MS is to the cell border between the serving cell and the neighbouring cell.

In order to identify the distribution of the MSs in a cell, it is determined to which neighbouring cells each MS is adjacent. Further, an MS could be close to more than one neighbouring cell.

A serving cell BS receives the signal strength measurement from each MS and stores the result in an MS report list, which is a system global list. Each cell fetches data from the MS report list into an MS distribution list, not considering the identity of each MS. Data in the MS distribution list is used to calculate relative location of each MS in a cell.

In the preferred embodiment of the invention, measurements already performed are used in the method, avoiding changes in the air interface.

As mentioned above, an MS in busy mode is continuously measuring the signal strength from the serving cell and the signal strength from a number of neighbouring cells (perch frequencies) audible from the current MS, sending the information to the BS in an existing condition report. Consequently, neither change of the condition report nor modification of the MSs are needed.

When the condition report is received in the co-ordi- nating node such as a Base Station Controller (BSC) or an MSC, the neighbouring cells are identified by mapping the perch frequencies to their corresponding cells and the message is split up and stored in the global MS report list.

When the condition report is received in the BS, all entries in the MS report list are cleared for the specific MS and the new values are stored. When an MS call is terminated, all entries of that MS are also removed from the MS report list, because there will not be any condition reports sent from that MS. This assists in keeping the MS re- port list consistent with the MS distribution in the network .

If the signal strength is above a certain threshold, it is entered into the MS report list for the current cell . The threshold specifies when the mobile station is close enough to a neighbouring cell in order to provide a sufficient speech quality with its BS .

This list is continuously updated by the information reported by the MS. The list does not have to be sorted.

Each entry in the list contains the following: the identity of the MS having performed the measurements, the identity of the neighbouring cell for which the signal strength measurement has been performed, the actual measured signal strength, and the serving cell identity.

For each cell there is an MS distribution list con- taining the identity of the neighbouring cell for which the signal strength measurement has been performed, and the actual measured signal strength.

For each cell controlled by the co-ordinating node, a periodic update of the MS distribution list is performed, wherein the value of the neighbouring cell designation and signal strength from the MS report list is fetched.

The MS distribution list is sorted according to the neighbouring cell number, and within each neighbouring cell according to the measured signal strength. Before an update of a cell is performed, all MS distribution data for the specific cell are removed. Since the data in the MS report list is consistent, the MS distribution list will also be consistent with the MS distribution in the network.

Table 1 shows an MS distribution list for the cell C4 in FIG 1. However, the mobile stations A-D in the Table 1 are provided for the purpose illustration and is not necessary for the operation of the invention.

Table 1.

MS distribution list (Cell C4)

According to the measurement presented in Table 1, MS B has reported the strongest signal strength of 120 dB from the BS in the cell C5 and MS A has reported a signal strength of 115 dB from the BS in cell C2. Further, there are two measurements from MS C : 90 dB from the BS in C5 and 100 dB from the BS in C7. Finally, MS D has reported a sig- nal strength measurement of 80 dB from the BS in C7.

In the method according to the invention a dynamic control of a signal strength offset parameter between a serving cell and a target cell for a handover is provided, using a two threshold occupancy level control . Thus , the offset parameter defines a parallell adjustment of a logical border, i.e a cell to cell signal strength relation. This is used in a cell selection algorithm for traffic channel (TCH) assignment and locating, as means for adjust- ing the sufficient signal strength value between two neighbouring cells.

The number of idle TCHs in all cells where the CLS function is activated is monitored by the system. Its level is continuously compared to two other system parameters, namely two predetermined traffic channel occupancy levels. The first load threshold is the traffic load in a cell above which level CLS is initiated, indicating few idle TCH : s . The second load threshold is the traffic load in a cell above which level CLS is accepted, indicating the available capacity.

The main steps of the method according to an embodiment the present invention are illustrated by the flow chart in FIG 2. The traffic load of a serving cell, C4 in the embodiment, is measured at step 201. If a cell has a high traffic load, higher than the first load threshold, is determined at step 202 and, if so, the border of the cell should be moved inwards by changing the offset parameter, i.e the cell coverage should be reduced.

However, in order to decrease the coverage o_f the serving cell C4 it is required that at least one of the neighbouring cells has low traffic. Therefore, it is determined at step 203 if the traffic load of any neighbouring cell or cells of the serving cell C4 is lower than the second load threshold. If no such low traffic load cell is available, the method proceeds to the step 207.

From the neighbouring cells having a low traffic, indicating the ability to receive MSs with maintained quality of service, a neighbouring cell having the highest number of signal strength measurements is selected at step 204. According to the Table 1, it is the neighbouring cell C5. However, if there are two or more cells, C5 and C7 in this example, having the "highest" number of signal strength measurements the first one represented in the list of these cells, i.e the cell C5 is selected. In an alternative embodiment, the cell having most idle traffic channels is selected, i.e the cell C7.

Hence, the cell coverage of said neighbouring cell C5 should be increased and the cell coverage of the serving cell C4 should be decreased a corresponding amount in a direction from said neighbouring cell. This is obtained by changing the offset.

In order to find out how many MSs that will be moved from the serving cell into a neighbouring cell, the MS distribution list is scanned for the current cell. For each neighbouring cell, the number of measurements having sufficient signal strength is counted.

In this embodiment the neighbouring cell that has the highest number of signal strength measurements (that is the highest number of MSs close to the cell border) is chosen for the current cell C4 to move its cell border towards, because the largest off-loading effect is gained there. It is determined how much the cell border is to be adjusted in order to move a sufficient number of MSs from the serving cell into the chosen neighbouring cell.

The MSs that are close to the neighbouring cell are sorted according to signal strength in the MS distribution list. This means that the MS that is closest to the cell border is placed first in the list, and the MS that has the longest distance to the cell border is placed last in the list .

Distance is measured in dB and since a change of the cell border could be calculated in dB, the system could predict that a change of the cell border in X dB would affect Y number of MSs .

This means that if the system wants to move Y number of MSs from the serving cell into the neighbouring cell, the system knows how many dB the cell border and hence the offset should be changed. In the example illustrated in FIG 1 it is decided to reduce the occupancy level in cell C4 by two TCHs and as a result, the cell coverage of said neighbouring cell C5 is increased by moving a first border extending between a point 2 and a point 3 towards the serving cell C4 at step 205, wherein the border then extends between another pair of points 2 ' and 3 ' , and the cell coverage of the serving cell C4 is decreased a corresponding amount in a direction from said neighbouring cell at step 206. Thus, the MSs B and C are handed over and become served by the neighbouring cell C5.

In another situation also illustrated in FIG 1 it is decided to reduce the occupancy level in cell C4 by three TCHs. According to the measurements presented in Table 1, it is most adequate to move two cell borders of the cell C4 , both in relation to the cell C5 , which is the first cell in the list having the highest number of signal strength measurements, and the cell C7 , which is the second cell in the list having the same highest number of signal strength measurements. Consequently, the cell coverage of the neighbouring cell C5 is increased by moving the border extending between the point 2 and the point 3 towards the serving cell C4 , and the cell coverage of the neighbouring cell C7 is increased by moving a second border extending between a point 4 and a point 5 towards the serving cell C4 at step 205. As a result the first border then extends between the points 2 ' and 3 ' , and the second border extends between still another pair of points 4' and 5' . The cell coverage of the serving cell C4 is decreased a correspond- ing amount in each respective direction from said neighbouring cells C5 and C7 at step 206. Thus, this is obtained by changing the offset between the cells C4 and C5, and C4 and C7, respectively, wherein the MSs B and C then are served by the neighbouring cell C5 and the MS D is served by the cell C7. Further, a check if all cells are handled is performed at step 207. If there are any cells not yet handled by the CLS method according to the invention, the next cell C5 is selected at step 208 and the same procedure as for the cell C4 is performed. However, if all cells have been handled the method waits a delay time CLSDelay at step 209 before it proceeds to the next cell, i.e the cell Cl of the plurality of cells C1-C7 associated with the co-ordinating node . In another embodiment of the invention, a check is performed if the neighbouring cell determined to be the receiving cell or cells, C5 (and C7) in this example, has enough valid traffic channels to receive the required number of mobile stations intended to be handed over from the serving cell C4. This check is necessary to prevent the neighbouring cell to become overloaded. Four alternative embodiments are available to solve this problem.

According to the first alternative the neighbouring cell has a limit allowing it to receive a number of mobile stations up to its first load threshold. The second alternative involves handovers of mobile stations until the current serving cell and the receiving neighbouring cell become equally load balanced. The third alternative implies handovers in order to obtain a load in the neighbouring cell below its first load threshold but above its second load threshold. According to the fourth alternative a maximum number of mobile stations are handed over, however, not causing the serving cell to go below its second load threshold at the same time as the neighbouring cell C5 will exceed its second load threshold.

Reference is again made to step 202. If the current cell does not have high traffic, it is checked at step 210 if the traffic load of the current cell is low, i.e lower than the second load threshold. If the traffic is not low it is considered to be normal and the method proceeds to the step 207. If not all cells have been handled the next cell in the sequence of cells is selected at step 208 and handled according to the CLS method. However, if the traffic load is considered to be low at step 210, it is deter- mined at step 211 whether the coverage of said first cell has been changed in at least one direction from a neighbouring cell, i.e the offset and hence the cell borders have been changed earlier due to CLS, the original cell borders of the current cell are re-established at step 212. Hence, the coverage of the neighbouring cell or cells previously changed in association with the current serving cell is correspondingly decreased. On the other hand, if the coverage of said first cell has not been changed the method proceeds to the step 207. If the cell coverage has been changed during a long time, longer than a predetermined period of time, the cellular network is not planed properly because of for example a change in the traffic load in the area. Hence, a rearrangement of the cellplanning is needed. In an alterna- tive embodiment of the invention, a timer means is provided, indicating when it is time to rearrange the cell- planning. For example, when calls involving the MSs B, C, and D are terminated the occupancy level of the cell C4 is lower than the first load threshold, the cell coverage of C4, C5 and C7 can then be re-established. Thus, the offset parameters and hence the cell borders of the cell C4 are moved, both in relation to the cell C5 and the cell C7. Consequently, the cell coverage of the serving cell C4 is increased at step 212 and the cell coverage of the neigh- bouring cell C5 is decreased by moving the border extending between the point 2 ' and the point 3 ' from the serving cell C4 , and the cell coverage of the neighbouring cell C7 is decreased by moving the second border extending between the point 4' and a point 5' from the serving cell C4 at step 213. As a result the first border then extends between the original points 2 and 3, and the second border extends between the other original pair of points 4 and 5. In the current system this is obtained by changing the offset between the cells C4 and C5 , and C4 and C7 , respectively. In another embodiment of the invention, instead of using the signal strength, the location of the mobile stations in relation to the cell borders is determined. This is performed by means of a stand-alone positioning system as for example the Global Positioning System (GPS) , or with mobile system measurements, for example Timing Advance

(TA) . When the location of the MS is calculated by the MS itself the data can be transmitted to the system (MSC/BSC) via Short Message Service (SMS) or via Unstructured Supplementary Service Data (USSD) . Based on the current offset, position data and transmitted power of the serving cell base station (C4) and neigbouring cells' base stations (Cl, C2 , C3 , C5, C6 , C7) , the cellborder position is predicted based on known radio propagation models. The distance from each MS to the re- spective cellborder is calculated and the CLS method proceeds in a similar manner as described according to the previous embodiments. In order to calculte the "amount" of the cell border movements, the distance is converted to an offset change calculated in dB, again using known radio propagation models.

Although the invention has been described by way of specific embodiments thereof it should be apparent that the present invention provides a method and system for cell load sharing in a cellular mobile radio communications sy- stem that fully satisfies the aims and advantages set forth above, and alternatives, modifications and variations are apparent to those skilled in the art.

In relation to known techniques, the invention suggests a faster and thus more effective method for sharing traffic load between cells. The method describes towards which adjacent cells the border should be moved and how much the border should be moved. This is obtained while preserving the good aspects of known techniques, e.g. stability against oscillation. The air interface between MS and the BS will not be changed which results in the fact that no modification of the MS is needed.

The problem should be applicable for other cellular systems, for example WCDMA, still within the scope of the present invention as claimed.

Claims

CLAIMS 1. A method of load sharing between a first cell (C4) and its neighbouring cells (Cl , C2 , C3 , C5 , C6 , C7) in a cellular mobile radio system, wherein each cell serves a number of mobile stations (1,A,B,C,D) , c h a r a c t e r i z e d by the steps of : determining if the traffic load of said first cell (C4) is higher than a first load threshold (201,202); if so, determining if the traffic load of the neighbouring cells (Cl, C2, C3 , C5, C6, C7) is lower than a second load threshold (203) ; if so, selecting from neighbouring cells (C2,C5,C7) having a traffic load lower than said second load threshold, at least one neighbouring cell (C5;C7) having the highest number of mobile stations (B,C) , associated with said first cell (C4), with a signal strength above a signal strength threshold (204) ; increasing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4) (205) ; and decreasing the cell coverage of said first cell (C4) to a corresponding degree in a direction from said at least one neighbouring cell (C5;C7) (206).

2. A method according to claim 1, c h a r a c t e r i z e d by, before the step of increasing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4), the further step of: if there are two or more neighbouring cells (C5,C7) having the highest number of mobile stations, associated with said first cell (C4), with a signal strength above said signal strength threshold (204), selecting at least one neighbouring cell in order, starting from the first neighbouring cell (C5) , stored in an MS report list of said first cell (C4) .

3. A method according to claim 1, c h a r a c t e r i z e d by, before the step of increasing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4), the further step of: if there are two or more neighbouring cells (C5,C7) having the highest number of signal strength measurements selecting at least one neighbouring cell in order, starting from the cell (C7) having the highest number of idle traffic channels.

4. A method according to any of the preceding claims, c h a r a c t e r i z e d by, before the step of increasing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4), the further step of: determining how much the cell coverage of each cell (C4,C5,C7) should be changed in order to decrease the traffic load of said first cell (C4) to become lower than said first load threshold.

5. A method according to claim 4, c h a r a c t e r i z e d in that the change of the cell coverage is measured in dB, wherein a particular increase/decrease of the cell coverage of the first cell (C4) a number of dB and a corresponding decrease/increase of the cell coverage of said at least one neighbouring cell (C5;C7) causes handover of a particular number of mobile stations between said first cell (C4) and said neighbouring cell (C5;C7) .

6. A method according to claim 4 or 5 , c h a r a c t e r i z e d in that the cell coverage of said at least one neighbouring cell (C5;C7) is increased towards said first cell (C4) (205) , in order to receive at said neighbouring cell (C5;C7), a number of mobile stations up to a maximum level of its first load threshold.

7. A method according to claim 4 or 5 , c h a r a c t e r i z e d in that the cell coverage of said at least one neighbouring cell (C5;C7) is increased towards said first cell (C4) (205), in order to receive at said neighbouring cell (C5;C7), a number of mobile stations causing the first cell (C4) and the the receiving neighbouring cell (C5;C7) to become equally load balanced.

8. A method according to claim 4 or 5 , c h a r a c t e r i z e d in that the cell coverage of said at least one neighbouring cell (C5;C7) is increased towards said first cell (C4) (205) , in order to receive at said neighbouring cell (C5,C7), a number of mobile stations, obtaining a load in the neighbouring cell (C5,C7) below its first load threshold but above its second load threshold.

9. A method according to claim 4 or 5 , c h a r a c t e r i z e d in that the cell coverage of said at least one neighbouring cell (C5;C7) is increased towards said first cell (C4) (205) , in order to receive at said neighbouring cell (C5;C7), a number of mobile stations not causing the serving cell (C4) to go below its second load threshold at the same time as the neighbouring cell (C5;C7) will exceed its second load threshold.

10. A method according to any of the preceding claims, c h a r a c t e r i z e d by, before the step of determining if the traffic load of the neighbouring cells

(Cl, C2, C3 , C5, C6, C7) is lower than a second load threshold (203), the further steps of: determining if the traffic load of said first cell (C4) is lower than its second load threshold (210) ; if so determining whether the coverage of said first cell (C4) has been changed in relation to at least one neigh- bouring cell (C5;C7) (211) ; if so increasing the coverage of the first cell towards said at least one neighbouring cell (C5;C7) (212); and correspondingly, decreasing the coverage of said at least one neighbouring cell (C5;C7) (213) .

11. A method according to any of the preceding claims, c h a r a c t e r i z e d in that the cell coverage of a cell is changed by modifying the signal strength relationship between a first cell (C4) and a neighbouring cell (C5) .

12. A method according to any of the preceding claims, c h a r a c t e r i z e d by the further steps of: ckecking if the cell coverage has been changed during a predetermined period of time, and if so, generating a signal indicating that a rearrangement of the cellplanning is needed.

13. A method of load sharing between a first cell (C4) and its neighbouring cells (Cl, C2 , C3 , C5 , C6 , C7) in a cellular mobile radio system, wherein each cell serves a number of mobile stations (1,A,B,C,D), c h a r a c t e r i z e d by the steps of : determining if the traffic load of said first cell (C4) is higher than a first load threshold (201,202); if so, determining if the traffic load of the neighbouring cells (C1,C2,C3,C5,C6,C7) is lower than a second load threshold (203) ; if so, selecting from neighbouring cells (C2,C5,C7) having a traffic load lower than said second load threshold, at least one neighbouring cell (C5;C7) having the highest number of mobile stations (B,C), associated with said first cell (C4), within a distance threshold from a cell border between the first cell (C4) and said at least one neighbouring cell (C5;C7) (204) ; increasing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4) (205) ; and decreasing the cell coverage of said first cell (C4) to a corresponding degree in a direction from said at least one neighbouring cell (C5;C7) (206) .

14. A method according to claim 13, c h a r a c t e r i z e d by, before the step of increasing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4), the further step of: if there are two or more neighbouring cells (C5,C7) having the highest number of mobile stations, associated with said first cell (C4), within said distance threshold (204), selecting at least one neighbouring cell in order, starting from the first neighbouring cell (C5) , stored in an MS report list of said first cell (C4) .

15. A method according to claim 13, c h a r a c t e r i z e d by, before the step of increas- ing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4) (205), the further step of : if there are two or more neighbouring cells (C5,C7) having the highest number of mobile stations within said distance threshold selecting at least one neighbouring cell in order, starting from the cell (C7) having the highest number of idle traffic channels.

16. A method according to any of the claims 13-15, c h a r a c t e r i z e d by, before the step of increasing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4) (205), the further step of : determining how much the cell coverage of each cell (C4,C5,C7) should be changed in order to decrease the traffic load of said first cell (C4) to become lower than said first load threshold.

17. A method according to claim 16, c h a r a c t e r i z e d in that the change of the cell coverage is measured in a unit of length, wherein a particular increase/decrease of the cell coverage of the first cell (C4) a particular distance and a corresponding decrease/increase of the cell coverage of said at least one neighbouring cell (C5;C7) causes handover of a particular number of mobile stations between said first cell (C4) and said neighbouring cell (C5;C7) .

18. A method according to claim 16 or 17, c h a r a c t e r i z e d in that the cell coverage of said at least one neighbouring cell (C5;C7) is increased towards said first cell (C4) (205), in order to receive at said neighbouring cell (C5;C7), a number of mobile stations up to a maximum level of its first load threshold.

19. A method according to claim 16 or 17, c h a r a c t e r i z e d in that the cell coverage of said at least one neighbouring cell (C5;C7) is increased towards said first cell (C4) (205) , in order to receive at said neighbouring cell (C5,-C7), a number of mobile stations causing the first cell (C4) and the the receiving neighbouring cell (C5;C7) to become equally load balanced.

20. A method according to claim 16 or 17, c h a r a c t e r i z e d in that the cell coverage of said at least one neighbouring cell (C5,-C7) is increased towards said first cell (C4) (205'), in order to receive at said neighbouring cell (C5,C7), a number of mobile stations, obtaining a load in the neighbouring cell (C5,C7) below its first load threshold but above its second load threshold.

21. A method according to claim 16 or 17, c h a r a c t e r i z e d in that the cell coverage of said at least one neighbouring cell (C5;C7) is increased towards said first cell (C4) (205), in order to receive at said neighbouring cell (C5;C7), a number of mobile stations not causing the serving cell (C4) to go below its second load threshold at the same time as the neighbouring cell (C5;C7) will exceed its second load threshold.

22. A method according to any of the claims 13-21, c h a r a c t e r i z e d by, before the step of determining if the traffic load of the neighbouring cells (C1,C2, C3, C5,C6, C7) is lower than a second load threshold (203), the further steps of: determining if the traffic load of said first cell (C4) is lower than its second load threshold (210) ; if so determining whether the coverage of said first cell (C4) has been changed in relation to at least one neighbouring cell (C5;C7) (211); if so increasing the coverage of the first cell towards said at least one neighbouring cell (C5;C7) (212) ; and correspondingly, decreasing the coverage of said at least one neighbouring cell (C5;C7) (213).

23. A method according to any of the claims 13-22, c h a r a c t e r i z e d by the further steps of : checking if the cell coverage has been changed during a predetermined period of time, and if so, generating a signal indicating that a rearrangement of the cellplanning is needed.