AU4967800A

AU4967800A - A method for cell load sharing in a cellular mobile radio communications system

Info

Publication number: AU4967800A
Application number: AU49678/00A
Authority: AU
Inventors: Robert Karlsson; Lars Stroberg; Christer Akesson
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1999-05-21
Filing date: 2000-05-18
Publication date: 2000-12-12
Also published as: JP2003500952A; SE9901846D0; EP1179274A1; SE515898C2; CN1351806A; SE9901846L; WO2000072618A1

Description

WO 00/72618 PCT/SEOO/00984 1 TITLE: A METHOD FOR CELL LOAD SHARING IN A CELLULAR MOBILE RADIO COMMUNICATIONS SYSTEM 5 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 10 load sharing, wherein the cell coverage of the cells is dy namically changed. Description of the Prior Art In a cellular mobile radio communications system or 15 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 20 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 25 tions systems is that the stochastic behaviour of the traf fic in some cases causes congestion even in a correctly di mensioned cellular system. Cell Load Sharing (CLS) is a feature that provides increased network capacity by distri buting the traffic load between neighbouring cells at tem 30 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 35 lancing is achieved by dynamically moving the borders be tween any two cells such that an overloaded cell becomes smaller and the neighbouring cell larger. Hence, when a WO 00/72618 PCT/SE00/00984 2 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 5 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 dy namically varying the thresholds, the effective areas of the cells are either increased or decreased and the cell 10 shapes are changed. The effective area of the overloaded cell is decreased to a point such that fewer mobile sta tions are handled in the overloaded cell and a higher num ber of mobile stations is handled in neighbouring cells. According to US-A-5 241 685, the movement of the cell 15 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 neigh bours' cell borders before enough mobile stations are moved 20 out from the cell having high traffic load. Further, the cell border is only moved a small prede termined step each time, in order to avoid a considerable amount of handovers. The algorithm is relatively slow and a risk is im 25 pending that the cell load sharing feature would be acti vated late, and, consequently, only has time to provide a marginal effect. Summary of the Invention 30 It is an object of the present invention to provide an improved method of cell load sharing in a cellular mo bile radio communications system, handling temporary load peaks in cells without adding additional cell resources, while still maintaining the quality of service in the sy 35 stem.

WO 00/72618 3 PCT/SEOO/00984 This object is accomplished by a method of load shar ing according to the present invention between a first cell and its neighbouring cells in a cellular mobile radio sy stem,, wherein each cell serves a number of mobile sta 5 tions. It is determined if the traffic load of a first cell is higher than a first load threshold. If so, it is deter mined 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 10 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 15 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 20 load of mobile stations located in a border region between adjacent cells in the communications system. It is deter mined 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 25 second load threshold, if so, at least one neighbouring cell having the highest number of mobile stations, asso ciated with said first cell, within a distance threshold from a cell border between the first cell and the neigh bouring cell is selected from the neighbouring cells having 30 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 35 invention it is determined how much the cell coverage of WO 00/72618 4 PCT/SEOO/00984 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 5 cient handling of fluctuations in the traffic load, wherein the cell coverage of each cell is changed in steps suffi ciently small to prevent oscillation and sufficiently large to be fast. 10 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 embodi ment are made to the accompanying drawings, in which 15 FIG 1 is an illustrative view of a portion of a cel lular network, and FIG 2 is a flow chart illustrating a first embodiment of the method according to the present invention. 20 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 25 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 30 network comprising 7 cells Cl-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 35 cellular network, the mobile station has to be in contact WO 00/72618 5 PCT/SEOO/00984 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 5 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 10 equipment in order to operate in a proper way. However, this is well known by the skilled man and will not be de scribed in further detail herein. Hence, in the following description, numerous spe cific details, such as the number of cells etc., are pro 15 vided in detail in order to give a more thorough descrip tion of the present invention. It will be obvious for those skilled in the art that the present invention may be prac tised without these specific details. Some well-known fea tures such as base stations, mobile service switching cen 20 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. 25 Each base station transmits a list of base stations or channels to be measured to the mobile station. The mo bile 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 30 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 35 changing load or other operating conditions.

WO 00/72618 6 PCT/SEOO/00984 Generally, the MS is continuously measuring and re porting the signal strength from the cell within which it is located, and from its neighbouring cells. If the MS re ceives a better signal strength from a neighbouring cell 5 than from the current serving cell, a handover is per formed. Hence, it is possible to determine the location of a particular MS between a serving cell BS and each neigh bouring cell BS, by using the signal strength measurements. 10 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 15 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 re port list, which is a system global list. Each cell fetches 20 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, 25 measurements already performed are used in the method, avoiding changes in the air interface. As mentioned above, an MS in busy mode is continuous ly measuring the signal strength from the serving cell and the signal strength from a number of neighbouring cells 30 (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 35 nating node such as a Base Station Controller (BSC) or an WO 00/72618 7 PCT/SEOO/00984 MSC, the neighbouring cells are identified by mapping the perch frequencies to their corresponding cells and the mes sage is split up and stored in the global MS report list. When the condition report is received in the BS, all 5 entries in the MS report list are cleared for the specific MS and the new values are stored. When an MS call is termi nated, all entries of that MS are also removed from the MS report list, because there will not be any condition re ports sent from that MS. This assists in keeping the MS re 10 port list consistent with the MS distribution in the net work. 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 15 enough to a neighbouring cell in order to provide a suffi cient 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 20 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 25 taining the identity of the neighbouring cell for which the signal strength measurement has been performed, and the ac tual measured signal strength. For each cell controlled by the co-ordinating node, a periodic update of the MS distribution list is performed, 30 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.

WO 00/72618 8 PCT/SEOO/00984 Before an update of a cell is performed, all MS dis tribution data for the specific cell are removed. Since the data in the MS report list is consistent, the MS distribu tion list will also be consistent with the MS distribution 5 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 neces sary for the operation of the invention. 10 Table 1. MS distribution list (Cell C4) NCELL MS SS C2 A 115 dB C5 B 120 dB C5 C 90 dB C7 C 100 dB C7 D 80 dB According to the measurement presented in Table 1, MS 15 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 20 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 25 offset parameter defines a parallell adjustment of a logi cal 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- WO 00/72618 9 PCT/SEOO/00984 ing the sufficient signal strength value between two neigh bouring cells. The number of idle TCHs in all cells where the CLS function is activated is monitored by the system. Its level 5 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 10 cell above which level CLS is accepted, indicating the available capacity. The main steps of the method according to an embodi ment the present invention are illustrated by the flow chart in FIG 2. 15 The traffic load of a serving cell, C4 in the embodi ment, 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 20 cell coverage should be reduced. However, in order to decrease the coverage of the serving cell C4 it is required that at least one of the neighbouring cells has low traffic. Therefore, it is deter mined at step 203 if the traffic load of any neighbouring 25 cell or cells of the serving cell C4 is lower than the sec ond 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, in dicating the ability to receive MSs with maintained quality 30 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 35 measurements the first one represented in the list of these WO 00/72618 10 PCT/SEOO/00984 cells, i.e the cell C5 is selected.In an alternative em bodiment, the cell having most idle traffic channels is se lected, i.e the cell C7. Hence, the cell coverage of said neighbouring cell C5 5 should be increased and the cell coverage of the serving cell C4 should be decreased a corresponding amount in a di rection from said neighbouring cell. This is obtained by changing the offset. In order to find out how many MSs that will be moved 10 from the serving cell into a neighbouring cell, the MS dis tribution list is scanned for the current cell. For each neighbouring cell, the number of measurements having suffi cient signal strength is counted. In this embodiment the neighbouring cell that has the 15 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 20 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 25 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 30 predict that a change of the cell border in X dB would af fect 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 35 offset should be changed.

WO 00/72618 11 PCT/SEOO/00984 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 5 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 10 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, 15 it is most adequate to move two cell borders of the cell C4, both in relation to the cell CS, 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 20 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 25 between a point 4 and a point 5 towards the serving cell C4 at step 205. As a result the first border then extends be tween 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 30 ing amount in each respective direction from said neigh bouring cells CS 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 35 by the cell C7.

WO 00/72618 12 PCT/SEOO/00984 Further, a check if all cells are handled is per formed 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 5 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 C1 of the plurality of cells C1-C7 associated with the co-ordinating node. 10 In another embodiment of the invention, a check is performed if the neighbouring cell determined to be the re ceiving cell or cells, C5 (and C7) in this example, has enough valid traffic channels to receive the required num ber of mobile stations intended to be handed over from the 15 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 20 stations up to its first load threshold. The second alter native involves handovers of mobile stations until the cur rent serving cell and the receiving neighbouring cell be come equally load balanced. The third alternative implies handovers in order to obtain a load in the neighbouring 25 cell below its first load threshold but above its second load threshold. According to the fourth alternative a maxi mum 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 30 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 35 it is considered to be normal and the method proceeds to WO 00/72618 13 PCT/SEOO/00984 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 traf fic load is considered to be low at step 210, it is deter 5 mined at step 211 whether the coverage of said first cell has been changed in at least one direction from a neigh bouring 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. 10 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. 15 If the cell coverage has been changed during a long time, longer than a predetermined period of time, the cel lular network is not planed properly because of for example a change in the traffic load in the area. Hence, a re arrangement of the cellplanning is needed. In an alterna 20 tive embodiment of the invention, a timer means is pro vided, 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 25 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 30 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 35 213. As a result the first border then extends between the WO 00/72618 14 PCT/SEOO/00984 original points 2 and 3, and the second border extends be tween the other original pair of points 4 and 5. In the current system this is obtained by changing the offset be tween the cells C4 and C5, and C4 and C7, respectively. 5 In another embodiment of the invention, instead of using the signal strength, the location of the mobile sta tions 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 10 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 Supple mentary Service Data (USSD). 15 Based on the current offset, position data and trans mitted power of the serving cell base station (C4) and neigbouring cells' base stations (C1, C2, C3, C5, C6, C7), the cellborder position is predicted based on known radio propagation models. The distance from each MS to the re 20 spective cellborder is calculated and the CLS method pro ceeds 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 25 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 30 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 sug gests a faster and thus more effective method for sharing 35 traffic load between cells. The method describes towards WO 00/72618 15 PCT/SEOO/00984 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. sta bility against oscillation. 5 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 10 present invention as claimed.

Claims

1. A method of load sharing between a first cell (C4) and its neighbouring cells (C1,C2,C3,C5,C6,C7) in a cellu lar mobile radio system, wherein each cell serves a number 5 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, 10 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 15 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 20 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). 25

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 increas ing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4), the further step 30 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 35 one neighbouring cell in order, starting from the first WO 00/72618 17 PCT/SEOO/00984 neighbouring cell (C5), stored in an MS report list of said first cell (C4).

3. A method according to claim 1, 5 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), the further step of: if there are two or more neighbouring cells (C5,C7) 10 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. 15

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 increas ing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4), the further step of: 20 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. 25

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 30 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). WO 00/72618 18 PCT/SEOO/00984

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 5 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 10 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. 15

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 20 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. 25

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 30 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. WO 00/72618 19 PCT/SEOO/00984

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 (C1,C2,C3,C5,C6,C7) is lower than a second load threshold 5 (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 10 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). 15

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 20 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 25 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 30 (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), c h a r a c t e r i z e d by the steps of: WO 00/72618 20 PCT/SEOO/00984 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 5 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 10 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 15 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). 20

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 increas ing the cell coverage of said at least one neighbouring cell (C5;C7) towards said first cell (C4), the further step 25 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, 30 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 35 ing the cell coverage of said at least one neighbouring WO 00/72618 21 PCT/SEOO/00984 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 5 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, 10 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: determining how much the cell coverage of each cell 15 (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, 20 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 25 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, 30 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. 35 WO 00/72618 22 PCT/SE00/00984

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 5 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, 10 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) 15 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 20 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 25 (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 determin ing if the traffic load of the neighbouring cells 30 (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 WO 00/72618 23 PCT/SEOO/00984 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 5 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, 10 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. 15