Classifications

• • 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
  • H04W16/02—Resource partitioning among network components, e.g. reuse partitioning
  • H04W16/06—Hybrid resource partitioning, e.g. channel borrowing
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/16—Performing reselection for specific purposes
  • H04W36/22—Performing reselection for specific purposes for handling the traffic

Definitions

• TITLE A METHOD FOR CELL LOAD SHARING IN A CELLULAR MOBILE RADIO COMMUNICATIONS SYSTEM
• 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.
• each cell has a set of neighbouring cells, wherein each cell serves a number of mobile stations.
• 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.
• CLS Cell Load Sharing
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 .
• FIG 1 is an illustrative view of a portion of a cellular network
• FIG 2 is a flow chart illustrating a first embodiment of the method according to the present invention.
• a cell load sharing (CLS) method 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.
• TDMA Time Division Multiple Access
• 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.
• BS base station
• the mobile station 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.
• 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.
• GSM global system for mobile communications
• a mobile station 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.
• 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.
• measurements already performed are used in the method, avoiding changes in the air interface.
• 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.
• 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.
• BSC Base Station Controller
• MSC Mobility Management Entity
• 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.
• 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.
• 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.
• 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.
• 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 .
• 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.
• TCH traffic channel
• 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 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.
• the method proceeds to the step 207.
• a neighbouring cell having the highest number of signal strength measurements is selected at step 204.
• the neighbouring cell C5 is selected.
• the cell having most idle traffic channels is selected, i.e the cell C7.
• 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.
• the MS distribution list is scanned for the current cell. For each neighbouring cell, the number of measurements having sufficient signal strength is counted.
• 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 .
• the system knows how many dB the cell border and hence the offset should be changed.
• 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.
• the MSs B and C are handed over and become served by the neighbouring cell C5.
• the first border then extends between the points 2 ' and 3 '
• 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.
• 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.
• a check if all cells are handled is performed at step 207.
• 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 .
• 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.
• 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.
• 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.
• 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.
• the coverage of the neighbouring cell or cells previously changed in association with the current serving cell is correspondingly decreased.
• 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.
• a timer means is provided, indicating when it is time to rearrange the cell- planning.
• the cell coverage of C4, C5 and C7 can then be re-established.
• 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.
• 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.
• this is obtained by changing the offset between the cells C4 and C5 , and C4 and C7 , respectively.
• 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 TA
• MSC/BSC Short Message Service
• USSD Unstructured Supplementary Service Data
• 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.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Mobile Radio Communication Systems (AREA)

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• 1999
  • 1999-05-21 SE SE9901846A patent/SE515898C2/sv not_active IP Right Cessation
• 2000
  • 2000-05-18 EP EP00931862A patent/EP1179274A1/en not_active Withdrawn
  • 2000-05-18 JP JP2000619955A patent/JP2003500952A/ja active Pending
  • 2000-05-18 WO PCT/SE2000/000984 patent/WO2000072618A1/en not_active Application Discontinuation
  • 2000-05-18 CN CN00807892A patent/CN1351806A/zh active Pending
  • 2000-05-18 AU AU49678/00A patent/AU4967800A/en not_active Abandoned

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